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We found that the particle RA-QD02-0115 returned by the Hayabusa spacecraft from near-Earth asteroid 25143 Itokawa contains the iron carbide haxonite (Fe 21.9-22.7 Co 0.2-0.3 Ni 0.2-0.8 )C 6 and several Fe,Ni alloys, including multi-domain tetrataenite and spinodally decomposed taenite. Ellipsoidal to nearly spherical voids occur throughout the particle and suggest the presence of a fluid phase during textural and chemical equilibration of the host rock within the parent asteroid of 25143 Itokawa. The calculated solubility of carbon in Fe,Ni metal indicates that the carbide formed at temperatures larger than 600 °C during thermal metamorphism of the LL-chondritic mineral assemblage. Haxonite formed metastably with respect to graphite and cohenite, probably due to its high degree of lattice match with neighboring taenite, a low cooling rate at peak metamorphic temperatures, and the hindered nucleation of graphite. Thermodynamic equilibrium calculations indicate that the fluid present was dry (H 2 O-poor) and dominated by methane. The reactive fluid most plausibly had an atomic H/C ratio of 4–5 and was derived from the reduction of macromolecular, insoluble organic matter (IOM) that initially co-accreted with water ice. The initial presence of water is a necessary assumption to provide sufficient hydrogen for the formation of methane from hydrolyzed IOM. Metallic iron was in turn partially oxidized and incorporated into the ferromagnesian silicates during the high-temperature stage of metamorphism. An exemplary bulk reaction from unequilibrated material on the left to an equilibrated assemblage on the right may be written as: 330 CH 0.8 O 0.2(IOM)  + 500 H 2 O (ice/g)  + 681 Fe (in alloy)  + 566 FeSiO 3(in Opx)  → 300 CH 4(g)  + 32 H 2(g)  + 5 Fe 23 C 6(in Hx)  + 566 Fe 2 SiO 4(in Ol) (Opx = orthopyroxene, Hx = haxonite, Ol = olivine, g = fluid species). The best estimate of the fluid/rock ratio in the region of the LL parent body where RA-QD02-0115 formed is about 3 × 10 −3 and corresponds to an initial ice/rock ratio of about 7 × 10 −3 (both by mass). [ABSTRACT FROM AUTHOR]

The concept of index mineral based metamorphic zones was first introduced by George Barrow in 1912 and the Barrovian metamorphic zones continue to be used as a framework for describing regional metamorphism. Pressure, temperature and protolith composition are widely recognized as primary controls on index mineral distribution. Today, metamorphic fluid flow is also recognized as an important driver of metamorphic reactions. The aim of this study is to establish if and how metamorphic fluids control index mineral distribution during Barrovian metamorphism. We use samples from Barrow's type locality in Glen Esk, SE Scottish Highlands, to study possible relationships between veining and index mineral distribution. In addition to petrographic and textural observations, we use whole-rock compositions, mineral compositions and oxygen isotope analyses. At low grade, in the chlorite zone and most of the biotite zone, no correlation between veining and index mineral distribution is seen. At higher grade, in the garnet and staurolite zones, index mineral abundance is shown to be higher adjacent to veins. These trends coincide with other mineralogical, chemical and isotopic changes in the vein-proximal rock, indicative of fluid-rock interaction. Kyanite distribution is homogeneous in the kyanite zone. However, we show that this too relates to extensive fluid-rock interaction. Garnet-, staurolite and kyanite-bearing selvedges are common in the sillimanite zone. However, sillimanite distribution is unrelated to these selvedges, which supports models arguing that sillimanite formed during a separate metamorphic event. We infer fluid flow from high grade to low grade because the fluid was out of isotopic equilibrium with the lower grade rocks, but in equilibrium with the higher grade rocks. We conclude that fluid flow played a major role in the stabilization and distribution of Barrovian index minerals in Glen Esk, and that the importance of fluid flow was greater at higher metamorphic grades. [ABSTRACT FROM AUTHOR]

Geofluids (2010) 10, 58–72 Chloride-rich brines are increasingly recognized as playing an important role in high pressure and temperature metamorphic and magmatic systems. The origins of these saline multicomponent fluids are debated, but experimental evidence suggests that regardless of their origin they must be important agents of rock alteration and mass transfer wherever they occur. Studies of the solubility of quartz in H2O, CO2–H2O and salt–H2O solutions provide a framework for understanding the role of brines in the deep crust and upper mantle. While quartz solubility in the system SiO2–H2O–NaCl–CO2 is maximal at a given high pressure and temperature if the solvent is pure H2O, the decline in quartz solubility with NaCl content (salting-out) is less severe than in CO2–H2O fluids at comparable H2O activities. Moreover, at lower pressures, quartz solubility initially salts in at low salt contents, before reaching a maximum and then declining. The behavior of quartz solubility in salt–H2O solutions has not yet been fully explained and is the subject of active debate. Experimental investigations of the solubility of some other rock-forming oxides and silicates show enhancements due to NaCl addition. As illustrated by the well-studied CaO–Al2O3–SiO2–NaCl–H2O system, enhancements initially increase to maxima, and then decline. This behavior can be explained by formation of a range of hydrated aqueous complexes and clusters with specific NaCl:H2O stoichiometries. In contrast, solubilities of calcium salts, including calcite, fluorite, fluorapatite and anhydrite, rise monotonically with increasing NaCl, implying complexing to form anhydrous ionic solutes and/or ion pairs. The experimental studies offer new insights into fluid-rock interaction in a range of settings, including carbonatite–fenite complexes, granulite-facies metamorphism, porphyry ore deposits and aluminum-silicate vein complexes in high-grade metamorphic terranes. [ABSTRACT FROM AUTHOR]

The production of large volumes of fluid from metabasic rocks, particularly in greenstone terranes heated across the greenschist–amphibolite facies transition, is widely accepted yet poorly characterized. The presence of carbonate minerals in such rocks, commonly as a consequence of sea-floor alteration, has a strong influence, via fluid-rock buffering, on the mineral equilibria evolution and fluid composition. Mineral equilibria modelling of metabasic rocks in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O (NCaFMASCH) is used to constrain the stability of common metabasic assemblages. Calculated buffering paths on T– XCO2 pseudosections, illustrate the evolution of greenstone terranes during heating across the greenschist-amphibolite transition. The calculated paths constrain the volume and the composition of fluid produced by devolatilization and buffering. The calculated amount and composition of fluid produced are shown to vary depending on P– T conditions, the proportion of carbonate minerals and the XCO2 of the rocks prior to prograde metamorphism. In rocks with an initially low proportion of carbonate minerals, the greenschist to amphibolite facies transition is the primary period of fluid production, producing fluid with a low XCO2. Rocks with greater initial proportions of carbonate minerals experience a second fluid production event at temperatures above the greenschist to amphibolite facies transition, producing a more CO2-rich fluid ( XCO2 = 0.2–0.3). Rocks may achieve these higher proportions of carbonate minerals either via more extensive seafloor alteration or via infiltration of fluids. Fluid produced via devolatilization of rocks at deeper crustal levels may infiltrate and react with overlying lower temperature rocks, resulting in external buffering of those rocks to higher XCO2 and proportions of carbonate minerals. Subsequent heating and devolatilization of these overlying rocks results in buffering paths that produce large proportions of fluid at XCO2 = 0.2–0.3. The production of fluid of this composition is of importance to models of gold transport in Archean greenstone gold deposits occurring within extensive fluid alteration haloes, as these haloes represent the influx of fluid of XCO2 = 0.2–0.3 into the upper crust. [ABSTRACT FROM AUTHOR]

The Tharsis massive sulphide deposit, one of the major VMS-type deposits in the Iberian pyrite belt (IPB) was severely deformed by the Variscan tectono-thermal events. The question of whether or not these events affected the metal distribution in the deposit has been addressed by simultaneously studying the mineral parageneses (Tharsis stockwork) and the fluid circulation (at local and regional scales). The results are:(1)The early paragenesis in the stockwork (Q1 quartz–pyrite–chlorite–phengite±cobaltite±ankerite) was strongly overprinted by a late post-kinematic mineral deposition, including new quartz veins (Q3 quartz) and base metal sulphides (chalcopyrite, sphalerite, Bi and Te minerals, pyrite and galena) and gold.(2)At a regional scale, fluids accompanying the peak metamorphism conditions (ca. 300 MPa, ca. 300 °C) were of C–O–H–N–NaCl type, CO2-dominated with CH4 and N2, and are considered to be “metamorphic” on the basis of microthermometry and geochemistry. The late- to post-kinematic evolution (“retrograde” stage) was characterised by a pressure drop, down to 40 MPa (lithostatic to hydrostatic transition), and a heat input leading to temperatures ≥430 °C, then decreasing to temperature around 170 °C. Fluids of the “retrograde” type exhibit both dilution of the C–O–H–N–NaCl fluid by a low salinity “meteoric” water and progressive loss of volatile components.(3)Fluids of the retrograde type pervasively percolated through the Tharsis stockwork and were responsible for the strong mineral overprint on the early (deformed) paragenesis. All the measurable fluid inclusions (f.i.) record these late fluids. There are primary fluid inclusions in Q1, but they are systematically imploded due to the external overpressure generated by the Variscan tectonic events. Although base metal distribution in the stockwork is basically the result of the “retrograde” fluid circulation, it remains unclear whether these metals were newly introduced into the stockwork from elsewhere or simply redistributed from the existing primary assemblages in the massive sulphide bodies. [Copyright &y& Elsevier]

AbstractA combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well-preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N2-bearing NaCl-rich solutions, whereas it changed into CO2-dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low-salinity fluids were involved. In situ UV-laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ18OVSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB-like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra-high-pressure (UHP) eclogites in the Dabie-Sulu area. However, the age-corrected initial εNd(t) is - 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N2 content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism. [ABSTRACT FROM AUTHOR]

Fluid inclusion salinities from quartz veins in the Otago Schist, New Zealand, range from 1.0 to 7.3 wt% NaCl eq. in the Torlesse terrane, and from 0.4 to 3.1 wt% NaCl eq. in the Caples terrane. Homogenization temperatures from these inclusions range from 124 to 350 °C, with modal values for individual samples ranging from 163 to 229 °C, but coexisting, low-salinity inclusions exhibiting metastable ice melting show a narrower range of T h from 86 to 170 °C with modes from 116 to 141 °C. These data have been used in conjunction with chlorite chemistry to suggest trapping conditions of ≈350–400 °C and 4.1–6.0 kbar for inclusions showing metastable melting from lower greenschist facies rocks, with the densities of many other inclusions reset at lower pressures during exhumation of the schist. The fluid inclusion salinities and Br/Cl ratios from veins from the Torlesse terrane are comparable to those of modern sea-water, and this suggests direct derivation of the vein fluid from the original sedimentary pore fluid. Some modification of the fluid may have taken place as a result of interaction with halogen-bearing minerals and dehydration and hydration reactions. The salinity of fluids in the Caples terrane is uniformly lower than that of modern sea-water, and this is interpreted as a result of the dilution of the pore fluid by dehydration of clays and zeolites. The contrast between the two terranes may be a result of the original sedimentary provenance, as the Torlesse terrane consists mainly of quartzofeldspathic sediments, whilst the Caples terrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydration during diagenesis, and hence may provide more fluid via dehydration at higher grades. [ABSTRACT FROM AUTHOR]

Muscovite and biotite dehydration-melting reactions near the peak of metamorphism played a significant role in the reaction and fluid history of the Rangeley Formation in southwestern New Hampshire, USA. Evidence for in situ melting includes: (1) the consistency among theoretical phase equilibria, observed reaction textures, and the inferred P–T conditions; (2) disseminated, centimeter-scale, leucocratic quartz + plagioclase + muscovite pods; (3) diffusion and growth zoning of major and trace elements in garnet that are best explained as the result of high-T muscovite and biotite breakdown; and (4) oxygen isotope evidence that high-T back-reaction of K-feldspar to muscovite near peak metamorphic conditions did not involve an isotopically disequilibrium (externally derived) fluid. Isotopically equilibrated fluids were apparently stored in melt pockets and then reused as the melts crystallized, thereby driving retrogression. Prograde muscovite dehydration-melting reactions further imply P≥4 kbar at T≤ ∼650°C, so that loading occurred before the peak of metamorphism at T ∼725°C. Oxygen isotope compositions of retrograde garnet that grew during cooling between T ∼650°C and T ∼550°C are consistent with closed-system models, indicating that previous back-reaction of K–feldspar to muscovite did not disturb the isotope compositions of the rocks. Late-stage growth of additional retrograde garnet, staurolite, and chlorite at T ∼475°C requires infiltration of externally derived H2O, but this retrograde infiltration did not affect garnet and staurolite isotope compositions, as expected for differing rates of infiltration-driven hydration vs isotope alteration. Late-stage infiltration continued after garnet and staurolite growth ceased, as evidenced by systematic differences in isotope trends near the base of the nappe for minerals with fast oxygen isotope diffusion rates (quartz, muscovite, and biotite) vs minerals with slow diffusion rates (garnet, staurolite, and sillimanite). This infiltration may reflect the dewatering of structurally lower levels after nappe emplacement. If so, then nappe emplacement occurred at T ∼475°C. [ABSTRACT FROM AUTHOR]