Decoupling between Mg and Ca isotopes in alkali basalts: Implications for geochemical differentiation of subduction zone fluids

Abstract

Chemical metasomatism of the mantle wedge by slab-derived fluids is an important mechanism for the compositional features of mafic magmas above oceanic subduction zones. The possible physicochemical differentiation would occur during the transport of metasomatic agents from the slab to the wedge. This process is indicated by decoupling between Mg and Ca isotopes in alkali basalts from the South Qinling Orogen in central China. These basalts exhibit ocean island basalts (OIB)-like trace element distribution patterns, variable (87Sr/86Sr)i ratios from 0.7040 to 0.7059, positive εNd(t) values of 3.3 to 3.5 and εHf(t) values of 6.2 to 6.6. They have extremely low SiO2 contents, high CaO contents and CaO/Al2O3 ratios. They also show light Ca isotopes with δ44Ca values of 0.64 to 0.96‰, similar to the sedimentary carbonate. In addition, igneous carbonate minerals are abundant in the basalts. They exhibit shifts in δ13C (−5.9 to −1.8‰) and δ18O (16.2 to 18.0‰) towards the sedimentary carbonate, but fall within the ranges of global carbonatite. These geochemical features indicate the mantle source of basaltic magmas was generated through metasomatism of the mantle wedge by slab-derived carbonated melts. However, these basalts have unusual heavy Mg isotope compositions with δ26Mg values of −0.36 to 0.03‰ relative to the mantle values of −0.25 ± 0.07‰, in contrast to typical sedimentary carbonated melts with light Mg isotope compositions. This points to the decoupling between Mg and Ca isotopes, which may be caused by the incongruent MgCa isotope fractionation during the metasomatism. The slab-derived fluid may undergo significant differentiation during their metasomatism of the overlying mantle wedge, and this process may have completely removed the carbonate effect on Mg isotopes but retained Ca isotopes as the “fingerprint” of the recycled carbonate. Our model calculation results further reveal that when the carbonated melts would be differentiated at considerable degrees (F > 0.1), the decoupling of MgCa isotopes begins to appear. Therefore, the MgCa isotopes decoupling in the basalts provide new insights into the differentiation behaviors of subduction zone fluids. This differentiation process may influence the composition of metasomatic agents and thus the resultant mafic igneous rocks.