Fluid-present Conditions Research Articles

Petrogenesis of A-type leucocratic granite magmas: An example from Delbegetei massif, Eastern Kazakhstan

The genesis of leucogranite magmas is important issue of geodynamics, petrology and ore geology because leucogranites are associated with collisional belts, partial melting of sedimentary source rocks, and may host rare metal (Sn, W, Li, NbTa, and Be) mineralization. To establish the petrogenesis of large leucogranite intrusions, detailed studies of petrography, mineralogy, fluid regime, and the material and isotopic composition of rocks are required. The paper reports results of the studies of the Delbegetei massif of the intrusion in Eastern Kazakhstan. The massif is composed predominantly of leucocratic granites, while syenogranites are subordinate. Rocks of the massif belong to shoshonitic and high-K calc-alkaline series; demonstrate a predominance of K over Na, high ferroan and high contents of LREE and HFSE; which allows them to be classified as A-type granites. The age of Delbegetei massif, estimated by the UPb zircon dating, varies in the range 249–240 Ma, which correspond the Early-Middle Triassic. The differences in rock composition and in temperatures of zircon saturation allow supposing that syenogranites and leucogranites formed from different parental magmas. Syenogranite magma formed as a result of partial melting of metamorphosed volcanic rocks (andesidacites or dacites) with possible influence of mafic magmas. Leucogranite magma formed as a result of fluid-present partial melting of metaterrigenous sedimentary rocks. Leucogranite magma underwent the feldspars differentiation in the fluid-present conditions. This led to composition variations of leucogranites. Analysis of the geological position, age and composition of the rocks allows concluding that the Delbegetei massif formed at the Early Triassic in an intraplate geodynamic setting and that the activity of the Siberian mantle plume is the most probable reason for their formation.

Peritectic Melting of Mica in Fault‐Related Pseudotachylite Melts and Potassium Mass Balance as an Indicator of Fluid‐Absent Source Conditions.

AbstractPseudotachylites are generally considered to be produced by flash melting of the most fusible minerals, with compositions often dominated by biotite mica. We present phase fraction calculations for 237 pseudotachylite analyses from 28 localities spanning a range of host rock compositions from granites to peridotites. Pseudotachylite matrix compositions cannot be reproduced by a simple linear mixture of the minerals in their host rocks and commonly show a potassium deficit in pseudotachylites which contain significant contributions from biotite or muscovite. This is strongly indicative of peritectic melting of mica under fluid‐absent conditions. Occasionally, a negative contribution from an aluminosilicate phase is required but with a positive contribution from potassium feldspar. This is most consistent with peritectic melting of micas under fluid present conditions. The present data therefore suggest that, while fluid absent flash melting might be the most common mode of formation of pseudotachylite, in some instances pseudotachylites can be produced in the presence of a free fluid. Potassium mass balance of pseudotachylites might therefore be a diagnostic indicator of fluid‐absent conditions in their source regions during seismogenic rupture. We give some examples of observational evidence of peritectic liquidus phases in pseudotachylites and suggest that the significance of these phases might have been overlooked in previous studies.

REE partition among zircon, orthopyroxene, amphibole and garnet in a high-grade metabasic system

AbstractA 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.

Origin of primitive ultra-calcic arc melts at crustal conditions — Experimental evidence on the La Sommata basalt, Vulcano, Aeolian Islands

To interpret primitive magma compositions in the Aeolian arc and contribute to a better experimental characterization of ultra-calcic arc melts, equilibrium phase relations have been determined experimentally for the La Sommata basalt (Som-1, Vulcano, Aeolian arc). Som-1 (Na2O+K2O=4.46wt.%, CaO=12.97wt.%, MgO=8.78wt.%, CaO/Al2O3=1.03) is a reference primitive ne-normative arc basalt with a strong ultra-calcic affinity. The experiments have been performed between 44 and 154MPa, 1050 and 1150°C and from NNO+0.2 to NNO+1.9. Fluid-present conditions were imposed with H2O–CO2 mixtures yielding melt H2O concentrations from 0.7 to 3.5wt.%. Phases encountered include clinopyroxene, olivine, plagioclase and Fe-oxide. Clinopyroxene is slightly earlier than olivine in the crystallization sequence. It is the liquidus phase at 150MPa, being joined by olivine on the liquidus between 44 and 88MPa. Plagioclase is the third phase to appear in the crystallization sequence and orthopyroxene was not found. Experimental clinopyroxenes (Fs7–16) and olivines (Fo78–92) partially reproduce the natural phenocryst compositions (respectively Fs5–7 and Fo87–91). Upon progressive crystallization, experimental liquids shift towards higher SiO2 (up to ~55wt.%), Al2O3 (up to ~18wt.%) and K2O (up to ~5.5wt.%) and lower CaO, MgO and CaO/Al2O3. Experimental glasses and natural whole-rock compositions overlap, indicating that progressive crystallization of Som-1 type melts can generate differentiated compositions such as those encountered at Vulcano. The low pressure cotectic experimental glasses reproduce glass inclusions in La Sommata clinopyroxene but contrast with glass inclusions in olivine which preserve basaltic melts more primitive than Som-1. Phase relations for the La Sommata basalt are identical in all critical aspects to those obtained previously on a synthetic ultra-calcic arc composition. In particular, clinopyroxene+olivine co-saturation occurs at very low pressures (≤100MPa). Ultra-calcic arc compositions do not represent primary mantle melts but result from the interaction between a primary mantle melt and clinopyroxene-bearing rocks in the arc crust. At Vulcano, primitive ultra-calcic end-member melts were generated between 250 and 350MPa in the lower magma accumulation zone by reaction between hot primitive melts and wehrlitic or gabbroic lithologies. At Stromboli, golden pumices and glass inclusions with an ultra-calcic affinity were also generated at shallow pressures, between 150 and 250MPa, suggesting that the interaction model is of general significance in the Aeolian arc.

Fluid‐induced breakdown of monazite in medium‐grade metasedimentary rocks of the Pontremoli basement (Northern Apennines, Italy)

AbstractThe last (decompression) stages of the metamorphic evolution can modify monazite microstructure and composition, making it difficult to link monazite dates with pressure and temperature conditions. Monazite and its breakdown products under fluid‐present conditions were studied in micaschist recovered from the cuttings of the Pontremoli1 well, Tuscany. Coronitic microstructures around monazite consist of concentric zones of apatite + Th‐silicate, allanite and epidote. The chemistry and microstructure of the monazite grains, which preserve a wide range of chemical dates ranging from Upper Carboniferous to Tertiary times, suggest that this mineral underwent a fluid‐mediated coupled dissolution–reprecipitation and crystallization processes. Consideration of the chemical zoning (major and selected trace elements) in garnet, its inclusion mineralogy (including xenotime), monazite breakdown products and phase diagram modelling allow the reaction history among accessory minerals to be linked with the reconstructed P–T evolution. The partial dissolution and replacement by rare earth element‐accessory minerals (apatite–allanite–epidote) occurred during a fluid‐present decompression at 510 ± 35 °C. These conditions represent the last stage of a metamorphic history consisting of a thermal metamorphic peak at 575 °C and 7 kbar, followed by the peak pressure stage occurring at 520 °C and 8 kbar. An anticlockwise P–T path or two clockwise P–T loops can fit the above P–T constraints. The former path may be related to a context of late Variscan strike‐slip‐dominated exhumation with minor Tertiary (Alpine‐related) reworking and fluid infiltration, while the latter requires an Oligocene–Miocene fluid‐present tectono‐metamorphic overprint on the Variscan paragenesis.

Metamorphic evolution of LT-UHP eclogite from the south Dabie orogen, central China: An insight from phase equilibria modeling

Low-temperature ultrahigh pressure (LT-UHP) eclogites from the south Dabie orogen are distributed in the Zhujiachong and Anqiling zones and their peak mineral assemblages and P–T paths are still disputable. The rocks consist commonly of garnet, omphacite, epidote, phengite, amphibole, quartz/coesite and rutile with or without glaucophane, kyanite, talc and paragonite. Garnet porphyroblasts in the representative sample (D901) from the Zhujiachong zone exhibit core–mantle zoning with slightly increasing pyrope (Xpy) and decreasing grossular (Xgr) contents, and mantle–rim zoning with rapidly increasing Xpy and decreasing Xgr. Garnet shows diverse rim compositions which can be classified into group-1 that show high Xpy and low Xgr values, being an indicative character of LT-UHP eclogite, and group-2 that show lower Xpy and higher Xgr than in group-1. Phengite shows higher Si contents in the Anqiling zone than in the Zhujiachong zone. Epidote, paragonite and amphibole usually occur as porphyroblasts that contain inclusions of garnet, omphacite and rutile. Pseudosections calculated using THERMOCALC for the representative samples suggest that the mineral assemblage in the pressure peak stage would be garnet+omphacite+lawsonite+talc+phengite+coesite in the model system. In this assemblage, Xpy steadily increases as temperature rises and Si in phengite increases with pressure rising, whereas Xgr is very insensitive as pressure changes. The peak P–T conditions for low-T UHP eclogites can be determined using the isopleths of maximum Xpy and Si in phengite in P–T pseudosections. Using this approach, the peak P–T conditions of eclogite were estimated to be ∼30kbar/615°C for the representative sample in the Zhujiachong zone, and ∼32kbar/623°C or ∼36kbar/640°C for samples in the Anqiling zone. The pre-peak prograde metamorphic evolution during subduction was modeled according to the garnet growth zoning in sample D901, where the core–mantle zoning suggests a flat P–T vector dominated by heating, representing a slower subduction process, and the mantle–rim zoning indicates a steep P–T vector dominated by increasing pressure, pointing to a faster subduction process. The post-peak exhumation of the LT-UHP eclogite involves two stages: the early-stage exhumation is dominated by lawsonite dehydrations, forming the porphyroblasts of epidote, paragonite and glaucophane under fluid-present conditions, and the late-stage exhumation occurs under fluid-absent conditions, forming hornblendic amphibole and plagioclase triggered by fluid-infiltration. As a consequence of the metamorphic evolution especially during the early-stage exhumation, the observed mineral assemblages in LT-UHP eclogites cannot well match those at their peak stages.

The Blacktail Creek Tuff: an analytical and experimental study of rhyolites from the Heise volcanic field, Yellowstone hotspot system

The magma storage conditions of the 6.62 Ma Blacktail Creek Tuff eruption, belonging to the Heise vol- canic field (6.62-4.45 Ma old) of the Yellowstone hotspot system, have been investigated by combining thermobaro- metric and experimental approaches. The results from dif- ferent geothermometers (e.g., Fe-Ti oxides, feldspar pairs, apatite and zircon solubility, and Ti in quartz) indicate a pre-eruptive temperature in the range 825-875 °C. The temperature estimated using two-pyroxene pairs varies in a range of 810-950 °C, but the pyroxenes are probably not in equilibrium with each other, and the analytical results of melt inclusion in pyroxenes indicate a complex history for clinopyroxene, which hosts two compositionally dif- ferent inclusion types. One natural Blacktail Creek Tuff rock sample has been used to determine experimentally the equilibrium phase assemblages in the pressure range 100- 500 MPa and a water activity range 0.1-1.0. The experi- ments have been performed at fluid-present conditions, with a fluid phase composed of H 2 O and CO 2 , as well as at fluid- absent conditions. The stability of the quartzo-feldspathic phases is similar in both types of experiments, but the pres- ence of mafic minerals such as biotite and clinopyroxene is

Ultra-oxidized rocks in subduction mélanges? Decoupling between oxygen fugacity and oxygen availability in a Mn-rich metasomatic environment

The manganese ore of Praborna (Italian Western Alps) is embedded within a metasedimentary sequence belonging to a subduction mélange equilibrated at high-pressure (HP) conditions (ca. 2GPa) during the Alpine orogenesis. The pervasive veining of the ore and the growth of “pegmatoid” HP minerals suggest that these Mn-rich rocks strongly interacted with slab-derived fluids during HP metamorphism. These rocks are in textural and chemical equilibrium with the veins and in contact with sulphide- and magnetite-bearing metabasites at the bottom of the sequence. They contain braunite (Mn2+Mn3+6SiO12), quartz, pyroxmangite (Mn2+SiO3), and minor hematite, omphacite, piemontite and spessartine-rich garnet. Sulphides are absent in the Mn-rich rocks, whereas sulphates (barite, celestine) occur together with As- and Sb-oxides and silicates. This rock association provides an excellent natural laboratory to constrain the redox conditions in subducting oceanic slab mélanges at HP and fluid-present conditions. Similarly to Fe-bearing minerals, Mn oxides and silicates can be regarded as natural redox-sensors. A thermodynamic dataset for these Mn-bearing minerals is built, using literature data as well as new thermal expansion parameters for braunite aud pyrolusite, derived from experiments. Based on this dataset and the observed assemblages at Praborna, thermodynamic calculations show that these mélange rocks are characterised by ultra-oxidized conditions (∆FMQ up to +12.7) if the chemical potential of oxygen (or the oxygen fugacity fO2) is accounted for. On the other hand, if the molar quantity of oxygen is used as the independent state variable to quantify the bulk oxidation state, the ore appears only moderately oxidized and comparable to typical subduction-slab mafic eclogites. Such an apparent contradiction may happen in rock systems whenever oxygen is improperly considered as a perfectly mobile component. In the Earth's mantle, redox reactions take place mainly between solid oxides and silicates, because O2 is a negligible species in the fluid phase. Therefore, the description of the redox conditions of most petrological systems requires the introduction of an extensive variable, namely the oxygen molar quantity (nO2). As a consequence, the oxygen chemical potential, and thus fO2, becomes a dependent state variable, not univocally indicative of the redox conditions of the entire rock column of a subduction zone, from the dehydrating oceanic crust to the overlying mantle wedge. On a more general basis, the comparison of fO2 retrieved from different bulk compositions and different phase assemblages is sometimes challenging and should be undertaken with care. From the study of mélange rocks at Praborna, the distribution of oxygen at subduction zones could be modelled as an oxidation gradient, grading from a maximum in the subducted altered oceanic crust to a minimum in the overlying peridotites of the mantle hanging-wall.

Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study

Two mafic eruptive products from Vesuvius, a tephrite and a trachybasalt, have been crystallized in the laboratory to constrain the nature of primitive Vesuvius magmas and their crustal evolution. Experiments were performed at high temperatures (from 1000 to ≥1200°C) and both at 0·1 MPa and at high pressures (from 50 to 200 MPa) under H2O-bearing fluid-absent and H2O- and CO2-bearing fluid-present conditions. Experiments started from glass except for a few that started from glass plus San Carlos olivine crystals to force olivine saturation. Melt H2O concentrations reached a maximum of 6·0 wt % and experimental fO2 ranged from NNO – 0·1 to NNO + 3·4 (where NNO is nickel–nickel oxide buffer). Clinopyroxene (Mg# up to 93) is the liquidus phase for the two investigated samples; it is followed by leucite for H2O in melt <3 wt %, and by phlogopite (Mg# up to 81) for H2O in melt >3 wt %. Olivine (Fo85) crystallized spontaneously in only one experimental charge. Plagioclase was not found. Upon progressive crystallization of clinopyroxene, glass K2O and Al2O3 contents strongly increase whereas MgO, CaO and CaO/Al2O3 decrease; the residual melts follow the evolution of Vesuvius whole-rocks from trachybasalt to tephrite, phonotephrite and to tephriphonolite. Concentrations of H2O and CO2 in near-liquidus 200 MPa glasses and primitive melt inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was reconstructed by the olivine-added experiments. They show that the primitive Vesuvius melts are trachybasalts (K2O ∼ 4·5–5·5 wt %, MgO = 8–9 wt %, Mg# = 75–80, CaO/Al2O3 = 0·9–0·95) that crystallize Fo-rich olivine (90–91) as the liquidus phase between 1150 and 1200°C and from 300 to <200 MPa. Primitive Vesuvius melts are volatile-rich (1·5–4·5 wt % H2O and 600–4500 ppm CO2 in primitive melt inclusions) and oxidized (from NNO + 0·4 to NNO + 1·2). Assimilation of carbonate wall-rocks by ascending primitive magmas can account for the disappearance of olivine from crystallization sequences and explains the lack of rocks representative of olivine-crystallizing magmas. A correlation between carbonate assimilation and the type of feeding system is proposed: carbonate assimilation is promoted for primitive magma batches of small volumes. In contrast, for longer-lived, large-volume, less frequently recharged, hence more evolved, cooler reservoirs, magma–carbonate interaction is limited. Primitive magmas from Vesuvius and other Campanian volcanoes have similar redox states. However, the Cr# of Vesuvius spinels is distinctive and therefore the peridotitic component in the mantle source of Vesuvius differs from that of the other Campanian magmas.

Interaction of chemical and physical processes during deformation at fluid-present conditions: a case study from an anorthosite–leucogabbro deformed at amphibolite facies conditions

We present microstructural and chemical analyses of chemically zoned and recrystallized plagioclase grains in variably strained samples of a naturally deformed anorthosite–leucogabbro, southern West Greenland. The recorded microstructures formed in the presence of fluids at mid-crustal conditions (620–640 °C, 7.4–8.6 kbar). Recrystallized plagioclase grains (average grain size 342 μm) with a random crystallographic orientation are volumetrically dominant in high-strain areas. They are characterized by asymmetric chemical zoning (An80 cores and An64 rims) that are directly associated with areas exhibiting high amphibole content and phase mixing. Analyses of zoning indicate anisotropic behaviour of bytownite plagioclase with a preferred replacement in the \( \left\langle {0 10} \right\rangle \) direction and along the (001) plane. In areas of high finite strain, recrystallization of plagioclase dominantly occurred by bulging recrystallization and is intimately linked to the chemical zoning. The lack of CPO as well as the developed asymmetric zoning can be explained by the activity of grain boundary sliding accommodated by dissolution and precipitation creep (DPC). In low-strain domains, grain size is on average larger and the rim distribution is not related to the inferred stress axes indicating chemically induced grain replacement instead of stress-related DPC. We suggest that during deformation, in high-strain areas, pre-existing phase mixture and stress induced DPC-caused grain rotations that allowed a deformation-enhanced heterogeneous fluid influx. This resulted in local plagioclase replacement through interface-coupled dissolution and precipitation and chemically induced grain boundary migration, accompanied by bulging recrystallization, along with neocrystallization of other phases. This study illustrates a strong interaction and feedback between physical and chemical processes where the amount of stress and fluids dictates the dominant active process. The interaction is a cause of deformation and external fluid infiltration with a result of strain localization and chemical re-equilibration at amphibolite facies conditions.

Complexity in the behavior and recrystallization of monazite during high-T metamorphism and fluid infiltration

A detailed study of monazite grains in silica-undersaturated and quartz-bearing Mg–Al metapelite from the Oygarden Group of islands, east Antarctica, reveals a complex history of growth and recrystallization during two separate events in the Neoproterozoic and earliest Cambrian. Monazite grains from garnet-poor and garnet-rich metapelite preserve core domains that have ages corresponding to growth and/or recrystallization at granulite facies (P≈9–1.0GPa, T≥850–900°C) conditions during the Rayner Structural Episode between 930 and 890Ma. High-Th rims (≤22wt.% ThO2) that are in textural equilibrium with sapphirine–orthopyroxene symplectites formed after garnet during late-Rayner decompression, occur on monazite grains in garnet-rich assemblages, and give electron microprobe ages (883±18Ma) within error of core domains (903±14Ma). These high-Th rim domains are interpreted to have formed through recrystallization of liberated inclusions via a process dominated by coupled dissolution–reprecipitation reactions facilitated by transient fluid films on the large surface areas in the symplectite and not by new growth. In garnet-poor metapelite, monazite grains that have grain boundaries in textural equilibrium with the granulite-facies assemblage also show alteration to higher-Th compositions on rims and along vein-like fractures. This compositional shift is accompanied by partial- to complete-resetting of ages to ~500Ma, along with minor modification of core domain ages. Textures and patterns of chemical age resetting are interpreted to also be the result of a coupled dissolution–reprecipitation reaction process, but in contrast to garnet-bearing assemblages, this alteration occurred during a fluid influx at lower temperatures during regional ‘Pan African’ tectonism. This study highlights the susceptibility of monazite to resetting, with fluid-present conditions dramatically increasing the potential for recrystallization.

The origin of Eo- and Neo-himalayan granitoids, Eastern Tibet

Two distinctive modes of post-collisional magmatism are evident along a transect through the eastern Himalaya at ca. 92°E. The first comprises Eocene igneous complexes emplaced into the central Tethyan Himalaya Series (THS) (Dala granitoids, 44.1±1.2Ma) and the core of the Yala-Xiangbo dome (Yala-Xiangbo granitoids, 42±5Ma). Trace element characteristics and Rb–Sr and Sm–Nd isotopes indicate that the Dala granitoids formed from sub-equal mixtures of Gangdese-type magmas and rocks of the Greater Himalayan Crystallines and/or THS. The Yala-Xiangbo granitoids are geochemically complex but interpreted to be evolved equivalents of the Dala granitoid suite. The undeformed Dala plutons constrain the timing of deformation of THS metasediments to be >44Ma and their compositions indicates that components of the Indian foreland sequences had been accreted to the hanging wall of the main Himalayan decollement by that time. The second magmatic mode comprises Miocene intrusions (Arunachal- and Tsona-types) similar in most respects to the High Himalayan Leucogranites documented throughout the range. Although fluid-absent melting of metapelites appears to be the dominant origin of Miocene Himalaya leucogranites, thermometry and trace element data for the Arunachal-type intrusions suggest that they formed under fluid-present conditions. Sr and Nd-isotopes indicate that Lesser Himalayan metasediments were involved during the partial melting that produced the Arunachal-type leucogranites.

Brittle-plastic deformation in initially dry rocks at fluid-present conditions: transient behaviour of feldspar at mid-crustal levels

We present detailed microstructural and chemical analyses from an initially dry anorthositic rock deformed during wet amphibolite facies conditions. Three different domains representing the microstructural variation of the deformed samples are investigated in detail in terms of fracture morphology and mode, grain characteristics and chemistry of present phases. Results show transient deformational behaviour where a close interaction between brittle, plastic and fluid-assisted deformation mechanisms can be observed. Our analysis allows us to describe the succession, interrelationships and effects of active mechanisms with progressively increasing strain in three so-called stages. In Stage 1, initial fracturing along cleavage planes promoted fluid influx that caused fragmentation and chemical reactions, producing fine-grained mineral assemblages in the fractures. Deformation twins and dislocations developed in clast pieces due to stress relaxation. Passive rotation of conjugate fracture sets and interconnection of intracrystalline fractures formed micro-shear-zones, constituting Stage 2. Microstructures and grain relationships indicate the activity and fluctuation between fracturing, dissolution-precipitation creep, grain boundary sliding and locally dislocation creep, reflecting the transient behaviour of brittle and plastic deformation mechanisms. Further rotation and widening of fractures into overall foliation parallel shear-bands (Stage 3) promoted strain partitioning into these areas through increased fluid influx, influence of fluid-assisted grain boundary sliding, phase mixing and presence of weak phases such as white mica. We suggest that local differences in fluid availability, volume fraction of weak phases produced by fluid present metamorphic reactions coupled with volume increase and local variations in stress concentration induced transient brittle-plastic behaviour. The studied shear-zone represents an example of the transformation of a rigid dry rock to a soft wet rock during deformation through syntectonic fracturing.

Experimental breakdown of staurolite + inclusions of albite and muscovite at 800°C and 0.1 GPa

AbstractExperimental breakdown of staurolite + inclusions of albite and muscovite (representing an aluminous Fe-rich metapelitic composition) occurs within the KNFMASH system at 800°C and 0.1 GPa under fluid-present conditions over periods of 2 days to 8 weeks. The overall melt-producing reaction is staurolite + muscovite + plagioclase + vapour = hercynite + corundum + mullite + quartz + liquid, and involves three reactions; staurolite + vapour = hercynite + corundum + quartz + liquid, albite + vapour = mullite + liquid, and muscovite + vapour = mullite + liquid. Melt (glass) compositions are siliceous (77.2–64.5 wt.% SiO2), peraluminous (A/CNK = 1.5–5.5), and have granite-trondjhemite compositions in terms of normative Ab-Ab-Or contents. Textures indicate that quartz formed later than corundum. The corundum + quartz + hercynite assemblage after staurolite is considered to be metastable and only likely to be found in nature where staurolite-bearing rocks have undergone rapid heating and melting followed by quenching.

Li diffusion in zircon

Diffusion of Li under anhydrous conditions at 1 atm and under fluid-present elevated pressure (1.0–1.2 GPa) conditions has been measured in natural zircon. The source of diffusant for 1-atm experiments was ground natural spodumene, which was sealed under vacuum in silica glass capsules with polished slabs of zircon. An experiment using a Dy-bearing source was also conducted to evaluate possible rate-limiting effects on Li diffusion of slow-diffusing REE+3 that might provide charge balance. Diffusion experiments performed in the presence of H2O–CO2 fluid were run in a piston–cylinder apparatus, using a source consisting of a powdered mixture of spodumene, quartz and zircon with oxalic acid added to produce H2O–CO2 fluid. Nuclear reaction analysis (NRA) with the resonant nuclear reaction 7Li(p,γ)8Be was used to measure diffusion profiles for the experiments. The following Arrhenius parameters were obtained for Li diffusion normal to the c-axis over the temperature range 703–1.151°C at 1 atm for experiments run with the spodumene source: $$ D_{\text{Li}} = 7.17 \times 10^{ - 7} { \exp }( - 275 \pm 11\,{\text{kJmol}}^{ - 1} /{\text{RT}}){\text{m}}^{2} {\text{s}}^{ - 1}.$$ Diffusivities are similar for transport parallel to the c-axis, indicating little anisotropy for Li diffusion in zircon. Similar Li diffusivities were also found for experiments run under fluid-present conditions and for the experiment run with the Dy-bearing source. Li diffusion is considerably faster than diffusion of other cations in zircon, with a smaller activation energy for diffusion. Although Li diffusion in zircon is comparatively rapid, zircons will be moderately retentive of Li signatures at mid-crustal metamorphic temperatures, but they are unlikely to retain this information for geologically significant times under high-grade metamorphism.

Partitioning of trace elements among coexisting crystals, melt, and supercritical fluid during isobaric crystallization and melting

The distribution of trace elements among coexisting crystals, melt, and supercritical fluid during melting and crystallization is a critical constraint for understanding the evolution of magmatic systems, including the origin and development of continental and oceanic crust. Although trace-element partitioning between crystals and melt during Rayleigh fractional crystallization or melting is well-known, partitioning among co-existing melt, crystals, and supercritical fluid during anatexis or crystallization is less explored despite the ubiquity of magmatic fluids. Here we develop the trace-element differential equations governing solid-melt-fluid equilibria for melting and crystallization under fluid-present conditions and provide analytical solutions for fractional and equilibrium crystallization and melting. A compilation of solid-fluid and melt-fluid distribution coefficients for about 30 trace elements in olivine, clinopyroxene, garnet, plagioclase, alkali feldspar, biotite, amphibole, apatite, and silicic melts is provided. Forward modeling demonstrates the conditions under which fluid-melt-solid partitioning will impact trace-element signatures in magmatic systems. We show that for trace elements soluble in aqueous fluids, the composition of a melt derived by fluid-present fractional crystallization or by fluid-present fractional melting will be significantly different than in otherwise comparable fluid-absent systems. Ignoring the partitioning of soluble elements into the fluid phase leads to large errors in concentrations (over 100%) and ratios and consequent misinterpretation of the trace-element character of source material and/or the processes of fractional crystallization and melting. Although significant in any setting involving fluid-present equilibria, this analysis may have a most profound influence on fluid-present subduction zone magma generation and the evolution of shallow level fluid-saturated silicic magmatic systems.

Ti diffusion in zircon

Chemical diffusion of Ti under anhydrous conditions at 1 atm and under fluid-present elevated pressure (1.1–1.2 GPa) conditions has been measured in natural zircon. The source of diffusant for 1-atm experiments was a ZrO 2–TiO 2–zircon mixture, with experiments run in crimped Pt capsules. Diffusion experiments done in the presence of H 2O–CO 2 fluid were run in a piston-cylinder apparatus, using a source of ground TiO 2, ZrSiO 4 and SiO 2, with oxalic acid added to produce H 2O–CO 2 vapor and partially melt the solid source material, yielding an assemblage of rutile + zircon + melt + vapor. Nuclear reaction analysis (NRA) with the resonant nuclear reaction 48Ti(p,γ) 49V was used to measure diffusion profiles for both sets of experiments. The following Arrhenius parameters were obtained for Ti diffusion normal to cover the temperature range 1350–1550 °C at 1 atm: D Ti = 3.33 × 10 2 exp ( − 754 ± 56 kJ mol − 1 / R T ) m 2 s − 1 . Ti diffusivities were found to be similar for experiments run under fluid-present conditions. A fit to all of the data yields the Arrhenius relation D = 1.34 × 10 2 exp(− 741 ± 46 kJ mol − 1/RT) m 2 s − 1. These data suggest that zircon should be extremely retentive of Ti chemical signatures, indicating that the recently-developed Ti-in-zircon crystallization geothermometer [Watson, E.B., Harrison, T.M., 2005. Zircon thermometer reveals minimum melting conditions on earliest Earth. Science 308, 841–844] will be quite robust in preserving temperatures of zircon crystallization. Titanium diffuses somewhat faster in zircon than larger tetravalent cations U, Th, and Hf, but considerably more slowly than Pb, the REE, and oxygen; hence Ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost.

Mid-cretaceous crustal shortening: evidence from a regional-scale ductile shear zone in the Coastal Range of central Chile (32° S)

In the Coastal Range of central Chile, widespread Early Cretaceous volcanism associated with extensional volcanosedimetary intra- or back-arc basins and subsequent basin closures, uplift, and increased erosion/exhumation rates during the mid-Cretaceous suggests a major change from a mainly extensional tectonic regime to a relatively contractional regime and resultant crustal shortening. The author documents the contractional Silla del Gobernador shear zone (SGSZ), which developed at the western boundary of the Coastal Range in central Chile (32° S). This structure corresponds to a high-strain ductile and cataclastic shear zone that developed under low-grade (greenschist facies) metamorphic and fluid-present conditions, which indicates EW-NWW crustal shortening in a compressional (transpressional) regime. UVLAMP 40Ar/ 39Ar laserprobe dating of neoformed white mica during mylonitic deformation suggests a maximum age for the reverse ductile shearing of 109±11 Ma. An inverse isochron age of 97.8±1.5 Ma from biotite samples of a mylonitized granodiorite suggests the minimum age of deformation. These ages constrain the ductile deformation age to approximately 100 Ma (mid-Cretaceous), coeval with high exhumation/erosion rates that appear to represent uplift of the Coastal Range. The uplift and crustal shortening of the Coastal Range of central Chile has been associated with high spreading rates from the SE Pacific and southern Atlantic convergence during a change from an extensional regime developed during the Early Cretaceous to a more compressional regime that started during the mid-Cretaceous. In this sense, the SGSZ records this tectonic regime change.

Crustal melting in the alborán domain: constraints from xenoliths of the Neogene Volcanic Province

Abstract Metapelitic xenoliths enclosed in the Crd-Grt-bearing lavas of the Neogene Volcanic Province of SE Spain retain evidence of partial melting and relevant information on the mechanisms and P-T conditions of crustal anatexis, preserved by rapid exhumation and cooling during eruption. Both at El Joyazo and Mazarron, microstructures show that anatexis was accompanied by foliation development, implying that the xenoliths represent portions of a deforming crystalline basement, partially molten before being enclosed in the dacite. At El Joyazo, the xenoliths have a marked restitic composition, are made of Bt-Pl-Sil-Grt-graphite (±Ilm, Crd, Her, Qtz), and contain abundant leucogranitic glass. Primary glass inclusions in all minerals indicate that the whole restite assemblage crystallised in the presence of melt, which is only possible by a disequilibrium melting mechanism due to very rapid heating rates. Variable degrees of graphite crystallinity point to syn-anatectic crystallisation of graphite, implying that the main stage of anatexis took place under fluid-present conditions. Further melting of biotite to hercynitic spinel was probably fluid-absent. Mass balance calculations among glasses, xenoliths and probable metapelitic protoliths from the basement of the Betic Cordillera indicate degrees of melting in the range of 35–60 wt. %. Crustal anatexis took place at 5–7 kbar, 850 ± 50 °C, and was followed by a further melting stage at T>900°C, probably when the xenoliths were already incorporated into the dacite. Calculated pressures approximate the actual Moho depth in the region (ca. 21 km), and suggest that partial melting of the xenoliths occurred close to the crust-mantle boundary. The very high temperatures, the absence of HP relicts, and the syn-anatectic pseudomorphs of sillimanite after andalusite observed in many xenoliths at Mazarron, are difficult to reconcile with a model of decompression melting, and rather suggest regional scale (isobaric) heating related to emplacement at shallow depth of asthenospheric mantle and/or mantle derived magmas.

Abrupt transition in H2O activity in the melt-present zone of a thermal aureole: Evidence from H2O contents of cordierites

The water content of cordierite was analyzed in seven samples of pelitic hornfels from successive contact-metamorphic zones in the Etive thermal aureole, Scotland, using secondary ion mass spectrometry. Together with independent estimates of peak metamorphic pressure and temperature, these results were used to calculate an H 2 O activity ( a H 2 O) in each sample. The calculated a H 2 O values are considered to reflect peak metamorphic conditions. In the middle aureole calculated a H 2 O values are high, close to unity, suggesting fluid-present conditions. In the innermost aureole they are low, a H 2 O appears to be abrupt and occurs at higher grade than the melt-in isograd. The results are compatible with the operation of an up-temperature sequence of reactions in pelitic and semipelitic rocks involving, progressively, (1) dehydration, followed by (2) fluid-present partial melting, followed by (3) fluid-absent partial melting. The consumption of H 2 O by fluid-present melting reactions provides a mechanism for reducing a H 2 O at the transition. A xenolith within the Quarry Diorite appears to have crystallized in the presence of magmatically derived hydrous fluid, but wall-rock hornfels as close as 15 m from the contact have not.