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11.
G. R. Byerly E. Jarosewich B. Mason R. S. Clarke 《Meteoritics & planetary science》1988,23(4):359-360
Abstract— A 664 gm stone fell into suburban Baton Rouge in November 1987. The stone has a nearly complete fusion crust and was undamaged upon impact. The ordinary chondrite contains equilibrated olivine, Fa28, and largely equilibrated bronzite, Fs23. Some bronzite is still compositionally zoned and polysynthetically twinned, thus requiring petrologic classification of the meteorite as a type 4. A major element analysis of the meteorite falls exclusively into the field for the LL compositional group. 相似文献
12.
Xiaogang Xie Gary R. Byerly Ray E. Ferrell Jr. 《Contributions to Mineralogy and Petrology》1997,126(3):275-291
IIb trioctahedral chlorite in the Barberton greenstone belt (BGB) metavolcanic rocks was formed during pervasive greenschist
metamorphism. The chem‐ical composition of the chlorite is highly variable, with the Fe/(Fe+Mg) ratio ranging from 0.12 to
0.8 among 53 samples. The chemical variation of the chlorite results from the chemical diversity of the host rock, especially
the MgO content of the rock, but major details of the variation pattern of the chlorite are due to the crystal structure of
the chlorite. All major cation abundances in the chlorite are strongly correlated with each other. Sil‐icon increases with
Mg and decreases with Fe, while AlIV and AlVI decrease with Mg and increase with Fe2+. A complex exchange vector explains over 90% of the chlorite compositional variation: Mg4SiFe2+
−3AlVI
−1 AlIV
−1, which has 3 parts Fe-Mg substitution coupled with one part tschermakite substitution. This ratio is required to maintain
the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets. The subtle change in Al
substitution in chlorite implies that AlVI is preferentially ordered in the M(4) site, and about 84% of the AlVI present is in the M(4) sites when they are nearly filled with AlVI. Based on 47 analyzed chlorite-bearing rock samples, chlorite (Chl) composition is strongly correlated with the MgO content
of the host rock. Calculated correlation coefficients are +0.91 for SiO2Chl-MgORock, −0.87 for Al2O3Chl-MgORock, +0.89 for MgOChl-MgORock, and −0.85 for FeOChl-MgORock. Only weak correlations have been found between chlorite oxides and other oxides of rock (between same oxides in chlorite
and rock: SiO2−0.67, Al2O3 + 0.59, FeO −0.41). However, MgOChl is saturated at about 36 wt% in rocks that have MgO above 22 wt%.The MgOChl is about 5 wt% when the host rock approaches 0 wt% of MgO. This implies that Mg substituting into the chlorite is approximately
limited to 1.5–9.2 Mg atoms per formula unit and 1.0–3.2 AlIV. Chlorite geothermometers can not be applied to all BGB samples. However, the empirical chlorite geothermometer based on
AlIV of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition
of the chlorite from basaltic and dacitic samples in this study. An estimated temperature of about 320°C for the greenschist
metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers.
Received: 22 January 1996 / Accepted: 15 August 1996 相似文献
13.
We report precise 207Pb/206Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 +/- 3 and 3416 +/- 5 Ma and contain older crustal components represented by a 3504 +/- 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 +/- 5 and 3225 +/- 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from 3323 +/- 4 to 3522 +/- 4 Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma. coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma. exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times. 相似文献
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Abstract— Glassy microspherules recently reported from the late Eocene of central Mississippi are physically and chemically similar to impact-produced microspherules, especially those from Molino de Cobo, Spain, leading several investigators to conclude that these samples document another late Eocene impact event. A careful evaluation of the sample site and spherule compositions, however, suggests that these glasses are modern contaminants. Similar contaminants could be widespread throughout industrialized countries. 相似文献