Role of H2O in subduction-zone magmatism: New insights from melt inclusions in high-Mg basalts from central Mexico

TLDR: This paper showed that fluxing of the wedge with an H 2 O-rich component from the subducted slab is important in formation of large ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength elements (HFSE).

Abstract: Although there is a growing body of data on H 2 O in arc magmas, there is still considerable uncertainty about the relationship between H 2 O and various incompatible elements during enrichment of the mantle wedge by subduction processes. We report data for H 2 O, other volatiles (CO 2 , S, Cl), and trace elements in olivine-hosted melt inclusions from high-Mg basalts in central Mexico that exhibit varying degrees of subduction-related enrichment. Most melt inclusions were trapped at low pressure, but rare inclusions (Mg# 65-78, olivine hosts Fo 8 5 - 9 0 ) trapped at upper to middle crustal pressures (1-6 kbar) contain high CO 2 (250-2120 ppm). The high-pressure inclusions indicate magmatic H 2 O contents from 1.3 to 5.2 wt%. Enrichment of H 2 O relative to Nb correlates positively with K/Nb, Ba/Nb, and La/Nb, indicating a clear link between H 2 O) and trace element enrichment of the mantle wedge. Our results show that fluxing of the wedge with an H 2 O-rich component from the subducted slab is important in formation of magmas that are enriched in large ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength elements (HFSE). In contrast, magmas with low LILEs and LREEs relative to HFSEs have relatively low H 2 O, and must have formed largely by decompression melting of unmodified mantle. Our data for volcanoes <50 km apart show evidence of significant variability in the composition of H 2 O-rich subduction components that are added to the mantle wedge beneath central Mexico.