Metal-sulfide Melt Research Articles

Significance of high field strength and rare earth element distributions in deciphering the evolution of the inner solar system

Major processes affecting high field strength (HFSE) and rare earth (REE) element ratios in planetary basalts are degree of melting, separation of metal-sulfide melt fractions, addition and loss of silicate melt, ilmenite fractionation, and subduction. Fractional crystallization of planetary magma oceans has left a surviving imprint on only three bodies for which we have data: the Moon, Vesta, and the angrite parent body. Thorium mobilization in aqueous fluids may account for decoupling of Th and Nb in planetary systems, and this is especially notable on Earth but also possible on Mars, the Moon and some asteroids. On Earth, HFSE and REE ratios in young basalts characterize hydrated (HM), enriched (EM) and depleted (DM) mantle sources, associated with, respectively, subduction, mantle plumes and ocean ridges. Terrestrial hydrated and depleted mantle were in existence by at least 4 Ga and possibly they may have been produced in a stagnant lid tectonic regime before 3 Ga. Also, removal of Nb in metal-sulfide melts can force the composition of silicate restitic material into the hydrated mantle field on HFSE-REE graphs, thus not requiring hydration. Such an origin is probable for “hydrated” mantle in primitive achondrites and plutonic angrites. The record of all three types of mantle in basalts from other bodies in the Solar System indicates the three mantle reservoirs are not diagnostic of plate tectonics, but can be produced in stagnant lid settings.Enriched mantle is thus far recognized only in Earth and possibly Mars. There are at least two enriched mantle reservoirs in Earth: a primordial (>4 Ga) reservoir, perhaps hidden in the D” layer above the core and rarely sampled by basalts, and a recycled plate reservoir (<3 Ga), perhaps located in the two LLSVPs commonly sampled by oceanic island basalts. Between 3 and 2 Ga, the recycled enriched mantle reservoir became established in Earth, possibly in response to the widespread propagation of subduction. On Mars enriched mantle shows depleted radiogenic isotopic signatures and requires a multistage process to decouple trace element and isotopic systems.Although there are several processes by which Nb can be fractionated from Ta in planetary bodies, the low Nb/Ta (<15) characteristic of some planetary and asteroid basalts may reflect separation of a metal-sulfide melt enriched in Nb, which may or may not produce a core. This fractionation must occur early during a relatively reduced stage of planetary evolution (IW-3 to IW-5) such that Nb behaves as a chalcophile or siderophile element. If the average Nb/Ta ratio of both primitive and depleted mantle is equal to 15, production of basaltic magma in the terrestrial mantle through time has not fractionated Nb from Ta. On the other hand, if the Nb/Ta in primitive mantle equals 17, Nb must be fractionated from Ta before 4 Ga, perhaps by partitioning into the core during or soon after planetary accretion when reducing conditions may have existed.

Genesis of Diamond in Metal–Carbon and Metal–Sulfur–Carbon Melts: Evidence from Experimental Data

The experimental data on diamond growth in the Fe–Ni–S–C and Fe–S–C systems with a sulfur content of 5–14 wt % at 5.5 GPa and 1300–1350°C are reported. Colorless and light yellow diamond crystals with a weight of 0.1–0.8 ct were synthesized. It is shown in the Fe–S–C system that at 5.5. GPa diamond may crystallize in a very narrow temperature range, from 1300 to 1370°C. Based on comparative analysis of the experimental data and the results of the study of native iron inclusions in natural diamonds from kimberlite pipes, it is suggested that diamond genesis may be partly controlled by the pre-eutectic (by the concentration of sulfur in relation to metal) metal–sulfide melt.

Iron carbide as a source of carbon for graphite and diamond formation under lithospheric mantle P-T parameters

Experimental modeling of natural carbide-involving reactions, implicated in the graphite and diamond formation and estimation of the iron carbide stability in the presence of S-bearing fluids, sulfide melts as well as mantle silicates and oxides, was performed using a multi-anvil high-pressure split-sphere apparatus. Experiments were carried out in the carbide-sulfur (Fe3C-S), carbide-sulfur-oxide (Fe3C-S-SiO2-MgO) and carbide-sulfide (Fe3C-FeS2) systems, at pressure of 6.3GPa, temperatures in the range of 900–1600°C and run time of 18–40h. During the interaction of cohenite with S-rich reduced fluid or pyrite at 900–1100°C, extraction of carbon from carbide was realized, resulting in the formation of graphite in assemblage with pyrrhotite and cohenite. At higher temperatures complete reaction of cohenite with newly-formed sulfide melt was found to produce metal-sulfide melt with dissolved carbon (Fe64S27C9 (1200°C)–Fe54S40C6 (1500°C), at.%), which acted as a crystallization medium for graphite (1200–1600°C) and diamond growth on seeds (1300–1600°C). Reactions of cohenite and oxides with S-rich reduced fluid resulted in the formation of graphite in assemblage with highly ferrous orthopyroxene and pyrrhotite (900–1100°C) or in hypersthene formation, as well as graphite crystallization and diamond growth on seeds in the Fe-S-C melt (1200–1600°C). We show that the main processes of carbide interaction with S-rich fluid or sulfide melt are recrystallization of cohenite (900–1100°C), extraction of carbon and iron in the sulfide melt, and graphite formation and diamond growth in the metal-sulfide melt with dissolved carbon. Our results evidence that iron carbide can act as carbon source in the processes of natural graphite and diamond formation under reduced mantle conditions. We experimentally demonstrate that cohenite in natural environments can be partially consumed in the reactions with mantle silicates and oxides, and is absolutely unstable in the presence of S-bearing reduced fluid or sulfide melt at temperatures higher than 1100°C, under lithospheric mantle pressures.

A AB-complex iron meteorite containing low-Ca clinopyroxene: northwest Africa 468 and its relationship to lodranites and formation by impact melting

Northwest Africa 468 (NWA 468) is a new ungrouped, silicate-rich member of the IAB complex of nonmagmatic iron meteorites. The silicates contain relatively coarse (∼300 μm) grains of low-Ca clinopyroxene with polysynthetic twinning and inclined extinction. Low-Ca clinopyroxene is indicative of quenching from high temperatures (either from protoenstatite in a few seconds or high-temperature clinoenstatite in a few hours). It seems likely that NWA 468 formed by impact melting followed by rapid cooling to ≤660°C. After the loss of a metal-sulfide melt from the silicates, sulfide was reintroduced, either from impact-mobilized FeS or as an S 2 vapor that combined with metallic Fe to produce FeS. The O isotopic composition (Δ 17O = −1.39‰) indicates that the precursor material of NWA 468 was a metal-rich (e.g., CR) carbonaceous chondrite. Lodranites are similar in bulk chemical and O isotopic composition to the silicates in NWA 468; the MAC 88177 lodranite (which also contains low-Ca clinopyroxene) is close in bulk chemical composition. Both NWA 468 and MAC 88177 have relatively low abundances of rare earth and plagiophile elements. Siderophiles in the metal-rich areas of NWA 468 are similar to those in the MAC 88177 whole rock; both samples contain low Ir and relatively high Fe, Cu, and Se. Most unweathered lodranites contain ∼20 to 38 wt.% metallic Fe-Ni. These rocks may have formed in an analogous manner to NWA 468 (i.e., by impact melting of metal-rich carbonaceous chondrite precursors) but with less separation of metal-rich melts from silicates.

Petrology of the Asuka 881931 ureilite with special reference to crystallization of interstitial silicate melt

Abstract— The Asuka 881931 meteorite is an unbrecciated ferroan ureilite and consists mainly of equi—granular olivine and pigeonite grains, a metal—sulfide network, interstitial silicates, and glass. Peripheral portions of equigranular olivine grains are often replaced by fine‐grained forsterite—metal aggregates and sometimes by fine‐grained enstatite—metal aggregates. These aggregates may have been produced from the equigranular olivine by reduction. Peripheral portions of equigranular pigeonite grains also are sometimes replaced by fine‐grained orthopyroxene aggregates with tiny patches of Si‐rich glass and may have been produced from the pigeonite by reduction reaction with silicate melt. Interstitial silicates are mainly orthopyroxene, magnesian pigeonite, high‐Ca pyroxene (diopside/fassaite), and CaO‐poor enstatite; and they crystallized from interstitial silicate melt. Interstitial glass is classified into two types—‐Si‐poor and Si‐rich. The Si‐poor glass is always in contact with equigranular olivine, but the Si‐rich glass never contacts equigranular olivine and is in contact with pyroxene and the metal—sulfide network. Both types of glass were produced from an original interstitial silicate melt, but the Si‐poor glass formed mainly by fractional crystallization of pyroxenes, and the Si‐rich glass may have formed by addition of Si mainly from nearby metal—sulfide melt, as well as crystallization of pyroxenes. The Si‐poor and Si‐rich melts were finally quenched as interstitial glasses under rapid cooling conditions.

Electrical conductivity, internal temperatures and thermal evolution of the moon

The electrical conductivity of olivine and pyroxene is a strong function of the fugacity of oxygen in the atmosphere with which the mineral is in equilibrium. Lunar temperature profiles calculated from data on the electrical conductivity of these two minerals at oxygen fugacities similar to those which exist in the Moon indicate considerably higher temperatures for the lunar interior than obtained from conductivity data collected under normal atmospheric conditions. These high interior temperatures, the extensive differentiation associated with the formation of the lunar maria, and the radioactive element content of the Moon indicate that the Moon accreted at temperatures between 600 and 1000°C. Gravitational heating during accretion would lead to melting of at least the outer 200 km of the Moon and would produce conditions favourable to separation of a metal-sulfide melt sufficient to form a core of 200–300 km radius. Such a core would reach the center of the Moon within a few million years after accretion. This core could produce the remanent magnetization observed in the surface rocks. Dynamo action would cease with the cessation of convective motion within the core as the temperature of the surrounding mantle increased due to radioactive heating. With the radioactivity assumed in the present model and the high accretion temperature, this event would require less than 2 b.y., but more than 1.6 b.y.