Data report: electron microprobe investigation of primary minerals in basalts from the West Philippine Sea Basin (Ocean Drilling Program LEG 195, Site 1201)

The basement cored at Site 1201 (West Philippine Basin) during Ocean Drilling Program Leg 195 consists of a 91 m-thick sequence of basalts, mostly pillow lavas, and perhaps one sheet lava flow, with a few intercalations of hyaloclastite and interpillow sedimentary material. Hydrothermal alteration pervasively affected the basalt sequence, giving rise to a variety of secondary minerals such as K-Fe-Mg-clay minerals, oxyhydroxides and clay minerals mixtures, natrolite group zeolites, analcite, alkali feldspar, and carbonate. The primary minerals of pillow and sheet basalts that survived the intense hydrothermal alteration were investigated by electron microprobe with the aim of characterizing their chemical composition and variability. The primary minerals are mostly plagioclase, ranging in composition from bytownite through labradorite to andesine, chromian-magnesian-diopside and spinels, both Ti-magnetite (partially maghemitized) and chromian-spinel. Overall, the chemical features of the primary minerals of Site 1201 basalts correspond to the primitive character of the bulk rocks, suggesting that the parent magma of these basalts was a mafic tholeiitic magma that most likely only suffered limited fractional crystallization, and crystallized at high temperatures (slightly below 1200°C) and under increasing ƒO2 conditions. The major element composition of clinopyroxene suggests a back-arc affinity of the mantle source of Site 1201 basement.