Recycling oceanic crust: Isotopic and elemental constraints from a high pressure terrain

Dale, Christopher (2005). Recycling oceanic crust: Isotopic and elemental constraints from a high pressure terrain. PhD thesis The Open University.



This study investigates the Re-Os and trace element budgets of gabbros, eclogites and metabasalts from subducted oceanic crust, at whole-rock and mineral scales. These data test the hypothesis that elevated 187OS/188OS ratios in some OIB evolved from the high Re/Os ratios of subducted oceanic crust recycled into their mantle source.

Re-Os data from metastable gabbros and corresponding eclogites from the Zermatt-Saas ophiolite (ZSO), which have undergone very high-pressure metamorphism (>2.0 GPa), yield a best-fit regression consistent with the crust formation age (164 Ma). ZSO eclogitic metabasalt data lie above this regression line, with a mean Re abundance of 440 ppt, substantially lower than current data for MORB glasses (~1000 ppt). This suggests that Re was lost from the basaltic crust (resulting in lower Re/Os ratios as found in a previous study), while low-strain gabbro bodies remain closed with respect to the Re-Os system during subduction. Re-Os data from pillow basalts of biotite, garnet and kyanite grades from the Sulitjelma ophiolite also indicate substantial Re loss, implying that this process may occur at lower P-T conditions during initial subduction.

Modelling the Os and Pb isotope evolution of a complete section of oceanic crust over 2 Ga implies that HIMU-type OIB can be produced by recycling >40% oceanic crust into the mantle source. Crucially, the effect of the gabbroic portion on the evolution of the whole crust is to considerably reduce 206Pb/204Pb ratios without altering the Os isotope evolution significantly. This permits long residence times for recycled crust in the mantle without generating Pb isotopic values higher than known OIB.

For the first time, mineral separates from gabbro and gabbroic eclogite have been analysed for Re and Os. Sulphide dominates the Os budget, while Re is mainly hosted in plagioclase for gabbro, and garnet for gabbroic eclogite.

Trace element measurements on the same mineral and bulk-rock samples are used to assess trace element fluxes during subduction. The most notable conclusion of this work is that U depletion occurs neither in the metabasalts nor metagabbros.

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