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To study the kinetics of the spinel-to-garnet transformation in peridotite, we conducted reaction experiments in the garnet peridotite stability field (3.2 GPa, 1020-1220 °C, for 0.6-30 h) using a single spinel crystal embedded in monomineralic orthopyroxene powder or in a mixture of powdered orthopyroxene and clinopyroxene. The growth textures observed in the reaction rim between the spinel crystal and the polycrystalline pyroxenes show that the reaction rim grew in both the spinel and pyroxenes directions, suggesting mobility of both SiO2 and R2O3 components (where R is a trivalent cation). Olivine grains formed only in the presence of monomineralic orthopyroxene and were present in some domains without forming reaction rims. Based on a diffusion-controlled growth model, the growth kinetics of the garnet reaction rim can be described by [x(t)]2 = k0 exp(-H*/RT)t, where x(t) is the rim width at time t, R is the gas constant, T is the absolute temperature, and H* is the activation enthalpy of reaction; k0 and H* are, respectively, k0 = 10-19.8 ± 4.9 m2/s and H* = 171 ± 58 kJ/mol. The development of a garnet reaction rim around a spinel core has been observed in alpine-type peridotitic rocks and mantle xenoliths. The reaction rims experimentally produced in this study are characteristic of corona textures observed in natural rocks, and the experimentally measured growth rate of the rims places important constraints on dynamic transformation processes involving spinel and garnet in peridotite. However, to reconstruct the P-T-t history of the corona texture based on these elementary processes, additional detailed studies on the textural evolution and quantitative kinetics of the garnet-rim growth stage are required.

Reaction rims contain a wealth of information that can be used to decipher the P-T-t-X history of metamorphic and metasomatic rocks. One of the most important parameters that controls reaction rim growth is the presence of volatiles, which can affect rim thicknesses, phase stabilities and the development of rim microstructures. In this study, reaction rim growth experiments were performed between periclase and quartz at anhydrous to water-saturated conditions at 3–4 kbar and 1100–1300 °C. Controlled minute amounts of water were added through OH-doped periclase, which enabled us to perform experiments at controlled water-undersaturated conditions. At anhydrous conditions, no reaction rim formed at all implying that water acts as a catalyst, and a minimum fluid threshold is needed to initiate metamorphic reactions. At water-undersaturated conditions extremely small variations in water content are sufficient to change reaction rim growth rates by multiple orders of magnitude. This implies that reaction rims have the potential to monitor variations in the amount of water at those grain boundaries that serve as fast pathways for component transport at water-undersaturated conditions during metamorphic and metasomatic reactions in natural systems, allowing them to be used as sensitive “geohygrometers”. Additionally, the effect of water on relative layer thicknesses may provide an application for reaction rim microstructures to be used as new physico-chemical gauges that will allow us to discriminate between water-undersaturated and water-saturated conditions during metamorphic events.

Au-Hg-Ag phases have been described from a variety of metallogenic orebodies and the placer deposits derived from them. In many documented placer deposits, the phases typically occur intergrown as 'secondary' rims to primary Au-Ag grains. The origin of these rims has been ascribed to supergene redistribution reactions during deposition or to the effects of amalgamation (i.e. use of mercury) during mining for gold. Difficulties in determining compositions and crystal structures on such a small scale have made full characterisation of these phases problematic. This paper describes a new occurrence of these phases, found by accident during investigation of a historical concentrate of 'osmiridium' containing a number of gold grains from beach sands at Waratah Bay, in southern Victoria, Australia. The phases occur as rims to gold grains and are intergrown on a scale of tens of micrometres or less. Application of electron microprobe analysis (EPMA) and limited electron back-scattered diffraction (EBSD) was required to characterise them. These techniques revealed the presence of the approved mineral weishanite (Au-Hg-Ag) and a phase with compositional range Au2Hg-Au3Hg surrounding primary Au-Ag (electrum) containing trace amounts of Hg. EBSD analysis showed weishanite is hexagonal P63/mmc and Au2Hg to be hexagonal P63/mcm. Comparison with published data from other localities (Philippines, British Columbia and New Zealand) suggests weishanite has a wide compositional field. Textures shown by these phases are difficult to interpret, as they might form by either supergene processes or by reaction with anthropogenic mercury used during mining. However, in the absence of any historical evidence for the use of mercury for gold mining at Waratah Bay, we consider the formation of the Au-Hg phases is most probably due to supergene alteration of primary Au-Ag alloy containing small amounts of Hg. In addition to revealing some of the reaction sequences in the development of these secondary Au-Hg-Ag rims, this paper illustrates methods by which these phases can be more fully characterised and thereby better correlated with the Au-Hg synthetic system.

The Sin Quyen deposit in northwestern Vietnam is composed of Fe-Cu-LREE-Au ore bodies hosted in Proterozoic metapelite. There are massive and banded replacement ores with variable amounts of monazite-(Ce) and chevkinite-(Ce) crystals, which have undergone fluid-induced alteration. Monazite-(Ce) and chevkinite-(Ce) were deposited from high-temperature fluids in the early ore-forming stage, but became thermodynamically unstable, and thus were altered to other phases in later ore-forming stages. The alteration of monazite-(Ce) formed a three-layered corona texture, which commonly has relict monazite-(Ce) in the core, newly formed fluorapatite in the mantle, and newly formed allanite-(Ce) in the rim. In some cases, the original monazite-(Ce) was completely consumed, forming a core of polygonal fluorapatite crystals rimmed by allanite-(Ce) crystals. The formation of allanite-(Ce) and fluorapatite at the expense of monazite-(Ce) indicates that the later-stage fluids had high Ca/Na ratios and relatively low temperatures. Chevkinite-(Ce) was variably replaced by an assemblage of allanite-(Ce) + aeschynite-(Ce) ± bastnasite-(Ce) ± columbite-(Fe) ± ilmenite. The replacement of chevkinite-(Ce) by mainly allanite-(Ce) and aeschynite-(Ce) required low-temperature, Ca-, LREE-, and Nb-rich metasomatic fluids, probably with relatively low fO2 Mass-balance calculations were made to investigate the hydrothermal element mobility. It is assumed that Th was immobile during the alteration process of monazite-(Ce). Light (and middle) REE from La to Tb, U, As, and Ge were variably lost relative to Th, while heavy REE from Dy to Lu, HFSE (e.g., Nb, Ta, Zr, and Hf) and Sr were variably gained relative to Th. Regarding the alteration of chevkinite-(Ce), some major elements in chevkinite-(Ce), such as Ti, La, and Ce, were obviously removed from the system during alteration, whereas Ca, Al, Nb, U, and HREE were needed to be variably supplied by the metasomatic fluids. Concerning the hydrothermal mobility of trace elements, previous studies demonstrated that REE and HFSE can be commonly reserved in the system during alteration, consistent with the traditionally assumed immobile nature of these elements. In contrast, this study shows that REE and HFSE can be mobilized on at least the hundreds of micrometers scale. This may be related to the high flux and strong chemical reactivity of the metasomatic fluids.

Mafic to felsic adakitic intrusive and extrusive rocks in the Torud-Ahmad Abad magmatic belt occur south-southeast of Shahrood (north of the Central Iran Zone). These adakites, in the form of dykes and other hypabyssal igneous bodies, are emplaced into late Neoproterozoic amphibolite and mylonitized granites and a thick sequence of Paleocene to middle Eocene volcanic and volcano-sedimentary rocks. These high silica and low silica adakites (HSA and LSA) span a range of lithologies including basaltic andesite, andesite, trachyandesite, dacite, trachydacite, and dacite. The adakites are composed mainly of calcic pyroxene, Ca-Na amphibole, and plagioclase phenocrysts, with minor biotite and titanomagnetite. In these intrusions, iron-titanium oxides crystallized late (within the groundmass) or occur as secondary phases. One of the interesting features of these rocks is the opacitization (magnetite rich selvage) of ferromagnesian phenocrysts, such as hornblende, which is a localized replacement reaction. The intensity and color of these opacitized margins depend on the extent of titanomagnetite-magnetite formed. The SEM-EDS analysis results show that most of magnetite formed at temperatures >500°C related to pressure quenching of the melt due to emplacement and differential volcanic degassing. The phenomenon of opacitization is due to decreasing stability of Fe2+ and hydrous ferromagnesian phenocrysts, such as amphiboles, to form less hydrous to anhydrous pseudomorphs (selvage) in the near-surface environment with oxidative reactions linked to pressure quenching and differential devolatilization of H2 from the melt during hypabyssal emplacement. The rapid decrease in pressure during magma ascent causes hornblende instability and thus helps to create these opaque rims (opacitized) on ferromagnesian phases (magnetite and titanomagnetite-rich assemblage). Hornblende breakdown - destabilization increases due to melt degassing - devolatilization [decrease in P(H2O)] during the process of ascent-emplacement with reduction of magmatic total pressure (decompression) and/or melt oxidation due to differential degassing of H2.

Solid-solid mineral reaction rates are influenced by the microfabrics of reactant phases and concurrent deformation. To investigate this interplay in carbonate systems, we performed annealing and deformation experiments on polycrystalline and single-crystal calcite and magnesite, forming dolomite (Dol) and magnesio-calcite (Mg-Cal). At a fixed temperature of T=750°C and confining pressure of P=400 MPa, samples were either annealed for 29 h, or deformed in triaxial compression or torsion for 18 h using a Paterson-type gas deformation apparatus. At the contact interface of the starting reactants, Dol reaction rims and polycrystalline Mg-Cal layers were formed. The widths of the layers were in the ranges 4-117 and 30-147 µm, respectively, depending on the microstructure of starting materials and experimental conditions. Annealing experiments with polycrystalline reactants in contact with each other resulted in a ∼22-fold increase in Dol rim thickness compared to a contact between two single crystals and a larger Mg-Cal layer width by a factor of 5 (cf. Helpa et al. 2014). This suggests that the microstructure of magnesite controls migration of the reaction front. For polycrystalline starting materials, axial stress accelerated Mg-Cal growth rates but not Dol growth rates. Highly strained torsion samples showed Dol formation along grain boundaries in Mg-Cal as well as in the polycrystalline calcite reactant. A reduction of Dol rim thickness between polycrystalline reactants deformed in torsion is possibly caused by concurrent grain coarsening of polycrystalline magnesite. Dol and Mg-Cal growth kinetics between single crystals were unaffected by the addition of ∼0.3 wt% water. The experiments demonstrate that Dol reaction kinetics strongly correlate with magnesite reactant grain sizes, while Mg-Cal growth depends on the calcite reactant grain sizes. The dolomite-forming mineral reaction kinetics are not significantly affected by concurrent deformation. In contrast, deformation enhances Mg-Cal formation, especially at small calcite grain sizes that promote efficient grain boundary diffusion. Therefore, the fastest reactions forming Dol and Mg-Cal in nature are expected to occur in very fine-grained reactants. Concurrent deformation may drastically enhance reaction kinetics if grain size reduction of the reactants occurs by, for example, cataclasis or dynamic recrystallization.

A mafic amphibole-bearing granulite with porphyroblastic garnet was investigated to evaluate: (1) the rare earth element (REE) partition among garnet, zircon, orthopyroxene and amphibole during the metamorphic evolution; (2) the significance of the REE distribution along lobes and bights of reabsorbed garnet rim; and (3) REE distribution coefficient values (DREE) suggestive of chemical equilibrium, assuming garnet as a reference. The results have been compared with those deriving from an intermediate granulite containing porphyroblastic garnet, without amphibole. Porphyroblastic garnet from both samples is rimmed by a continuous corona formed during post-peak decompression characterized by REE-enriched lobes and REE-poor bights. The amphiboles from corona have various REE abundances, reflecting a different dissolution rate of original garnet rim. The initial slow rate of garnet dissolution caused high REE concentration in the new garnet rim due to intra-crystalline diffusion, leading to the formation of REE-poorer amphiboles in corona. Subsequently, under an increasing geothermal gradient and fluid-present conditions, the faster dissolution of garnet determined the formation of bights and the transfer of REEs towards the corona. The timing of garnet growth and its dissolution were checked by U–Pb zircon ages. The zircons dated from 339 Ma to 303 Ma in two rock types combined with the garnet domains (core, outer core, rim) show similar distribution of patterns relative to heavy rare earth elements for zircon and garnet (DHREEzrn/grt), suggesting chemical equilibrium. Zircons dated at c. 300 Ma do not appear in equilibrium with REE-rich garnet lobes, and younger zircons (278 Ma) show a new equilibrium with REE-poor garnet bights. On this basis, the DHREEamph/grt values obtained in specific textural sites might be interpreted as suggestive of equilibrium under granulite conditions.

Petrographic and whole-rock geochemical study of biotite-muscovite gneiss was determined in order to interpret the metamorphic evolution of the Basement Complex of Southwestern, Nigeria. The gneiss shows a millimetric banding, and in some cases the quartzo-feldspathic bands running up to 10 cm. The gneiss has mineral assemblage biotite + plagioclase + quartz + garnet + K-feldspar + muscovite + chlorite + ilmenite ±titanite. Chlorite occurs along cleavage planes of biotite, and in some cases forms reaction rims around porphyroblasts of garnet. K-feldspar crystals are surrounded by muscovite. Titanite crystals are sub-idioblastic to xenoblastic in form, and have inclusions of ilmenite. Titanite, where present, occurs in close association with biotite and opaque minerals (ilmenite). Also, titanite forms a reaction rim around apatite. Mylonitic texture, fine-grained matrix of mica and quartz ribbons were observed. In addition, there is stretching of the quartz crystals. The SiO 2 content is greater than 60 wt %, while CaO ranges from 3.05-6.91 wt %. The M 1 foliation comprise of mineral biotite some of which are included in the opaque mineral, M 2 represents the metamorphism which gave rise to porphyroblasts of ilmenite, while the M 3 gave rise to foliations that forms a wraparound structure on the porphyroblasts of ilmenite. The last metamorphism gave rise to retrograde minerals; chlorite, titanite, and muscovite. The study suggests that this area of the Basement Complex has been subjected to multiple deformations, as well as multiple episodes of metamorphism. The structures observed are similar to those associated with shear zone environment.

Highly restitic garnet-kyanite-phlogopite metapelitic schists from the Goshen Dome of western Massachusetts contain: a population of prograde monocrystalline, megacrystic garnet, some with significant P in substitution for Si; precipitates of hydroxylapatite and rutile; and <1 µm zircon crystals of undetermined origin and abundance on the order of 105/mm3. The unusual P content and the abundant internal precipitate suite are similar to features reported in garnet from ultrahigh-pressure (UHP) and mantle settings, suggesting a potential (U)HP origin for the garnet megacrysts. Zircon included in megacrysts is surrounded by radial fractures, indicating in situ volumetric expansion or new growth. Cores display rare earth element (REE) profiles and cathodoluminescence (CL) zoning consistent with magmatic growth, and yield only Paleozoic dates (447-404 Ma). The embayed core-rim boundary is marked by a several micrometers wide band of CL-dark zircon enriched in Y, P, U, and Th that is interpreted as the accumulation of redistributed xenotime component from the original zircon rim during metamorphism. Outside of this band, the rim has elevated Hf, Th/U much less than 1, and steep heavy REE profiles. The metamorphic rims yield concordant dates from 400 to 381 Ma. Matrix zircon grains have magmatic cores (1726-415 Ma) with similar core-rim boundaries enriched in Y, P, U, and Th. Metamorphic rims on matrix zircon yield slightly younger dates (393-365 Ma) and are compositionally heterogeneous. The difference between the youngest core and oldest rim indicates a short interval (ca. 4 Ma) between deposition of detrital zircon and the onset of metamorphism in the earliest Acadian. The oldest zircon rim dates are found within phosphatic garnet megacrysts of possible very high-pressure origin. The compositional uniformity of these rims indicates equilibrium with a single source; the anomalous composition suggests a combination of dissolution-reprecipitation and new growth of zircon that is derived from garnet. The range in both composition and dates indicates that matrix zircon rims formed in response to local changes in mineralogy and fluid/melt composition and/or availability. New growth of zircon on these grains cannot be confirmed, suggesting that dissolution-reprecipitation reactions during continued metamorphism may be the dominant mechanism that formed these rims. The data collectively suggest that dissolution-reprecipitation may be a common mechanism for producing metamorphic rims on zircon that does not require additional Zr and Hf, which are limited within most metamorphic settings.

Amphibole + phlogopite + diopside bearing veins are observed in a large number of upper mantle xenoliths, but the composition of the melt that forms them is poorly constrained. Recent data from the Heldburg Phonolite, Central Germany, has shown that phonolite melt will react with olivine and orthopyroxene xenocrysts to form reaction rims of amphibole + phlogopite + diopside at mid-lower crustal pressures. This is the first example of where a melt has reacted with peridotite to form the mineralogy of the metasomatic veins. It is therefore necessary to explore whether a phonolite melt could be the parent melt that forms amphibole + phlogopite + diopside metasomatic veins. Experimental reactions between single crystals of olivine and orthopyroxene with phonolite melt were conducted at upper mantle conditions of 1.0–1.5 GPa and 900–1,000 °C. Melt water contents were varied from anhydrous to >12 wt. H 2 O. Olivine reacts to form phlogopite reaction rims with overgrowths of diopside <1,000 °C or rims of secondary olivine >1,000 °C. Orthopyroxene reacts to form amphibole with epitaxial diopside overgrowths <1,000 °C. No reaction rims form when the bulk melt H 2 O is lower than ~3.8 wt%. Pressure has little effect over the small range tested. These experiments reproduce reaction rims on olivine and orthopyroxene observed in the Heldburg Phonolite, Central Germany, and suggest that a relatively narrow range of temperatures and melt water contents is required for rim formation. The compositions of rim amphibole, phlogopite and diopside from the experiments have very similar compositions to those from Heldburg but do not match those from metasomatic veins. Phenocrysts from Heldburg are similar to the metasomatic veins, suggesting that a phonolite could potentially form the veins if vein formation is dominated by crystallization rather than reaction and replacement of wall rock phases.