Hydrothermal spinel, corundum and diaspore in lower oceanic crustal troctolites from the Hess Deep Rift
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Aluminous spinel, corundum and diaspore are reported from intensely altered parts of primitive troctolites recovered from IODP Site U1415 at the Hess Deep Rift. The spinel is green-colored, has an irregular shape, has low Cr concentrations, and is so distinct from primary igneous chromite. Corundum and diaspore occur mainly at the rims of green spinel grains with a texture suggesting a sequential replacement of spinel by corundum, and then corundum by diaspore. The green spinel is associated with anorthite and pargasite, which is overgrown by tremolite that forms coronitic aggregates with chlorite around olivine. These petrographic observations are supported by pressure–temperature pseudosections, which predict spinel + pargasite stability field, and tremolite/hornblende + chlorite field at lower temperature conditions. From these pseudosections and simplified system phase diagrams, estimated formation temperature conditions calculated at 2 kbar are 650–750 °C for spinel + pargasite, 410–690 °C for tremolite/hornblende + chlorite, 400–710 °C for corundum, and <400 °C for diaspore. Because the aluminous spinel occurs in the domains that were previously occupied by magmatic plagioclase, and because spinel-bearing rocks characteristically have high Al2O3/CaO and Al2O3/SiO2 ratios, it is likely that the stabilization of spinel was caused by the loss of Ca2+ and SiO2(aq) in high-temperature hydrothermal fluids. The results of this study suggest that (1) the concentrations of aluminous phases in the lower oceanic crust are presently underestimated, and (2) chemical modification of the lower oceanic crust due to high-temperature hydrothermal metasomatic reactions could be common near spreading axes.
KeywordsSpinel Corundum Diaspore Troctolite Oceanic crust Hydrothermal alteration
The samples used in this study were provided by IODP. Our special thanks go to the shipboard scientists, staff and crew of IODP Expedition 345 for scientific discussions, technical support and a successful voyage. Our thanks also go to Masaki Mifune for permission to use a Raman spectrometer in his laboratory at Okayama University, and Chen Zhu and his team at Indiana University for compiling and updating the SUPCRT data file and script formulation. The manuscript was significantly improved by the thoughtful comments from Riccardo Tribuzio, an anonymous reviewer and Editor Othmar Müntener. TN was supported by funds from Japan Drilling Earth Science Consortium, Japan Agency for Marine-Earth Science and Technology, and JSPS KAKENHI Grant Number 25400515. RM acknowledges the Norwegian Research Council, the CGB and Rolf B. Pedersen for support. RPW and BW were supported by post-cruise funds award # 062246-00003B.
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