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Marsh (Contrib Miner Petrol 166:665-690, 2013 (See CR48)) again claims that crystal-free basalt magmas are unable to differentiate in crustal magma chambers and regards layered intrusions as primarily due to the repeated emplacement of crystal suspensions. He ignores an earlier critique of his unconventional inferences (Latypov, J Petrol 50:1047-1069, 2009 (See CR37)) as well as a wealth of petrographic, geochemical and experimental evidence supporting the dominant role of fractional crystallization in the solidification of layered intrusions. Most tellingly, the cryptic variations preserved in the Skaergaard and many other basaltic layered intrusions would require an exceedingly implausible sequence of phenocrystic magmas but are wholly consistent with in situ fractional crystallization. A major flaw in Marsh's hypothesis is that it dismisses progressive fractional crystallization within any magma chamber and hence prohibits the formation of crystal slurries with phenocrysts and melts that change systematically in composition in any feeder system. This inherent attribute of the hypothesis excludes the formation of layered intrusions anywhere., Author(s): Rais Latypov[sup.1] , Tony Morse[sup.2] , Brian Robins[sup.3] , Richard Wilson[sup.4] , Grant Cawthorn[sup.1] , Christian Tegner[sup.4] , Marian Holness[sup.5] , Charles Lesher[sup.4] [sup.6] , Steve Barnes[sup.7] , Brian [...]

Crystal clusters are common in both extrusive and plutonic rocks, but the mechanisms by which they form are not well-constrained. Following a consideration of the physics of nucleation, we outline the expected microstructural characteristics of clusters formed by heterogeneous nucleation and those formed by synneusis, together with the ways they might evolve during subsequent grain growth and textural equilibration. By combining analysis of the microstructures in experimental chromite-basalt charges with a detailed microstructural analysis of the UG2 chromitite of the Bushveld layered intrusion using EBSD, we argue that the UG2 chromitite formed by settling and accumulation of single grains and clusters comprising randomly oriented grains produced by the aggregation of previously isolated chromite crystals. Although there is no evidence of epitaxy, at least some of the lowermost chromite grains of the main UG2 chromitite may have nucleated heterogeneously on the silicate grains forming the floor, with subsequent accumulation and sintering of individual grains or clusters. The reduced thickness of chromitites on the steep and overhanging parts of the floor is thus due to the relative difficulty of sticking more grains to the existing layer in these orientations. The absence of any fining-upwards of grains in either the main UG2 chromitite or the associated stringer can be accounted for if both layers were formed by the settling and accumulation of clusters as well as single grains. Comparison with examples of clustered chromite grains in extrusive rocks suggests that aggregation by synneusis is a widespread magmatic process. The 'chicken-wire' texture formed by clustered chromite grains commonly found in olivine-rich cumulates is argued to also be formed by gravitational settling, with the possible exception of clusters of chromite grains in relatively thin seams argued to be the result of metasomatism, which may instead have formed by impingement during in situ growth. [ABSTRACT FROM AUTHOR]

The current debate on the origin of platinum-group element (PGE) reefs in layered intrusions centres mostly on gravity settling of sulphide liquid from overlying magma versus its introduction with interstitial melt/fluids migrating upward from the underlying cumulate pile. Here, we show that PGE-rich chromitite seams of the Rum Eastern Layered Intrusion provide evidence for an alternative origin of such deposits in layered intrusions. These laterally extensive 2-mm-thick chromitite seams occur at the bases of several cyclic mafic-ultramafic units and show lithological and textural relationships suggesting in situ growth directly at a crystal-liquid interface. This follows from chromitite development along the edges of steeply inclined culminations and depressions at unit boundaries, even where these are vertically oriented or overhanging. High concentrations of PGE (up to 2-3 ppm Pd + Pt) are controlled by fine-grained base-metal sulphides, which are closely associated with chromitite seams. The following sequence of events explains the origin of the PGE-rich chromitite seams: (a) emplacement of picritic magma that caused thermal and mechanical erosion of underlying cumulate, followed by in situ growth of chromite against the base, (b) precipitation of sulphide droplets on chromite grains acting as favourable substrate or catalyst for sulphide nucleation, (c) the scavenging of PGE by sulphide droplets from fresh magma continuously brought towards the base by convection. Since the rate of magma convection is [10.sup.5]-[10.sup.7] times higher than that of the solidification (km/year to km/day versus 0.5-1.0 cm/year), the in situ formed sulphide droplets can equilibrate with picritic magma of thousands to million times their own volume. As a result, the sulphide-bearing rocks are able to reach economic concentrations of PGE (several ppm). We tentatively suggest that the basic principles of our model may be used to explain the origin of PGE-rich chromitites and classical PGE reefs in other layered mafic-ultramafic intrusions. Keywords Chromitite seams * Sulphide liquid * Platinum-group elements * In situ growth * Convection * Layered intrusions, Introduction Mafic-ultramafic layered intrusions are 'fossilised' natural laboratories for the study of magma chamber processes that provide important constraints on many fundamental principles of igneous and ore petrology. These intrusions [...]

The activity of volatile-rich fluids may be important in the evolution of basaltic magmatic systems and associated precious metal ore formation. There is evidence for Cl-rich fluids within the Stillwater Complex (Montana, U.S.A.), which have been linked to platinum and palladium mineralization in the economically important Johns-Manville (J-M) Reef ore body. We present the first data set for heavy halogens (Cl, Br, and I) and natural noble gas isotopes in bulk rock and mineral separates from the Peridotite Zone and the Olivine-Bearing Zone I of the Stillwater Complex, including samples from the J-M Reef and G Chromitite bodies. Our data reveal concentrations of 4 to 13 500 ppm for Cl, 26 ppb to 360 ppm for Br, and <1 ppb to 9 ppm I over the whole sample set. Cl, Br, and I correlate well with each other implying a shared process and/or distribution in mineral species. Br/Cl and I/Cl ratios span a range from 0.3 to 35 × 10−3 and 5 to 900 × 10−6 by weight, respectively, encompassing MORB-like to more enriched compositions, particularly for Br/Cl. High-Br/Cl ratios compared to MORB in some Stillwater samples suggest fractionation of halogens during the exsolution of a volatile-rich fluid to explain the most Br-enriched samples. More generally, the presence of minerals such as scapolite, hornblende, and apatite in the most halogen-enriched samples suggests that the halogen-bearing fluids were derived from the cooling of the intrusion rather than late-stage (low-temperature) metamorphism. The combined halogen abundance and noble gas isotope data set imply that crustal contamination may have played a limited role in the crystallization of pegmatoids and the G Chromitite but is not required to account for the halogen budget of the J-M Reef. High-halogen contents in the sulfide-bearing J-M Reef and associated lithologies are consistent with the influence of fluid-related activity during platinum-group element (PGE)-Reef formation, lending weight to the hydromagmatic model for mineralization in the Stillwater intrusion. Our new data also imply chalcophile tendencies of Br and I over Cl in sulfides in natural systems, hinting at the importance of sulfide liquid interaction with halogen-rich fluids in the formation of sulfide-hosted precious metal ore deposits. [ABSTRACT FROM AUTHOR]

Hydrothermal alteration of oceanic crust represents a globally significant geochemical exchange between the crust and oceans, introducing volatile elements into the oceanic lithosphere and removing latent heat from crystallization. The extent to which hydrothermal systems penetrate the lower oceanic crust is debated and is significant for competing models of lower oceanic crust formation. In this study, we apply halogen and noble gas geochemistry in order to better understand the fluid sources and conditions during a hydrothermal alteration in a suite of gabbros sampled from drill cores in the mid‐ (hole GT3A) and lower‐ (hole GT1A) crust of the Oman Ophiolite. Low Br/Cl in mid‐crustal samples is controlled by amphibole formed at relatively high fluid/rock ratios. Lower crustal samples, meanwhile, have Br/Cl ranging from seawater to much higher values (<5 × 10−3). These require alteration at highly variable fluid/rock ratios from ∼1 to <0.01 in the lower crust. Noble gas isotopes also suggest alteration at relatively high and low fluid/rock ratios in the mid‐ and lower‐crust, respectively. Indicators of alteration and proxies for fluid/rock ratios (loss on ignition, Pb/Ce, Br/Cl, 40Ar/36Ar96Ma) vary systematically with distance from major fault zones in the lower crust with the strongest correlation in the least altered minerals. Together with age constraints derived from K‐Ar decay, these data indicate faults were major conduits, supplying seawater to the lower crust roughly contemporaneous with the formation of the ophiolite, and commencing at the magmatic stage. These results suggest that hydrothermal alteration of the lower crust is primarily controlled by major fault structures. Plain Language Summary: After a new oceanic crust is formed by volcanic activity at mid‐ocean ridges, it cools and reacts with circulating seawater. However, there is controversy over whether water penetrates to the lower oceanic crust, or whether this is cooled by conduction alone. This is important for theories of how the lower oceanic crust is formed. Investigating the lower crust is difficult since few samples have been recovered by deep seafloor drilling. However, ophiolites–sections of oceanic crust, uplifted onto continental margins during tectonic collisions–offer a potential window into processes in the deep oceanic crust. In this study, we use halogens and noble gases as chemical fingerprints for the origin of water in rocks from the Oman Ophiolite. We show that rocks from the mid‐oceanic crust and from the lower‐oceanic crust interacted with water under markedly different conditions and that the supply of water to the lower crust was very variable. Rocks close to large fractures cutting through the lower crust were altered by seawater ∼96 Myr ago. Further from these fractures, compositions are increasingly different from seawater and water was scarce. This provides important evidence that large fractures delivered water to the lower crust and may be important for cooling new oceanic crust globally. Key Points: Gabbros from the Oman Ophiolite record hydrothermal circulation in both the upper and lower oceanic crustHalogen and noble gas systematics suggest alteration under high and low fluid/rock ratiosFluid supply to the lower crust is variable and controlled by major fault zones commencing at the magmatic stage [ABSTRACT FROM AUTHOR]