Previous   |    Next

doi:10.2204/iodp.proc.345.109.2014

Inorganic geochemistry

Geochemistry analysis was completed on three gabbroic rock pieces (one clinopyroxene oikocryst-bearing troctolite, one orthopyroxene-bearing olivine gabbro, and one olivine-bearing gabbronorite) and three sand cuttings from drilling-induced disaggregated gabbro from lithologic Units I and II in Hole U1415I. Sample selection criteria were established during discussion among representatives from all expertise groups within the shipboard scientific party. Major and trace elements were determined by inductively coupled plasma–atomic emission spectroscopy (ICP-AES), 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), and selected data are shown in Figures F29 and F30. Major and trace elements are reported on a volatile-free basis.

Gabbroic rock

Hole U1415I gabbroic rock has loss on ignition (LOI) values ranging from 1.0–1.6 wt% for mildly altered gabbro in the lower part of the borehole (Intervals 16 and 22) to 3.2 wt% in the most altered sample (345-U1415I-2R-1, 0–4 cm). Measured LOI appears directly correlated to the water content of the analyzed samples (e.g., ~1 wt% H₂O in Sample 345-U1415I-4R-1, 50–52 cm) (Fig. F29). These values are consistent with the degree of alteration estimated from thin section descriptions (see “Metamorphic petrology”). Hole U1415I gabbro has low CO₂ contents (<0.1 wt%) and variable S compositions (290–560 ppm). No systematic variations of the volatile elements with the mineral mode and/or analyzed elements were observed.

Hole U1415I gabbroic rock has orthopyroxene contents ranging from 0 to 8 vol% (see “Igneous petrology” and thin section descriptions in “Core descriptions”). The occurrence of orthopyroxene in oceanic gabbro successions is commonly interpreted as indication of crystallization within an evolved system (Shipboard Scientific Party, 1993). In contrast to the variability in modal orthopyroxene contents, Hole U1415I gabbroic rock displays a very restricted range of values for major element compositions (Fig. F30). The studied plutonic rock has primitive compositions with high Mg# (81–82) and low Fe₂O₃ (4.9–5.8 wt%) and SiO₂ (47.7–49.5 wt%) contents and display some minor variations in CaO and Al₂O₃ that can be related to variations in the plagioclase/clinopyroxene ratio in the primary mineral assemblage, with Al₂O₃ mainly concentrated in plagioclase and CaO dominant in clinopyroxene in plagioclase-poor samples. Hole U1415I gabbroic rock has relatively high and variable Cr (700–1110 ppm) and Ni (150–300 ppm) contents and low TiO₂ (0.16–0.18 wt%) concentrations. Together with their high Mg#, these compositions suggest crystallization from primitive magmas for Hole U1415I gabbroic rock, the geochemistry of which is similar to that of the gabbronorite sampled in Hole U1415H (Fig. F30).

Drilling-induced disaggregated gabbros

The three samples of drilling-induced disaggregated gabbro have homogeneous volatile contents, with LOIs ranging from 3.5 to 3.7 wt%, H₂O contents ranging from 2.8 to 3 wt%, and CO₂ contents of ~0.05 wt% (Fig. F29). These values are consistent with the abundance of altered minerals observed in the sampled material (see thin section descriptions in “Core descriptions” and “Metamorphic petrology”). These volatile contents are similar to those of the overlying orthopyroxene-bearing olivine gabbro Sample 345-U1415I-2R-1, 0–4 cm (e.g., LOI = 3.2 wt%). The similarity in volatiles between this sample and the drilling-induced disaggregated gabbro points toward a similar alteration history for the plutonic rock sampled above 27.5 mbsf in rubble Unit I (see “Igneous petrology”).

The drilling-induced disaggregated gabbro has low Mg# (79) and high TiO₂ (~0.4 wt%) and incompatible lithophile element contents (e.g., Y = 10 ppm) compared to neighboring gabbro, suggesting that it comprises fractions of more evolved gabbroic material. The gabbro is also distinguished by higher Na₂O (2.5–2.6 wt%) and Sr (~100 ppm) relative to Hole U1415I gabbros (Na₂O = ~1.6–1.9 wt%; Sr = ~70 ppm). These values are consistent with the high degrees of alteration of the drilling-induced disaggregated gabbro, which is characterized by, for instance, the abundance of secondary plagioclase (see “Metamorphic petrology” and thin section descriptions in “Core descriptions”). However, the results for the drilling-induced disaggregated gabbro probably also reflect contamination of the sampled material by drilling mud mixed with seawater. Our data provide further evidence of this contamination during drilling. The drilling-induced disaggregated gabbro has high V (140 ppm) and Zn (140–160 ppm) compared to neighboring gabbro (V = ~40 ppm; Zn = ~30 ppm). These elements are enriched in the antirust coatings used on drilling equipment (Zn) and in the drill bit components (V in tungsten carbide), and the high V and Zn values suggest a non-negligible contamination by drilling materials during downhole disaggregation of the sampled plutonic rocks (Fig. F30E). Because the extent of this chemical contamination cannot be deduced from our data set, the composition of the two disaggregated gabbro samples was not used for petrogenetic interpretation of the Hole U1415I plutonic series.

Top of page   |    Previous   |    Next