Mantle-to-crust metal transfer by nanomelts

Abstract

The transfer of chalcophile metals across the continental lithosphere has been traditionally modeled based on their chemical equilibrium partitioning in sulfide liquids and silicate magmas. Here, we report a suite of Ni-Fe-Cu sulfide droplets across a trans-lithospheric magmatic network linking the subcontinental lithospheric mantle to the overlying continental crust. Petrographic characteristics and numerical calculations both support that the sulfide droplets were mechanically scavenged from the mantle source during partial melting and transported upwards by alkaline magmas rising through the continental lithosphere. Nanoscale investigation by high-resolution transmission electron microscopy (HR-TEM) documents the presence of galena (PbS) nanoinclusions within the sulfide droplets that are involved in the mantle-to-crust magma route. The galena nanoinclusions show a range of microstructural features that are inconsistent with a derivation of PbS by exsolution from the solid products of the Ni-Fe-Cu sulfide liquid. It is argued that galena nanoinclusions crystallized from a precursor Pb(-Cu)-rich nanomelt, which was originally immiscible within the sulfide liquid even at Pb concentrations largely below those required for attaining galena saturation. We suggest that evidence of immiscibility between metal-rich nanomelts and sulfide liquids during magma transport would disrupt the classical way by which metal flux and ore genesis are interpreted, hinting for mechanical transfer of nanophases as a key mechanism for sourcing the amounts of mantle-derived metals that can be concentrated in the crust.

Study on Microinclusions of Metals and Minerals in Rocks: Problems in Interpreting Research Findings, and Their Application to the Study of Magmatic Systems of Kamchatka and the Stanovoy Fold Belt

“…Намечающееся в последнее время интенсивное развитие «микрометаллогенического» подхода к изучению петрологии и металлогенического потенциала горных пород требует разработки надежных методов определения химического состава и фазовой принадлежности обнаруживаемых в этих породах микровключений. Эти методы направлены как на прецизионное определение химического состава микровключений [например, 35,54], так и на экспрессную полуколичественную оценку состава с целью их минералогической идентификации, продемонстрированную, в частности, в настоящей статье. Ее использование при изучении анкарамитов и содержащихся в них перидотитовых ксенолитов из Авачинского вулкана на Камчатке, а также пород основного-ультраосновного массива Ильдеус в Становой складчатой области позволило получить новые данные о петрологических процессах в мантийно-коровом субстрате под активной вулканической дугой и об особенностях эволюции рудно-магматической системы под древней активной континентальной окраиной.…”

Carbonated magmatic sulfide systems: Still or sparkling?

“…This concept triggered a cascade of subsequent studies and gained increasing popularity ( 26 – 28 ), supporting previous studies on the potential role of volatiles in metal transport ( 29 – 33 ). The adherence of sulfide globules to gas bubbles may enhance their transport, especially in shallow magmatic-hydrothermal systems ( 34 ); however, the extension of this “bubble” model of sulfide mobilization to deep systems ( 10 , 11 ), where gas bubbles are unlikely to exist due to their high solubilities at high pressures, might not be suitable.…”

“…This hypothesis was initially put forward based on experimental studies that demonstrated the strong affinity of vapor bubbles for sulfide globules ( 21 ). Originally confined to experimental studies and numerical modeling, only a few years later this compound droplet model was gradually applied to natural systems ( 10 , 11 , 24 , 25 ). According to this hypothesis, as magma ascends, the adherence of vapor bubbles to the surfaces of sulfide globules results in the formation of relatively buoyant compound droplets that resemble hot-air balloons transporting metal-rich sulfide cargos ( Fig.…”

“…Allegedly, these voids were later filled with highly fractionated silicate melt or secondary phases due to pressure-driven infiltration ( 24 , 25 ). This concept, initially deemed pressure independent ( 27 ), was further extrapolated to lower crustal levels ( 10 , 11 ). It was proposed that the adherence of supercritical CO 2 liquid to sulfide globules might also play a pivotal role in extracting sulfide liquid from the mantle and facilitating its transport across the lithosphere.…”

Nanomaterial accumulation in boiling brines enhances epithermal bonanzas

“…In the field of economic geology, nanomaterials may include atomic clusters, mineral nanoparticles (1–100 nm; NPs hereafter), nanominerals, nanofluids, and nanomelts, usually constituted by a wide suite of metals 1 – 3 . In the last decade, an ever-increasing amount of evidence has documented nanomaterials in hydrothermal solutions 4 , thus questioning the traditional paradigm that ligands (e.g., chloride- and bisulfide-bearing) are the sole efficient metal carriers in ore-forming fluids 5 .…”