Stephen J. Barnes

The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions

A series of calculations have been carried out to evaluate the effect on cumulus mineral compositions of solidification of trapped intercumulus liquid in orthocumulates. The calculation assumes local equilibrium between phases, and that the system remains chemically closed during crystallization of the trapped liquid. The latter assumption is held to be valid on a scale of tens to hundreds of centimeters. It is not necessary to know the composition of the trapped liquid, as the calculation only requires an estimate of FeO content and trapping temperature.The change in composition of a mineral from that of the initially precipitated cumulus crystals to the final composition after complete solidification is termed the “trapped liquid shift”. Its magnitude depends on the modal proportions of cumulus phases and the initial porosity, and is only weakly dependent on initial phase compositions. Trapped liquid shifts are significant when compared with mineral composition changes occurring during fractional crystallization. Crystallization of 30% trapped liquid gives rise to shifts of up to 10 mol. percent in Mg number of olivine or pyroxene. The size of the shift becomes greater when the initial cumulus assemblage has a lower total FeO+MgO content, and vice versa.As a result of the relationship between trapped liquid shift and cumulus mode, mineral composition variations and trends may be generated in sequences of cumulates which originally had constant compositions of cumulus minerals. For example, in a cyclic unit grading from a pyroxenitic base to an anorthositic top, crystallization of a uniform proportion of trapped liquid will result in an apparent iron enrichment trend from bottom to top of the cycle, as has been observed in the Upper Critical Zone of the Bushveld Complex.

The distribution of chromium among orthopyroxene, spinel and silicate liquid at atmospheric pressure

A series of experiments has been carried out in which a synthetic silicate melt, of composition equivalent to a “U-type” Bushveld Complex parent liquid, was equilibrated with bronzitic orthopyroxene and chromite spinel between 1334 and 1151°C over a range of oxygen fugacities between the nickel-nickel oxide and iron-wustite buffers. The partition coefficient for Cr between bronzite and melt increases with falling temperature along a given oxygen buffer, and decreases with falling oxygen fugacity at a given temperature, showing an overall range from 1.1 to 11.7. The Cr content of the melt in equilibrium with spinel (Cr solubility) decreases with falling temperature and increases with lower oxygen fugacity. This variation may be quantified in terms of temperature-dependent solubility of Cr3+ combined with a changing ratio of Cr3+ to Cr2+ in the melt. Iso-oxidation curves for Cr are approximately parallel to Fe-Si-O buffer curves and to curves for equal Fe3+/Fe2+ determined by Hill and Roeder (1974), and agree within experimental error with the results of Schreiber and Haskin (1976). When the changing oxidation state of Cr is allowed for, the orthopyroxene partition coefficient may be expressed as the sum of the temperature-dependent coefficient for Cr3+ and a partition coefficient of about 1 for Cr2+. The experimental results are used to construct a series of model curves for liquid and bronzite compositional variations during fractional crystallization of a Bushveld parent liquid. Trends for Cr variation are shown to depend critically upon oxygen fugacity, and on whether the liquid is saturated with chromite. The position of the peritectic between chromite and orthopyroxene is shown to be very sensitive to oxygen fugacity within one and a half log units of the QFM buffer. This observation may explain the contrasting distribution of chromite seams in the Bushveld Complex, where chromite occurs within bronzitite-norite sequences, and in the Stillwater Complex in which chromite is restricted to olivine cumulate layers.

Partitioning of the platinum group elements and gold between silicate and sulphide magmas in the Munni Munni Complex, Western Australia

Abstract The Munni Munni Complex in the Archaean Pilbara Block contains PGE mineralization in the form of disseminated sulphides within pyroxenite. The uppermost sulphide-bearing layer shows a characteristic vertical distribution of PGE and Au. Peak values for Pd, Pt, and the other PGEs are systematically offset by up to 5 metres below the horizon of maximum S, Ni, and Cu abundance. The Main Sulphide Zone of the Great Dyke of Zimbabwe, which occurs in an identical stratigraphic setting, shows very similar features. This distribution of PGEs with respect to the sulphide peak cannot be explained in terms of postcumulus mobilisation from an initially homogenous sulphide layer, for a number of reasons: the offset profiles are remarkably consistent along several km of strike; remobilization would be expected to affect Pd and Au more than the other PGEs; and underlying sulphide concentrations show coincident trends for all the PGE, Au, Cu, Ni, and S. The offset between PGE and Ni-Cu-S peaks has been interpreted by workers on the Great Dyke as the result of fractional sulphide segregation, combined with different values for sulphide liquid-silicate melt partition coefficient (SSD) for the various PGEs, Au, Ni, and Cu. A series of model calculations has been performed in an attempt to apply this interpretation to the Munni Munni case, with the object of making quantitative estimates of SSD values for the PGEs and Au. Results in general support the conclusions of Great Dyke studies that apparent SSDs decrease in the order Ir = Ru = Rh = Os ≥ Pd > Pt > Au > Cu = Ni. However, it proves impossible to duplicate some highly regular and persistent features of the Munni Munni offset sulphide layer with any model that assumes constant values for these SSDs. The inadequacy of the fractional sulphide segregation model implies that sulphide liquid is not the sole collector of the PGEs. The observed behaviour of the PGEs requires a sudden increase in the apparent affinity of the PGEs for the sulphide fraction at the base of the main sulphide peak, unattributable to any significant change in parent magma composition or crystallization conditions. Natural profiles can be modelled successfully by describing this apparent affinity of PGE for the sulphide fraction in terms of widely variable “virtual” partition coefficients. This behaviour can be explained if the silicate melt, and consequently the co-existing sulphide melt, is saturated with respect to solid PGE phases. The association of PGE with sulphides is necessary because of the extremely low concentrations of PGE in the silicate melt, which makes it kinetically impossible to nucleate solid phases even at substantial degrees of supersaturation. Pre-concentration by several orders of magnitude due to partitioning into sulphide liquid allows solid PGE phases to nucleate within sulphide droplets, giving rise to an apparently very strongly PGE-enriched sulphide fraction.

Are Bushveld U-type parent magmas boninites or contaminated komatiites?

Three distinct categories of magmas — Bushveld “U-type” parent magmas, boninites, and siliceous high magnesium basalts from Archaean greenstone belts —share the distictive geochemical characteristics of high MgO (9%–19%), low TiO2(less than 1%) and high SiO2(greater than 52%). Boninites are generally thought to form by hydrous melting of metasomatized, previously depleted upper mantle, while siliceous high magnesium basalts (SHMB) in greenstone belts have recently been recognized as the products of combined fractionation and crustal contamination of komatiites. Both these mechanisms can apparently give rise to similar end products, and both mechanisms have been proposed for the petrogenesis of Bushveld U-type magmas.A detailed comparison of the three magma types, using data drawn from the literature, shows a broad area of overlap in major elements and most trace elements. U-type magmas are generally intermediate in composition between SHMB and boninites. U-type magmas differ significantly from boninites, and are more similar to SHMB, in three important respects: their relatively high abundances of rare earth elements and degree of light rare earth enrichment; higher FeO/MgO ratio for a given MgO content; and Sm/Nd isotopic systematics indicative of crustal contamination. BU magmas are therefore more likely to be extreme examples of contaminated komatiitic parents than primary “boninitic” mantle melts. The striking similarity in major element chemistry of the three groups may be due to the near-coincidence in compositional space of the mediumpressure, hydrous olivine-orthopyroxene phase boundary, which controls the composition of boninites, with the lowpressure anhydrous phase boundary which controls differentiated SHMB and U-type magmas.

Maia X-ray fluorescence imaging: Capturing detail in complex natural samples

Motivated by the challenge of capturing complex hierarchical chemical detail in natural material from a wide range of applications, the Maia detector array and integrated realtime processor have been developed to acquire X-ray fluorescence images using X-ray Fluorescence Microscopy (XFM). Maia has been deployed initially at the XFM beamline at the Australian Synchrotron and more recently, demonstrating improvements in energy resolution, at the P06 beamline at Petra III in Germany. Maia captures fine detail in element images beyond 100 M pixels. It combines a large solid-angle annular energy-dispersive 384 detector array, stage encoder and flux counter inputs and dedicated FPGA-based real-time event processor with embedded spectral deconvolution. This enables high definition imaging and enhanced trace element sensitivity to capture complex trace element textures and place them in a detailed spatial context. Maia hardware and software methods provide per pixel correction for dwell, beam flux variation, dead-time and pileup, as well as off-line parallel processing for enhanced throughput. Methods have been developed for real-time display of deconvoluted SXRF element images, depth mapping of rare particles and the acquisition of 3D datasets for fluorescence tomography and XANES imaging using a spectral deconvolution method that tracks beam energy variation.

Archean komatiite volcanism controlled by the evolution of early continents

Significance Komatiites are rare, ultra-high-temperature (∼1,600 °C) lavas that were erupted in large volumes 3.5–1.5 bya but only very rarely since. They are the signature rock type of a hotter early Earth. However, the hottest, most extensive komatiites have a very restricted distribution in particular linear belts within preserved Archean crust. This study used a combination of different radiogenic isotopes to map the boundaries of Archean microcontinents in space and time, identifying the microplates that form the building blocks of Precambrian cratons. Isotopic mapping demonstrates that the major komatiite belts are located along these crustal boundaries. Subsequently, the evolution of the early continents controlled the location and extent of major volcanic events, crustal heat flow, and major ore deposit provinces. The generation and evolution of Earth’s continental crust has played a fundamental role in the development of the planet. Its formation modified the composition of the mantle, contributed to the establishment of the atmosphere, and led to the creation of ecological niches important for early life. Here we show that in the Archean, the formation and stabilization of continents also controlled the location, geochemistry, and volcanology of the hottest preserved lavas on Earth: komatiites. These magmas typically represent 50–30% partial melting of the mantle and subsequently record important information on the thermal and chemical evolution of the Archean–Proterozoic Earth. As a result, it is vital to constrain and understand the processes that govern their localization and emplacement. Here, we combined Lu-Hf isotopes and U-Pb geochronology to map the four-dimensional evolution of the Yilgarn Craton, Western Australia, and reveal the progressive development of an Archean microcontinent. Our results show that in the early Earth, relatively small crustal blocks, analogous to modern microplates, progressively amalgamated to form larger continental masses, and eventually the first cratons. This cratonization process drove the hottest and most voluminous komatiite eruptions to the edge of established continental blocks. The dynamic evolution of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmosphere, while also providing a fundamental control on the distribution of major magmatic ore deposits.

The role of fluids in the metamorphism of komatiites, Agnew nickel deposit, western Australia

The Agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy. Olivine + tremolite + chlorite + cummingtonite ±enstatite assemblages occur at the margin of the ultramafic lens, giving way to olivine + anthophyllite, olivine + talc and olivine + antigorite assemblages successively inwards. These rocks are interpreted as having crystallized from komatiitic lavas, and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates. The relative modal abundances of metamorphic olivine, tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith. Peak metamorphic conditions are estimated at 550° C, based on garnet-biotite thermometry, at a maximum pressure of 3 kb. This temperature falls within the narrow range over which metamorphic olivine may co-exist with enstatite, anthophyllite, talc or antigorite depending upon the fugacity of water in the metamorphic fluid. The observed mineralogical zonation is therefore attributed to infiltration by CO2-rich fluids, generated by decarbonation of talc-carbonate rocks formed during pre-metamorphic marginal alteration of the ultramafic lens. Metamorphic fluids were essentially binary mixtures of water and CO2, with minor H2S having a maximum partial pressure less than 1 percent of total pressure. Enstatite-bearing assemblages formed in the presence of CO2-rich fluids at fluid: rock volume ratios close to one, while anthophyllite, talc and antigorite bearing assemblages formed in the presence of progressively more water-rich fluids at progressively lower fluid-rock ratios.

Role of late magmatic fluids in Merensky-type platinum deposits: A discussion

A number of features of platinum group element (PGE)-rich layers in layered intrusions provide evidence for the involvement of late magmatic fluids. The presence of pegmatitic textures, intergrowth of sulfides with intercumulus volatile-rich phases, high Cl content in volatile-bearing phases, and association with graphite and fluid inclusions have been taken by some authors as evidence for postcumulus transport and deposition of PGE by a fluid phase of late magmatic derivation. We argue that these apparently hydrothermal features are the result of postcumulus processes superimposed on layers whose extensive, uniform PGE concentrations were formed by magmatic cumulus processes. The association with pegmatites and volatiles arises from the relatively low solidus temperatures of orthocumulate layers, which cause these layers to act as traps for fluids migrating through the crystal pile. These fluids are mostly derived by vapor exsolution from fractionated intercumulus melt and are consequently enriched in CI. Redissolution of these fluids into vapor-undersaturated trapped liquid in orthocumulate layers causes recrystallization and formation of gabbro pegmatites. Immiscible sulfide-oxide liquid solidifies at near-solidus temperatures in the interstitial pore space between early-crystallizing silicate minerals, and it is forced into areas occupied by late-crystallizing CI-bearing silicates. This accounts for intergrowths of Cu and PGE-rich sulfides with amphiboles and micas, a ubiquitous feature of sulfide-bearing gabbros. Further exsolution of a vapor phase during late stages of crystallization causes deuteric alteration, formation of fluid inclusions, and possible local redistribution of sulfides and PGE. Cooling of this vapor phase may lead to precipitation of graphite at or below the orthocumulate solidus. Graphite deposition may be catalyzed by sulfides and PGE.

Three-dimensional morphology of magmatic sulfides sheds light on ore formation and sulfide melt migration

The morphology of magmatic sulfides in igneous cumulates is controlled by the wetting properties of sulfide liquids against silicates. The formation of nickel sulfide ores, the behavior of sulfide liquids during mantle melting, and potentially the segregation of the Earth9s core, are all controlled by the ability of sulfide liquids to migrate through the pore space of partially molten silicates. Three-dimensional X-ray tomographic images of sulfide aggregates in komatiitic olivine cumulates indicate that sulfide liquids have a limited tendency to wet olivine crystals, forming interconnected networks only in the absence of silicate melt. Consequently, the ability of sulfide liquids to migrate through the pore space of olivine cumulates is limited. We conclude that disseminated sulfide ores in komatiites formed by accumulation of transported sulfide blebs a few millimeters in size, and not by settling of sulfide-olivine aggregates, and that sulfides accumulated in the proportions in which they are now found, rather than by percolation through cumulate pore space. It is unlikely that sulfide droplets can be entrained and carried from the mantle at low degrees of partial melting. Our results also support the hypothesis that segregation of the Earth9s core took place from a magma ocean, rather than by percolation of sulfidic melt through partially molten mantle.

Platinum ore in three dimensions : insights from high-resolution X-ray computed tomography

Platinum-group elements (referred to as PGE, comprising Pt, Pd, Rh, Ru, Ir, and Os) are strategic metals with a wide variety of industrial applications. Most of the worlds PGE production is mined from large mafic-ultramafic intrusions such as the Bushveld Complex in South Africa, which currently provides 75% of the worlds Pt production. The PGE mineralization is found within distinctive layers, tens to hundred of centimeters thick but extending laterally for many tens of kilometers, where the PGE occur at low parts per million levels as platinum-group minerals (PGM) and in solid solution within disseminated base-metal sulfides. There is still heated debate at the most fundamental level about the mode of formation of this class of deposit; genetic models range from primary magmatic sulfide collection to concentration by migrating halogen-rich fluids. A crucial line of evidence is the spatial relationship between the PGM, which are the most important PGE-bearing phases, and the base-metal sulfide aggregates or blebs. So far, all observations have been carried out using two-dimensional mineralogical studies where textural relationships with other minerals are ambiguous, and with statistical limitations owing to sampling of trace phases intersecting random surfaces. We present the first detailed three-dimensional in situ analysis of the PGM at the sample scale using high-resolution X-ray computed tomography coupled with conventional microscopic and mineralogical study. We find a striking and highly consistent relationship of PGM grains with the edges of complex-shaped magmatic sulfide blebs, and the intersection of these blebs with chromite-silicate grain boundaries. These new three-dimensional observations strongly support an orthomagmatic model coupled with nucleation and growth of PGM at the margins of sulfide liquid droplets.