The evolution and ascent paths of mantle xenolith-bearing magma: Observations and insights from Cenozoic basalts in Southeast China

Abstract

Studies have shown that mantle xenolith-bearing magmas must ascend rapidly to carry mantle xenoliths to the surface. It has thus been inferred inadvertently that such rapid ascending melt must have undergone little crystallization or evolution. However, this inference is apparently inconsistent with the widespread observation that xenolith-bearing alkali basalts are variably evolved with Mg# ≤72. In this paper, we discuss this important, yet overlooked, petrological problem and offer new perspectives with evidence.We analyzed the Cenozoic mantle xenolith-bearing alkali basalts from several locations in Southeast China that have experienced varying degrees of fractional crystallization (Mg# = ~48–67). The variably evolved composition of host alkali basalts is not in contradiction with rapid ascent, but rather reflects inevitability of crystallization during ascent. Thermometry calculations for clinopyroxene (Cpx) megacrysts give equilibrium temperatures of 1238–1390 °C, which is consistent with the effect of conductive cooling and melt crystallization during ascent because TMelt > TLithosphere. The equilibrium pressure (18–27 kbar) of these Cpx megacrysts suggests that the crystallization takes place under lithospheric mantle conditions. The host melt must have experienced limited low-pressure residence in the shallower levels of lithospheric mantle and crust. This is in fact consistent with the rapid ascent of the host melt to bring mantle xenoliths to the surface.