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J. JOHN LOWE STEPHEN LOWE ALISON J. FOWLER ROBERT E. M. HEDGES TIM J. F. AUSTIN 《Boreas: An International Journal of Quaternary Research》1988,17(3):355-369
Accelerator mass spectrometry measures of the radiocarbon activity of various chemical fractions prepared from Late Devensian Lateglacial lake sediments from the site of Llyn Gwernan, near Cader Idris, North Wales are presented and assessed. These are compared with radiocarbon measurements obtained by radiometric (decay) counting which were reported earlier from the same site and are considered in the light of pollen-stratigraphic information. The potential advantages of accelerator radiocarbon measurements to the assessment of the chronology and correlation of Lateglacial lake sediments are evaluated. 相似文献
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Magmatic Evolution of the Skye Igneous Centre, Western Scotland: Modelling of Assimilation, Recharge and Fractional Crystallization 总被引：3，自引：2，他引：1
The Skye igneous centre, forming part of the British Tertiarymagmatic province, developed over a 7 Myr period (61–54Ma) and is characterized by a complex suite of lavas, hypabyssaland intrusive rocks of picritic to granitic composition. Theintrusion of magma from mantle to crust at 2x10–3km3/yr(6 Mt/yr) advected magmatic heat of roughly 0·2 GW averagedover the period of magmatism supporting an ‘excess’heat flux of about 130 mW/m2, or about twice the present-dayaverage continental heat flow. The volume of new crust generatedat Skye (15000 km3) spread over the present-day area of Skyecorresponds to 9 km of new crust. The geochemical evolutionof the Skye magmatic system is constrained using the Energy-ConstrainedRecharge, Assimilation, and Fractional Crystallization (EC-RAFC)model to understand variations in the Sr- and Pb-isotopic andSr trace-element composition of the exposed magmatic rocks withtime. The character (composition and specific enthalpy) of bothassimilant and recharge magma appears to change systematicallyup-section, suggesting that the magma reservoirs migrated toprogressively shallower levels as the system matured. The modelof the magma transport system that emerges is one in which magmabatches are stored initially at lower-crustal levels, wherethey undergo RAFC evolution. Residual magma from this stagethen migrates to shallower levels, where mid-crustal wall rockis assimilated; the recharge magma at this level is characterizedby an increasingly crustal signature. For some of the stratigraphicallyyoungest rocks, the data suggest that the magma reservoirs ascendedinto, and interacted with, upper-crustal Torridonian metasediments. KEY WORDS: assimilation; EC-RAFC model; geochemical modelling; magma recharge; Skye magmatism 相似文献
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Abstract— Diogenites are recognized as a major constituent of the howardite, eucrite and diogenite (HED) meteorite group. Recently, several papers (Mittlefehldt, 1994; Fowler et al, 1994, 1995) have identified trace-element systematics in diogenites that appeared to mimic simple magmatic processes that involved large degrees of crystallization (up to 95% orthopyroxene) of basalt with extremely high normative hypersthene. Such a crystallization scenario linking all the diogenites is highly unlikely. The purpose of this study is to explore other possible models relating the diogenites. Computational major-element melting models of a variety of different potential bulk compositions for the eucrite parent body (EPB) mantle indicate that these compositions show a similar sequence in residuum mineral assemblage with increasing degrees of partial melting. Numerous bulk compositions would produce melts with Mg# appropriate for diogenitic parent magmas at low to moderate degrees of partial melting (15% to 30%). These calculations also show that melts with similar Mg# and variable incompatible element concentrations may be produced during small to moderate degrees of EPB mantle melting. The trace-element characteristic of the orthopyroxene in diogenites does not support a model for large amounts of fractional crystallization of a single “hypersthene normative” basaltic magma following either small-scale or large-scale EPB mantle melting. Small degrees of fractional crystallization of a series of distinct basaltic magmas are much more likely. Only two melting models that we considered hold any promise for producing different batches of “diogenitic magmas.” The first model involves the fractional melting of a homogeneous source that produces parental magmas to diogenites with an extensive range of incompatible elements and limited variations in Mg#. There are several requirements for this model to work. The first requirement of this model is that the Dorthopyroxene/melt must change during melting or crystallization to compress the range of incompatible elements in the calculated diogenitic magmas. The second prerequisite is that either some of the calculated diogenitic magmas are parental to eucrites or the Mg# in diogenitic magmas are influenced by slight changes in oxygen fugacity during partial melting. The second model involves batch melting of a source that reflects accretional heterogeneities capable of generating diogenitic magmas with the calculated Mg# and incompatible element contents. Both of these models require small to moderate degrees of partial melting that may limit the efficiency of core separation. 相似文献
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Large-ion lithophile element characteristics of an amphibolite facies to granulite facies transition at Gruinard Bay, North-west Scotland 总被引：1，自引：0，他引：1
M. B. FOWLER 《Journal of Metamorphic Geology》1986,4(3):345-359
Abstract Lewisian grey gneisses from Gruinard Bay, North-west Scotland retain mineralogical and geochemical evidence for Scourian horn-blende-granulite facies metamorphism, and they may be used to assess current models of elemental depletion at granulite grade. Their 'immobile'major and trace element geochemistry is indistinguishable from that of Lewisian amphibolite and pyroxene-granulite facies counterparts. The K, Rb, Th and U contents of the Gruinard Bay gneisses are depleted relative to amphibolite facies gneisses, but generally the abundances of these elements are above those of comparable pyroxene granulites. U and Th have reached an advanced stage of depletion, but allanite appears to be crucial in maintaining significantly higher U and Th abundances at Gruinard Bay than in pyroxene granulites. K and Rb loss is less extreme, and depends on the stability of the rock-forming minerals: K-feldspar; biotite; and, amphibole. Early removal of K and Rb has resulted in a small rise in K/Rb, but further preferential Rb loss would have been required to generate the characteristically high K/Rb ratios of Lewisian pyroxene granulites.
The residence of U and Th in the accessory minerals of granulite facies gneisses, which are often correlated with the residua of intracrustal partial melting, renders unlikely their extreme incompatibility required by such models. Even if such phases are ignored, high mineral-melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism. On the other hand, the advanced stage of U and Th depletion reached in Gruinard Bay gneisses, which were still partly hydrous, severely restricts the role played by CO2 -dominated fluids and a hydrous medium is preferred. 相似文献
The residence of U and Th in the accessory minerals of granulite facies gneisses, which are often correlated with the residua of intracrustal partial melting, renders unlikely their extreme incompatibility required by such models. Even if such phases are ignored, high mineral-melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism. On the other hand, the advanced stage of U and Th depletion reached in Gruinard Bay gneisses, which were still partly hydrous, severely restricts the role played by CO
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