Showing papers in "Contributions to Mineralogy and Petrology in 2002"

Igneous zircon: trace element composition as an indicator of source rock type

Abstract: Trace element abundances in igneous zircons, as determined by electron microprobe and laser-ablation microprobe ICPMS analysis, are shown to be sensitive to source rock type and crystallisation environment. The concentrations of 26 trace elements have been determined for zircons from a wide range of different rock types and reveal distinctive elemental abundances and chondrite-normalised trace element patterns for specific rock types. There is a general trend of increasing trace element abundance in zircons from ultramafic through mafic to granitic rocks. The average content of REE is typically less than 50 ppm in kimberlitic zircons, up to 600–700 ppm in carbonatitic and lamproitic zircons and 2,000 ppm in zircons from mafic rocks, and can reach per cent levels in zircons from granitoids and pegmatites. Relatively flat chondrite-normalised REE patterns with chondrite-normalised Yb/Sm ratios from 3 to 30 characterise zircons from kimberlites and carbonatites, but Yb/Sm is commonly over 100 in zircons from pegmatites. Th/U ratios typically range from 0.1 to 1, but can be 100–1000 in zircons from some carbonatites and nepheline syenite pegmatites. The geochemical signatures characteristic of zircon from some rock types can be recognised in bivariate discriminant diagrams, but multivariate statistical analysis is essential for the discrimination of zircons from most rock types. Classification trees based on recursive partitioning techniques provide a rapid means of relating parent rock type to zircon trace element analysis; zircons from many rock types can be discriminated at confidence levels of 75% or more. These trees allow recognition of the provenance of detrital zircons from heavy mineral concentrates, and significantly enhance the usefulness of zircon in regional crustal studies and as an indicator mineral in mineral exploration.

Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in zircon: application to porphyry copper deposits of northern Chile

Abstract: Major- and trace-element compositions of zircons and whole rocks from 14 barren and seven ore-bearing calc-alkaline intrusions from the Chuquicamata-El Abra porphyry copper belt of northern Chile have been measured in situ by excimer laser ablation (ELA) ICP-MS. These data permit the Ce(IV)/Ce(III) ratio within zircon to be calculated using a lattice-strain model for mineral-melt partitioning of Ce(IV) and Ce(III). Zircon Ce(IV)/Ce(III) and EuN/EuN* ratios, and by inference magmatic oxidation states, generally increase from older, mafic to younger, felsic units. Within this sequence, porphyry copper mineralization is directly associated only with intrusions with zircon Ce(IV)/Ce(III)>300 and EuN/EuN*>0.4. Such trends can be understood in terms of interdependent relations between oxygen fugacity, sulfur speciation and solubility, and chalcophile element partitioning in silicate magmas. Because zircon occurs in most calc-alkaline intrusions and is resistant to subsolidus alteration, zircon Ce(IV)/Ce(III) ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic-hydrothermal Cu±Au mineralization. The approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks.

Mesozoic lithosphere destruction beneath the North China Craton: evidence from major-, trace-element and Sr–Nd–Pb isotope studies of Fangcheng basalts

Abstract: Major- and trace-element as well as Sr–Nd–Pb isotopic data of the Mesozoic Fangcheng basalts provide an insight into the nature of their mantle source and the secular evolution of the lithospheric mantle beneath the North China Craton. Fangcheng basalts include alkali basalt and olivine tholeiite, both characterized by high Mg (Mg#=65–72), Si, and Ca and low K+Na, Ti, and P. They are extremely enriched in LREEs ((La/Yb)N=39.3–49.3) and LILEs (Ce, Rb, Ba, U, Th) and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with slightly negative Pb anomaly. Correspondingly, these basalts are exceedingly high in e Sr (74.0~81.5) and low in e Nd (–13.1~–14.2) and 206Pb/204Pb (<17.8). Since crustal contamination during the magma ascent is insignificant, the Fangcheng basalts could reflect the nature of its mantle source. The isotopic data of these basalts cannot be explained by mixing of typical mantle components, but can be accounted for by interaction of an old lithospheric mantle with the lower/middle crust. Therefore, we consider that these basalts originated from the Mesozoic lithospheric mantle, which evolved from its Paleozoic counterpart through extensive interaction with a crust-derived melt. We propose that this melt was generated from the melting of the subducted lower crust of the Yangtze Craton. This peculiar Mesozoic lithospheric mantle somehow was in turn replaced later by the hot and thin Cenozoic lithospheric mantle.

The role of an H2O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California

Abstract: This paper presents analyses of the trace element abundances and isotopic compositions in primitive lavas from the Mt. Shasta region, N California. These data are combined with estimates of pre-eruptive H2O contents and constraints from experimental petrology to develop a model of subduction zone magmatism. These lavas share geochemical characteristics of high-Mg andesites from the Setouchi volcanic belt in SW Japan and Adak-type high-Mg andesites of the western Aleutian arc. Estimates of the pre-eruptive water contents of the Shasta region lavas range from 8 wt % H2O. The pre-eruptive H2O content and an inferred melt of a harzburgitic residue are used to carry out a mass balance for the relative contributions from a mantle-derived melt and slab-derived fluid-rich component. We assume that elements are contributed either from melting of mantle peridotite or from a subduction-related fluid-rich component. Estimated fluid-rich component compositions are characterized by strong light rare earth element (LREE) enrichments ([La/Gd]N=3 to 7) and variable heavy rare earth element (HREE) depletions ([Dy/Yb]N=1 to 3). Sr and Ba abundances vary by approximately a factor of 2.5 in the fluid compositions calculated for the Mt. Shasta region lavas and large ion lithophile element (LILE) abundances are similar to those calculated by Stolper and Newman (1994) and Eiler et al. (2000). The major elements in the fluid-rich component are H2O (~55–68 wt%), Na2O (~25–33 wt%) and K2O (~5–13 wt%). This composition may be that of a supercritical fluid or a low-degree melt of the slab that has reacted with the overlying mantle wedge. Although the slab beneath Mt. Shasta is inferred to be hot (~600 – 650 °C), the calculated fluid-rich components do not resemble a pure slab melt. The calculated isotopic composition of the fluid-rich component is bimodal. One component has 87Sr/86Sr=0.7028 and eNd=+8, and is most similar to a MORB source. The second component has more radiogenic 87Sr/86Sr=0.7038 and eNd=+1 and is most similar to a sediment. These fluid-rich components probably represent a mixture of fluids and melts from the slab (serpentinized mantle, altered basalt, and sediment).

Geochemistry of oceanic carbonatites compared with continental carbonatites: mantle recycling of oceanic crustal carbonate

Abstract: Major and trace element and Sr–Nd–Pb–O–C isotopic compositions are presented for carbonatites from the Cape Verde (Brava, Fogo, Sao Tiago, Maio and Sao Vicente) and Canary (Fuerteventura) Islands. Carbonatites show pronounced enrichment in Ba, Th, REE, Sr and Pb in comparison to most silicate volcanic rocks and relative depletion in Ti, Zr, Hf, K and Rb. Calcio (calcitic)-carbonatites have primary (mantle-like) stable isotopic compositions and radiogenic isotopic compositions similar to HIMU-type ocean island basalts. Cape Verde carbonatites, however, have more radiogenic Pb isotope ratios (e.g. 206Pb/204Pb=19.3–20.4) than reported for silicate volcanic rocks from these islands (18.7–19.9; Gerlach et al. 1988; Kokfelt 1998). We interpret calcio-carbonatites to be derived from the melting of recycled carbonated oceanic crust (eclogite) with a recycling age of ~1.6 Ga. Because of the degree of recrystallization, replacement of calcite by secondary dolomite and elevated ∂13C and ∂18O, the major and trace element compositions of the magnesio (dolomitic)-carbonatites are likely to reflect secondary processes. Compared with Cape Verde calcio-carbonatites, the less radiogenic Nd and Pb isotopic ratios and the negative Δ7/4 of the magnesio-carbonatites (also observed in silicate volcanic rocks from the Canary and Cape Verde Islands) cannot be explained through secondary processes or through the assimilation of Cape Verde crust. These isotopic characteristics require the involvement of a mantle component that has thus far only been found in the Smoky Butte lamproites from Montana, which are believed to be derived from subcontinental lithospheric sources. Continental carbonatites show much greater variation in radiogenic isotopic composition than oceanic carbonatites, requiring a HIMU-like component similar to that observed in the oceanic carbonatites and enriched components. We interpret the enriched components to be Phanerozoic through Proterozoic marine carbonate (e.g. limestone) recycled through shallow, subcontinental–lithospheric–mantle and deep, lower-mantle sources.

Graphitization in a high-pressure, low-temperature metamorphic gradient: a Raman microspectroscopy and HRTEM study

Abstract: The graphitization of carbonaceous material (CM) in a high-pressure metamorphic gradient is characterized along a cross section in the Schistes Lustres formation, Western Alps. Along this 25-km cross section, both the CM precursor and the host-rock lithology are homogeneous, and the prograde evolution of the pressure–temperature metamorphic conditions from the lower blueschist-facies (13 kbar, 330 °C) to the eclogite-facies (20 kbar, 500 °C) is tightly constrained by literature data. Raman microspectroscopy shows that at the micrometre scale, this process is progressive and continuous with increasing metamorphic grade, and that the structure of CM is very sensitive to temperature variations. At the nanometre scale (HRTEM), the CM is composed of a mixture of a microporous phase and an onion-ring like phase, both known as non-graphitizing under the effect of temperature at ambient pressure. The HP–LT graphitization produces structurally and microtexturally heterogeneous CM. With increasing metamorphic grade, the graphitization of the two types of CM proceeds up to the triperiodic graphite stage because of microtextural and structural changes that are specific to each type of CM. The microporous material is progressively transformed into graphite through a macroporous transitional stage. In this case, graphitization mainly occurs on the pore walls as a result of pore growth. In the case of concentric onion-ring like material, graphitization occurs in the regions with the largest radius of curvature, i.e. on the outer part of the ring. In comparison with 1-bar experiments, pressure seems to induce microtextural changes, which allows the subsequent structural modifications of the starting material.

An experimental study of element partitioning between magnetite, clinopyroxene and iron-bearing silicate liquids with particular emphasis on vanadium

Abstract: Mineral-melt partition coefficients of vanadium and a series of divalent trace elements (Ni, Co, Mn, Sr) have been determined for ferrobasaltic bulk compositions at one atmosphere. Experiments were performed at constant temperature (1,068 °C) and oxygen fugacity from 0.7 log units below to 2.6 log units above the NNO buffer (NNO–0.7 to NNO+2.6). All experiments were saturated in clinopyroxene and titanomagnetite. Partition coefficients for divalent cations between the liquid and these two minerals are found to be controlled by the ionic radius of the cation and the composition of the coexisting liquid, coefficients being significantly higher in more polymerised melts. Vanadium partitioning is strongly dependent on oxygen fugacity, decreasing by approximately one order of magnitude with increasing \( f_{O_2 } \) from NNO–0.7 to NNO+2.6 for both clinopyroxene and magnetite. Based upon thermodynamic modelling of the relative proportions of V3+, V4+ and V5+ in our liquids, this behaviour is inferred to be dominated by partitioning of V3+, despite the fact that this valence state is predicted to occur in low relative abundance. Derived values of \( {\rm D}_{{\rm V}^{{\rm 3 + }} } \) show no systematic dependence on melt polymerisation, but do show a systematic dependence on mineral composition. In particular, our data and those of the literature are combined to show that \( {\rm D}_{{\rm V}^{{\rm 3 + }} }^{{\rm Cpx/Liq}} \) increases significantly as clinopyroxenes become more iron-rich. The partition coefficients for vanadium determined in this study have been used to model the V concentration of liquid and magnetite as a function of differentiation in a ferrobasaltic system at different oxygen fugacities. These results show that extreme enrichments of V2O5 in magnetite will only occur for a relatively small range of \( f_{O_2 } \) , between NNO and NNO–1.5. The results of our modelling are shown to be consistent with observations made on the V-rich magnetite layers of the Bushveld intrusion.

Trace element geochemistry of phlogopite-rich mafic mantle xenoliths: Their classification and their relationship to phlogopite-bearing peridotites and kimberlites revisited

Abstract: Kimberlite magmas from the Kimberley area of South Africa have sampled two main types of phlogopite-rich mafic xenoliths which represent deep mantle segregations from highly alkaline melts. The first group corresponds to the MARID rocks characterised by the mineral association mica (phlogopite)–amphibole (K-richterite)–rutile–ilmenite–clinopyroxene and the second group consists of the PIC rocks characterised by the mineral association mica (phlogopite)–ilmenite–clinopyroxene–minor rutile. The two groups are clearly distinguished from one another by their mineral paragenesis, by the major element composition of their phlogopite and ilmenite, by the trace element content of their clinopyroxene and by their clinopyroxene and whole rock Sr and Nd isotope ratios. The combined major and trace element variations are interpreted to indicate a genetic relationship between the PIC rocks and group I kimberlite magma, and between the MARID rocks and group II kimberlite magma. The two types of parental melts percolated through, and metasomatised, the upper mantle beneath the Kimberley area as indicated by the trace element characteristics of the clinopyroxenes of the studied phlogopite-bearing peridotites.

Rare earth element diffusion in a natural pyrope single crystal at 2.8 GPa

Abstract: Volume diffusion rates of Ce, Sm, Dy, and Yb have been measured in a natural pyrope-rich garnet single crystal (Py71Alm16Gr13) at a pressure of 2.8 GPa and temperatures of 1,200–1,450 °C. Pieces of a single gem-quality pyrope megacryst were polished, coated with a thin layer of polycrystalline REE oxide, then annealed in a piston cylinder device for times between 2.6 and 90 h. Diffusion profiles in the annealed samples were measured by SIMS depth profiling. The dependence of diffusion rates on temperature can be described by the following Arrhenius equations (diffusion coefficients in m2/s): \(\)\(\matrix{ {\log _{10} D_{{\rm Yb}} = ( - 7.73 \pm 0.97) - \left( {343 \pm 30\;{\rm kJ}\;{\rm mol}^{{\rm - 1}} /2.303RT} \right)} \cr {\log _{10} D_{{\rm Dy}} = ( - 9.04 \pm 0.97) - \left( {302 \pm 30\;{\rm kJ}\;{\rm mol}^{{\rm - 1}} /2.303RT} \right)} \cr {\log _{10} D_{{\rm Sm}} = ( - 9.21 \pm 0.97) - \left( {300 \pm 30\;{\rm kJ}\;{\rm mol}^{{\rm - 1}} /2.303RT} \right)} \cr {\log _{10} D_{{\rm Ce}} = ( - 9.74 \pm 2.84) - \left( {284 \pm 91\;{\rm kJ}\;{\rm mol}^{{\rm - 1}} /2.303RT} \right)} \cr } \) . There is no significant influence of ionic radius on diffusion rates; at each temperature the diffusion coefficients for Ce, Sm, Dy, and Yb are indistinguishable from each other within the measurement uncertainty. However, comparison with other diffusion data suggests that there is a strong influence of ionic charge on diffusion rates in garnet, with REE3+ diffusion rates more than two orders of magnitude slower than divalent cation diffusion rates. This implies that the Sm–Nd isotopic chronometer may close at significantly higher temperatures than thermometers based on divalent cation exchange, such as the garnet–biotite thermometer. REE diffusion rates in pyrope are similar to Yb and Dy diffusion rates in diopside at temperatures near the solidus of garnet lherzolite (~1,450 °C at 2.8 GPa), and are an order of magnitude faster than Nd, Ce, and La in high-Ca pyroxene at these conditions. At lower temperatures relevant to the lithospheric mantle and crust, REE diffusion rates in garnet are much faster than in high-Ca pyroxene, and closure temperatures for Nd isotopes in slowly-cooled garnets are ~200 °C lower than in high-Ca pyroxene.

Unexpectedly high-PGE chromitite from the deeper mantle section of the northern Oman ophiolite and its tectonic implications

Abstract: Unusually high, platinum-group element (PGE) enrichments are reported for the first time in a podiform chromitite of the northern Oman ophiolite. The chromitite contains ≤1.5 ppm of total PGE, being highly enriched in the IPGE subgroup (Ir, Os and Ru) and strongly depleted in the PPGE subgroup (Rh, Pt and Pd). Its platinum-group minerals (PGMs) are classified into three types arranged in order of abundance: (1) sulphides (Os-rich laurite, laurite–erlishmanite solid solution and an unnamed Ir sulphide), (2) alloys (Os–Ir alloy and Ir–Rh alloy), and (3) sulpharsenides (irarsite and hollingworthite). The high PGE concentrations are observed only in a discordant chromitite deep in the mantle section, which has high-Cr# (>0.7) spinel with an olivine matrix. All the other types of chromitite (in the Moho transition zone (MTZ) and concordant pods in the deeper mantle section) are poor in PGEs and tend to have spinels with lower Cr# (up to 0.6). This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced low-Cr#, PGE-poor chromitites at the Moho transition zone and, to a lesser extent, within the mantle, possibly beneath a fast-spreading mid-ocean ridge. The second chromitite-forming event involves higher degree partial melting, which produced high-Cr#, PGE-rich discordant chromitite in the upper mantle, possibly in a supra-subduction zone setting.

I-type plutonism in a continental back-arc setting: Miocene granitoids and monzonites from the central Aegean Sea, Greece

Abstract: Miocene extension in the Aegean Sea was accompanied by the intrusion of granitoid and monzonitic magmas at upper crustal levels. With increasing distance from the trench, the metaluminous to slightly peraluminous I-type intrusives show a systematic regional variation in their compositions: medium- to high-K granodiorites in the WSW, high-K low-silica granites in the central region and high-K high-silica granites, monzonites, monzogranites and composite lamprophyric-monzonitic dikes in the ENE. The granodioritic and low-silica granitic plutons comprise similar ranges of SiO2 (63.5–71.6 wt%) and initial Nd–Sr and O isotopic values [eNd(I)=–7.0 to –9.8; 87Sr/86Sr(I)=0.7092–0.7115; δ18O=9.7–11.5‰], but display significant differences in their chemical compositions. Compared with the granodiorites, the low-silica granites have higher abundances of large ion lithophile, light rare earth and some high-field strength elements. Initial isotopic signatures of the high-silica granites in the ENE [eNd(I)=–9.0 to –9.6; 87Sr/86Sr(I)=0.7128–0.7144; δ18O=10.8–12.0‰] are slightly more 'crust-like' than those of the low-silica granites. In chemical variation diagrams, samples from the high-silica granites continue the trends defined by those from the low-silica granites. It is suggested that all granitoid magmas originated by dehydration melting of metaluminous older crustal sources, most probably metagreywackes. U–Pb systematics of zircon populations in conjunction with Zr–SiO2 systematics and zircon saturation temperatures suggest a high proportion of restitic or assimilated crustal material in these magmas. Monzonitic to monzogranitic rocks show chemical and isotopic characteristics [eNd(I)=–0.8 to –3.6; 87Sr/86Sr(I)=0.7059–0.7071; δ18O=9.4–10.6‰] that are significantly different from those of the granitoids. A small portion of an older crustal component in these magmas is indicated by U–Pb systematics of zircon populations. Lamprophyric dikes display chemical and isotopic signatures [eNd(I)≈+0.5; 87Sr/86Sr(I)≈0.7044; δ18O≈8.9‰] that are compatible with an origin from either a mantle source previously enriched by slab-derived hydrous fluids/melts as suggested by negative Nb–Ta anomalies or a new K-rich basaltic underplate. Abundant composite dikes indicate the coexistence of and mingling/mixing between lamprophyric melts and monzonitic magmas.

Minor- and trace-element zoning in plagioclase: implications for magma chamber processes at Parinacota volcano, northern Chile

Abstract: Textural and compositional zoning in plagioclase phenocrysts in a sample from Parinacota volcano (Chile) was investigated using backscattered electron images and electron microprobe analysis of major and trace elements. Large (2 mm) oscillatory zoned crystals (type I) with resorption surfaces of moderate An discontinuities (⩽10% An) and decreasing trace-element contents (Sr, Mg, Ti) towards the rim reflect melt differentiation and turbulent convection in the main magma body. Early recharge with a low-Sr mafic magma is seen in the core. Small-scale Sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years. Smaller crystals (type II) with low trace-element/An ratio reflect the influence of an H2O-rich melt probably from a differentiated boundary layer. Closed-system in-situ crystallisation, mafic magma recharge and the role of a water-rich differentiated boundary layer can be distinguished from the An–trace element relationships. Crystals apparently move relatively freely between different parts and regimes in the magma chamber, evidence for "convective crystal dispersion". High-Sr type II crystals indicate an earlier input of Sr-rich mafic magma. Recharge of two distinct mafic magma types is thus identified (high-Sr and low-Sr), which must have been present – at increasing recharge rates with time – in the plumbing system throughout the volcano's history.

Water solubility in orthopyroxene

Abstract: The solubility of water in pure enstatite was measured on samples synthesized at 1,100 °C and pressures to 100 kbar. Enstatite crystals were grown under water-saturated conditions from a stoichiometric mixture of high-purity SiO2 and Mg(OH)2. Water contents were calculated from polarized FTIR spectra measured on oriented single crystals. The water solubility in orthoenstatite increases with pressure to 867±35 ppm H2O by weight at 75 kbar. At 100 kbar, in the stability field of high-clinoenstatite, a water solubility of 714±35 ppm was observed. The water solubility in enstatite at 1,100 °C can be described by the equation cH2O=AfH2O exp(–PΔV/RT), where f H2O is water fugacity, A=0.0204 ppm/bar and ΔV=12.3 cm3/mol. The infrared spectra of the hydrous enstatite crystals show a sharp, intense band at 3,363 cm–1 and a broad, weaker band at 3,064 cm–1. Both bands are strongly polarized parallel c. Most likely, pairs of protons attached to non-bridging oxygen atoms substitute for Mg2+. In order to investigate the effect of chemical impurities on water solubility in enstatite, an additional series of experiments was carried out with gels doped with Al, B, or Li as starting material. Whereas, the presence of Li and B had no detectable effect on water solubility, the addition of about 1 wt% Al2O3 increased water solubility in enstatite from 199 to 1,100 ppm at 1,100°C and 15 kbar. In the infrared spectra of these aluminous samples, additional bands occur in the range from 3,450 to 3,650 cm–1. Similar bands are also observed in natural, aluminous orthopyroxenes and are most likely caused by protons coupled with Al according to the substitution of Al3++H+ for Si4+. A series of hydrous annealing experiments on a natural, gem-quality aluminous enstatite from Tanzania yielded water solubilities generally consistent with the results from the synthetic model systems. The results presented here imply that pure enstatite has a similar storage capacity for water as olivine; however, aluminous orthopyroxenes in the mantle may dissolve much larger amounts of water comparable with the entire mass of the present hydrosphere. Moreover, the mechanism of aluminum substitution in orthopyroxenes, i.e., the distribution of Al between tetrahedral and octahedral sites, may be a potential probe of water fugacity.

The andesite aqueduct: perspectives on the evolution of intermediate magmatism in west-central (105–99°W) Mexico

Abstract: Intermediate calc-alkaline magma (52–65% SiO2) in western-central Mexico is the focus of this paper, and the typically porphyritic andesites (57–65% SiO2) form large central volcanoes, whereas basaltic andesites (52–57% SiO2) are less porphyritic, and they are found as cones and flows but are absent from central volcanoes. Several studies of experimental phase equilibria on these lavas relate water concentration to the phenocryst assemblages and to the degree of crystallinity, so that the abundance, composition and variety of phenocrysts can be used to constrain the amount of water dissolved in the magmas. Thus, the plagioclase-rich andesites of Volcan Colima, Mexico, become so as a result of decompressional crystallisation at ~950 °C (the pyroxene phenocryst temperature), and lose their dissolved water (2.5 to 4.5 wt% H2O) which is inversely proportional to the modal abundance of plagioclase. The feeding magma to V. Colima, North America's most productive central volcano, is represented by hornblende lamprophyre, a lava type without plagioclase phenocrysts which requires at least 6 wt% water to reproduce the phenocryst assemblage. Thus, degassing of the V. Colima magmas, and of those of the other central volcanoes in the western-central Mexican volcanic belt, contributes essentially all their dissolved water to the conduit or to the atmosphere. The source of this magmatic water is related to the source of the intermediate magmas. For some this must lie in the mantle, as the incorporation of hornblende-lherzolite nodules in a hydrous andesite with hornblende phenocrysts could only have occurred while ascending through the mantle. Consistent with a mantle source is the composition of the olivine phenocrysts in Mexican lavas with 10 to 5% MgO, which is in the mantle range of Fo88–92. Accordingly, basaltic andesites and andesites with >5% MgO are candidates for a mantle source. The equilibration of intermediate magmas with the mantle, as illustrated by the experiments of various workers, requires that the magmas be hydrous at pressure. An additional constraint is that the activity of silica in the mantle must be equal to that in the hydrous magma at equilibrium. Using published and new experiments to define RTlnγSiO2 in hydrous liquids, this quantity is shown to vary as a function of liquid composition (H2O, MgO, Na2O+K2O), and it approaches zero for quartz-saturated hydrous liquids. Using appropriate values of RTlnγSiO2 for three intermediate lavas, the amount of water required to equilibrate with an olivine-orthopyroxene mantle source is calculated, and within error indicates that only the most silica-rich magma is at water saturation in the mantle, in agreement with published experimental work. Hydrous intermediate magmas, ascending from their hornblende-lherzolite source regions (~1 to 1.5 GPa) along the hydrous adiabat, may not encounter any phase boundaries until 0.2–0.4 GPa because of the increase in the thermal stability of hornblende in water-undersaturated magmas. Therefore, the phenocryst assemblages of hornblende-free andesites equilibrate at low pressures. The virtual absence of basalt in west-central Mexico ( 50% crystallinity, evidently also an eruptible limit for V. Colima andesitic lavas. If the lower limit of water dissolved in Mexican intermediate magmas is accepted as that required for phenocryst equilibration (~6 wt% water), and the upper limit as saturation in the mantle source at 1 GPa (~16 wt%) then, with an estimate of the volcanic and plutonic magma delivery rate (km3/106 year) per km of volcanic arc, the flux of water returned from the mantle along the 35,000-km, global subduction-related arc system can be estimated. Measurements of the volcanic flux are woefully few, and estimates from Mexico, the Lesser Antilles and central America show a range from 4 to 20 km3/106 year×km which, if subtracted from the isotopically constrained continental growth rate, gives the plutonic flux rate. This suggests that, of the magma flux ascending to the continental crust, only about a fifth reaches the surface. If the dissolved magmatic water limits are coupled with the volcanic and plutonic emplacement rates, then the amount of water returned by magmatism to the crust is crudely in balance with that subducted.

High-resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images: new constraints on oscillatory zoning

Abstract: Oscillatory zoning in plagioclase is investigated at small scale (≤10 µm) using accumulated backscattered electron (BSE) images as high-resolution imaging method. Combined with electron microprobe quantitative analysis, gray-value profiles across these images can be calibrated for An-content with a resolution of 0.5 mol% An. Applied to oscillatory-zoned crystals, this new application of BSE imaging allows better characterization of zoning patterns along a profile and quantification of wavelength, amplitude, and shape of the oscillations. We also obtain high-resolution information on the morphology of growth zones boundaries. This approach allows us to better classify the different types of "oscillations" and concentric zoning. Dissolution is more frequent than usually recognized. Major resorption surfaces crosscut several growth zones. Irregular 5–10-µm saw-tooth zones are delimited by faint wavy dissolution surfaces and must be distinguished from small-scale oscillations (≤1–3 µm) with straight boundaries. This suggests at least two mechanisms for the formation of these zoning patterns: faint oscillations are probably caused by local kinetic control whereas wavy dissolution surfaces involve magma chamber dynamics.

Peridotites from the Mariana Trough: first look at the mantle beneath an active back-arc basin

Abstract: Two dives of the DSV Shinkai 6500 in the Mariana Trough back-arc basin in the western Pacific sampled back-arc basin mantle exposures. Reports of peridotite exposures in back-arc basin setting are very limited and the lack of samples has hindered our understanding of this important aspect of lithospheric evolution. The Mariana Trough is a slow-spreading ridge, and ultramafic exposures with associated gabbro dykes or sills are located within a segment boundary. Petrological data suggest that the Mariana Trough peridotites are moderately depleted residues after partial melting of the upper mantle. Although some peridotite samples are affected by small-scale metasomatism, there is no evidence of pervasive post-melting metasomatism or melt-mantle interaction. Spinel compositions plot in the field for abyssal peridotites. Clinopyroxenes show depletions in Ti, Zr, and REE that are intermediate between those documented for peridotites from the Vulcan and Bouvet fracture zones (the American- Antarctic and Southwest Indian ridges, respectively). The open-system melting model indicates that the Ma- riana Trough peridotite compositions roughly corre- spond to theoretical residual compositions after 7% near-fractional melting of a depleted MORB-type upper mantle with only little melt or fluid/mantle interactions. The low degree of melting is consistent with alow magma budget, resulting in ultramafic exposure. We infer that the mantle flow beneath the Mariana Trough Central Graben is episodic, resulting in varying magma supply rate at spreading segments.

Fluid-mobile trace element constraints on the role of slab melting and implications for Archaean crustal growth models

Abstract: We use published and new trace element data to identify element ratios which discriminate between arc magmas from the supra-subduction zone mantle wedge and those formed by direct melting of subducted crust (i.e. adakites). The clearest distinction is obtained with those element ratios which are strongly fractionated during refertilisation of the depleted mantle wedge, ultimately reflecting slab dehydration. Hence, adakites have significantly lower Pb/Nd and B/Be but higher Nb/Ta than typical arc magmas and continental crust as a whole. Although Li and Be are also overenriched in continental crust, behaviour of Li/Yb and Be/Nd is more complex and these ratios do not provide unique signatures of slab melting. Archaean tonalite-trondhjemite-granodiorites (TTGs) strongly resemble ordinary mantle wedge-derived arc magmas in terms of fluid-mobile trace element content, implying that they-did not form by slab melting but that they originated from mantle which was hydrated and enriched in elements lost from slabs during prograde dehydration. We suggest that Archaean TTGs formed by extensive fractional crystallisation from a mafic precursor. It is widely claimed that the time between the creation and subduction of oceanic lithosphere was significantly shorter in the Archaean (i.e. 20 Ma) than it is today. This difference was seen as an attractive explanation for the presumed preponderance of adakitic magmas during the first half of Earth's history. However, when we consider the effects of a higher potential mantle temperature on the thickness of oceanic crust, it follows that the mean age of oceanic lithosphere has remained virtually constant. Formation of adakites has therefore always depended on local plate geometry and not on potential mantle temperature.

A thermodynamic model for Fe-Mg dioctahedral K white micas using data from phase-equilibrium experiments and natural pelitic assemblages

Abstract: The purpose of this study is to derive a solid-solution model for potassic white micas (KWM) encountered in rocks of various bulk compositions, over a wide range of P–T conditions. A compilation of phengite compositions lead us to propose a seven-thermodynamic-component (muscovite, Fe2+-Al-celadonite, Mg-Al-celadonite, annite, phlogopite, pyrophyllite and paragonite) ionic solid-solution model which accounts for the Tschermak, Fe–Mg, di/trioctahedral, pyrophyllitic and paragonitic substitutions observed in nature. A four-site mixing model with symmetric Margules parameters to model the Tschermak substitutions, asymmetric Margules parameters to model the other substitutions, and ideal intersite interaction has been adopted. In contrast to previous models, the relevant thermodynamic data and solid-solution properties are calibrated with independent sets of published experiments conducted for the KMASH, KFASH, KFMASH, and KNASH systems, as well as about 200 natural data involving KWM assemblages. The constraints span a wide range of pressure and temperature conditions (150 to 750 °C, 0.5 to 30 kbar), so that our model does not need to be extrapolated outside the calibration range to be used for P–T thermobarometric purposes. The calculated thermodynamic data are interconsistent with the TWQ thermodynamic database and solid-solution models, including that recently published for chlorites.

Mapping the thermal structure of solid-media pressure assemblies

Abstract: The refractory oxides MgO and Al2O3 are the most commonly used insulator/filler materials in solid-media pressure assemblies. These oxides react with one another at high temperatures and pressures, forming a well-defined layer of spinel (~MgAl2O4) at the contact. The spinel layer widens in proportion to the square root of time at a rate that also depends systematically upon temperature and pressure. On the basis of 44 piston-cylinder runs spanning 1,200–2,000 °C and 1.0–4.0 GPa, we present a general relationship describing the width (ΔX) of the spinel layer as a function of time (t, in s), temperature (T, in K) and pressure (P, in GPa): \(\Delta {\rm X} = \left[ {{\rm 8}{\rm .58} \times {\rm 10}^{{\rm 11}} \cdot \exp \left( { - 48865/{\rm T} - 2.08 \cdot {\rm p}^{1/2} } \right) \cdot {\rm t}} \right]^{1/2}\) . If the pressure and duration of an experiment are known (as is usually the case) this calibration makes it possible to calculate the temperature to within a few degrees at any location in a solid-media assembly where MgO and Al2O3 are in contact (at T above ~1,200 °C) – simply by measuring the width of the spinel layer with an optical microscope. Application of this "reaction-progress" thermometer to the 13- and 19-mm diameter piston-cylinder assemblies used in the RPI lab confirms generally parabolic axial T gradients with acceptably broad hot spots. Three-dimensional maps of the 19-mm assembly reveal a radial component to the thermal field, with somewhat higher temperatures near the graphite heating element (i.e., a saddle-shaped hot region). Two exploratory experiments in a multi-anvil apparatus at 14 GPa (1,700 and 1,975 °C) confirm that the reaction-progress technique will work at pressures well above 4 GPa. The piston-cylinder-based calibration predicts ΔX to within a factor of two in the two multi-anvil runs, and relative changes in T along the assembly can be readily mapped. However, additional high-pressure calibration points will be needed before the thermometer can be used in quantitative multi-anvil applications. The spinel reaction-progress thermometer is easily implemented, and should allow other researchers to map the thermal structures of their own assemblies in a single experiment with one thermocouple.

Time scales of magma storage and differentiation of voluminous high-silica rhyolites at Yellowstone caldera, Wyoming

Abstract: Ion microprobe dating of zircons from post-collapse rhyolites at Yellowstone caldera reveals the time scales of crystallization and storage of silicic magma in a differentiating magma reservoir, the role of recycling of crystals from the caldera-forming magmatism, and the timing and efficacy of crystal-melt separation. Zircons in the voluminous (~900 km3) Central Plateau Member lavas, which progressively erupted between 70 to 160 ka, yield 238U-230Th disequilibrium ages dominantly spanning the range from those of their respective eruptions to ~200 ka; mean zircon ages range to ca. 60,000 years before eruption. When considered together with the trace element and Sr- and Nd-isotope compositions of their host melts, the age distributions of the CPM zircons show that the rhyolites are cogenetic and differentiated tens of thousands of years prior to eruption from an evolving magma reservoir. Thus, the post-caldera CPM rhyolites were not erupted from a long-standing body of rhyolitic magma left over from the caldera-forming eruption, nor do they represent significant remobilization of the plutonic roots of the caldera. Rather, the CPM magma was generated and differentiated by episodes of effective crystal-melt separation at ~200 and ~125 ka and, sustained by thermal inputs, stored for timescales on par with estimates for other voluminous caldera-related rhyolites.

Synorogenic melting of mafic lower crust: constraints from geochronology, petrology and Sr, Nd, Pb and O isotope geochemistry of quartz diorites (Damara orogen, Namibia)

Abstract: Quartz diorites represent the earliest (ca. 540 Ma) and most primitive plutonic rocks in the Pan African Damara belt and they pre-date the main phase of high-T regional metamorphism. Two suites of synorogenic quartz diorites are unusual among Damaran intrusive rocks in their elemental and isotopic features. Comparison of the diorite compositions with melts from amphibolite-dehydration melting experiments points to a garnet-bearing meta-tholeiite, probably enriched in K2O, as a likely source rock. Partial melting processes generated mafic (ca. 50 wt% SiO2) quartz diorites in the deep crust at temperatures of between 1,000 and 1,100 °C, based on comparison with experimental results and similar temperature estimates based on P2O5 solubility in mafic rocks. Subsequently, the quartz diorites evolved by multistage, polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses. Although their chemical characteristics (high LILE, low HFSE) resemble those of other quartz diorites with calc-alkaline affinities, they differ in their enriched Sr (initial 87Sr/86Sr: 0.70943–0.71285), Nd (initial e Nd: –9.1 to –15.2 ) and O (δ18O: 6.8–8.1‰) isotope compositions. Neodymium model ages (TDM) that range from 1.7 to 2.2 Ga and large variation in 207Pb/204Pb relative to 206Pb/204Pb indicates involvement of ancient crustal material. Lead (206Pb/204Pb: 17.08–17.23, 207Pb/204Pb: 15.53–15.62, 208Pb/204Pb: 37.71–38.16) isotope compositions are strongly retarded, indicating that the source underwent a pre-Pan-African U/Pb fractionation and U depletion. It is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust. This contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during Pan-African times with characteristics modified by subduction of oceanic crust and sedimentary rocks.

Oxygen isotope study of the Long Valley magma system, California: isotope thermometry and convection in large silicic magma bodies

Abstract: Products of voluminous pyroclastic eruptions with eruptive draw-down of several kilometers provide a snap-shot view of batholith-scale magma chambers, and quench pre-eruptive isotopic fractionations (i.e., temperatures) between minerals. We report analyses of oxygen isotope ratio in individual quartz phenocrysts and concentrates of magnetite, pyroxene, and zircon from individual pumice clasts of ignimbrite and fall units of caldera-forming 0.76 Ma Bishop Tuff (BT), pre-caldera Glass Mountain (2.1–0.78 Ma), and post-caldera rhyolites (0.65–0.04 Ma) to characterize the long-lived, batholith-scale magma chamber beneath Long Valley Caldera in California. Values of δ18O show a subtle 1‰ decrease from the oldest Glass Mountain lavas to the youngest post-caldera rhyolites. Older Glass Mountain lavas exhibit larger (~1‰) variability of δ18O(quartz). The youngest domes of Glass Mountain are similar to BT in δ18O(quartz) values and reflect convective homogenization during formation of BT magma chamber surrounded by extremely heterogeneous country rocks (ranging from 2 to +29‰). Oxygen isotope thermometry of BT confirms a temperature gradient between "Late" (815 °C) and "Early" (715 °C) BT. The δ18O(quartz) values of "Early" and "Late" BT are +8.33 and 8.21‰, consistent with a constant δ18O(melt)=7.8±0.1‰ and 100 °C temperature difference. Zircon-melt saturation equilibria gives a similar temperature range. Values of δ18O(quartz) for different stratigraphic units of BT, and in pumice clasts ranging in pre-eruptive depths from 6 to 11 km (based on melt inclusions), and document vertical and lateral homogeneity of δ18O(melt). Worldwide, five other large-volume rhyolites, Lava Creek, Lower Bandelier, Fish Canyon, Cerro Galan, and Toba, exhibit equal δ18O(melt) values of earlier and later erupted portions in each of the these climactic caldera-forming eruptions. We interpret the large-scale δ18O homogeneity of BT and other large magma chambers as evidence of their longevity (>105 years) and convection. However, remaining isotopic zoning in some quartz phenocrysts, trace element gradients in feldspars, and quartz and zircon crystal size distributions are more consistent with far shorter timescales (102–104 years). We propose a sidewall-crystallization model that promotes convective homogenization, roofward accumulation of more evolved and stagnant, volatile-rich liquid, and develops compositional and temperature gradients in pre-climactic magma chamber. Crystal + melt + gas bubbles mush near chamber walls of variable δ18O gets periodically remobilized in response to chamber refill by new hotter magmas. One such episode of chamber refill by high-Ti, Sr, Ba, Zr, and volatile-richer magma happened 103–104 years prior to the 0.76-Ma caldera collapse that caused magma mixing at the base, mush thawing near the roof and walls, and downward settling of phenocrysts into this hybrid melt.

In-situ UV-laser 40Ar/39Ar geochronology of a micaceous mylonite : an example of defect-enhanced argon loss

Abstract: Muscovite and biotite from a crustal-scale mylonite zone (Pogallo Shear Zone, southern Alps) were investigated using furnace step-heating and in-situ UV-laser ablation 40Ar/39Ar geochronology. Undeformed muscovite porphyroclasts yield 40Ar/39Ar plateau ages of 182.0±1.6 Ma, whereas in-situ UV-laser ablation 40Ar/39Ar dating and furnace step-heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger. The range of 40Ar/39Ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra-grain scale. These microstructurally controlled segments are bound by either first-order lattice discontinuities, sub-microscopic structural defects and/or zones of high defect density, which create variable length-scales for intragranular argon diffusion. The observed deformational microstructures within muscovite and biotite acted as intra-grain fast diffusion pathways in the slowly cooled mylonitic rocks. Therefore, the high-spatial resolution 40Ar/39Ar data record the initial and final closure to argon diffusion over a time span of about 60 Ma.

Coupled evolution of back-arc and island arc-like mafic crust in the late-Neoproterozoic Agardagh Tes-Chem ophiolite, Central Asia: evidence from trace element and Sr–Nd–Pb isotope data

Abstract: We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia, south-west of Lake Baikal (50.5°N, 95°E). The majority of samples are high-alumina basalts and basaltic andesites having island-arc affinities. They were derived from an evolved parental magma (Mg#≥0.60, Cr~180 ppm, Ni~95 ppm) by predominantly clinopyroxene fractionation. The parental magma developed from a primary mantle melt by fractionation of about 12% of an olivine+spinel assemblage. The island-arc rocks have high abundances of incompatible trace elements (light rare-earth element abundances up to 100 times chondritic, chondrite-normalised (La/Yb)n=14.6–5.1) and negative Nb anomalies (Nb/La=0.37–0.62), but low Zr/Nb ratios (7–14). Initial eNd values are around +5.5, initial Pb isotopic compositions are 206Pb/204Pb=17.39–18.45, 207Pb/204Pb=15.49–15.61, 208Pb/204Pb=37.06–38.05. Enrichment of large-ion lithophile elements within this group is significant (Ba/La=11–130). Another group of samples consists of back-arc basin-related volcanic rocks. They are most likely derived from the same depleted mantle source as the island-arc rocks, but underwent higher degrees of melting (8–15%) and are not influenced by slab components. They have lower abundances of incompatible trace elements, flat rare-earth element patterns [(La/Yb)n=0.6–2.4] and higher eNd values (+7.8 to +8.5). Negative Nb anomalies are absent (Nb/La=0.81–1.30), but Zr/Nb is high (21–48). At least three components are necessary to explain the geochemical evolution of the volcanic rocks: (1) an enriched (ocean island-like) component characterised by a high Nb concentration (up to 30 ppm), an absent negative Nb anomaly, a low Zr/Nb ratio (~6.5), a low eNd value (around 0), and radiogenic 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb; (2) a back-arc basin component similar to N-MORB with a flat rare-earth element pattern and a high eNd value (around +8.5); and (3) an island-arc component from a mantle source which was modified by the downgoing slab. Crystal fractionation superimposed on mixing and source contamination by subducted sediments is suitable to explain the observed geochemical data. The most likely geodynamic environment to produce these characteristics is a young, intra-oceanic island-arc system and an associated back-arc basin.

Abyssal peridotites >3,800 Ma from southern West Greenland: field relationships, petrography, geochronology, whole-rock and mineral chemistry of dunite and harzburgite inclusions in the Itsaq Gneiss Complex

Abstract: Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to ~500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite–harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of ca. 3,650 Ma, but with an age of ca. 3,800 Ma most likely. The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at ca. 3,800 Ma. This is interpreted to have occurred during the development of a 3,810–3,790-Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. This northern part of the Itsaq Gneiss Complex is the most favourable for the geochemical study of early Archaean protoliths because it experienced peak metamorphism only within the amphibolite facies with little or no in-situ melt segregation, and contains some areas that have undergone little deformation since ca. 3,800 Ma. Most of the ultramafic enclaves are thoroughly altered, and largely comprise secondary, hydrous phases. However, the centres of some enclaves have escaped alteration and comprise dunite and harzburgite with >95% olivine (Fo89–91) + orthopyroxene (En89) + Al-spinel (Cr8–20) assemblages. The dunites and harzburgites are massive or irregularly layered and are olivine-veined on 5–10-m to 10-cm scales. Their whole rock major and rare earth element, and olivine and spinel compositions differ significantly from xenoliths representing the Archaean cratonic lithospheric mantle, but are typical of some modern abyssal peridotites. The harzburgites and dunites show both LREE depleted and enriched patterns; however, none show the massive REE depletion associated with the modelled removal of a komatiite. They are interpreted as being the products of small degrees of melt extraction, with some showing evidence of refertilisation. These Greenland dunites and harzburgites described here are currently the best characterised 'sample' of the early Archaean upper mantle.

Onset of magmatic accretion within a magma-poor rifted margin: a case study from the Platta ocean-continent transition, eastern Switzerland

Abstract: Exhumation of subcontinental mantle rocks and its exposure at the seafloor is known from different magma-poor passive continental margins. However, the transition from largely amagmatic passive rifting to seafloor spreading is still poorly documented. In this contribution we use MOR-type gabbroic and basaltic rocks to characterize the magmatism associated with the formation of an ancient ocean-continent transition preserved in the Platta nappe, eastern Switzerland. Gabbros form individual small intrusions into exhumed serpentinized subcontinental mantle rocks. Mineral and bulk-rock chemistry and simple modeling indicate that each gabbro body records different magmatic processes ranging from predominantly fractional crystallization to solidification without fractionation. Mg numbers and Ni contents of equilibrium olivine calculated from basalts and gabbros indicate that few mafic rocks are primary melts but most represent fractionated compositions ranging from T- to N-MORB. Whereas most mafic rocks may be explained by low to moderate degrees of melting of an N-MORB-type mantle, the source of some basalt is enriched in incompatible elements. This compositional variation seems to correlate with the spatial distribution of the mafic rocks within the ocean-continent transition whereby mafic rocks with T-MORB signatures occur close to the continental margin whereas N-MORB signatures are predominantly found oceanwards. As in an opening system time and space are closely linked, the chemical evolution of the mafic rocks along the ocean-continent transitions suggests continuous thinning of the subcontinental mantle and associated uplift of the underlying asthenosphere during the time between the crustal and the lithospheric breakup.

A crystal size-distribution study of the Kiglapait layered mafic intrusion, Labrador, Canada: evidence for textural coarsening

Abstract: Crystal size distributions (CSD) of plagioclase, olivine and clinopyroxene from the Kiglapait intrusion, Labrador were measured for a series of 13 samples spanning most of the exposed intrusion. All CSDs lack small crystals and are slightly curved to the right, concave up. All CSDs of a mineral sweep out a fan, as expected where there is relatively little variation in phase proportion. CSD dispersion can be replicated using the communicating neighbours model of textural coarsening (Ostwald ripening). Most of the textural variation in the intrusion is the result of variable degrees of coarsening. It is proposed that coarsening occurred in the outer part of the crystallisation boundary layer where release of latent heat and changes in the intercumulus liquid composition resulted in a reduction in undercooling, suppression of nucleation and coarsening. Particularly coarse-grained areas reflect focusing of circulating magmatic fluids during coarsening. The effects of crystal settling were not observed in the CSDs, even within single well-developed layers. They were probably concealed by the pervasive textural coarsening. CSDs do not suggest that mechanical compaction has played an important role in the formation of texture, although it is very likely to have occurred. However, samples from the lower part of the intrusion show textural evidence of pressure-solution compaction.

Plume dynamics beneath the African plate inferred from the geochemistry of the Tertiary basalts of southern Ethiopia

Abstract: Southern Ethiopian flood basalts erupted in two episodes: the pre-rift Amaro and Gamo transitional tholeiites (45-35 million years) followed by the syn-extensional Getra-Kele alkali basalts (19-11 million years). These two volcanic episodes are distinct in both trace element and isotope ratios (Zr/Nb ratios in Amaro/Gamo lavas fall between 7 and 14, and 3-4.7 in the Getra-Kele lavas whereas 206Pb/204Pb ratios fall between 18-19 and 18.9-20, respectively). The distinctive chemistries of the two eruptive phases record the tapping of two distinct source regions: a mantle plume source for the Amaro/Gamo phase and an enriched continental mantle lithosphere source for the Getra-Kele phase. Isotope and trace element variations within the Amaro/Gamo lavas reflect polybaric fractional crystallisation initiated at high pressures accompanied by limited crustal contamination. We show that clinopyroxene removal at high (0.5 GPa) crustal pressures provides an explanation for the common occurrence of transitional tholeiites in Ethiopia relative to other, typically tholeiitic flood basalt provinces. The mantle plume signature inferred from the most primitive Amaro basalts is isotopically distinct from that contributing to melt generation in central Ethiopian and Afar. This, combined with Early Tertiary plate reconstructions and similarities with Kenyan basalts farther south, lends credence to derivation of these melts from the Kenyan plume rather than the Afar mantle plume. The break in magmatism between 35 and 19 Ma is consistent with the northward movement away from the Kenya plume predicted from plate tectonic reconstructions. In this model the Getra-Kele magmatism is a response to heating of carbonatitically metasomatised lithosphere by the Afar mantle plume beneath southern Ethiopia at this time.

Very high-density carbonic fluid inclusions in sapphirine-bearing granulites from Tonagh Island in the Archean Napier Complex, East Antarctica: implications for CO 2 infiltration during ultrahigh-temperature (T>1,100 C) metamorphism

Abstract: The ultrahigh-temperature (UHT) metamorphism of the Napier Complex is characterized by the presence of dry mineral assemblages, the stability of which requires anhydrous conditions. Typically, the presence of the index mineral orthopyroxene in more than one lithology indicates that H2O activities were substantially low. In this study, we investigate a suite of UHT rocks comprising quartzo-feldspathic garnet gneiss, sapphirine granulite, garnet–orthopyroxene gneiss, and magnetite–quartz gneiss from Tonagh Island. High Al contents in orthopyroxene from sapphirine granulite, the presence of an equilibrium sapphirine–quartz assemblage, mesoperthite in quartzo-feldspathic garnet gneiss, and an inverted pigeonite–augite assemblage in magnetite–quartz gneiss indicate that the peak temperature conditions were higher than 1,000 °C. Petrology, mineral phase equilibria, and pressure–temperature computations presented in this study indicate that the Tonagh Island granulites experienced maximum P–T conditions of up to 9 kbar and 1,100 °C, which are comparable with previous P–T estimates for Tonagh and East Tonagh Islands. The textures and mineral reactions preserved by these UHT rocks are consistent with an isobaric cooling (IBC) history probably following an counterclockwise P–T path. We document the occurrence of very high-density CO2-rich fluid inclusions in the UHT rocks from Tonagh Island and characterize their nature, composition, and density from systematic petrographic and microthermometric studies. Our study shows the common presence of carbonic fluid inclusions entrapped within sapphirine, quartz, garnet and orthopyroxene. Analysed fluid inclusions in sapphirine, and some in garnet and quartz, were trapped during mineral growth at UHT conditions as 'primary' inclusions. The melting temperatures of fluids in most cases lie in the range of –56.3 to –57.2 °C, close to the triple point for pure CO2 (–56.6 °C). The only exceptions are fluid inclusions in magnetite–quartz gneiss, which show slight depression in their melting temperatures (–56.7 to –57.8 °C) suggesting traces of additional fluid species such as N2 in the dominantly CO2-rich fluid. Homogenization of pure CO2 inclusions in the quartzo-feldspathic garnet gneiss, sapphirine granulite, and garnet–orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of –34.9 to +4.2 °C. This translates into very high CO2 densities in the range of 0.95–1.07 g/cm3. In the garnet–orthopyroxene gneiss, the composition and density of inclusions in the different minerals show systematic variation, with highest homogenization temperatures (lowest density) yielded by inclusions in garnet, as against inclusions with lowest homogenization (high density) in quartz. This could be a reflection of continued recrystallization of quartz with entrapment of late fluids along the IBC path. Very high-density CO2 inclusions in sapphirine associated with quartz in the Tonagh Island rocks provide potential evidence for the involvement of CO2-rich fluids during extreme crustal temperatures associated with UHT metamorphism. The estimated CO2 isochores for sapphirine granulite intersect the counterclockwise P–T trajectory of Tonagh Island rocks at around 6–9 kbar at 1,100 °C, which corresponds to the peak metamorphic conditions of this terrane derived from mineral phase equilibria, and the stability field of sapphirine + quartz. Therefore, we infer that CO2 was the dominant fluid species present during the peak metamorphism in Tonagh Island, and interpret that the fluid inclusions preserve traces of the synmetamorphic fluid from the UHT event. The stability of anhydrous minerals, such as orthopyroxene, in the study area might have been achieved by the lowering of H2O activity through the influx of CO2 at peak metamorphic conditions (>1,100 °C). Our microthermometric data support a counterclockwise P–T path for the Napier Complex.

An experimental study of B-, P- and F-rich synthetic granite pegmatite at 0.1 and 0.2 GPa

Abstract: Extreme enrichment in H2O, B, P and F is characteristic of many evolved granites and pegmatites. We report experimental phase relations of a synthetic peraluminous pegmatite spiked with P2O5 ,B 2O3 and F (5 wt% of each), Rb2O, Cs2O (1 wt% of each) and Li2O (0.5 wt%). Experiments were carried out in H2O-satu- rated conditions in cold-seal rapid-quench pressure vessels at 0.1-0.2 GPa. Crystallisation starts at about 820 C with berlinite and topaz. Quartz appears at 700- 750 C. Topaz is replaced by muscovite at about 600 C. At near-solidus temperatures (450-500 C) amblygonite, lacroixite and a Cs-bearing aluminosilicate crystallise. In all charges aluminosilicate melt coexists with low-density hydrous fluid and hydrosaline melt. The latter is strongly enriched in Na3AlF6 and H3BO3 components. Experi- mental evidence of the liquid immiscibility and mineral reactions documented in our study offers new explana- tions of many enigmatic features of natural pegmatites.