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doi:10.2204/iodp.proc.335.103.2012

Geochemistry

During Expedition 335, whole-rock chemical analyses were performed on three granoblastic basalts from Cores 335-1256D-235R through 238R and on one basalt, five granoblastic basalts, and two gabbroic rocks retrieved during junk basket runs. These samples are representative of the rock types recovered from Hole 1256D during Expedition 335. They were chosen from the least altered parts of the core and rock samples, as far as possible from hydrothermal veins and magmatic intrusions, in order to obtain the best estimate of primary compositions. Where possible, a thin section was taken next to the geochemistry sample for detailed petrographic characterization (see “Igneous petrology” and “Alteration and metamorphism” for thin section petrographic description). The 11 rock samples were analyzed for major and trace element concentrations and H₂O and CO₂ contents following standard shipboard procedures (see “Geochemistry” in the “Methods” chapter [Expedition 335 Scientists, 2012b]). Table T7 lists the geochemical analyses collected during Expedition 335.

Basalt

Sample 335-1256D-Run02-EXJB, a basalt, was retrieved from Hole 1256D at a maximum depth of 923 mbsf, during junk basket Run 2. The sample was prepared from roughly 1 cm size gravel, by careful separation of the gravel from the sand that constituted most of the junk basket material. No thin section was taken.

Sample 335-1256D-Run02-EXJB is characterized by a loss on ignition (LOI) of 1.22 wt%, H₂O of 2.6 wt%, and CO₂ of 0.12 wt%. These values suggest that this sample has been altered and are the highest H₂O and CO₂ measured during Expedition 335. As illustrated in Figure F30, LOI and H₂O in all measured samples are positively correlated, although the measured H₂O is significantly higher than LOI values. LOI slightly underestimates the total volatile content because of the conversion of Fe²⁺ to Fe³⁺ during heating of the sample powders to 1020°C (see “Geochemistry” in the “Methods” chapter [Expedition 335 Scientists, 2012b]). However, this cannot be the sole explanation for this difference. During Expedition 335, the unresolved overlap of the water peak with sulfur in gas chromatography leads to a systematic 20%–30% overestimate of water contents in standards (see “Geochemistry” in the “Methods” chapter [Expedition 335 Scientists, 2012b]) and to a large uncertainty in the actual water content of the measured samples. Indeed, the measured water content in Sample 335-1256D-Run02-EXJB is significantly higher than published H₂O values for Hole 1256D samples (H₂O from 0.58 to 1.65 wt% in variably altered basalt; Shilobreeva et al., 2011). The water measurements are therefore considered as qualitative and will not be further discussed.

The measured CO₂ concentration in Sample 335-1256D-Run02-EXJB (0.116 wt%) is higher than other samples measured during Expedition 335 (0.03–0.07 wt%) (Fig. F30). This slightly higher value does not correlate with higher Ca and Sr contents compared to Site 1256 basalts or to Expedition 335 samples, which would indicate the presence of carbonate minerals in this altered sample. It is, however, in the same range of values as the basalt that constitutes the upper volcanic section of Hole 1256D (0.05–0.25 wt%; Shilobreeva et al., 2011). This suggests some variability in the CO₂ content in the volcanic section.

Sample 335-1256D-Run02-EXJB overlaps in major and trace element composition with the basalt sampled in the volcanic section of Hole 1256D (above 1004 mbsf) (Figs. F31, F32, F33). It has a typical mid-ocean-ridge basalt (MORB) tholeiite composition with SiO₂ concentrations of 50.4 wt%, (Na₂O + K₂O) of 2.91 wt%, Mg# (cationic Mg/(Mg + Fe) ratio with all Fe as Fe²⁺) of 58, and relatively low trace element contents (e.g., Zr = 77 ppm, Y = 32 ppm, and Sr = 88 ppm). However, Sample 335-1256D-Run02-EXJB is distinguished by low concentrations of CaO and Zn and higher concentrations of Ba (24 ppm) relative to the previously analyzed Hole 1256D basalts at a given MgO concentration. These differences suggest slight albitization of plagioclase (Ca loss and Na gain) associated with the circulation of hydrothermal fluids (leaching of Zn and transport of fluid mobile elements such as Ba). Similar chemical variations were previously observed in the lower part of the volcanic section and are associated with low-temperature alteration of basalt and precipitation of hydrous minerals (Wilson, Teagle, Acton, et al., 2003; Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Alt et al., 2010). The lack of a thin section does not allow confirmation of this interpretation for Sample 335-1256D-Run02-EXJB. However, these similarities in composition lead us to suggest that Basalt 335-1256D-Run02-EXJB is derived from the lower part of the drilled volcanic section at Hole 1256D.

Granoblastic basalts

The granoblastic basalts have LOI of –0.369 to 0.273 wt% and H₂O of 0.54 to 1.2 wt%, and their CO₂ contents vary between 0.01 and 0.048 wt% (Fig. F30). The small to negative LOI, together with the low CO₂ contents, shows that these rocks were not heavily affected by late alteration processes.

The cored granoblastic basalts and the junk basket samples overlap in composition. They have 7.2–8.7 wt% MgO, 48.5–50.8 wt% SiO₂, 13.9–15.1 wt% Al₂O₃, 9.7–12.4 wt% FeO, and 11.4–12.3 wt% CaO (Fig. F31). As illustrated in Figures F32 and F33, they have relatively homogeneous compositions in most minor (e.g., Sc = 47–51 ppm) and trace (e.g., Ba = 9–12 ppm; Sr = 71–82 ppm; Y = 19–33 ppm) elements and Zr (31–58 ppm). Only Cr shows more variable concentrations, ranging from 88 to 165 ppm. These concentrations do not follow the trend of other data; they are interpreted as evidence of variable amounts of a minor Cr-rich phase.

The samples analyzed overlap in composition with those of the sheeted dike complex and of the granoblastic dikes previously sampled in Hole 1256D, which span the entire range of composition of the basalt that composes the volcanic section. Similar to the dikes sampled during previous expeditions (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006), the composition of the granoblastic basalts sampled during Expedition 335 is consistent with fractionation of the MORB melts, which formed the entire section of oceanic crust sampled by Hole 1256D.

Two of the junk basket samples, 335-1256D-Run12-RCJB-Rock B and 335-1256D-Run19-RCJB-Rock C, are distinguished by slightly higher FeO (13.3–14.2 wt%) and lower CaO (9.6–10.2 wt%) and Ca# (cationic Ca/(Ca + Na) = 66–69) compared to other Expedition 335 granoblastic basalts (FeO = 9.7–12.3 wt%; CaO = 11.4–12.3 wt%; Ca# = 72–75) and Hole 1256D granoblastic dikes (Fig. F31). Sample 335-1256D-Run12-RCJB-Rock B also has the highest TiO₂ (1.6 wt%) and V (480 ppm) concentrations. Higher oxide modal contents could explain the variations of FeO, TiO₂, and V (see Thin sections in “Core descriptions”) but not the lower values of Ca#. Samples 335-1256D-Run12-RCJB-Rock B and 335-1256D-Run19-RCJB-Rock C are both crosscut by dioritic hornblende–rich veins. Although Expedition 335 shipboard analyses were carried out on material collected as far as possible from those veins, samples may have been contaminated by these more evolved melts. This process is generally associated with an increase in incompatible trace element concentrations, but we do not observe significant changes of the minor and trace element concentrations in these samples. Another possible explanation of these slight differences in composition is a higher degree of interaction with hydrothermal fluids in these samples compared to the other granoblastic basalts sampled during Expedition 335. This process would have led to albitization of the plagioclase. This explanation is consistent with the higher LOI of these two samples, the highest of the granoblastic basalts sampled during Expedition 335. Petrographic description of the thin section does not provide supporting evidence for this interpretation, although both samples show some degree of hydration. Further constraints (e.g., in situ chemical analyses) will require shore-based studies.

A common characteristic of the granoblastic basalts sampled during Expedition 335 is their depleted Cu, Zn, Zr, and Y compositions compared to those of previously sampled basaltic samples in Hole 1256D. These elements plot at the low concentration end of the field of composition of the granoblastic dikes and sheeted dikes (Figs. F32, F33). These variations in composition appear correlated to depth and will be discussed later in this section (see “Downhole chemical variations”).

Gabbroic rocks

Two gabbroic rocks were analyzed during Expedition 335: Sample 335-1256D-Run11-EXJB, an olivine gabbronorite, and Sample 335-1256D-Run20-RCJB-Rock C, an olivine gabbro. They were retrieved from Hole 1256D during junk basket runs. Compared to granoblastic basalts, both samples overlap in composition for CO₂ (0.035–0.07 wt%) but have high LOI values (0.815–1.16 wt%), which are most likely related to secondary alteration mineral assemblages observed in these samples (chlorite and minor smectite; see Thin sections in “Core descriptions”) and imply that they were more affected by hydrothermal alteration compared to the neighboring granoblastic basalts (see “Alteration and metamorphism”).

The analyzed gabbros have relatively high concentrations of MgO (11.21–12.24 wt%), Cr (450–750 ppm), and Ni (230–350 ppm) and low concentrations of trace elements such as TiO₂ (0.69–0.72 wt%) and Y (16 ppm) compared to granoblastic basalts (Figs. F31, F32, F33, F34). In contrast with Gabbro 1 and Gabbro 2 from Expedition 312, Expedition 335 gabbroic rocks are quite distinct from the granoblastic basalts with respect to trace elements. They overlap in composition with gabbros sampled in other oceanic environments, and their compositions are consistent with formation as cumulates from a parental MORB melt. Their relatively high Mg# (70–72) and distinctly high Ni (230–350 ppm) reflect the slight variations in modal olivine in the samples.

As illustrated in Figure F34, Expedition 335 gabbroic rocks overlap in composition with the least evolved gabbros previously sampled in Hole 1256D. These gabbros were found only in the Gabbro 1 interval. This characteristic could be used to infer the origin of the junk basket samples as being in the Gabbro 1 interval. All less evolved gabbros in Hole 1256D are characterized by the presence of olivine. Olivine was also described during Expedition 335 (see “Igneous petrology”) in the samples from the Gabbro 2 interval, but these samples were not analyzed. Therefore, existing geochemical data cannot rule out an origin of the two analyzed gabbroic rocks from this interval or even from below.

Downhole chemical variations

The lower part of Hole 1256D, below 1340 mbsf, is characterized by strong chemical variations with, for example, Mg# ranging from 42 to 72 ppm and Zr from 23 to 117 ppm (Fig. F35). These changes in composition mainly reflect the changes in rock types from the low-Mg#, trace element–rich sheeted and granoblastic dikes and dike screens to the higher Mg# and trace element–depleted gabbroic rocks of Gabbro 1 and Gabbro 2. However, there are significant depth-dependent trace element variations in the granoblastic basalts.

There is a general downhole trend of decreasing incompatible element contents (i.e., Zr and, to a lesser extent, Y) in the granoblastic basalts, with Zr ranging from 73 to 110 ppm above Gabbro 1, from 47 to 86 ppm in Dike Screen 1, and from 15 to 58 ppm in Dike Screen 2 (Fig. F35). At a smaller scale, granoblastic basalts sampled within Dike Screens 1 and 2 define a trend of increasing Zr with depth, with the lowest Zr values always found in the upper part of the dike screens. This pattern is particularly marked at the bottom of Hole 1256D, where the most depleted granoblastic basalts (15 ppm Zr; Yamasaki et al., 2009) were sampled at 1502 mbsf, just below Gabbro 2. Expedition 335 samples show increasing Zr contents from ~30 ppm at 1507–1512 mbsf to 58 ppm in Sample 335-U1256D-238R-1W, 15–17 cm, at 1518 mbsf. These downhole variations in incompatible element contents are illustrated by changes in Zr/Y; the most depleted samples from below Gabbro 2 have low Zr/Y (~1.5) compared to the sheeted dikes and granoblastic dikes above Gabbro 1 (Zr/Y = 2–3). Zr and Y are generally considered as not being affected by alteration and can be used as indicators of magmatic processes. The relative depletion of the more incompatible Zr relative to Y is commonly interpreted as evidence of partial melting. The observed variations can be explained either by changes of composition or degree of partial melting at the source or as evidence for local remelting of the granoblastic basalts. Because no petrologic or chemical observations support a change within the mantle source, we favor local remelting of the granoblastic basalts. We interpret the systematic depletion observed in the granoblastic basalts just below both Gabbro 1 and Gabbro 2 as evidence of small degrees of partial melting, probably caused by the gabbroic intrusions into the partially hydrated dikes. The degree of partial melting is probably minor, as the effects on the major element compositions of the dikes are undetectable. However, partial melting appears to modify the incompatible element composition of the granoblastic basalts over significant distances: 10 m below Gabbro 1 and at least 18 m below Gabbro 2. This implies a perturbation of the local thermal conditions over a significant time period.

We also observe lower Zn and Cu compositions in the granoblastic basalts cored at the bottom of Hole 1256 compared to the sheeted dikes and granoblastic dikes sampled above Gabbro 1 (Fig. F35). These elements are commonly mobilized during hydrothermal alteration. Below Gabbro 2, granoblastic basalts have Cu and Zn contents of 20–47 and 25–47 ppm, respectively, compared to 50–100 ppm for Cu and 50–150 ppm for Zn in the granoblastic dikes and sheeted dikes in the upper part of the borehole. A general decrease in Cu and Zn contents was also observed in the lower part of the sheeted dike complex in Hole 504B (Alt et al., 1996). These elements are remobilized during high-temperature hydrothermal alteration (>350°C), which leaches Cu and Zn out of the rock (e.g., Alt et al., 1996). At the bottom of Hole 1256D, this process appears to affect mostly (only?) the granoblastic basalts. Hole 1256D gabbros do not show evidence of Cu and Zn depletion compared to other gabbroic suites: for example, Hole 1256D gabbroic rocks have Zn of 25–73 ppm compared to 10–70 ppm Zn in the gabbros and olivine gabbros sampled at Atlantis Massif (30°N Mid-Atlantic Ridge; Godard et al., 2009). They also have relatively high concentrations of these elements compared to the neighboring granoblastic basalts. This may imply that the injection of Gabbro 1 and Gabbro 2 occurred during or after this stage of high-temperature hydrothermal alteration. Although this hypothesis is consistent with the remelting process suggested to explain Zr/Y downhole variations, it cannot be conclusively demonstrated with the available shipboard data.

Most of the material sampled during Expedition 335 was recovered during junk basket runs, with no direct indication of their original depth. However, all of the granoblastic basalt samples are depleted in Zr, Y, Cu, and Zn with low Zr/Y, similar to those of Dike Screen 2 (i.e., below 1497.5 mbsf). Therefore, the geochemistry of the granoblastic basalts sampled by the junk basket Runs 12–20 provides evidence that they come from the bottom of the borehole (at the Gabbro 2 lower interface and below) rather than from shallower levels. Although the gabbro geochemistry does not provide similarly strong indicators, gabbros sampled with the Expedition 335 junk baskets are also most likely derived from the same interval from the bottom of Hole 1256D below Gabbro 2.

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