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

摘要

Although there is a growing body of data on H₂O in arc magmas, there is still considerable uncertainty about the relationship between H₂O and various incompatible elements during enrichment of the mantle wedge by subduction processes. We report data for H₂O, other volatiles (CO₂, 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_85–90) trapped at upper to middle crustal pressures (1–6 kbar) contain high CO₂ (250–2120 ppm). The high-pressure inclusions indicate magmatic H₂O contents from 1.3 to 5.2 wt%. Enrichment of H₂O relative to Nb correlates positively with K/Nb, Ba/Nb, and La/Nb, indicating a clear link between H₂O and trace element enrichment of the mantle wedge. Our results show that fluxing of the wedge with an H₂O-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 H₂O, 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 H₂O-rich subduction components that are added to the mantle wedge beneath central Mexico.