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  • 1
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    Decoding polyphase migmatites using geochronology and phase equilibria modelling (2014)
    Wiley
    Details
    Publication Date: 2014-11-14
    Description: In this study, in situ U–Pb monazite ages and Lu–Hf garnet geochronology are used to distinguish mineral parageneses developed during Devonian–Carboniferous and Cretaceous events in migmatitic paragneiss and orthogneiss from the Fosdick migmatite–granite complex in West Antarctica. SHRIMP U–Pb monazite ages define two dominant populations at 365–300 Ma (from cores of polychronic grains, dominantly from deeper structural levels in the central and western sectors of the complex) and 120–96 Ma (from rims of polychronic grains, dominantly from the central and western sectors of the complex, and from monochronic grains, mostly from shallower structural levels in the eastern sector of the complex). For five paragneisses and two orthogneisses, Lu–Hf garnet ages range from 116 to 111 Ma, c. 12–17 Ma older than published Sm–Nd garnet ages of 102–99 Ma from three of the same samples. Garnet grains in the analysed samples generally have Lu-enriched rims relative to Lu-depleted cores. By contrast, for three of the same samples individual garnet grains have flat Sm concentrations consistent with high-temperature diffusive resetting. Lutetium enrichment of garnet rims is interpreted to record the breakdown of a Lu-rich accessory mineral during the final stage of garnet growth immediately prior to the metamorphic peak, and/or the preferential retention of Lu in garnet during breakdown to cordierite in the presence of melt concomitant with the initial stages of exhumation. Therefore, garnet is interpreted to be part of the Cretaceous mineral paragenesis and the Lu–Hf garnet ages are interpreted to record the timing of close-to-peak metamorphism for this event. For the Devonian–Carboniferous event, phase equilibria modelling of the metasedimentary protoliths to the paragneiss and a diatexite migmatite restrict the peak P–T conditions to 720–800°C at 0.45–1.0 GPa. For the Cretaceous event, using both forward and inverse phase equilibria modelling of residual paragneiss and orthogneiss compositions, the P–T conditions after decompression are estimated to have been 850–880°C at 0.65–0.80 GPa. These P–T conditions occurred between c. 106 and c. 96 Ma, determined from Y-enriched rims on monazite that record the timing of garnet and biotite breakdown to cordierite in the presence of melt. The effects of this younger metamorphic event are dominant throughout the Fosdick complex. This article is protected by copyright. All rights reserved.
    Print ISSN: 0263-4929
    Electronic ISSN: 1525-1314
    Topics: Geosciences
    Published by Wiley
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  • 2
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    Retraction (2012)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2012-07-24
    Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Anderson, Travis M -- Neiwert, Wade A -- Kirk, Martin L -- Piccoli, Paula M B -- Schultz, Arthur J -- Koetzle, Thomas F -- Musaev, Djamaladdin G -- Morokuma, Keiji -- Cao, Rui -- Hill, Craig L -- New York, N.Y. -- Science. 2012 Jul 20;337(6092):290. doi: 10.1126/science.337.6092.290-a.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22822129" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 3
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    A late-transition metal oxo complex: K7Na9[O=PtIV(H2O)L2], L = [PW9O34]9 (2004)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2004-11-27
    Description: Terminal mono-oxo complexes of the late transition metal elements have long been considered too unstable to synthesize because of repulsion between the oxygen electrons and the mostly filled metal d orbitals. A platinum(IV)-oxo compound flanked by two polytungstate ligands, K7Na9[O=Pt(H2O)L2], L = [PW9O34(9-)], has now been prepared and isolated at room temperature as air-stable brown crystals. X-ray and neutron diffraction at 30 kelvin revealed a very short [1.720(18) angstrom] Pt-O bond and no evidence of a hydrogen atom at the terminal oxygen, ruling out a better precedented Pt-OH complex. Density functional theory and spectroscopic data account for the stability of the Pt(IV)-oxo unit by electron withdrawal into delocalized orbitals of the polytungstates.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Anderson, Travis M -- Neiwert, Wade A -- Kirk, Martin L -- Piccoli, Paula M B -- Schultz, Arthur J -- Koetzle, Thomas F -- Musaev, Djamaladdin G -- Morokuma, Keiji -- Cao, Rui -- Hill, Craig L -- GM-057378/GM/NIGMS NIH HHS/ -- New York, N.Y. -- Science. 2004 Dec 17;306(5704):2074-7. Epub 2004 Nov 25.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Chemistry, Emory University, Atlanta, GA 30322, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/15564312" target="_blank"〉PubMed〈/a〉
    Keywords: Chemistry, Physical ; Crystallization ; Crystallography, X-Ray ; Electrons ; Fourier Analysis ; Hydrogen-Ion Concentration ; Ligands ; Molecular Structure ; Neutron Diffraction ; Oxidation-Reduction ; Oxygen/*chemistry ; Physicochemical Phenomena ; Platinum/chemistry ; Platinum Compounds/chemical synthesis/*chemistry/isolation & purification ; Spectrum Analysis ; Temperature ; Tungsten/chemistry ; Tungsten Compounds/chemical synthesis/*chemistry/isolation & purification
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 4
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    Fluid generation and evolution during exhumation of deeply subducted UHP continental crust: petrogenesis of composite granite–quartz veins in the Sulu belt, China (2017)
    Wiley
    Details
    Publication Date: 2017-03-04
    Description: Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high-pressure (HP) mineral assemblage of dominantly quartz+phengite+ allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si-in-phengite barometry) and temperatures of 850–780 °C (Ti-in-zircon thermometry) at 2.5 GPa (~20 °C lower at 2.1 GPa). Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at c . 221 Ma. This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt, consistent with the HP conditions retrieved from the granite. The ε Hf ( t ) values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite, whereas δ 18 O values of new zircon are similar in the veins and the crustal rocks. These data are consistent with zircon growth from a blended fluid generated internally within the gneiss and the eclogite, without any ingress of fluid from an external source. However, at the peak metamorphic pressure, which could have reached 7 GPa, the rocks were likely fluid absent. During initial exhumation under UHP conditions, exsolution of H 2 O from nominally anhydrous minerals generated a grain boundary supercritical fluid in both gneiss and eclogite. As exhumation progressed, the volume of fluid increased allowing it to migrate by diffuse porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite. This produced a blended fluid intermediate in its isotope composition between the two end members, as recorded by the composite veins. During exhumation from UHP (coesite) eclogite to HP (quartz) eclogite facies conditions, the supercritical fluid evolved by dissolution of the silicate mineral matrix, becoming increasingly solute-rich, more ‘granitic’ and more viscous until it became trapped. As crystallization began by diffusive loss of H 2 O to the host eclogite concomitant with ongoing exhumation of the crust, the trapped supercritical fluid intersected the solvus for the granite–H 2 O system, allowing phase separation and formation of the composite granite–quartz veins. Subsequently, during the transition from HP eclogite to amphibolite facies conditions, minor phengite breakdown melting is recorded in both the granite and the gneiss by K-feldspar+plagioclase+biotite aggregates located around phengite and by K-feldspar veinlets along grain boundaries. Phase equilibria modelling of the granite indicates that this late-stage melting records P–T conditions towards the end of the exhumation, with the subsolidus assemblage yielding 0.7–1.1 GPa at 〈670 °C. Thus, the composite granite–quartz veins represent a rare example of a natural system recording how the fluid phase evolved during exhumation of continental crust. The successive availability of different fluid phases attending retrograde metamorphism from UHP (coesite) eclogite to amphibolite facies conditions will affect the transport of trace elements through the continental crust and the role of these fluids as metasomatic agents interacting with the mantle wedge in the subduction channel. This article is protected by copyright. All rights reserved.
    Print ISSN: 0263-4929
    Electronic ISSN: 1525-1314
    Topics: Geosciences
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  • 5
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    Magnetic order inTbCo2Zn20andTbFe2Zn20 (2010)
    American Physical Society
    Details
    Publication Date: 2010-04-08
    Print ISSN: 1098-0121
    Electronic ISSN: 1550-235X
    Topics: Physics
    Published by American Physical Society
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  • 6
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    Magmatic Apatite: A Powerful, Yet Deceptive, Mineral (2015)
    Mineralogical Society of America
    In: Elements
    Details
    Publication Date: 2015-06-02
    Description: Apatite may be a minor constituent in magmatic rocks but it is a powerful research tool because it is ubiquitous and it incorporates magmatic water, halogens, S, C, and trace elements including Sr, U, Th, and the rare earth elements. Recent advances in experimental and analytical methodologies allow geologists to analyze apatite textures and compositions in great detail. This information improves understanding of the behavior of volatiles and trace elements both in terrestrial igneous melts and their related fluids and in extraterrestrial bodies, such as the Moon and Mars. With more research, the petrological power of apatite can only increase with respect to understanding eruptive, pluton-building, and mineralizing magmatic systems.
    Print ISSN: 1811-5209
    Electronic ISSN: 1811-5217
    Topics: Geosciences
    Published by Mineralogical Society of America
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  • 7
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    From Source to Sink: Petrogenesis of Cretaceous Anatectic Granites from the Fosdick Migmatite-Granite Complex, West Antarctica (2016)
    Oxford University Press
    Details
    Publication Date: 2016-08-30
    Description: Anatectic granites from the Fosdick migmatite–granite complex yield U–Pb zircon crystallization ages that range from 115 to 100 Ma, with a dominant grouping at 107–100 Ma, which corresponds to the timing of dome formation during the regional oblique extension that facilitated exhumation of the complex. The occurrence of leucosome-bearing normal-sense shear zones in migmatitic gneisses indicates that suprasolidus conditions in the crust continued into the early stages of doming and exhumation of the complex. The structure allows access to variably oriented granites in networks of dykes at deeper structural levels and subhorizontal sheeted granites at shallower structural levels within the complex. This feature allows an evaluation of the mechanisms that modify the composition of granite melts in their source and of granite magmas during their ascent and emplacement using whole-rock major, trace element and Sr and Nd isotope compositions, zircon Hf and O isotope compositions, and phase equilibria modelling of potential source rocks. Geochemical variability within the granites is attributed to source heterogeneity and blending of melts, which themselves are consistent with derivation from regional metasedimentary and metaplutonic source materials. The granites typically contain coarse blocky K-feldspar and/or plagioclase grains within interstitial quartz, and have low Rb/Sr ratios and large positive Eu anomalies. These features are inconsistent with the composition of primary crustal melts derived from metasedimentary and metaplutonic source materials, but consistent with early fractional crystallization of feldspar and subsequent drainage of the fractionated melt. Processes such as peritectic mineral entrainment and accessory mineral dissolution, entrainment and crystallization did not have any significant influence on the major and trace element composition of the granites. The granites in the networks of dykes are interpreted to represent choking of magma transport channels through the middle crust as the rate of magma flow declined during doming and exhumation, whereas the sheeted granites record collapse of subhorizontal, partially crystallized layers of magma by filter pressing and melt exfiltration during vertical shortening associated with doming and exhumation. These processes separated feldspar-rich residues from evolved melt. Based on the results of this study, caution is urged in estimating melt proportion from the volume of granite retained in migmatitic gneiss domes, as the granites may not represent liquid compositions.
    Print ISSN: 0022-3530
    Electronic ISSN: 1460-2415
    Topics: Geosciences
    Published by Oxford University Press
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  • 8
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    Olivine + Quartz + Water {+/-} HCl At Mid-Crustal Conditions: Controls On the Growth of Fibrous Talc As Determined From Hydrothermal Diamond Anvil Cell EXPERIMENTS (2017)
    Mineralogical Association of Canada
    Details
    Publication Date: 2017-07-08
    Description: Many talc deposits are formed by metasomatism of dolostone or ultramafic rocks during metamorphism or hydrothermal alteration. In these deposits, talc commonly exhibits a platy habit; rarely, it forms fibers. In previous studies, the origin of fibrous talc has been explained by the secondary replacement of fibrous amphiboles during retrograde metamorphism. In this study, experiments have been performed in the MgO–FeO–SiO 2 –H 2 O–HCl system using a hydrothermal diamond anvil cell (HDAC). San Carlos olivine (Fo 90 ) + Brazilian quartz + aqueous fluid [deionized water or 0.1–10 m m (milli-molal) HCl] were used as starting materials and were reacted for approximately 2 to 6 hours at temperatures of 400–655 °C and pressures of 0.1–1.2 GPa. All experiments produced talc as the only new phase, but with both fibrous and platy habits. The talc fibers (length:width ≥ 25:1; average length ~40 μm) grew on the surface of the olivine and on the gasket walls. Maximum talc growth rates in these experiments ranged from 10 –11 to 10 –13 moles per second or 10 –5 to 10 –6 moles per second per square meter. Experiments in this study demonstrate that the growth of fibrous talc does not require a fibrous precursor. Experimental results suggest that the mechanism for talc fiber growth is related to initial nucleation density of talc on the substrate and the degree of supersaturation. Conditions that appear to inhibit the growth of fibrous talc include a large surface area of olivine relative to quartz; an increase in the density of the aqueous fluid; and the chemical isolation of olivine by talc armoring. Decreases in aqueous fluid density and lower olivine surface area relative to quartz are consistent with the production of fibrous talc.
    Print ISSN: 0008-4476
    Topics: Geosciences
    Published by Mineralogical Association of Canada
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  • 9
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    40Ar/39Ar thermochronology of high-pressure granulite nappes in the southern Brasilia Belt, Brazil: Implications for Nappe Exhumation (2010)
    American Journal of Science
    Details
    Publication Date: 2010-12-01
    Print ISSN: 0002-9599
    Electronic ISSN: 1945-452X
    Topics: Geosciences
    Published by American Journal of Science
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  • 10
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    Topaz: a single-crystal diffractometer for the spallation neutron source (2006)
    International Union of Crystallography
    Details
    Publication Date: 2006-08-06
    Print ISSN: 0108-7673
    Electronic ISSN: 2053-2733
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by International Union of Crystallography
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