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Inorganic geochemistry

One olivine-bearing gabbronorite was selected in Hole U1415H (Sample 345-U1415H-1R-1, 17–23 cm) for geochemical analysis. Sample selection was based on discussion among representatives from all expertise groups within the shipboard scientific party. Inductively coupled plasma–atomic emission spectroscopy (ICP-AES) was used for determining major and trace element concentrations, and gas chromatography was used for S, H2O, and CO2 quantifications. Results are reported in Table T1 in the “Geochemistry summary” chapter (Expedition 345 Scientists, 2014c). Major and trace elements are reported on a volatile-free basis.

Sample 345-U1415H-1R-1, 17–23 cm, has a loss on ignition of 1.1 wt%, H2O of 1.4 wt%, and CO2 of 0.14 wt%. These values are slightly higher than those reported for the gabbronorite sampled in Hole U1415E (Sample 345-U1415E-1R-1, 44–47 cm); however, thin section descriptions indicate that Hole U1415H olivine-bearing gabbronorite is less altered (~5% versus 10% for Hole U1415E gabbronorite; see thin section descriptions in “Core descriptions”). These higher volatile contents may reflect variations in the alteration products of both samples and, in particular, the complete replacement of primary olivine in Sample 345-U1415H-1R-1, 17–23 cm, by hydrous-rich phases such as serpentine (which contains as much as 13 wt% H2O stoichiometrically). Alternatively, the higher volatile content may indicate centimeter-scale alteration heterogeneities in the degree of alteration within the Hole U1415H olivine-bearing gabbronorite.

Sample 345-U1415H-1R-1, 17–23 cm, is characterized by a high Mg# (cationic Mg/[Mg + Fe] – all Fe as Fe2+) of 81, high Ni (150 ppm) and Cr (1000 ppm) abundances, and low TiO2 (0.3 wt%) and Y (5 ppm) content compared to Hole U1415E gabbronorite. These compositions may reflect the presence of olivine in the primary mineral assemblage (see “Igneous petrology”), and the high Cr content points toward a petrogenetic link with a relatively primitive melt.

Hole U1415H gabbronorite is significantly less evolved than previously sampled Hess Deep gabbro and gabbronorite (e.g., Gillis, Mével, Allan, et al. 1993; Pedersen et al., 1996; Kirchner and Gillis, 2012) and has compositional similarities with primitive gabbros sampled at Pito Deep (Perk et al., 2007).