Lithospheric Xenoliths Record Thermal and Magmatic Signature of Rift Development Beneath Ross Island, Antarctica
Abstract
The West Antarctic Rift System produced extensive volcanism along the Transantarctic mountain range that delivered crustal and mantle xenoliths to the surface. Previous studies of Antarctic xenoliths largely focused on evaluating major and trace element variations to determine the compositions of mantle reservoirs beneath the rift system, which could have been influenced by a mantle plume. To unravel the thermal, chemical and dynamic history of the subantarctic lithosphere, we conducted a geochemical investigation of olivine and pyroxene in mantle wehrlites and plagioclase in a crustal anorthosite retrieved from Ross Island, Antarctica. Monomineralic crustal anorthosite xenoliths (An# 69) are present in alkali basalts that also host wehrlitic peridotite xenoliths. Olivines in the wehrlites are protogranular and range from ~150-1000μm in diameter, with Mg#s of 88. All olivine grains exhibit reverse Ca, Cr and Al zoning (e.g., see Figure). Application of Ca-in-olivine thermometry to mineral cores recovers temperatures (Ts) of 726-860°C, while rim Ts are 972-1045°C; taken at face value the T distribution suggests a late-stage heating event. Minor amounts of unzoned Cr-rich spinel (Cr# 62) are interstitial in the wehrlites and record Ts of 966-1017°C according to an olivine-spinel thermometer. Two types of clinopyroxenes exist in the wehrlite: those with increasing concentrations of Cr, Al, and Ti moving from the core to the rim, and those with decreasing concentrations of Cr, Al, and Ti moving from the core to the rim. The discrepant zoning profiles may reflect subsolidus reequilibration after heating superimposed on inherited magmatic signatures, which is suggested by the dependence of the sense of zoning to the proximity of clinopyroxene to Cr-rich spinel. Future efforts will focus on investigation of crustal xenoliths with more diverse mineralogies and applying inverse methods to mantle and crustal samples to reconstruct their T-time histories. To better understand the development of the West Antarctic Rift, we will evaluate our findings in the context of geodynamic models simulating lithospheric thinning.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51F0116B
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGY;
- 3651 Thermobarometry;
- MINERALOGY AND PETROLOGY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY