Hydrous Fluid Phase Research Articles

Co–Ni decoupling indicates fluid exsolution during the formation of podiform chromitites: Insights from the Luobusa ophiolite, southern Tibet

Podiform chromitites typically occur in harzburgites near the petrological Moho in ophiolite sections, and are petrologically and economically significant. Hydrous fluids may have an important role in the formation of podiform chromitites, although unambiguous evidence for the involvement of such fluids is rare. To examine whether hydrous fluids are involved in the formation of podiform chromitites, we measured the Ni, Co, Mn, and Zn contents of olivines and chromites in representative samples from the Luobusa ophiolite, southern Tibet, taking account of the different geochemical behaviors of these elements in melts and hydrous fluids. In the harzburgites, the olivine (Ol) has a limited compositional range (2605–2919 ppm Ni, 117–135 ppm Co, 736–1005 ppm Mn, and 28.9–35.7 ppm Zn), whereas the chromite (Chr) has a relatively variable composition (718–1239 ppm Ni, 364–493 ppm Co, 1314–2733 ppm Mn, and 1411–1900 ppm Zn). From the dunite lenses to the dunite envelopes, and to the chromitites, both olivine and chromite exhibit significant decreases in Co (Ol = 137 to 45 ppm; Chr = 542 to 168 ppm), Mn (Ol = 889 to 273 ppm; Chr = 2426 to 862 ppm), and Zn (Ol = 37.7 to 4.0 ppm; Chr = 1746 to 206 ppm) contents, and an increase in Ni contents (Ol = 3009 to 5946 ppm; Chr = 413 to 1462 ppm). These features cannot be fully explained by subsolidus re-equilibration, partial melting, fractional crystallization, and melt–rock reactions. Olivine relicts in chromite indicate the dissolution of pre-existing olivine in dunitic channels. The olivine dissolution may have been due to the water-rich nature of the parental melts of the chromitites. These observations and mass-balance calculations suggest that, during the formation of podiform chromitites, chromite crystallization was accompanied by olivine dissolution and exsolution of a hydrous fluid phase. This resulted in the preferential transfer of Ni into the melt and olivine relicts, and Co, Mn, and Zn into the fluid phase. As such, the chromite and olivine in the chromitites became Ni-rich and Co-, Mn-, and Zn-poor, which led to Ni–Co decoupling.

Paleoarchean variole-bearing metabasalts from the East Pilbara Terrane formed by hydrous fluid phase exsolution and implications for Archean greenstone belt magmatic processes

Archean greenstone belts contain low grade metamorphic volcanic sequences with localised examples of preserved original volcanic textures and relict magmatic minerals, which are invaluable for inferring volcanic processes and by extension magma genesis. A feature observed in many Archean greenstone belts but rare in modern volcanic rocks is variolitic texture with millimetre- to centimetre-sized leucocratic globular structures (varioles), in mafic lavas. Models proposed to explain this texture include rapid undercooling, magma mingling and liquid immiscibility. Here, we investigate samples of Paleoarchean variole-bearing pillow lavas from the ca. 3469 Ma Mount Ada Basalt of the Pilbara Supergroup, Western Australia. The samples contain relict magmatic clinopyroxene and spinel and preserve sharp contacts between varioles and host. No plagioclase is observed in the samples. The clinopyroxene mineral composition is indistinguishable between in the varioles and host. This is despite the variole material having lower MgO and Fe2O3(total) and higher SiO2 and Al2O3 concentrations. However, the variole and host materials have indistinguishable TiO2, Cr2O3, NiO2 and Sm-Nd isotope systematics. These observations are consistent with an immiscibility reaction from a parental melt to a hydrous fluid phase and conjugate mafic liquid, where minerals that formed before the reaction become incorporated into both endmembers where they continued to grow by diffusive exchange between the hydrous fluid and silicate melt. Two lines of evidence indicate that hydrous flux exsolution occurred due to H2O saturation in the precursor magmas, including: (1) the lack of plagioclase, for which crystallisation is supressed at high H2O content; and (2) Nb/Th < 7, which indicates a hydrated mantle source. The variole-bearing pillow lavas are part of the 10-Myr-duration Coongan Subgroup volcanic cycle and occur interspersed with tholeiite basalts and minor komatiites of the Mount Ada Basalt and mafic to felsic volcanic rocks of the Duffer Formation. These distinctly different volcanic rock types, which have εNdi −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites, indicate that there was variation in magma genesis, which included hydrous fluxed melting likely genetically related to delamination, during the 10 Myr formation of the Coongan Subgroup volcanic cycle, as part of the Paleoarchean East Pilbara Terrane formation.

The stability and composition of sulfate melts in arc magmas

The stability field and composition of immiscible sulfate melts in equilibrium with silicate magmas has been determined using experiments over a range of crustal pressures, allowing an assessment of their possible role in transporting sulfur into the sub-volcanic arc and porphyry copper deposit systems. Experimental starting materials were based on natural trachyandesite and trachydacite compositions, with 3.5–7 wt% H2O and 3.5–5.5 wt% sulfur added to produce large, analyzable amounts of sulfate phases. Conditions ranged over 800–1200 °C, 0.2-1GPa and ƒO2 > NNO + 2.5. Sulfate melts formed at temperatures above 1000 °C at 0.75 and 1 GPa and above 900 °C at 0.2 GPa, suggesting some pressure dependence on their stability. At temperatures below 1100 °C sulfate melts and anhydrite crystals commonly coexist. Sulfate melts quenched to an intergrowth that was difficult to prepare for analysis. However, the composition was approximated by EPMA and further constrained by mass balance calculations. Sulfate melts were dominated by CaO and SO3, but also contained, in order of decreasing abundance, Na2O, K2O, MgO, FeO, Cl and P2O5. Chlorine showed a particular preference for the sulfate melt relative to the coexisting silicate melt, and calculated partition coefficients for sulfate/silicate melts were 5–13 at 1200 °C, 0.75–1 GPa. Experimental data show that, in the absence of an exsolved, hydrous fluid phase, sulfate melts can form in natural arc magmas at near-liquidus temperatures ≥ 1000 °C, assuming that magmas are oxidized and contain sufficient sulfur (> 2000–3000 ppm S). These results suggest that sulfate melt could be an important component in transporting sulfur as well as chlorine to shallow levels in the crust for hydrous magmas under a specific range of conditions. Both the non-quenchable and water-soluble nature of sulfate melts (and anhydrite) make them difficult to identify, unless trapped as mineral inclusions similar to the “wormy anhydrite” trapped in high-temperature amphiboles from Yanacocha, Peru.

Anomalously old biotite40Ar/39Ar ages in the NW Himalaya

Biotite 40Ar/39Ar ages older than corresponding muscovite 40Ar/39Ar ages, contrary to the diffusion properties of these minerals, are common in the Himalaya and other metamorphic regions. In these cases, biotite 40Ar/39Ar ages are commonly dismissed as “too old” on account of “excess Ar.” We present 32 step-heating 40Ar/39Ar ages from 17 samples from central Himachal Pradesh Himalaya, India. In almost all cases, the biotite ages are older than predicted from cooling histories. We document host-rock lithology and chemical composition, mica microstructures, biotite chemical composition, and chlorite and muscovite components of biotite separates to demonstrate that these factors do not offer an explanation for the anomalously old biotite 40Ar/39Ar ages. We discuss possible mechanisms that may account for extraneous Ar (inherited or excess Ar) in these samples. The most likely cause for “too-old” biotite is excess Ar, i.e., 40Ar that is separated from its parent K. We suggest that this contamination resulted from one or several of the following mechanisms: (1) 40Ar was released during Cenozoic prograde metamorphism; (2) 40Ar transport was restricted due to a temporarily dry intergranular medium; (3) 40Ar was released from melt into a hydrous fluid phase during melt crystallization. Samples from the Main Central Thrust shear zone may be affected by a different mechanism of excess-Ar accumulation, possibly linked to later-stage fluid circulation within the shear zone and chloritization. Different Ar diffusivities and/or solubilities in biotite and muscovite may explain why biotite is more commonly affected by excess Ar than muscovite.

Genesis of chromium-rich kyanite in eclogite-facies Cr-spinel-bearing gabbroic cumulates, Pohorje Massif, Eastern Alps

Natural kyanites with Cr2O3 contents >1 wt% are very rare and known only from high- P environments, for example, eclogite-facies gabbroic cumulates containing Cr-spinel from the Pohorje Massif, Eastern Alps, Slovenia. In these rocks, turquoise-colored Cr-rich kyanites are present in two different textural types. A first type has formed as blocky crystals of several hundred micrometers in size around clusters of small drop-like Cr-spinels. This kyanite shows a highly irregular Cr distribution and may contain up to 15.6 wt% Cr2O3, which is one of the highest Cr2O3 contents reported so far. A second type is present as part of reaction coronas around large red-brownish Cr-spinel and forms deep-blue needle-like crystals that rarely exceed 100 μm in size. This kyanite contains up to 8.2 wt% Cr2O3 and is associated with Cr-rich corundum (≤9.1 wt% Cr2O3) and Cr-Al-rich pargasite (≤3.9 wt% Cr2O3). The formation of kyanite around Cr-spinel droplets is interpreted to be the result of increasing P-T conditions during prograde metamorphism where Cr-spinel and plagioclase or quartz react to Cr-kyanite, ±garnet, ±omphacite. In contrast, the formation of kyanite associated with Cr-rich corundum and Cr-rich pargasite within coronas around Cr-spinel occurred in an early stage of the retrogressive evolution of the gabbroic cumulates at eclogite-facies conditions of ~2.5 GPa and 750–800 °C triggered by the influx of H2O-rich fluids. The driving force for developing these coronas was an increase in the chemical potential of silica caused by the infiltrating hydrous fluid phase. P-T estimates using matrix mineral assemblage place the peak metamorphic conditions close to the quartz/coesite transition with temperatures in the range of 750–810 °C and pressures of ~2.9 GPa.

Spheroidal textures in igneous rocks – Textural consequences of H2O saturation in basaltic melts

When melts reach H2O saturation before they are emplaced at the Earth’s surface, they exsolve a hydrous fluid phase. At elevated pressure and high temperature, a hydrous fluid is not pure H2O but can contain some quantities of silicate and metal oxide solute, notably if fluid saturation occurs at high pressure. Chemically and physically, fluid-saturated basalts behave like any other system with liquid immiscibility. Textures that can result after solidification are globules of former fluid dispersed in a former silicate melt, or silicate melt orbs embedded in a former fluid matrix. The Troodos ophiolite on the island of Cyprus, generated in a back arc basin above a wet intra oceanic subduction zone, carries in its magmatic stratigraphy lithologies distinctly orbicular and globular in shape. Many melt compositions parental to the ophiolite were H2O-saturated near their liquidus temperatures before they reached their depths of emplacement within the ophiolite sequence. Spheroids are not uncommon in magmatic rocks but still debated as to how they form. We propose that globular and orbicular textures in igneous worldwide may result when melts reach H2O saturation and exsolve an immiscible solute-bearing H2O-dominated fluid phase at elevated temperature and pressure, before or while they are emplaced.

Chloride partitioning and solubility in hydrous phonolites from Erebus volcano: A contribution towards a multi-component degassing model

We present results from a series of experiments on the partitioning of chlorine between a hydrous fluid phase and a phonolitic melt that we then use to model specific aspects of volcanic degassing. Experiments were performed from 250 to 10MPa on a phonolite from Erebus lava lake, Antarctica, at 1000°C near the QFM solid buffer. Only one of 48 experimental samples shows coexistence of low-density vapour and high-density brine in the fluid phase while 35 samples are unambiguously in the vapour field. Large uncertainties in the H2O–NaCl phase diagram at the P–T considered do not allow us to assign reliable phase behaviour to the remaining samples. We select a vapour-dominated subset of our dataset to establish an empirical HCl solubility law valid at low pressures and salinities. This law is incorporated into a thermodynamical model of degassing, which we use to compute equilibrium temperatures from gas measurements made at Erebus in 2010. The quiescent lake activity features cyclic temperature variations between 1000 and 1050°C, which is consistent with thermal convection within the shallow plumbing system. Backward tracking of representative gas compositions and temperatures shows the evolution with pressure of the molar ratio of SO2/HCl in the gas, a quantity that is often measured in volcanic plumes. Model outputs suggest that this ratio can vary by up to an order of magnitude when the pressure changes from atmospheric to 100MPa, depending on degassing style (coupled vs. decoupled ascent of gas and melt).

Fluid-present melting of sulfide-bearing ocean-crust: Experimental constraints on the transport of sulfur from subducting slab to mantle wedge

To constrain the sulfur enrichment of arc magma source-regions and the agent of sulfur transport from subducting slab to mantle wedge, here we report experimental measurements of sulfur content at sulfide saturation (SCSS) of slab-derived hydrous partial melts at 2.0 and 3.0GPa and from 800 to 1050°C, using Ni–NiO (NNO) and Co–CoO (CCO) external oxygen fugacity (fO2) buffers. A synthetic H2O-saturated MORB with 1wt.% S (added as pyrite) was used as starting material. All experiments produced pyrrhotite- and fluid-saturated assemblages of silicate glass, clinopyroxene, garnet, quartz, and rutile (plus amphibole at 2GPa/800°C and phengite at 3GPa/850°C). The silicate partial melt composition evolves from rhyolitic to rhyodacitic compositions with increasing temperature and melting degree in equilibrium with an eclogitic residue, showing substantial decrease in SiO2 and Mg# and increase in FeOT, TiO2 and Na2O. At all temperatures melt sulfur concentrations are very low, with an average of 110±50ppm S, similar to previous measurements at lower pressures. Melt SCSS appears to be mainly controlled by the melt composition, the activity of water, aH2O and the sulfur fugacity, fS2 (calculated from the composition of pyrrhotite). Mass-balance calculations show that the proportion of bulk sulfur dissolved in the silicate melt is negligible (<0.005wt.% of the bulk sulfur). In contrast, diminishing proportion of pyrrhotite with increasing temperature suggests that the fluid phase at equilibrium may contain as much as 10–15wt.% S at ⩾1050°C, and more than 40wt.% of the bulk sulfur initially present in the slab may be transferred to the aqueous fluid. Our data also suggest that fluid/melt sulfur partitioning increases with increasing temperature, from ∼300 at 900°C to ∼1200 at 1050°C, whereas pressure appears to have less of an effect. With respect to fO2, no real difference of fluid/melt S partitioning, within data uncertainties, between NNO and CCO at 2 and 3GPa was observed. This is interpreted as a consequence of comparable S speciation in the hydrous fluid phase in fO2 conditions ranging from NNO−0.5 to CCO−1.0 at 2 and 3GPa and 850 to 1050°C. Under these reducing conditions, the hydrous fluid phase is composed mainly of H2S and H2O, and to a lesser extent of S2 and H2, and S in melt is dissolved almost totally as S2− (i.e., S6+/Stotal∼0). Our study suggests that slab-derived aqueous fluids with reduced sulfur species could be sufficient vectors for the transport of sulfur from the slab to the mantle wedge and only ∼0.3 to 1.5wt.% of slab fluids are required for the mantle wedge enrichment in sulfur over background mantle concentrations. Also, depending on the initial amount of sulfur in the slab, recycling of sulfide to the deep mantle may be favored in cold to intermediate subduction zones, whereas warm subduction zones will cause fluid-mediated slab-mantle wedge transfer of sulfur.

Origin and geodynamic evolution of late Cenozoic potassium-rich volcanism in the Isparta area, southwestern Turkey

Post-collisional potassic-rich volcanism of Gölcük Volcano in the Isparta area of southwestern Turkey consists of two groups: (i) extracaldera lavas, corresponding mainly to Pliocene activity; and (ii) intracaldera lavas and pyroclastics (ignimbrite flows and ash/pumice fall deposits) formed during the Quaternary. Extracaldera volcanic rocks mainly comprise lamprophyre (minette), basaltic trachyandesite, trachyandesite, and trachyte. A close relationship exists between the silica content and phenocryst type in the extracaldera volcanics such that trachyte–trachyandesites with SiO2 < 57 wt% and basaltic trachyandesites are characterized mainly by mafic phenocryst phases (e.g. pyroxene, amphibole, biotite–phlogopite). These features suggest suppression of plagioclase crystallization under high H2O pressure conditions. Intracaldera volcanics are composed of tephriphonolitic dikes, remnants of lava flows and domes at the caldera rim, and a trachytic lava dome on the caldera floor. The Gölcük flows and pyroclastics are mainly characterized by strong incompatible element enrichment in large ion lithophile elements (LILEs; e.g. Cs, Ba, U, and Th) relative to K, Rb, and high-field strength elements (e.g. Nb, Ta, and Ti). We conclude that the Gölcük lavas were derived from a metasomatized lithospheric mantle source containing phlogopite–amphibole garnet peridotite; the latter resulted from metasomatism by a hydrous fluid phase related to subducted sediments and oceanic crust. The parental magma for the extracaldera volcanics was lamprophyric, and that for the intracaldera volcanics was basanitic. All the geological and geochemical data show that the alkaline Gölcük lavas display a gradual decrease in silica content with decreasing eruption age, indicating that, in the Isparta volcanic province, the asthenospheric melt component became more important over time. In the extracaldera volcanics, 87Sr/86Sr isotope ratios of the evolved trachyte–trachyandesites range between 0.70366 and 0.70504, whereas these ratios are lower in the less evolved basaltic trachyandesite and lamprophyres, varying in narrow ranges around 0.70365 and between 0.70374 and 0.70453, respectively. The 143Nd/144Nd values lie between 0.51264 and 0.51273 in trachyandesites, 0.51267 and 0.51273 in basaltic trachyandesites, and 0.51270 and 0.51274 in lamprophyres. In the intracaldera lavas, the 87Sr/86Sr isotope ratio is 0.70361 in the tephriphonolite and 0.70388 in the intracaldera trachytic lava dome. The 143Nd/144Nd isotope ratio is 0.51274 in the analysed tephriphonolitic flow and 0.51271 in the intracaldera trachytic lava dome; these values are higher than that of trachyte–trachyandesites of the extracaldera volcanics. The Sr–Pb isotopic signatures indicate that crustal contamination was significant for the evolved extracaldera volcanics, but was negligible for the intracaldera volcanics. The εNd values of the Gölcük volcanics range between 0 and 2.0. The low Sr isotope ratios and positive εNd values are characteristic features of a depleted mantle source. The isotopically depleted and incompatible enriched nature of the Gölcük lavas point to recent enrichment processes prior to partial melting of the mantle source. Conversely, their radiogenic lead isotope compositions (206Pb/204Pb = 19.19–19.54, 207Pb/204Pb = 15.64–15.67, 208Pb/204Pb = 39.12–39.49) indicate an enriched mantle source region.

Stable isotope and chemical systematics of pseudotachylyte and wall rock, Homestake shear zone, Colorado, USA: Meteoric fluid or rock‐buffered conditions during coseismic fusion?

A hydrous fluid phase is critical in controlling effective stress and fault mechanics, and influencing the mineralogy and strength of materials within fault zones. Oxygen and hydrogen isotope and chemical analysis of wall rock gneiss, pseudotachylyte, and selected minerals in gneiss and pseudotachylyte from the Homestake shear zone was used to assess whether melting occurred in the presence of meteoric water or involved only minor amounts of H2O derived from micas in wall rock gneiss. Bulk pseudotachylyte has slightly lower δ18OSMOW than the whole rock protolith. δD for one bulk pseudotachylyte is essentially identical to biotite in gneiss; δD for two samples is lower by ∼20‰. Bulk pseudotachylyte has lower SiO2 and K2O, and higher Al2O3, FeO, MgO, CaO, and H2O, than gneiss. The lower SiO2 of pseudotachylyte compared to gneiss is explained by physical segregation of 25 to 72 volume % of quartz clasts from the mobile melt phase. Samples of gneiss and pseudotachylyte define a SiO2‐δ18O mixing line between quartz and the most SiO2‐ and 18O‐depleted pseudotachylyte. Physical segregation of quartz (highest oxygen isotope composition in the pseudotachylyte‐gneiss system) accounts for the slightly lower oxygen isotope composition of bulk pseudotachylyte relative to gneiss. The similar δD of pseudotachylyte and biotite from gneiss in one sample is consistent with dehydration melting of biotite during frictional heating and dissolution of biotite‐derived H2O in the melt. Late devitrification of glass and formation of greater amounts of fine‐grained muscovite, accompanied by 10–30% loss of hydrogen as H2O, results in lower δD values in other samples. In general, melt generation occurred in a fault zone closed to infiltration of meteoric water. There was no free, H2O‐rich pore fluid present at the time of slip to potentially influence the behavior of the fault.

Kinetics of iron oxidation-reduction in hydrous silicic melts

The kinetics of Fe oxidation-reduction in two hydrous rhyolitic melts, one metaluminous and the other peralkaline, have been studied at 800°C, 2kb, for melt water contents from ~5 wt% to saturation and fO2 between NNO-2 and NNO + 3 (NNO = nickel-nickel oxide redox buffer). The metaluminous melt (~1 wt% FeOT) reached redox equilibrium after 10 hours and the peralkaline one (~3 wt% FeOT) after 3 hours. The kinetics of Fe oxidation and reduction are similar and unaffected by the presence or absence of a hydrous fluid phase. No redox front is observable in the glass as the Fe3+/Fe2+ ratio evolves, implying that the Fe redox kinetics in hydrous silicic melts is rate-limited neither by the diffusion of H2 nor by the mobilities of divalent cations, as observed for anhydrous basaltic melts. We propose a two-step reaction mechanism that involves: (1) virtually instantaneous diffusion of H2 in the sample, followed by (2) slower structural/chemical reorganizations around Fe atoms. The overall redox process involving iron and hydrogen in Fe-poor, H2O-rich melts is thus reaction-limited and obeys a first-order logarithmic rate law. The relatively slow kinetics of oxidation/ reduction explains why melt Fe3+/Fe2+ can be readily quenched in laboratory experiments. Simulation of oxidation of magmas due to H2 exchange with wall rocks is performed using these new kinetics laws and two DH2 values extracted from the literature. We demonstrate that the metaluminous composition is not significantly modified whereas the peralkaline composition undergoes important and fast changes of Fe3+/Fe2+ during short processes such as ascent prior to Plinian-style eruptions

The Fluid-absent Partial Melting of a Zoisite-bearing Quartz Eclogite from 1middle dot0 to 3middle dot2 GPa; Implications for Melting in Thickened Continental Crust and for Subduction-zone Processes

Fluid-absent melting experiments on a zoisiteand phengite-bearing INTRODUCTION eclogite (omphacite, garnet, quartz, kyanite, zoisite, phengite and Zoisite or clinozoisite is present in many high-pressure rutile) were performed to constrain the melting relations of these eclogites, and many occurrences are known from the hydrous phases in natural assemblages, as well as the melt and Scandinavian Caledonides (e.g. Holsnoy: Austrheim & mineral compositions produced by their breakdown. From 1·0 to Mork, 1988; Jamtveit et al., 1990; Western gneiss region: 3·2 GPa the solidus slopes positively from 1·5 GPa at 850°C to Griffin et al., 1985; Seve Nappe: Kullerud et al., 1990). 2·7 GPa at 1025°C, but bends back at higher pressures to 975°C Indeed, most eclogites worldwide contain minor amounts at 3·2 GPa. The melt fraction is always low and the melt of zoisite or other hydrous phases, according to the compositions always felsic and become increasingly so with increasing compilations of Smith (1988) and Carswell (1990). During pressure. The normative Ab–An–Or compositions of the initial prograde eclogitization, zoisite forms by breakdown of melts vary from tonalites at 1·0 GPa to tonalite–trondhjemites at the anorthite component of plagioclase in the presence 1·5 GPa, adamellites at 2·1 and 2·7 GPa, and to true granites of a hydrous fluid phase. The origin of these hydrous at 3·2 GPa. At pressures 2·5 GPa zoisite and phengite break fluids is controversial. One possibility is that they are down more or less simultaneously. At 3·2 GPa and 1000°C zoisite introduced from below during continental collision, for is unreacted whereas phengite is absent, so that the first formed melt example, if wet continental sedimentary rocks are deeply at these conditions is granitic. Our experiments show that if subducted. During subduction of oceanic crust zoisite sufficiently high temperatures (of the order of 1000°C) are attained, forms by prograde metamorphism of hydrothermally zoisiteand phengite-bearing eclogites can produce small fractions altered oceanic crust, which in its upper levels contains of silicic melts of a wide range of compositions. These melts are low-temperature hydrous Ca-rich phases (e.g. Poli & rich in water and, probably, in Sr and other incompatible elements, Schmidt, 1997). At the gabbro to dyke transition zone so that they can act as metasomatic agents in the mantle wedge. the temperature and pressure are high enough to allow

Up with Bromine

ATMOSPHERIC SCIENCE Major volcanic eruptions eject particles and gases into the stratosphere, including the halogens chlorine (Cl) and bromine (Br), both of which contribute to the catalytic destruction of ozone, but the magnitude of the volcanic contribution of these gases is uncertain. Although enlarged polar ozone holes were observed after the Pinatubo eruption in 1991, these were attributed primarily to the effects of sulfate aerosols and other particles from the eruption on the formation of polar stratospheric clouds. Most of the volcanic Cl (which forms HCl) is likely scavenged before it reaches the stratosphere, but the potential effects of Br, which is much more potent at destroying ozone than Cl, are not well studied. Bureau et al . now present a series of experiments which suggest that the Pinatubo eruption and other comparable eruptions could inject as much Br into the atmosphere as the annual budget from all other natural and anthropogenic sources. Their results show that in magmas, Cl, Br, and I partition strongly into a hydrous fluid phase, which is responsible for driving violent eruptions. Bromine seems less likely to be washed out in the eruption column than Cl, although further study of this is needed. — BH Earth Planet. Sci. Lett . 183 , 51 (2000).

Comparison of conventional and garnet-aluminosilicate-quartz O isotope thermometry; insights for mineral equilibration in metamorphic rocks

Oxygen isotope thermometry based on fractionations among quartz, garnet, kyanite, sillimanite, and potassium feldspar in amphibolite to granulite facies schists and gneisses was compared to thermometry based on mineral equilibria (“conventional thermometry”). The former approach takes advantage of the exchange between quartz and more refractory minerals such as garnet and the aluminosilicates, and their potential for preserving peak oxygen isotopic fractionations. Although isotopic thermometry using these phases is potentially an approach for circumventing the problem in slowly cooled rocks of retrograde oxygen isotopic exchange with micas and feldspars, and of Fe and Mg exchange between garnet and biotite, the results are accurate or geologically meaningful in only certain settings. In most cases, the results of isotopic thermometry must be evaluated with the aid of quantitative forward models for calculating the amount of retrograde diffusive exchange among micas, feldspars, and quartz. Two models used here predict quartz isotopic compositions that differ by only 0.1 per mil and, in most cases, reproduce to within 0.2 per mil the measured isotopic composition of quartz. Excellent agreement between isotopic and conventional thermometry is obtained for mid-to-upper amphibolite facies rocks that have experienced one period of recrystallization and mineral growth, and if they are coarse-grained or cooled in the absence of a hydrous fluid phase. Isotopic temperatures of 850 °C are in excellent agreement with conventional thermometry in migmatitic granulite facies metapelites if cooling occurred under anhydrous conditions. In other cases, isotopic temperatures in granulite facies rocks are higher than conventional temperatures, a reasonable result in view of the tendency for retrograde cation exchange between garnet and biotite. In samples that have undergone multiple periods of recrystallization or metamorphism, isotopic temperatures from coexisting quartz, garnet, and aluminosilicates may be modified by diffusional exchange of quartz with micas and feldspars aided by recrystallization, by retrograde reactions involving the latter phases, or by open-system interaction with externally derived fluids. External fluid infiltration, combined with recrystallization of quartz and aluminosilicate, may result in isotopic reversals between garnet and matrix phases. Conventional thermobarometry in such complex rocks, based in large part on mineral equilibria involving garnet, may be erroneously assigned geological significance in the absence of insights provided by oxygen isotope analysis.

Solubility of water in the α, β and γ phases of (Mg,Fe) 2 SiO 4

The solubility of hydroxyl in the α, β and γ phases of (Mg,Fe)2SiO4 was investigated by hydrothermally annealing single crystals of San Carlos olivine. Experiments were performed at a temperature of 1000° or 1100 °C under a confining pressure of 2.5 to 19.5 GPa in a multianvil apparatus with the oxygen fugacity buffered by the Ni:NiO solid-state reaction. Hydroxyl solubilities were determined from infrared spectra obtained of polished thin sections in crack-free regions ≤100 μm in diameter. In the α-stability field, hydroxyl solubility increases systematically with increasing confining pressure, reaching a value of ∼20,000 H/106Si (1200 wt ppm H2O) at the α-β phase boundary near 13 GPa and 1100 °C. In the β field, the hydroxyl content is ∼400,000 H/106Si (24,000 wt ppm H2O) at 14–15 GPa and 1100 °C. In the γ field, the solubility is ∼450,000 H/106Si (27,000 wt ppm H2O) at 19.5 GPa and 1100 °C. The observed dependence of hydroxyl solubility with increasing confining pressure in the α phase reflects an increase in water fugacity with increasing pressure moderated by a molar volume term associated with the incorporation of hydroxyl ions into the olivine structure. Combined with published results on the dependence of hydroxyl solubility on water fugacity, the present results for the α phase can be summarized by the relation COH = A(T)f nH2Oexp(−PΔV/RT), where A(T) = 1.1 H/106Si/MPa at 1100 °C, n = 1, and ΔV = 10.6×10–6 m3/mol. These data demonstrate that the entire present-day water content of the upper mantle could be incorporated in the mineral olivine alone; therefore, a free hydrous fluid phase cannot be stable in those regions of the upper mantle with a normal concentration of hydrogen. Free hydrous fluids are restricted to special tectonic environments, such as the mantle wedge above a subduction zone.

Partitioning of phosphorus between melt and fluid in the system haplogranite-H 2O-P 2O 5

The partitioning of phosphorus between hydrous fluid and silicate melt has been studied in the system NaAl-Si 3O 8-KAlSi 3O 8-SiO 2-H 2O-P 2O 5 at 1–5 kbar and 700–1100°C. Experiments were mostly carried out in rapid-quench cold-seal pressure vessels. Both quenched fluid and glass phase were analyzed for phosphorus and major elements by ICP-AES. Equilibrium partition coefficients were bracketed by using starting materials which initially contained all of the phosphorus either in the fluid or in the melt phase. Depending on pressure and temperature, measured partition coefficients ( D = C fluid C melt ; concentrations in ppm by weight) vary over two orders of magnitude, from ∼0.02 to ∼2. D increases with decreasing temperature and increasing pressure. The dependence of phosphorus distribution on pressure and temperature appears to be largely controlled by the properties of the fluid. The partition coefficients can be fitted to the equation ln D = 0.991 − 2.576 ln V, where V (in cm 3 g −1) is the specific volume of pure water at the respective pressure and temperature. This model predicts that during degassing of eruptive rhyolite melts on the surface no significant amount of phosphorus is lost, i.e. phosphorus does not behave as a volatile element under these conditions. The behavior of phosphorus during the final stages of crystallization in a water-saturated granitic melt is controlled by the emplacement depth of the pluton. At pressures below 1 kbar, only very small amounts of phosphorus enter the hydrous fluid phase. However, above 4 kbar, phosphorus partitions in favour of the fluid ( K D > 1).

Carpholite, sudoite, and chloritoid in low-grade high-pressure metapelites from Crete and the Peloponnese, Greece

In the high-pressure metamorphic Phyllite-Quartzite Unit of Crete and the Peloponnese, aluminous metasediments contain assemblages with Fe-Mg-carpholite, sudoite, chloritoid, pyrophyllite, chlorite, garnet, white mica and quartz. Variations of parageneses and mineral reactions in response to varying pressure, temperature and activity of H 2 O have been studied in detail. In Eastern Crete (≃300 ° C, 8 kbar), metapelites with high Mg/Fe 2+ are characterized by sudoite-bearing assemblages with chlorite or pyrophyllite. In rocks with low Mg/Fe 2+ , chlorite+pyrophyllite occur instead, suggesting the (continuous) reaction sudoite+quartz=chlorite+pyrophyllite+H 2 O. Additional carpholite rarely occurs in chlorite-pyrophyllite schists, probably through the hydration reaction chlorite+pyrophyllite+H 2 O=carpholite+quartz. Progress of this reaction requires relatively high a H2O , otherwise chlorite+pyrophyllite persist. In Central Crete (≃350 ° C, 9 kbar), the paragenesis chlorite+pyrophylite is no longer stable and chloritoid appears as a widespread rock-forming mineral, occasionally associated with carpholite, through the reactions carpholite=chloritoid+quartz+H 2 O, chlorite+pyrophyllite+I2O=chloritoid+carpholite+quartz. As in Eastern Crete, reactions involving carpholite proceed only in the case of relatively high a H2O ; the estimated values range between 1.0 and 0.7. This variability of H2O is reflected in AFM diagrams by the compositional shift of the three-phase assemblages carpholite-chloritoid-pyrophyllite and carpholite-chloritoid-chlorite. At lower a H2O , probably the normal case inCrete, no carpholite appears and chloritoid directly forms after chlorite+pyrophyllite=chloritoid+quartz+H 2 O. In this case, no minerals or parageneses diagnostic for high-P/low-T metamorphism appear. The same parageneses as in Central Crete have been observed in Western Crete (400 ° C, 10 kbar) and in the Peloponnese (≃450 ° C, 17 kbar), but with carpholites richer in Mg. For these rocks, AFM three-phase parageneses show fixed mineral compositions, suggesting equilibrium with a hydrous fluid phase of constant composition. The highest grade of metamorphic evolution of metapelites from the Phyllite-Quartzite Unit is reflected by the appearance of almandine-rich garnet in the Peloponnese

Structural and metamorphic controls on the distribution of zircon in an evolving quartzofeldspathic migmatite: an example from the Reynolds Range, central Australia

Three types of zircon occur in a complexly deformed and variably migmatized quartzofeldspathic gneiss from the Reynolds Range, central Australia. The oldest type is inherited from the granitic precursor of the gneiss, and is overgrown by a second group of zircon grains that formed during prograde, granulite facies metamorphism. Partial melting of the gneiss resulted in solution of both the inherited and metamorphic zircon. No new zircon growth accompanied crystallization of the partial melt, suggesting loss of zirconium–rich residual fluids. Hydrous, amphibolite facies retrogression of the gneiss and its migmatized variants during late shearing produced new, idiomorphic zircon in both the shear zone and its wall rocks.Important implications of this study are that (i) zircon has a tendency to dissolve if it comes into direct contact with a melt produced from anhydrous biotite breakdown in a quartzofeldspathic granulite, (ii) melt crystallization is not necessarily accompanied by zircon growth, and (iii) euhedral zircon can grow from a hydrous fluid phase under subsolidus, amphibolite facies conditions, e.g. within shear zones.

Reaction-enhanced formation of eclogite-facies shear zones in granulite-facies anorthosites

Abstract Infiltration of a dry granulite-facies anorthosite complex by a hydrous fluid phase under high pressure was crucial to start (a) an eclogitization process which led to the breakdown of plagioclase and the formation of either dense or hydrated minerals and (b) dynamic recrystallization of plagioclase in shear zones. Reaction products of the eclogitization process are related spatially to cracks and shear zones. The reduction in grain size during dynamic recrystallization, the foliation development in shear zones and the occurrence of new ‘soft’ and/or dense minerals changed the mechanical properties (e.g. ductility) of the rocks considerably. The initiation of detachment and thrusting in the lower crust may be an important large scale consequence of such localized processes.

Ordering and unmixing of alkali feldspars in a porphyritic granite and its country rocks

50 alkali feldspars from a Variscan porphyritic granite and 27 from two country rocks of Caledonian and Variscan age respectively have been investigated for their bulk chemistries and their structural states. Ordering is illustrated in thebc-plot afterStewart andWright (1974); unmixing is illustrated in a newly designed diagram with the Or-contents of the bulk feldspar and that of the unmixed K-rich phase on the coordinates. The feldspars from the granite have a greater variability with regand to chemistry, degree of ordering and degree of unmixing than those of the country rocks. Ordering and unmixing depend mainly on the action of a hydrous fluid phase. During the in situ formation of the granite, water must have accumulated in the granitic magma and must have remained in the granite after consolidation. Temperatures derived by means of the two feldspar geothermometer indicate selective loss of sodium during the postmagmatic stage and before unmixing of the alkali feldspars. Loss of sodium makes unmixing increasingly sluggish.