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991.
N. M. Sushchevskaya A. N. Evdokimov B. V. Belyatsky V. A. Maslov D. V. Kuz’min 《Geochemistry International》2008,46(1):1-16
Petrological and geochemical data obtained on the Quaternary lavas of volcanoes at Spitsbergen Island indicate that the rocks were produced via the deep-seated crystallization of parental alkaline magmas at 8–10 kbar. The character of clinopyroxene enrichment in incompatible elements indicates that the mineral crystallized from more enriched melts than those inferred from the composition of the host lavas. These melts were close to the parental melts previously found as veinlets in mantle hyperbasite xenoliths in the lavas. According to the character of their enrichment in Pb and Sr radiogenic isotopes and depletion in Nd, the basalts from Spitsbergen Island define a single trend with the weakly enriched tholeiites of the Knipovich Ridge, a fact suggesting the closeness of the enriched sources beneath the continental margin of Spitsbergen and beneath the spreading zone. Magmatic activity at Spitsbergen was related to the evolution of the Norwegian-Greenland basin, which evolved in pulses according to the shift of the spreading axes. The most significant of the latter events took place in the Neogene, when the Knipovich Ridge obtained its modern position near the western boundary of Spitsbergen. Early in the course of the evolution, the emplacement of alkaline melts generated at Spitsbergen into the oceanic mantle could form the enriched mantle, which was later involved in the melting process beneath the spreading zone. 相似文献
992.
Wenjun Yong E. Dachs A. C. Withers E. J. Essene 《Contributions to Mineralogy and Petrology》2008,155(2):137-146
The low-temperature heat capacity (C
p) of Si-wadeite (K2Si4O9) synthesized with a piston cylinder device was measured over the range of 5–303 K using the heat capacity option of a physical
properties measurement system. The entropy of Si-wadeite at standard temperature and pressure calculated from the measured
heat capacity data is 253.8 ± 0.6 J mol−1 K−1, which is considerably larger than some of the previous estimated values. The calculated phase transition boundaries in the
system K2O–Al2O3–SiO2 are generally consistent with previous experimental results. Together with our calculated phase boundaries, seven multi-anvil
experiments at 1,400 K and 6.0–7.7 GPa suggest that no equilibrium stability field of kalsilite + coesite intervenes between
the stability field of sanidine and that of coesite + kyanite + Si-wadeite, in contrast to previous predictions. First-order
approximations were undertaken to calculate the phase diagram in the system K2Si4O9 at lower pressure and temperature. Large discrepancies were shown between the calculated diagram compared with previously
published versions, suggesting that further experimental or/and calorimetric work is needed to better constrain the low-pressure
phase relations of the K2Si4O9 polymorphs.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
993.
Jürgen Konzett Daniel J. Frost Alexander Proyer Peter Ulmer 《Contributions to Mineralogy and Petrology》2008,155(2):215-228
Experiments have been conducted in the P-T range 2.5–15 GPa and 850–1,500°C using bulk compositions in the systems SiO2–TiO2–Al2O3–Fe2O3–FeO–MnO–MgO–CaO–Na2O–K2O–P2O5 and SiO2–TiO2–Al2O3–MgO–CaO–Na2O to investigate the Ca-Eskola (CaEs Ca0.5□0.5AlSi2O6) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO2 ± TiO2 ± kyanite as a function of P, T, and bulk composition. The results show that CaEsss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative
anorthite and low diopside contents. In this study, a maximum of 18 mol% CaEsss was found at 6 GPa and 1,350°C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison,
no significant increase in CaEsss with increasing P could be observed. If the formation of oriented SiO2-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these
textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites
cooled from temperatures ≥750°C. The presence of clinopyroxene with approx. 4 mol% CaEsss in an experiment conducted at 2.5 GPa/850°C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes
are not necessarily restricted to ultrahigh pressure metamorphic conditions.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
994.
Li-Sr-Nd isotope signatures of the plume and cratonic lithospheric mantle beneath the margin of the rifted Tanzanian craton (Labait) 总被引:3,自引:2,他引:1
Sonja Aulbach Roberta L. Rudnick William F. McDonough 《Contributions to Mineralogy and Petrology》2008,155(1):79-92
Lithium concentrations and isotopic compositions of olivine and 87Sr/86Sr and 143Nd/144Nd of coexisting clinopyroxene from peridotite xenoliths from the Quaternary Labait volcano, Tanzania, document the influence
of rift-related metasomatism on the ancient cratonic mantle. Olivines show negative correlations between Fo content and both
δ7Li and Li concentrations. Olivines in iron-rich peridotites (Fo85–87) have high Li concentrations (3.2–4.8 ppm) and heavy δ7Li (+5.2 to +6.6). In contrast, olivines in ancient, refractory peridotites have lower Li concentrations (∼2 ppm) and relatively
light δ7Li (+2.6 to +3.5). This reflects mixing between ancient, refractory cratonic lithosphere and asthenosphere-derived rift magmas.
A uniquely fertile, deformed, high-temperature garnet lherzolite, interpreted to be from the base of the lithosphere, has
a 87Sr/86Sr of 0.7029 and 143Nd/144Nd of 0.51286, similar to HIMU oceanic basalts. It provides the best estimate of the Sr–Nd isotope composition of the upwelling
mantle (i.e., plume, sensu lato) underlying this portion of the East African Rift, and is slightly less radiogenic compared
to previous estimates of the plume that were based on rift basalts. Although elevated δ7Li are not exclusive to HIMU source regions, the data collectively indicate that the plume beneath Labait has HIMU characteristics
in Sr, Nd and Li isotope composition.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
995.
Anthony C. Withers Marc M. Hirschmann 《Contributions to Mineralogy and Petrology》2008,156(5):595-605
Olivine crystals were grown in the presence of a hydrous silicate fluid during multi-anvil experiments at 8 GPa and 1,000–1,600°C.
Experiments were conducted both in a simple system (FeO–MgO–SiO2–H2O) and in a more complex system containing additional elements (CaO–Na2O–Al2O3–Cr2O3–TiO2–FeO–MgO–SiO2–H2O). Silica activity was buffered by the presence of either pyroxene (high a
SiO2) or ferropericlase (low a
SiO2), and was buffered by the presence of Ni + NiO or Fe + FeO, or constrained by the presence of Fe2O3. Raman spectroscopy was used to identify pyroxene polymorphs in the run products. Clinoenstatite was present in the 1,000°C
experiment, and enstatite in experiments at 1,400–1,520°C. The H2O content of olivine was measured using secondary ion mass spectroscopy, and infrared spectroscopy was used to investigate
the nature of hydrous defects. The H2O storage capacity of olivine decreases with increasing temperature at 8 GPa. In contrast to previous experimental results
at ≤2 GPa, no significant effect of varying oxygen fugacity is evident, but H2O storage capacity is enhanced under conditions of low silica activity. No significant growth of low wavenumber (<3,400 cm−1) peaks, generally associated with high at low pressure, was observed in the FTIR spectra of olivine from the high experiments. Our experiments show that previous high pressure H2O storage capacity measurements for olivine synthesized under more oxidizing conditions than the Earth’s mantle are not likely
to be compromised by the of the experiments. However, the considerable effect of temperature on H2O storage capacity in olivine must be taken into account to avoid overestimation of the bulk upper mantle H2O storage capacity. 相似文献
996.
R. B. Trumbull M.-S. Krienitz B. Gottesmann M. Wiedenbeck 《Contributions to Mineralogy and Petrology》2008,155(1):1-18
Tourmaline is widespread in metapelites and pegmatites from the Neoproterozoic Damara Belt, which form the basement and potential
source rocks of the Cretaceous Erongo granite. This study traces the B-isotope variations in tourmalines from the basement,
from the Erongo granite and from its hydrothermal stage. Tourmalines from the basement are alkali-deficient schorl-dravites,
with B-isotope ratios typical for continental crust (δ11B average −8.4‰ ± 1.4, n = 11; one sample at −13‰, n = 2). Virtually all tourmaline in the Erongo granite occurs in distinctive tourmaline-quartz orbicules. This “main-stage”
tourmaline is alkali-deficient schorl (20–30% X-site vacancy, Fe/(Fe + Mg) 0.8–1), with uniform B-isotope compositions (δ11B −8.7‰ ± 1.5, n = 49) that are indistinguishable from the basement average, suggesting that boron was derived from anatexis of the local
basement rocks with no significant shift in isotopic composition. Secondary, hydrothermal tourmaline in the granite has a
bimodal B-isotope distribution with one peak at about −9‰, like the main-stage tourmaline, and a second at −2‰. We propose
that the tourmaline-rich orbicules formed late in the crystallization history from an immiscible Na–B–Fe-rich hydrous melt.
The massive precipitation of orbicular tourmaline nearly exhausted the melt in boron and the shift of δ11B to −2‰ in secondary tourmaline can be explained by Rayleigh fractionation after about 90% B-depletion in the residual fluid.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
997.
Hong-Fu Zhang Steven L. Goldstein Xin-Hua Zhou Min Sun Jian-Ping Zheng Yue Cai 《Contributions to Mineralogy and Petrology》2008,155(3):271-293
The ages of subcontinental lithospheric mantle beneath the North China and South China cratons are less well-constrained than
the overlying crust. We report Re–Os isotope systematics of mantle xenoliths entrained in Paleozoic kimberlites and Mesozoic
basalts from eastern China. Peridotite xenoliths from the Fuxian and Mengyin Paleozoic diamondiferous kimberlites in the North
China Craton give Archean Re depletion ages of 2.6–3.2 Ga and melt depletion ages of 2.9–3.4 Ga. No obvious differences in
Re and Os abundances, Os isotopic ratios and model ages are observed between spinel-facies and garnet-facies peridotites from
both kimberlite localities. The Re–Os isotopic data, together with the PGE concentrations, demonstrate that beneath the Archean
continental crust of the eastern North China Craton, Archean lithospheric mantle of spinel- to diamond-facies existed without
apparent compositional stratification during the Paleozoic. The Mesozoic and Cenozoic basalt-borne peridotite and pyroxenite
xenoliths, on the other hand, show geochemical features indicating metasomatic enrichment, along with a large range of the
Re–Os isotopic model ages from Proterozoic to Phanerozoic. These features indicate that lithospheric transformation or refertilization
through melt-peridotite interaction could be the primary mechanism for compositional changes during the Phanerozoic, rather
than delamination or thermal-mechanical erosion, despite the potential of these latter processes to play an important role
for the loss of garnet-facies mantle. A fresh garnet lherzolite xenolith from the Yangtze Block has a Re depletion age of
∼1.04 Ga, much younger than overlying Archean crustal rocks but the same Re depletion ages as spinel lherzolite xenoliths
from adjacent Mesozoic basalts, indicating Neoproterozoic resetting of the Re–Os system in the South China Craton. 相似文献
998.
Spatial variations in grain-size parameters (i.e. grain-size trends) contain information on sediment transport patterns. Analytical procedures have been proposed using the grain-size trend to determine net sediment transport pathways. In the first part of this study, the fundamentals of the theory are presented through methods for analysing 1D and 2D variations. The methods used are critically discussed, while pointing out some severe problems. So far, these methods suffer from limitations leading to serious interpretational errors, making it necessary to take account of two kinds of uncertainties. Inputs uncertainties are linked to the physical sediment properties as well as procedures of sampling and analysis. Model uncertainties are then discussed for each step of the grain-size trend analysis. The validity of Sediment Trend Analysis under natural conditions is tested against published field studies to determine the most appropriate variation trend to use in a specific environment. Proposals are given for each step of the procedure for optimal use of the method using a Quality Assurance (QA) approach. Further developments are proposed, such as integration into a Geographic Information System. 相似文献
999.
We present the software program THERIA_G, which allows for numerical simulation of garnet growth in a given volume of rock
along any pressure–temperature–time (P–T–t) path. THERIA_G assumes thermodynamic equilibrium between the garnet rim and the rock matrix during growth and accounts for
component fractionation associated with garnet formation as well as for intracrystalline diffusion within garnet. In addition,
THERIA_G keeps track of changes in the equilibrium phase relations, which occur during garnet growth along the specified P–T–t trajectory. This is accomplished by the combination of two major modules: a Gibbs free energy minimization routine is used
to calculate equilibrium phase relations including the volume and composition of successive garnet growth increments as P and T and the effective bulk rock composition change. With the second module intragranular multi-component diffusion is modelled
for spherical garnet geometry. THERIA_G allows to simulate the formation of an entire garnet population, the nucleation and
growth history of which is specified via the garnet crystal size frequency distribution. Garnet growth simulations with THERIA_G
produce compositional profiles for the garnet porphyroblasts of each size class of a population and full information on equilibrium
phase assemblages for any point along the specified P–T–t trajectory. The results of garnet growth simulation can be used to infer the P–T–t path of metamorphism from the chemical zoning of garnet porphyroblasts. With a hypothetical example of garnet growth in a
pelitic rock we demonstrate that it is essential for the interpretation of the chemical zoning of garnet to account for the
combined effects of the thermodynamic conditions of garnet growth, the nucleation history and intracrystalline diffusion.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
相似文献
F. GaidiesEmail: |
1000.
Mauro Lo Cascio Yan Liang Nobumichi Shimizu Paul C. Hess 《Contributions to Mineralogy and Petrology》2008,156(1):87-102
The grain-scale processes of peridotite melting were examined at 1,340°C and 1.5 GPa using reaction couples formed by juxtaposing
pre-synthesized clinopyroxenite against pre-synthesized orthopyroxenite or harzburgite in graphite and platinum-lined molybdenum
capsules. Reaction between the clinopyroxene and orthopyroxene-rich aggregates produces a melt-enriched, orthopyroxene-free,
olivine + clinopyroxene reactive boundary layer. Major and trace element abundance in clinopyroxene vary systematically across
the reactive boundary layer with compositional trends similar to the published clinopyroxene core-to-rim compositional variations
in the bulk lherzolite partial melting studies conducted at similar P–T conditions. The growth of the reactive boundary layer takes place at the expense of the orthopyroxenite or harzburgite and
is consistent with grain-scale processes that involve dissolution, precipitation, reprecipitation, and diffusive exchange
between the interstitial melt and surrounding crystals. An important consequence of dissolution–reprecipitation during crystal-melt
interaction is the dramatic decrease in diffusive reequilibration time between coexisting minerals and melt. This effect is
especially important for high charged, slow diffusing cations during peridotite melting and melt-rock reaction. Apparent clinopyroxene-melt
partition coefficients for REE, Sr, Y, Ti, and Zr, measured from reprecipitated clinopyroxene and coexisting melt in the reactive
boundary layer, approach their equilibrium values reported in the literature. Disequilibrium melting models based on volume
diffusion in solid limited mechanism are likely to significantly underestimate the rates at which major and trace elements
in residual minerals reequilibrate with their surrounding melt.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献