Geochemical characteristics of fast-spreading lower oceanic crust: an example of troctolites at the Hess Deep Rift (IODP Expedition 345)

Abstract:

Troctolites were recovered during IODP Expedition 345 (Dec 2012 - Feb 2013), which targeted plutonic rocks from fast-spread lower oceanic crust at the Hess Deep Rift. The troctolites are divided into three groups based on textural diversity; “skeletal olivine-bearing”, “fine-grained” and “coarse-grained” troctolites.

For major- and trace-element compositions of olivine and plagioclase, the skeletal olivine-bearing and fine-grained troctolites are more evolved (olivine Fo and NiO contents, 83 to 86 and 0.08 to 0.2 wt.%, respectively, and plagioclase An content, 77 to 84) than coarse-grained ones (olivine Fo and NiO contents, 87to 89 and 0.2 to 0.3 wt.%, respectively, and plagioclase An content, 85 to 90). Clinopyroxenes show scattered chemical compositions in the skeletal olivine-bearing and fine-grained troctolites, down to the scale of a single thin section. Clinopyroxenes, some of which show no Eu anomaly, are clearly zoned in the coarse-grained troctolites: the rims are chemically more evolved than the cores.

The skeletal olivine-bearing and fine-grained troctolites record signs of melt invasion, which caused the chemical heterogeneity of the clinopyroxenes. In contrast, the coarse-grained troctolites show no such sign, and contain the zoned clinopyroxenes, which were co-precipitated with plagioclases. MORB (mid-ocean ridge basalt) melts experience multi-stage evolution, including fractional crystallization and melt/troctolite interaction during migration. Such various melt migration processes in the lower oceanic crust possibly cause regional diversity of MORB chemistry.