Arc Magma Genesis from Melting of Mélange Diapirs

Abstract:

Alkaline basalts occur in many subduction-related volcanic settings, including the Sunda, Izu-Bonin, Honshu, Aeolian, and Aleutian arcs, yet their origin continues to be debated. Recent studies have suggested that buoyant material (mélange) from the slab-wedge interface may rise into the hot corner of the mantle wedge as low-density plumes or diapirs, where it will melt or induce mantle melting due to dehydration. High-pressure mélange rocks represent a mixture of mafic, ultramafic, and sedimentary components, and are often dominated by chlorite. Mélange rocks are also enriched in accessory phases such as monazite, zircon, and rutile, which host a variety of trace elements. We present results from experimental melting of chlorite-rich mélange material at mantle wedge conditions that reproduce many of the compositional features of subduction-related lavas.

Piston cylinder experiments were performed at conditions appropriate for mantle wedge diapirs (1030–1280 °C, 1.5–2.5 GPa) using natural mélange rocks from Syros, Greece. Experimental melts derived from omphacite-epidote-phengite bearing chlorite schists range in composition from basaltic trachyandesites to trachydacites to more alkaline melt compositions (50.7–60.73 wt% SiO₂, 7.48–12.93 wt% Na₂O+K₂O). All of the experimental melts are characterized by high alumina contents (16.79–21.36 wt% Al₂O₃). Minerals coexisting with these melts include clinopyroxene, amphibole (at lower T) or olivine (at higher T), garnet (at higher P), ilmenite and/or rutile, and zircon.

Trace element patterns in our experimentally produced melts are similar to those from arc volcanoes worldwide. Experimental melts are enriched in large ion lithophile elements (Cs, Rb, Ba, K, Pb, Sr) and depleted in high field strength elements (Nb, Ta, and Ti). Zirconium and Hf are enriched relative to the rare earth elements (REE), which show flat to heavy REE depleted patterns depending on the presence of residual garnet. Thorium is fractionated from U, suggesting the presence of a light REE-rich accessory phase rich in Th. Similarity of the major and trace elements of experimental melts with those found in arcs suggests that many of the geochemical features unique to volcanic arcs can potentially be explained by melting of chlorite-rich mélange rocks in mantle wedge plumes or diapirs.