Fluxing of mantle carbon as a physical agent for metallogenic fertilization of the crust

Daryl E Blanks,Santiago Tassara,Marco L Fiorentini,Andrea Giuliani,Marilena Moroni,Adrian J Boyce,José M González-Jiménez,David A Holwell,Elena Ferrari

doi:10.1038/s41467-020-18157-6

Abstract

Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth’s mantle, including economically important metals like nickel, copper and platinum-group elements. However, transport of these metals within silicate magmas primarily occurs within dense sulfide liquids, which tend to coalesce, settle and not be efficiently transported in ascending magmas. Here we show textural observations, backed up with carbon and oxygen isotope data, which indicate an intimate association between mantle-derived carbonates and sulfides in some mafic-ultramafic magmatic systems emplaced at the base of the continental crust. We propose that carbon, as a buoyant supercritical CO2 fluid, might be a covert agent aiding and promoting the physical transport of sulfides across the mantle-crust transition. This may be a common but cryptic mechanism that facilitates cycling of volatiles and metals from the mantle to the lower-to-mid continental crust, which leaves little footprint behind by the time magmas reach the Earth’s surface.

Highlights

Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth’s mantle, including economically important metals like nickel, copper and platinum-group elements
The low density of the bubble is sufficiently high to overcome the relative density contrast between sulfide/magnetite and the host silicate magma, allowing for effective and potentially rapid upward transport of the metal-bearing phases. This process is analogous to industrial froth floatation, which is used to concentrate dense minerals in ore processing at ambient or relatively low confining pressures. Such a physical mechanism has not been demonstrated at pressures equivalent to the base of the continental crust, arguably one of the most important parts of the system where sulfide blebs enriched in mantle-derived chalcophile and siderophile metals have to cross a major physical barrier, the Moho discontinuity
The magmatic carbonate-sulfide occurrences discussed here are consistently hosted by alkaline volatile-rich ultramafic–mafic rocks, which commonly display accessory P and Te minerals

Summary

Introduction

Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth’s mantle, including economically important metals like nickel, copper and platinum-group elements. Hydrous volatile phases have been demonstrated experimentally to provide a viable mechanism of upward physical transport for relatively dense sulfide liquid droplets[12,13] and magnetite crystals[14] This process is reflected in the recent identification of sulfides in magmatic systems associated with coarse-grained hydrous silicate caps[15,16,17]. The low density of the bubble is sufficiently high to overcome the relative density contrast between sulfide/magnetite and the host silicate magma, allowing for effective and potentially rapid upward transport of the metal-bearing phases This process is analogous to industrial froth floatation, which is used to concentrate dense minerals in ore processing at ambient or relatively low confining pressures. The process of compound droplet floatation is independent of pressure and a plausible, yet far unrecognized, mechanism for transporting sulfide in the lower crust should sulfide and volatile supersaturation occur[13]

Methods

Results

Conclusion