Oxygen and hydrogen isotope heterogeneity of clinopyroxene megacrysts from Nushan Volcano, SE China

Abstract

Clinopyroxene megacrysts hosted by alkali basalts are a common form of mantle material found in Nushan volcano, Anhui Province, SE China. Compositionally, the clinopyroxene megacrysts are Al-rich augites with no intra-mineral chemical zoning. Oxygen and hydrogen isotopic ratios are constant within single clinopyroxene megacrysts, but display large variation among different samples ( δ 18O=4.9∼6.5‰; δ D=−45∼−111‰). Regular covariations among major elements and REEs and similar 87Sr/ 86Sr (0.70309 to 0.70346) and 143Nd/ 144Nd (0.512846 to 0.513010) ratios indicate that these clinopyroxene megacrysts were derived from the same source and crystallized in a parent magma similar in composition to their host basalts. Oxygen isotope data constrain the mantle source to be slightly 18O-depleted, likely resulting from interaction with low δ 18O oceanic crust. Oxygen and hydrogen isotope data define two different compositional groups for the Nushan clinopyroxene megacrysts, suggesting that the parent magma experienced different petrogenetic processes in the course of its evolution to two groups (group I and group II) and crystallized in two different magma chambers possibly located at different depths. The slight increase of δ 18O and H 2O, and constant δ D with Mg# values of group I megacrysts imply a simple fractional crystallization process; the relative large increase of δ 18O and H 2O and decrease of δ D with Mg# values of group II megacrysts suggest possible lower continental crust contamination and magma degassing during their crystallization. High water contents of some group II clinopyroxenes decoupled from the others possibly resulted from interaction with host basaltic magmas during their ascent to the surface. Therefore, major element, REE content and O–H–Sr–Nd isotopic compositions suggest the following petrogenetic stages: (1) mantle source contamination due to interaction with low δ 18O material likely being subducted oceanic crust; (2) crystallization of group I clinopyroxene by simple crystal fractionation; (3) crystallization of group II megacrysts during magmatic contamination by lower continental crustal material with high δ 18O and water degassing from the evolving magma; (4) increase of water content and hydrogen isotopic composition re-equilibration of some group II megacrysts likely due to the interaction with host basaltic magmas during their ascent to the surface.