Proportion Of Melt Research Articles

The role of phosphorus in crystallisation processes of basalt: An experimental study

To evaluate the effects of phosphorus on differentiation of evolved basaltic magmas, a series of one-atmosphere experiments on a ferrobasaltic composition were carried out over a range of P 2O 5 contents and at oxygen fugacities from 2 log 10 units below to 2 log 10 units above the fayalite-magnetite-quartz (FMQ) buffer curve. The experiments were performed isothermally, the investigated variables being the amount of added P 2O 5 and the oxygen fugacity (f o 2 ). The results confirm the high solubility of phosphorus in basaltic magmas and show that, at fixed temperature, the progressive addition of phosphorus causes: 1. (1) the disappearance of olivine at reducing conditions, 2. (2) the disappearance of magnetite at oxidising conditions, 3. (3) the stabilisation of pigeonite throughout the studied range of f o 2 , 4. (4) an increase in the modal plagioclase/pyroxene ratio, and 5. (5) increased melt proportion and large changes in the composition of the coexisting melt, in particular the SiO 2 content. The destabilisation of magnetite with increasing P 2O 5 content may be accounted for by the formation of Fe 3+(PO 4) 3− complexes, rather than by a large change of the redox ratio of the melt; we suggest that the formation of Fe 3+(PO 4) 3− complexes dominates that of P-O- M complexes (where M is a network-modifying cation). The effect of P on the modal proportions of plagioclase and pigeonite may help to explain the mineralogy of some anorthositic rocks and KREEP basalts, as well as the presence of pigeonite as an intercumulus phase in the Skaergaard intrusion (resulting from enrichment of the trapped liquid in phosphorus). These new results thus provide insights into the effects of phosphorus in lunar and terrestrial systems, as well as providing information regarding the structural role of phosphorus in silicate melts.

Boron partitioning in the upper mantle: An experimental and ion probe study

Piston cylinder experiments were performed on two natural spinel lherzolites, from Lherz (French Pyrénées) and Kilbourne Hole (USA), in order to determine the distribution coefficients of boron between minerals and melt for upper mantle conditions. Additionally, a MORB glass and its phenocrysts were taken as a low pressure analogue. Boron concentration measurements were made by ion probe, the only technique suitable for in situ (∼ 10 μm) boron concentration measurements in the range 0.1–1 ppm. The two starting peridotites samples have low bulk boron concentrations of 0.96 and 0.81 ppm. Boron distribution coefficients were determined from (1) direct measurement of melt and crystals in contact, and (2) calculations from profiles of several tens of measurements made using 10μm diameter spots containing variable proportions of crystals and melt. Distribution coefficients between minerals and melt decrease in the following order: D cpx-melt (0.117) > D sp-melt (0.08) ⪢ D ol-melt (0.034) > = D opx-melt (0.027). The limited ranges of composition, pressure, and temperature investigated mimic natural conditions for basalt genesis but sharply limit the present data for determining variations of partition coefficients with P, T, and X. However, boron behavior seems to follow roughly that of aluminum, since the Al-richest minerals are the ones with the highest boron distribution coefficients. The D ol-melt values increase with pressure, which is similar to what has been observed for the distribution coefficient of aluminum between olivine and melt. Boron is, therefore, strongly incompatible during partial melting in the mantle. Using these distribution coefficients and assuming that MORB have been produced by 5–20% partial melting, the depleted mantle source of MORB is estimated to contain ∼0.05–0.3 ppm B. These values correspond to the lower limit for the mean boron content of chondrites (0.55 ± 0.3 ppm) but are in accordance with a simple budget of boron between mantle, crust, and seawater. In fact, a simple mass balance calculation, where all the boron hosted in the crust and in seawater is presumed to have been extracted from an upper mantle of chondritic original boron content, yields a boron content for the depleted upper mantle of ∼0.3 ppm at present. Because in situ boron concentration measurements under the ppm level at the μm scale are now possible with the ion probe, and because of its strong partitioning in melts and possible partitioning in fluids, boron is a potentially very powerful geochemical tracer for the study of mantle processes such as basalt genesis or metasomatism.

Fluid-absent melting and the roles of fluids in the lithosphere: a slanted summary?

The presence or absence, nature and compositions of lithospheric fluids are crucial factors in the evolution of igneous and metamorphic rocks. Despite this, confusion still surrounds perceptions of the degrees of fluid involvement in many geological settings. However, experimental and field investigations constrain these roles, and we are able to make some valid generalizations. Studies of prograde metamorphism have highlighted the spatial heterogeneity of fluid concentrations in many crustal environments. This commonly indicates internal lithological control of volatile activities. Under fluid-present conditions, crustal fluids do not appear to mix readily; fluid flow is generally structurally or lithologically controlled. The transition from amphibolite to granulite facies is characterized by a decrease in a H2O, as well as a temperature increase. In many terranes, these phenomena correlate with the appearance of migmatites. Some granulites are not migmatitic but have chemical and mineralogical characteristics of melt-depleted restites. This indicates efficient extraction of melts. Incongruent melting reactions involving the breakdown of crystalline hydrates, in crustal rocks, produce granitoid melts plus denser restites that are nearly or actually anhydrous. Experimental and theoretical treatments of fluid-absent melting show that micas and amphiboles decompose at 700 to 900°C, for mid- to lower-crustal pressures. The melts resemble the compositions of S- and I-type granitoids. Pervasive aqueous fluids could not generally have been present during this melting, since exposed granulite terranes are not characterized by high melt proportions, and the magmas themselves are water-undersaturated. Likewise, the general presence of very H 2O-poor pervasive fluids is unlikely, since this would allow only subsolidus dehydration at normally attainable crustal temperatures. In this case the amphibolite-granulite transition is possible at relatively low temperatures, and the resultant rocks would not contain evidence of in-situ melting. The granulite-granite connexion and effective crustal differentiation seem viable only during high-temperature, fluid-absent metamorphism. Data on continental geotherms show that the lower crust is not entirely granulitic. Granulites are produced in regions of anomalously high heat flow (the mantle connexion). Thus, inferences on the nature of the lower crust should not be entirely based on studies of exposed granulite terranes. Mineral assemblages in eclogites also indicate widespread internal buffering of volatile activities, and probable fluid-absent conditions. Mantle-derived basaltic magmas too are volatile-bearing and it is plausible that volatile-fixing phases also occur in the mantle. Volatiles may largely be introduced into the mantle through subduction processes and only released during the melting attending the breakdown of hydrous minerals, as in the crust.

BBe systematics in subduction-related metamorphic rocks: Characterization of the subducted component

The mobility of B and Be in H 2O-rich fluids and felsic silicate liquids produced during metamorphism of subducted oceanic slab and sediments has been investigated through analysis of subduction-zone metamorphic rocks of the Catalina Schist, California. In metasedimentary rocks, B/Be and the range in B/Be decrease with increasing metamorphic grade (mean = 72, std. dev. = 41 for lowest-grade lawsonite-albite fades rocks; mean = 21, std. dev. = 11 for higher-grade greenschist and epidote-amphibolite facies equivalents). This decrease to more uniformly low B/Be may be attributed to the preferential removal of B in H 2O-rich fluids produced by devolatilization reactions over the approximate temperature interval of 350–600°C. Metamafic rocks do not show pronounced decrease in B/Be with increasing metamorphic grade; however, all metamafic samples have B/Be< 30, lower than values for many altered seafloor basalts. In amphibolite-grade exposures, felsic leucosomes and pegmatites reflecting partial melting have low B/Be similar to their metasedimentary and metamafic hosts, which presumably experienced prior reduction in B/Be during lower temperature devolatilization. This evidence for B and Be mobility during high-P/T metamorphism complements studies of B-Be systematics in arc volcanic rocks in further characterizing mechanisms by which slab-derived elements can be added to the source regions of arc lavas. Before subducted mafic and sedimentary rocks reach Wadati-Benioff zone depths beneath arcs ( 80–150 km), the B/ Be of these rocks is likely to have decreased to <30. Thus, highly fractionated, slab-derived hydrous fluids may be necessary to generate the high-B/ Be signatures observed in many arcs (B/Be of up to ~200). The B-Be data, together with previously presented stable isotope data for the Catalina Schist, demonstrate that subduction-zone metamorphic processes are capable of homogenizing presubduction variability in the concentrations of particularly “fluid-mobile” elements in rocks and may, through mixing, produce fluids which trend toward uniform trace element and isotopic compositions. These homogeneous fluids could infiltrate parts of the mantle wedge and contribute to the characteristic trace element and isotopic signatures of arc-magma source regions. In hotter subduction zones ( e.g., involving subduction of young, hot oceanic lithosphere ), silicate melts derived from previously devolatilized sedimentary and mafic rocks may contribute relatively low-B/Be signatures to arc source regions. Thus, significant variations, among arcs, in the ranges of B/Be observed in front-rank volcanoes (e.g., for the Bismarck arc, B/Be = 20–190; for the Cascades arc, B/ Be < 5) may be related in part to varying thermal structure, which could govern both the B/Be of hydrous fluids and the relative proportions of hydrous fluid and silicate melt derived from the subducted slab and sediments.

Carbonatite metasomatism in the northern Tanzanian mantle: Petrographic and geochemical characteristics

Peridotite xenoliths from the Olmani cinder cone, northern Tanzania, possess distinctive mineralogical and chemical features interpreted to result from interaction of ultra-refractory peridotite residues with carbonatite melts. Chief among these are as follows: (1) The presence of unusually low Al 2O 3 clinopyroxene and the lack enstatite in refractory dunites (with olivines up to Fo 94). (2) The presence of monazite and F-rich apatite in refractory harzburgite and wehrlite xenoliths, respectively. (3) LREE enrichment and strong Ti depletion relative to Eu in all but one peridotite. Ca/Al ratios of the clinopyroxene-bearing dunites are some of the highest ever measured for peridotites (up to 10.8, relative to chondritic Ca/Al of 1.1). In addition, Zr/Hf and Ca/Scratios correlate positively, increasing with greater influence of carbonatite on the whole rock. (4) Clinopyroxene-bearing samples have a restricted range of isotopic compositions (ε Nd = +3.1to+3.9, 87Sr/ 86Sr= 0.7034to0.7035), whereas the monazite-bearing harzburgite has lower ε Nd (+0.8) at similar 87Sr/ 86Sr. The isotopic compositions of the former are similar to young, isotopically primitive east African carbonatites and basalts, suggesting the metasomatism occurred recently and that the carbonatites responsible for the metasomatism were ultimately derived from the asthenosphere. Inferred trace element signatures of carbonatite melts responsible for modal metasomatism of these and other peridotites include: high La/Yb, Nb/La and Ca/Al, very high Zr/Hf and very low Ti/Eu; features similar to those of erupted carbonatites and consistent with partition coefficients for silicates in equilibrium with carbonatite melts. These trace element systematics are used to illustrate that many LREE-enriched spinel peridotite xenoliths may have been affected by addition of very small amounts (⩽2%) of carbonatite. Such metasomatism will have small effects on major element compositions of peridotites (e.g., Mg#, Ca/Al) relative to peridotites from the literature can be modeled as mixtures between carbonatite and refractory peridotite, the modally metasomatized Olmani peridotites and SE Australian wehrlites require an open-system style of metasomatism, perhaps due to higher proportions of carbonatite melt.

The role of lithospheric mantle in continental flood volcanism: Thermal and geochemical constraints

Continental flood basalts (CFB) are commonly said to form by direct melting of metasomatized lithospheric mantle, either during major lithospheric extension or when a mantle plume impinges on the base of the lithosphere. We tested these ideas in a thermomechanical model that combines lithospheric dynamics and mantle convection. Dry melting was assumed, and the proportions of melt from different source regions were monitored. In all cases, &gt;96% of melt was found to come from asthenosphere or plume, with minimal amounts from continental lithosphere. During passive lithosphere extension the total amount of melt is small, and the proportion from the lithosphere is &lt;3%. During plume interaction and concurrent extension, magmas formed during the first half of a magmatic episode contain virtually no lithospheric melt, and during the second half the proportion reaches only ∼2%. The proportion of lithosphere heated above its solidus is greater but never exceeds 3–4% of the total source. These results were used to test the concept that the distinctive chemical signature of CFB is a result of melting of lithospheric mantle. Our calculations show that if CFB are hybrid magmas and the proportion of lithospheric melt is less than 10–30%, the composition required for the lithospheric source is unrealistic (e.g., εNd &lt; −40; Nb/U negative). Crustal contamination of asthenospherederived magmas can explain many of the characteristics of CFB if the primary magmas were produced by melting in plumes at sublithosphere depths (&gt;100 km) and have high concentrations of both MgO (&gt;20%) and incompatible trace elements (K2O ∼ 1%). The very low Nb and Ta concentrations in certain CFB cannot, however, be explained by this process. Ratios of Nb to elements such as La or U are lower in many flood basalts and picrites than in all likely source materials: they are almost as low as in most rocks from the continental crust, and they are far lower than in peridotites from the lithospheric or asthenospheric mantle. Another process must therefore fractionate Nb and Ta. We suggest that this takes place during the passage of magma through the lithospheric mantle, perhaps because of differences in the reaction rates of minerals in metasomatized peridotite. The probability that CFB are hybrid magmas containing material from aesthenospheric and lithospheric mantle and in many cases from continental crust, as well as the possibility that some elemental ratios change during magma‐lithosphere interaction, casts serious doubt on the reliability of such rocks as probes of the lithospheric mantle.

Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites

The generation of trondhjemites and tonalites on a massive scale during the Archean (3.8-2.5 Ga ago) marked the transition from a simatic to a sialic crust, and represents the magmatic contribution to cratonization. Petrogenetic models for the origin of these rocks based on their highly fractionated, HREE-depleted rare earth patterns suggest a mafic crustal source, either through a process of partial melting of amphibolite, garnet-amphibolite, or eclogite, in which hornblende and/or garnet are essential residual phases, or by hornblende-controlled fractionation of hydrous basaltic magma. A series of vapor-absent (i.e., Pfluid< Ptotal) melting experiments on four natural basaltic compositions were conducted at 8, 16, 22 and 32 kbar in order to assess the validity of models for the origin of Archean granitoids which assume a mafic crustal source. Melt compositions produced by 10–40% melting are tonalitic-trondhjemitic at all pressures investigated; residual assemblages are amphibole+plag±opx±FeTi at 8 kbar, garnet+cpx±amphibole±plag±opx±FeTi oxide at 16 kbar, and garnet+cpx±rutile at 22 and 32 kbar. REE patterns for most of the trondhjemitic-tonalitic partial melts, calculated on the basis of estimated modal proportions of melt and residual phases, are highly fractionated (LaYb is 30–50), heavy rare earth-depleted (YbN is 1–10) when garnet is present to some extent in the residue; these REE patterns are similar to those of trondhjemitic and tonalitic gneisses from several Archean “grey gneiss” and granite-greenstone terrains. A consideration of estimated Archean geotherms with respect to the experimental P-T conditions indicates that a temporally diminishing Archean geotherm might have progressively swept through a P-T regime in which trondhjemitic-tonalitic melts could have been generated initially from a water-saturated (Pf=Pt) to undersaturated (Pf<Pt) amphibolite source by partial melting at 5–8 kbar. Subsequent relaxation of the geotherm through the mid- to late-Archean would have produced similar melts by vapor-absent melting of garnet-amphibolite at 16 kbar and eclogite at 22–32 kbar. However, the degree of melting required to produce melts of trondhjemitic-tonalitic composition increases with pressure, 10–15% melting being appropriate at 8 kbar in a amphibole-dominated residue, but 25–35% melting being required at 22–32 kbar, where garnet dominates the residue. Supposing the tendency for melt segregation and/or magma mobilization mechanisms to be more effective at higher degrees of melting, an origin by partial melting of eclogite seems to be the most likely source for massive plutonic trondhjemite-tonalite contributions to the juvenile continents. Such a source is consistent with the generation of trondhjemite-tonalite protocontinental cores in any number of plausible Archean tectono-thermal scenarios, though not necessarily in a conventional subduction-zone setting.

Alkaline hybrid mafic magmas of the Yampa area, NW Colorado, and their relationship to the Yellowstone mantle plume and lithospheric mantle domains

The Yampa volcanic field (late Miocene) consists of about 70 outcrops of monogenetic cinder cones, lavas, dykes, volcanic necks and hydrovolcanic pyroclastic deposits and is situated in the most northerly part of the Rio Grande rift. Contemporaneous extension in this part of the rift was small, but there is geological and geophysical evidence that, by the late Miocene, the area was underlain by hot asthenosphere convected by the Yellowstone mantle plume. The Yampa rocks are mafic and chemically diverse, including basanites, alkali basalts, potassic trachybasalts, hawaiites and shoshonites. About half the rocks bear the xenocryst suite feldspar, pyroxene, Fe−Ti oxide, amphibole, biotite. There is a tendency for xenocryst-free rocks to be the most mafic, interpreted to indicate that the xenocrysts are cognate, and represent cumulate material from fractional crystallization of the magmas in deep crustal magma chambers. The elemental and isotopic (Nd and Sr) variations can be modelled by mixing variable proportions of partial melts of local lithospheric mantle with an OIB end-member formed by partial melting of asthenosphere. The OIB end-member appears to have the elemental and isotopic composition of typical Northern Hemisphere OIB, in particular the plume-derived basanites of Loihi seamount, Hawaii. The OIB end-member at Yampa is interpreted to have been derived from mantle convected in the Yellowstone mantle plume.

Nd, Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera, Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initialɛNd values (∼1ɛNd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similarɛNd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higherɛNd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from lowɛNd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The lowɛNd magmas most likely formed via the incorporation of lowδ18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higherδ18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have lowɛNd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing “basification” of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.

Development of the Long Valley, California, magma chamber recorded in precaldera rhyolite lavas of Glass Mountain

Glass Mountain, California, consists of >50 km3 of high-silica rhyolite lavas and associated pyroclastic deposits that erupted over a period of >1 my preceding explosive eruption of the Bishop Tuff and formation of the Long Valley caldera at 0.73 Ma. These “minimum-melt” rhyolites yield Fe-Ti-oxide temperatures of 695–718°C and contain sparse phenocrysts of plagioclase+quartz+magnetite+apatite±sanidine, biotite, ilmenite, allanite, and zircon. Incompatible trace elements show similar or larger ranges within the Glass Mountain suite than within the Bishop Tuff, despite a much smaller range of major-element concentrations, largely due to variability among the older lavas (erupted between 2.1 and 1.2 Ma). Ratios of the most incompatible elements have larger ranges in the older lavas than in the younger lavas (1.2–0.79 Ma), and concentrations of incompatible elements span wide ranges at nearly constant Ce/Yb, suggesting that the highest concentrations of these elements are not the result of extensive fractional crystallization alone; rather, they are inherited from parental magmas with a larger proportion of crustal partial melt. Evidence for the nature of this crustal component comes from the presence of scarce, tiny xenocrysts derived from granitic and greenschist-grade metamorphic rocks. The wider range of chemical and isotopic compositions in the older lavas, the larger range in phenocryst modes, the eruption of magmas with different compositions at nearly the same time in different parts of the field, and the smaller volume of individual lavas suggest either that more than one magma body was tapped during eruption of the older lavas or that a single chamber tapped by all lavas was small enough that the composition of its upper reaches easily affected by new additions of crustal melts. We interpret the relative chemical, mineralogical, and isotopic homogeneity of the younger Glass Mountain lavas as reflecting eruptions from a large, integrated magma chamber. The small number of cruptions between 1.4 and 1.2 ma may have allowed time for a large magma body to coalesce, and, as the chamber grew, its upper reaches became less affected by new inputs of crustal melts, so that trace-element trends in magmas erupted after 1.2 Ma are largely controlled by fractional crystallization. The extremely low Sr concentrations of Glass Mountain lavas imply extensive crystallization in chambers at least hundreds of cubic kilometers in volume. The close similarity in Sr, Nd, and Pb isotopic ratios between the younger Glass Mountain lavas and unaltered Bishop Tuff indicates that they tapped the same body of magma, which had become isotopically homogenous by 1.2 Ma but continued to differentiate after that time. From 1.2 to 0.79 Ma, volumetric eruptive rates may have exceeded rates of differentiation, as younger Glass Mountain lavas become slightly less evolved with time. Early-erupted Bishop Tuff is more evolved than the youngest of the Glass Mountain lavas and is characterized by slightly different trace element ratios. This suggests that although magma had been present for 0.5 my, the composiional gradient exhibited by the Bishop Tuff had not been a long-term, steady-state condition in the Long Valley magma chamber, but developed at least in part during the 0.06-my hiatus between extrusion of the last Glass Mountain lava and the climactic eruption.

The evolution and tectonic consequences of a tonalitic magma layer within Archean continents

The petrological and tectonic consequences of early continent formation by localized open-system fractional crystallization of an almost global shallow-level tholeiitic magma layer are investigated. A near-surface tonalitic–trondhjemitic–granodioritic (TTG) magma "ocean", evolved by such a process near mantle sinks, would rapidly crystallize a stably floating solid upper crust several kilometres thick and could therefore be a long-lived feature of Archean geology. Given likely mantle heat fluxes, crystallization of the molten layer resulting from secular cooling and movement from mantle-upwelling to -downwelling regions would normally be from the top downward. In areas of extremely low mantle heat flow, crystallization from the base upward is also possible.The partially molten layer will convect with cells on a much smaller scale than those in the underlying mantle. This convection will influence tectonic patterns in the overlying solid crust, the history of which will be largely independent of specific mantle convection patterns. Small-amplitude surface topography is predicted. Intrusion of mafic or ultramafic magmas to high levels would not be possible through tonalite with a melt proportion greater than about 40% and is likely therefore to be concentrated at cooler, downwelling zones of the small-scale convection system. These two predictions together suggest that Archean greenstone belts form over local downwarping zones of TTG crust. Early, shelf-facies sedimentation is related to convection-induced topographic relief, whereas later, higher energy sedimentation is related to gravitational instability resulting from localized loading of the crust.So long as a continental magma layer is continually replenished by tholeiite melt, its composition remains essentially tonalitic. When it ceases to be replenished, fractionation will give rise to granitic melts that through their lower density will increase the gravitational instability of the Archean crust.

Shape and microstructure of microgranitoid enclaves: Indicators of magma mingling and flow

At Tarandore Point, 328 km south of Sydney, New South Wales, Australia, two tonalite phases are in contact with a gabbroic diorite, all belonging to the Moruya Batholith. Lobate to crenulate contacts, pillowing of the more mafic phase in the more felsic phase, net-veining of the more mafic phase by the more felsic, local chilling of the more mafic phase against the more felsic, and an abundance of rounded to elongate microgranitoid enclaves all suggest mingling of magmas at the site of emplacement. Therefore, all phases were emplaced before the others solidified, and an order of intrusion cannot be determined. The variety of microgranitoid enclaves suggests that repeated magma mingling has occurred. Linear geochemical trends indicate either magma mixing or restite unmixing as a prime cause of variation in the members of the Moruya suite of granitoids. Direct evidence of mixing is not present, but evidence that the magmas were substantially liquid at the time of emplacement sugests that mixing is possible. Restite unmixing is unlikely to be the prime process, because near-minimum melts, which would occur in all granitoid phases according to that model, should mix easily, rather than mingling. In fact, mingling has occurred between tonalite phases of very similar composition, which would not be expected if the liquid parts of enclaves were minimum melts. Available experimental and theoretical data on mixing in both magma chambers and conduits cannot be applied directly to the occurrence at Tarandore Point, owing to inadequately constrained parameters, but mixing generally should be favoured by higher temperatures, which is consistent with our inference that relatively high proportions of melt were present in the granitoid phases at the time of emplacement. Conduit mixing may be reflected in a local zone of strong to extreme elongation of enclaves, in which the distortion has been accomplished by magmatic flow without solid-state deformation, as evidenced by strong dimensional preferred orientations, coupled with a lack of microstructural evidence of intragranular plastic strain. This suggests that elongate enclaves should not be used as indicators of solid-state deformation or as tectonic strain markers without microstructural evidence that suitably strong intracrystalline plastic deformation has taken place.

The significance of Mo for the geochemistry of the upper mantle

Northwest Africa 6693 is a new type of achondrite, with a unique combination of oxygen-isotopic composition (low Δ17O: −1.08‰; also δ17O = 1.19‰) and FeO-rich, low mg bulk composition. A mode (in vol%) shows 70% pyroxene, 16% olivine and 13% feldspar, along with 0.6% Cr-spinel, and 0.4% NiFe metal (awaruite). Its coarse-poikilitic texture, with pigeonite oikocrysts up to 14 mm, as well as the subchondritic MgO/SiO2 of the rock’s bulk composition, indicate origin as an igneous cumulate. The cumulus phases included pigeonite and olivine, and the parent magma was probably also saturated with feldspar, which occurs mainly as anhedral, yet optically continuous, grains intergrown with the pyroxene. The mafic silicates are uniformly ferroan: pigeonite near En57Wo3.2 and olivine near Fo49. The feldspar is uniformly albitic, near Ab92, except for a single tiny grain of Ab57Or43. However, the albite features diverse K/Ca (Or/An) ratios: ranging from consistently ∼0.46 in one end of the oblong NWA 6693 stone, to 5.2 in an olivine-rich enclave that consists mostly of micrographic olivine–feldspar intergrowth. Also, siderophile and incompatible element data show heterogeneity among samples from different regions of this large cumulate. The rock was probably neither an orthocumulate nor an adcumulate, and the proportion of “trapped liquid” probably varied from place to place. After initial crystallization, a shock event caused very minor brecciation, and pervasively mobilized linear-arcuate trails of microinclusions (minute oxides, mostly) and bubbles. A minor proportion of additional melt was formed within, and/or infiltrated into, the rock and formed discrete overgrowth mantles, recognizable based on unusual scarcity of microinclusions, on some pyroxenes. Final cooling, based on mineral-equilibration temperatures, occurred at a moderate rate by intrusive-igneous standards.Olivine, metal, and sulfide phases are all very Ni-rich (e.g., olivine NiO averages 0.77 wt%). Evidently the partitioning of NiO into the parent melt was extraordinarily high, which suggests a commensurately high oxygen fugacity. The V/(Al + Cr) ratio within spinel suggests that fO2 was IW + 2. The bulk-rock composition features strong depletions in sulfur and chalcophile elements, but nonvolatile lithophile elements are only subtly fractionated from chondritic. Even siderophile element concentrations are near-chondritic; Co, Ni, Ir and Os are all at 0.7–1.0 × CI chondrites, and Au at 0.55 × CI. The limited fractionation of the siderophile elements may reflect igneous processing in a parent body so pervasively oxidizing that FeNi metal did not play its usual planetary role as agent for efficient sequestration of siderophile elements. More generally, the limited fractionation among nonchalcophile, nonvolatile elements suggests that the parent melt was not produced by extensive fractional crystallization; i.e., the high FeO and low mg of NWA 6693 were probably in large measure already properties of the original primary magma, produced from an extremely oxidized (FeO-rich) variety of primitive material. NWA 6693 indicates that high oxygen fugacity inherited from oxidized-chondritic building blocks may persist within small bodies, despite melting extensive enough to engender igneous cumulates.

Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa

Abstract Dehydration‐melting reactions, in which water from a hydrous phase enters the melt, leaving an anhydrous solid assemblage, are the dominant mechanism of partial melting of high‐grade rocks in the absence of externally derived vapour. Equilibria involving melt and solid phases are effective buffers of aH2,o. The element‐partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which, for most cases, aH2o is driven to lower values. The mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at T± 850°C is relatively small (10–20%), and probably insufficient to mobilize a partially melted rock body.Granulite facies metapelite, biotite gneiss and metabasic gneiss in Namaqualand contain coarse‐grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals. The segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration‐melting reactions: Bt + Sil + Qtz + PI = Grt ° Crd + Kfs + L (metapelite), Bt + Qtz + Pl = Opx + Kfs + L (biotite gneiss), and Hbl + Qtz = Opx + Cpx + Pl + L (metabasic gneiss). The size, shape, distribution and modes of segregations suggest only limited migration and extraction of melt. Growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity. Also, they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. The preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that H2O in the melt was released as vapour on crystallization.This model, combined with the natural observations, suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt.

Experimental determination of the fluid-absent melting relations in the pelitic system

In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als. The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C. We demonstrate that, even under fluid-absent conditions, a large proportion (≈40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites. The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.

Constraints on melting and magma production in the crust

Major intrusions of granitic rocks are found in several tectonic settings and, in all cases, crustal melts may contribute to the volumes of granitic magma. High-grade metamorphism and partial melting of the crust take place predominantly under fluid-absent conditions. We present a model for calculating the amounts of melt that may be formed by fluid-absent breakdown of micas and amphiboles in common crustal rock types (pelitic, quartzofeldspathic, intermediate and mafic). Melt proportions depend mainly on the kind of source rock, the pressure at which melting takes place, the temperature and the hydrous mineral (H 2O) content of the source. As a consequence of the pressure dependence of water solubility in silicate melts, any given source rock will produce more melt, by a given fluid-absent reaction, at lower pressure. At a given pressure, higher-temperature reactions can produce more melt from a given source rock. Based on a survey of the compositions of common rock types, we show that the amounts of melt can vary from < 10to> 50vol.%. Thus, crustal rocks vary widely in their “fertility” as magma sources, depending on the types and amounts of hydrous phases they contain. In general, muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation. Biotite breakdown in pelites occurs at higher temperature and, because most high-grade pelites (below granulite grade) are biotite-rich, can yield up to about 50 vol.% of granitoid melt. Rocks of intermediate composition and hornblende-rich mafic rocks are potentially highly fertile magma sources also, provided that the high temperatures necessary for biotite and hornblende breakdown are realized. Pyroxene-rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile. Data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with < 30and probably< 20vol.% melt. The pressure-temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria.

Rise and fall of a basalt-trachyte-rhyolite magma system at the Kane Springs Wash Caldera, Nevada

Magmas erupted at the Kane Springs Wash volcanic center record the buildup and decay of a silicic magma chamber within the upper crust between 14.1 and 13.2 Ma ago. Intrusion of a variety of mantle-derived basaltic magmas into the crust sustained the system thermally, but only alkali basalts appear to be parental. Fractionation of alkali basalt, together with 10–20% contamination by partial melts of the lower crust, generated trachyandesite magmas. Mafic trachytes, with magma temperatures of ∼1,000° C, were initially generated from trachyandesites at depths greater than 15 km. Continued fractionation combined with assimilation of upper crustal melts at a depth of 5–10 km produced more evolved trachytes and high-silica rhyolites. These silicic magmas erupted as the Kane Wash Tuff 14.1 Ma ago from a chamber zoned from fayalite-bearing alkali rhyolite near 820° C at the roof to a trachytic “dominant volume”. Initial ash flows of the Kane Wash Tuff, Member V1, are metaluminous, whereas later cooling units, Members V2 and V3, are mildly peralkaline and have higher Fe, Zr, and Hf and lower Ca, Th/Ta, Rb/ Zr, and LREE/HREE. Less than 1 % upper crustal component was involved in generation of Members V2 and V3 from trachytic magma. Eruption of ∼130 km3 of magma resulted in collapse of the Kane Springs Wash caldera. Trachytic magma from deeper levels of the system was extruded onto the caldera floor shortly afterward, forming a central trachyte/syenite complex. Replacement of this magma by hotter, more mafic magma may have induced additional melting of the already heated chamber walls, as high-silica rhyolites that erupted in the “moat” surrounding the central complex have a large crustal component. Early moat rhyolites had temperatures near 800° C and, in contrast to the Kane Wash Tuff, are ferroedenite-bearing, have higher Al, K/Na, Th/Ta, and Ba, and have lower Fe, REE, and Zr. Fractional crystallization of this magma within the cooling and crystallizing magma chamber formed biotite-bearing rhyolite in isolated pockets. The most evolved of these had temperatures near 700° C, elevated F contents, H2O contents of ∼5 wt.%, Rb> 500 ppm, chondrite-normalized LREE/HREE <1, and formed vapor-phase topaz. Declining temperatures and Cl/ F from the Kane Wash Tuff through the moat rhyolites may reflect decreasing basalt input into the base of the system and increasing proportions of upper crustal melts in the silicic magmas.

The case for a melt matrix in plagioclase‐POIK mesosiderites

The plagioclase‐POIK mesosiderites, Bondoc, Budulan, and Mincy, have poikilitic matrix textures exactly like igneous rocks including the olivine zone of the Palisades sill and Apollo 17 mare basalts. They contain olivine grains with embayed margins characteristic of resorption in liquid rather than coronas formed during solid state reactions. There is a nonrandom (i.e., nonmetamorphic) arrangement of pyroxene phases in the matrix: orthopyroxene is enclosed poikilitically by plagioclase, but inverted pigeonite is interstitial and locally subophitic to plagioclase, as in an igneous crystallization sequence. These mesosiderites are therefore reclassified from subgroup 3B (highly recrystallized matrix) to 4B (igneous matrix). The 4A mesosiderites have fine‐grained intergranular textures because they are richer in plagioclase than subgroup 4B, so that plagioclase crystallized earlier than in 4B, but also because they contained abundant nuclei associated with relict plagioclase clasts. The only 4A melt‐rock mesosiderite, Pinnaroo, with an intergranular‐poikilitic matrix texture transitional between 4A and 4B mesosiderites, contains very few plagioclase clasts. Over half the mesosiderites contain melt matrix or melt‐rock clasts. The heterogeneity within mesosiderites as a whole and within specific mesosiderites (e.g., Estherville) may be due to variations in the proportions of hot melt and cold clasts incorporated.

Natural partial melting of syenite blocks from Ascension Island

Blocks of coarse-grained syenite included in a trachyandesite lava flow on Ascension Island exhibit features attributable to partial melting. The liquid so formed has been quenched to a glass and the chemical variation of the glass is considered to depend on the amount of melting, the mineralogy and modal proportions of each block — especially the presence or absence of quartz — and the chemical composition of the minerals subjected to melting. Partial melting of quartz-syenites such as these could have produced magmas of composition similar to that of the comendites and granites found on Ascension but differences in major element and minor element chemistry between the glass and the comendites and granites suggest otherwise.

Rare earth element systematics of hydrous liquids from partial melting of basaltic eclogite a re-evaluation

The origin of orogenic andesitic magmas is tested by calculations of REE fractionation in hydrous melts derived from partial melting of subducted ocean basalt in eclogite facies. New data on the subsolidus phase proportions of basaltic eclogite, the enrichment of LREE in altered ocean basalts, and experimentally determined REE partition coefficients ( K D's ) between garnet and melt have been included in trace element fractionation equations. Non-modal melting of phases combined with variation in K D's during melting is a unique feature of these calculations. Variation of K D , melting proportions, initial proportion of subsolidus phases, degree of melting, and initial REE concentrations yield a wide range of input parameters that produce REE profiles in partial melts of basaltic eclogite matching REE profiles of some orogenic andesites. The positive correlation of REE concentration with silica content for many andesitic suites can be accounted for by non-modal melting if quartz (or a similar phase with low REE K D values) melts at a high melting proportion and garnet melts at a low melting proportion during the first stages of fusion. However, no mineralogic fractionation scheme can account for REE/silica systematics if REE K D values are linearly decreasing with increasing melting. Earlier workers who have used similar calculations to discredit the eclogite fractionation model have set overly strict, and sometimes incorrect, constraints concerning the range in REE K D values for garnet, the subsolidus proportions of phases in basaltic eclogite, and the relative concentrations of REE in subducted ocean crust undergoing partial melting.