Rb/Sr Geochemistry and Geochronology on Lawsonite from the Schiste Lustrés, France
Abstract
Lawsonite is a water-rich mineral that is indicative of high pressure/low temperature conditions. This project seeks to constrain the age of lawsonite growth and determine its fluid source and reaction history. Lawsonite has previously only been dated via Lu-Hf [1]. This is part of a broader study exploiting the abundance of lawsonite in these subduction complex metasediments (Schistes Lustrés, Western Alps); having equilibrated at ~350°C and 1.2-1.3 GPa (~40 km depth; [2]), they represent ideal targets for investigating fluid-rock interactions along the downdip end of the seismogenic interface [3,4].
Samples collected just a few meters apart were selected for preliminary analysis. One sample - typical of the rock matrix in this outcrop - contained fresh grains of black lawsonite (with graphite), calcite, ankerite, micas and albite, as well as accessory zircon and pyrite. The second sample - a creamy white colored vein cross-cutting the rock fabric - contained large cm-scale lawsonite crystals with quartz, calcite and traces of white mica. After crushing, lawsonite fractions were separated using Frantz, heavy liquids, hand-picking, and acid leaching; still lawsonite purity varied. Certain mineral inclusions in both samples did not lie on the lawsonite isochron; primary sedimentary calcite and low-grade phyllosilicates are suspected to be the cause. These non-lawsonite separates were excluded from isochrons. An isochron from the black matrix lawsonite yields an initial 87Sr/86Sr of 0.7124, while the white vein lawsonite yields an initial 87Sr/86Sr of 0.7145. This indicates that the vein lawsonite did not inherit its strontium directly from the surrounding local matrix, but more likely from an external radiogenic fluid source. A preliminary age for the white vein lawsonite is ca. 56 to 61 Ma, which agrees well with published 40Ar/39Ar ages of 55 to 62 Ma [2]. A preliminary isochron for the black matrix lawsonite yields a surprising apparent age of 188.4 Ma ± 2.7 (n=3, MSWD=0.51). This could reflect a protlith age (somehow remarkably surviving the Alpine orogeny), or mixing between non-equilibrated mineral reservoirs. [1] Mulcahy et al. 2009, Geology, v. 37, p. 978-990; [2] Agard et al. 2002, JMG, v. 20, p. 599-618 ; [3] Audet and Burgmann, 2014, Nature, 510, 389-392; [4] Agard et al. 2018, Lithos, 320-321, 537-561.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43E0134R
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY;
- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY