Incipient carbonate melting drives metal and sulfur mobilization in the mantle

Abstract

We present results from high-pressure, high-temperature experiments that generate incipient carbonate melts at mantle conditions (~90 kilometers depth and temperatures between 750° and 1050°C). We show that these primitive carbonate melts can sequester sulfur in its oxidized form of sulfate, as well as base and precious metals from mantle lithologies of peridotite and pyroxenite. It is proposed that these carbonate sulfur-rich melts may be more widespread than previously thought and that they may play a first-order role in the metallogenic enhancement of localized lithospheric domains. They act as effective agents to dissolve, redistribute, and concentrate metals within discrete domains of the mantle and into shallower regions within Earth, where dynamic physicochemical processes can lead to ore genesis at various crustal depths.

Fig. 1.. Back-scattered electron micrographs of experimental sample M21-107 (2.5 GPa, 1050°C—peridotite) at different magnifications.

(A) Overview of experimental charge. The carbonate melt is pooled at the top right and displays typical quench structure with sub-micrometer scale quench crystals in a fine-grained network. A clinohumite rim (dark gray) surrounds the residual peridotite highlighting the widespread activity of the melt throughout the experiment. (B) Magnified view of the yellow box in (A): Tiny droplets of sulfide are visible along grain boundaries, along which melt has migrated. (C) Clinohumite is interpreted to have formed because of a reaction between olivine (Olv) grains and the percolating melt. The black box shows the location of the TEM lamella, which was extracted for TEM analysis. This location is also highlighted by the white box in (A).

Fig. 2.. Back-scattered electron micrographs of M21-107 (2.5GPa, 1050°C—pyroxenite) at different magnifications.

(A) Low-magnification overview: Carbonate melts occur along the edges of the experimental charge and are rimmed by olivine (Olv), which has grown as an incongruent melting product as phlogopite began to melt. Clinopyroxene (Cpx), ilmenite (Ilm), and phlogopite (Phl) are present throughout the charge. The yellow box highlights the position of the focused ion beam lamella used for TEM analysis. (B) Magnified view of the carbonate melt quench structure encroaching into the olivine grains. The quenched phases consist of fluorite, brucite, and calcite (identified using TEM-EDS).

Fig. 3.. Transmission electron micrographs and compositional maps of M21-107.

(A) TEM image of the peridotite lamella [see Fig. 1 (A and C) for location] showing sulfides in cross-sectional view. The TEM image highlights high-density inclusions within both sulfide blebs. These sulfides occur along an olivine-clinohumite grain boundary and dislocations induced by the growth of the sulfide blebs during quenching are highlighted by the white arrows. A low-density rim surrounds both sulfides and was identified as carbonate melt by TEM-EDS (B). (B) TEM-EDS of the same sulfide blebs: A thin veneer of carbonate melt (green) surrounds both sulfides. (C) High-angle annular dark-field (HAADF) STEM of the pyroxenite lamella (see Fig. 2A). The STEM image shows two sulfide blebs with inner high-density inclusions surrounded by quenched carbonate melt. Graphite is also present in this quenched melt-sulfide complex due to the reduction of carbonate melt to graphite [dark red in (D)]. (D) Four distinct quenched phases were identified with TEM-EDS in the carbonatite that formed during melting of the pyroxenite sample: calcite (CaCO₃), calcium fluoride (CaF₂), brucite [Mg(OH)₂], and graphite (C).

Fig. 4.. Transmission electron micrograph and compositional maps of M21-107 peridotite sulfide bleb.

(A) HAADF image of the left sulfide bleb from Fig. 3A. The PGE-rich high-density inclusion is visible as the bright white area contained within the sulfide. (B to F) EDS maps of the sulfide bleb showing the distribution of sulfur (B), platinum (C), iridium (D), iron (E), and nickel (F). There is no detectable sulfur within the PGE inclusion (cf. table S7).

Fig. 5.. Transmission electron micrograph compositional maps of M21-107 pyroxenite sulfide bleb.

Two distinct Fe-rich PGE inclusions are contained within the sulfide. Compositions of the PGE inclusion within the left sulfide bleb are provided in table S7. EDS maps of (A) iron, (B) sulfur, (C) platinum, and (D) rhodium.

Fig. 6.. Chalcophile element compositions of olivine and neighboring clinohumite in M21-107 peridotite.

Carbonate melts make their way through silicate rocks by a dissolution and precipitation mechanism, evidenced by the transport of Ni, Cu, Co, and Fe from the silicate minerals to the melt. Typically, carbonate melts do not preferentially include chalcophile elements. However, their strong depletions in clinohumite suggest that sulfate-rich carbonate melts are responsible for this preferential partitioning. The depletion of Fe within the clinohumite is balanced by increased Mg concentrations (A), testifying to the exchange of elements with a percolating melt. The black arrows in (A) to (C) highlight the effect of passing melt on the olivine-clinohumite pairs. (B) The depletion of Ni and Cu in clinohumite relative to olivine. (C) The depletion of Ni and Co in clinohumite relative to olivine.

Fig. 7.. Synchrotron micro x-ray fluorescence (XRF) map of M21-107 peridotite and accompanying S-XANES.

(A) The synchrotron micro-XRF map highlights compositional variations across the sample. Sulfur (red) is concentrated along grain boundaries and within the carbonate melt pool. The clinohumite rim also shows strong depletions in iron. (B) S-XANES spectrum of the carbonate melt, taken from the white square in (A). Most of the sulfur within the carbonate melt is present as oxidized sulfate (S⁶⁺).

Fig. 8.. PiFM map of M21-107 pyroxenite at different magnifications.

(A) PiFM map acquired at the sulfate peak (1100 cm⁻¹ wave number): The bright gold regions correspond to sulfate, which is predominately found along grain boundaries between silicate minerals. (B) PiFM map of the red box in (A) combining spectra for olivine (green), mica (blue, phlogopite), and sulfate (red). (C) A high-magnification PiFM map of the red box in (B): Here, interstitial carbonate melt (black) is identified in addition to olivine, mica, and sulfate. The sulfate peaks correspond to aqueous sulfate anions dissolved in the carbonate melt.

Fig. 9.. Schematic diagram of melting processes in the lithospheric mantle.

(A) Incipient melts are likely widespread throughout the lithospheric mantle where small amounts of CO₂ and H₂O significantly depress the solidus of peridotite and pyroxenite to temperatures of ~950°C at 2.5 GPa. The first formed melts are carbonatitic in composition, provided oxidized conditions persist. The lithospheric mantle is oxidized enough at depths less than 200 km, and carbon occurs as carbonates in preference to its reduced form of diamond or graphite. (B) The oxidized carbonate melt can interconnect at tiny volumes and, as documented here, transport sulfur as sulfate. This effectively mobilizes sulfur from its mantle reservoir to shallower depths within Earth. The addition of sulfate anions to the melt likely causes a chemical potential gradient and chalcophile elements (Cu, Fe, Ni, and Co) partition strongly from olivine into the sulfate-carbonate melt, along with PGEs. These highly mobile low-degree melts quickly leave their mantle sources and rise toward the crust, where they will react and metasomatize the deep lower crust, enriching it with precious elements that are essential for ore genesis.