Oxidation state and metasomatism of the lithospheric mantle beneath the Rae Craton, Canada: strong gradients reflect craton formation and evolution

Alan B Woodland,Bruce A Kjarsgaard,D Graham Pearson,Heidi E Höfer,Carolin Gräf,Theresa Sandner,Hans-Michael Seitz

doi:10.1038/s41598-021-83261-6

Alan B Woodland, Bruce A Kjarsgaard + Show 5 more

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https://doi.org/10.1038/s41598-021-83261-6

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Abstract

We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~ 100 km depth could have locally destabilised any pre-existing diamond or graphite.

Highlights

We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield
One factor allowing the long-term (> 1 Ga) preservation of such crustal rocks is the existence of a thick lithospheric root that may extend to 250 km depth or more and which is chemically and physically distinct from the surrounding a sthenosphere[3,4,5,6]
Somerset Island peridotites were previously studied by Irvine et al.[18] and Bragagni et al.[19] primarily for whole-rock Re-Os and PGE geochemistry and comprise five spinel peridotites, ten garnet peridotites and ten garnet-spinel peridotites

Summary

Introduction

We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. Garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. To occur at deeper levels, suggestive of a chronologically and structurally layered lithosphere, and are thought to represent more juvenile mantle, possibly related to metasomatic and/or magmatic interactions from regionalscale underplating during the Kivalliq–Nueltin event at 1.77–1.7 G a9

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