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991.
Ordering and exsolution processes in Or-rich alkali feldspar megacrysts from the Eldzhurtinskiy granite (Caucasus) 总被引:3,自引:0,他引:3
G. Witt-Eickschen C. Evangelakakis H. A. Seck H. Kroll A. G. Gurbanov 《Contributions to Mineralogy and Petrology》1996,124(1):71-81
The extremely young (2.5 Ma) I-type Eldzhurtinskiy granite complex (Central Caucasus) is uniform with respect to modal composition,
major and trace element chemistries of bulk rocks and mineral phases. In contrast, it reveals two types of alkali feldspar
megacrysts differing in tetrahedral Al-content (2t1) and exsolution microtextures:
1. Alkali feldspar megacrysts (Or70An2Ab28) from the top of the body consist of ideally coherent intergrowths of fine-scale regular Or- and Ab-rich lamellae. The exsolved
K-feldspar host is monoclinic (2t1=0.7), the exsolved Na-rich phase consists of Albite- and/or Pericline-twinned albite.
2. Megacrysts from greater depths have the same bulk composition, but the exsolved Ab-rich phase occurs in the form of optically
visible, broad lamellae and patches of low albite. In addition, the K-rich host yields a higher degree of (Al, Si) ordering
(2t1=0.8). The evolution of the distinct types of megacrysts reflects differences in the cooling history within the upper and
lower part of the granite body. The occurrence of the coherent lamellae in the megacrysts from the top of the body is attributed
to exsolution under dry conditions during fast cooling, whereas coarsening of lamellae and formation of albite patches in
the megacrysts from the lower part are caused by fluid-feldspar interaction. The transition zone in the body between the two
types of megacrysts is sharp (in a depth interval of 100–200 m) and not related to shear zones.
Received: 12 June 1995 / Accepted: 29 January 1996 相似文献
992.
The first occurrence of platinum group minerals (PGM) in a chromite deposit in the Eastern Desert, Egypt 总被引:1,自引:0,他引:1
M. A. Elhaddad 《Mineralium Deposita》1996,31(5):439-445
The platinum-group mineralogy (PGM) of the chromitite from Gebel Lawi, in the southeastern desert has been investigated.
The most abundant base metal sulfides (BMS) associated with the Lawi chromite are pentlandite, millerite and heazlewoodite.
The major platinum-group minerals identified were as follows: laurite (IrOsRu)S2, osmian iridium (OsIr), hollingworthite (RhAsS),
tellurian arsenopalladinite (PdTeSbAs), potarite (PdHg) besides cuprian palladian gold (CuPdAu), a Pd-Sb-Hg and HgTe phases.
Laurite and osmian iridium occur preferentially in chromite. Os-Ir commonly forms composite PGM with laurite. Hollingworthite
and tellurian arsenopalladinite are included within serpentine and, close to the base-metal sulfides, the cuprian palladian
gold shares boundaries with chromite. Potarite together with the Pd-Sb-Hg and HgTe phases are embedded in serpentine. Palladium
is the most abundant PGE in the Gebel Lawi chromite.
A paragenetic sequence of PGM formation is described. Textural evidence indicates that Os-, Ir- and Ru-bearing PGM formed
early and were followed by Rh- and Pd-bearing PGM. The concentration of all five PGE could be magmatic, but much of the PGE
mineralogy except for laurite and osmian iridium in the center of chromite grains, has been modified by subsequent processes.
At later stages, the environment became Te-, Sb-, As- and Hg-rich, which finally led to the formation of low-temperature alteration
minerals.
Received: 24 April 1995 / Accepted: 28 March 1996 相似文献
993.
Chr Samtleben A. Munnecke T. Bickert J. Pätzold 《International Journal of Earth Sciences》1996,85(2):278-292
The Silurian of Gotland, Sweden, consists of 440 m of carbonate deposits. Repeatedly, uniform sequences of micritic limestones
and marls are interrupted by complex-structured reefs and by adjacent platform sediments. Generally, the alteration of facies
is interpreted as the result of sea-level fluctuations caused by a gradual regression with superimposed minor transgressive
pulses. The purpose of this study is a facies interpretation based on both field observations and stable isotope measurements
of brachiopod shells. Approximately 700 samples from stratigraphically arranged localities in different facies areas have
been investigated. The carbon and oxygen isotopes show principally parallel curves and a close relationship to the stratigraphic
sequence. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods
dominated by reefs and extended carbonate platforms. The oxygen isotope ratios are interpreted to reflect paleosalinity changes
due to varying freshwater input, rather than to paleotemperature. Carbon isotope ratios are believed to have been connected
to global changes in the burial of organic carbon in black shales during periods of euxinic deep water conditions. Consequently,
the facies succession on Gotland results from global paleoclimatic conditions. Changes in terrigenous input due to different
rates of weathering and freshwater runoff, rather than sea-level fluctuations, control the carbonate formation of the Silurian
on Gotland. 相似文献
994.
Trace element and isotope geochemistry of depleted peridotites from an N-MORB type ophiolite (Internal Liguride,N. Italy) 总被引:6,自引:0,他引:6
E. Rampone A. W. Hofmann G. B. Piccardo R. Vannucci P. Bottazzi L. Ottolini 《Contributions to Mineralogy and Petrology》1996,123(1):61-76
Mantle peridotites of the Internal Liguride (IL) units (Northern Apennines) constitute a rare example of the depleted lithosphere
of the Jurassic Ligurian Tethys. Detailed chemical (ICP-MS and SIMS techniques) and isotopic investigations on very fresh
samples have been performed with the major aim to constrain the timing and mechanism of their evolution and to furnish new
data for the geodynamic interpretation. The data are also useful to discuss some general geochemical aspects of oceanic-type
mantle. The studied samples consist of clinopyroxene-poor spinel lherzolites, showing incipient re-equilibration in the plagioclase-facies
stability field. The spinel-facies assemblage records high (asthenospheric) equilibration temperatures (1150–1250° C). Whole
rocks, and constituent clinopyroxenes, show a decoupling between severe depletion in highly incompatible elements [light rare
earth elements (LREE), Sr, Zr, Na, Ti] and less pronounced depletion in moderate incompatible elements (Ca, Al, Sc, V). Bulk
rocks also display a relatively strong M(middle)REE/H(heavy)REE fractionation. These compositional features indicate low-degree
(<10%) fractional melting, which presumably started in the garnet stability field, as the most suitable depletion mechanism.
In this respect, the IL ultramafics show strong similarity to abyssal peridotites. The Sr and Nd isotopic compositions, determined
on carefully handpicked clinopyroxene separates, indicate an extremely depleted signature (87Sr/86Sr=0.702203–0.702285; 143Nd/144Nd=0.513619–0.513775). The Sm/Nd model ages suggest that the IL peridotites melted most likely during Permian times. They
could record, therefore, the early upwelling and melting of mid ocean ridge basalt (MORB) type asthenosphere, in response
to the onset of extensional mechanisms which led to the opening of the Western Tethys. They subsequently cooled and experienced
a composite subsolidus evolution testified by multiple episodes of gabbroic intrusions and HT-LP retrograde metamorphic re-equilibration, prior to their emplacement on the sea floor. The trace element chemistry of IL peridotites
also provides useful information about the composition of oceanic-type mantle. The most important feature concerns the occurrence
of Sr and Zr negative anomalies (relative to “adjacent” REE) in both clinopyroxenes and bulk rocks. We suggest that such anomalies
reflect changes in the relative magnitude of Sr, Zr and REE partition coefficients, depending on the specific melting conditions.
Received: 15 February 1995/Accepted: 4 August 1995 相似文献
995.
Experimental determination of activities in FeTiO3-MnTiO3 ilmenite solid solution by redox reversals
Solid solutions of (Fe,Mn)TiO3 were synthesized, mostly at 0.10 XMn intervals, at 1 bar, 900°C and log f
O
2 = –17.50. Analysis by EMP indicate an ideal stoichiometry for the Fe-Mn ilmenites with (Fe+Mn) = Ti = 1.000 when normalized
to 3 oxygens. Their unit cell volume increases linearly with XMn. The composition of Fe-Mn ilmenite coexisting with metallic Fe and rutile was reversed at 1 bar, 700–900°C and fixed f
O
2 in a gas-mixing furnace. Oxygen fugacity was controlled by mixing CO2 and H2 gas and was continuously monitored with an yttrium-stabilized zirconia electrolyte. Solution properties of Fe-Mn ilmenite
were derived from the experimental data by mathematical programming (Engi and Feenstra, in preparation) including notably
the results of Fe-Mn exchange experiments between ilmenite and garnet (Feenstra and Engi, submitted) and anchoring the standard
state properties to the updated thermodynamic dataset of Berman and Aranovich (1996). The thermodynamic analysis resulted
in positive deviations from ideality for (Fe,Mn)TiO3 ilmenite, which is well described by an asymmetric Margules model with WH
FeFeMn = –9.703 and WH
FeMnMn = –23.234 kJ/mol, WS
FeFeMn = –19.65 and WS
FeMnMn = –22.06 J/(K·mol). The excess free energy for Fe-Mn ilmenite derived from the redox reversals is larger than in the symmetric
ilmenite model (WG
FeMn = +2.2 kJ/mol) determined by O'Neill et al. from emf measurements on the assemblage iron-rutile-(Fe,Mn)ilmenite.
Received: 10 January 1996 / Accepted: 11 July 1996 相似文献
996.
997.
Polycyclic aromatic hydrocarbons (PAH), important constituents of petroleum products, are known to enter freshwater systems via industrial effluents. In the present study the possible toxicity of automobile effluent presumably containing PAH was demonstrated in the freshwater prawn Macrobrachium lamarrei. The prawns were exposed for 1, 8, 15 and 28 days to two sublethal concentrations (0.75% and 3.75%) of automobile effluent. A decrease was noted in the total protein content in haemolymph and hepatopancreas of prawns exposed for 28 days to both these sublethal concentrations. SDS-PAGE revealed that the effluent caused reduction in high molecular weight proteins (106-90 kD) of the haemolymph. The exposure to automobile effluent resulted in changes in the lamellar structure of the gills and shrinkage of hepatopancreatic cell types. Thus it is imperative to discard such wastes carefully ensuring that there is no entry into aquatic ecosystems.author for correspondence 相似文献
998.
Luis E. Vallejo 《Engineering Geology》1997,48(3-4):159-160
999.
N. V. Sobolev F. V. Kaminsky W. L. Griffin E. S. Yefimova T. T. Win C. G. Ryan A. I. Botkunov 《Lithos》1997,39(3-4):135-157
The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO2 content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m2 conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle. 相似文献
1000.
Understanding the formation of laminated, organic-rich sediments is an essential topic for researchers interested in fossil fuels, biogeochemical cycles, Earth's environmental history and global change. Biologists have very recently demonstrated that some marine phytoplankton blooms actively govern their own sedimentation by the formation of sticky transparent gels that facilitate rapid aggregation, accelerated sinking and efficient export flux. Here we present fossil evidence of unfragmented, low-diversity phytoplankton assemblages preserved as sedimentary laminae and irregular flocs that are attributable to a similar phytoplankton-driven sedimentary mechanism we term ‘self-sedimentation’. The geological evidence suggests that self-sedimentation precludes significant heterotrophic grazing, propels the formation of some conspicuous hemipelagic sedimentary laminae and results in efficient carbon and opal flux to the sediments. We suggest that the self-sedimentation phenomenon may have broad implications for the geological history of biogeochemical cycling, oceanic ecological dynamics, and abrupt atmospheric/environmental change. Broader recognition of the self-sedimentation phenomenon as explicitly defined here is a prerequisite to testing these unconventional hypotheses. 相似文献