Major element compositions of primary minerals (olivine, spinel, and amphibole) were determined using an electron probe microanalyzer with wavelength dispersive X-ray spectrometry (JEOL JXA-8600SX) at Niigata University (Japan). Operating conditions were 15-kV accelerating voltage, 13-nA beam current, and ~1-µm beam diameter, using the oxide ZAF (atomic number, absorption, and fluorescence) matrix correction program. Averaged major element compositions for olivine, spinel, and amphibole in selected samples are listed in Tables T1, T2, and T3.
Olivine crystals that escaped serpentinization occur in olivine gabbro, olivine gabbronorite, troctolite, and impregnated dunite. Fo content of the olivine crystals varies from 89.2 to 91.2 in an impregnated dunite (Sample 209-1271B-10R-1, 43–46 cm) that is more Fe rich than harzburgites from Holes 1272A and 1274A (Fig. F4). Fo content of olivine in troctolite, olivine gabbro, and olivine gabbronorite is much lower and varies from 85.6 to 88.6.
Spinel occurs in most rocks. Cr# of spinel (= 100 x Cr/[Cr + Al] molar ratio) ranges from 38.9 to 62.7 in dunites, 46.9 to 59.1 in impregnated dunites, and 46.3 to 57.6 in troctolites, olivine gabbros, and olivine gabbronorite (Table T2). Spinel in a harzburgite (Sample 209-1271B-5R-1, 64–67 cm) has the lowest Cr# of 37.4 ± 0.5. Mg# of spinel (= 100 x Mg/[Mg + Fe2+] molar ratio) negatively correlates with Cr# (Fig. F5). Spinels in dunites lay at the extreme Cr rich end of the range for abyssal peridotites (Dick and Bullen, 1984). Spinels in troctolites, olivine gabbros, and olivine gabbronorite tend to have lower Mg#s than those in dunites, impregnated dunites, and harzburigite.
In Figures F6 and F7, spinel TiO2 content positively correlates with Cr# and Fe3+# (= Fe3+/[Cr + Al + Fe3+] molar ratio) from dunite through troctolite toward olivine gabbro/gabbronorite, whereas some high-Cr# spinels in dunites and troctolites from Unit III have extremely low TiO2 and Fe3+#. The former variation is very similar to spinels in peridotites and plutonic intrusions from Hess Deep Site 895 (Arai and Matsukage, 1996; Dick and Natland, 1996).
Figure F8 shows the relationship between Fo content of olivine and Cr# of spinel for Hole 1271B in comparison with harzburgites from Holes 1272A and 1274A and a general trend for mantle peridotite (i.e., the olivine-spinel mantle array proposed by Arai, 1994). Olivine and spinel from troctolite, olivine gabbro, and olivine gabbronorite are plotted out of the olivine-spinel mantle array, with lower Fo contents relative to the impregnated dunite in Unit II and harzburgites from Holes 1272A and 1274A. The compositional ranges for troctolite, olivine gabbro, and olivine gabbronorite coincide with those from Hess Deep Site 895 (Arai and Matsukage, 1996). A dunite (Sample 209-1271B-10R-1, 43–46 cm) from Hole 1271B has olivine with higher Fo content than others and plots inside the olivine-spinel mantle array, similar to harzburgites from other holes.
Amphiboles in Hole 1271B peridotite and plutonic rocks are mostly edenitic in composition, spanning between tremolite and pargasite end-members (Fig. F9). Amphiboles in dunite, troctolite, and olivine gabbro are characterized by lower TiO2 (0.04–0.7 wt%) and higher Cr2O3 (0.3–2.4 wt%) contents, in contrast to those in brown amphibole gabbros with higher TiO2 (1.4–2.7 wt%) and lower Cr2O3 (<0.05 wt%). Mg# of amphibole ranges 0.92 in dunite, 0.91–0.92 in troctolite, 0.88–0.90 in olivine gabbro, and 0.83–0.90 in brown amphibole gabbro.
Figure F10 shows the downhole compositional variation of olivine and spinel in Hole 1271B. Mineral compositions vary systematically as a function of depth and lithology. The Fo content of olivine decreases from Unit II toward the center of Unit III, whereas Cr# of spinel increases from Unit I to Unit III with a maximum in the center of Unit III. Cr# of spinel from the dunite in Unit III is greater than those in Units I and IV. TiO2 weight percent and Fe3+# of spinel vary widely with lithology and stratigraphic position. Spinels in harzburgite, dunites, and impregnated dunites in Units I and II have low TiO2 contents, <0.05 wt% (Fig. F11). In Unit III spinel, TiO2 contents vary from 0.02 to 0.40 wt% for dunite and impregnated dunite, 0.34 to 0.69 wt% for troctolites, and 0.68 to 2.29 wt% for olivine gabbros and olivine gabbronorite. TiO2 weight percent and Fe3+# of spinel significantly increase in the upper and lower parts of Unit III, where olivine gabbro, olivine gabbronorite, and troctolite are present. It should be noted that both downhole lithological and compositional variations are symmetric, with a center axis at the Unit III dunite horizon.