Hydrothermal spinel, corundum and diaspore in lower oceanic crustal troctolites from the Hess Deep Rift

Abstract

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 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 Al 2 O 3 /CaO and Al 2 O 3 /SiO 2 ratios, it is likely that the stabilization of spinel was caused by the loss of Ca 2+ and SiO 2 (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.

Formation of Igneous Layering in the Lower Oceanic Crust From the Samail Ophiolite, Sultanate of Oman

“…Very high Cl contents in hastingsites and pargasites within these veins (up to 5 wt%, Currin et al, 2018) provide evidence for the presence of highly saline, seawater-derived fluids at depth. Evidence for deep hydrothermal activity at high and very high temperatures in layered gabbros from fast-spreading systems has been reported in several studies (Garrido et al, 2001;Bosch et al, 2004;Koepke et al, 2014;Nozaka et al, 2016). Coogan et al (2006) outlined that hydrothermal activity is channeled to focused fluid flow zones, hence not affecting the entire crust homogeneously.…”

Mineralogical and geochemical study of rodingites and associated serpentinized peridotite, Eastern Desert of Egypt, Arabian-Nubian Shield

“…Petrographic studies reveal that corundum and diaspore formed by replacement of green spinel; in partly rodingitized samples, corundum rims green spinel that becomes smaller and more irregular as the amount of diaspore and corundum increases. The reaction pathway from gabbro to highly replaced rodingite-like rock appears to involve early stabilization of green spinel driven by decreasing activity of CaO and SiO 2 and increasing activity of Al 2 O 3 in high-temperature hydrothermal fluids (Nozaka et al, 2016). When the system reaches corundum saturation, Fe 2 O 3 activity also begins to decrease.…”

Convective Instability in Intraplate Oceanic Mantle Caused by Amphibolite-Derived Garnet-Pyroxenites—A Xenolith Perspective (Hyblean Plateau, Sicily)

“…A MORB-type igneous protolith, probably a troctolite gabbro, may give reason to both the high Al and Mg of this rock and the distribution of several trace elements. Scientific reports of marine geology enterprises often report on troctolite bodies in different oceanic settings [69], including core complexes [70].…”

“…Hydrothermal metasomatic reactions can strongly modify the composition of the oceanic crust, giving also rise to significant concentration of aluminous phases, such as Al-spinel, corundum and even diaspore [69]. Notwithstanding the very low solubility of Al in aqueous fluids, the aluminum transport is thought to be greatly enhanced by hot, saline fluids [83].…”