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doi:10.2204/iodp.proc.345.113.2014

Inorganic geochemistry

Chemical analyses were performed on 13 olivine gabbro and orthopyroxene-bearing olivine gabbro samples and on five troctolite samples. Sample selection was based on discussion with representatives from all expertise groups within the shipboard scientific party. Inductively coupled plasma–atomic emission spectroscopy was used for determining major and trace element concentrations, and gas chromatography was used for S, H₂O, and CO₂ quantification. Selected data are shown in Figures F61, F62, and F63 and fully reported in Table T1 in the “Geochemistry summary” chapter (Gillis et al., 2014c). Major and trace elements are reported on a volatile-free basis. The lowest part of the borehole (from Core 345-U1415P-21R through Core 23R) was not sampled for geochemistry because high-quality inductively coupled plasma–atomic emission spectroscopy major and trace element measurements could not be collected during the transit back to Panama.

Multitextured Layered Gabbro Series

The 13 analyzed olivine gabbro samples were selected from between 12 and 64 mbsf in lithologic Unit II, which represents the Multitextured Layered Gabbro Series. Minor orthopyroxene (<1%–5%) was observed in seven olivine gabbro samples from Intervals 3, 18, 21, and 25 (see “Igneous petrology” and thin section descriptions in “Core descriptions”).

Hole U1415P olivine gabbro and orthopyroxene-bearing olivine gabbro is altered to various degrees, with loss on ignition (LOI) ranging from 1.7 to 4.8 wt%. The water content in gabbro (1.6–4.5 wt%) correlates with LOI (Fig. F61). In contrast to the water concentrations, CO₂ (0.03–0.20 wt%) and S (190–800 ppm) contents are low and cannot be correlated with any of the other analyzed elements or with optically determined igneous and metamorphic observations reported in the thin section descriptions. Small sulfide minerals were observed within the Multitextured Layered Gabbro Series (see “Metamorphic petrology”); however, the sampling strategy goal of an average characterization of the cores was too wide-ranging to detect small-scale correlations between S concentration and these observed phases.

Hole U1415P olivine gabbro and orthopyroxene-bearing olivine gabbro have primitive compositions, with high Mg# (78–85), low SiO₂ (43.9–50.1 wt%) and TiO₂ (0.1–0.3 wt%) contents, and incompatible lithophile element (e.g., Y = 3–11 ppm) contents. Similar to the gabbroic rock previously sampled during Expedition 345, the gabbro displays a wide range of Al₂O₃ (15.7–23.7 wt%) and CaO (9.8–20.9 wt%) compositions that contrast with their relatively restricted range of Mg# (Fig. F62). These variations mainly reflect variations in the plagioclase and clinopyroxene modes of the analyzed gabbroic samples. The wide range of Sc (40–130 ppm) and V (9–36 ppm) contents are also attributed to differences in mineral mode, as these elements are more compatible in clinopyroxene compared to the other major mineral phases crystallized in the analyzed samples (plagioclase and olivine; see thin section descriptions in “Core descriptions”). Three olivine gabbro samples from Sections 345-U1415P-4G-1, 7R-1, and 8R-1 are significantly enriched in Cr (>1550 ppm) compared to adjacent olivine gabbro and orthopyroxene-bearing olivine gabbro (~150–810 ppm) (Fig. F62). These high Cr contents probably reveal the presence of minor spinel, a Cr-rich phase observed in some thin sections (see “Igneous petrology” and thin section descriptions in “Core descriptions”). High Cr values were also reported for three olivine gabbro samples in Hole U1415J; however, the occurrence of spinel was only rarely optically observed in Hole U1415J gabbro (see visual core descriptions in “Core descriptions”).

Overall, olivine gabbro and orthopyroxene-bearing olivine gabbro sampled in Hole U1415P overlap in major and trace element compositions (Figs. F62, F63; see also Table T1 in the “Geochemistry summary” chapter [Gillis et al., 2014c]). The primitive geochemical characteristics of both olivine gabbro and orthopyroxene-bearing olivine gabbro are surprising because the occurrence of orthopyroxene in primary mineral assemblages is generally considered as an indicator of crystallization from evolved melts within the mid-ocean-ridge basalt gabbroic suites.

Troctolite

Five troctolite samples were selected from Cores 345-U1415P-16R through 20R from the Troctolite Series (lithologic Unit III) in the lower part of Hole U1415P (below ~64.2 mbsf).

Similar to previously analyzed Site U1415 plutonic rock, LOI (5.1–9.7 wt%) and H₂O contents (5.2–9.6 wt%) of Hole U1415P troctolite correlate positively, at approximately 1:1 (Fig. F61). In addition, both LOI and H₂O concentrations are similar to the most altered troctolite in Hole U1415J and thus are significantly higher compared to those of other drilled plutonic lithologies. The extremely low measured CO₂ (0.03–0.09 wt%) and S (161–352 ppm) abundances cannot be correlated with other shipboard-acquired mineralogical and/or geochemical data sets.

Hole U1415P troctolite is MgO rich (18–30.8 wt%) and is distinguished from olivine gabbro by its higher Mg# (87–89) and high Ni abundance (700–1170 ppm) (Fig. F62; see also Table T1 in the “Geochemistry summary” chapter [Gillis et al., 2014c]), reflecting its elevated olivine content. The troctolite has variable Al₂O₃ compositions (11.8–21.1 wt%) indicative of variations in the plagioclase mode, and overall it has lower CaO concentrations (8.1–12.4 wt%) compared to neighboring gabbro, reflecting its lower clinopyroxene fraction. Hole U1415P troctolite has very low incompatible elements with TiO₂ (<0.04 wt%), Sc (3–9 ppm), V (14–23 ppm), and Y (<3 ppm) (Fig. F63). Together with their refractory character, these compositions suggest precipitation from a primitive mantle melt for the Hole U1415P troctolite. As expected for primitive cumulates, Hole U1415P troctolite is Cr rich (120–1065 ppm); however, it is not as Cr-rich as some of the olivine gabbro samples, as would be expected for the most primitive end-member of a mid-ocean-ridge-cumulate suite (Fig. F62). These compositions may indicate either a bias in sampling Cr-rich phases in the troctolite series or a more complex petrogenetic history compared to a simple crystal fractionation process.

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