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71.
S. N. Feldstein Rebecca A. Lange Torsten Vennemann James R. O'Neil 《Contributions to Mineralogy and Petrology》1996,126(1-2):51-66
Complete chemical analyses, including ferric and ferrous iron, H2O contents and δD values for 16 phlogopite and biotite and 2 hornblende separates are presented. Samples were obtained from
volcanic rocks from four localities: (1) phlogopite phenocrysts from minette lavas from the western Mexico continental arc,
(2) biotite and hornblende phenocrysts from andesite lavas from Mono Basin, California, (3) phlogopite and biotite from clinopyroxenite
nodules entrained in potassic lavas from the East African Rift, Uganda, and (4) phlogopite phenocrysts from a wyomingite lava
in the Leucite Hills, Wyoming. The Fe2O3 contents in the micas range from 0.8 to 10.5 wt%, corresponding to 0.09 to 1.15 Fe3+ per formula unit (pfu). Water contents vary from 1.6 to 3.0 wt%, corresponding to 1.58 to 3.04 OH pfu, significantly less
than would be expected for a site fully occupied by hydroxyl. Cation- and anion-based normalization procedures provide accurate
mineral formulae with respect to most cations and anions, but are unable to generate accurate estimates of Fe3+/FeT, and overestimate OH at the expense of O on the hydroxyl site. These inaccuracies are present despite acceptable adjusted
totals and stoichiometric calculated site occupancies. The phlogopite and biotite phenocrysts in arc-related lavas from western
Mexico and eastern California have the highest Fe3+/FeT ratios (56–87%), reflecting high magmatic oxygen fugacities (ΔNNO = +2 to +5), in contrast to those from Uganda (25–40%)
and the Leucite Hills (23%). There is no correlation between the OH content and the Fe3+/FeT ratio in the micas. Values of KMg/Fe2+D (± 2σ errors) were calculated for three phlogopite-olivine pairs (0.12 ± 0.12, 0.26 ± 0.14, 0.09 ± 0.12), two biotite-hornblende
pairs (0.73 ± 0.08 and 1.22 ± 0.10) and a single phlogopite-augite pair (1.15 ± 0.12). Values of KF/OHD for two biotite and
hornblende pairs could not be determined without significant error because of the extremely low F contents (< 0.2 wt%) of
the four phases. The δD values obtained in this study encompass a large range (−137 to −43‰). The phlogopite and biotite separates
from Uganda have δD values of −70 to −49‰, which overlap those believed to represent “primary” mantle. There is a larger range
in δD values (−137 to −43‰) for phlogopite phenocrysts from western Mexico minette lavas, although their range in δ18O values (5.2–6.2‰) is consistent with “normal” mantle. It is unlikely, therefore, that the variable δD values reflect heterogeneity
in the mantle source region of the minette magmas. Nor can the extremely low δD values reflect degassing of H2 or H2O since almost 100% loss of dissolved water in the magma is required, an unrealistic scenario given the stability of the hydrous
phenocrysts. The very low δD values of the Mascota minette phlogopites require that the hydrogen be introduced from an external
source (e.g., meteoric water). Whatever the process responsible for the observed hydrogen isotope composition, it had no effect
on the δ18O value, f
O
2, a
H
2O or bulk composition of the host magmas.
Received: 5 January 1995 / Accepted: 19 March 1996 相似文献
72.
Nd and Sr isotopic variations of Early Paleozoic oceans 总被引:4,自引:0,他引:4
We report143Nd/144Nd and87Sr/86Sr isotopic data for Lower Paleozoic phosphatic brachiopod and conodont fossils. The data appear to represent the isotopic values of Early Paleozoic seawaters. We show that different paleoceanic water masses can be distinguished on the basis of their εNd signatures. Two sides of what is classically considered one circulating Iapetus Ocean have different εNd signatures from at least the Middle Cambrian until the Late Middle Ordovician. We infer two ocean basins between North America and Baltica separated by an island and/or shoal circulation barrier. Thus, it appears necessary to redefine the area of the Iapetus Ocean. The εNd signature of the redefined smaller Iapetus Ocean ranges from −5 to −9 and the εNd signature of the larger, coeval Panthalassa Ocean, including part of what was formerly called the Iapetus Ocean, ranges from −10 to −20. The first time that the εNd values are identical in these two water masses is coincident with the onset of the Taconic Orogeny of North America. The paleogeographic geometry we infer from this work is consistent with paleogeographic reconstructions having the Baltica continent at very high latitudes in the Early/Middle Ordovician. The εNd and faunal distribution support temperature-controlled conodont faunal provinciality. A rough mean age for exposed continental crust in the Early Paleozoic can be obtained from the average εNd value of Early Paleozoic Oceans. The data suggest that the mean age of the crust as a function of time has remained essentially constant or even decreased during the past 500 Ma, and suggest substantial additions of new crust to the continents through the Phanerozoic. 相似文献
73.
Stephen R. Getty Jane Selverstone Brian P. Wernicke Stein B. Jacobsen Elaine Aliberti Daniel R. Lux 《Contributions to Mineralogy and Petrology》1993,115(1):45-57
Integrated petrologic and Sm–Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P–T studies indicate a single stage of metamorphism with final equilibration at 600°C and 8–9 kbar. The Sm–Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A40Ar/39Ar age for this hornblende indicates cooling through 525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm–Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent ages of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at 128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America. 相似文献
74.
Mads Faurschou Knudsen Bo Holm Jacobsen Niels Abrahamsen 《Physics of the Earth and Planetary Interiors》2003,135(1):55-73
A robust finite-element technique is presented for computation of both the internal demagnetization effects and magnetic terrain effects in bodies with arbitrary shape and arbitrary susceptibility distribution. This method facilitates a flexible analysis of the palaeomagnetic deflection problem. Tests on geologically realistic settings of highly magnetic rocks demonstrate that deflections of several degrees may occur even for relatively simple two-dimensional models. Similarly, the magnetic intensity may well be biased by 5-15% by demagnetization effects. The present paper focuses on deflections and intensity variations inside the magnetized body, where we find a systematic shallowing of inclination for bodies with a horizontal elongation. Because the bodies sampled at a typical palaeomagnetic site will have a dominant direction of elongation, the magnetic deflection effect will tend to impose a systematic bias which doesn’t average out. An inversion-based procedure for elimination of the deflection effect is presented. It requires that the magnetic body is quite homogeneous and that its surface geometry is known, as may be the case for historical lava flows. Tests demonstrate that in order to recover both ambient palaeofield direction and the effective susceptibility at blocking temperature it is necessary to sample near strong topographic elements in the magnetic body. Since the surface geometry rarely is known it is proposed as an alternative to inversion that an effective susceptibility is assessed and a horizontal slab correction is applied for samples taken far from topographical features. When shape geometry is unknown and no correction applied, palaeomagnetic conclusions must take into account the possible bias from internal demagnetization and magnetic terrain effects. 相似文献
75.
Effects of speciation on equilibrium fractionations and rates of oxygen isotope exchange between (PO4)aq and H2O 总被引:1,自引:0,他引:1
The effects of phosphate speciation on both rates of isotopic exchange and oxygen isotope equilibrium fractionation factors between aqueous phosphate and water were examined over the temperature range 70 to 180°C. Exchange between phosphate and water is much faster at low pH than at high pH, an observation that is similar to what has been observed in the analogous sulfate-water system. Oxygen isotope fractionations between protonated species like H3PO4 and H2PO4− that are dominant at relatively low pH and species like PO43− and ion pairs like KHPO4− that are dominant at relatively high pH, range between 5 and 8‰ at the temperatures of the experiments. In aqueous phosphate systems at equilibrium, 18O/16O ratios increase with increasing degree of protonation of phosphate. This effect can be explained in part by the relative magnitudes of the dissociation constants of the protonated species. Under equilibrium conditions, carbonate in solution or in solid phases concentrates 18O relative to orthophosphate in solution or in solid phases at all temperatures, supporting the traditional view that biogenic phosphate is precipitated in near oxygen isotope equilibrium with body/ambient aqueous fluids with no attendant vital effects. 相似文献
76.
Laug Andreas Haberzettl Torsten Pannes Andre Schwarz Anja Turner Falko Wang Junbo Engels Stefan Rigterink Sonja Börner Nicole Ahlborn Marieke Ju Jianting Schwalb Antje 《Journal of Paleolimnology》2021,66(3):171-186
Journal of Paleolimnology - The Tibetan Plateau, also known as the “Water Tower of Asia” because of its function as a water storage and supply region, responds dramatically to modern... 相似文献
77.
Torsten Henkel Thomas Stephan Elmar K. Jessberger Peter Hoppe Roger Strebel Sachiko Amari Roy S. Lewis 《Meteoritics & planetary science》2007,42(7-8):1121-1134
Abstract— Thirteen presolar silicon carbide grains—three of supernova (SN) origin and ten of asymptotic giant branch (AGB) star origin—were examined with time‐of‐flight‐secondary ion mass spectrometry (TOF‐SIMS). The grains had been extracted from two different meteorites—Murchison and Tieschitz—using different acid residue methods. At high lateral resolution of ~300 nm, isotopic and elemental heterogeneities within the micrometer‐sized grains were detected. The trace elemental abundances, when displayed in two‐element correlation plots, of Li, Mg, K, and Ca show a clear distinction between the two different meteoritic sources. The different concentrations might be attributed to differences of the host meteorites and/or of extraction methods whereas the stellar source seems to be less decisive. In one SN grain with 26Mg‐enrichment from extinct 26Al, the acid treatment, as part of the grain separation procedure, affected the Mg/Al ratio in the outer rim and therefore the inferred initial 26Al/27Al ratio. A second SN grain exhibits a lateral heterogeneity in 26Al/27Al, which either is due to residual Al‐rich contamination on the grain surface or to the condensation chemistry in the SN ejecta. 相似文献
78.
Crustal root beneath the highlands of southern Norway resolved by teleseismic receiver functions 总被引:1,自引:0,他引:1
L. Svenningsen N. Balling B. H. Jacobsen R. Kind K. Wylegalla J. Schweitzer 《Geophysical Journal International》2007,170(3):1129-1138
Teleseismic data have been collected with temporary seismograph stations on two profiles in southern Norway. Including the permanent arrays NORSAR and Hagfors the profiles are 400 and 500 km long and extend from the Atlantic coast across regions of high topography and the Oslo Rift. A total of 1071 teleseismic waveforms recorded by 24 temporary and 8 permanent stations are analysed. The depth-migrated receiver functions show a well-resolved Moho for both profiles with Moho depths that are generally accurate within ±2 km.
For the northern profile across Jotunheimen we obtain Moho depths between 32 and 43 km (below sea level). On the southern profile across Hardangervidda, the Moho depths range from 29 km at the Atlantic coast to 41 km below the highland plateau. Generally the depth of Moho is close to or above 40 km beneath areas of high mean topography (>1 km), whereas in the Oslo Rift the crust locally thins down to 32 km. At the east end of the profiles we observe a deepening Moho beneath low topography. Beneath the highlands the obtained Moho depths are 4–5 km deeper than previous estimates. Our results are supported by the fact that west of the Oslo Rift a deep Moho correlates very well with low Bouguer gravity which also correlates well with high mean topography.
The presented results reveal a ca . 10–12 km thick Airy-type crustal root beneath the highlands of southern Norway, which leaves little room for additional buoyancy-effects below Moho. These observations do not seem consistent with the mechanisms of substantial buoyancy presently suggested to explain a significant Cenozoic uplift widely believed to be the cause of the high topography in present-day southern Norway. 相似文献
For the northern profile across Jotunheimen we obtain Moho depths between 32 and 43 km (below sea level). On the southern profile across Hardangervidda, the Moho depths range from 29 km at the Atlantic coast to 41 km below the highland plateau. Generally the depth of Moho is close to or above 40 km beneath areas of high mean topography (>1 km), whereas in the Oslo Rift the crust locally thins down to 32 km. At the east end of the profiles we observe a deepening Moho beneath low topography. Beneath the highlands the obtained Moho depths are 4–5 km deeper than previous estimates. Our results are supported by the fact that west of the Oslo Rift a deep Moho correlates very well with low Bouguer gravity which also correlates well with high mean topography.
The presented results reveal a ca . 10–12 km thick Airy-type crustal root beneath the highlands of southern Norway, which leaves little room for additional buoyancy-effects below Moho. These observations do not seem consistent with the mechanisms of substantial buoyancy presently suggested to explain a significant Cenozoic uplift widely believed to be the cause of the high topography in present-day southern Norway. 相似文献
79.
B.N. Maehlum K. Måseide K. Aarsnes A. Egeland B. Grandal J. Holtet T.A. Jacobsen N.C. Maynard F. Søraas J. Stadsnes E.V. Thrane J. Trøim 《Planetary and Space Science》1980,28(3):259-278
A “mother-daughter” rocket was launched from Andøya, Norway, February 1 1976 over two auroral structures. The “daughter” payload carried a 10keV electron accelerator and the “mother” carried a series of diagnostic instruments for monitoring optical and wave effects generated through beam-atmospheric interactions and production of secondary electrons.The experimental details are presented in this paper together with a survey of some of the results. This paper is also intended as a reference for a series of accompanying papers. 相似文献
80.