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Cerro Uturuncu is a long-dormant, compositionally monotonous, effusive dacitic volcano in the Altiplano–Puna Volcanic Complex (APVC) of SW Bolivia. The volcano recently gained attention following the discovery of an ∼70 km diameter Interferometric Synthetic Aperture Radar (InSAR) anomaly roughly centred on its edifice. Uturuncu dacites, erupted over the past ∼1 Myr, invariably have a phase assemblage of plagioclase–orthopyroxene–biotite–ilmenite–magnetite–apatite–zircon and rhyolite glass. To better constrain storage conditions of the dacite magmas and to help understand their relationship with the observed deformation, petrological experiments were performed in cold-seal hydrothermal vessels. Volatile-saturated (PH2O = PTOTAL and PH2O + PCO2 = PTOTAL) phase equilibria experiments were run between 50 and 250 MPa and 760 and 900°C at fO2 ∼ Ni–NiO. Two synthetic starting compositions were investigated based on a typical Uturuncu dacite whole-rock and its rhyolitic groundmass glass. Pre-eruptive magma storage conditions have been estimated by comparing results from the experiments with natural phase assemblages, modes, and mineral and glass compositions. H2O-saturated experiments constrain storage pressures to 100 ± 50 MPa, equivalent to 1·9–5·7 km below surface. In the dacite, natural phase assemblages are reproduced at 870°C, 100 MPa with both orthopyroxene and biotite stabilized concurrently. Natural glass chemistries are most closely replicated at 50 MPa at 870°C, reflecting the role of decompression crystallization prior to eruption. In H2O-saturated rhyolite experiments the natural phase assemblage is most closely replicated at 870°C, 50 MPa. Isothermal, mixed volatile dacite experiments at 870°C further constrain storage pressures to 110 ± 10 MPa. Assuming that there has been no dramatic change in the eruptive behaviour of Uturuncu in the last 270 kyr, pre-eruptive storage of dacite magmas at ∼100 MPa precludes their role in producing the large diameter deformation anomaly. If deformation is a result of magmatic intrusion, then intrusion of less evolved magmas into deeper, mid-crustal storage regions is a more probable explanation. Intrusion within the Altiplano–Puna Magma Body (APMB), the extent of which is roughly coincident with the APVC, is most likely. It is proposed that dacite magmas form from andesitic parents, via fractionation and/or assimilation, within the APMB. Dacites then rise buoyantly to shallow storage levels where they stall and crystallize prior to eruption. Microlites form during subsequent ascent from the storage region to the surface. [ABSTRACT FROM PUBLISHER]

Developing approaches to determine the modes of transport and weathering of mafic detrital minerals in natural sedimentary environments is critical to our understanding of sediment production, dispersal and provenance, as well as carbon capture under natural and enhanced weathering regimes. We integrated the characterisation of morphological and surface textures with the surface composition of recent detrital clinopyroxene grains concentrated in a sandy coastal area from the western North Island of New Zealand and a rocky shore area of Santa Maria Island in the Azores Archipelago. Using the compactness shape descriptor, 341 grains were subdivided into elongated, elongated angular, euhedral, angular and subangular groups, with each group further characterised using morphological and microtextural indicators of abrasion, breakage and dissolution/chemical weathering. In both studied environments, the clinopyroxenes are dominated by elongated to angular shapes with flat cleavage surfaces and conchoidal fractures. This is consistent with dominant subaqueous transport by rivers and longshore currents for New Zealand, and dominant wave action with limited sediment supply by rivers along the rocky shore of Santa Maria. More abundant subabraded and abraded shapes with bulbous and elongated depression microtextures are observed in New Zealand, which indicates additional effects by aeolian transport that are not seen in Santa Maria. Consistent with semi-quantitative EDS spot analyses that document fresh clinopyroxenes surfaces, chemical weathering textures are very rare to absent. In Santa Maria, sampling of recent beach sand was complemented by ∼125 kyr-old palaeobeach clinopyroxenes, but these yielded similar textural results without evidence for chemical weathering. However, the surface of these older clinopyroxenes includes small adhering smectite (typically <50 μm in width and a few μm in thickness) interpreted to reflect incipient cementation with buffering of acidic fluids by more reactive lithic fragments in the deposits. The lack of chemical weathering but pervasive evidence for mechanical breakage of clinopyroxenes indicate that, at the sand size, these minerals break faster than they dissolve due to frequent subaqueous and/or aeolian reworking in the studied high-energy environments and temperate climatic conditions. [ABSTRACT FROM AUTHOR]

The Titanium (Ti) isotope compositions of felsic rocks are heavier than their mafic counterparts, and alkaline magmas develop heavier Ti isotope compositions compared to other magma series during magmatic differentiation. Both observations are interpreted to reflect the preferential sequestration of light Ti isotopes in Fe-Ti oxides, such as rutile, ilmenite and titanomagnetite. However, such interpretations so far rely on whole rock studies of cogenetic magmatic samples and the detailed mechanics of oxide-melt equilibrium on the Ti isotope composition of magmas is poorly constrained. To address this, we have measured the Ti isotope composition of co-existing Fe-Ti oxides and groundmass or silicate melt in both natural lavas from contrasting tectonic settings (Heard Island and Santorini), and experimental run products (rutile-melt). All Fe-Ti oxide phases are consistently isotopically lighter than their respective host groundmass or silicate melt, with the magnitude of Δ49/47Ti oxide-melt increasing from rutile to ilmenite, and titanomagnetite. The observed difference in Ti isotope fractionation between rutile-melt experiments and ilmenite-groundmass pairs is primarily reflective of small differences in their Ti O bond length, with ilmenite being isotopically lighter (Δ49/47Ti ilmenite-melt extrapolated to 1000 K = −0.600 ± 0.035‰) compared to rutile (Δ49/47Ti rutile-melt at extrapolated to 1000 K = −0.404 ± 0.099‰) due to slightly longer Ti O bonds in ilmenite. In contrast, the variation in Δ49/47Ti observed between titanomagnetite-groundmass pairs increases as a function of increasing TiO 2 content (increasing ulvöspinel component) in titanomagnetite, with Δ49/47Ti titanomagnetite-melt values extrapolated to 1000 K ranging from −0.811 to −1.451‰ in Ti-rich titanomagnetite (21–23 wt.% TiO 2 ; Usp 66-73) from Heard Island compared to −0.673‰ to −0.863 in Santorini (14–15 wt.% TiO 2 ; Usp 45-49). We interpret this to result from a weaker and distorted crystal lattice due to changes in the local cationic environment resulting from exchange of smaller Fe3+ ions with larger Fe2+ ions during magnetite-ulvöspinel solid solution. Our results are consistent with fractionation factors inferred from Ti isotopic analyses of mineral separates and ab-initio calculations. We use these fractionation factors in combination with Ti isotope fractionation factors for silicate minerals recalculated from previous studies, along with petrologic information to model the behaviour of Ti isotopes during partial melting of Earth's mantle as well as reproducing the observed variation in δ49/47Ti of differentiated magmas from distinct geodynamic settings. [ABSTRACT FROM AUTHOR]

Abstract Low-gradeplatinum-group element mineralisation in the Volspruit Sulphide Zone is sulphide-poor (<5 vol. %), distributed over a ~60 m-thick horizon in the lowermost cumulates of the northern limb of the Bushveld Complex. Unlike any other platinum-group element (PGE) deposit of the Bushveld Complex, the Volspruit Sulphide Zone is hosted exclusively within harzburgitic and dunitic cumulates in the Lower Zone of the Rustenburg Layered Suite. Here, we present a petrological investigation on the distribution of PGEs and chalcophile metals in mineralised pyroxenite cumulates from the Volspruit Sulphide Zone, to determine the origin of the PGE mineralisation in ultramafic cumulates and evaluate whether Volspruit-style mineralisation could occur in the stratigraphically lowest, ultramafic portions of other layered intrusions. Electron microscopy of pyroxenite cumulates revealed (1) chromite inclusions containing dolomite, albite, monazite, Pb-chlorides, base metal sulphides and Pt-As minerals, (2) the presence of exotic microxenocrysts (<300 μm diameter) in the pyroxenite matrix such as grains of CaCO 3 , U-Th-oxide and Mn-ilmenite, and (3) base metal sulphide assemblages enclosing grains of primary galena, sphalerite and Pb-chlorides. Systematic mapping of high-density mineral assemblages in pyroxenite cumulates across the Volspruit Sulphide Zone identified 196 precious metal mineral grains (Pt-, Pd-, Rh-, Au- or Ag-minerals), 98 Pb-sulphide grains (± Se, Cl), 27 Pb-chloride grains (± K, Se, Te, S), as well as 1 grain of Pb-telluride, 1 monazite grain and 1 grain of U–Pb-Th oxide. Trace element analyses of base metal sulphides reveal the highest S/Se values in pyrrhotite and chalcopyrite yet recorded in the Bushveld Complex. While some base metal sulphides are enriched in PGEs, the overall low-grade of the deposit and inferred fertile ultramafic magma(s) require relatively low R-factors (mass of silicate to sulphide melt) compared to other sulphide-poor PGE deposits, with a calculated R factor of ~500–3000. We consider that the presence of exotic inclusions in chromite, exotic microxenocrysts, and Pb/Zn/Cl grains enclosed within primary base metal sulphide assemblages provide strong evidence for crustal contamination in the Volspruit Sulphide Zone. The Malmani dolomite and the Black Reef quartzite within the lower Chuniespoort Group (2.2–2.4 Ga) are the most likely source of xenocrysts, assimilated in a staging chamber beneath the main Grasvally chamber, in which the Volspruit Sulphide Zone developed. It is possible that the Malmani dolomite contained an enrichment of Pb, Zn, Cl, and S minerals prior to assimilation. The assimilation of dolomite and limestone would locally increase the fO 2 of the magma, triggering chromite crystallisation. The sudden removal of Fe from the melt, coupled with the addition of external sulfur triggered saturation of an immiscible sulphide melt in the ultramafic Volspruit magma. Chromite and base metal sulphides were subsequently emplaced into the main Grasvally magma chamber as a crystal-bearing slurry. Therefore, we consider it is possible for PGE mineralisation to occur in the ultramafic portion of any layered intrusion intruding in the vicinity of carbonate units. Even if this style of mineralisation in the lowermost portions of layered intrusions is sub-economic, it may reduce the grade or opportunity for PGE mineralisation higher up in the local magmatic stratigraphy, or in later magma emplacement events sourced from the same reservoir. The technique of specifically searching for microxenocrysts could be applied beyond layered intrusion research, to identify the range of crustal contaminants in other magmatic systems where macro-scale xenoliths are neither sampled nor preserved. Graphical abstract Unlabelled Image Highlights • We document a ~60 m thick horizon of sulphide-poor PGE mineralisation in ultramafic cumulates • Exotic mineral assemblages are enclosed within the core of chromite grains • Microxenocrysts occur within the pyroxenite matrix • Pb-sulphides, Pb-chlorides and sphalerite are enclosed within magmatic sulphide blebs • Pyrrhotite contains S/Se ≤21,566 – the highest values recorded in the northern Bushveld Complex [ABSTRACT FROM AUTHOR]