Previous   |    Next

doi:10.2204/iodp.proc.345.110.2014

Inorganic geochemistry

Chemical analyses were performed on 1 aphyric basalt and 24 plutonic rocks (including 14 orthopyroxene- and olivine-bearing gabbroic rocks), 8 samples from troctolite-dominated intervals, and 2 samples of drill cuttings. 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 quantification. Selected data are shown in Figures F100, F101, F102, and F103 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.

Aphyric basalt

One aphyric basalt was sampled from the rubble unit in ghost Core 345-U1415J-2G (Sample 345-U1415J-2G-1, 0–13 cm). The basalt is strongly altered (~30%; see “Metamorphic petrology” and thin section descriptions in “Core descriptions”), with a loss on ignition (LOI) of 1.86 wt% and 1.8 wt% H₂O, and is characterized by a relatively high Mg# of 69, high Cr (520 ppm) and Ni (210 ppm) contents, and low incompatible trace element contents (e.g., Y = 24 ppm). Major element geochemistry defines this rock as a tholeiitic basalt, similar in composition to the primitive MORB previously sampled in the Hess Deep area at Site 894 (Shipboard Scientific Party, 1993) and at the more primitive end of the East Pacific Rise basalt recovered along the Northern Escarpment of the Hess Deep Rift (Mg# = 44–66; Stewart et al., 2002).

Gabbroic rock

Olivine gabbro, clinopyroxene oikocryst-bearing gabbro, and gabbro

The analyzed gabbroic samples comprise three gabbro samples, one clinopyroxene oikocryst-bearing olivine gabbro sample, and nine olivine-bearing and olivine gabbro samples (>5% olivine); minor orthopyroxene (0.1%–4%) is reported in the gabbroic rock sampled in Intervals 5, 11, 32, 38, and 42 in lithologic Unit II (see “Igneous petrology” and thin section descriptions in “Core descriptions”). All samples are altered to various degrees, with LOIs ranging from 1.1 to 5.9 wt%. The water content (1.2–5.3 wt%) correlates with the LOI (Fig. F100). CO₂ content is low (0.01–0.4 wt%) and does not appear to correlate with any of the other analyzed elements or with any igneous and alteration features reported in the thin section descriptions. Sulfur is highly variable (160–1740 ppm) in the analyzed gabbro. Minor pyrite, pentlandite, and chalcopyrite were observed in Hole U1415J (see “Metamorphic petrology”). The observed variations in sulfur compositions are probably related to the amount of sulfide in the analyzed samples.

Hole U1415J gabbro has primitive compositions, with Mg# 81–87 and 130–490 ppm Ni (Fig. F101). The gabbro overlaps in composition of major and trace elements (Figs. F102, F103) with the gabbroic rocks sampled in Hole U1415I, with 45–50 wt% SiO₂, 0.1–0.3 wt% TiO₂, and 1.6–6.5 ppm Y. No systematic compositional variations were observed downhole (Fig. F101) or between the oikocryst-bearing gabbro, olivine gabbro, and gabbro samples (see Table T1 in the “Geochemistry summary” chapter [Gillis et al., 2014c]). Compared to the relative minor changes in Mg#, strong variations in CaO, Al₂O₃, and Na₂O as well as in trace element compositions, in particular Sc, were observed (e.g., Al₂O₃ = 13–25 wt%) (Fig. F102). These variations mainly reflect local changes in the primary plagioclase- and clinopyroxene-dominated modal composition of the samples, with variable amounts of olivine. Na₂O and Al₂O₃ are concentrated mainly in plagioclase, CaO and Sc are mainly concentrated in clinopyroxene (the former is abundant in plagioclase also), and olivine is depleted in all of these elements.

The gabbroic rocks sampled in Hole U1415J have Cr compositions ranging from 200 to 850 ppm, which is typical of primitive gabbroic series (e.g., Cannat, Karson, Miller, et al., 1995; Godard et al., 2009), except for three samples characterized by low Cr concentrations (<100 ppm). These low concentrations may reflect a change in the mobility of Cr during alteration; the Cr-depleted samples were recovered between 99 and 104 mbsf, where cataclasites and high-temperature alteration (500°–600°C) are reported (see “Metamorphic petrology”). Three samples are distinguished by their high Cr content (2100–2500 ppm) (Fig. F101). These variations could not be correlated with any igneous and alteration features observed in thin sections (see thin section descriptions in “Core descriptions”) or with other analyzed elements. These high Cr concentrations may reflect the occurrence of Cr-rich clinopyroxene and/or of a Cr-rich minor phase (e.g., chromite) in the samples. The origin of the high Cr contents of these olivine gabbro samples will be further investigated using onshore technical facilities (e.g., by electron probe microanalyzer).

Troctolitic intervals

Eight samples were collected from the troctolite-dominated intervals from lithologic Units II and III: three clinopyroxene oikocryst-bearing troctolite samples (Intervals 24, 38, and 47) in Unit II and two troctolite samples (Intervals 58 and 73), one clinopyroxene-bearing troctolite sample (Interval 66), one olivine-bearing anorthosite sample, and one chromitite sample (345-U1415J-18R-1, 67–72 cm; see “Igneous petrology”) in Unit III. The LOI (2.7–9.1 wt%) and H₂O content (2.2–7.9 wt%) of the troctolitic samples are correlated (Fig. F100); on average, they are higher than that of Hole U1415J gabbro. The highest water contents were observed in samples having the highest olivine content in their primary assemblage (45%–50% olivine in Samples 345-U1415J-13R-1, 38–44 cm, and 19R-1, 15–19 cm). These samples reflect the water-rich composition of the alteration minerals replacing olivine, such as serpentine (~13 wt% H₂O) (see “Metamorphic petrology” and thin section descriptions in “Core descriptions”). CO₂ (0.01–0.3 wt%) and sulfur (310–1160 ppm) contents are highly variable and similar to those of Hole U1415J gabbro.

Olivine-bearing anorthosite, troctolite, and clinopyroxene oikocryst-bearing troctolite overlap in composition with Hole U1415J gabbro for most major elements (Figs. F101, F102). Similar to gabbro, they display highly variable CaO, Na₂O, and Al₂O₃ contents that mainly reflect the variable plagioclase/clinopyroxene ratios in the analyzed samples. For example, olivine-bearing anorthosite is distinguished by its high Al₂O₃ content (26.8 wt%) compared to Hole U1415J troctolitic samples (Al₂O₃ = 12.0–21.4 wt%). Olivine-bearing anorthosite, troctolite, and clinopyroxene oikocryst-bearing troctolite also overlap in composition with gabbro for Cr (365–680 ppm), Zn (18–48 ppm), and Cu (11–110 ppm), although plagioclase-rich olivine-bearing samples from the cataclasite interval at the bottom of Unit II (~38 mbsf) have low Cr contents (<35 ppm) (Fig. F101). In contrast to the Cr-depleted gabbros sampled at the bottom of Hole U1415J, no indication is apparent of a change in the conditions of alteration in these samples compared to neighboring samples; only the presence of talc could indicate a different alteration pathway that may have affected Cr mobility (see “Metamorphic petrology” and thin section descriptions in “Core descriptions”). It must be noted also that gabbro (Sample 345-U1415J-8R-3, 35–38 cm) sampled next to the most Cr-depleted troctolite (Sample 8R-3, 26–29 cm) has a composition typical of the gabbro sampled in Hole U1415J (590 ppm), indicating that the Cr composition in this rock is controlled by highly localized processes.

Troctolite has Mg# ranging from 81 to 89 and high Ni contents (260–1490 ppm) (see Table T1 in the “Geochemistry summary” chapter [Gillis et al., 2014c]). Ni is highly concentrated in olivine, and the most Ni rich samples are also the ones that have the highest fraction of olivine in their primary assemblage (e.g., Samples 345-U1415J-13R-1, 39–41 cm, and 19R-1, 18–19 cm; see thin section descriptions in “Core descriptions” and Table T1 in the “Geochemistry summary” chapter [Gillis, et al., 2014c]). Troctolite has low TiO₂ (<0.1 wt%); Sc (<13 ppm); and V, Y, and Zr (<20 ppm) compared to Hole U1415J gabbro (Fig. F103; see Table T1 in the “Geochemistry summary” chapter [Gillis, et al., 2014c]). These low concentrations, together with their refractory composition, suggest that the troctolite probably precipitated from less evolved melts than the neighboring gabbro.

Chromitite (Sample 345-U1415J-18R-1, 67–72 cm) is distinguished by its enrichment in Fe₂O₃ (30.5 wt%) and MgO (22.8 wt%) and depletion in SiO₂ (31 wt%) and TiO₂ (0.02 wt%) compared to other rock sampled in the troctolitic intervals in Hole U1415J. These compositions are consistent with its modal composition (60% olivine and 40% oxide). This sample was interpreted as a chromitite whose composition was modified during late hydrothermal alteration (see “Igneous petrology” and “Metamorphic petrology”). Chemical data indicate that the oxides present in the sample are Fe rich and Ti poor, probably magnetite. The sample’s trace element composition is similar to that of Hole U1415J troctolite, including its Cr content (640 ppm).

Drill cuttings

Two samples of sandy material were selected from ghost Core 345-U1415J-2G in Unit I (see “Metamorphic petrology”). The samples comprise abundant altered rock material and have high LOI (2.7–3.3 wt%) and H₂O (3.4–3.9 wt%) and CO₂ (0.07–0.14 wt%) contents, even when compared to the altered drilling-induced disaggregated gabbro sampled in Hole U1415I. The samples of drill cuttings have lower Mg# (76), lower Ni (150 ppm) content, and higher TiO₂ (0.6–0.7 wt%) and incompatible trace element (e.g., Y = ~12–13 ppm) contents compared to plutonic rock sampled in Hole U1415J, which suggests that they comprise more evolved gabbroic material. Compared to all of the plutonic rock sampled at Site U1415, these drill cuttings are characterized by enrichment in Na₂O (2.6–3 wt%) and Ba (70–100 ppm), indicative of the presence of higher fractions of altered minerals (Na-rich albitized plagioclase and chlorite) and, probably, contamination by the Ba- and clay-rich drilling muds mixed with seawater used during coring operations. Therefore, these results should be treated with caution for petrogenetic interpretations.

Top of page   |    Previous   |    Next