39 Structural Geology (Posters)

Added due to amount of discipline abstracts submitted.

Wednesday, 11 March 2026
9:00 AM - 12:00 PM
RCC, Lower Level Hall

Effect of Composition on the Melt Topology of Two Common Mafic Rocks During Deformation

Paper number: 39-1
Program: Technical Sessions
Time: 9:00 AM – 9:00 AM

Authors

Abstract

Diabase and amphibolite are common rocks in subducting slabs which could be sources of magma during subduction. These rocks have similar bulk compositions, but only the amphibolite contains a hydrous phase which may affect the mechanisms that allow melt to migrate from the source rock. In order to determine how the difference in composition between diabase and amphibolite affect melt topology, melt segregation, and the strength of the rock, we deformed cylinders of diabase and amphibolite at a constant strain rate (10-5/s) at a temperature of 950oC and pressure of 1.5 GPa. Melt content was varied by letting the cylinders sit hydrostatically at these conditions for four different times (0,16,40 or 80 hours) prior to deformation, which varied the starting melt contents in the diabase (3-7 vol%) and amphibolite (2-6 vol%). Diabase strengths increased from 480 MPa to 850 MPa as a function of increasing melt content; whereas, amphibolite strengths decreased from 255 MPa to 145 MPa as a function of increasing melt content. Melt in diabase is located in vertical melt pockets in pyroxene grains and along pyroxene-feldspar grain boundaries. Melt in amphibolite is located in melt pools along pyroxene-feldspar grain boundaries. The difference in strengths is likely due to the different melting reactions: the diabase reaction produces melt which strips water from the phases in the rock and does not modify the grain size, whereas, the amphibolite melting reaction releases significant water and produces fine-grained reaction products. These results indicate that some melting reactions could cause strengthening of the lower crust.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1

doi: 10.1130/abs/2026SE-411756

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