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doi:10.2204/iodp.proc.345.111.2014

Inorganic geochemistry

Chemical analyses were performed on two basaltic samples from Hole U1415N. Samples were selected by representatives from all expertise groups within the shipboard scientific party. Sample 345-U1415N-2R-1, 3–5 cm, is moderately olivine phyric and Sample 4R-1, 12–15 cm, is sparsely olivine and plagioclase phyric. Geochemical samples were cut within areas visibly depleted in phenocrysts; in contrast, the associated thin sections were selected in zones where the minerals could easily be optically characterized. Major and trace elements were determined by inductively coupled plasma–atomic emission spectroscopy, and gas chromatography was used for S, H₂O, and CO₂ quantifications. Results are reported in Table T1 in the “Geochemistry summary” chapter (Gillis et al., 2014c). Major and trace elements are reported on a volatile-free basis.

Hole U1415N basalt has high loss on ignition (LOI; 1.9–2.6 wt%) and H₂O content (2.1–2.7 wt%), indicating their advanced degree of alteration (~17%–40%; see “Metamorphic petrology” and thin section descriptions in “Core descriptions”). Nevertheless, this basalt plots in a narrow range of SiO₂ (47.7–49.3 wt%) and has low TiO₂ (0.9–1.2 wt%), Na₂O (2.1–2.9 wt%), and K₂O (<0.04 wt%) contents (Figs. F12, F13), as is expected from tholeiitic mid-ocean-ridge basalt. This suggests that the basalt’s alkali content was not strongly affected by interaction with seawater-derived fluids.

In contrast to their relatively narrow range of SiO₂ contents, Hole U1415N basalts have relatively variable compositions for most other major elements. Sample 345-U1415N-2R-1, 3–5 cm, is characterized by high MgO (14.7 wt%), Ni (530 ppm), and Cr (1140 ppm) contents (Fig. F13; see also Table T1 in the “Geochemistry summary” chapter [Gillis et al., 2014c]). The high MgO and Ni concentrations reflect the primary modal olivine content, and the high Cr content reflects the occurrence of Cr-spinel in the analyzed sample (see “Igneous petrology” and thin section descriptions in “Core descriptions”). These data most likely reflect the accumulation of olivine and chromite rather than representing an exotic picrobasaltic liquid composition. Sample 345-U1415N-4R-1, 12–15 cm, is characterized by relatively high Al₂O₃ (17.9 wt%) and CaO (11.8 wt%) contents, which may reflect the presence of plagioclase phenocrysts in the sample (see “Igneous petrology” and thin section descriptions in “Core descriptions”).

Both samples have low incompatible trace element contents (e.g., Y = ~21–27 ppm, Zr = ~39–55 ppm), similar to basalt recovered in Hole U1415J and typical of primitive mid-ocean-ridge basalt compositions (Figs. F12, F13). Site U1415 basalts overlap in composition with basalts recovered at Ocean Drilling Program Site 894 (Scientific Party, 1993; Allan et al., 1996) and plot at the most depleted and primitive end of the field of Eastern Pacific Rise basaltic compositions (Fig. F13).

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