Oceanic and super-deep continental diamonds share a transition zone origin and mantle plume transportation

Luc S Doucet,Hamed Gamal El Dien,Zheng-Xiang Li

doi:10.1038/s41598-021-96286-8

Abstract

Rare oceanic diamonds are believed to have a mantle transition zone origin like super-deep continental diamonds. However, oceanic diamonds have a homogeneous and organic-like light carbon isotope signature (δ13C − 28 to − 20‰) instead of the extremely variable organic to lithospheric mantle signature of super-deep continental diamonds (δ13C − 25‰ to + 3.5‰). Here, we show that with rare exceptions, oceanic diamonds and the isotopically lighter cores of super-deep continental diamonds share a common organic δ13C composition reflecting carbon brought down to the transition zone by subduction, whereas the rims of such super-deep continental diamonds have the same δ13C as peridotitic diamonds from the lithospheric mantle. Like lithospheric continental diamonds, almost all the known occurrences of oceanic diamonds are linked to plume-induced large igneous provinces or ocean islands, suggesting a common connection to mantle plumes. We argue that mantle plumes bring the transition zone diamonds to shallower levels, where only those emplaced at the base of the continental lithosphere might grow rims with lithospheric mantle carbon isotope signatures.

Highlights

Rare oceanic diamonds are believed to have a mantle transition zone origin like super-deep continental diamonds
The positions of these deposits relative to plume magmatism and the large low sheared wave velocity province (LLSVP)[2] suggest that they are related to mantle plume e vents[2,3,4,5]
We show that most oceanic diamonds found in ophiolite belts are not associated with classic ophiolitic sequences that represent the normal oceanic lithosphere formed at mid-ocean ridges; instead, they are associated with plume-modified oceanic large igneous provinces (O-LIPs) or ocean island basalts (OIBs) lithospheric fragments preserved in ophiolite belts

Summary

Introduction

Rare oceanic diamonds are believed to have a mantle transition zone origin like super-deep continental diamonds. A small fraction (1%) of the continental diamonds have mineral inclusions that suggest a deeper origin of between ∼ 300 and 1000 km depths[6], and are known as super-deep or sub-lithospheric continental diamonds[7]. These oceanic diamonds share similar characteristics to super-deep continental diamonds in that they, in general, are microdiamonds (< 1 mm in size[35], except for the exceptionally large CLIPPIR diamonds36) and are associated with a range of ultra-high-pressure and highly reduced mineral phases (e.g., coesite, kyanite, UHP nitride, SiC, Ni–Mn alloys, Fe–Si and Fe–C)[12,15,16]. It is widely accepted that such diamonds (and related ultra-high pressure and reduced mineral phases) formed from an oxidized, CO3-rich melt[37,38,39,40,41] or reduced fluids produced when subducted slabs melted after entering the transition zone, where the pressure is 15–16 GPa, temperature ~ 1600 °C, and volatile contents 1–1.5 wt%36,42,43

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