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doi:10.2204/iodp.proc.304305.103.2006

Geochemistry

During Expedition 304, chemical analyses were performed on 1 piece of ultramafic rubble, 2 harzburgites, 7 basalts, 1 basaltic breccia, 7 diabases, and 6 gabbroic rocks from Hole U1309B and on 2 basalts, 11 diabases, 59 gabbroic rocks, 2 peridotites, and 1 wehrlite from Hole U1309D. During Expedition 305, 8 olivine-rich troctolites, 119 gabbroic rocks, and 4 diabases were analyzed. Gabbros, olivine, and olivine-bearing gabbros are the most common rock types at Site U1309. Sample selection was based on discussion among representatives from all expertise teams within the expedition scientific party. ICP-AES was used for determining major and trace element concentrations, and gas chromatography was used for H2O and CO2. Samples are representative of the rocks recovered from Site U1309 (see “Igneous petrology” and “Metamorphic petrology” for characterization of the lithologic units). Tables T10 and T11 list all analytical data (including H2O and CO2 contents) collected during Expeditions 304 and 305.

One water sample was taken at seafloor at the beginning of Expedition 304 (see “Operations”) and analyzed onboard for chemical composition (see “Appendix A”). Two water samples were taken at the bottom of Hole U1309D at the beginning of each logging session during Expedition 305 (at ~2396 and ~1215 mbsf; see “Operations”) and analyzed onboard for physical properties and chemical composition (see “Appendix B”).

Basalts and diabases

Basalts were recovered from the upper sections of Holes U1309B and U1309D. Two basalt and diabase intervals are present in Hole U1309B, and three are present in Hole U1309D above 401.3 mbsf (see “Igneous petrology”). Above 401.3 mbsf, Site U1309 basalts are characterized by loss on ignition (LOI) values ranging from –0.05 to 1.45 wt% and H2O contents ranging from 0.56 to 2.00 wt% (Fig. F213). Site U1309 upper diabases are distinguished by more variable, but still low, LOI and H2O contents (LOI = –0.28–1.90 wt%; H2O = 0.79–2.33 wt%). LOI and H2O contents are positively correlated; however, LOI slightly underestimates the total volatile content due to the conversion of Fe2+ to Fe3+ during heating of the sample powders to ~1000°C (see the “Methods” chapter). The relatively low LOI and H2O values are consistent with the thin section descriptions and XRD results (see “Metamorphic petrology”) that show that Site U1309 basalts and diabases are modified to different extents by alteration with secondary precipitation of tremolite and actinolite and, more rarely, chlorite. Because tremolite and actinolite have low H2O contents (<2.3 wt%) relative to chlorite (>10 wt%), the volatile content of Site U1309 basalts and diabases is not a direct reflection of the degree of alteration of the rock but more likely reflects the mode of the secondary assemblage. Site U1309 diabases sampled above 401.3 mbsf have low CO2 (<0.01–0.1 wt%), except for Sample 304-U1309B-5R-2, 90–94 cm, which has slightly higher CO2 (0.13 wt%). Site U1309 basalts sampled above 401.3 mbsf have higher but variable CO2 contents ranging from 0.02 to 0.49 wt% (Table T10). These slightly higher CO2 contents do not correlate with LOI, H2O, or any major elements. In addition, thin section descriptions and XRD results (see “Metamorphic petrology”) do not show evidence of a different type of alteration, such as carbonate precipitation, that could explain a higher CO2 contents in these samples. Therefore, we interpret this difference in CO2 contents as a primary feature of these rocks.

Diabases sampled below 401.3 mbsf (Expedition 305) have high H2O contents (1.8 and 2.7 wt%) (Fig. F213) compared with diabases from the upper part of Hole U1309D, whereas their CO2 contents are relatively low (0.01–0.05 wt%) (Table T11). The relatively high H2O contents of Sample 305-U1309-94R-3, 22–26 cm (2.7 wt%), is consistent with the thin section descriptions (Sample 94R-3, 34–36 cm), showing a total alteration of ~30%.

Most basalts and diabases sampled in Hole U1309B and in the upper part of Hole U1309D to 401.3 mbsf (during Expedition 304) are similar in SiO2 and alkali contents (SiO2 = 49–53 wt%; Na2O + K2O = 2.2–3.7 wt%). They have 5.7–10.4 wt% MgO, 7.1–15.8 wt% Fe2O3, 12.7–15.7 wt% Al2O3, 9.5–12.8 wt% CaO, and 1.1–2.6 wt% TiO2. These values are typical of tholeiitic basalt compositions, except for two samples from Hole U1309B that have a slightly more andesitic composition (Figs. F15, F214). Basalt and diabase trace element contents range 62–184 ppm Zr, 240–400 ppm V, 38–47 ppm Sc, and 30–71 ppm Y (Fig. F215). These elements are considered immobile during alteration (e.g., Hébert et al., 1990). Zr and Y are incompatible elements, preferentially partitioned into the liquid during partial melting or fractional crystallization. The increase in these incompatible elements in Site U1309 diabases and basalts at relatively constant Zr/Y ratio (1.6–3) suggests an increasing degree of fractionation of silicate phases, the most Zr- and Y-enriched diabases and basalts being the most differentiated (Fig. F215). Moderately incompatible elements such as Ti, V, and Sc show a more complex evolutionary trend. Figure F216 shows that V and Sc both show a strong positive correlation with Ti. Ti, V, and Sc concentrate in clinopyroxene; however, Ti and V also have a strong affinity for oxide minerals, especially for magnetite. The increases in Ti and V correlate with an increase in Fe content and may result from local increases in the abundance of titanomagnetite (Fig. F215). These observations are consistent with visual core and thin section descriptions (see “Igneous petrology”) and with physical property measurements (see “Physical properties”) that show that high Fe diabases and basalts were sampled in areas where MS, as measured by the MST, was high (Fig. F217) and where magnetite was more abundant. Site U1309 basalts and diabases have Sr ranging from 80 to 130 ppm and Ba <38 ppm. These elements are highly incompatible, but they are also mobile in hydrothermal fluids, which may explain some scattering in Ba content. Sr contents correlate with Ti, Y, and Zr, but they are low compared to MAR volcanic glasses (Fig. F215).

Using the classification of Le Maitre et al. (1989), the diabases sampled during Expedition 305 from 471 to 1378 mbsf have basaltic compositions (Fig. F15). Trace element concentrations are somewhat scattered (compatible elements: 159–357 ppm Cr and 82–125 ppm Ni; moderately incompatible elements: 0.63–1.72 wt% Ti, 179–383 ppm V, and 30–45 ppm Sc; incompatible elements: 50–160 ppm Zr and 22–60 ppm Y), probably because of slight degrees of fractionation (Fig. F214; Table T11).

Major and trace element compositions of Site U1309 diabases and basalts sampled during Expeditions 304 and 305 span most of the field of basalt glasses from the entire MAR (Figs. F214, F215). All samples are slightly CaO and Al2O3 poor and Na2O rich compared to basaltic glass compositions from the MAR (Fig. F214). These variations may be related to the pervasive greenschist alteration observed, to different extents, in all diabases and basalts in Holes U1309B and U1309D (see “Metamorphic petrology”). Major elements in diabases show the largest scatter. The analyzed diabases are fine to medium grained and sparsely plagioclase phyric (see “Site U1309 thin sections” in “Core descriptions”), and their compositional variability may reflect the primary mode of the selected sample and/or the degree of differentiation of the crystallizing melt, in addition to alteration. For example, two diabases from igneous Unit 20 (Expedition 304) plot outside the field defined by the other Site U1309 basalt and diabase (Fig. F214). Sample 304-U1309B-5R-1, 127–129 cm, is distinguished by slightly higher SiO2 (52 wt%) and Al2O3 (14.5 wt%) and low MgO (6.85 wt%), whereas Sample 5R-2, 90–94 cm, has lower SiO2 (48.9 wt%) and Al2O3 (12.7 wt%) and higher MgO (9.91 wt%). These variations are explained, in part, by small variations in modal compositions; Sample 304-U1309B-5R-1, 127–129 cm, has a higher plagioclase/clinopyroxene ratio than Sample 5R-2, 90–94 cm, and is therefore relatively Al rich and Mg poor compared to the latter. These variations are also correlated with TiO2 and trace element values, indicating that Sample 304-U1309B-5R-1, 127–129 cm, is relatively more evolved. We also observe a decrease in trace element content from fine-grained margins to the coarser grained centers of diabase bodies, such as in Diabase D-5 (Unit 42 and 44, Hole U1309D) (see “Igneous petrology”). We interpret these variations as reflecting the mode of the samples selected in the coarser grained areas.

Sample 304-U1309B-14R-1, 107–109 cm, a plagioclase-phyric basalt (Unit 40), differs from the other Site U1309 basalts and diabases in having slightly higher SiO2 (53.11 wt%) and Na2O (3.2 wt%) and lower Fe2O3 (7 wt%), although other major and trace element contents plot in the same field as that of the other basalt and diabase samples from Site U1309. The high SiO2 together with high MgO (8.5 wt%) is poorly understood. Detailed studies on more samples should provide a means to explain these slight differences in composition.

Basaltic breccia

We analyzed a basaltic breccia from Hole U1309B (Unit 15), Sample 304-U1309B-3R-1, 71–74 cm. This sample is composed of relatively fresh clasts of basalt and diabase in a heavily altered matrix (see “Site U1309 thin sections” in “Core descriptions”). It displays LOI and volatile contents similar to those of the diabase and basalt samples (LOI = 1.99 wt%; H2O = 2.57 wt%; CO2 = 0.10 wt%). It is characterized by high MgO (13.1 wt%) and low Al2O3 (10.4 wt%). Compared to Site U1309 diabases and basalts, it displays, on average, lower incompatible element contents (Fig. F215). However, in contrast to Site U1309 basalts and diabases, Cr and Ni contents are above the detection limit in this sample (Cr = 480 ppm and Ni = 370 ppm). Cr and Ni are compatible elements concentrated in the most primitive melts. Sample 304-U1309B-3R-1, 71–74 cm, is composed of basaltic clasts embedded in a matrix composed mainly of secondary tremolite and actinolite (see “Metamorphic petrology”). These minerals are depleted in Al but have high Mg contents compared to the average composition of Site U1309 basalts and diabases. The high MgO, Ni, and Cr contents of Sample 304-U1309B-3R-1, 71–74 cm, suggests either that primitive material could have been mixed within the primary assemblage that formed this matrix and/or that the hydrothermal fluids that flowed through the basalt breccia and induced its alteration were in equilibrium with serpentinite.

Gabbroic rocks

We analyzed 6 gabbroic rocks from Hole U1309B and 59 from Hole U1309D (to 401.3 mbsf) during Expedition 304, as well as 119 gabbroic samples from Hole U1309D (to 1378 mbsf [Core 305-U1309D-287R]) during Expedition 305. Gabbroic rocks include olivine-rich troctolite, troctolite, olivine gabbro, olivine-bearing gabbro, gabbro and gabbronorite, microgabbro, oxide gabbro, and leucocratic gabbro. For simplicity, some rock types classified in the visual core and thin section descriptions were grouped: gabbro, cataclastic gabbro, medium-grained gabbro, and coarse-grained gabbro are referred to as gabbro; troctolitic gabbro, olivine gabbro, medium-grained troctolitic gabbro, layered olivine-gabbro and troctolite, troctolite with olivine-gabbro bands, medium-grained olivine-gabbro, and alternating troctolite and olivine-gabbro sequence are referred to as olivine gabbro. Olivine-bearing gabbros are referred to as gabbros, except for samples having ≥5% olivine based on thin section descriptions, which are referred to as olivine gabbros. Thin section estimates of modal composition indicate that Sample 304-U1309D-8R-2, 123–129 cm, is a troctolitic interval in an interval of troctolitic gabbro; it is referred to as a troctolite. Orthopyroxene-bearing gabbro and gabbronorite are plotted with gabbro in all figures. Olivine-rich troctolite intervals that comprise olivine-rich troctolite and dunite are described separately.

Gabbro and troctolite

From Hole U1309B, we selected two gabbro samples from Unit 28 above a serpentinized peridotite interval (Unit 32; 61 mbsf), one gabbro below this interval (Unit 56), and one olivine gabbro sample each from Units 46, 50, and 53. From Hole U1309D, we selected 17 gabbro samples, 25 olivine gabbros, 5 troctolites, 2 orthopyroxene-bearing gabbros, 3 oxide gabbros, and a late magmatic leucocratic dike crosscutting an olivine-bearing gabbro unit (Unit 89; Expedition 304).

Gabbroic rock comprises abundant plagioclase and clinopyroxene with various amounts of olivine (see “Igneous petrology”). Major and trace element compositions mainly reflect the mode of these phases in the rocks. For example, high plagioclase abundance leads to elevated Al2O3 at lower MgO, whereas high olivine contents cause high MgO at low CaO.

Site U1309 gabbros and orthopyroxene-bearing gabbros down to 401.3 mbsf (Expedition 304) are characterized by LOI values ranging from 0.3 to 1.74 wt% and H2O ranging from 0.6 to 1.9 wt% (Fig. F213). Oxide gabbros have comparable H2O (0.70–1.07 wt%) but lower LOI (–0.28–1.05 wt%). The conversion of Fe2+ to Fe3+ during heating of the sample powders to ~1000°C (see the “Methods” chapter) leads to a strong underestimation of the volatile contents in these Fe-rich samples (Fe2O3 > 22 wt%). Olivine gabbros and troctolites are characterized by more scattered and higher LOI and H2O values, with LOI ranging from 0.2 to 9.6 wt% and H2O contents ranging from 0.55 to 10.6 wt%. These LOI and H2O values are consistent with the thin section descriptions and XRD results (see “Metamorphic petrology”) that show that the analyzed gabbro samples were modified to different extents by alteration, as shown by secondary crystallization of tremolite, actinolite, and, more rarely, brown hornblende in all gabbroic rocks. Alteration of olivine to talc and/or chlorite is commonly complete in olivine gabbros and in troctolites. All gabbroic rocks display low CO2 contents (<0.23 wt%).

Hole U1309D gabbros below 401.3 mbsf (Expedition 305) have H2O contents between the detection limit of 0.15 and 7.3 wt% (Figs. F213, F218), which reflects differences in the style and degree of alteration (see "Discussion"). Most gabbroic rocks display CO2 contents near the detection limit of 0.04 wt% (Table T11). Only Samples 305-U1309D-101R-3, 0–14 cm, 111R-2, 6–14 cm, and 158R-1, 11–17 cm, have CO2 contents of ~0.2 wt%, consistent with carbonate veining in those parts of the core (see “Metamorphic petrology”). The volatile data are consistent with thin section descriptions and XRD results (see “Metamorphic petrology”) that show that the analyzed gabbro samples were modified to different extents by alteration, as shown by secondary formation of talc, tremolite, actinolite, chlorite, zeolite, and serpentine minerals.

Gabbroic rocks sampled in Hole U1309B and above 401.3 mbsf in Hole U1309D (Expedition 304) comprise gabbros, orthopyroxene-bearing gabbros, olivine gabbros, troctolites, and olivine-rich troctolites. The gabbros have 47.5–53.5 wt% SiO2, 5–12 wt% MgO, 0.14–0.49 wt% TiO2, 15.2–23.5 wt% Al2O3, and 2.9–7.4 wt% Fe2O3. Orthopyroxene-bearing gabbros have similar compositions to gabbros: 51.8–53 wt% SiO2, 10.4–12.2 wt% MgO, 0.2–0.3 wt% TiO2, 15.6–16.45 wt% Al2O3, and 4.4–6.4 wt% Fe2O3. Olivine gabbros overlap in composition with the gabbros, having 4.5–26.5 wt% MgO, <0.3 wt% TiO2, 10.5–26.4 wt% Al2O3, and 2.8–9.9 wt% Fe2O3. The troctolites have the most primitive compositions, with 9–36.8 wt% MgO, <0.18 wt% TiO2, 8–26.9 wt% Al2O3, and 3.3–10.4 wt% Fe2O3 (Fig. F219). Texturally and compositionally, all of these samples represent cumulate compositions and comprise abundant plagioclase and clinopyroxene, with various proportions of olivine (see “Site U1309 thin sections” in “Core descriptions”). The most extreme variations due to variations in plagioclase content are in Sample 304-U1309D-48R-1, 111–119 cm, an anorthositic troctolite (Unit 108; see “Igneous petrology”) that has 29.2 wt% Al2O3 and 4.25 wt% MgO.

Below 401.3 mbsf (Expedition 305), the gabbros have 41.8–53.2 wt% SiO2, 3.0–36.1 wt% MgO, 0.07–7.9 wt% TiO2, 7.1–23.6 wt% Al2O3, and 4.0–29.1 wt% Fe2O3 (Figs. F219, F220). Troctolites have the most primitive compositions (Figs. F219, F220). Al2O3 (7.1–21.9 wt%), MgO (11.9–36.1 wt%), CaO (4.1%–10.5%), and Na (0.4–2.3 wt%) show a large range of concentrations due to the modal variations in olivine and plagioclase, whereas SiO2 (41.8–46.6 wt%), Fe2O3 (6.7–11.1 wt%), and TiO2 (0.07–0.14 wt%) are less scattered. Olivine gabbros show the largest range in major element composition among the gabbroic rocks. They tend to have more primitive compositions than gabbros and are more evolved than troctolites (43.6–50.9 wt% SiO2, 10.4–23.6 wt% Al2O3, 7.2–24.0 wt% MgO, 7.2–15.1 wt% CaO, and 0.11–0.36 wt% TiO2) (Figs. F219, F220) but still overlap with the other gabbroic rocks. Olivine-bearing gabbros (48.1–52.3 wt% SiO2, 14.1–21.4 wt% Al2O3, 8.9–12.9 wt% MgO, 13.1–15.2 wt% CaO, and 0.18–0.56 wt% TiO2) (Figs. F219, F220), in general, overlap the compositional field defined by the gabbros (49.0–53.2 wt% SiO2, 14.0–22.2 wt% Al2O3, 7.4–11.8 wt% MgO, 10.7–14.7 wt% CaO, and 0.19–1.72 wt% TiO2) (Figs. F219, F220) but have, on average, more primitive compositions. Orthopyroxene-bearing gabbros, which are included here for simplicity in the gabbro group, overlap the gabbros in composition but tend to be slightly more SiO2 rich. Four microgabbros were analyzed (Samples 305-U1309D-103R-1, 15–23 cm, 113R-2, 145–149 cm, 117R-4, 23–27 cm, and 181R-1, 56–62 cm) from 511 and 881 mbsf. They fully overlap the compositional range defined by the gabbros and olivine gabbros (Figs. F219, F220).

Compared to gabbros sampled during Leg 153 (Agar et al., 1997) and Leg 209 (Kelemen, Kikawa, Miller, et al., 2004) from the MAR and from Hole 735B (Dick, Natland, Miller, et al., 1999) on the SWIR, Site U1309 gabbroic rocks have mostly highly primitive compositions indicated by high Mg# (molar Mg/[Mg + Fe]) from ~52 to 90 and low TiO2 (<0.49 wt%) and Na2O (0.1–3.7 wt%) (Fig. F219, F220). The only exceptions are the oxide gabbros and leucocratic gabbros.

The oxide gabbros and oxide-bearing gabbros have compositions similar to other Site U1309 gabbroic rocks, except for the high TiO2 (up to 7.9 wt%) and Fe2O3 (up to 29.2 wt%) and low Al2O3 (<13.2 wt%) and SiO2 (<47.72 wt%) in the most oxide rich samples. The positive correlation between Fe2O3 and TiO2 (Figs. F221) indicates that the variation in these two elements is primarily related to oxide abundance in Site U1309 gabbroic rocks.

Leucocratic gabbros are outstanding in terms of major and trace element composition. They show evolved, highly variable compositions.

The late magmatic leucocratic dike (Sample 304-U1309D-36R-3, 80–82 cm) sampled during Expedition 304 clearly stands out in comparison to the different gabbroic groups, having lower MgO (4.85 wt%) and CaO (4.8 wt%) and much higher SiO2 (63.1 wt%) and Na2O (7.95 wt%). This sample comprises mainly plagioclase and actinolite with minor oxides and carbonates (see “Site U1309 thin sections” in “Core descriptions”), and its chemical composition reflects the modal abundance of these minerals. The average plagioclase composition was calculated using a mass balance equation relating modes and whole-rock and typical mineral compositions (Deer et al., 1992) in the system comprising the elements CaO-FeO-MgO-Al2O3-SiO2. The calculated plagioclase composition is An07. This value is consistent with extinction angles of plagioclase twins, which give anorthite contents of ~10 (see “Site U1309 thin sections” in “Core descriptions”), and this observation suggests that plagioclase in Sample 304-U1309D-36R-3, 80–82 cm, was anorthite poor before alteration (and albitization).

The four leucocratic gabbros (Samples 305-U1309D-93R-1, 11–16 cm, 116R-3, 67–77 cm, 128R-3, 38–48 cm, and 158R-1, 11–17 cm) sampled during Expedition 305 display elevated concentrations of Na2O (3.3–5.0 wt%), K2O (0.05–0.10 wt%), and P2O5 (1.3–2.8 wt%) (Figs. F219, F220; Table T11).

Trace element geochemistry of olivine gabbros, orthopyroxene-bearing gabbros, and gabbros sampled during Expedition 304 is consistent with the primitive compositions defined by the major elements. They have high concentrations of compatible elements, with Ni up to 840 ppm and Cr ranging from 150 to 1400 ppm; Sample 304-U1309D-48R-1, 44–53 cm, an olivine gabbro, has the highest Cr content of all gabbroic rocks (3060 ppm). In contrast, oxide gabbros and the late magmatic leucocratic dikes have low Cr (<190 ppm) and Ni (<166 ppm). The troctolites have, on average, higher Ni and Cr contents (1040–1625 ppm and 1075–2980 ppm, respectively). Ni is concentrated in olivine, and the whole-rock composition in the gabbroic rocks mainly indicates the abundance of this mineral. Cr contents are variable, although we note that the highest Cr contents are found among the most primitive olivine gabbros and troctolites. Cr is mainly concentrated in oxides and, more particularly, in spinels, although clinopyroxene may also contain Cr (up to 0.4 wt% Cr2O3). Shipboard thin section descriptions do not show evidence for spinel in these samples. Cr may reflect variations in the Cr content in clinopyroxenes and/or in the modal fraction of this mineral. However, there is no correlation between Cr and any other major and trace elements. This suggests that spinel may be present as a microphase that is not evenly distributed in these samples.

Moderately incompatible elements V and Sc covary positively with TiO2 in Site U1309 gabbros, orthopyroxene-bearing gabbros, olivine gabbros, and troctolites sampled during Expedition 304, with V ranging from 19 to 201 ppm and Sc from 3 to 53 ppm (Fig. F216). Oxide gabbros have significantly higher V (615–1287 ppm), an element that is highly compatible in oxides. Sc shows a very good anticorrelation with Al2O3 in Site U1309 gabbroic rocks (Fig. F222). The Sc budget in Site U1309 gabbroic rocks is primarily controlled by the abundance of clinopyroxene, given the compatibility of Sc in clinopyroxene. The Al2O3 budget of Site U1309 gabbroic rocks is primarily related to the modal abundance of plagioclase. The negative correlation between Sc and Al2O3 is, therefore, an indicator of changes in the clinopyroxene/​plagioclase ratio of these rocks. When the plagioclase/pyroxene ratio is low, bulk rock Sc content is higher and Al2O3 content is lower. Together with positive Sc-Ti covariation (Fig. F216), these relations are consistent with a simple mass balance involving corresponding increases in the proportion of clinopyroxene plus oxides as plagioclase proportion decreases. Interestingly, Sc does not covary with Al2O3 in Site U1309 troctolites. In these rocks, Al2O3 decreases at more or less constant Sc values. This is consistent with the increase of the proportion of olivine when plagioclase decreases in these rocks (see “Site U1309 thin sections” in “Core descriptions”). Sample 304-U1309D-4R-2, 112–113 cm, a gabbro, is slightly Ti rich compared to gabbros with similar Sc and V contents. This is consistent with thin section descriptions that show that rutile, a Ti-rich microphase, is present in this sample.

Except for three samples from the upper part of Hole U1309D (gabbro Zone 1; see “Igneous petrology”), Site U1309 gabbroic rocks from Expedition 304 have low incompatible element content with Y < 16 ppm and Zr < 17 ppm that covary with TiO2. Oxide gabbros are slightly enriched in some trace elements, with ~25 ppm Zr and 23–86 ppm Y. The three Zone 1 gabbros are Sample 304-U1309D-5R-3, 114–118 cm, an olivine gabbro that has 28 ppm Y and 22 ppm Zr, and two gabbros, Samples 4R-2, 112–113 cm, and 7R-3, 102–105 cm, that have 71 and 120 ppm Y and 97 and 20 ppm Zr, respectively. These samples show significant scatter in Zr and Y contents (Figs. F223, F224). The scattered diverging trends indicate that there could be (at least) two controls on their trace element concentration. One control may simply be enrichment in each of these incompatible elements due to increasing concentration of both elements in the parental melts during fractional crystallization. The variability of Y and Zr contents may also be related to the presence of minor phases concentrating these elements (e.g., zircon, titanite, and xenotime).

Site U1309 gabbros from Expedition 304 (including oxide gabbros) have Ba contents of less than the detection limit to 21 ppm and Sr contents of 27–166 ppm (Table T10; Fig. F223). Sr and Ba are incompatible elements, although Sr is more concentrated in plagioclase than in other silicate phases. These elements are also highly mobile during alteration. Sr variations correlate with Al2O3, an element abundant in plagioclase. Variable Ba content in the Site U1309 mafic rock suite is probably due to postemplacement processes (see “Metamorphic petrology”).

One leucocratic dike (Sample 304-U1309D-36R-3, 80–82 cm) was sampled during Expedition 304. It has low Sc (~1 ppm) but V content close to those of other Site U1309 gabbroic rocks. The low Sc is consistent with the absence of clinopyroxene in this sample. This sample has significantly higher Zr (328 ppm), Y (141 ppm), Ba (83 ppm), and Sr (94 ppm) contents than other gabbroic rocks sampled during Expeditions 304 and 305. The high Zr is consistent with the observation of zircon in this sample (see “Site U1309 thin sections” in “Core descriptions”). These concentrations are much higher than Y and Zr concentrations observed for mid-ocean-ridge basalt magmas. These high concentrations suggest that this sample was in equilibrium with an evolved trace-element-enriched melt (Fig. F224).

In general, the trace element geochemistry of Hole U1309D gabbroic rocks below 401.3 mbsf (Expedition 305) is similar to gabbros recovered during Expedition 304 and consistent with the primitive compositions defined by major elements. Troctolites and olivine gabbros have relatively high concentrations of compatible elements, with Ni from 150 to 1575 ppm and Cr ranging from 4090 to 120 ppm (Fig. F225; Table T11). In contrast, oxide gabbros and leucocratic gabbros have very low Cr (below detection limit to 195 ppm) and Ni (below detection limit to 140 ppm). Olivine-bearing gabbros and gabbros fall in between the olivine gabbros and oxide gabbros in Ni and Cr concentration. Oxide gabbros have significantly higher V (85–1980 ppm). Sc shows a very good anticorrelation with Al2O3 in Site U1309 gabbroic rocks (Fig. F226).

Except for leucocratic gabbros, most Hole U1309D gabbroic rocks below 401.3 mbsf (Expedition 305) have low incompatible element content with <24 ppm Y and Sr contents of 32–287 ppm (Fig. F223). Leucocratic gabbros are characterized by high incompatible trace element contents, with concentrations up to 1180 ppm Zr and 12–301 ppm Y. They also show significant scatter in Zr and Y contents. Leucocratic gabbros have extremely high incompatible trace element concentrations, like Y and Zr (Figs. F224, F226; Table T11), compared with the other gabbroic rocks and MAR-basalts, pointing to the highly fractionated compositions of those liquids.

On average, olivine gabbro and troctolite display lower incompatible element concentrations than gabbro, although they overlap in composition. The oxide gabbros overlap in composition with the most trace element enriched gabbros (Fig. F226). These compositions fall into the range of concentrations previously observed in gabbros in Hole 735B (Dick et al., 2000) and from Legs 153 (Casey, 1997) and 209 (Kelemen, Kikawa, Miller, et al. 2004) (Figs. F223, F224). Overall, the compositions of most of the gabbros analyzed at Site U1309 plot at the most primitive end of the field defined by comparable oceanic gabbros. Based on their major and trace element compositions, Site U1309 gabbros form a coherent suite with diabase and basalts sampled at the same site. This suggests a cogenetic origin for these mafic rocks.

Olivine-rich troctolite

Fourteen samples were selected from lithologic units comprising olivine-rich troctolites. Samples from these intervals, cored during Expedition 304, grade from serpentinized dunite with small amounts of interstitial clinopyroxene and/or plagioclase to wehrlitic and olivine-rich troctolitic intervals at ~132 (Units 57–58), ~300–311 (Units 136–147), and ~330 (Units 167–173) mbsf. They comprise three dunites with variable amounts of interstitial plagioclase, one wehrlite, and two olivine-rich troctolites. Eight olivine-rich troctolite samples were analyzed in two intervals, between 670 and 690 mbsf (Samples 305-U1309D-136R-2, 4–14 cm, and 140R-2, 11–19 cm) and 1095 to 1233 mbsf (Samples 227R-3, 73–78 cm, 234R-2, 63–68 cm, 240R-2, 84–92 cm, 242R-2, 84–92 cm, 248R-2, 5–11 cm, and 256R-2, 88–94 cm) during Expedition 305.

The H2O and CO2 contents in the olivine-rich troctolitic samples range between 0.7 and 14.94 wt% and 0.09 and 0.75 wt%, respectively (Figs. F213, F218; Table T11). High H2O contents in most samples from olivine-rich troctolite intervals point to the presence of serpentine minerals replacing olivine (Fig. F213). Samples 305-U1309D-227R-3, 73–78 cm, and 248R-2, 5–11 cm, have the lowest H2O contents of all olivine-rich rocks recovered during Expeditions 304 and 305. Carbon dioxide contents are low in the olivine-rich troctolites (<0.14 wt% CO2), with the exception of Sample 305-U1309D-140R-2, 11–19 cm, which has 0.75 wt% CO2 (Table T11).

The olivine-rich rocks sampled in Units 57–58, 136–147, and 167–173 during Expedition 304 have low SiO2 (41.6–42.8 wt%) and TiO2 (<0.07 wt%), similar to peridotites sampled during Expedition 304. They have 0.9–8.4 wt% Al2O3 and 0.8–5.8 wt% CaO. These elements concentrate in plagioclase and clinopyroxene, and their variations reflect the modes of these minerals. They overlap in composition in MgO and Fe2O3 contents with peridotites sampled during Expedition 304, although they have more variable MgO (34.5–42.2 wt%) and higher Fe2O3 (10–15 wt%), with the highest Fe2O3 characterizing the dunites (12.45–15 wt%). MgO concentrates in olivine, and ultramafic rock samples with significant amounts of plagioclase and clinopyroxene (high Al2O3 and CaO content) have lower olivine contents and therefore display the lowest MgO. However, Fe2O3 variations are not associated with modal changes. This suggests that the Fe composition of the minerals that compose the olivine-rich troctolitic rocks and, in particular, that of olivine, varies.

The olivine-rich rocks sampled in Units 57–58, 136–147, and 167–173 have high Cr (2422–3140 ppm) and overlap in composition with Expedition 304 peridotites, but they have lower Ni (1750–2310 ppm). The Cr values are consistent with thin section observations that show the presence of various amounts of spinels in these samples. Ni concentrates in olivine, and the low Ni content of these samples may reflect their low olivine mode. The ultramafic rocks have ~4 ppm Ba, 3–16 ppm Sr, 28–36 ppm V, 6–14 ppm Sc, and <5 ppm Y. These trace element contents are similar to those of the most olivine rich gabbroic rocks sampled during Expedition 304.

Below 401.3 mbsf (Expedition 305), the olivine-rich troctolites have low SiO2 (40.8–47.4 wt%), Al2O3 (3.3–6.4 wt%), and TiO2 (0.05–0.46 wt%) contents and high MgO (30.0–38.4 wt%) and Fe2O3 (10.5–14.2 wt%) contents compared to the gabbroic rocks from Hole U1309D (Figs. F219, F220). The Mg# of these rocks is homogeneously high (Figs. F220, F225), ranging only between 84.3 and 86.0 (except for Sample 305-U1309D-140R-2, 11–19 cm). Compatible trace element concentrations are high, with Ni ranging from 1170 to 2008 ppm and Cr ranging from 248 to 3219 ppm (Fig. F225). The extremely high Cr contents point to the presence of abundant spinel, which is confirmed by hand specimen and thin section descriptions (see “Igneous petrology”). In Sample 305-U1309D-227R-3, 73–78 cm, abundant chromite was identified by XRD (see “Metamorphic petrology”). Moderately incompatible and incompatible trace element contents are, in general, low (<73 ppm V, <20 ppm Sc, <30 ppm Sr, <7.8 ppm Y, and less than detection limit of Zr) and very similar to the ultramafic rocks recovered during Expedition 304 (Figs. F225, F226; Table T11). The primitive composition of the olivine-rich troctolites may indicate that they are cumulates marked by higher concentrations of compatible trace elements such as Mg, Cr, and Ni. However, these samples have significantly higher Ni content than gabbroic rocks with similar Mg#, which suggests either a different magmatic crystallization trend or a more Ni enriched parent magma.

The olivine-rich troctolitic intervals have compositions intermediate between the most primitive gabbroic rocks and peridotites sampled during Expedition 304. Alternatively, they may represent the most extreme product of melt-rock interaction with (or impregnation by) a basaltic melt as proposed by Kelemen, Kikawa, Miller, et al. (2004) for Site 1275 troctolites.

Ultramafic rocks

We selected one piece of ultramafic rubble and two serpentinized harzburgite samples from Hole U1309B. The two serpentinized harzburgites (Unit 32) are representative of a 4 m thick interval that represents the only presence of peridotites in Hole U1309B (see “Igneous petrology” and “Downhole measurements”). We also sampled three ultramafic samples in Hole U1309D. The uppermost sample in Hole U1309D, Sample 304-U1309D-10R-1, 107–111 cm, was taken from a 15 cm long interval composed of ultramafic material cut by several altered gabbro dikes at ~61 mbsf (Unit 28). This sample was taken as far as possible from the gabbroic dikes, from the leftovers of the microbiology sample. The modal composition of this sample, determined in hand specimen, is that of a wehrlite with >60% olivine, ~35% pyroxene (probably mostly clinopyroxene), and <2% plagioclase. A serpentinized dunite (Unit 77) and a serpentinized harzburgite (Unit 79) were sampled in a ~2.50 m ultramafic interval within the gabbro section at ~171 mbsf.

Bulk rock analyses of ultramafic rocks from Site U1309 show that the composition of these samples was modified by hydrothermal alteration, which added variable amounts of volatile constituents to the original ultramafic assemblage (Fig. F213). The peridotites sampled in Holes U1309B and U1309D have LOI ranging from 8.9 to 11.73 wt%. These values are consistent with visual core descriptions and XRD results (see “Metamorphic petrology”) that show that these samples have abundant lizardite. Sample 304-U1309D-31R-1, 25–28 cm, a dunite, and Sample 31R-2, 11–17 cm, a harzburgite, have higher LOIs (12.78 and 12.85 wt%, respectively) and CO2 (1.24 and 1.28 wt%, respectively). High CO2 may indicate late precipitation of carbonates. This is consistent with visual core description and thin section descriptions that show evidence for carbonate veinlets cutting the peridotites and carbonate secondary precipitation within the serpentine mesh (see “Metamorphic petrology”). The ultramafic rubble (Sample 304-U1309B-1R-3, 17–21 cm) sampled at the top of Hole U1309B, which also shows evidence of alteration to talc, has a significantly lower LOI value (7.5 wt%). Unit 28 wehrlite from Hole U1309D has the same LOI value (7.5 wt%) but much higher CO2 (1.19 wt%) compared to the Hole U1309B ultramafic samples (0.14–0.19 wt%). These relatively low H2O and high CO2 contents are consistent with thin section descriptions that show the presence of carbonate veins as well as pervasive talc alteration, although the sample was less altered than Sample 304-U1309B-1R-3, 17–21 cm, as relics of olivines are preserved in the latter. LOI is positively correlated with water content in the alteration intervals in ultramafic rocks, with H2O ranging between 10.89 and 13.15 wt% where lizardite is predominant and 8.2 and 7 wt% where talc is predominant.

Hole U1309B harzburgites

In spite of their high volatile content, bulk rock compositions of Hole U1309B harzburgites primarily reflect the relative abundance of their primary phases, olivine and pyroxene (Fig. F227). They have low Al2O3 (~0.8 wt%) and CaO (0.2–0.42 wt%) compared to Leg 153 peridotites (Casey, 1997) (Fig. F228). Both Al2O3 and CaO are concentrated in pyroxenes, with CaO primarily in clinopyroxene, so variations in Al2O3 and CaO reflect clinopyroxene content and, therefore, the relative degree of fertility of peridotites. The low CaO and Al2O3 contents of Hole U1309B harzburgites suggest that, prior to alteration, these peridotites may have been more refractory than Leg 153 peridotites. Together with low clinopyroxene contents, high Mg# is considered as an indicator of high degrees of partial melting of peridotite, yet Hole U1309B harzburgites have the same Mg# as Leg 153 peridotites (90–91) (Fig. F229). Hole U1309B harzburgites are characterized by higher Fe2O3 contents (9–10.1 wt%) compared to tectonically emplaced and abyssal peridotites with the same MgO contents (44.5–45.6 wt%) (Fig. F230). This slight Fe enrichment explains their relatively low Mg# and may result from melt-rock reactions involving Fe-Mg exchange with an olivine-saturated basaltic melt. Sample 304-U1309B-11R-1, 100–104 cm, which displays the highest Fe content, was sampled <5 cm from a gabbroic dike.

Harzburgite from Hole U1309B has ~40 ppm V, ~10 ppm Sc, 2500–2650 ppm Ni, and 125–150 ppm Co (Fig. F231). Ni and Co are preferentially partitioned into olivine, and V and Sc are preferentially partitioned into clinopyroxene. Variations in these elements in Hole U1309B harzburgites mainly reflect the modal proportions of olivine and clinopyroxene. V and Sc contents in Hole U1309B harzburgites are high compared to those of Leg 209 peridotites with the same Al content. Other trace element concentrations in Hole U1309B harzburgites fall in the range of Leg 209 peridotites. Hole U1309B harzburgites are depleted in TiO2 (<0.02 wt%), Zr (<1.5 ppm), and Sr (<2 ppm) compared to Leg 153 peridotites (Fig. F231). These elements are moderately to highly incompatible, and their range in concentration suggests that Hole U1309B harzburgites underwent degrees of partial melting similar to Leg 209 peridotites and higher than Leg 153 peridotites. V and Sc partition preferentially into clinopyroxene, and their correlation with CaO and Al2O3, both proxies for the proportion of pyroxene present, suggests the presence of a small amount of clinopyroxene preserved in these rocks. This is consistent with thin section descriptions that show a small amount of relict clinopyroxene.

Hole U1309B ultramafic rubble

Sample 304-U1309B-1R-3, 17–21 cm (Unit 1), is a fragment recovered at the top of the drilled section in Hole U1309B. It is altered to talc and serpentine (see “Metamorphic petrology”), and its composition primarily reflects the relative abundance of these alteration phases (Fig. F227). It is significantly enriched in SiO2 (60 wt%) and depleted in MgO (33 wt%) and Fe2O3 (6.7 wt%) relative to harzburgite from the same hole. It is also strongly depleted in CaO, suggesting that serpentinization removed Ca from the pyroxene that was originally present in the peridotite protolith. Sample 304-U1309B-1R-3, 17–21 cm, is depleted in trace elements that are mobile during alteration, such as Sr (below detection limit), relative to peridotites from Legs 153 and 209. Like Ca, these elements appear to have been removed from the peridotite by alteration. This sample is also depleted in Y (<1 ppm), Zr (<1.5 ppm), and, to a lesser extent, TiO2 (0.03 wt%), V (21 ppm), and Sc (7 ppm). It has high values in Cr (1940 ppm) and Ni (2400 ppm). These elements are considered to be relatively immobile during alteration (e.g., Hébert et al., 1990). Therefore, they can be used as indicators of protolith composition. V, Sc, and Cr variations and, to a lesser extent, Zr, Ti, and Y variations correlate with Al2O3 variations in Hole U1309B harzburgites (Fig. F231). Zr, V, Sc, Ti, and Y behave as incompatible elements during partial melting, whereas Cr and Ni behave as compatible elements and Al2O3 may be used as an indicator of the protolith pyroxene content. The low Al2O3 (0.62 wt%) and trace element contents of Sample 304-U1309B-1R-3, 17–21 cm, suggest that the protolith was composed of a depleted peridotite, probably more similar to or slightly more refractory than Hole U1309B serpentinized harzburgite found at 60 mbsf.

Hole U1309D wehrlite (~61 mbsf, Unit 28)

Sample 304-U1309D-10R-1, 107–111 cm (~61 mbsf; Unit 28), is a wehrlite with low SiO2 (42.9 wt%), Al2O3 (1.2 wt%), and TiO2 (<1 wt%) and high Fe2O3 (15 wt%) and MgO (36.4 wt%) compared to those of Expedition 304 gabbroic rocks (Fig. F219), although Expedition 304 gabbroic rocks have similar Mg# (83). It is grouped together with the olivine-rich troctolites (see “Igneous petrology”). Its composition may indicate that this wehrlite is a primitive cumulate distinguished by higher concentrations of compatible trace elements such as Cr (2670 ppm) and Ni (1240 ppm). Its incompatible trace element contents (98 ppm V, 6 ppm Y, 17 ppm Zr, and <25 ppm Sc) are similar to those of Site U1309 olivine gabbros and troctolites. The high concentrations of V and Sc can be attributed to the presence of significant amounts of cumulate clinopyroxene. Spinel is observed in thin sections of wehrlite (see “Igneous petrology” and “Metamorphic petrology”), which accounts for the high Cr concentration. The high Ni content of Hole U1309D wehrlite may reflect, in part, its high olivine content. However, it should be noted that this sample plots outside of the Ni-Mg# trend shown by the most primitive of Site U1309 gabbroic rocks, suggesting mixing between a peridotite and a more mafic material (Fig. F229). In addition, thin section description suggests that olivines were deformed at high temperature. This suggests that Hole U1309D wehrlite could have formed as a product of reaction between peridotite and a basaltic melt, as proposed for peridotites with gabbroic impregnation from ODP Site 1271 and for Site 1275 troctolites, which are similarly characterized by low Mg# (as low as 85) but have relatively high Ni contents compared to cumulate rocks of the same Mg# (Kelemen, Kikawa, Miller, et al., 2004).

Hole U1309D Unit 77 dunite and Unit 79 harzburgite

Two serpentinized peridotites were sampled in a ~2.50 m ultramafic interval within the gabbro section at ~171 mbsf. This ultramafic interval is cut by several altered gabbro dikes. Sample 304-U1309D-31R-1, 25–28 cm, a dunite (Unit 77), and Sample 31R-2, 11–17 cm, a harzburgite (Unit 79), were taken as far as possible (>20 cm) from these gabbroic dikes.

The bulk rock compositions of these samples primarily reflect the relative abundance of their primary phases, olivine and pyroxene. They have 43.8–44.16 wt% SiO2, 43.39–43.46 wt% MgO, and 0.02–0.04 wt% TiO2. The dunite is distinguished by lower Al2O3 (~0.2 wt%) compared to the harzburgite (0.87 wt% Al2O3). Al2O3 is concentrated in pyroxenes and, to a lesser extent, in spinel in plagioclase-depleted samples, so its variations between the dunite and harzburgite samples mainly reflect pyroxene content. Both samples have high CaO content (1.15 wt%) compared to Hole U1309B harzburgites. Together with high CO2, these relatively high CaO contents probably indicate late carbonate precipitation, as shown by visual core description and thin section descriptions. Both samples display relatively high Fe2O3 content (10.03–10.84 wt%) and low Mg# (88.9–89.6), the more Fe rich sample being the dunite.

Sample 304-U1309D-31R-2, 11–17 cm, the harzburgite, has trace element compositions comparable to Hole U1309B harzburgites, with 0.6 ppm Y, 52 ppm V, 12 ppm Sc, 2350 ppm Ni, 2603 ppm Cr, and 125–150 ppm Co. The dunite, Sample 304-U1309D-31R-1, 25–28 cm, has higher Ni content (3080 ppm) but lower Cr and V contents (1009 ppm and 19 ppm, respectively). Ni concentrates in olivine, whereas Cr and V concentrate in spinel. According to thin section descriptions, Sample 304-U1309D-31R-1, 25–28 cm, is composed of 98% olivine and 2% spinel. Whereas the high Ni content of Sample 304-U1309D-31R-1, 25–28 cm, is consistent with thin section description, the sample’s low Cr and V contents suggest that a smaller amount of spinel was present in the geochemistry sample. Both samples have higher Sr contents (5–32 ppm) than Hole U1309B harzburgites. This Sr enrichment may be related to the carbonate alteration observed in these two samples.

Except for chemical variations related to carbonate alteration, the compositions of Samples 304-U1309D-31R-1, 25–28 cm, and 31R-2, 11–17 cm, are comparable to those of Hole U1309B peridotites (Fig. F231). The low Al2O3 and incompatible element contents of the two samples suggest that, prior to alteration, these peridotites were more refractory than Leg 153 peridotites. However, like Hole U1309B harzburgites, they are enriched in Fe compared to tectonically emplaced and abyssal peridotites with the same MgO contents, the dunite being the more Fe enriched (Fig. F230). This slight Fe enrichment explains their relatively low Mg#. We interpret these variations as resulting from melt-rock interactions involving Fe-Mg exchange with an olivine-saturated, basaltic melt.

Discussion

Basaltic rocks

Basalt and diabase sampled at Site U1309 are tholeiitic basalts to basaltic andesites with compositions that overlap basalt glasses from the entire MAR. One of the most striking features of Site U1309 basalts and diabases is their wide range of compositions, compared to the full suite of MAR volcanic glasses. All Site U1309 basalts and diabases were modified to different extents by alteration to tremolite and actinolite. This process may explain part of the compositional variation, especially for mobile elements including Ba, K, Sr, and, to a lesser extent, Na and Ca (for example, when plagioclase is albitized). However, elements such as Zr, Ti, Y, Sc, and V, which are assumed immobile during hydrothermal alteration (e.g., Staudigel et al., 1996), also show a wide range of compositions. These variations can be present within the same diabase (Fig. F232). In spite of their large range in composition, Site U1309 diabase and basalt have relatively constant trace element ratios (e.g., Zr/Y varies from 1.6 to 3) and are interpreted to reflect relatively uniform parental magma compositions (Fig. F214). The analyzed diabases are fine to medium grained and sparsely plagioclase phyric (see “Site U1309 thin sections” in “Core descriptions”), and their compositional variability may reflect the phenocryst abundance of the selected sample and/or the degree of differentiation of the crystallizing melt. Plagioclase-rich diabases have higher trace element contents, suggesting that these samples crystallized from a more evolved melt.

Ti and Fe contents in Site U1309 diabases are also affected by oxide abundance, as shown by the correlation between Ti and V, which concentrate in oxides, and Fe content (Fig. F217). This is consistent with higher magnetic susceptibilities measured in these intervals, suggesting high magnetite contents in high-Fe basalts and diabases.

Gabbroic rocks

Site U1309 gabbroic rocks including gabbro, olivine gabbro, troctolite, and gabbronorite have compositions that are among the most primitive sampled anywhere along the MAR, as indicated by high Mg# (67–87) and low TiO2 (<0.72 wt%), Na2O (0.3–3.6 wt%), and trace element contents (e.g., ~11 ppm Y and ~17 ppm Zr) on average. Most of the compositional variations observed in Site U1309 gabbroic rocks are consistent with a simple mass balance involving corresponding increases in the proportion of clinopyroxene plus oxide as plagioclase proportion decreases, except in the most olivine rich gabbroic rocks (essentially in troctolites). In these rocks, the proportion of olivine increases when plagioclase decreases. Site U1309 gabbroic rocks can be interpreted as cumulates, which are related, together with Site U1309 basalts and diabases, through crystal fractionation processes to a common parental magma (see “Petrogenesis of gabbroic rocks”).

Three gabbroic rocks, all sampled from the upper part from Hole U1309D, show distinctive enrichments in Zr, Y, and, to a lesser extent, Ti. These enrichments are not correlated to major or other trace element variations. On the basis of the thin section description, they are interpreted as being associated with the presence of minor phases concentrating these elements (e.g., zircon, rutile, and titanite).

Oxide gabbros are distinguished by their high TiO2 (up to 7.9 wt%) and Fe2O3 (up to 29.2 wt%) contents and low Mg# (23–62). The strong variations in Fe at constant Mg observed in oxide gabbros mainly reflects the modal content of Fe-rich oxides in these samples (Fig. F233). These gabbros also have slightly higher trace element contents (~25 ppm Zr and 23–86 ppm Y), which suggest they may have crystallized from a slightly more evolved melt.

The gabbroic section is cut by several leucocratic gabbros. The composition of these samples clearly stands out in comparison to the other gabbroic groups having low MgO (3–6 wt%) and high alkali (3.4–8 wt%). Sample 304-U1309D-36R-3, 80–82 cm, a late magmatic leucocratic dike, is further distinguished by its low CaO (4.8 wt%) and much higher SiO2 (63.1 wt%) and Na2O (7.95 wt%) contents. The composition of this sample reflects its modal assemblage, which is mainly albite-rich plagioclase and actinolite. The high Zr is consistent with the observation of zircon in this sample (see “Site U1309 thin sections” in “Core descriptions”). Leucocratic gabbros are also characterized by their high trace element contents, with 141–301 ppm Y and 190–1180 ppm Zr. These values are significantly higher than Y and Zr concentrations observed in Site U1309 gabbroic rocks and in mid-ocean-ridge basaltic glass. This suggests that these samples were in equilibrium with an evolved trace element–enriched melt. Highly evolved dikes were also observed at Site 1275, indicating that rocks with such enriched compositions are present elsewhere in the oceanic crust (Kelemen, Kikawa, Miller, et al., 2004).

Several olivine-rich intervals comprising olivine-rich troctolite, dunite, and wehrlite are observed below 300 mbsf. The olivine-rich troctolitic intervals and the wehrlite sampled at ~61 mbsf in Hole U1309D (Unit 32) have compositions intermediate between the most primitive gabbroic rocks and peridotite sampled from the core. They have low SiO2 (<45 wt%) and TiO2 (<0.1 wt%), similar to other peridotites sampled from Site U1309. They have highly variable Al2O3 (0.9–8.4 wt%) and CaO (0.8–5.8 wt%) that reflects the mode of plagioclase and clinopyroxene in these rocks. They overlap in composition in MgO and Fe2O3 contents with Site U1309 peridotites, although they have more variable MgO (30–42.2 wt%) and higher Fe2O3 (10–15 wt%), with the highest Fe2O3 characterizing the dunites (12.45–15 wt%). All of these olivine-rich samples have higher trace element contents than the peridotites (2–8 ppm Y and 7–17 ppm Zr). They have high but variable Cr and Ni contents (1200–3127 ppm Cr and 1170–2311 ppm Ni). Texturally and compositionally, these olivine-rich rocks are similar to the ultramafic rocks sampled at Site 1275 (Kelemen, Kikawa, Miller, et al., 2004). They are interpreted either as ultramafic cumulates or, alternatively, the most extreme product of melt-rock interaction with (or impregnation by) a basaltic melt (see “Petrogenesis of gabbroic rocks”).

Ultramafic rocks

Ultramafic rocks sampled at Site U1309 have strikingly different compositions, and, for several samples, their origin is uncertain. Bulk rock analyses show that their compositions were modified by hydrothermal alteration, which added variable amounts of volatile constituents to the original ultramafic assemblage. The highest LOI (10.35–12.52 wt%) and H2O (10.89–13.15 wt%) contents are observed where lizardite is predominant, yet the bulk rock compositions of these ultramafic rocks primarily reflect the relative abundance of their primary phases, olivine and pyroxene. In contrast, where volatile contents are lower and talc is predominant (H2O = 7–8.2 wt%), the bulk rock compositions mainly reflect the alteration phases, as in Sample 304-U1309B-1R-3, 17–21 cm, the ultramafic rubble sampled at the top of Hole U1309B. When a small amount of carbonate is present, the ultramafic rocks have slightly higher CO2 and CaO contents than the other ultramafic samples.

Four of the ultramafic samples, two serpentinized harzburgite samples from Hole U1309B and two serpentinized peridotites (Units 77 and 79) sampled in a ~2.50 m ultramafic interval within the gabbro section at ~171 mbsf in Hole U1309D, have highly depleted compositions similar to those of serpentinized peridotites sampled during Leg 209. They have low Al2O3 (<0.8 wt%) and CaO (0.2–0.42 wt%; except for carbonate-altered samples) (Fig. F228), suggesting that, prior to alteration, these peridotites were poor in clinopyroxene. These samples are characterized by higher Fe2O3 values (9–10.8 wt%) compared to tectonically emplaced and abyssal peridotites with the same MgO contents (43.4–45.6 wt%) (Fig. F230). All of these samples have high Ni contents (2360–3080 ppm) and low incompatible element contents (<1.7 ppm Y and <1 ppm Zr). They likely represent residual peridotites that, prior to alteration, had chemical compositions comparable to those of Leg 209 peridotites, apart from relative FeO enrichment. Their high Fe contents may result from melt-rock reactions involving Fe-Mg exchange with an olivine-saturated, basaltic melt. It should be noted that these peridotites were sampled in ultramafic intervals crosscut by multiple gabbroic dikes.

Alteration (downhole variation in H2O and CO2)

As water in the primary rocks is only present in “trace amounts,” the water content of the altered rocks mirrors the extent of alteration and/or the assemblage of secondary water-bearing phases, which are mainly a product of olivine alteration. In detail, tremolite and actinolite have low H2O contents (<2.3 wt%), talc has intermediate H2O contents (~5 wt%), and chlorite and serpentine minerals have high H2O contents (>10 wt%) (Fig. F213).

In general, olivine gabbros are more volatile rich than the pyroxene- and plagioclase-rich gabbros. Figure F218 shows a clear decrease in water contents from 400 to 500 mbsf. High water contents of >2.5 wt% are always correlated to primary olivine-rich rock types like olivine gabbros and olivine-rich troctolites. This suggests that serpentine minerals are the main water-bearing phases in these altered rocks. The upper 401.3 m of Hole U1309D drilled during Expedition 304 shows, on average, much higher and more scattered water contents than the lower 1000 m, where the average water content is <0.5 wt%. This is consistent with visual core and thin section descriptions showing that the total degree of alteration decreases downhole. It has to be noted that the samples taken for the geochemical analysis are usually taken from the least altered core intervals and therefore show a bias to more unaltered compositions compared to the average degree of alteration.

Compared to other serpentinized ultramafic rocks from Legs 153 and 209 (Casey, 1997; Kelemen, Kikawa, Miller, et al., 2004), the CO2 contents of the ultramafics in Hole U1309D are very low. Because pervasive carbonate precipitation is commonly related to late-stage hydrothermal alteration, it is likely that the serpentinized olivine-rich rock types from Hole U1309D are relatively unaffected by seawater.

Overall geochemical variations of Hole U1309D gabbroic rocks from troctolite to gabbro

Figures F220 and F225 show covariation of Mg# and selected major and minor elements in Hole U1309D gabbros and peridotites, respectively. Whole-rock geochemical composition of gabbros reflects modal contents and elemental compositions of constituent minerals. Whole-rock Mg# can be used as a first-order indication of the degree of differentiation with some caution, as in the case of oxide gabbros. MgO and Fe2O3 contents of other gabbroic rocks correlate well (Fig. F233). Their Mg#s decrease in order of olivine-rich troctolite, troctolite, olivine gabbro, and gabbro. This decrease correlates with a decrease of modal content of olivine and reflects a decrease in Mg# of these constituent phases.

SiO2 contents of the gabbroic rocks increase with decreasing Mg#. SiO2 contents of main constituent minerals increase in order of olivine, plagioclase, and pyroxenes (Fig. F220A). Therefore, the increasing trend of the SiO2 of gabbroic rocks is interpreted as a decrease in modal content of olivine and an increase in plagioclase and pyroxenes with a decrease of Mg#.

TiO2 and MnO contents broadly increase with decreasing Mg# (Fig. F220B, F220D). Because TiO2 and MnO are basically incompatible elements, these elements concentrate in the liquid phase as a result of differentiation. Thus, these trends are appropriate for cumulates of normal fractional crystallization. It is also noted that olivine gabbros show relatively large scattering of both elements, which is probably caused by modal variation of constituent minerals. MnO contents of the olivine gabbros are more scattered than TiO2 contents. TiO2 partitions only into clinopyroxene, whereas MnO partitions into olivine and clinopyroxene, except in oxide gabbros. Therefore, scattering of the MnO content in the gabbroic rocks, except for oxide gabbros, reflects modal variations of both olivine and clinopyroxene. However, there is also a possibility of scattering of Mg# at the same MnO (and TiO2) content, caused by significant modification of the Mg# of those rocks by later magmatic or hydrothermal processes, or change in MnO by alteration at constant Mg#.

Al2O3 variations of Hole U1309D gabbroic rocks show an essentially similar tendency with CaO variations. Al2O3 and CaO contents increase with decreasing Mg# in relatively primitive lithologies (i.e., peridotite, olivine-rich troctolite, and troctolite) and relate to the (clinopyroxene + plagioclase)/olivine ratios. Al and Ca contents decrease with decreasing Mg# in gabbros (Fig. F220C). CaO contents increase with decreasing Mg# in order of peridotite, olivine-rich troctolite, and troctolite (Fig. F220E). This tendency corresponds to a modal increase of plagioclase within those lithologies (see “Igneous petrology”), but CaO contents of gabbros decrease with decreasing Mg#. Although the decreasing trend of CaO in the Hole U1309D gabbros can be explained by enrichment of the albite component in plagioclase with advance of differentiation, modal content of plagioclase also increases with decreasing Mg# (see “Downhole chemical variation within Hole U1309D,” below). Therefore, decrease of modal content of clinopyroxene is probably the main cause of CaO and Al2O3 decreases for the gabbros. Olivine gabbro shows significant scattering due to modal variation of plagioclase.

Cr and Ni contents show clear fractional crystallization trends from ultramafic rocks to evolved lithologies (Fig. F225A, F225C). Because Cr partitions into spinel and clinopyroxene, spinel-containing lithologies (i.e., olivine-rich troctolites and troctolites) show the highest contents of Cr (>2000 ppm).

V and Y contents increase with decreasing Mg# and show normal trends of cumulates with differentiation of magma (Fig. F225B, F225D). Some of the olivine gabbros show trends of lower contents in Y and V compared to the main trend. Because V and Y partition into clinopyroxene in olivine gabbros, this low V and Y trend may correspond to lower modal content of clinopyroxene in those rocks. Oxide gabbros show significantly higher contents of V and Y. In basaltic systems, V strongly partitions into oxide minerals; hence, higher content of V in the oxide gabbros directly correlates to higher abundance of oxide minerals. Y also partitions toward the oxide minerals and clinopyroxenes.

Downhole chemical variation within Hole U1309D

Figures F234 and F235 show the downhole variation of Mg#, TiO2, MnO, and Al2O3 over the whole 1415.5 m section drilled during Expeditions 304 and 305. The figure is based on 219 whole-rock analyses. Gabbroic rock compositions are characterized by significant variations in Mg# at ~600 and ~1100 mbsf. Mg# variations are correlated to abrupt changes in TiO2 and MnO content and in lithology. In the upper part of the section, Mg# and TiO2 do not show a significant downhole trend, although MnO increases continuously downhole. From 600 to 1100 mbsf, Mg# increases downhole but TiO2 and MnO do not show significant downhole trends. Below 1100 mbsf, gabbroic rocks are dominantly olivine-rich troctolites that are characterized by high Mg#, low TiO2, and high MnO.

At a smaller scale, TiO2 shows variable downhole contents, increasing from 0 to ~300 and from ~300 to ~1020 mbsf. In the simple case of differentiation of one magma body, a reverse chemical trend would be expected. These variations probably correspond to modal content of clinopyroxene in the gabbroic rocks. Numerous crosscutting relations are observed throughout the entire section; the observed chemical variations may be related to these intrusive relations. However, because of the limited sampling, the correspondence between the descriptive units and detailed geochemical features is uncertain. At the base of structural Unit 2 (see “Structural geology”), three fault zones have been described (at 695, 746, and 785 mbsf). Although the Mg# shows scattering around those fault zones, systematic change is not observed across the fault zones. The major change in MG# occurs above, at ~600 mbsf.

Al2O3 content decreases from 0 to 600 mbsf and shows broadly constant values from 600 to 1120 mbsf, although the downhole variation shows relatively large scatter. Because plagioclase is the only main phase that contains significant amounts of Al2O3, the downhole variation probably corresponds to modal content of plagioclase. Figure F235 is a comparison between whole-rock Al2O3 content and modal content of plagioclase based on thin section observations. Although modal content of plagioclase shows broad variability, it broadly corresponds to downhole Al2O3 variation.

Petrogenesis of gabbroic rocks

The range of composition of the gabbroic rocks sampled at Site U1309 spans highly primitive olivine gabbros with Mg# >85 to highly evolved leucocratic gabbros with high SiO2 and trace element contents.

In general, whole-rock geochemical evolution of gabbroic rocks is the result of complex changes in modal composition and changes in compositions of individual constituent minerals with advance of differentiation. Therefore, precise examination of crystallization processes without mineral chemistry is difficult. However, whole-rock geochemical variations correlate with whole-rock Mg# as mentioned above, suggesting that whole-rock compositions (except for Fe-rich oxide gabbro) are useful for examination of first-order crystallization processes.

The order of fractional crystallization of olivine tholeiite magmas under anhydrous conditions at relatively low pressures (Green and Ringwood, 1967; Walker et al., 1979; Grove and Bryan, 1983; Tormey et al., 1987; Juster et al., 1989) is

ol + spl – ol + pl + spl – ol + cpx + pl –

cpx + opx + pl,

where

  • ol = olivine,
  • spl = spinel,
  • pl = plagioclase,
  • cpx = clinopyroxene, and
  • opx = orthopyroxene.

These modes are consistent with the modal composition evolution from olivine-rich troctolites to gabbros. Low-pressure conditions for Hole U1309D gabbroic rocks are also supported by the cotectic crystallization of olivine and plagioclase as observed in troctolites (although not in all olivine-rich intervals). In basaltic systems, the cotectic relation of olivine and plagioclase disappears at ~7 kbar (Kushiro and Yoder, 1966; Presnall et al., 1978).

This suggests that the suite of gabbroic rocks sampled at Site U1309 may represent a single crystallization suite, the olivine-rich troctolites representing the first crystallization product of an olivine tholeiite magma. Yet olivine-rich troctolites have, on average, higher Ni contents than olivine-rich gabbros with similar Mg#, which is not consistent with a simple crystallization model. In addition, it should be noted that the texture of these samples with rounded olivine in a plagioclase-rich matrix (see “Igneous petrology”) suggests that these samples were formed by a complex multistage magmatic process. Alternatively, in these rocks, olivine and spinel may represent relics of mantle minerals. Texturally and compositionally, the most olivine rich rocks sampled in Site U1309 olivine-rich troctolite intervals are similar to the ultramafic rocks sampled at Site 1275 (Kelemen, Kikawa, Miller, et al., 2004).

Orthopyroxene-bearing gabbro and gabbronorite were sampled in Hole 1309D, mainly between ~800 and ~1000 mbsf. They may have formed as the late product of crystallization from an olivine tholeiite liquid following the olivine-clinopyroxene-plagioclase crystallization. Yet the Mg# of orthopyroxene-bearing gabbros and gabbronorites overlap the same range of values as olivine gabbros and gabbros, which is inconsistent with crystallization from a more evolved melt. In addition, they do not show trace element enrichments compared to gabbros as expected for late products of differentiation.

Comparison with other oceanic gabbros

Figure F236 is a whole-rock molecular Mg/(Mg + Fe) and Ca/(Ca + Na) diagram of Hole U1309D gabbros. Fields of other drill cores from MAR gabbros (Casey, 1997; Kelemen, Kikawa, Miller, et al., 2004) and the SWIR (Dick et al., 2000) are also shown in the figure for comparison. The gabbroic rocks from Hole U1309D plot on the primitive end of MAR gabbros, suggesting that Hole U1309D gabbros and other MAR gabbros could have been derived from relatively similar parent magmas and crystallized under similar conditions. One of the most significant aspects of Hole U1309D gabbros in terms of geochemical features is the presence of a series of rock types that span a compositional range from very primitive to moderately evolved. The expanded range in composition has rarely been noted in oceanic settings.

A second salient feature is the ubiquity of oxide gabbros throughout Hole U1309D (Fig. F233). A similar presence has been reported for Hole 735B, Atlantis Bank (Dick et al., 1999), in the MARK area (Cannat, Karson, Miller, et al., 1995), and at 15°N at the MAR (Leg 209; Kelemen, Kikawa, Miller, et al., 2004). Oxide gabbros may make up a common component of gabbroic rocks at slow-spreading ridges.