The origin of low-Ca olivine from ultramafic xenoliths and host basaltic lavas in a back-arc setting, James Ross Island, Antarctic Peninsula

The James Ross Island Volcanic Group (JRIVG) at the northern tip of the Antarctic Peninsula is composed of various types of volcanic rocks erupted during the Late Miocene to Late Pleistocene. The sub-aerial lavas are the most abundant erupted products within the suite and are represented by alkali olivine basalts that contain significant amounts of ultramafic xenoliths. Precise determination of chemical compositions of olivine by electron microprobe and LA-ICP-MS reveals more than one compositionally distinct olivine population in the xenoliths and the host lavas. The majority of the olivine grains from the xenolith suite are remarkable for their high Mg/Fe ratios (>Fo₈₈) and low-Ca contents (<500 ppm) and are similar in composition to the mantle olivine, while some others, despite their similarly low-Ca abundances, are characterized by significantly lower Mg/Fe (<Fo₈₅) reflecting some degree of differentiation. We interpret the co-existence of both olivine types with low-Ca contents as reflecting a magmatic cumulate origin of the xenoliths through olivine-dominated fractional crystallization from a H₂O-rich parent magma, during which the presence of water affects the partitioning behavior of elements and reduces the D_CaO ^olivine/melt significantly. The cores of the most primitive olivine macrocrysts (>Fo₈₈) from the basaltic lavas also have low Ca contents compared to MORB olivine at similar Fo, indicating an arc-melt like volatile content of the primary magma. Evaluation of minor and trace element relative abundances in olivine further indicate that the alkaline basalts in the JRIVG are the products of peridotite-dominated partial melting of a volatile-rich mantle source with signatures of mantle hydration most probably promoted by preceding subduction events. The results from olivine chemistry, when combined with the evaluation of primary melt compositions, appear to be consistent with the view that the primary magmas from which the JRIVG basalts were derived are the results of partial melting of a mantle domain that has experienced hydrous silicate melt metasomatism through interaction of peridotitic upper mantle rocks with melts of a slab-derived component, most probably generated by dehydration melting of subducted oceanic crust.