60-39 Nanoscale Observations of Cryptocrystalline Pseudotachylyte Matrix: Evidence for Ripplocations and Complex Fluid-Rock Interactions

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

Presenting Author:

Authors:

Abstract:

Numerous studies have characterized the mm- to micron-scale features of pseudotachylyte using optical petrography and SEM BSE imaging; however, nanoscale features of the cryptocrystalline matrix are less well known. Pseudotachylytes from the Ikertôq shear zone in western Greenland include 1–100-micron amygdules filled by a complex mixture of secondary hydrothermal minerals including quartz, orthoclase, albite, calcite, dolomite, ankerite, barite, pyrite, and Fe- and Ti-oxides. The matrix and amygdules are crosscut by microshears filled by a later generation of calcite and barite. Amygdules and quartz survivor clasts are typically rimmed by diffuse potassium-rich halos at a scale of tens of microns. To explore the nature of matrix chemical heterogeneity, we used TEM to investigate nanoscale mineralogy. Eight 70-nm-thick lamellae were milled from pseudotachylyte matrix using a Helios 5 UC FIB-SEM at Virginia Tech NanoEarth. The lamellae were analyzed using TEM for imaging and scanning TEM mode to collect EDS spectra and chemical maps. The results show the K-rich matrix halos are crystalline at the submicron scale and consist of distinct minerals with a hypidiomorphic texture including albite, quartz, and K-rich phases orthoclase and phengite, in addition to less abundant Fe-Ti-oxides, calcite, dolomite, apatite, amphibole, and pyrite. We interpret the following sequence: (1) Flash melting followed by quench cooling generating cryptocrystalline matrix, vesicles, microlite sprays up to 100 microns in length (plagioclase, magnetite, and amphibole), and unmelted to partially melted survivor clasts dominated by quartz; (2) phases of hydrothermal mineralization filling vesicles and nanoporosity; (3) late-stage microfractures and calcite-barite vein fill. The most interesting microstructural observation was the presence of ripplocations recorded by 80 nm, V-shaped arrays in euhedral phengite. Ripplocations, first discovered a decade ago, are defects generated by atomic-scale buckling in layered materials. They have rarely been reported from natural geologic materials, and this is the first report from a pseudotachylyte. They were likely generated by small volumetric strain during hydrothermal mineralization and may have provided new nanoscale porosity to further aid fluid transmission. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9506

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.