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GERHARD SCHMIDT 《Meteoritics & planetary science》1997,32(6):761-767
Abstract— Seven large (10 g) impact melt rock samples from boreholes from the Boltysh impact crater (Ukraine) and six samples from the East Clearwater crater (Canada) were analyzed for Os, Ir, Ru, Rh, Pd, Re and Au by the nickel sulfide technique in combination with neutron activation. Earlier analyses of Clearwater East impact melt rocks have shown that they are strongly enriched in Ir, Os, Pd and Re. In this work, I confirm earlier findings and demonstrate similarly high enrichments of Rh and Ru. The average Os/Ir, Ru/Ir, Pd/Ir, Rh/Ir and Ru/Rh ratios of the melt rock samples from Clearwater East are CI-chondritic and yield an average Ir content of 25.2 ± 6.5 ng/g relative to an average upper crust concentration of 0.03 ± 0.02 ng/g Ir. The amount of meteoritic component corresponds to 4 to 7% of a nominal CI component for Clearwater East. The impact melt rock samples from a bore hole from Boltysh are low in Ir with an average of 0.2 ± 0.1 ng/g. The CI-normalized abundances increase from the refractory to the more volatile siderophile elements (Os < Ir < Ru < Rh ～ Pd ～ Au ～ Ni ～ Co). Because of the low Ir anomaly and uncertainties in making corrections (correlations are weak) for indigenous siderophile elements, no clear projectile assignment can be made. 相似文献
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Phase Relations of an Armalcolite-Phlogopite Lamproite from Smoky Butte, Montana: Applications to Lamproite Genesis 总被引：1，自引：0，他引：1
Suprasolidus phase relations at pressures from 8 to 30 kb andtemperatures from 950 to 1380C have been determined experimentallyfor a glassy armalcolite–phlogopite lamproite from thechilled margin of a medium–grained lamproite from SmokyButte, Montana: The armalcolite-phlogopite lamproite has microphenocrystsof olivine in a groundmass of phlogopite, sanidine, armalcolite,clinopyroxene, chromite, priderite, apatite, and abundant glass.The lamproite is SiO2-rich and has high F/H2O relative to lamproitesthat have been investigated in previous experimental studies.Our data show that with decreasing temperature from the liquidusat pressures above 12 kb, melt coexists successively with:olivine; orthopyroxene + clinopyroxene; orthopyroxene + clinopyroxene+ phlogopite; clinopyroxene +phlogopite; and clinopyroxene +orthopyroxene + K-richterite. Below 12 kb, the assemblage successionis: olivine; olivine + clinopyroxene; olivine + clinopyroxene+ phlogopite; and olivine +clinopyroxene + phlogopite + armalcolite.The main difference from the natural paragenesis is that therock does not contain any orthopyroxene—a feature thatis rather remarkable inasmuch as it has 16% normative hypersthene—andthe rock differs also in that it contains sanidine and priderite.In the experiments, sanidine is observed only as ghostlike domainsin some of the glass and appears to have formed during quenching. The solid phases crystallized experimentally are generally compositionallysimilar to the minerals in the rock. These similarities andthe experimental phase relations support the concept of a rapidinitial magma ascent with only a small temperature drop andcrystallization of olivine, but not of orthopyroxene. At lowerpressures, less than 12 kb, it appears that the magma ascendedmore slowly with a larger temperature drop suggested by thesimilarity of the experimentally determined sequence of assemblagesto the paragenesis of the rock. No quasi-invariant multiphase-saturation point was found suchas might be indicative of pressure and temperature conditionsfor formation of the lamproite magma by eutectic-type partialmelting of a mantle source. The occurrence of olivine, orthopyroxene,and clinopyroxene near the liquidus, and the high proportionof normative hypersthene in the melt suggest that lherzoliteor harzburgite was probable in the magma source rock. The highSiO2 and MgO contents of the Smoky Butte lamproites may indicatethat orthopyroxene was a source mineral even though it did notcrystallize under near-surface conditions. The curve definingthe appearance of phlogopite appears at progressively lowertemperatures from the liquidus as pressure increases, so itwould appear that either phlogopite was not the mantle K-reservoir,or it was entirely consumed during the partial melting process.The composition of the near-liquidus glass in the experimentsis likely to be the composition of the bulk rock less the verysmall amounts of olivine + clinopyroxene + orthopyroxene crystallizedwithin a few degrees below the liquidus. From the inferred compositionof this glass, anhydrous phlogopite is a potential mineral.The principal variable that determines whether phlogopite crystallizesas a near-liquidus mineral is F/H2O; low values of this ratiopromote the presence of phlogopite as a near-liquidus mineralwhereas high values deter its crystallization. The common practiceof adding H2O but not F in experiments to compensate for degassingmay obscure the role of phlogopite in the evolution of lamproitemagmas. 相似文献
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Up-temperature flow of surface-derived fluids in the mid-crust: the role of pre-orogenic burial of hydrated fault rocks 总被引：1，自引：0，他引：1
The Walter‐Outalpa shear zone in the southern Curnamona Province of NE South Australia is an example of a shear zone that has undergone intensely focused fluid flow and alteration at mid‐crustal depths. Results from this study have demonstrated that the intense deformation and ductile shear zone reactivation, at amphibolite facies conditions of 534 ± 20 °C and 500 ± 82 MPa, that overprint the Proterozoic Willyama Supergroup occurred during the Delamerian Orogeny (c. 500 Ma) (EPMA monazite ages of 501 ± 16 and 491 ± 19 Ma). This is in contrast to the general belief that the majority of basement deformation and alteration in the southern Curnamona Province occurred during the waning stages of the Olarian Orogeny (c. 1610–1580 Ma). These shear zones contain hydrous mineral assemblages that cut wall rocks that have experienced amphibolite facies metamorphism during the Olarian Orogeny. The shear zone rock volumes have much lower δ18O values (as low as 1‰) than their unsheared counterparts (7–9‰), and calculated fluid δ18O values (5–8‰) consistent with a surface‐derived fluid source. Hydrous minerals show a decrease in δD(H2O) from ?14 to ?22‰, for minerals outside the shear zones, to ?28 to ?40‰, for minerals within the shear zones consistent with a contribution from a meteoric source. It is unclear how near‐surface fluids initially under hydrostatic pressure penetrate into the middle crust where fluid pressures approach lithostatic, and where fluid flow is expected to be dominantly upward because of pressure gradients. We propose a mechanism whereby faulting during basin formation associated with the Adelaidean Rift Complex (c. 700 Ma) created broad hydrous zones containing mineral assemblages in equilibrium with surface waters. These panels of fault rock were subsequently buried to depths where the onset of metamorphism begins to dehydrate the fault rock volumes evolving a low δ18O fluid that is channelled through shear zones related to Delamerian Orogenic activity. 相似文献
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Experimental Petrology of a Highly Potassic Magma 总被引：3，自引：0，他引：3
The melting behaviour of a highly potassic biotite mafuriteof the Central African olivine leucitite kindred has been studiedexperimentally as a function of pressure (to 30kb) temperature,and water content (0%, 5%, 15%, 25%, and 40% H2O). Olivine isthe liquidus phase up to 30 kb for all water contents studiedexcept for anhydrous (clinopyroxene on the liquidus) and 15%H2O (phlogopite on the liquidus) conditions. Analyses of phasescrystallizing from the biotite mafurite show that pressure hasvery little effect on the composition of clinopyroxene whichis extremely calcium-rich, and low in Al2O3 and TiO2 for allconditions investigated. Phlogopite has low TiO2 content andtitanphlogopite cannot be a refractory phase in the upper mantlecausing Ti-depletion in partial melts in equilibrium with titanphlogopite.There are apparently no conditions where the extremely potassicbiotite mafurite could be a partial melt from pyrolite but derivationfrom an olivine+clinopyroxene+phlogopite+ilmenite assemblageoccurring as ‘enriched’ patches in the upper mantle,is possible. Liquids in equilibrium with phlogopite as a residualphase at 30 kb would be olivine nephelinites with approximately5% K2O, Na2O/K2O 1 and TiO2 > 5+. Crystal elutriation withtransported residual phlogopite reacting (phlogopite+liquid1 olivine+liquid 2) at lower pressures provides a mechanismfor K-enrichment and generating Na2O/K2O < 1. 相似文献
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MARTIN KLUG STEFFI SCHMIDT OLE BENNIKE OLIVER HEIRI MARTIN MELLES BERND WAGNER 《Boreas: An International Journal of Quaternary Research》2009,38(1):59-71
A 2.9 m long sedimentary record was studied from a small lake, here referred to as Duck Lake, located at 76°25'N, 18°45'W on Store Koldewey, an elongated island off the coast of Northeast Greenland. The sediments were investigated for their geophysical and biogeochemical characteristics, and for their fossil chironomid assemblages. Organic matter began to accumulate in the lake at 9.1 cal. kyr BP, which provides a minimum age for the deglaciation of the basin. Although the early to mid-Holocene is known as a thermal maximum in East Greenland, organic matter accumulation in the lake remained low during the early Holocene, likely due to late plant immigration and lack of nutrient availability. Organic matter accumulation increased during the middle and late Holocene, when temperatures in East Greenland gradually decreased. Enhanced soil formation probably led to higher nutrient availability and increased production in the lake. Chironomids are abundant throughout the record after 9.1 cal. kyr BP and seem to react sensitively to changes in temperature and nutrient availability. It is concluded that relative temperature reconstructions based on biogeochemical data have to be regarded critically, particularly in the period shortly after deglaciation when nutrient availability was low. Chironomids may be a suitable tool for climatic reconstructions even in those high arctic environments. However, a better understanding of the ecology of chironomids under these extreme conditions is needed. 相似文献
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Time constants for the evolution of sea spray droplets 总被引：4，自引：0，他引：4
EDGAR L ANDREAS 《Tellus. Series B, Chemical and physical meteorology》1990,42(5):481-497
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Chlorite and associated minerals from the volcanogenic Taveyanne metasediment of the western Helvetic nappes, Switzerland, were investigated by electron microprobe (EMP) and transmission electron microscopy (TEM) in order to determine their textural and chemical evolution during low-temperature metamorphism. EMP analyses of chloritic material from sub-greenschist facies outcrops show a decrease of Si and Σ(Ca, Na, K) with increasing metamorphic grade. A number of conclusions may be drawn from combined TEM images and analytical electron microscopy (AEM) data. 1 In diagenetic-grade samples, chlorite crystals (observed maximum defect-free distance=80 nm) always contain some 1 nm layers (with a maximum of 29% of all layers) and less frequently some 0.7 nm berthierine-like layers. With increasing grade, the amounts of 1 and 0.7 nm layers decrease, and most chlorite from the epizone is structurally pure or contains less than 2% of 1 nm layers. 2 A positive correlation was found between the amount of 1 nm layers and the Ca+K+Na content, indicating that the 1 nm layers are saponite. 3 Observations and calculations suggest that the transformation reaction of saponite to chlorite takes place by the replacement of the interlayer cations in saponite by brucite-like layers resulting in a local volume decrease. In contrast, the destruction of berthierine has only minor influence on the local bulk volume. These results confirm recent studies which show that the change in composition measured by EMP of diagenetic-grade chloritic material are mainly the result of mixtures of chlorite and saponite. The use of chlorite ‘geothermometry’ in such systems is greatly influenced by the presence of saponite and hence is not based on reaction equilibria, even though temperatures calculated in this study agree with temperatures derived from other methods. Therefore, chlorite evolution should be treated as a kinetically controlled grade indicator and developed as a qualitative scale similar to the illite crystallinity index. 相似文献