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Phase relations and fractionation sequences in potassic magma series modelled in the system CaMgSi2O6-KAlSiO4-Mg2SiO4SiO2-F2O-1 at 1 bar to 18 kbar
No full textLiquidus phase relations in the system diopside-kalsilite-forsterite-quartz with 3 wt% F were examined at 1 bar and the locations of important invariant points were determined at 18 kbar. At all pressures within this range a large liquidus field for fluorphlogopite (Phl) exists, and has a large influence on both melting and fractionation processes. One eutectic point was found to the silica-rich side of the plane Lc-Fo-Di at Di(1)Ks(30)Fo(2)Qz(67), where a melt coexists with San, Qz, Phl and Di at 840 degrees C and 1 bar. Another eutectic point must exist in the silica-poor part of the system because the phase topology determines that thermal barriers must exist. At this point a feldspathoid, either Lc or Ks, must coexist with Fo, Phl and a Ca-bearing phase such as Di. The exact location and phase assemblage were not determined, but the equilibrium melt must have a composition rich in Di (>29 wt%) and extremely poor in Qz (<8 wt%). The composition of the first eutectic moves towards lower SiO2 contents with increasing pressure (Di(3)Ks(40)Fo(1)Qz(56) at 18 kbar), whereas the second does not exist at 18 kbar due to the disappearance of Lc as a stable liquidus phase. Liquids which coexist with mafic minerals such as En, Fo, Phl and Di are important for the genesis of potassium-rich mafic rocks by partial melting in the mantle and for the early stages of fractional crystallisation. The equilibrium melt at the invariant point Fo + En + Phl + Di + L at 1125 degrees C is very poor in Fo and Di components at atmospheric pressure (Di(5)Ks(37)Fo(5)Qz(53)), whereas at Is kbar the melt contains large amounts of Fo and Di (Di(19)Ks(31)Fo(28)Qz(21)), and has a composition close to that of natural lamproites. Kamafugites do not correspond to melts in this system under any of the studied conditions, and appear to require CO2 in the source. Fractionation processes from primitive: potassic basanite melts are controlled principally by the size (and not the mere presence) of the liquidus phase field for phlogopite: at high pressures where the Phl field is large, olivine is eliminated early from the fractionating assemblage and Cpx + Phl fractionation may lead to relatively silica-rich rock differentiates such as trachytes. At low pressures, extensive olivine and restricted Phl crystallisation prevents silica enrichment in the melt, resulting in phonolitic differentiates. Later crystallisation of alkali feldspar accentuates the trends laid down in the early stages of fractionation
The effect of crystal orientation on the wetting behaviour of silicate melts on the surfaces of spinel peridotite minerals
No full textThe effect of crystal anisotropy on wetting angles of equilibrium silicate melts on crystal faces of spinet, diopside. enstatite and olivine has been determined experimentally by the sessile melt drop technique. The anisotropy (A) over tilde gamma(SL), of solid-liquid interfacial energies (gamma(SL) (max')-gamma(SL) (min)) can be related to the wetting angles, psi, by (A) over tilde gamma(SL) proportional to \cos psi (max) - cos psi (min)\ = Pw ((A) over tilde gamma(SL)). Normalising to the smallest wetting angle gives values of P-w for diopside = 0.0728, olivine = 0.0574, orthopyroxene=0.01521, and spinel=0.0075. Crystal anisotropy influences grain-scale morphology of small-degree partial melts. permeability and the melt connectivity threshold, phi(c). Results show that, at sufficient melt fractions. diopside should increase permeability in a peridotitic matrix, whereas enstatite should lower it. Despite its low anisotropy, spinet contributes positively to permeability and phi(c) because of its high surface energies. These results suggest that harzburgitic mineral matrices typical of the suberatonic mantle should impede the movement of low-degree partial melts, whereas melts should flow more easily through spinet lherzolites
Cs-Rb-Ba systematics in phengite and amphibole: an assessment of fluid mobility at 2.0 GPa in eclogites from Trescolmen, Central Alps
No full textEclogites from Trescolmen that contain abundant hydrous minerals (phengite, amphibole, paragonite, zoisite, talc, apatite) show petrographic evidence for fluid infiltration under conditions of 2.0 to 1.8 GPa, 650 degreesC. Large ion lithophile elements (LILE, e.g. Cs, Rb, Ba and Sr) were analysed by in-situ techniques in all eclogite mineral phases in order to characterize the behaviour of fluid-mobile elements at high pressure. In-situ analysis of carefully-chosen metamorphic assemblages circumvents the problem of partial late-stage alteration, which can severely influence the calculated element budgets of whole-rock samples. Phengite is the dominant host for Cs, Rb, and Ba in both eclogite and adjacent garnet mica schist samples, and incorporates > 90% of the budgets of these elements in whole rocks. LILE contents of phengites in phengite-rich rocks are likely to record the Cs/Rb and Ba/Rb ratios of their host rock protoliths. The LILE patterns of eclogite are consistent with protoliths derived from basalt that underwent seafloor alteration, whereas those of mica schist are almost identical to average upper continental crust. In contrast, LILE patterns of eclogite samples that lack phengite, but do contain amphibole, are unlike any plausible protolith, but are identical to those of amphibole in phengite-bearing samples. This observation points to homogenization of the LILE in different lithologies, which we correlate with petrographic evidence for fluid infiltration. Because phengite in garnet mica schist has a strong capacity to buffer the fluid with respect to Cs, Rb, and Ba, homogenization of amphiboles is best explained by fluid infiltration from the surrounding metapelites into eclogite bodies, implying at least metre-scale fluid mobility. The amphibole homogenization can be most easily modelled by a pervasive open-system fluid flux through the eclogites, possibly facilitated by ductile deformation during the early stages of uplift. Simple calculations give minimum fluid-rock ratios of similar to0.001 to 0.004. Demonstration of the mobility of very small volumes of fluid through eclogite is an important prerequisite of many subduction zone models that try to explain across-are variations in trace element geochemistry. The low fluid-rock ratios from this study are not in contrast with oxygen isotope heterogeneities reported from other eclogite localities. Fluid mobile elements such as Cs, Rb and Ba are more sensitive indicators of small volume, fluid-rock interaction and are therefore potentially valuable for understanding fluid infiltration processes in systems where oxygen isotope shifts are not large enough to be detectable
Cs-Rb-Ba systematics in phengite and amphibole: an assessment of fluid mobility at 2.0 GPa in eclogites from Trescolmen, Central Alps
No full textEclogites from Trescolmen that contain abundant hydrous minerals (phengite, amphibole, paragonite, zoisite, talc, apatite) show petrographic evidence for fluid infiltration under conditions of 2.0 to 1.8 GPa, 650 degreesC. Large ion lithophile elements (LILE, e.g. Cs, Rb, Ba and Sr) were analysed by in-situ techniques in all eclogite mineral phases in order to characterize the behaviour of fluid-mobile elements at high pressure. In-situ analysis of carefully-chosen metamorphic assemblages circumvents the problem of partial late-stage alteration, which can severely influence the calculated element budgets of whole-rock samples. Phengite is the dominant host for Cs, Rb, and Ba in both eclogite and adjacent garnet mica schist samples, and incorporates > 90% of the budgets of these elements in whole rocks. LILE contents of phengites in phengite-rich rocks are likely to record the Cs/Rb and Ba/Rb ratios of their host rock protoliths. The LILE patterns of eclogite are consistent with protoliths derived from basalt that underwent seafloor alteration, whereas those of mica schist are almost identical to average upper continental crust. In contrast, LILE patterns of eclogite samples that lack phengite, but do contain amphibole, are unlike any plausible protolith, but are identical to those of amphibole in phengite-bearing samples. This observation points to homogenization of the LILE in different lithologies, which we correlate with petrographic evidence for fluid infiltration. Because phengite in garnet mica schist has a strong capacity to buffer the fluid with respect to Cs, Rb, and Ba, homogenization of amphiboles is best explained by fluid infiltration from the surrounding metapelites into eclogite bodies, implying at least metre-scale fluid mobility. The amphibole homogenization can be most easily modelled by a pervasive open-system fluid flux through the eclogites, possibly facilitated by ductile deformation during the early stages of uplift. Simple calculations give minimum fluid-rock ratios of similar to0.001 to 0.004. Demonstration of the mobility of very small volumes of fluid through eclogite is an important prerequisite of many subduction zone models that try to explain across-are variations in trace element geochemistry. The low fluid-rock ratios from this study are not in contrast with oxygen isotope heterogeneities reported from other eclogite localities. Fluid mobile elements such as Cs, Rb and Ba are more sensitive indicators of small volume, fluid-rock interaction and are therefore potentially valuable for understanding fluid infiltration processes in systems where oxygen isotope shifts are not large enough to be detectable
Trace element and Nd–Sr isotopic composition of ultramafic lamprophyres from the East Antarctic Beaver Lake area
No full textThe trace element and Nd-Sr isotopic compositions of Cretaceous (110-117 Ma) ultramafic lamprophyres from the Beaver Lake area in East Antarctica, which are developed as sill, dyke and plug intrusions. have been investigated. Rare earth elements of lamprophyres are strongly fractionated, with LREE > 100 times chondrite, whereas HREE are < 10 times chondrite, presumably indicating the presence of residual garnet in the source region. A characteristic feature of the Beaver Lake rocks is low concentrations of Zr and Hf (Zr, 50-150 ppm: Hf, 1.0-5.0 ppm) and to a lesser extent Nb (17-90 ppm) which cause strong negative anomalies in the normalized trace element patterns. Eruption age-corrected Nd isotope values vary within narrow ranges (epsilon Nd-(t) = + 2.1 to + 4.3), whereas Sr isotope values show more variation (Sr-87/Sr-86((t)) = 0.704336-0.706431), similar to the isotopic compositions of other ultramafic lamprophyres. The genesis of the ultramafic lamprophyres is explained as a result of the gradual widening of the Lambert-Amery rift during the Phanerozoic. An increase in the geothermal gradient from cold continental conditions beneath the Archean block to the west may have resulted in grazing of the peridotite solidus at depths greater than 110 km. The initial carbonate-rich melts migrated upwards and froze as carbonate-bearing veins in the overlying lithosphere. Later upward and outward migration of the asthenosphere beneath the rift caused remelting of the veined region, resulting in melts of ultramafic lamprophyre composition with a mixed geochemical abundance and isotope signature from carbonate-rich veins and depleted peridotite wall-rock. The CO2-rich component of the lamprophyres is derived principally from the vein assemblage, from which phlogopite gives rise to the variable Sr isotopes, and baddeleyite to the differing Zr and Hf abundances. The difference between the lamprophyre facies of the Beaver Lake intrusions can be explained by minor amounts of olivine and Cr-spinel fractionation. (C) 2001 Elsevier Science B.V, All rights reserved
Rutile/melt partition coefficients for trace elements and an assessment of the influence of rutile on the trace element characteristics of subduction zone magmas
No full textFractionation of some or all of the high field strength elements (HFSE) Nb, Ta, Zr, Hf, and Ti relative to other trace elements occurs in igneous rocks from convergent margins and in the average continental crust, and is generally attributed to a process occurring during subduction. The experimental partitioning of an extensive array of trace elements between rutile/melt pairs is presented which enables the effect of rutile during melting in subduction zones to be directly assessed. D-Nb and D-Ta are in the range 100-500, D-Zr and D-Hf are about 5, whereas all other trace elements analyzed have D-rutile/melt less than 0.1. Published D patterns for Nb and Ta between rutile and water-rich fluids are similar to those for melt, whereas the values for Zr and Hf are significantly higher. D-Nb and D-Ta values for clinopyroxene and garnet are much lower than for rutile, and cannot cause the fractionation of HFSE from other elements seen in island area. The presence of rutile in the subducted slab residue during dehydration may be essential in the production of the geochemical signatures of are magmas, whereas that of the continental crust, including higher Zr/Sm, may be produced by melting of eclogite. Copyright (C) 1999 Elsevier Science Ltd
Trace element abundances in rutiles from eclogites and associated garnet mica schists
No full textWe present electron microprobe and laser ablation microprobe (LAM) data for a range of high field strength (Zr, Nb, Me, Sri, Sb, Hf, Ta, W) and other trace elements (Al, Si, Ca, V, Cr, Mn, Fe, Pb, Th, U) in rutile from eclogites and garnet mica schists, from Trescolmen. Central Alps. Most analysed rutiles are homogeneous (at least for Nb, Cr, W, Zr, V and Fe), both on a single grain scale and between grains from a single thin section. Concentrations of V, Zr, Nb, Sb and W determined by both electron and laser ablation microprobe techniques yield similar results and confirm the reliability of the analytical methods within estimated precision. Measurements of trace element contents of coexisting phases in eclogites and their modal abundances show that rutile is the dominant carrier ( > 90% of whole rock content) for Ti, Nb, Sb, Ta and W as well as an important carrier (5-45% of the whole rock content) for V, Cr, Me and Sri. The crystallographic implications are that, for relatively rigid crystal sites such as in rutile, trace elements with a similar ionic radius are preferred over trace elements with the same charge but deviating size. Our results demonstrate the utility of rutile chemistry in the following applications: (1) By using a combination of the measured TiO2 content of the whole rock and the trace element concentration of rutile, precise whole rock data on elements that are either difficult to analyze by conventional techniques such as XRF or solution ICP-MS (Nb, Sb, Ta, W) or may be susceptible to late stage alteration (Sb) can be estimated. (2) Trace element contents of detrital rutile grains are a potentially powerful toot for sedimentary provenance studies since they reflect key element ratios (e.g,, Nb/TiO2 and Cr/TiO2) of their source rocks. In addition, measurements of trace elements in detrital rutiles might help distinguish possible source rocks, e.g., high-grade metamorphic rocks such as eclogites and high-pressure granulites from hydrothermal ore deposits and kimberlites. In view of the dominance of rutile in the Sb budget of subducting oceanic crust, and the enrichment of Sb in the slab component of subduction zones. additional experimental studies on Sb-partitioning between rutile and fluid are needed in order to understand the behaviour of Sb in subduction zones. (C) 2002 Elsevier Science B.V All rights reserved
Magmatic modification and metasomatism of the subcontinental mantle beneath the Vitim volcanic field (East Siberia): evidence from trace element data on pyroxenite and peridotite xenoliths from Miocene picrobasalt
No full textThe genesis of several groups of pyroxenite xenoliths from Miocene picrobasalt of the Vitim volcanic field are considered on the basis of petrology and mineral chemical data, including trace element analyses of minerals by ion probe. The pyroxenites and related xenoliths can be classified into three groups consisting of 10 sets of xenoliths: Group I (Cr-diopside series) is made up by three subdivisions; (Ia) common Cr-diopside garnet and spinel websterites, (Ib) Al-poor Cr-diopside websterites, (Ic) modally metasomatized Cr-diopside Iherzolites. Group II (Al-augite series) consists of four further xenolith types; (IIa) Cr-rich and (IIb) Cr-poor megacrystic pyroxenite assemblages, (IIc) Al-rich garnet clinopyroxenites, and (IId) amphibole and/or phlogopite-bearing rocks and associated orthopyroxenites. Group III consists of xenoliths all characterized by Ca-rich clinopyroxene (garnet clinopyroxenite and gabbros, garnet granulites and spinel websterites) thought to have a similar depth of origin near the crust-mantle boundary. Trace element data and textural criteria allow the distinction of three melt types operating in the mantle: (1) Cr-rich melt migrating along small fractures and forming Cr-diopside pyroxenite veins as the result of percolation fractionation; (2) picrobasaltic to alkaline basaltic melt fractionating in large hydraulic fractures or magma chambers and producing host volcanics, clinopyroxene megacrysts and megacrystic pyroxenites (Groups IIa-IIb); and (3) high field strength element (HFSE)-enriched smaller volume melts, which formed the hydrous mineral-bearing xenoliths of Groups I and II (Ic and IId). The megacrystic pyroxenites (IIa-IIb) were crystallized in magma chambers or large fractures at various depths during multistage polybaric fractionation. The high-temperature Cr-rich websterites (Group IIa) correspond to the deepest level at near 30 kbar. The garnet websterites and megacrystic clinopyroxenites crystallized at intermediate levels, whereas the most differentiated megacrystic clinopyroxenites with ilmenite and phlogopite inclusions are from the shallowest levels at 15-11 kbar. The HFSE-rich melt resulted in amphibole- and phlogopite-rich assemblages crystallized in veins and interstitially in peridotites, and the development of ilmenite-phlogopite-bearing orthopyroxenites in a reaction zone between these. Pyroxenites of Groups I and II represent samples of an intensely veined sub-Vitim lithosphere: the preservation of textural and chemical disequilibrium indicates that the enrichment immediately preceded xenolith sampling and is related to the upward and outward migration of the asthenosphere-lithosphere boundary beneath the developing Baikal Rift. The formation of lower pressure xenoliths (Group III) is related to both of the first two melt types mentioned above. Spinel websterites of Group III are genetically related to the Group Ia melt crystallized near the crust-mantle boundary, and the mafic garnet granulites of Group III to intermediate fractionation products (gabbro-norites) of similar melts. Garnet gabbros are associated with the most fractionated megacrystic pyroxenites. (C) 2000 Elsevier Science B.V. All rights reserved
Internal Differentiation of the Archean Continental Crust: Fluid-Controlled Partial Melting of Granulites and TTGAmphibolite Associations in Central Finland
No full textFault bound blocks of granulite and enderbite occur within upper amphibolite-facies migmatitic tonalitictrondhjemiticgranodioritic (TTG) gneisses of the Iisalmi block of Central Finland. These units record reworking and partial melting of different levels of the Archean crust during a major tectonothermal event at 2627 Ga. Anhydrous mineral assemblages and tonalitic melts in the granulites formed as a result of hydrous phase breakdown melting reactions involving amphibole at peak metamorphic conditions of 811 kbar and 750900C. A nominally fluid-absent melting regime in the granulites is supported by the presence of carbonic fluid inclusions. The geochemical signature of light rare earth element (LREE)-depleted mafic granulites can be modelled by 1030 wt partial melting of an amphibolite source rock leaving a garnet-bearing residue. The degree of melting in intermediate granulites is inferred to be less than 10 wt and was restricted by the availability of quartz. Pressuretemperature estimates for the TTG gneisses are significantly lower than for the granulites at 660770C and 56 kbar. Based on the PT conditions, melting of the TTG gneisses is inferred to have occurred at the wet solidus in the presence of an H(2)O-rich fluid. A hydrous mineralogy, abundant aqueous fluid inclusions and the absence of carbonic inclusions in the gneisses are in accordance with a water-fluxed melting regime. Low REE contents and strong positive Eu anomalies in most leucosomes irrespective of the host rock composition suggest that the leucosomes are not melt compositions, but represent plagioclasequartz assemblages that crystallized early from felsic melts. Furthermore, similar plagioclase compositions in leucosomes and adjacent mesosomes are not a migmatite paradox, as both record equilibration with the same melt phase percolating along grain boundaries
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