MINERAL COMPOSITIONS

Mineral compositions were analyzed on a JEOL JXA-8900R Superprobe equipped with five wavelength-dispersive spectrometers (WDS) at the Ocean Research Institute of the University of Tokyo, Japan. Silicate and oxide minerals were analyzed using a focused beam, an accelerating voltage of 15 kV, and a beam current of 12 nA. To measure NiO contents in olivine, Ni was analyzed with a counting time of 60 s at peak position with 30 s of background measurement. The standard deviation of X-ray counting for NiO analysis in olivine was <10%. Other elements were analyzed with a counting time of 20 s at peak position and 10 s of background measurement. Fe²⁺ and Fe³⁺ contents in spinel were calculated using the method of Droop (1987).

Chemical compositions of olivine grains are listed in Table T1 and shown on an NiO vs. Mg# (Mg/[Mg+Fe]x100) diagram (Fig. F2). In general, the centers of phenocrysts (PC) (Table T1) have Mg-rich compositions (Mg# > 80) that are higher than phenocryst peripherals, microphenocrysts, and groundmass olivine grains. However, even groundmass olivine grains (GM) (Table T1) have relatively Mg-rich compositions, generally higher than Mg# ~80 (olivine grains poorest in Mg are found in the groundmass of Sample 187-1154A-2R-1, 46-49 cm; Mg# = 79.81 ± 1.15 [1- statistical variation]). Compositional variations of olivine in each sample (at least in each thin section) are very low, and differences of Mg# between PC and GM are <4-5. Olivine grains in Indian-type basalts are generally richer in magnesium than those in Pacific-type basalts.

NiO contents in olivine are also relatively constant, corresponding to restricted variations in Mg#. Central parts of olivine phenocrysts contain 0.2 to 0.3 wt% NiO, and olivine grains in the groundmass contain 0.1 to 0.2 wt% NiO (Fig. F2).

Chemical compositions of plagioclase grains are listed in a supplementary data file (see the "Supplementary Material" contents list). Because compositional zoning occurs to some extent in plagioclase, compositional variations within plagioclase grains are significantly higher than in olivine grains. In general, the central parts of plagioclase phenocrysts have higher Ca compositions than phenocryst peripherals, microphenocrysts, or groundmass plagioclase. Oscillatory zoning is developed in some large phenocrysts (Fig. F3A), and a few analyzed grains have reversed zoning.

Results of linear compositional profiles for larger plagioclase phenocrysts are shown in Figure F3. In most phenocrysts (with the exception of a few samples with oscillatory zoning patterns), the central parts of phenocrysts have the highest An contents (Fig. F3B) and compositions change abruptly near the peripheral regions of phenocrysts (within ~0.1 mm of the rim). As shown in Figure F3B, phenocryst centers have compositions An₈₀ and the peripheral zones of phenocrysts have An_60-70. A few grains, particularly in Sample 187-1155A-2R-1, 14-16 cm, have striking reversed zonation compositional patterns. It is noteworthy that plagioclases with both normal (Fig. F3C) and reversed (Fig. F3D) zoning coexist. In Sample 187-1155A-2R-1, 14-16 cm, plagioclase phenocrysts with reversely zoned compositional patterns (Fig. F3D) are larger (1-1.5 mm in size) than those with normal zoning (Fig. F3C) (~0.5 mm in size). Even in reversely zoned plagioclase phenocrysts, peripheral zones (within ~0.1 mm of rim) show normal zoning.