Anhydrite assimilation by ultramafic melts of the Bushveld Complex, and its consequences to petrology and mineralization

Yudovskaya, M.A., Sluzhenikin, S.F., Costin, G., Shatagin, K.N., Dubinina, E.O., Grobler, D.F., Ueckermann, H., and Kinnaird1, J.A.

in “Metals, Minerals and Society”, Special Publication no. 21, Society of Economic Geologists, Chapter 9,  pp. 177-206

Abstract

Sulfate assimilation by mafic to ultramafic melt is thought to be an important process in the genesis of magmatic PGE-Ni-Cu deposits. We consider petrological indicators and possible mechanisms of anhydrite assimilation by ultramafic melts of the northern limb of the Bushveld Complex. On farm Turfspruit, an anhydrite-bearing sedimentary raft of the Duitschland Formation separates the Platreef from underlying Lower zone peridotites. The proportion of anhydrite across the raft increases from negligible in corundum-sillimanite-magnetite hornfels at the base to 95 to 100% in anhydrite marble at the top. Underlying Lower zone peridotites lack anhydrite, whereas overlying Platreef pyroxenites contain both widespread interstitial to euhedral anhydrite as well as spherical to irregularly shaped anhydrite inclusions in association with olivine chadacrysts inside oikocrystic orthopyroxene. Olivine chadacryst compositions (Mg# 79–81 and 0.33–0.46 wt % NiO) support their pristine liquidus origin, although an association of Al-enriched orthopyroxene and interstitial anorthite indicates exchange reactions involving anhydrite and aluminosilicates from hornfels. Plagioclase from the anhydrite-contaminated rocks has an Sr isotope initial ratio (Sri) of 0.7047 to 0.7063, similar to the compositions of Bushveld early primitive magmas, in agreement with a relatively non-radiogenic signature of the anhydrite-bearing contaminant with Sri of 0.7057 to 0.7094. The range of Sri of plagioclase from the underlying Lower zone peridotites (0.7040– 0.7067) and from the Turfspruit platinum reefs just below the Main zone contact (0.7068–0.7084) supports their correlation and synchronous emplacement with the Lower zone and the top of the Upper Critical zone in the western and eastern limbs of the Bushveld. The δ34S values of anhydrite (12.2–14.5‰) and a coexisting pyrrhotite-millerite-chalcopyrite sulfide assemblage (6.2–7.8‰) in a hornfelsed raft and overlying pyroxenites are interpreted to have resulted from open system isotopic exchange, indicating closure temperatures of 750° to 820°C. The assimilation of sedimentary anhydrite is interpreted to be an important component of contact-style mineralization of the Platreef at Turfspruit that took place through the erosion and disintegration of footwall rocks by dynamic pulses of hot magmas. Chemical dissolution, thermal decomposition, and melting of sulfate-bearing rafts or xenoliths are viable assimilation processes that result in the saturation of silicate melt with sulfate, exsolution of immiscible sulfate melts, crystallization of cumulus and interstitial anhydrite, and precipitation of contact-style sulfide mineralization at the base of the intrusion. Reef-style mineralization at the top of the Platreef shows contrastingly negligible compositional and isotopic evidence of sulfate assimilation.

Yudovskaya, M.A., Sluzhenikin, S.F., Costin, G., Shatagin, K.N., Dubinina, E.O., Grobler, D.F., Ueckermann, H., and Kinnaird1, J.A. (2018): Anhydrite assimilation by ultramafic melts of the Bushveld Complex, and its consequences to petrology and mineralization. In “Metals, Minerals and Society”, Editors Antonio M. Arribas and Jeffrey L. Mauk. SEG Special publications no. 21, pp. 177-206.

https://www.segweb.org/store_info/SP/SP-21-Additional-Product-Info.pdf

Ding, S. and Dasgupta, R.

Abstract: The sulfur (S) and copper (Cu) contents of primitive mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) are similar, although the latter are thought to be derived from hotter mantle. To reconcile the sulfur and chalcophile element budgets of OIB, we developed a model to describe the behavior of sulfide and Cu during decompression melting of mantle by combining experimental constraints on decompression melting at different excess mantle potential temperatures, T_P, between 1400 and 1650°C, and empirical sulfur content at sulfide saturation (SCSS) models, which take into account the effect of Ni and Cu present in the equilibrium sulfide melt. Model calculations at T_P = 1450–1650°C were applied to explain the S and Cu inventory of high-Mg# ‘reference’ OIB. Modeling indicates that partial melts relevant to OIB generation have higher SCSS than those of primitive MORB, because of the positive effect of temperature on SCSS. Therefore, for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle. Calculation of SCSS along melting adiabats at T_P = 1450–1550°C, with variable initial S content of peridotite, indicates that sulfide-undersaturated primitive Icelandic basalts with ∼720 ppm S and 74–115 ppm Cu can be generated by 10–25 wt % melting of peridotite containing 100–150 ppm S. The S and Cu budgets of OIB that are thought to represent low-degree melts can be satisfied by (1) peridotite partial melting if a sulfide-saturated partial melt with a Ni content in the sulfide melt ≥25–30 wt % is derived from relatively cold mantle (T_P ≤ 1450°C), or (2) if primitive melts parental to OIB are enriched in S (>1500 ppm), or (3) an extremely low (≤1%) degree of melting is applicable. Alternatively, if the Ni content in the equilibrium sulfide in the peridotitic mantle is ≤20–25 wt %, mixing of partial melts, derived from low-degree melting of MORB-eclogite and metapelite, with S-depleted peridotite partial melts may be necessary to reconcile the measured S and Cu contents in the low-F (<10%, where F is melt fraction) basalts from Galapagos spreading center, Lau Basin and Loihi for T_P of 1450–1650°C. In this last case, sulfides, equivalent to 50–100 ppm S in the peridotite mantle, can be exhausted by 1–9 wt % partial melting. The total S inventory of the heterogeneous mantle source of these basalts is higher because of the presence of subducted eclogite ± sediments. Our analysis also suggests that compared with peridotite, which is likely to become sulfide-free during partial melting owing to the high SCSS for its partial melts, subducted MORB-eclogite and metapelite probably play important roles in retaining sulfide in the Earth’s shallow mantle, owing to low SCSS in their partial melts and high initial sulfide abundances.

Ding, S. & Dasgupta, R. (2018). Sulfur inventory of ocean island basalt source regions constrained by modeling the fate of sulfide during decompression melting of a heterogeneous mantle. Journal of Petrology 59, 1281–1308. doi:10.1093/petrology/egy061

A non-van der Waals 2D Material from natural titanium mineral ore Ilmenite

Puthirath Balan, A., Radhakrishnan, S., Kumar, R., Neupane, R., Sinha, S.K., Deng, L., de los Reyes, C.A., Apte, A., Rao, B.M., Paulose, M., Vajtai, R., Chu, C.W., Costin, G., Marti, A.A., Varghese, O.K., Singh, A.K., Tiwary, C.S., Anantharaman, M.R., Ajayan, P.M

Chemistry of Materials  https://pubs.acs.org/doi/10.1021/acs.chemmater.8b01935

Abstract

Two dimensional (2D) materials from naturally occurring minerals are promising and possess interesting physical properties. A new 2D material ‘Ilmenene’ has been exfoliated from the naturally occurring titanate ore ilmenite (FeTiO3) by employing liquid phase exfoliation in dimethylformamide (DMF) solvent by ultrasonic bath sonication. Ilmenene displays [001] orientation which is confirmed by transmission electron microscopy (TEM). Probable charge transfer excitation from Fe2+Ti4+ to Fe3+Ti3+ results in ferromagnetic (FM) ordering along with the antiferromagnetic (AFM) phase accompanied by enhanced anisotropy due to surface spins. The 2D nature and bandgap states help ilmenene form a heterojunction photocatalyst with titania nanotube arrays, capable of broad spectrum light harvesting and separating/transferring the photo-generated charges effectively for solar photoelectrochemical water splitting.

Puthirath Balan, A., Radhakrishnan, S., Kumar, R., Neupane, R., Sinha, S.K., Deng, L., de los Reyes, C.A., Apte, A., Rao, B.M., Paulose, M., Vajtai, R., Chu, C.W., Costin, G., Marti, A.A., Varghese, O.K., Singh, A.K., Tiwary, C.S., Anantharaman, M.R., Ajayan, P.M., 2018. A non-van der Waals 2D Material from natural titanium mineral ore Ilmenite. Chem. Mater. acs.chemmater.8b01935. https://doi.org/10.1021/acs.chemmater.8b01935

https://pubs.acs.org/doi/10.1021/acs.chemmater.8b01935

Impurity controlled crystal growth in low dimensional bismuth telluride

Sharifi, T., Yazdi, S., Costin, G., Apte, A., Coulter, G., Tiwary, C., and Ajayan, P.M.

Chemistry of Materials,  https://pubs.acs.org/doi/10.1021/acs.chemmater.8b02548

ABSTRACT

Topological insulators, such as layered Bi2Te3, exhibit extraordinary properties, manifesting profoundly only
at nanoscale thicknesses. However, it has been challenging to synthesize these structures with controlled thickness. Here,
control over the thickness of solvothermally grown Bi2Te3 nanosheets is demonstrated by manipulating the crystal growth
through selected and controlled impurity atom addition. By a comprehensive analysis of growth mechanism and the intentional
addition of Fe impurity, it is demonstrated that the nucleation and growth of few-layer nanosheets of Bi2Te3 can be
stabilized in solution. By optimizing the Fe concentration, nanosheets thinner than 6 nm, and as thin as 2 nm, can be
synthesized. Such thicknesses are smaller than the anticipated critical thickness for the transition of topological
insulators to the quantum spin Hall regime.

Sharifi, T., Yazdi, S., Costin, G., Apte, A., Coulter, G., Tiwary, C., and Ajayan, P.M. (2018). Impurity controlled crystal growth in low dimensional bismuth telluride. Chemistry of Materials, p. acs.chemmater.8b02548. doi: 10.1021/acs.chemmater.8b02548

https://pubs.acs.org/doi/10.1021/acs.chemmater.8b02548

Tsuno, K., Grewal, D.S., Dasgupta, R. (2018). Core-mantle fractionation of carbon in Earth and Mars: The effects of sulfur. Geochimica et Cosmochimica Acta 238: 477-495.

Abstract: Constraining carbon (C) fractionation between silicate magma ocean (MO) and core-forming alloy liquid during early differentiation is essential to understand the origin and early distribution of C between reservoirs such as the crust-atmosphere,
mantle, and core of Earth and other terrestrial planets. Yet experimental data at high pressure (P)-temperature (T) on the
effect of other light elements such as sulfur (S) in alloy liquid on alloy-silicate partitioning of C and C solubility in
Fe-alloy compositions relevant for core formation is lacking. Here we have performed multi-anvil experiments at
6–13 GPa and 1800–2000 °C to examine the effects of S and Ni on the solubility limit of C in Fe-rich alloy liquid as well
as partitioning behavior of C between alloy liquid and silicate melt (Dc). The results show that C solubility in the alloy
liquid as well as Dc decreases with increasing in S content in the alloy liquid. Empirical regression on C solubility in
alloy liquid using our new experimental data and previous experiments demonstrates that C solubility significantly increases
with increasing temperature, whereas unlike in S-poor or S-free alloy compositions, there is no discernible effect of Ni on C
solubility in S-rich alloy liquid.
Our modelling results confirm previous findings that in order to satisfy the C budget of BSE, the bulk Earth C undergoing
alloy-silicate fractionation needs to be as high as those of CI-type carbonaceous chondrite, i.e., not leaving any room for
volatility-induced loss of carbon during accretion. For Mars, on the other hand, an average single-stage core formation at
relatively oxidized conditions (1.0 log unit below IW buffer) with 10–16 wt% S in the core could yield a Martian mantle with
a C budget similar to that of Earth’s BSE for a bulk C content of 0.25–0.9 wt%. For the scenario where C was delivered to
the proto-Earth by a S-rich differentiated impactor at a later stage, our model calculations predict that bulk C content in the
impactor can be as low as ~0.5 wt% for an impactor mass that lies between 9 and 20% of present day Earth’s mass. This value
is much higher than 0.05–0.1 wt% bulk C in the impactor predicted by Li et al. (Li Y., Dasgupta R., Tsuno K., Monteleone B.,
and Shimizu N. (2016) Carbon and sulfur budget of the silicate Earth explained by accretion of differentiated planetary
embryos. Nat. Geosci. 9, 781–785) because C-solubility limit of 0.3 wt% in a S-rich alloy predicted by their models is significantly
lower than the experimentally derived C-solubility of 1.6 wt% for the relevant S-content in the core of the impactor.