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doi:10.2204/iodp.proc.330.105.2012

Geochemistry

Igneous rocks

We analyzed 33 samples of igneous rocks from Hole U1374A for concentrations of major elements and several trace elements (Table T11) by inductively coupled plasma–atomic emission spectroscopy (see “Geochemistry” in the “Methods” chapter [Expedition 330 Scientists, 2012a] for information on analytical procedures, instrumentation, and data quality). Most of the samples were collected from lava lobes and clasts in the breccias that dominate this site. Three samples were from stratigraphic units identified unambiguously as flows (Units III, IV, and VI; see “Igneous petrology and volcanology”). Four were from intrusive sheets within Units XVI–XIX (Samples 330-U1374A-52R-2 [Piece 3, 35–37 cm]; 55R-2 [Piece 1, 6–8 cm]; 58R-4 [Piece 1, 64–66 cm]; and 64R-2 [Piece 2B, 37–39 cm], respectively). One sample (72R-2 [Piece 1, 28–31 cm]) was from a lithologic unit at the base of Unit XIX that could be either a lava flow or an intrusive sheet (see “Igneous petrology and volcanology”).

As at Sites U1372 and U1373, total weight percentages for the major element oxides vary significantly, from 92.29 to 102.20 wt%. Possible reasons for this variation are discussed in “Geochemistry” in the “Methods” chapter (Expedition 330 Scientists, 2012a). To better compare results with one another and with data from the literature, we normalized the raw major element values to 100 wt% totals. The normalized values are presented below the raw data in Table T11 and are used in the figures and in the discussion below.

Weight loss on ignition (LOI) varies from 0.6 wt% to 5.0 wt%. Only one Site U1374 sample (the lowermost sample from Unit XIX) has a LOI value of <1 wt%, which is typical for unaltered basalt (e.g., Rhodes, 1996). The majority have LOI values of <2.5 wt% and indicate moderate overall levels of alteration. However, LOI values for 10 samples are >3 wt%, which indicate comparatively high levels of alteration. These results are generally consistent with the petrography of the rocks (see “Alteration petrology” and “Igneous petrology and volcanology”). Of the group of elements measured, K₂O is typically the most affected by alteration (e.g., see “Geochemistry” in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]). No correlation is present between LOI and K₂O, but the sample with the highest LOI, a lava clast from the sedimentary breccia of Unit IX (Sample 330-U1374A-13R-1 [Piece 1, 5–6 cm]) has markedly higher K₂O (1.87 wt%) than the other samples (0.35–1.20 wt%). This sample also has the highest Na₂O (4.33 wt% vs. 2.39–3.82 wt% for the other samples), and it is possible that its Na₂O concentration also has been elevated by alteration. Besides this sample, K₂O has probably been affected to variable extents by alteration in a number of the other samples because K₂O does not correlate with TiO₂, P₂O₅, Zr, or Y (this is true whether or not the highest-K₂O sample is excluded).

In a total alkalis (Na₂O + K₂O) vs. SiO₂ diagram (Fig. F61), data for Site U1374 samples overlap considerably with data from Sites U1372 and U1373. However, on average the Site U1374 rocks tend to be slightly lower in SiO₂ at similar total alkali concentrations. This combination of characteristics makes the Site U1374 samples slightly more alkalic as a group. For example, alkalinity, a measure of the distance of a data point above or below the alkalic-tholeiitic dividing line in Figure F61, averages 1.8 for Site U1374 compared to 0.9 for both Site U1373 (located on the same seamount as Site U1374) and Site U1372 (on Canopus Guyot). Likewise, no transitional basalts were found, in contrast to the several analyzed for Sites U1372 and U1373. Data for 21 of the Site U1374 samples fall in the field of alkalic basalt, just as the great majority of samples from Sites U1372 and U1373 do. However, data for 12 Site U1374 samples lie in the field of basanite and tephrite. One of these samples is the high-LOI Unit IX sample mentioned above, whose high alkali content appears to be at least partly a result of alteration. This sample has 45.77 wt% SiO₂, and its original composition (i.e., prior to alteration) is likely to have been more basaltic. The other Site U1374 samples possessing basanitic-tephritic compositions in Figure F61 all have <45 wt% SiO₂. Seven have total alkali contents between 4.24 and 4.96 wt%. Six of these samples are from Unit X, and the seventh is from a clast in the sedimentary breccia of Unit XI. The remaining four samples with data points in the basanite-tephrite field have lower total alkali contents (3.25–3.69 wt%) and are from Units XIII, XIV, XVIII, and XIX. The Unit XVIII sample is from an intrusive sheet.

Values of Mg number (Mg# = 100 × Mg²⁺/[Mg²⁺ + Fe²⁺], assuming Fe₂O₃/FeO = 0.15) vary from 65.8 to 35.1, whereas Ni contents range from 215 to 39 ppm. Averages for both quantities are relatively low (49.5 for Mg number and 76 ppm for Ni), illustrating the rather evolved nature of the Site U1374 rocks as a group. Except for one sample, MgO concentrations range from 2.78 to 8.54 wt%, with an average of 5.88 wt%. The exception is Unit III Sample 330-U1374A-3R-2 (Piece 4D, 108–110 cm), which has 10.15 wt% MgO. Like its even higher MgO counterparts from Sites U1372 and U1373 (where MgO ranges to 13.21 and 11.78 wt%, respectively; see “Geochemistry” in the “Site U1372” chapter [Expedition 330 Scientists, 2012b] and “Geochemistry” in the “Site U1373” chapter [Expedition 330 Scientists, 2012c]), this sample is highly olivine-augite phyric and likely to contain excess olivine. However, its CaO/Al₂O₃ ratio (0.58) is rather low for its MgO content, suggesting that it does not contain excess augite.

In an Al₂O₃ vs. MgO diagram (Fig. F62A), the Site U1374 data again largely overlap the field defined by samples from Sites U1372 and U1373 and indicate a relatively minor overall role for fractionation of plagioclase. Many of the Site U1374 samples also have similar CaO and CaO/Al₂O₃ values to those of the two previous sites. However, most of the samples from the uppermost 240 m of Site U1374 (i.e., most samples from Units III–XIII) have lower CaO and CaO/Al₂O₃ for their MgO values (e.g., Fig. F62B). The very low CaO/Al₂O₃ of one sample from Site U1372 is probably a result of alteration (see “Geochemistry” in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]). Scandium contents are low for these and other Site U1374 samples, averaging 15 ppm. In contrast, the average value of Sc for Site U1373 is 28 ppm (Sc data were not obtained for Site U1372). These characteristics suggest that clinopyroxene was a significant factor in the evolution of the Site U1374 magmas, particularly those supplying units in the upper half of the hole. The rocks from Unit X, which are all aphyric, and several other aphyric samples have relatively high Fe₂O₃^T (total iron as Fe₂O₃), with values between 14.17 and 16.15 wt% (Fig. F62C). Because fractionation of clinopyroxene does not lead to significant iron enrichment, a high relative iron content may be a feature inherited from the parental magmas of these samples. Sample 330-U1374A-12R-4 (Piece 12, 113–115 cm), an aphyric clast from the Unit IX sedimentary breccia, has unusually high CaO (15.63 wt%) and CaO/Al₂O₃ (0.98). It also has rather high LOI (4.0 wt%), and we suspect that its CaO content may have been increased by alteration, much like a similarly high CaO/Al₂O₃ sample from Site U1372 (Fig. F62B; see “Geochemistry” in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]).

As with major elements, Site U1374 trace element data overlap data for Sites U1372 and U1373 (e.g., Fig. F63). However, Site U1374 samples exhibit greater overall variability in Ba, P₂O₅, Sr, Zr, and Y relative to TiO₂, ranging to higher Ba, Sr, and Y and to both higher and lower P₂O₅ and Zr for a given TiO₂ content. These characteristics are consistent with some combination of variability in the amount of partial melting and in source composition at Site U1374 and between Site U1374 and the two previous drill sites. The slightly more alkalic nature of the Site U1374 rocks is also consistent with differences in partial melting or source composition between this site and Sites U1372 and U1373. Sample 330-U1374A-64R-2 (Piece 2B, 37–39 cm), from an intrusive sheet in the upper part of Unit XIX, has an anomalously high Sr concentration (914 ppm vs. 402–739 ppm for the other Site U1374 rocks). The sample is aphyric, so its high Sr content is not a product of accumulation of plagioclase or another common mineral. Strontium can be affected by some types of submarine alteration, and this sample’s LOI value (4.1 wt%) is one of the higher values measured for Site U1374. However, concentrations of elements more sensitive to alteration are not unusually high or low (e.g., K₂O = 1.02 wt%, CaO = 11.15 wt%, and P₂O₅ = 0.37 wt%), rendering alteration an unlikely explanation for the sample’s anomalous Sr content. Two Site U1373 samples have high Sr for their TiO₂ concentrations, but they also have high Ba (see “Geochemistry” in the “Site U1373” chapter [Expedition 330 Scientists, 2012c]), whereas this sample has a Ba concentration (199 ppm) close to the Site U1374 average (192 ppm). We have no reason to suspect an analytical artifact. Presently, we have no explanation for the high Sr concentration of this sample.

Although Sites U1373 and U1374 are located on the same seamount within 10.4 km (5.6 nmi) of each other and the samples from each site overlap in major and trace element composition, we cannot correlate any of the Site U1374 samples with those from Site U1373. Presumably, the igneous rocks sampled at the two sites represent distinct eruptive events.

Downhole variations reveal little systematic behavior in the upper part of Hole U1374A. The full range of Mg numbers (35.1–65.8) is observed in the uppermost 100 m in Units III–X (Fig. F64A). Below this level the olivine-phyric samples from Units XI and XII and the topmost part of Unit XIII exhibit relatively high Mg numbers (56.0–60.2), whereas samples from greater depths all have lower values. A rough trend of decreasing Mg number is evident from Units XII–XV, where a local low of 43.9 is seen, below which values fluctuate between 44.9 and 53.4. Variations in Ni correspond broadly with those in Mg number, consistent with variable control by mafic phases, principally olivine. The incompatible elements Ba, Sr, Zr, TiO₂, and Y show no simple overall downhole trend (e.g., Fig. F64C), nor do incompatible-element ratios that serve as indicators of variation in partial melting or source heterogeneity, such as Ba/Y and Zr/Ti (Fig. F64D, F64E). Barium can be affected by some types of alteration, but neither Ba nor Ba/Y correlates with, for example, LOI or K₂O. Titanium content varies irregularly in Units III–X, reaching high values (3.56–3.78 wt%) in Unit X (Fig. F64C). In Unit XI, the concentration drops significantly (to 2.84 wt%) and then gradually increases in Units XI–XV, mirroring the downhole decrease in Mg number and Ni in this interval. The highest TiO₂ concentration (3.91 wt%) is near the bottom of the hole in Unit XIX.

The Zr/Ti ratios of most of the Site U1374 samples vary within a rather restricted range between 0.010 and 0.014 (Fig. F64E), but six of the seven samples from Unit XIII have distinctly higher values between 0.015 and 0.017. Similarly high Zr/Ti values are found in the upper part of the hole in the Unit III lava flow and in a clast from the sedimentary breccia of Unit IX. However, unlike the Unit III and IX samples, the high Zr/Ti in Unit XIII is not accompanied by a comparable or greater elevation in the Ba/Y ratio. This difference implies that the high Zr/Ti ratios of the Unit XIII samples are probably not solely a result of smaller extents of partial melting than those that produced the lower Zr/Ti rocks. Because Ba is more incompatible than Zr and Y is less incompatible than Ti, smaller amounts of partial melting normally produce greater relative increases in magmatic Ba/Y than in Zr/Ti. As a consequence, the comparatively high Zr/Ti values in Unit XIII may at least partly be a source feature. In contrast to the upper six samples of Unit XIII, the lowermost sample has a Zr/Ti value of only 0.011. We infer that the lava lobe from which this sample was taken belongs to a different magma type than that represented by the other samples of this unit. In addition to the multiple samples collected from Unit XIII, we also collected multiple (six) samples from Unit X. Unit X offers an interesting contrast to Unit XIII in its relative homogeneity (Fig. F64B, F64C, F64E).

The four aphyric samples collected from intrusive sheets are compositionally not very different from one another, except that the Unit XVI sheet sample has lower Ba/Y and slightly lower Zr/Ti and Zr/Y than those in Units XVII–XIX (e.g., Fig. F64D, F64E); the Unit XIX sample has anomalously high Sr, as discussed above. Excluding Sr in this sample, the major and trace element compositions of the sheet samples are within the overall range exhibited by the lava samples. However, we are unable to correlate any of the sheets with specific lava flows, lobes, or clasts, including the aphyric units higher in Hole U1374A.

Carbon, organic carbon, nitrogen, and carbonate

Six samples of sediment were taken from Core 330-U1374A-1R (1–6 mbsf) for determination of total carbon, total nitrogen, carbonate (as CaCO₃), and total organic carbon. All of the samples were disturbed by the rotary bit used to drill the hole. Carbonate content ranges from 61.1 to 85.1 wt%, slightly less than values measured for samples from Site U1372. Total organic carbon content in the Site U1374 samples is low, ranging from 0.05 to 0.81 wt% (values of >2 wt% are generally considered high, whereas values of <0.5 wt% are considered low). Total nitrogen content is extremely low (from 0.016 wt% to below detection). Total carbon ranges from 7.4 to 10.6 wt%.

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