Equilibrium Partial Melting Research Articles

Mechanism of Formation of Premian Volcanic Rocks in Sawu'er Region, Xinjiang, China: Constraints from Rare Earth Elements

A large quantity of Premian volcanic rocks formed in Sawu'er region of Xinjiang, China. Based on the analysis of the rare earth elements' (REEs) geochemical characteristics of the volcanic rocks, the correlativity of REE and major elements, and diagrams of covariant relation of REE, combined with the research results of the regional geological setting, petrology, and petrochemistry characteristics, it is inferred that the mechanism of the formation of Premian basic and intermediate volcanic rocks is mainly attributed to equilibrium partial melting, and the magmatic fractional crystallization is not significant, whereas the rock-forming processes of medium-acidic volcanic rocks is influenced by both partial melting and fractional crystallization. The REE geochemical research provided important evidence for the mechanism of magmatism of the Premian post-collision stage and the geodynamical evolution process.

Geochemistry and petrogenesis of tholeiitic intrusions of possible Bushveld-age in the Vredefort Dome, South Africa

The tholeiitic group of intrusions that occurs in the core and collar rocks of the Vredefort dome comprises three related intrusions, namely the Wittekopjes norite, the Parsons Rust dolerite-norite and the Reebokkop dolerite. The texture of these intrusions varies from hypidiomorphic-granular in the norite to the distinctive poikilitic character of the dolerite-norite to hypidiomorphic to subophitic texture of the dolerite. The major constituent minerals, pyroxene and plagioclase, are normally zoned. Chemical variations in the normally zoned minerals indicate a differentiating magma. The tholeiitic group comprises siliceous high-MgO tholeiitic rocks with a strong noritic character. The characteristic geochemistry of the group includes high MgO, SiO 2, Cr and Ni contents; low enrichment in light and heavy rare earth elements (REE) with regard to average chondrite composition; absence of significant fractionation of light REE with regard to heavy REE; and low TiO 2, P 2O 5, K 2O, Sr, Zr, Y, and Nb contents. Geochemical variation within the sills conforms to normal differentiation trends. To explain the relatively high SiO 2 content in these mafic intrusions two mechanisms can be proposed: incongruent melting of enstatite in a depleted mantle source will generate forsteritic olivine and progressively silica enriched melts; or alternatively, crystallisation of a high percentage of olivine could result in silica enrichment of an ultramafic magma. Two possible scenarios for the generation of a noritic magma with composition similar to that of the Wittekopjes norite are proposed. Firstly, derivation of a noritic magma by means of fractional crystallisation of approximately 50% olivine from an ultramafic Bushveld-type magma. Secondly, derivation of a primary noritic magma from depleted mantle material with near chondritic REE composition, by approximately 42% equilibrium partial melting. The mantle material is similar to the depleted peridotite nodules in South African kimberlite. It is illustrated that the Parsons Rust dolerite-norite and the Reebokkop dolerite can be derived from such a noritic magma by means of fractional crystallisation of olivine, pyroxene and plagioclase. The tholeiitic group is compared to other mafic intrusions in the Vredefort dome. It differs from the tholeiitic Annas Rust dolerite and associated rocks, and can, just like the epidiorite, be classified as calc-alkaline. Considering field, petrographical and geochemical evidence it is postulated that the tholeiitic group is of Bushveld age.

Origin of ureilites inferred from a SIMS oxygen isotopic and trace element study of clasts in the Dar al Gani 319 polymict ureilite

Secondary ion mass spectrometer (SIMS) oxygen isotope analyses were performed on 24 clasts, representing 9 clast types, in the Dar al Gani (DaG) 319 polymict ureilite with precisions better than 1‰. Olivine-rich clasts with typical ureilitic textures and mineral compositions have oxygen isotopic compositions that are identical to those of the monomict ureilites and plot along the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Other igneous clasts, including plagioclase-bearing clasts, also plot along the CCAM line, indicating that they were derived from the ureilite parent body (UPB). Thus, we suggest that some of the plagioclase-bearing clasts in the polymict ureilites represent the “missing basaltic component” produced by partial melting on the UPB. Trace element concentrations (Mg, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr and Ba) in ureilitic plagioclase and glass from 13 clasts were obtained by using the SIMS high mass resolution method. The trace element contents of the plagioclase generally show monotonic variations with anorthite content (mol%) that are consistent with partial melting and fractional crystallization. Incompatible trace element concentrations (K, Ti, and Ba) are low and variable for plagioclase with An > 40, indicating that the parental magmas for some clasts were derived from a depleted source. We performed partial melt modeling for CI and CM precursor compositions and compared the results to the observed trace element (K, Ba, and Sr) abundances in the plagioclase. Our results indicate that (1) the UPB evolved from a alkali-rich carbonaceous chondritic precursor, (2) parent melts of porphyritic clasts could have formed by 5–20% equilibrium partial melting and subsequent fractional crystallization, and (3) parent melts of the incompatible trace element-depleted clasts could be derived from fractional melting, where low degree (<10%) partial melts were repeatedly extracted from their solid sources. Thus, both the oxygen isotopic and trace element compositions of the plagioclase bearing clasts in DaG-319 suggest that the UPB underwent localized low degree-partial melting events. The partial melts could have been repeatedly extracted from the precursor, resulting in the formation of the olivine-pigeonite monomict ureilites as the final residue.

226Ra-excess during the 1631-1944 activity period of Vesuvius (Italy) : A model of alpha-recoil enrichment in a metasomatized mantle and implications on the current state of the magmatic system

The origin of the 226Ra-excess during the last cycle of Vesuvius activity was investigated by high-resolution γ-spectrometry, TIMS and EDXRF. Lavas display high initial 226Ra-excess (500–1000%), similar ( 230Th/ 232Th ) activity ratios (0.87–0.91) and most samples show significant 238U-excess. During the period 1631–1944 the initial absolute 226Ra-excess reached the highest values (19–44 dpm g −1) recorded for earth volcanoes. Crystal fractionation and particularly leucite floating did not cause the 226Ra-excess in spite of the high 226Ra activity (21–85 dpm g −1) in leucite. The presence of phlogopite in the mantle source, documented by field and petrological evidences on local mantle-derived xenoliths, rules out that equilibrium partial melting can be responsible for the 226Ra-excess. This primary feature may be explained by a multistage process involving metasomatic mantle fluids (MMFs) flowing through a mantle wedge where U is concentrated as U-accessory minerals deposited along microfractures. Fluids, passing through the mantle wedge, are supplied of 226Ra, 230Th and 234U by α-decay recoil of parent nuclides from U-enriched microfractures. This model calculates that the ascent time of fluids through the mantle wedge was ≤ 12 ka. Successively MMFs mixed with mantle-derived melts, giving rise to 226Ra-enriched magmas, which entered the Vesuvius plumbing system less than 7 ka. Crystal fractionation did not affect extensively the initial 226Ra/Ba ratio, which varied in the 1631–1944 period according to a pattern reflecting periodic inputs of 226Ra-enriched magma, variable reservoir volumes and residence times in magmatic chamber(s). The temporal trend of the reservoir volumes, extrapolated to the present time, indicates a volume of magma of ∼ 0.021 km 3, stored most probably in a shallow chamber.

On the thermo‐kinetic consequences of slab melting

REE abundances in melts produced by partial melting of a hornblende‐bearing eclogite were examined using a dynamic disequilibrium‐melting model and following the P‐T path of a young subducting slab. Model calculations show that melts produced by small degrees of slab melting at 700∼900°C are more likely affected by sluggish kinetics, and have lower La/Yb ratios, higher Li2O and Na2O abundances than melts produced by large degrees of melting at higher temperatures. The extent of disequilibrium depends on deformation and melting histories of the slab, and effective grain sizes and transport properties of the minerals. The LREE abundances in the large‐degree melts are broadly similar to those in adakites. If kinetics is important in slab melting, the degrees of melting required to generate adakites may be higher than values predicted by equilibrium partial melting models and REE abundances in the melts.

Interpretation of trace element and isotope features of basalts: relevance of field relations, petrology, major element data, phase equilibria, and magma chamber modeling in basalt petrogenesis

The concentrations and ratios of the major elements determine the physical properties and the phase equilibria behavior of peridotites and basalts in response to the changing energy contents of the systems. The behavior of the trace elements and isotopic features are influenced in their turn by the phase equilibria, by the physical character of the partial melting and partial crystallization processes, and by the way in which a magma interacts with its wall rocks. Concentrating on the trace element and isotope contents of basalts to the exclusion of the field relations, petrology, major element data, and phase equilibria is as improvident as slaughtering the buffalo for the sake of its tongue. The crust is a cool boundary layer and a density filter, which impedes the upward transfer of hot, dense “primary” picritic and komatiitic liquids. Planetary crusts are sites of large-scale contamination and extensive partial crystallization of primitive melts striving to escape to the surface. Escape of truly unmodified primitive melts to the surface is a rare event, requiring the resolution of daunting problems in chemical and mechanical engineering. Primary status for volumetrically abundant basalts such as mid–ocean ridge basalt, ocean island basalt, and continental flood basalts is denied by their low-pressure cotectic character, first remarked upon on petrological grounds in 1928 and on experimental grounds in 1962. These basalt liquids are products of crystal–liquid separation at low pressure. Primary status for these common basalts is further denied by the phase equilibria of such compositions at elevated pressures, when the required residual mantle mineralogy (magnesian olivine and orthopyroxene) is not stable at the liquidus. It is also denied by the picritic or komatiitic nature of partial melts of candidate upper-mantle compositions at high pressures—a conclusion supported by calculation of the melt composition, which would need to be extracted in order to explain the chemical variation between fertile and residual peridotite in natural ultramafic rock suites. The subtleties of magma chamber partial crystallization processes can produce an astounding array of “pseudospidergrams,” a small selection of which have been explored here. Major modification of the trace element geochemistry and trace element ratios, even those of the highly incompatible elements, must always be entertained whenever the evidence suggests the possibility of partial crystallization. At one extreme, periodically recharged, periodically tapped magma chambers might undergo partial crystallization by ∼95% consolidation of a succession of small packets of the magma. Refluxing of the 5% residual melts from such a process into the main body of melt would lead to eventual discrimination between highly incompatible elements in that residual liquid comparable with that otherwise achieved by 0.1 to 0.3% liquid extraction in equilibrium partial melting. Great caution needs to be exercised in attempting the reconstruction of more primitive compositions by addition of troctolite, gabbro, and olivine to apparently primitive lava compositions. Special attention is focussed on the phase equilibria involving olivine, plagioclase (i.e., troctolite), and liquid because a high proportion of erupted basalts carry these two phases as phenocrysts, yet the equilibria are restricted to crustal pressures and are only encountered by wide ranges of basaltic compositions at pressures less than 0.5 GPa. The mere presence of plagioclase phenocrysts may be sufficient to disqualify candidate primitive magmas. Determination of the actual contributions of crustal processes to petrogenesis requires a return to detailed field, experimental, and forensic petrologic studies of individual erupted basalt flows; of a multitude of cumulate gabbros and their contacts; and of upper-mantle outcrops.

Minor Phases as Carriers of Trace Elements in Non-Modal Crystal–Liquid Separation Processes II: Illustrations and Bearing on Behaviour of REE, U, Th and the PGE in Igneous Processes

Minor phases which strongly concentrate selected trace elements, termed here ‘carrier-phases’, release relatively large amounts of those elements to the liquid phase when they are eliminated during partial melting and glean relatively large amounts of those elements when they first appear during progressive crystallization. It is characteristic of such relationships that concentrations of the selected trace elements in the bulk residues of partial melting will rise to a peak somewhat before the last of the carrier-phase is eliminated during progressive melting. In the liquids produced during equilibrium partial melting a corresponding peak in the concentration of the trace element occurs at the point where the carrier-phase is eliminated; the corresponding peak in trace element concentration in the liquids produced by accumulated perfect fractional melting is found somewhat above that point. These peaks become more sharply accentuated as the distribution coefficient of the trace element into the carrier-phase increases. The highest trace element concentration in a partial melt liquid product is found in the small drop of liquid produced during perfect fractional melting at the point where the carrier-phase is eliminated. Still higher concentrations may be found in the first cumulates containing the carrier-phase which precipitate during perfect fractional crystallization but the corresponding liquids do not contain exceptionally high concentrations. Under favourable conditions a large proportion of the available mass of a trace element in a magmatic system may be transferred from the solid to the liquid phases or vice versa with only a small change in the mass fraction of liquid in and energy content of the system. Within that range, separation of otherwise very similarly behaved trace elements becomes possible. Further complexities arise and the opportunities for separation increase when two carrier-phases compete with differing success for the same group of trace elements.

Incompatible trace element partitioning and residence in anhydrous spinel peridotites and websterites from the Ronda orogenic peridotite

We report solution-ICPMS analyses of Rb, Ba, Th, U, Nb, Ta, REE, Sr, Zr and Hf for acid-leached minerals of anhydrous spinel peridotites and websterites from the Ronda peridotite (S. Spain). The same elements were also analyzed by LA-ICPMS in the silicates of three peridotites. The results obtained by solution-ICPMS and LA-ICPMS are similar for the less (HREE) and the most incompatible (Rb–Ba) elements, and provide comparable inter-element distribution coefficients ( D xt/cpx) for these elements. However, moderately incompatible elements (typically LREE) show significant discrepancies between solution and in situ analyses. D opx/cpx and D ol/cpx for these elements are generally lower for solution than for in situ analyses. D xt/cpx for MREE, HREE, Zr and Hf are consistent with experimental values. In contrast, D xt/cpx for highly incompatible elements and LREE are higher than expected from available experimental data and/or crystal-chemical considerations. The observed D xt/cpx for the most incompatible trace elements may be explained by very small amounts of melt/fluid, or solid, inclusions trapped in these minerals. Inclusions would affect both solution- and LA-ICPMS data, but their proportion would be less important for LA-ICPMS analyses. We show with a mixing model that an extremely small amount of equilibrium partial melt (typically 0.01–0.1%) trapped in minerals is sufficient to increase the D opx/cpx for HIE and LREE by a factor of 5–20 and the D ol/cpx by two or three orders of magnitude. Similar effects may be produced by sub-percent amounts of HIE-rich fluids of solid microphases. Such very small volumes of inclusions may pass unnoticed during mineral handpicking and LA-ICPMS analysis. Hence, D xt/cpx for HIE and LREE should be considered cautiously when mineral analyses are used to constrain melt processes and mantle composition. Mass balance calculations were performed for a nominally anhydrous spinel harzburgite sample. Similar to previous studies, the mass balance indicates important discrepancies for HIE between peridotite composition reconstructed from mineral analyses (bulk and in situ) and whole rock composition. The major silicate minerals are the main repositories for REE, Zr and Hf (>75% of the whole rock budget), and also host ≥65% of Th and U. In contrast, more than 80% of the budget of Rb, Ba and Nb, and about 60% of Ta and Sr, is hosted by micro-components in grain boundaries (GBC) or trapped in minerals (inclusions). Alone, the GBC accounts for 50% of the budget of Nb and Ta. The inclusions are an important repository for Rb (39%), Nb (40%) and Sr (49%). The GBC and inclusion repositories display very similar trace element signatures, suggesting that they were once a single repository (<1 wt%) now re-distributed in different textural components. This repository could be a combination of hydrous phases and/or Ti oxides, and/or melt/fluid inclusions of mantle origin.

Petrogenesis of silicate inclusions in the Weekeroo Station IIE iron meteorite: differentiation, remelting, and dynamic mixing

The Weekeroo Station IIE iron meteorite contains a variety of felsic and mafic inclusions enclosed in an FeNi-metal host. Petrographic, EMP, and SIMS data suggest that the petrogenesis of the silicates was complex, and included differentiation, remelting, FeO-reduction, and dynamic mixing of phases. Differentiation produced a variety of olivine-free inclusion assemblages, ranging from pyroxene + plagioclase + tridymite with peritectic compositions, to coarse orthopyroxene, to plagioclase + tridymite and its glassy equivalent. Individual phases have similar trace-element abundances and patterns, despite large variations in inclusion textures, modes, and bulk compositions, probably as a result of mechanical separation of pre-existing phases in an impact event that dynamically mixed silicates with the metallic host. Trace-element data imply that augite and plagioclase grains in different inclusions crystallized from the same precursor melt, characterized by relatively unfractionated REE abundances of ∼20–30 × CI-chondrites except for a negative Eu anomaly. Such a precursor melt could have been produced by ∼2–5% equilibrium partial melting of an H-chondrite silicate protolith, or by higher degrees of partial melting involving subsequent fractional crystallization. Glass appears to have formed by the remelting of pre-existing plagioclase and orthopyroxene, in a process that involved either disequilibrium or substantial melting of these phases. During remelting, silicate melt reacted with the FeNi-metal host, and FeO was reduced to Fe-metal. Following remelting and metal-silicate mixing, inclusions apparently cooled at different rates in a near-surface setting on the parent body; glass- or pigeonite-bearing inclusions cooled more rapidly (≥2.5°C/hr between 1000–850°C) than pigeonite-free, largely crystalline inclusions. The results of this study point to two likely models for forming IIE iron meteorites, both involving collision between an FeNi-metal impactor and either a differentiated or undifferentiated silicate-rich target of H-chondrite affinity. Each model has difficulties and it is possible that both are required to explain the diverse IIE group.

Are ureilites residues from partial melting of chondritic material? The answer from MAGPOX

Abstract— Calculations performed using MAGPOX show that no bulk compositions having chondritic Ca/Al ratio and within the range of chondritic Si/Mg ratios can produce the olivine‐pigeonite ureilites (which constitute 65% of those for which modal abundances are known) as residues of single‐stage equilibrium partial melting. Calcium/aluminum ratios of 2–3.5 × CI are required. In addition, all the ureilites could not have formed from a single composition at various degrees of reduction, because they show no correlation between pigeonite/olivine ratio and mg ratio. Materials with various Al/Mg ratios, ranging from subchondritic to superchondritic, are required. If these materials are primitive (i.e., created by nebular processes rather than planetary igneous processes), they are unknown in the meteorite record. Excess accretion (relative to chondrites) of 5–10 mol% of a high‐temperature condensate component, which was itself almost completely depleted in corundum due to early fractionation, could create the necessary compositions. The plausibility that such processes occurred on a parent‐body sized scale is difficult to assess. In contrast, lodranites can be produced as residues of ∼3–30% equilibrium partial melting of an average ordinary chondritic composition at the appropriate level of reduction. Although many features of ureilites suggest that they are relatively primitive residues produced by low degrees of melting of chondritic materials, and thus resemble lodranites and other groups of primitive achondrites, their predominantly pigeonite + olivine mineralogy remains difficult to explain within this simple scenario.

Glasses in mantle xenoliths from western Victoria, Australia, and their relevance to mantle processes

Mantle-derived xenoliths from western Victoria, Australia, contain glass in patches and veinlets, and as secondary inclusions within xenolith minerals. Glass patches are commonly associated with primary clinopyroxene, spinel and in some cases relict pargasitic amphibole, phlogopite, apatite or calcitic carbonate. The patches and veins are filled with secondary microphenocrysts of olivine, clinopyroxene and spinel. The siliceous, aluminous, alkali-rich glasses display ranges in major element compositions which are similar to those of glasses in mantle xenoliths worldwide. Textures and compositions of glass and associated phases in the Victorian xenolith suite argue strongly against origins involving equilibrium partial melting of spinel peridotite, or migration of exotic metasomatic melts. Metasomatism of the southeastern Australian lithosphere is expressed by formation of amphibole, phlogopite, and apatite, and increased abundances of clinopyroxene at the expense of orthopyroxene, in lherzolite or harzburgite. Partial or complete melting of this metasomatic assemblage occurred immediately prior to, or during, entrainment of the xenolith in the host magmas, and locally produced disequilibrium liquids, now preserved as glass. Melting in the xenoliths was caused by the thermal and decompressional effects of transport in the host magma, or by thermal and metasomatic effects in the lithosphere associated with adjacent intrusion of magmas. Large inter-xenolithic variation in glass compositions was caused by variations in the nature and proportions of the precursor metasomatic assemblage, by reaction of the melts with primary orthopyroxene in the lherzolites and harzburgites, and by rapid, disequilibrium crystallization from the melts of an assemblage of olivine, clinopyroxene and spinel in all samples.

Integrated Compositions of Perfect Fractional Liquid and Solid Products are Identical, with Respect to Ideal Trace Element Behaviour, to Simple Equilibrium (Batch) Products in Very Special Cases

of liquid involved are zero and unity; and that the Partial melting of natural rocks is likely to proceed with bulk crystal–liquid distribution coefficient, d, is assumed the pooling of liquids which have been produced by a constant throughout each of the processes. range of mass fractions of liquid development (O’Hara, Given the first two conditions listed above, the average 1985, 1995; Richter, 1986; Langmuir, 1989; Eggins, mass fraction of liquid, f av, developed in linear planform 1992; Plank & Langmuir, 1992; O’Hara & Fry, 1996). circumstances must be ( f max+ f min)/2, and given the third Partial crystallization of magmas in magma chambers condition this expression must have the value 0·5 (circular may involve the integration of residual liquids from a planform circumstances are mentioned briefly below). range of mass fractions of liquid survival either through The relationship between C L, the concentration of an the crystallization zone or across the crystallizing surface element in the equilibrium (batch) partial melting (EPM ) (Langmuir, 1989; O’Hara & Fry, 1996). The conor equilibrium partial crystallization (EPC) liquid, and centrations of ideally behaved trace elements in an inC S, the concentration of the element in the source tegrated liquid or solid and that in the product of a simple material which began to undergo partial melting or process operating at the same average mass fraction of crystallization, is obtained from liquid in the system are, in general, significantly different (O’Hara, 1995; O’Hara & Fry, 1996). However, under CL CS = 1

The Aurora volcanic field, California-Nevada: oxygen fugacity constraints on the development of andesitic magma

The Aurora volcanic field, located along the northeastern margin of Mono Lake in the Western Great Basin, has erupted a diverse suite of high-K and shoshonitic lava types, with 48 to 76 wt% SiO2, over the last 3.6 million years. There is no correlation between the age and composition of the lavas. Three-quarters of the volcanic field consists of evolved (<4 wt% MgO) basaltic andesite and andesite lava cones and flows, the majority of which contain sparse, euhedral phenocrysts that are normally zoned; there is no evidence of mixed, hybrid magmas. The average eruption rate over this time period was ∼200 m3/km2/year, which is typical of continental arcs and an order of magnitude lower than that for the slow-spreading mid-Atlantic ridge. All of the Aurora lavas display a trace-element signature common to subduction-related magmas, as exemplified by Ba/Nb ratios between 52 and 151. Pre-eruptive water contents ranged from 1.5 wt% in plagioclase-rich two-pyroxene andesites to ∼6 wt% in a single hornblende lamprophyre and several biotite-hornblende andesites. Calculated oxygen fugacities fall within –0.4 and +2.4 log units of the Ni-NiO buffer. The Aurora potassic suite follows a classic, calc-alkaline trend in a plot of FeOT/MgO vs SiO2 and displays linear decreasing trends in FeOT and TiO2 with SiO2 content, suggesting a prominent role for Fe-Ti oxides during differentiation. However, development of the calc-alkaline trend through fractional crystallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric iron that its oxygen fugacity would have fallen several log units below that of the Ni-NiO buffer. Nor can fractionation of hornblende be invoked, since it has the same effect as titanomagnetite in depleting the residual liquid in ferric iron, together with a thermal stability limit that is lower than the eruption temperatures of several andesites (∼1040–1080°C; derived from two-pyroxene thermometry). Unless some progressive oxidation process occurs, fractionation of titanomagnetite or hornblende cannot explain a calc-alkaline trend in which all erupted lavas have oxygen fugacites ≥ the Ni-NiO buffer. In contrast to fractional crystallization, closed-system equilibrium crystallization will produce residual liquids with an oxygen fugacity that is similar to that of the initial melt. However, the eruption of nearly aphryic lavas argues against tapping from a magma chamber during equilibrium crystallization, a process that requires crystals to remain in contact with the liquid. A preferred model involves the accumulation of basaltic magmas at the mantle-crust interface, which solidify and are later remelted during repeated intrusion of basalt. As an end-member case, closed-system equilibrium crystallization of a basalt, followed by equilibrium partial melting of the gabbro will produce a calc-alkaline evolved liquid (namely, high SiO2 and low FeOT/MgO) with a relative f O 2 (corrected for the effect of changing temperature) that is similar to that of the initial basalt. Differentiation of the Aurora magmas by repeated partial melting of previous underplates in the lower crust rather than by crystal fractionation in large, stable magma chambers is consistent with the low eruption rate at the Aurora volcanic field.

Granite-migmatite genetic link: the example of the Manaslu granite and Tibetan Slab migmatites in central Nepal

In central Nepal, the Tibetan Slab is made up of biotite-gneisses (metapelites and metagreywackes), orthogneisses (metagranites) and migmatites. Melanosomes are generally biotite-(± muscovite)-bearing, but locally they may be tourmaline-rich when associated with boron-rich granitic material. Leucosomes occur as lenses conformable with the foliation, veins, patches, or as fillings in shear zones and extensional structures. Field relationships, and mineralogical and chemical data show that three processes may have contributed to the formation of the Tibetan Slab leucosomes: metamorphic differentiation or disequilibrium partial melting (low-Zr tonalitic leucosomes), in-situ equilibrium partial melting (high-Zr leucosomes and some granitic leucosomes) and injection of externally-derived melts (most granitic and some tonalitic leucosomes). The Manaslu pluton belongs to the High Himalayan leucogranite belt and was emplaced at the top of the Tibetan Slab. It corresponds to a muscovite-biotite leucogranite that has been assumed to derive from melting of the Tibetan Slab gneisses (Formation I). Phase relationships, a more magnesian chemistry of the ferromagnesian minerals from the Tibetan-Slab migmatites as compared to the Manaslu leucogranite, the microtextures of accessory phases, and trace-element compositions (lower U, Li, F and higher Sr, Eu, Y, Yb contents in the migmatite leucosomes) show that the in situ Himalayan migmatites, at the crustal level presently exposed, have not been produced under the same P-T-X H 2O conditions as the Manaslu leucogranite magma. While the Formation I was the probable source for the Manaslu granite, migmatites within the formation are not the remanants of a melting process from which the Manaslu granite was derived. Both the Tibetan Slab migmatites and the Manaslu leucogranite may be considered as evidence of dehydration and melting at deeper crustal levels, and of percolation of melts and hydrothermal fluids through the crust.

Estimation of the primary magma compositions of an igneous rock series petrogenetically associated with fractional crystallization with special reference to element abundance relationships

A primary magma not only represents the starting point of a fractional crystallization process, but also is the product of an equilibrium or fractional partial melting process in a mantle. Element abundance relationships in the primary magma obey both the law of power function for fractional crystallization and the law of fractional linear function for equilibrium partial melting. Based on this double nature of the primary magma, the authors advanced a principle to restore the primary magma composition from that of an igneous rock series with petrogenesis of fractional crystallization and put forward an approach of estimating the element abundance of the primary magma, exemplified by the rare-earth elements in the Andes volcanic rock series.

Superimposing Magma Chamber and Melting Regime Processes: How Easy is it to (Mis)Interpret the Liquid Products?

Choice of partial melting model. Less-thanperfect fractional melting processes (O'Hara 1993) yield liquid products which quite closely simulate products of equilibrium partial melting (EPM); a useful first approximation is that the partial melting event supplies a liquid which is an EPM product of the true source region composition. Choice of magma chamber process. Less-thanperfect fractional crystallisation processes (O'Hara 1993, O'Hara and Mathews 1981) in magma chambers yield liquid products which quite closely simulate those of equilibrium partial crystallisation (EPC) of the gross input to the magma chamber, including any materials assimilated from the roof and walls; a useful first approximation is that the partial crystallisation event supplies a liquid which is an EPC product (the equations are identical to EPM) of the average magma chamber input composition. Simplifying assumptions. In this formulation the space problem is ignored and contamination by assimilation of country rock or previously erupted lavas assumed to be non-existent. It is further assumed that the distribution coefficient, d, for each element remains constant throughout both processes and is the same in each process. A simple two-stage model The simplest twostage model of a 'real' system in which erupted

Melting Behaviour of Mantle Peridotites and Compositional Variations of Partial Melts at High Pressures

Determination of composition of melts formed by partial melting of peridotite at high pressures is important for understanding genesis of magmas in the upper mantle; however, the previous experimental studies had more or less shortcoming in the method and the results were not always satisfactory. Recently, a new method which seems to have less shortcoming than the previous methods was developed independently by two groups. In the new method, aggregates of diamond grains are placed on top of or between peridotite layers. Upon heating partial melts formed in the peridotite layers segregate into pore space between diamond grains, and after quencheing they can be analysed without quench problem (Johnson and Kushiro, 1992; Kushiro and Hirose, 1992, Baker et al., 1992; Hirose and Kushiro, 1993, Baker and Stolper, 1993). In the present studies, this method is applied to (1) equilibrium partial melting of a relatively fertile mantle peridotite, (2) stepwise fractional melting of the peridotite, and (3) melt segregation from the peridotite. The compositional variations of melts formed by these processes are determined.

Trace element geochemical effects of imperfect crystal-liquid separation

Abstract The strongly contrasted effects of melting or crystallization under perfect equilibrium conditions; of perfect fractional partial melting; and of perfect fractional crystallization upon the relative concentrations of the highly incompatible and highly compatible elements are well known and are surveyed briefly at the start. A major role for either of the two latter processes in the genesis of natural basaltic magmas and peridotitic residues is apparently excluded provided that the liquids or crystals are removed in infinitesimally small batches, the crystals and melts are perfectly separated and there is no flux of source material or residue in or out of the greater system during the process (either liquid or crystals must be removed from the immediate, or lesser, system if it is undergoing perfect fractional melting or perfect fractional crystallization). The consequences of small departures from (i) infinitesimal size of the batches of liquid or crystals removed, (ii) perfect separation of the crystal and liquid phases, or (iii) perfect closure of the greater system to fluxes of the source material and the residue are explored. These departures can very substantially reduce the differences between the trace element concentrations generated in the products and those expected in a perfect equilibrium process. These effects are illustrated here by means of 3D plots of the logarithm of relative concentration in the liquid phase as a function of the logarithm of the distribution coefficient and the logarithm of the mass fraction of liquid involved in each process. There are also significant complementary effects on the compositions of the residues or precipitates from these processes which are not illustrated in this paper, although the necessary equations and some worked examples are provided. With the values of parameters chosen in these examples, the fluxing of liquid or solid through a ‘box’ in which fractional crystallization or fractional melting is taking place proves to be by far the most important factor in modifying the results towards those of the equilibrium processes. Although much trace element behaviour in basalts can be quite successfully modelled using the perfect equilibrium melting relationships, it is unsafe to exclude on these grounds a major role for physical processes of fractional melting and fractional crystallization in the genesis of these magmas or to conclude that equilibrium partial melting is the principal physical process involved in their genesis.

Chemical exchange in migmatites during cooling

The chemistry of the leucosome components in limited-size migmatitic associations is generally believed to reflect the composition of the partial melt that could have been extracted to yield an anatectic magma. Several well-documented studies of migmatites indicate that the distributions of some trace elements in leucosome/melanosome pairs are inconsistent with equilibrium partial melting but are rather in agreement with chemical reequilibration during cooling. In migmatites, the liquid fraction does not leave the system. This may induce an alteration of the chemical composition of the whole migmatite system by extensive chemical exchange between the liquid and the solid residue during cooling. If this is the case, the leucosome and melanosome components are not representative of the chemical composition of the liquid and solid residue respectively, at the time of partial melting. A rough evaluation of the residences time of the melt fractions in cooling in-situ migmatitic bodies yields minimum estimates of 1 to 10 Ma. For such time spans the characteristics distances of diffusion distances of diffusion-induced chemical transfer exceed the sizes of individual mineral grains, which is consistent with a re-equilibration process. The release of latent heat of crystallization in cooling migmatitic systems and the presence of an interstitial liquid phase are critical in determining the extent and scale of chemical exchange. The mechanism involves (1) reducing the cooling rate and (2) promoting the efficiency of chemical diffusion through the liquid phase. Thus, some doubt is cast on the ability of small-scale migmatitic systems to provide relevant information about chemical mass balance during partial crustal melting.

Major- and trace-element constraints on the petrogenesis of a fault-related pseudotachylyte, western Blue Ridge province, North Carolina

A newly identified 6 cm thick, zoned pseudotachylyte from the western Blue Ridge province of North Carolina is interpreted to be the result of episodic melting on a reactivated brittle fault, previously active in the ductile regime. Kinematic indicators in cataclasites associated with the pseudotachylyte indicate top-to-the-northeast shear, whereas kinematic indicators in mylonites indicate top-to-the-northwest shear. Higher concentrations in the pseudotachylyte of Al 2O 3, K 2O, FeO and depletion in SiO 2 are explained by the total fusion of micas and partial fusion of K-feldspar (≈ 60%), leaving quartz and refractory phases as residual. Enrichment of the pseudotachylytes in Ba 2+, Rb +, Sr 2+ and Eu 2+ provide additional evidence for the preferential melting of K-feldspar. The trace element signature of the pseudotachylytes (similar REE patterns, Ba/K, K/Rb, Ba/Rb, Zr/Hf, Ti/Cr ratios) and entrained lithoclasts identify the source of the melt as cataclastically deformed mylonite of the hangingwall. The similarity of the REE patterns in the pseudotachylytes and the source rock is explained by the entrainment and/or dissolution of REE-bearing phases in the melt in proportion to its volume. These data indicate that coseismic flash melting is distinctly different from equilibrium partial melting and that trace elements can be used to identify the source rock and distinguish those phases that melt from those that remain residual.