COMPOSITIONAL VARIATIONS IN FRICTIONAL MELTS: A HRTEM STUDY OF GLASS-BEARING PSEUDOTACHYLYTES (ALPINE FAULT, NEW ZEALAND)

Melt generated during flash-friction in the Earth’s crust plays a critical role in the seismic and mechanical behavior of faults, where molten material can accumulate along a displacement surface and lubricate faulting. In this HRTEM study, we examine two natural pseudotachylyte-bearing schist samples from the Alpine Fault of New Zealand in order establish the compositional variations that occurred during melting and freezing and to discuss the properties of the molton layer. Sample 1, consisting of amphibolite schist comprised primarily of quartz-feldspar-biotite-chlorite-epidote-opaques, contains multiple crosscutting generations of pseudotachylyte veins. Sample 2, a quartz-feldspar-biotite-muscovite pelitic schist, contains a layered vein built up by multiple stages of melt generation. Detailed compositional data of whole rock, matrix, mineral and glass was obtained by combining ICP-OES, microprobe and analytical electron microscopy analyses. The schist samples contain similar concentrations of Si, Al, Na, Ti and Cr, but differ in the mafic schist containing more Fe, Mg and Ca, and notably less K than the pelitic lithology. During frictional melting, there is a tendency toward decreasing concentrations of Si, and increasing Al, Ca, Ti and K, reflecting the preferential breakdown of micas and feldspars. There is a notable discrepancy between the composition of pseudotachylyte matrix (bulk melt) and the composition of preserved glass; the latter showing significant enrichment in Si and corresponding depletion in other cations, particularly Fe and K. We suggest that the melt composition varied during generation and transport, and that the quenched glass preserves the late, more mature stages of the molton layer, which progressively increases its viscosity as the melt began to crystallize and lose its volatile content.