Trace element thermometry of garnet-clinopyroxene pairs

We present major and trace element data on coexisting garnet and clinopyroxene from experiments carried out between 1.3 and 10 GPa and 970 and 1400 °C. We demonstrate that the lattice strain model, which was developed for applications to mineral-melt partitioning, can be adapted to garnet-clinopyroxene partitioning. Using new and published experimental data we develop a geothermometer for coexisting garnet and clinopyroxene using the concentration of rare earth elements (REE). The thermometer, which is based on an extension of the lattice strain model, exploits the tendency of minerals at elevated temperatures to be less discriminating against cations that are too large or too small for lattice sites. The extent of discrimination against misfit cations is also related to the apparent elasticity of the lattice site on which substitution occurs, in this case the greater stiffness of the dodecahedral X-site in garnet compared with the eightfold M2-site in clinopyroxene. We demonstrate that the ratio of REE in clinopyroxene to that in coexisting garnet is particularly sensitive to temperature. We present a method whereby knowledge of the major and REE chemistry of garnet and clinopyroxene can be used to solve for the equilibrium temperature. The method is applicable to any scenario in which the two minerals are in equilibrium, both above and below the solidus, and where the mole fraction of grossular in garnet is less than 0.4. Our method, which can be widely applied to both peridotitic and eclogitic paragenesis with particular potential for diamond exploration studies, has the advantage over commonly used Fe-Mg exchange thermometers in having a higher closure temperature because of slow interdiffusion of REE. The uncertainty in the calculated temperatures, based on the experimental data set, is less than ±80 °C.J.P. is grateful to Rio Tinto for a Ph.D. studentship at the University of Bristol, BGI and Dave Dobson for access to their multi-anvil apparatus and Richard Hinton for assistance with the ion-microprobe analyses. J.B. acknowledges funding from ERC Advanced Grant CRITMAG and a Royal Society Wolfson Research Merit Award. This work has benefitted from discussion with Chris Smith, Russell Sweeney, John Schumacher, Susanne Skora, and Wim van Westrenen. We thank Yan Liang and an anonymous reviewer for thoughtful reviews of our manuscript

Volatiles contents, degassing and crystallisation of intermediate magmas at Volcan de Colima, Mexico, inferred from melt inclusions

In volatile-saturated magmas, degassing and crystallisation are interrelated processes which influence the eruption style. Melt inclusions provide critical information on volatile and melt evolution, but this information can be compromised significantly by post-entrapment modification of the inclusions. We assess the reliability and significance of pyroxene-hosted melt inclusion analyses to document the volatile contents (particularly H2O) and evolution of intermediate arc magmas at Volcán de Colima, Mexico. The melt inclusions have maximal H2O contents (≤4wt%) consistent with petrological estimates and the constraint that the magmas crystallised outside the amphibole stability field, demonstrating that pyroxene-hosted melt inclusions can preserve H2O contents close to their entrapment values even in effusive eruptions with low effusion rates (0.6m3s−1). The absence of noticeable H2O loss in some of the inclusions requires post-entrapment diffusion coefficients (≤1×10−13m2s−1) at least several order of magnitude smaller than experimentally determined H+ diffusion coefficient in pyroxenes. The H2O content distribution is, however, not uniform, and several peaks in the data, interpreted to result from diffusive H2O reequilibration, are observed around 1 and 0.2wt%. H2O diffusive loss is also consistent with the manifest lack of correlations between H2O and CO2 or S contents. The absence of textural evidence supporting post-entrapment H2O loss suggests that diffusion most likely occurred via melt channels prior to sealing of the inclusions, rather than through the host crystals. Good correlation between the melt inclusion sealing and volcano-tectonic seismic swarm depths further indicate that, taken as a whole, the melt inclusion population accurately records the pre-eruptive conditions of the magmatic system. Our data demonstrate that H2O diffusive loss is a second-order process and that pyroxene-hosted melt inclusions can effectively record the volatile contents and decompression-induced crystallisation paths of vapour-saturated magma

Experimental petrology constraints on the recycling of mafic cumulate:a focus on Cr-spinel from the Rum Eastern Layered Intrusion, Scotland

Reactive liquid flow is a common process in layered intrusions and more generally in episodically refilled magma chambers. Interaction between newly injected melt and cumulates, or crystal mushes, perturbs the liquid line of descent of the melt and modifies mineral chemistry and texture. We present insights into the effects of assimilation of mafic cumulate rocks (gabbro, troctolite) by cogenetic Mg-rich basalt liquid using one-atmosphere, controlled fO2 phase equilibrium experiments on picritic parental liquid to the Rum layered intrusion, Scotland. For picrite-only experiments at fO2 = QFM, Cr-spinel (Cr# = Cr/[Cr + Al + Fe3+] = 0.43; Fe# = Fe2+/[Mg + Fe2+] = 0.32) saturates at 1320 °C, olivine (Fo88) at ~1290 °C, plagioclase (An77) at 1200 °C, and clinopyroxene (Mg#: 0.81) at 1180 °C. In melting experiments on picrite + gabbro mixtures, plagioclase (1230 °C, An80) and clinopyroxene (1200 °C, Mg#: 0.85) saturation temperature and mode are increased significantly. Cr-spinel in these experiments has a distinctive, low Fe#. In melting experiments on picrite + troctolite mixtures, plagioclase (An86) saturates at 1240 °C and clinopyroxene (Mg#: 0.81) at 1170 °C. Al-rich spinel crystallizes at high temperature (>1220 °C) and becomes more Cr-rich upon cooling, reaching the highest Cr# = 0.47 at 1180 °C (0.54 at QFM-1.2). The experimental results confirm that plagioclase and clinopyroxene stability plays a major role in determining the composition of coexisting spinel. Comparing our experimental results to the Rum Eastern Layered Intrusion, we propose a model for the precipitation of spinel from picrite–troctolite hybrid melt that is compatible with the observed olivine, plagioclase, and clinopyroxene chemistry.ISSN:0010-7999ISSN:1432-096

The sources of granitic melt in Deep Hot Zones

A Deep Hot Zone develops when numerous mafic sills are repeatedly injected at Moho depth or are scattered in the lower crust. The melt generation is numerically modelled for mafic sill emplacement geometries by overaccretion, underaccretion or random emplacement, and for intrusion rates of 2, 5 and 10 mm/yr. After an incubation period, melts are generated by incomplete crystallisation of the mafic magma and by partial melting of the crust. The first melts generated are residual from the mafic magmas that have low solidi due to concentration of H2O in the residual liquids. Once the solidus of the crust is reached, the ratio of crustal partial melt to residual melt increases to a maximum. If wet mafic magma, typical of arc environments, is injected in an amphibolitic crust, the residual melt is dominant over the partial melt, which implies that the generation of I-type granites is dominated by the crystallisation of mafic magma originated from the mantle and not by the partial melting of earlier underplated material. High ratios of crustal partial melt over residual melt are reached when sills are scattered in a metasedimentary crust, allowing the generation of S-type granites. The partial melting of a refractory granulitic crust intruded by dry, high-T mafic magma is limited and subordinate to the production of larger amount of residual melt in the mafic sills. Thus the generation of A-type granites by partial melting of a refractory crust would require a mechanism of selective extraction of the A-type mel

Chemical differentiation by mineralogical buffering in crustal hot zones

Chemical diversity in convergent margin magmas is a product of igneous differentiation in crustal hot zones, vertically extensive regions characterised by a low-volume (<20%) mobile melt phase dispersed in a crystal-rich mush. Chemical reaction between buoyant, percolating melts and the surrounding mush leads to chemical buffering by the local mineral assemblage. Where this assemblage has low thermodynamic variance (e.g. six mineral phases plus melt and H2O-CO2 fluid) the resultant multiply saturated melts will show limited chemical variability. Plutonic xenoliths from many volcanic arcs, as well as exhumed arc crustal sections, testify to the ubiquity of low-variance, broadly gabbroic, mineral assemblages. Here I use the concept of multiple saturation to explore the chemical consequences of percolative reactive melt flow in crustal hot zones using data from published experimental studies on a wide variety of different starting materials. I show that the common, low-variance hornblende gabbronorite assemblage clinopyroxene-hornblende-orthopyroxene-magnetite-plagioclase-ilmenite (CHOMPI) coexists with fluid-saturated melt over a wide range of pressure (1–10 kb) temperature (800–1050 °C) and fluid composition (molar fraction H2O, XH2O, of 1.0 to 0.3). The CHOMPI stability field is bounded by the following: the appearance of garnet at high pressure, the hydrous haplogranite granite liquidus at low temperature, and amphibole breakdown at high temperature and low pressure. CHOMPI melts cover a wide compositional range (54–74 wt% SiO2; 4.4–0.1 wt% MgO) that can be parameterised in terms of five independent variables: pressure, temperature, fO2, molar CO2/H2O in the fluid and melt K2O content. The compositional diversity and broad stability field of CHOMPI-saturated melts make them extremely common in the rock record. Melt composition parameterisations can be inverted to recover pressure (±1.3 kb), temperature (±16 °C) and fluid molar CO2/H2O (±0.43) of CHOMPI-saturated melts. If a natural magma composition can be shown to lie on or close to the CHOMPI saturation surface then the conditions under which that melt was last in equilibrium with this mineral assemblage can be established. I apply this method of magma source thermobarometry and hygrometry to the most recent eruptions from 15 Cascades arc volcanic centres. Calculated pressures range from 1.3 to 5.8 kb (5–21 km depth) with significant along-arc variation. Temperatures correlate with pressure and match independent estimates of eruption temperatures from mineral thermometry with the exception of two eruptions where significant (≤10°C) cooling occurred during pre-eruptive magma storage. Fluid XH2O is in the range 0.47–0.92 and inversely correlates with pressure. Mineralogical buffering of melt chemistry in hot zones is proposed as an important mechanism of chemical differentiation in volcanic arcs. Mineralogical buffering can operate at the low-melt fractions observed in geophysical surveys of arc crust, providing an alternative to traditional concepts of assimilation-fractional crystallisation and liquid lines of descent that operate most effectively in melt-rich systems

Generation of mantle-derived basaltic andesites in volcanic arcs

Primary magmas in volcanic arcs exhibit wide compositional diversity on both local and global scales. Processes responsible for this diversity are generally ascribed to some combination of mantle melting or crustal differentiation processes. One widespread view is that arc magmagenesis is driven by combination of H2O-fluxed and decompression melting of peridotitic mantle wedge, and that primary, mantle-derived melts are high-MgO basalt. However, a variety of other mantle-derived primitive arc magmas, ranging in composition from high-Mg andesite to picrite, has been recognised and it remains unclear to what extent this diversity can be generated by mantle melting processes modulated, for example, by changes in the thermal state of the mantle wedge or the supply of fluid from the slab. Here we use high pressure and temperature experiments to constrain magma generation conditions of a primitive magnesian (8.8 wt% MgO) basaltic andesite from Klyuchevskoy volcano, Kamchatka arc, Russia. We use an inverse experimental approach to define a multiple saturation point on the liquidus surface of the basaltic andesite. The experimental multiple saturation point defines the pressure and temperature at which an erupted melt could have last been in equilibrium with a polymineralic source rock, such as mantle peridotite, and hence provides a robust estimate of magma source conditions. Equilibrium piston-cylinder experiments were carried out between 0.5 and 1.0 GPa under hydrous conditions (3 to 6 wt% added H2O) at fO2 = DNNO+1. We show that Klyuchevskoy basaltic andesite is multiply saturated with the lherzolite assemblage ol (Fo90) + clinopyroxene + orthopyroxene + Cr-spinel close to its liquidus (≥ 95% melt) in the pressure range of 0.6 to 1 GPa (23 to 36 km depth) and 1220-1240 ºC. Amphibole is present at temperatures just below the multiple saturation point (≤ 1200 °C). Our results show that basaltic andesite was produced by 8 to 11 wt% partial melting of amphibole-lherzolite source and therefore represents a primary, undifferentiated magma extracted from its source at near- Moho depths. These findings are in a good agreement with geophysical studies of Klyuchevskoy volcano that show magmas are supplied directly from a reservoir at near- Moho depths (25-30 km) through a sub-vertical, pipe-like feeder system. Coeval high-MgO basalts from the volcano may correspond to higher degree mantle melts extracted at slightly greater depths. Our results provide a tight constraint on the thermal structure of the mantle wedge beneath Klyuchevskoy. Experimental temperatures are higher than those calculated from steady-state thermal models suggesting that upwelling asthenosphere might directly impinge the Moho in a similar fashion to mid-ocean ridges. Intra-arc rifting promotes asthenospheric decompression beneath the Central Kamchatka Depression. The presence of amphibole in our experiments at temperatures up to 1200 °C indicates that dehydration melting of amphibole peridotite formed by metasomatism of mantle wedge by slab-derived fluids is the primary magma generating process. Amphibole stability exercises an important control on melting conditions. Integrating our results with published multiple-saturation experiments we show that Klyuchevskoy basaltic andesite is one of a family of primary arc magmas whose compositions depend on mantle wedge thermal structure and H2O activity