Deformation and Melt-Rock Interaction in Peridotites From the Atlantis II Transform, SWIR: Evidence for Diffuse Melt Percolation in Deep Lithospheric Mantle
Abstract
Abyssal peridotites that record both melt-rock interaction and crystal-plastic deformation provide the most direct means of assessing the interaction of tectonic processes and melt generation and transport processes in the mantle beneath mid-ocean ridges. We present a preliminary study of microstructural observations, crystallographic fabric analysis, modal compositions, and mineral chemistry for 10 spinel lherzolites from the Atlantis II Transform Fault on the ultraslow-spreading (~1.4 cm/year) Southwest Indian Ridge (SWIR). The samples were collected via submersible along a 1km traverse up the side of the valley. All preserve protogranular to weakly porphyroclastic textures characteristic of high-temperature (~1100-1250°C) deformation. Deformation microstructures include: a) patchy zones of recrystallized olivine, ortho-and clino- pyroxene, and b) a weakly developed pyroxene shape-preferred orientation. Electron backscatter diffraction (EBSD) analysis indicates that olivine, ortho- and clino-pyroxene exhibit a weak but consistent lattice-preferred orientation (LPO). The average olivine and orthopyroxene M-indices are ~0.1, comparable to peridotites from the upwelling zones of ophiolites. Clinopyroxene M-indices are very weak (~0.05). The samples also preserve microstructural evidence for melt-rock interaction, including: a) olivine "embayments" in orthopyroxene resulting in extremely complex grain shapes; b) thin (<0.1mm) interstitial clinopyroxene and spinel, oriented roughly parallel to the foliation and LPO slip direction; c) vermicular spinel replacing the edges of orthopyroxene porphyroclasts; and d) large (3-5mm) complex spinels interstitial to aggregates of ortho- and clino-pyroxene. These delicate microstructures are not deformed; thus at least some melt percolation and crystallization must postdate the high temperature deformation. The degree to which melt-rock interaction has overprinted the deformation varies from sample to sample, showing that the last stage of melt percolation was heterogeneously distributed on the scale of <1km. Two samples show plagioclase-rich melt impregnations and one contains a 5mm wide micrograbbro vein. Our study reveals the upwelling and emplacement history of these peridotites: 1) partial melting, along with deformation and melt-rock reaction from ~60-30km in the upwelling mantle, 2) accretion to the lithosphere at a depth of >25km beneath the SWIR while still upwelling (before "corner flow"); this depth is consistent with the expected mantle temperatures adjacent to a large-offset transform fault, 3) cessation of penetrative deformation, and onset of localized deformation in shear zones not sampled by this study, 4) heterogeneously distributed melt percolation while still in the spinel stability field at depths >25km, 5) minor late-stage intrusion of gabbro veins and interstitial melt in the plagioclase stability field (<25km depth), and 6) emplacement of peridotites to the wall of the Atlantis II Transform Fault. These samples record the cessation of both melting and penetrative high temperature deformation at a depth >25 km. Therefore, we suggest that the majority of melt delivery and heat supply may have been focused toward the ridge segment center and away from the segment end near the Atlantis II transform. The last recorded stage of significant melt percolation in these samples was diffuse and spatially heterogeneous.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.T52B..04A
- Keywords:
-
- 3615 Intra-plate processes (1033;
- 8415);
- 3621 Mantle processes (1038);
- 3625 Petrography;
- microstructures;
- and textures;
- 8120 Dynamics of lithosphere and mantle: general (1213);
- 8159 Rheology: crust and lithosphere (8031)