CO2-bearing Melts Research Articles

Redox state determination of eclogite xenoliths from Udachnaya kimberlite pipe (Siberian craton), with some implications for the graphite/diamond formation

The formation of diamonds within eclogitic rocks has been widely linked to the fate of carbon during subduction and, therefore, referred to conditions of pressure, temperature, and oxygen fugacity (fo2). Mantle-derived eclogite xenoliths from Udachnaya kimberlite pipes represent a unique window to investigate the formation of carbon-free, graphite–diamond-bearing and diamond-bearing rocks from the Siberian craton. With this aim, we exploited oxy-thermobarometers to retrieve information on the P–T–fo2 at which mantle eclogites from the Siberian craton equilibrated along with elemental carbon. The chemical analyses of coupled garnet and omphacitic clinopyroxene were integrated with data on their iron oxidation state, determined both by conventional and synchrotron 57Fe Mossbauer spectroscopy. The calculated fo2s largely vary for each suite of eclogite samples from 0.10 to − 2.43 log units (ΔFMQ) for C-free eclogites, from − 0.01 to − 2.91 (ΔFMQ) for graphite–diamond-bearing eclogites, and from − 2.08 to − 3.58 log units (ΔFMQ) for diamond-bearing eclogites. All eclogite samples mostly fall in the fo2 range typical of diamond coexisting with CO2-rich water-bearing melts and gaseous fluids, with diamondiferous eclogites being more reduced at fo2 conditions where circulating fluids can include some methane. When uncertainties on the calculated fo2 are taken into account, all samples essentially fall within the stability field of diamonds coexisting with CO2-bearing melts. Therefore, our results provide evidence of the potential role of CO2-bearing melts as growth medium on the formation of coexisting diamond and graphite in mantle eclogites during subduction of the oceanic crust.

Hydrous melts weaken the mantle, crystallization of pargasite and phlogopite does not: Insights from a petrostructural study of the Finero peridotites, southern Alps

This study reports petrostructural observations in the pargasite and phlogopite-bearing Finero peridotite massif (Italian Western Alps), which suggest that the pervasive foliation in this massif was formed by deformation concomitant with percolation of hydrous Si-rich melts: (1) diffuse contacts, but systematic parallelism between the pyroxenitic layers and the foliation of the peridotite (2) strong shape and crystal preferred orientations (SPO and CPO), but subhedral or interstitial shapes and weak intracrystalline deformation of the hydrous phases, (3) CPO, but interstitial shapes of the pyroxenes, (4) very coarse olivine grain sizes, which are correlated to the olivine abundance, and (5) elongated shapes, but weak intracrystalline deformation, and extremely weak and highly variable CPO of olivine. The pervasive deformation of the Finero peridotite occurred therefore under conditions that allowed coexistence of H2O–CO2-bearing melts, pargasite, and spinel, that is, temperatures of 980–1080 °C and pressures <2 GPa. The petrostructural observations suggest that the presence of hydrous melts results in accommodation of large amounts of deformation by stress-controlled dissolution–precipitation and advective transport of matter by the melts and in fast grain boundary migration in olivine. By consequence, it produces significant rheological weakening. Water contents in olivine are <4 ppm wt., implying limited contribution of hydration of olivine to weakening. In addition, the analysis of protomylonites composing the external domains of the shear zones that overprint the pervasive foliation indicates that the transition to melt-free conditions results in enhanced contribution of dislocation creep to the deformation. The associated increase of the peridotites' strength leads to onset of strain localization. The latter is not correlated to the local abundance in pargasite or phlogopite, implying that crystallization of amphiboles or phlogopite, even at concentrations of 25 vol.%, does not produce rheological weakening in the upper mantle.

Phase relations of phlogopite and pyroxene with magnesite from 4 to 8 GPa: KCMAS–H2O and KCMAS–H2O–CO2

To constrain the melting phase relationships of phlogopite and magnesite in the presence of clino- and orthopyroxene, we performed experiments in the K2O–CaO–MgO–Al2O3–SiO2–H2O (KCMAS–H2O) and K2O–CaO–MgO–Al2O3–SiO2–H2O–CO2 (KCMAS–H2O–CO2) systems at pressures of 4–8 GPa and temperatures from 1100 to 1600 °C. We bracketed the carbonate-free solidus between 1250 and 1300 °C at 4 and 5 GPa, and between 1300 and 1350 °C at 6, 7 and 8 GPa. The carbonate-bearing solidus was bracketed between 1150 and 1200 °C at 4, 5 and 6 GPa, and between 1100 and 1150 °C at 7 and 8 GPa. Below the solidus in both systems at 4–6 GPa, phlogopite is in equilibrium with enstatite, diopside, garnet (plus magnesite in the carbonate-bearing system) and a fluid. At 7 GPa, phlogopite coexists with KK-richterite, enstatite, diopside, garnet (plus magnesite in the carbonate-bearing system) and a fluid. KK-richterite is the only stable K-bearing phase at 8 GPa and coexists with enstatite, diopside, garnet (plus magnesite in the carbonate-bearing system) and a fluid. In KCMAS–H2O, phlogopite is present to ~100 °C above the solidus. Olivine forms at the solidus and coexists with enstatite, diopside, garnet and melt. At depth in a subcontinental lithospheric mantle keel, phlogopite would be stable with orthopyroxene, clinopyroxene and magnesite to ~5 GPa along a 40 mW/m2 geotherm. A hydrous, potassic and CO2-bearing melt that intrudes the subcontinental mantle can react with olivine, enstatite and garnet, crystallizing phlogopite, magnesite and potentially liberating a hydrous fluid.

Experimental simulation of bubble nucleation and magma ascent in basaltic systems: Implications for Stromboli volcano

The ascent of H2O- and H2O-CO2-bearing basaltic melts from the deeper to the shallower part of the Stromboli magmatic system and their vesiculation were simulated from decompression experiments. A well-studied “golden” pumice produced during an intermediate- to a large-scale paroxysm was used as starting material. Volatile-bearing glasses were synthesized at an oxygen fugacity ![Formula][1] ranging from NNO−1.4 to +0.9, 1200 °C and 200 MPa. The resulting crystal- and bubble-free glasses were then isothermally (1200 °C) decompressed to final pressures P f ranging between 200 and 25 MPa, at a linear ascent rate of 1.5 m/s (or 39 kPa/s) prior to be rapidly quenched. Textures of post-decompression glasses that were characterized by X-ray computed tomography result from different mechanisms of degassing that include bubble nucleation, growth, coalescence, and outgassing, as well as fragmentation. Homogeneous bubble nucleation occurs for supersaturation pressures (difference between saturation pressure and pressure at which bubbles start to form homogeneously, Δ P HoN) ≤ 50 MPa. In the CO2-free melts, homogeneous nucleation occurs as two distinct events, the first at high P f (200–150 MPa) and the second at low P f (50–25 MPa) near the fragmentation level. In contrast, in the CO2-bearing melts, multiple events of homogeneous bubble nucleation occur over a substantial P f interval along the decompression path. Bubble coalescence occurs in both H2O- and H2O-CO2-bearing melts and is the more strongly marked between 100 and 50 MPa P f. The CO2-free melts follow equilibrium degassing until 100 MPa P f and are slightly supersaturated at 60 and 50 MPa P f, thus providing the driving force for the second bubble nucleation event. In comparison, disequilibrium degassing occurs systematically in the CO2-bearing melts that retain high CO2 concentrations. Fragmentation was observed in some CO2-free charges decompressed to 25 MPa P f and is intimately associated with the occurrence of the second bubble nucleation event. Textures of H2O-CO2-bearing glasses reproduce certain critical aspects of the Stromboli natural textures (bubble number densities, shapes, sizes, and distributions) and chemistries (residual volatile concentrations). Average bubble sizes, bubble size distribution (BSD), and bubble number density (BND) data are used together to estimate that the “golden” pumice magmas ascend from their source region in 43 to 128 min. [1]: /embed/mml-math-1.gif

Carbon dioxide in silicate melts at upper mantle conditions: Insights from atomistic simulations

The detail of the incorporation of carbon dioxide in silicate melts at upper mantle conditions is still badly known. To give some theoretical guidance, we have performed first-principle molecular dynamics simulations (FPMD) to quantify the speciation and the incorporation of carbon dioxide in two CO2-rich silicate melts (~20wt.% CO2 at 2073K and 12GPa), a basaltic and a kimberlitic composition chosen in the CaO–MgO–Al2O3–SiO2 system. In the basaltic composition, carbon dioxide is incorporated under the form of a minority population of CO2 molecules and a prevailing population of carbonate ions (CO32−). In contrast, the amount of CO2 molecules is found to be very small in the kimberlitic melt. Moreover, a new (transient) species has been identified, the pyrocarbonate ion C2O52− issued from the reaction between CO2 and CO32−. With regard to the structure of the CO2-bearing melts, it is shown that the carbonate ions modify the silicate network by transforming some of the oxygen atoms into bridging carbonates, non-bridging carbonates, and free carbonates, with a distribution depending on the melt composition. In the basaltic melt a majority of carbonate ions are non-bridging or free, whereas in the kimberlitic melt, most of the carbonate ions are under the form of free carbonates linked to alkaline earth cations. Surprisingly, the addition of CO2 only has a weak influence on the diffusion coefficients of the elements of the melt. The consequence is that the strong enhancement of the electrical conductivity reported recently for carbonated basalts (Sifré et al., 2014, Nature 509, 81), can be reproduced by simulation only if one assumes that the ionic charges assigned to the elements of the melt depend, in a non-trivial way, on the CO2 content. Finally, a comparison of the FPMD calculations with classical molecular dynamics simulations using an empirical force field of the literature (Guillot and Sator, 2011, GCA 75, 1829) shows that the latter one needs some improvement.

Copper partitioning between felsic melt and H2O–CO2 bearing saline fluids

Analysis of fluid inclusions from porphyry copper deposits reveals that magmatic vapor and brine are vital for the removal of copper from arc magmas and its transport to the site of ore deposition. Experiments in melt–vapor–brine systems allow for investigation of the partitioning of copper between silicate melts and volatile phases at magmatic conditions. The presence of CO2 affects both the pressure at which a melt saturates with respect to volatile phases. Therefore, the partitioning of copper among felsic (rhyolitic) melt, vapor and brine in CO2-bearing experiments was examined to provide insights into copper partitioning and the generation of porphyry copper and related deposits.We present results from experiments performed at 800°C and 100MPa in CO2-bearing melt–vapor–brine systems with XCO2v+b=0.10 and 0.38. The compositions of vapor and brine inclusions, and run-product glasses, were determined during the course of this investigation. Microthermometric measurements of fluid inclusions show an increase in the salinity of the magmatic brine (∼65 to ∼70wt% NaCleq) and decrease in the salinity of the vapor (∼3.5 to ∼1wt% NaCleq) as XCO2 is increased from 0.10 to 0.38. The partitioning of copper between brine and vapor (DCub/v±2σ) increases from 25 (±6) at XCO2=0.10, to 100 (±30) at XCO2=0.38. The partitioning of copper between vapor and melt (DCuv/m±2σ) decreases from 9.6 (±3.3) at XCO2=0.10, to 2 (±0.8) at XCO2=0.38. These data demonstrate that copper partitioning in sulfur-free, CO2-bearing systems is controlled by the changes in the salinity of the vapor and brine that, in turn, are functions of XCO2. No change in the apparent equilibrium constants for Cu–Na exchange was observed in Fe-bearing experiments which supports a salinity-dependent model for copper partitioning.An existing model (MVPart) for ore metal partitioning between melt and volatile phases was modified to incorporate partitioning data from CO2-bearing experiments. The model can be used to predict trends in efficiency of removal of copper from melts into exsolving CO2-bearing magmatic volatile phases at a fixed pressure and temperature. The CO2-MVPart model predicts that CO2-rich (XCO2=0.38) magmatic vapor will remove half of the available copper from the melt, without a contribution from the brine, compared to low CO2 (XCO2≤0.10) systems. Thus, periods of CO2-rich vapor exsolution are not expected to be associated with efficient removal of copper from shallow felsic melts.

THE INTRAGRANITIC POTRERILLOS NYF PEGMATITES AND THEIR A-TYPE HOST GRANITES OF THE LAS CHACRAS - POTRERILLOS BATHOLITH, SIERRA DE SAN LUIS, ARGENTINA

The Potrerillos granitic pegmatites, in the Sierras Pampeanas Orientales of San Luis Province, Argentina, consist of mediumsized bodies with rounded, elliptical, or irregular outcrops varying from 10 to 60 m; vertically, they mostly look like thick lenses. Zoning is ill-defined, with a border zone that grades into a coarse-grained intermediate zone. The core zone also is poorly defined, normally containing milky to pink or gray quartz with large crystals of K-feldspar and, locally, pods of primary fluorite. The pegmatite-forming minerals are quartz, microcline and albite, with accessory biotite, muscovite, tourmaline, beryl, allanite, fluorite, fluorapatite, ilmenite, niobian rutile, rutile and pyrite. Rare primary species are monazite-(Ce), U-rich pyrochlore, scheelite, bismuthinite and molybdenite. Subordinate species are hematite, goethite, Mn oxides, clinobisvanite, bismuthite, and opal. The pegmatites are ascribed to the allanite–monazite type, REL–REE subclass, NYF family. Data on melt+fluid inclusion are consistent with crystallization of an aqueous, CO2-bearing melt at 475° < T < 600°C and ~3.0 to 3.7 kbar, that cooled down to a fluid of low to moderate salinity. The pegmatites are hosted in three dominant facies of two plutons of the Las Chacras – Potrerillos batholith, described as biotite porphyritic granite, biotite-bearing equigranular red granite, and muscovite-bearing equigranular red granite. All of them have high-K compositions, and are metaluminous to mildly peraluminous (ASI in the range 1 to 1.07); the least evolved porphyritic biotite granites of the Las Chacras pluton are depleted in SiO2 and enriched in TiO2, Fe2O3(T), CaO and MgO, poor in Nb, Rb, Th, and HREE, and enriched in P, Zr, Sr and Ba and LREE relative to the red equigranular muscovite bearing facies of the Potrerillos pluton. The asymmetric REE patterns likely reflect the control by accessory monazite-(Ce) and allanite-(Ce) in the porphyritic facies, contrasting with relative HREE enrichment in the muscovite-bearing equigranular granite. Both granite types display Eu/Eu* values <1, indicative of feldspar fractionation in the source; the HREE are strongly depleted, more accentuated in less evolved biotite porphyritic facies, likely reflecting garnet retained in the source. The crystallization-emplacement history of the suite spanned from the Upper Devonian to the Lower Carboniferous in a within-plate regime crossed by deep shear zones. The granite facies that host the pegmatites are redefined as A-type granites. The A-type magmas probably originated by partial melting of a depleted garnet-bearing lower crust, of likely granodiorite or tonalite composition in the minimum melt field of Opx + Cpx + Grt ± Qtz, triggered by mantle-derived heat but previously fertilized by alkali metasomatism produced by mantle degassing.

Carbon dioxide in silicate melts: A molecular dynamics simulation study

We have performed a series of molecular dynamics simulations aimed at the evaluation of the solubility of CO 2 in silicate melts of natural composition (from felsic to ultramafic). In making in contact within the simulation cell a supercritical CO 2 phase with a silicate melt of a given composition, we have been able to evaluate (i) the solubility of CO 2 in the P– T range 1473–2273 K and 20–150 kbar, (ii) the density change experienced by the CO 2-bearing melt, (iii) the respective concentrations of CO 2 and CO 3 2 - species in the melt, (iv) the lifetime and the diffusivity of these species and (v) the structure of the melt around the carbonate groups. The main results are the following: (1) The solubility of CO 2 increases markedly with the pressure in the three investigated melts (a rhyolite, a mid-ocean ridge basalt and a kimberlite) from about ∼2 wt% CO 2 at 20 kbar to ∼25 wt% at 100 kbar and 2273 K. The solubility is found to be weakly dependent on the melt composition (as far as the present compositions are concerned) and it is only at very high pressure (above ∼100 kbar) that a clear hierarchy between solubilities occurs (rhyolite < MORB < kimberlite). Furthermore at a given pressure the calculated solubility is negatively correlated with the temperature. (2) In CO 2-saturated melts, the proportion of carbonate ions ( CO 3 2 - ) is positively correlated with the pressure at isothermal condition and is negatively correlated with the temperature at isobaric condition (and vice versa for molecular CO 2). Furthermore, at fixed ( P, T) conditions the proportion of carbonate ions is higher in CO 2-undersaturated melts than in the CO 2-saturated melt. Although the proportion of molecular CO 2 decreases when the degree of depolymerization of the melt increases, it is still significant in CO 2-saturated basic and ultrabasic compositions at high temperatures. This finding is at variance with experimental data on CO 2-bearing glasses which show no evidence of molecular CO 2 as soon as the degree of depolymerization of the melt is high (e.g. basalt). These conflicting results can be reconciled with each other by noticing that a simple low temperature extrapolation of the simulation data predicts that the proportion of molecular CO 2 in basaltic melts might be negligible in the glass at room temperature. (3) The carbonate ions are found to be transient species in the liquid phase, with a lifetime increasing exponentially with the inverse of the temperature. Contrarily to a usual assumption, the diffusivity of carbonate ions into the liquid silicate is not vanishingly small with respect to that of CO 2 molecules: in MORB they differ from each other by a factor of ∼6 at 1473 K and only a factor of ∼2 at 2273 K. Although the bulk diffusivity of CO 2 is governed primarily by the diffusivity of CO 2 molecules, the carbonate ions contribute significantly to the diffusivity of CO 2 in depolymerized melts. (4) Concerning the structure of the CO 2-bearing silicate melt, the carbonate ions are found to be preferentially associated with NBO’s of the melt, with an affinity for NBOs which exceeds that for BOs by almost one order of magnitude. This result explains why the concentration in carbonate ions is positively correlated with the degree of depolymerization of the melt and diminishes drastically in fully polymerized melts where the number of NBO’s is close to zero. Furthermore, the network modifier cations are not randomly distributed in the close vicinity of carbonate groups but exhibit a preferential ordering which depends at once on the nature of the cation and on the melt composition. However at the high temperatures investigated here, there is no evidence of long lived complexes between carbonate groups and metal cations.

Carbon speciation in the asthenosphere: Experimental measurements of the redox conditions at which carbonate-bearing melts coexist with graphite or diamond in peridotite assemblages

The speciation of carbon in the Earth's mantle is largely governed by oxygen fugacity. Under reducing conditions carbon forms graphite, diamond or CH 4, while under more oxidising conditions carbonate minerals or CO 2-bearing melts or fluids are stable. Using an Ir–Fe alloy sliding redox sensor we have measured the oxygen fugacity at which carbon (graphite or diamond) oxidises to carbonate minerals or melts within mantle peridotite assemblages between 2.5 and 11 GPa at 1100–1600 °C. The experiments were performed up to temperatures where carbonate melts evolve towards more silicate-rich compositions. The dilution of the carbonate melt component lowers the relative fo 2, expanding the melt stability field with respect to reduced carbon. We parameterise our results for the a Ca-bearing system as a function of P (in bars), T (K) and mole fraction of CO 2 ( X CO 2 ) in the melt with the equation. Log f o 2 = 5 . 44 − 2138 0 / T + 0.0 78 P − 1 / T + Log X CO 2 This expression reproduces our experimental results to within 0.3 log units between 2.5 and 11 GPa. A further equation is fitted to Ca-free results. Studies indicate that garnet peridotite mantle rocks become progressively more reduced with depth due to the effects of pressure on the governing ferric/ferrous equilibria. It has been proposed that small degree carbonate-rich melts may form at depths up to 300 km beneath mid ocean ridges; however, if the current garnet peridotite oxy-thermometer is correct then our results indicate that the fo 2 of peridotitic mantle will remain in the diamond/graphite stability field up to at least 100–150 km depth. Only at depths shallower than 150 km would Fe 3+ in mantle silicates react with graphite to produce carbonate-rich melts in a redox melting process. Redox melting, therefore, limits the depth interval over which carbonate-rich melts can form beneath ridges. If current estimates for the effect of pressure on mantle fo 2 in the garnet peridotite stability field are correct, graphite or diamond should be the dominant host for carbon below 150 km, although Fe metal and carbides are also possible hosts in the deep mantle. In peridotitic mantle carbonates should only be stable at depths greater than 150 km in highly oxidised regions or as dilute melt species.

Continuous Gradations among Primary Carbonatitic, Kimberlitic, Melilititic, Basaltic, Picritic, and Komatiitic Melts in Equilibrium with Garnet Lherzolite at 3–8 GPa

Multianvil melting experiments in the system CaO–MgO–Al2O3– SiO2–CO2 (CMAS–CO2) at 3–8GPa, 1340–1800 � C, involving the garnet lherzolite phase assemblage in equilibrium with CO2-bearing melts, yield continuous gradations in melt composition between carbonatite, kimberlite, melilitite, komatiite, picrite, and basalt melts. The phase relations encompass a divariant surface in P–T space. Comparison of the carbonatitic melts produced at the low-temperature side of this surface with naturally occurring carbonatites indicates that natural magnesiocarbonatites could be generated over a wide range of pressures >2� 5GPa. Melts analogous to kimberlites form at higher temperatures along the divariant surface, which suggests that kimberlite genesis requires more elevated geotherms. However, the amount of water found in some kimberlites has the potential to lower temperatures for the generation of kimberlitic melts by up to 150 � C, provided no hydrous phases are present. Compositions resembling group IB and IA kimberlites are produced at pressures around 5–6GPa and 10GPa, respectively, whereas the compositions of some other kimberlites suggest generation at higher pressures still. At pressures <4GPa, an elevated geotherm produces melilitite-like meltinthe CMAS–CO2system ratherthan kimberlite. Even when a relatively CO2-rich mantle composition containing 0� 15 wt % CO2 is assumed, kimberlites and melilitites are produced by <1% melting and carbonatites are generated by even smaller degrees of melting of <0� 5%.

Long-lived postbreakup magmatism along the East Greenland margin: Evidence for shallow-mantle metasomatism by the Iceland plume

Research Article| February 01, 2004 Long-lived postbreakup magmatism along the East Greenland margin: Evidence for shallow-mantle metasomatism by the Iceland plume M. Storey; M. Storey 1Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar A.K. Pedersen; A.K. Pedersen 2Geological Museum, Øster Voldgade 5–7, DK-1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar O. Stecher; O. Stecher 3 Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar S. Bernstein; S. Bernstein 3 Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar H.C. Larsen; H.C. Larsen 3 Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar L.M. Larsen; L.M. Larsen 4Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar J.A. Baker; J.A. Baker 5Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar R.A. Duncan R.A. Duncan 6College of Oceanography, Oregon State University, Corvallis, Oregon 97331, USA Search for other works by this author on: GSW Google Scholar Author and Article Information M. Storey 1Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark A.K. Pedersen 2Geological Museum, Øster Voldgade 5–7, DK-1350 Copenhagen, Denmark O. Stecher 3 Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark S. Bernstein 3 Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark H.C. Larsen 3 Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark L.M. Larsen 4Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen, Denmark J.A. Baker 5Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark R.A. Duncan 6College of Oceanography, Oregon State University, Corvallis, Oregon 97331, USA Publisher: Geological Society of America Received: 03 Jul 2003 Revision Received: 06 Oct 2003 Accepted: 07 Oct 2003 First Online: 03 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2004) 32 (2): 173–176. https://doi.org/10.1130/G19889.1 Article history Received: 03 Jul 2003 Revision Received: 06 Oct 2003 Accepted: 07 Oct 2003 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. Storey, A.K. Pedersen, O. Stecher, S. Bernstein, H.C. Larsen, L.M. Larsen, J.A. Baker, R.A. Duncan; Long-lived postbreakup magmatism along the East Greenland margin: Evidence for shallow-mantle metasomatism by the Iceland plume. Geology 2004;; 32 (2): 173–176. doi: https://doi.org/10.1130/G19889.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract 40Ar/39Ar dating has identified a succession of middle Miocene (14–13 Ma) basaltic lavas in East Greenland that overlie Eocene flood basalts that were erupted during continental breakup ca. 56–55 Ma. The long postbreakup magmatic history (∼40 m.y.) of the East Greenland margin precludes a simple relationship between this later igneous activity and the track of the Iceland hotspot. Chemical and isotopic data suggest that the postbreakup magmas were produced from mantle that had been metasomatized by light rare earth element–enriched, H2O- and CO2-bearing melts originating from the Iceland plume. Episodic melting of recently metasomatized shallow mantle beneath Greenland and the North Atlantic can explain both the composition and the long-lived nature of postbreakup magmatism along the East Greenland margin, as well as lavas on Jan Mayen Island that have enriched, Icelandic-type isotopic signatures. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

SIMS analysis of volatiles in silicate glasses, 2: isotopes and abundances in Hawaiian melt inclusions

Ion microprobe measurements of the concentrations of H 2O, CO 2, F, S and Cl and the isotopic composition of hydrogen are reported for populations of olivine-hosted melt inclusions from five lava samples from the Hawaiian volcanoes Loihi, Kilauea, Mauna Loa and Koolau. After reheating of the melt inclusions and correction for the effects of post-entrapment modification, the melt inclusions have MgO contents ranging from 8.9% to 15.1% and averaging 11%, significantly higher in MgO than most submarine glasses. The melt inclusions show large ranges in H 2O (0.03–0.84 wt.%), CO 2 (5–862 ppm), F (308–1000 ppm), S (156–3330 ppm) and Cl (8 ppm to 1.11 wt.%), accompanied by large ranges in δD (−165‰ to +40‰). Laboratory reheating experiments on Loihi inclusions show that diffusive loss of hydrogen can occur from olivine-hosted melt inclusions on hour- to day-long time scales via proton diffusion through olivine, with consequent positive shifts in the D/H ratios of the residual hydrogen in the melt inclusion. Most melt inclusions from subaerial Kilauea and Mauna Loa samples have signatures of low H 2O (0.05–0.2 wt.%) and high δD (up to +40%) compared to published analyses of submarine glasses, suggesting diffusive H loss during slow cooling of inclusions shortly after eruption. Koolau melt inclusions have the lowest δD values yet measured in oceanic basalts (−61‰ to −165‰). Shallow-level degassing produces a H 2O– δD relationship in most Koolau melt inclusions which can be explained by open-system (Rayleigh) degassing with a vapor-melt D/H fractionation factor of 1.024, similar to previous estimates. Shallow degassing is also indicated in some inclusions by parallel depletions in H 2O and S, but degassed melt inclusions from all volcanoes display a wide range in CO 2 concentrations, indicating kilometer-scale vertical convection of melts within Hawaiian magma reservoirs. The measured δ 13C of three CO 2-bearing melt inclusions from Koolau volcano are depleted (−12‰ to −29‰) and correlated with δD, possibly consistent with open-system degassing of CO 2-rich magmas and subsequent mixing with less-degassed magmas. Assimilation of seawater-derived components is indicated in a small number of melt inclusions which exhibit high Cl and Cl/K ratios, with an extreme example from Loihi (1.11 wt.% Cl, 0.48 wt.% H 2O, δD=−118‰). A subset of melt inclusions have escaped the confounding effects of H diffusion, shallow degassing and crustal contamination, and provide evidence for heterogeneity of D/H ratios in the Hawaiian mantle, which appear to correlate with published radiogenic (Sr, Nd, Pb, Os) and oxygen isotope data. If the apparent Hawaiian δD variability can be confirmed to be a source signature, then the Hawaiian D/H data indicate that heterogeneities within the Hawaiian plume are required to be large in scale (tens of kilometers) and/or young in age (<1 Ga) in order to preserve hydrogen isotopic variability against the extremely rapid diffusivity of hydrogen in the mantle.

Noble gases in anhydrous lherzolites from the newer volcanics, southeastern Australia: a MORB-like reservoir in the subcontinental mantle

In order to understand the nature of noble gas reservoirs beneath continents, elemental and isotopic compositions of noble gases were analyzed from spinel-lherzolites with anhydrous mineral assemblages (i.e., olivine > clinopyroxene ∼ orthopyroxene > spinel); these are considered to be typical of the upper lithospheric mantle. The observed noble gas elemental and isotopic compositions are very similar to those found in MORBs. Characteristic features include (1) 3He/ 4He ratio of about 1 × 10 −5, (2) linearly correlated 20Ne/ 22Ne and 21Ne/ 22Ne ratios plotting on the well-established MORB-line, (3) relatively high 40Ar/ 36Ar ratios, and (4) correlated excesses in 129Xe and 136Xe with respect to atmospheric xenon. In addition, 3He to 22Ne ratios are close to those of a solar component, reinforcing the view that the Earth contains primordial helium and neon of solar composition. The MORB-like noble gases are trapped predominantly in CO 2-rich fluid inclusions, which are inferred to be of secondary origin. These gases probably are of metasomatic origin, having been introduced into the lithospheric mantle from the convective upper mantle by CO 2-bearing melts. The results indicate that a MORB-like noble gas reservoir may characterize the upper part of the mantle on a global scale. This reservoir may be underlain by a relatively less degassed mantle reservoir, as plume-like mantle noble gases also have been identified beneath southeastern Australia (Matsumoto et al., 1997). Thus, some form of chemical layering appears to be required to explain the noble gas signatures both in the subcontinental and suboceanic mantle.

Diamond precipitation and mantle metasomatism - evidence from the trace element chemistry of silicate inclusions in diamonds from Akwatia, Ghana

Trace element concentrations in the four principal peridotitic silicate phases (garnet, olivine, orthopyroxene, clinopyroxene) included in diamonds from Akwatia (Birim Field, Ghana) were determined using SIMS. Incompatible trace elements are hosted in garnet and clinopyroxene except for Sr which is equally distributed between orthopyroxene and garnet in harzburgitic paragenesis diamonds. The separation between lherzolitic and harzburgitic inclusion parageneses, which is commonly made using compositional fields for garnets in a CaO versus Cr2O3 diagram, is also apparent from the Ti and Sr contents in both olivine and garnet. Titanium is much higher in the lherzolitic and Sr in the harzburgitic inclusions. Chondrite normalised REE patterns of lherzolitic garnets are enriched (10–20 times chondrite) in HREE (LaN/YbN = 0.02–0.06) while harzburgitic garnets have sinusoidal REEN patterns, with the highest concentrations for Ce and Nd (2–8 times chondritic) and a minimum at Ho (0.2–0.7 times chondritic). Clinopyroxene inclusions show negative slopes with La enrichment 10–100 times chondritic and low Lu (0.1–1 times chondritic). Both a lherzolitic and a harzburgitic garnet with very high knorringite contents (14 and 21 wt% Cr2O3 respectively) could be readily distinguished from other garnets of their parageneses by much higher levels of LREE enrichment. The REE patterns for calculated melt compositions from lherzolitic garnet inclusions fall into the compositional field for kimberlitic-lamproitic and carbonatitic melts. Much more strongly fractionated REE patterns calculated from harzburgitic garnets, and low concentrations in Ti, Y, Zr, and Hf, differ significantly from known alkaline and carbonatitic melts and require a different agent. Equilibration temperatures for harzburgitic inclusions are generally below the C-H-O solidus of their paragenesis, those of lherzolitic inclusions are above. Crystallisation of harzburgitic diamonds from CO2-bearing melts or fluids may thus be excluded. Diamond inclusion chemistry and mineralogy also is inconsistent with known examples of metasomatism by H2O-rich melts. We therefore favour diamond precipitation by oxidation of CH4-rich fluids with highly fractionated trace element patterns which are possibly due to “chromatographic” fractionation processes.

Mantle fluids: Evidence from fluid inclusions

A total dissolution technique has been developed and used to identify and quantify the incompatible element contents of fluids trapped in inclusions in minerals from peridotite xenoliths using “fluids” in the generic sense (i.e., COH fluids and melts). Fluids from lherzolites, a wehrlite, and a harzburgite host important quantities of alkalis, Ba, U, Th, Pb, and contain Sr and Nd as well. Quantitative application of the technique shows that the CO2-rich fluid in a Iherzolite from Nunivak Island, AK, USA and the CO2-poor melt in a Iherzolite from San Carlos, Arizona, USA, have incompatible element compositions similar to each other differing only in their K and Ba contents. With the exception of their K contents, the trace element compositions of these fluids resemble those of carbonatites and kimberlites. Major and radiogenic isotope data from the lherzolite and a phlogopite harzburgite from Nunivak suggest that the fluid trapped in the Iherzolite is associated with a carbonatite melt, linked to hydrous metasomatism in the region. A more dilute CO2-bearing melt was identified in a wehrlite from Salt Lake Crater, HI, USA resembling Hawaiian alkali basalt in its incompatible element composition. The strontium, neodymium, and lead isotope composition of the fluids resemble those of the surrounding mantle and do not reflect their parent/daughter ratios. Lead isotope data for fluid-bearing clinopyroxene in the wehrlite suggest fluid influx was recent. Conventional and laser oxygen isotope analyses show that most fluid inclusion-bearing xenoliths examined are out of oxygen isotope equilibrium. Diffusion-based arguments suggest that fluid infiltration in these xenoliths occurred over the last 1–10 My. The fluids identified in this study will dominate the incompatible element budget of typical mantle peridotite if present in greater than sub-weight percent quantities.