Residual Melt Fractions Research Articles

Developing an Integrated Petrogenetic Model for Understanding REE Deportment of the Ampasibitika Intrusion and Associated Ion Adsorption Deposits

Abstract Alkaline–peralkaline igneous systems are promising sources of rare earth elements (REEs). Preservation bias has resulted in a gap in the geological record for alkaline–peralkaline magmatic systems, with the hypabyssal plumbing system linking magma chambers to extrusive volcanic rocks poorly represented. Large plutonic varieties of these systems are often proposed to have fed (now eroded) volcanoes, and current peralkaline volcanic systems obscure the plutonic system at depth. The alkaline to peralkaline Ampasibitika Intrusion in Madagascar is a rare example where the magmatic–volcanic interface between a deeper level magma reservoir and its genetically related caldera volcano is exposed. This c. 24 Ma sub-volcanic intrusive system comprises silica-undersaturated to silica-oversaturated units, of peralkaline to metaluminous and peraluminous characters, with varying styles of REE mineralisation, including supergene ion adsorption-style REE occurrences in the overlying weather profiles. There are two main intrusive suites: (1) the concentric Marginal Dyke Swarm (MDS) formed of quartz–microsyenite and peralkaline granite dykes (PGDs), and (2) the Ampasibitika Ring Dyke (ARD) comprising alkali feldspar syenites and subordinate nepheline syenites, trachytes and phonolites. We present new field observations and geochemical data to indicate that the MDS was emplaced as a series of low-viscosity, volatile-rich melt batches, which coalesced in the magma reservoir roof zone and intruded prior to caldera collapse, whereas the ARD was emplaced into the ring fault as a heterogeneous mix of variably evolved syenitic crystal mushes and phonolitic to trachytic-melt batches. As such, we suggest the MDS represents the residual melt fraction of the magma reservoir, whereas the ARD contains portions of the fractionating, silica-neutral to silica-undersaturated syenite, cumulate assemblage. In this revised framework, we assess the major and trace element geochemistry of amphibole- and clinopyroxene-group minerals to gain insight into the magmatic evolution of the Ampasibitika Intrusion and partitioning of REE between early cumulate and residual melt phases. Ultimately, the most REE-enriched units, the PGDs of the MDS, are identified as the product of the most volatile-rich, highly evolved melts from the roof zone of the magma reservoir. However, although REE enriched, the mineralogy does not always enable efficient release of REE for ion adsorption-style mineralisation; instead, lower REE-content protoliths with REE-host phases more amenable to decomposition release a greater proportion of REE.

Fe-Ti-Zr metasomatism in the oceanic mantle due to extreme differentiation of tholeiitic melts (Moa-Baracoa ophiolite, Cuba)

Ti-rich amphibole, Mg-rich ilmenite, baddeleyite, zirconolite, srilankite, and zircon are important high-field-strength elements (HFSE) bearing phases in the Potosí chromitite bodies located in the Moho Transition Zone of the Cretaceous Moa-Baracoa suprasubduction zone ophiolite (eastern Cuba). Such HFSE-bearing phases were found in the interaction zone between gabbroic intrusions and chromitite pods. In addition to HFSE-bearing minerals, the studied samples are composed of Fe3+ and Ti-rich chromite, olivine (Fo86-90), clinopyroxene (En44-49), plagioclase (An51-56), orthopyroxene (En84-94), F-rich apatite, and Fe-Cu-Ni sulfides. The studied ilmenite hosting Zr oxides (baddeleyite, zirconolite, and srilankite) contains up to 13 wt.% MgO. The Potosí zirconolite is the first record of this mineral in ophiolitic chromitites and non-metamorphic ophiolite units, and it has relatively high REE contents (up to 10 wt.% of REE2O3) and the highest concentrations in Y2O3 (up to 11 wt.%) reported so far in zirconolite from terrestrial occurrences. Zircon is observed forming coronas surrounding ilmenite grains in contact with silicate minerals, and is characterized by very low U and Pb contents. The zircons formed after high temperature Zr diffusion in ilmenite (exsolution) and a subsequent reaction along grain boundaries following crystallization. Finally, U-Pb dating of baddeleyite exsolutions within ilmenite yielded an average age of 134.4 ± 14 Ma, which provides the first ever dating for a metasomatic event in Potosí that matches well (within uncertainty) the formation age of the oceanic crust of the eastern Cuba ophiolite. We propose that the occurrence of HFSE- and REE-bearing minerals in the Potosí chromite deposit is the result of a two stage process: first, water-rich and HFSE-rich residual melts are produced by intercumulus crystal fractionation after an evolved MORB (BABB)-like melt; and secondly, these residual melt fractions escaped the solidifying mush and extensively reacted and metasomatized the surrounding chromitites, crystallizing HFSE- and REE-bearing minerals and Fe-Cu-Ni sulfides.

Crystallisation and zircon saturation of calc-alkaline tonalite from the Adamello Batholith at upper crustal conditions: an experimental study

The understanding of the geochemical and petrophysical evolution of magmas forming intermediate calc-alkaline batholiths at shallow crustal levels critically depends on knowledge of the phase equilibria relations along the liquid line of descent. Here, we present experimental results for a tonalitic system at a pressure of 200 MPa and under water-saturated conditions. Melting experiments were performed at temperatures between 700 and 1000 °C in externally heated HCM pressure vessels, with oxygen fugacity controlled close to the Ni–NiO buffer equilibrium (NNO) employing an argon–methane mixture as pressure medium and Co–Pd redox sensors to verify fO2 conditions. Natural rock powder of a medium-K tonalite from the Adamello Batholith in Northern Italy served as experimental starting material. Based on compositional data of stable phases in the run products and images of entire run charges, mass balance calculations as well as image processing were performed to investigate the evolution of the crystal/melt ratio with respect to temperature. Furthermore, compositional trends of minerals as well as the liquid line of descent of residual melts were obtained. Orthopyroxene, clinopyroxene and plagioclase were identified as near-liquidus phases (below 990 °C). At 900 °C, amphibole joins the solid-phase assemblage at the expense of clinopyroxene, indicating the existence of a peritectic relationship. After an initial near-linear decrease with temperature, residual melt fractions exhibit a plateau of 45–55 vol. % between 750 and 850 °C, followed by a rapid decrease coinciding with quartz saturation at 725 °C. Compositions of residual liquids evolve along a typical calc-alkaline differentiation trend with decreasing temperature (increasing SiO2 and decreasing TiO2, Al2O3, CaO, MgO and FeO contents) and become peraluminous below 900 °C. Intermediate to acidic rocks from the Adamello follow the experimental liquid line of descent indicating that the observed compositional spread of the natural intermediate composition rock record can be explained by low-pressure magma differentiation and liquid extraction. Experimentally determined zircon saturation levels are at low temperatures distinctly lower compared to existing and often used Zr-saturation models, but fully consistent with observed and modelled Zr-evolution trends from the natural rock record of the Southern Adamello Batholith inferring that zircon saturation in these intermediate to felsic plutonic rocks occurred at 800–830 °C corresponding to a melt fraction of about 50 vol. %.

Influence of water on granite generation: Modeling and perspective

Water influences the physics and chemistry of magmatic differentiation because it reduces the melting point, decreases melt viscosity, modifies phase equilibria, and controls how latent heat is released. Here, we explore how compositional trends of fractionating magmas can be used to evaluate the amount of water in the crystallizing system. Of interest is the role of water in generating silicic magmas such as granites and rhyolites. Water expands the crystallization window over which silicic melts are stable, such that granites are limited to small residual melt fractions (F < 0.2) under dry conditions but stable over a wider range of residual melt fractions (F < 0.6) at high water contents. Variations of SiO2 versus F are relatively insensitive to oxygen fugacity and can be used as a hygrometer for granitoids, where F is estimated through relative enrichments in incompatible elements, such as Th and K. We also show that generation of granitoids in arc environments follow distinctly different Mg#-SiO2 trends compared with anorogenic or intra-plate granites. We confirm that arc magmas differentiate at higher water contents and possibly higher oxygen fugacities than anorogenic/intra-plate magmas. Finally, Archean TTGs (tonalite, trondhjemite, and granodiorite) show similar Mg#-SiO2 systematics as Phanerozoic arc-related granitoids, suggesting similar petrogenetic physical and chemical conditions.

Mixing of crystal mushes and melts in the genesis of the Bogács Ignimbrite suite, northern Hungary: An integrated geochemical investigation of mineral phases and glasses

The 16.4Ma old Bogács Ignimbrite, located south of the Bükk Mountains, northern Hungary, was formed during a silicic ignimbrite flare-up in the Pannonian Basin that occurred from 20Ma to 13Ma. It comprises two main units, a lower, variably welded pumiceous and an upper, scoriaceous pyroclastic flow unit. Bulk chemistry of the juvenile clasts indicates a gradual change of geochemical character with an upward decreasing SiO2 content through the stratigraphic section. A detailed in-situ major and trace element investigation of the main mineral phases and glasses combined with petrogenetic model calculations reveals complex magma reservoir processes. Based on the major and trace element variability, six juvenile clast types were distinguished and each contain fresh glass fractions with distinct compositions. The mineral assemblage consists of plagioclase, orthopyroxene, biotite with minor and variable amounts of quartz, amphibole, ilmenite, zircon and allanite. The anorthite content of the plagioclases varies from 20 to 90mol%, whilst the Enstatite content of orthopyroxenes covers also a wide range from 40 to 90mol%. This large compositional variation can be detected even in single crystals. This extreme geochemical variability can be explained by mixing of crystal mush bodies evolved from both basaltic and more silicic magmas. The calcic plagioclases (An=80–90mol%) and magnesian orthopyroxenes (En=70–90mol%) clearly indicate the role of primitive mafic magmas in the growth of the silicic magma reservoir, even though no basaltic volcanic activity was associated with the Miocene silicic volcanism in the Pannonian basin. The prolonged crystallization in the mushy sills resulted in compositionally different residual melt fractions that moved upwards and accumulated in separated melt pods at the roof of the magma reservoir. Intermittent intrusions of mafic and intermediate magmas into this silicic magma system could have resulted in thorough stirring of the crystal mush bodies and the melt pods, leading to eruptive products having compositionally heterogeneous glass and mineral assemblage.

A model of episodic melt extraction for plumes

A model of melt segregation and extraction within rising plumes is proposed. It is based on two‐phase porous flow within the partially molten region, combined with only three extraction parameters. The conservation equations of mass, momentum, and energy are solved for a two‐phase melt matrix system. As a rising hot mantle region (plume) reaches the asthenosphere, decompression melting occurs, and the melt begins to percolate with respect to the matrix. Accumulation layers form, which might be the locus for the formation of buoyancy‐driven propagating dikes. As dike propagation requires a minimum dike length, melt extraction is parameterized by dex, ϕ1, and ϕ2. Here dex is the critical thickness of the partially molten layer in which a critical melt fraction ϕ2 is exceeded. If this condition is met within a certain region of the melt source region, melt might be extracted from that region in the form of one or several propagating dikes, leaving behind a region of residual melt fraction ϕ1. This simple extraction model is tested in one dimension for rising hot mantle flow. Depending on the chosen extraction parameters, multiple extraction events may be observed with a characteristic episodicity and a saw‐tooth‐like depth distribution. Exploring the parameter space shows that for values of dex and ϕ2 of a few kilometers and a few melt percent, respectively, typical extraction cycles have the order of 103–104 years, and they extract melt volumes per surface area of 50 to several hundred meters each. Tentatively assuming that eruptions are tied to mantle ascent at depth, the model is applied to observed eruption frequencies and multiple extraction depths, and values for ϕ2 of about 2% and dex of 3–5 km are derived.

Petrogenesis of Ilmenite-Bearing Symplectite Xenoliths from Vitim Alkaline Basalts and Yakutian Kimberlites, Russia

Ilmenite-bearing symplectite, typically ilmenite-clinopyroxene, occurs in rare mantle xenoliths from kimberlite and other alkaline rocks. Two major hypotheses have been proposed for their origin: (1) exolution from high-pressure pyroxene or garnet; and (2) cotectic or eutectic co-precipitation from evolved “proto-kimberlite” melt, producing megacrysts. Here we present new findings about these ilmenite-clinopyroxene symplectites in the Miocene picrobasalts and Pleistocene basanites of the Vitim volcanic field, central Siberia. For comparison, we have described unusual ilmenite-clinopyroxene symplectites from Yakutian kimberlites: the Mary pipe (Kuoisk field) and the Mir pipe (Malo-Botuobinsk field). Symplectite clinopyroxene from Vitim alkaline basalts corresponds to the low-Mg end of the clinopyroxene megacryst trend. They have an Mg# of 70-72 and contain 1.5-2.0 wt% TiO2. Ilmenites from Vitim symplectites are Cr-poor and contain 5.3-6.3 wt% MgO, whereas ilmenites from kimberlite symplectites contain 0.1-1.4 wt% Cr2O3 and 8-13 wt.% MgO. However, trace-element patterns of symplectitic clinopyroxene in both kimberlites and alkaline basalts are similar. Clinopyroxenes from Vitim symplectites have more evolved trace-element patterns (Lan = 3.1-3.8, normalized to primitive mantle) relative to the majority of megacrysts (Lan = 1.0-3.8). Modeling of fractional crystallization of clinopyroxene from alkaline basaltic melt indicates that clinopyroxenes from symplectites correspond to the residual melt fraction of 20-40%, which is high for eutectic crystallization. Symplectites from the Mary pipe can be divided into Cr-rich (Cr2O3 cpx = 0.20-0.39 wt.% and Cr2O3 ilm = 0.7-1.3 wt%) and Cr poor (Cr2O3 cpx = 0.04-0.16 wt.% and Cr2O3 ilm = 0.1-0.4 wt.%) subgroups. However trace-element patterns in clinopyroxenes from both groups of symplectites are similar. This may indicate a symplectite origin from different pulses of magma. Cr-enrichment in the Cr-rich group can be related to contamination by adjacent peridotites. Compositional and textural variations in symplectites from both the alkaline basalts and kimberlites reveal similarities in genesis and a clear connection with megacryst assemblages. We suggest that ilmenite-clinopyroxene symplectites characterize the initial stage of simultaneous precipitation of ilmenite and clinopyroxene. They crystallize after the majority of the clinopyroxene megacrysts and before normal Ilm-Cpx intergrowths and ilmenite megacrysts. This indicates a possible cotectic rather than eutectic precipitation of ilmenite and clinopyroxene in symplectites. Pressure and temperature estimations indicate that Ilm-Cpx symplectites from the Mir and Mary kimberlite pipes formed at pressures of 40-50 kbar, whereas those from Vitim basalts formed at lower pressures of 12-17 kbar. Coexisting Fe-Ti oxides in Vitim ilmenites from symplectites and megacrysts suggest equilibration temperatures of 690-1025°C at fO2 = 0.22-1.88 log units below the QFM buffer. This redox state is similar to that of spinel peridotites in the same localities.

Role of magma pressure, tectonic stress and crystallization progress in the emplacement of syntectonic granites. The A-type Estrela Granite Complex (Carajás Mineral Province, Brazil)

The Archaean, syntectonic, A-type Estrela Granite Complex (Carajás Mineral Province, Brazil) consists of three plutons emplaced in a greenstone sequence under low-pressure conditions (180< P<310 MPa). It is composed mainly of annite-, ferropargasite (±hedenbergite)- and ilmenite-bearing monzogranites. The contact aureole is affected by a subvertical penetrative schistosity conformable with the limits of the plutons. Meso- to microstructures and mineral reactions in the granites indicate that deformation occurred in a continuum from above-solidus to low- T subsolidus conditions. Two distinct planar structures are observed: (i) a concentrical primary foliation (S 0) corresponding to rhythmic, isomodal, phase layering associated with a faint grain shape fabric; it is horizontal in the centre and vertical towards the edges of the plutons; and (ii) a steep to subvertical foliation (S 1) associated with the deformation of S 0 and accompanied with emplacement of synplutonic dykes and veins of leucocratic granites and pegmatites. Emplacement, differentiation and consolidation of the Estrela Granite Complex are considered to result from a continuous evolution under decreasing temperatures in a single-stage strained crust (transpression), with two main periods. (1) The first period is controlled by body forces, and it corresponds to inflation with magma ponding. As long as the rheology is melt dominated, magma pressure is the critical parameter and almost no strain is recorded. With decreasing T, magmas crystallize and differentiate leading to a concentrical magmatic phase layering. The growing magmatic bodies are mechanically decoupled from the country rocks and their evolution depends on internal magma chamber processes. (2) For higher amount of crystallization (residual melt fraction F<0.5), the role of magma pressure becomes insignificant. Establishment of a continuous crystal framework leads to the coupling of plutons with their surroundings, and deformation in response to tectonic stress. Most of the strain is recorded during this period which starts from the rigid percolation threshold, and extends to subsolidus low-grade conditions. This leads to deformation of the partially crystallized volume and redistribution of fluid-enriched differentiated melts. The amount of crystallization through the rheological thresholds appears as the critical parameter determining the transition from magma-controlled processes (inflation and differentiation of the magma chamber, with development of a phase layering) to tectonic-controlled processes (deformation of the phase layering and redistribution of residual melts). This accounts for the fact that syntectonic plutons commonly display intermingled, boudinaged layers with distinct modal compositions and in some cases well-preserved rhythmic layering.

Crystallisation of fine- and coarse-grained A-type granite sheets of the Southern Oklahoma Aulacogen, U.S.A.

A-type felsic magmatism associated with the Cambrian Southern Oklahoma Aulacogen began with eruption of voluminous rhyolite to form a thick volcanic carapace on top of an eroded layered mafic complex. This angular unconformity became a crustal magma trap and was the locus for emplacement of later subvolcanic plutons. Rising felsic magma batches ponding along this crustal magma trap crystallised first as fine-grained granite sheets and then subsequently as coarser-grained granite sheets. Aplite dykes, pegmatite dykes and porphyries are common within the younger coarser-grained granite sheets but rare to absent within the older fine-grained granite sheets. The older fine-grained granite sheets typically contain abundant granophyre.The differences between fine-grained and coarse-grained granite sheets can largely be attributed to a progressive increase in the depth of the crustal magma trap as the aulacogen evolved. At low pressures (&lt;200MPa) a small increase in the depth of emplacement results in a dramatic increase in the solubility of H2O in felsic magmas. This is a direct consequence of the shape of the H2O-saturated granite solidus. The effect of this slight increase in total pressure on the crystallisation of felsic magmas is to delay vapour saturation, increase the H2O content of the residual melt fractions and further depress the solidus temperature. Higher melt H2O contents, and an extended temperature range over which crystallisation can proceed, both favour crystallisation of coarser-grained granites. In addition, the potential for the development of late, H2O-rich, melt fractions is significantly enhanced. Upon reaching vapour saturation, these late melt fractions are likely to form porphyries, aplite dykes and pegmatite dykes.For the Southern Oklahoma Aulacogen, the progressive increase in the depth of the crustal magma trap at the base of the volcanic pile appears to reflect thickening of the volcanic pile during rifting, but may also reflect emplacement of earlier granite sheets. Thus, the change in textural characteristics of granite sheets of the Wichita Granite Group may hold considerable promise as an avenue for further investigation in interpreting the history of this rifting event.

Developments of the Rayleigh equation: “crystal” a Pascal program for simulating fractional crystallization

An interactive program to simulate the fractional crystallization process has been implemented. The Pascal program, termed “Crystal”, uses an equation based on the trace elements of the parent and daughter magmas. “Crystal” is based on the development of the Rayleigh equation: C 1 C o =(1−Σy i) (Σy iK i Σy i−1) where the new variable y i represents the mass of phase i in the solid in relation to the initial mass. From this form of the equation, it is possible to determine the mass fraction of phase i in the solid x i without assigning a series of arbitrary values to x i or F (residual melt fraction). At the present time, using in a stochastic manner the Rayleigh equation, one or other of the equation variables has to be assigned arbitrarily by balancing the mass with the major elements. The main advantage of the method is that it prevents external factors from conditioning the trace-element model.

Small-scale variations and subtle zoning in granitoid plutons: the Liberty Hill pluton, South Carolina, U.S.A.

Imperfect flow separation and remixing of crystals and melt produced chemical, mineral and textural variability on the scale of several centimeters to meters in coarse-grained granitoids of the Liberty Hill pluton. This is believed to develop during emplacement because the resulting layering conforms to the geometry of the pluton and is better developed toward the margins. Mineral-specific, equilibrium and fractional crystallization regimes superimposed on the flow separation and remixing produced more complex compositional variations than expected from such granitoid cumulate processes alone. The process is chemically comparable to restite unmixing, but it is difficult to demonstrate that many minerals involved in the flow are restites. Local compositional inhomogeneities also formed by deformation and disaggregation of enclaves. Depending on whether the enclaves are interpreted as xenoliths, chilled mafic magma, or high-melting-point restite clots, they represent contamination by mechanical assimilation, magma mingling and mixing, or restite unmixing. Marginal granitoids, distinguished from the central granitoids by pink feldspar + biotite ± muscovite rather than white feldspar + amphibole + biotite, are more evolved but overlap the compositional range of the central granitoids. Mineralogic and textural similarity and a gradational contact between the two granitoids indicate they are related and that the pluton's subtle reverse zoning was produced by a continuous in situ process. Flow separation and remixing, operating more efficiently and on a larger scale, may have produced the more evolved composition of the marginal granitoids; however, proximity to the contact is more important than rock composition in determining mineralogy. There is slight geochemical evidence for contamination of the marginal granitoids by the wall rocks; compared to central granitoids they have small LREE depletions and HREE enrichments, which more nearly reflects the LREE depletion and HREE enrichment of the wall rocks. Cross-cutting contacts and slightly differing compositional trends indicate a later and distinct evolution for the finer-grained biotite granitoids. They have silica enrichment, incompatible-element enrichment (Rb, U, Th), compatible-element depletion (major elements, Sr, Ba), depleted HREE and, less frequently, LREE contents as compared to the central granitoids. The fine-grained granitoids could be a contemporaneous and unrelated magma or fractionally crystallized from the coarse-grained magma. Compositional, mineralogic and textural differences among the later finer-grained granitoids indicate either multiple magmas, periodic sampling and intrusion of successive residual melt fractions, or flow separation and remixing during emplacement.

Further developments of the Rayleigh equation for fractional crystallization

The equation developed by Neuman et al. [1] from the Rayleigh fractionation law is expressed as: C 1C/ 0= F (D − 1) Since the residual melt fraction F is not directly connected with the mass fraction of phase i in the solid x i, it is necessary to assign a series of arbitrary values to one of these variables. The modelling which results from using Rayleigh's equation is thus externally controlled and as such has little real significance. By introducing the variable y i to represent the mass of phase i in the solid in relation to the initial mass, we obtain F = 1 − Σy i and the previous equation can be rewritten: C 1C/ 0= (1 − Σy i) (Σy iK i/Σy i− 1) From this form of the equation it is possible to find the other system variables for known values of K i, C 1 and C 0 where K i represents the solid-liquid partition coefficient and C 1 and C 0 are the concentration of the differentiated and parent melts respectively. The system of simultaneous equations can be solved by least-squares methods and applied to the study of natural systems.

Double-diffusive convection in partially molten silicate systems: Its role during magma production and in magma chambers

In this paper, two problems involving double-diffusive convection in partially molten, silicate systems are investigated mathematically. The first problem concerns the role of double-diffusive convection during magma production. The systems Di-An and Di-An-Ab are used as examples. The calculations show that, in a growing layer of partial melt, heated from below, the thickness of the layer at which convection initiates depends strongly upon the bulk composition. For example, assuming reasonable values of the physical parameters, in a Di-rich system, convection will occur when the layer is ∼ 800 m thick, whereas in an An-rich system convection, driven by the destabilizing compositional gradient, will begin in a layer only centimeters thick. A qualitative investigation of the Di-An-Ab system suggests similar results, depending upon whether the bulk composition lies in the diopside or the plagioclase field, respectively. Moreover, if one considers a two-component, one-dimensional diapir of Di-An, an examination of the phase diagram indicates that vigorous compositional convection will occur in the partially molten diapir as it ascends, regardless of whether the bulk composition is Di-rich or An-rich. These results suggest that during magma production, convective processes will tend to homogenize the melt before it segregates from the source zone; however, the vigor of mixing is dependent upon the bulk composition of the source, among several other factors. Models of melt segregation should be modified to include double-diffusive processes. The second problem concerns the structure of a porous boundary layer that forms as a result of side-wall crystallization in a convecting magma chamber. An examination of the steady-state boundary layer equations for flow through a porous medium shows that the boundary layer structure may be of two types. If the residual melt fraction, upon crystallization at the wall, has negative compositional buoyancy, or if the negative thermal buoyancy at the cold wall exceeds the positive compositional buoyancy of the residual melt, the flow across the entire boundary layer will be downward. If the residual melt fraction has negative compositional buoyancy, the magma chamber will become stratified as a result of accumulation of a layer of dense, cold liquid on the floor; whereas if the melt fraction has positive compositional buoyancy, the boundary layer fluid will tend to be remixed into the interior of the magma chamber. If, however, the positive compositional buoyancy exceeds the negative thermal buoyancy, counterflowing boundary layers will occur; and the compositionally buoyant liquid will tend to be fractionated towards the top of the magma chamber. An examination of the Di-An and Di-An-Ab systems suggests that both types of boundary layer structures may occur in magma chambers. An approximate calculation of the rate of fractionation suggests that, for a range of parameters that may be representative of basaltic magmas, the flow in the compositional boundary layer may transport ∼ 10 −3 km 3/yr of low-density liquid to the top of the magma chamber.

Exhalative Sn and W deposits associated with mafic volcanism as precursors to Sn and W deposits associated with granites

Precambrian and Early Palaeozoic submarine exhalative Sn and W deposits are most commonly present in thick deep water sequences comprising pelites, carbonate, calcsilicate and cherty rocks, mafic volcanics and quartz-tourmaline rocks. The exhalative deposits are commonly low grade and, in places, the sequence is transgressed by granites with economic Sn or W mineralisation. These thick deep water sequences as components of sinking trenches are more likely than most other rocks to undergo high grade metamorphism and anatexis. Thus elements such as Sn, W, B, F, Mo, Bi, Li, Be and base metals associated with the exhalites rapidly partition into the anatectic melt, lower the liquidus and solidus, and are concentrated in the late stage aqueous fluids and residual melt fractions to form Sn or W deposits associated with highly fractionated granitic rocks.

Lewisian pyroxene gneisses from Barra and the geochemistry of the Archaean lower crust

Synopsis Tonalitic, pyroxene gneisses from the pre- 2.6 Ga Lewisian complex of Barra are characterized by: (1) low K, Rb and Th; high Sr and Ba; high K/Rb ratios and low Rb/Sr and K/Ba ratios; (2) enriched light REE and depleted heavy REE; (3) positive Eu anomalies. These features, when considered in conjunction with evidence that a possible Archaean lower/upper crust interface may occur on Barra, are compatible with two models for lower crustal evolution: (A) the purging of hydrous tonalitic precursors in the lower crust of H 2 O and K, Rb and Th by mantle-derived, CO 2 -rich fluids carrying Sr and Eu during granulite facies metamorphism; (B) the primary igneous crystallization under high P co 2 of new tonalitic additions to the crust, from which the hydrous residual melt fraction was expelled upwards. Pyroxene gneisses may be the repository of the extra Eu in the crust which is needed to complement the deficiency of that element in post-Archaean sediments and the upper crust.