Barren Island Volcano (NE Indian Ocean): Island-arc high-alumina basalts produced by troctolite contamination

Barren Island (BI) is a subduction-related volcanic island lying in the northeastern Indian Ocean, about 750 km north of the northern tip of Sumatra. Rising from a depth of ～2300 m on the Andaman Sea floor, BI has a submarine volume estimated at ～400 km³, but the island is just 3 km across, reaches a maximum elevation of 355 m, and has a subaerial volume of only ～1.3 km³. The first historical eruption began in 1787 when a cinder cone grew in the center of a pre-historical caldera 2-km in diameter and sent lava flows westward to reach the sea; activity continued intermittently until 1832. Two subsequent eruptions modified the central cone and also sent lava flows westward to reach the sea in 1991 and 1994–1995.A suite of 28 lava, scoria, and ash samples were investigated from various stages of the subaerial eruptive history of BI. Most are basalts (including all 10 samples from the 1994–1995 eruption) and basaltic andesites (including 7 of 8 samples from the 1991 eruption), but 2 pre-1787 andesites were also studied. On multi-element spider diagrams the BI suite shows subparallel trends for most elements that reflect an important role for fractional crystallization, along with the characteristic depletions of Nb–Ta and enrichments of K–Rb–Pb found in other subduction-related island-arc suites. The typical relative enrichment of Ba is not present, likely because the subducted sediments in the Andaman arc are not Ba-rich. Wide compositional ranges for Cs, Th, Rb, U, and Pb may trace different degrees of scavenging from the underlying volcanic pile.BI basalts and basaltic andesites have variable abundances of phenocrystic–microphenocrystic olivine plus Cr–Al–Mg spinel inclusions, plagioclase, and clinopyroxene, embedded in a matrix of glass, the same minerals, and titanomagnetite (mostly exsolved). The most remarkable mineralogical feature of certain BI basalts and basaltic andesites is the presence of abundant (to 40 vol.%) and large (to 5 mm) crystals of relatively homogeneous anorthitic plagioclase (to An_95.7). These have inclusions of Mg olivine (to Fo₇₉) and thin (10–150 μm) normally zoned margins that reach to the more sodic compositions of the plagioclase phenocryst and microphenocryst rims. Anorthitic plagioclase crystals are common at many subduction-related volcanoes. At BI, the anorthitic plagioclase and associated olivine crystals are thought to have entered the magmas through disaggregation of troctolitic crystal mushes or plutonic xenoliths. This process affected bulk-rock compositions in many ways, including raising Al₂O₃ contents to values as high as 22.8 wt.% and Eu / Eu* values up to 1.05. Compared to a large petrological and geochemical database for Indonesian volcanic rocks, the BI suite falls at the most depleted end for levels of K and incompatible trace elements, and Sr, Nd, and Pb isotopic ratios. Consequently, the BI suite defines an excellent primitive baseline against which Indonesian volcanic suites can be compared.