Assimilation of solids during ascent of magmas from the Bartoy Field of the Baikal Region, Siberia

Abstract

Most investigators ascribe mare basalt magma genesis to partial melting at depths of approximately 130 to greater than 400 km within the cumulate pile deposited from a lunar magma ocean. Mare basalts share with mid-ocean ridge basalts the characteristic of relative depletion in LREE and other incompatible trace elements that arises from melting within 'used' mantle, from which crust-forming elements have already been separated. Some mare basalt types do not show the classical, La-Nd depleted mare basalt REE distributions; however, some types are isotopically heterogeneous. These differences have been ascribed to assimilation, mainly AFC-style, of KREEPy highland material overlying the source region. Might such assimilation occur during magma ascent through the KREEPy material? To gain information from a terrestrial setting on possible assimilation during ascent, we have studied a suite of Quaternary nepheline-hawalites and nepheline-mugearites from the Bartoy cinder cone complex of the Baikal Rift, Siberia. The Bartoy magmas originated from greater than 80 km deep, and erupted through thick Archean crust. We find evidence for assimilation of approximately 31 wt. percent xenocrysts of garnet, aluminous clinopyroxene, kaersutite, and olivine, all presumably from the basalt source region, but no appreciable assimilation of overlying crust, consistent with isotopic constraints. Magmatic superheat made available by rapid ascent and decomposition accounts adequately for the energy of assimilation; no accompanying fractional crystallization is required or evident