Alan Bruce Thompson

Partial melting of subducting oceanic crust

The conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two-dimensional thermal models of subduction zones. For porosities of ~ 1% containing H20 the amount of partial melt generated at the wet basaltic solidus is limited to ~ 100 MPa) can be maintained by rocks close to, or above, their melting temperatures. In the absence of high shear stresses, substantial melting of the oceanic crust will only occur during subduction of very young (< 5 Ma) oceanic lithosphere. Partial melting of hydrated basalt (amphibolites) derived from the mid-ocean ridge has been proposed [e.g., 1-3] as being responsible for the generation of certain recent high-Al andesitic to dacitic volcanic rocks (adakites). Three of these volcanic suites (Mount St. Helens, southern Chile, and Panama) occur in volcanic arcs where oceanic crust < 25 Ma is being subducted at rates of 1-3 cm/yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments. However, oceanic lithosphere is not currently being subducted beneath Baja and New Guinea, where recent adakites are also present, suggesting that some adakite magmas may form by water-undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust. Further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately.

Melting of the continental crust: Some thermal and petrological constraints on anatexis in continental collision zones and other tectonic settings

Useful constraints on the depth-temperature range and degree of melting of lower crustal rocks have been deduced by combining the results from experimental petrology with plausible PTt paths calculated for continental collision zones. H2O is required to generate melts at the temperatures of crustal orogenesis. Our modeling has shown that residual free water from subsolidus dehydration reactions produces less than 0.5% granitic melt at the H2O-saturated solidus. Breakdown of hydrous minerals results in fluid-absent (dehydration)- melting. For average geotherms such melting of muscovite and biotite in metapelite will generate at most 25% granitic melt. Because of much higher solidus temperatures, fluid-absent melting of hornblende in metabasalt (amphibolite) will not occur in collision zones without augmented heat input from the mantle. Fluid-absent melting of epidote in metabasalt, and low Ca amphibole and biotite in metavolcanics, can produce up to 10% melt in continental collision zones. Minor melt production from dehydration-melting of chlorite, staurolite, chloritoid, or talc will occur at pressures greater than ∼0.6 GPa. The calculated paths envelope small regions or PT space, thus identifying which mineral reactions are important for crustal anatexis and at which PT conditions future experiments should be made. In a dynamically evolving crust during uplift, thermal buffering by melting reactions is a minor component of the heat bucket. Extension of thickened continental crust will not promote dehydration-melting without the incursion of mantle heat. Delamination of thickened eclogitic lower crust, or decompression of the asthenosphere in response to lithosphere thinning are the most promising mechanisms to induce extensive lower crustal melting including amphibolite.

Experimental melting of biotite + plagioclase + quartz ± muscovite assemblages and implications for crustal melting

Abstract. In order to understand the role of mica-rich rocks as a source of granite magmas, a series of melting experiments was performed on two different starting materials. The first composition is a model biotite gneiss consisting of 30 wt % biotite, 30 wt % plagioclase, and 40 wt % quartz. The second composition is a model two-mica pelites consisting of 15 wt % biotite, 15 wt % muscovite, 30 wt % plagioclase, and 40 wt % quartz. Experiments were performed under vapor-absent conditions at 1.0 GPa and between 750 o and 950oC. With only biotite in the starting material the volume of melt is always less than 15 vol % below 900oC and reaches 25 vol % at 950oC. In experiments that involve both biotite and muscovite in the starting material, the melt proportion increases up to 28 vol % at 825oC and reaches 60 vol % at 950oC. For the biotite-plagioclase-quartz (BPQ) assemblage, the solidus is located at 800oC at 1.0 GPa. The melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet + biotite + orthopyroxene + plagioclase + quartz. In addition, the experiments show that at 1.0 GPa biotite can be stable above 950oC. With both micas in the starting material (BPQM), the solidus at 1.0 GPa is located at 750oC. The melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet + sillimanite + biotite + quartz + plagioclase + Kfeldspar in experiments below 900oC. At 950oC the residuum consists of garnet + orthopyroxene + biotite + plagioclase. The melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz. Mineral modes of the source rocks, particularly the amount of quartz, are at least as important as the amount of available H20 in controlling the melt fraction generated during crustal anatexis.

Some thermal and tectonic models for crustal melting in continental collision zones

Summary Calculated geotherms and the pressure-temperature-time (PTt) paths followed by rocks during continental thickening episodes are interpreted with respect to the volumes of crustal melt that may be formed during orogenesis in the absence of heat transfer by mantle-derived melts. Particular attention is paid to a tectonic history that may characterize wider orogenic belts, such as are represented most obviously at present by Tibet. This comprises a period of crustal thickening, followed by an interval during which the crust is thinned by extensional strain, rather than by erosion. The amount of crustal melt produced depends strongly on the amount of water (free, and in hydrated minerals) contained in the lower crust. However, we may expect several (1–5) km3 of crustal melt per km2 of orogen if a crust of around average continental surface heat flux (60–70 mW m−2) is thickened by a factor of two. For the lower surface heat flux, partial melting of a sedimentary source would produce predominantly S-type granites and, with slightly higher geotherms, doubling of crustal thickness can lead to partial melting of amphibolites to give I-type granitic activity and calc-alkaline volcanism.

THERMAL EVOLUTION AND EXHUMATION IN OBLIQUELY CONVERGENT (TRANSPRESSIVE) OROGENS

Most P-T-t path models to date have considered a linear erosion rate for exhumation from burial depth, related to isostatic readjustment of crustal thickness. A few have discussed extension-enhanced exhumation. Erosional exhumation can only restore lower crustal rocks from the thickened mountain root to their previous original depth in the pre-collisional crust. One major assumption of all models to date is that the compressive forces responsible for crustal thickening cease before elevation and erosion begins. However, compression is often still active, even if the crustal thickening has stopped. Further compression of the thickened and strongly deformed orogenic root is responsible for forceful exhumation-extrusion of softened rocks upwards. Hence, the rate of exhumation is related to the rate of convergence of colliding plates. Extrusional exhumation can elevate buried rocks to any depth depending on the action of fault-shear systems. In the extrusional exhumation models examined here, the ascending rocks cool faster because they approach the zero temperature surface conditions more rapidly than by isostatic erosion. The exhumation rate is also governed by the angle between the plate boundary and the displacement vector (α), implying that the convergent plate boundaries are regarded as complex transpressive systems in which the degree of obliquity can be expressed by the ratio of pure to simple shear components. A low ratio of pure/simple shear typical for wrench-dominated plate boundaries, implies long-distance horizontal transport from the buried position in the original orogen, and longer periods during which metamorphic heating occurs. A high ratio of pure/simple shear characteristic for frontal-like convergence implies a rapid exhumation without significant heating. Thus granulites, Barrovian-type, and blueschist facies metamorphism are characterised by increasing angle of obliquity (α∼10°, α∼30°, α∼90°), respectively. The high-temperature limit at low ratio of pure/simple shear is the geotherm T∞ and lies at temperatures that would commonly be taken to indicate that addition of mantle heat was required to generate hot geotherms and P-T-t paths. Wrench faulting over 100′s of kilometres at reasonable slow convergence rates could lead to extensive dehydration melting and granulite formation in large-scale collisional orogens.

Fluid and enthalpy production during regional metamorphism

Models for regional metamorphism have been constructed to determine the thermal effects of reaction enthalpy and the amount of fluid generated by dehydration metamorphism. The model continental crust contains an average of 2.9 wt % water and dehydrates by a series of reactions between temperatures of 300 and 750° C. Large scale metamorphism is induced by instantaneous collision belt thickening events which double the crustal thickness to 70 km. After a 20 Ma time lag, erosion due to isostatic rebound restores the crust to its original thickness in 100 Ma. At crustal depths greater than 10 km, where most metamorphism takes place, fluid pressure is unlikely to deviate significantly from lithostatic pressure. This implies that lower crustal porosity can only be maintained if rock pores are filled by fluid. Therefore, porosities are primarily dependent on the rate of metamorphic fluid production or consumption and the crustal permeability. In the models, permeability is taken as a function of porosity; this permits estimation of both fluid fluxes and porosities during metamorphism. Metamorphic activity, as measured by net reaction enthalpy, can be categorized as endothermic or exothermic depending on whether prograde dehydration or retrograde hydration reactions predominate. The endothermic stage begins almost immediately after thickening, peaks at about 20 Ma, and ends after 40 to 55 Ma. During this period the maximum and average heat consumption by reactions are on the order 11.2·10−14 W/cm3 and 5.9·10−14 W/ cm3, respectively. The maximum rates of prograde isograd advance decrease from 2.4·10−8 cm/s, for low grade reactions at 7 Ma, to 7·10−10 cm/s, for the highest grade reaction between 45 and 58 Ma. Endothermic cooling reduces the temperature variation in the metamorphic models by less than 7% (40 K); in comparison, the retrograde exothermic heating effect is negligible. Dehydration reactions are generally poor thermal buffers, but under certain conditions reactions may control temperature over depth and time intervals on the order of 1 km and 3 Ma. The model metamorphic events reduce the hydrate water content of the crust to values between 1.0 and 0.4 wt % and produce anhydrous lower crustal granulites up to 15 km in thickness. In the first 60 Ma of metamorphism, steady state fluid fluxes in the rocks overlying prograde reaction fronts are on the order of 5·10−11 g/cm2-s. These fluid fluxes can be accommodated by low porosities (<0.6%) and are thus essentially determined by the rate of devolitalization. The quantity of fluid which passes through the metamorphic column varies from 25000 g/cm2, within 10 km of the base of the crust, to amounts as large as 240000 g/cm2, in rocks initially at a depth of 30 km. Measured petrologic volumetric fluid-rock ratios generated by this fluid could be as high as 500 in a 1 m thick horizontal layer, but would decrease in inverse proportion of the thickness of the rock layer. Fluid advection causes local heating at rates of about 5.9·10−14 W/cm3 during prograde metamorphism and does not result in significant heating. The amount of silica which can be transported by the fluids is very sensitive to both the absolute temperature and the change in the geothermal gradient with depth. However, even under optimal conditions, the amount of silica precipitated by metamorphic fluids is small (<0.1 vol %) and inadequate to explain the quartz veining observed in nature. These results are based on equilibrium models for fluid and heat transport that exclude the possibility of convective fluid recirculation. Such a model is likely to apply at depths greater than 10 km; therefore, it is concluded that large scale heat and silica transport by fluids is not extensive in the lower crust, despite large time-integrated fluid fluxes.

Thermally softened continental extensional zones (arcs and rifts) as precursors to thickened orogenic belts

Abstract Intra-continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence. This enables significant thickening with formation of an orogenic root. We have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra-continental extension with associated heating. Geologically this situation is appropriate to intra-continental rifts and back-arc basins. If thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak (soft) lower crust and sub-arc/rift mantle result. This weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot. These regions are associated with formation of granulites and metamorphic assemblages typical of high-temperature/low-pressure (HT/LP). If convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc/rift domain. Such sites are ideal for the formation and for the exhumation of metamorphic core complexes. Subsequent thickening during convergence leads to HT eclogites when the previous arc/rift was hot and to medium-T eclogites for a thickened “standard” geotherm. These P–T paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm. If the compression starts long after cessation of the extensional event and associated thermal anomaly, then the geotherm of the extended area relaxes and the whole region hardens. In this case, no homogeneous thickening occurs and deep continental roots cannot form.

Experimental approach to constrain second critical end points in fluid/silicate systems: Near-solidus fluids and melts in the system albite-H2O

Abstract Experimental investigations in the system albite-water have been carried out to evaluate the accuracy of a new procedure to determine major-element solubilities in fluid phases and water solubilities in melts. The system albite-water had been examined previously and some consensus has been achieved with respect to phase relations, solidi, and water solubilities in the melts. Above 15 kbar the system approaches a second critical end point, where a distinction between fluid and melt no longer can be made and the term solidus has to be reconsidered. All experimental runs were carried out at near-solidus conditions at 5-17 kbar and 625-775 °C. Experimental charges contained albite, water, and a layer of diamond crystals (grain size 50 mm). The pore space between the diamonds is preserved during the run indicating that the fluid was able to circulate throughout the entire capsule. During quenching, the material dissolved in the fluid precipitated between the diamond crystals and, as a result, could so be separated from the solid residue. The recovered capsules were directly embedded in epoxy and the diamond layers were analysed by laser ablation microprobe (LAM-ICP-MS). The new method allows the determination of a broad range of silicate/water ratios in aqueous fluids and hydrous melts. In some experimental runs at 15 and 17 kbar, fluids with approximately 40 wt% solute could be trapped leading to the conclusion that supercritical conditions were reached. Results furthermore indicate that albite dissolves non-stoichiometrically at 5 kbar producing a higher Na/Al and Si/Al in the solute than in the melt. Dissolution and melting at 10 kbar and higher pressures appears to be congruent within the limits of this method.

Extrusion tectonics and elevation of lower crustal metamorphic rocks in convergent orogens

In the proposed model, weak zones of rheologically homogeneous crust, continually compressed between rigid lithospheric indenting plates, can be exhumed upward by extrusion. The rate of exhumation is governed by the rate of convergence of approaching plates and by the width of the weak deformable zone. Modeled extrusion results in near-isothermal decompression during the rapid elevation history in narrow orogenic belts for realistic plate velocities. Such pressure-temperature-time paths exhibit distinct collapsed geochronologies and record maximum metamorphic temperatures ( T max ) at the maximum burial depth ( P max ).

Model systems for anatexis of pelitic rocks: II. Facies series melting and reactions in the system CaO-KAlO2-NaAlO2-Al2O3-SiO2-H2O

Subsolidus and melting reactions involving calcic plagioclase in pelitic assemblages in the K-Na-Ca model system occur at higher temperatures than their K-Na counterparts. For the most calcic plagioclase compositions observed in high-grade pelitic rocks (An25-An40) the equilibria are rarely extended by more than 30 ° C above those in KA1O2-NaAlO2-Al2O3-SiO2-H2O, although all discontinuities in facies inferred for the K-Na system are continuously displaced when they involve Ca-bearing plagioclase. The maximum pressure-temperature overlap between muscovite dehydration and initial melting reactions occurs in the pressure range of 4–6 kbar between about 640 ° and 720 ° C. This provides optimum conditions for anatectic melt generation in felsic rocks of the appropriate compositions progressively metamorphosed in kyanite-sillimanite facies series. Progressive regional metamorphism at pressures of 2–4 kbar, corresponding to andalusite-sillimanite facies series, shows little overlap between muscovite dehydration and initial melting reactions. Consequently anatectic melt generation in andalusite-sillimanite facies series would require the participation of biotite in dehydration-melting reactions. Felsic intrusive rock in andalusite-sillimanite terranes could have risen upward from their anatectic sites in high grade kyanite-sillimanite facies series at depth. Many andalusite-sillimanite facies series terranes culminating in migmatites could represent upward movement of kyanite-sillimanite facies series rocks to shallower depths with uplift rates faster than cooling rates.