Melt Percolation at the Base of the Pacific Lithosphere: Insights from Petit-spot Mantle Xenoliths
Abstract
Japanese petit-spot volcanoes have been interpreted as products of low-degree mantle melts extracted along fractures extending from the seismic low velocity zone (LVZ) associated with plate flexure1. The presence of volatile-rich silicate melts is infered to explain the seismic anomalies observed at ~80 km depth1,2 potentially corresponding either to: 1) the conditions being those required to stabilize low-degree volatile-rich melts or 2) to melts rising from the upper mantle, which are then trapped at the permeability boundary at the top of the asthenosphere. Recent studies call into question the presence of brittle fractures extending as far as the base of the lithosphere. For example, multiple saturation experiments3 suggest that petit-spot melts were last equilibrated at lithospheric conditions of ~2 GPa and ~1,280°C, suggesting that melts from the LVZ have percolated through the base of the lithosphere before being extracted to the surface. Such a process is supported by the occurrence of two mantle xenoliths enclosed in site A petit-spot lavas showing evidence for melt percolation and metasomatic enrichment4. However, it remains unclear if this process is ubiquitus in subduction zones. Here, we perform a systematic study of mantle xenoliths including samples collected at site B about 600 km southeast from the first locality. Several xenoliths have distinctive cpx and opx trace element patterns showing enrichment in incompatible elements (e.g. Nb, Ta, LREE) but depletion in HREEs, in comparison to cpx from abyssal peridotite. These results suggest equilibration with garnet (60-70 km), and indicate widespread metasomatic enrichment of the base of the Pacific lithosphere. These xenoliths demonstrate that melt percolation within the base of the lithosphere is possible within the context of plate flexure. Therefore, the lithosphere is not an impermeable barrier for melt percolation, as commonly assumed. The next part of the project will investigate the volatile content of these xenoliths, to test if metasomatic processes could modify the volatile budget at the base of the lithosphere.
Hirano et al. (2006) Science 313, 1426-1428 Kawakatsu et al. (2009) Science 324, 499-502 Machida et al. (2017) communications, DOI: 10.1038/ncomms14302 Pilet et al. (2016) Geoscience 9, 898-904- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMDI0150013G
- Keywords:
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- 3914 Electrical properties;
- MINERAL PHYSICS;
- 3621 Mantle processes;
- MINERALOGY AND PETROLOGY;
- 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS;
- 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS