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Title: Ar-Ar Thermochronology of Apollo 12033 Impact-Melt Breccia

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1349006
Report Number(s):
LLNL-PROC-717558
DOE Contract Number:
AC52-07NA27344
Resource Type:
Conference
Resource Relation:
Conference: Presented at: Lunar and Planetary Science Conference, Houston, TX, United States, Mar 20 - Mar 24, 2017
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; 58 GEOSCIENCES

Citation Formats

Crow, C A, Cassata, W S, Jolliff, B L, Zeigler, R A, Borg, L E, and Shearer, C K. Ar-Ar Thermochronology of Apollo 12033 Impact-Melt Breccia. United States: N. p., 2017. Web.
Crow, C A, Cassata, W S, Jolliff, B L, Zeigler, R A, Borg, L E, & Shearer, C K. Ar-Ar Thermochronology of Apollo 12033 Impact-Melt Breccia. United States.
Crow, C A, Cassata, W S, Jolliff, B L, Zeigler, R A, Borg, L E, and Shearer, C K. Tue . "Ar-Ar Thermochronology of Apollo 12033 Impact-Melt Breccia". United States. doi:. https://www.osti.gov/servlets/purl/1349006.
@article{osti_1349006,
title = {Ar-Ar Thermochronology of Apollo 12033 Impact-Melt Breccia},
author = {Crow, C A and Cassata, W S and Jolliff, B L and Zeigler, R A and Borg, L E and Shearer, C K},
abstractNote = {},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Jan 10 00:00:00 EST 2017},
month = {Tue Jan 10 00:00:00 EST 2017}
}

Conference:
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  • The authors have obtained 15 high resolution (21-51 step) {sup 40}Ar/{sup 39}Ar age spectra on six Apollo 15 impact melt rocks of different compositions using a continuous laser system on submilligram subsamples and on single crystal plagioclase clasts. Four of the six samples gave reproducible age spectra with well-defined intermediate temperature plateaus over 48% or more of the {sup 39}AR released; the plateaus are interpreted as crystallization ages. Samples 15304,7,69, 15294,6,21, and 15314,26,156 gave virtually identical plateau ages whose weighted mean is 3,870 {plus minus} 6 Ma. These three melt rocks differ in composition and likely formed in three separatemore » impact events. Sample 15356,9 gave replicate plateau ages that average 3,836 {plus minus} 12 Ma and date a fourth and younger impact event. The age spectra for samples 15308,9 and 15414,3,36 increase with increasing increment temperature and may have been formed in or affected by impacts at about 2,700 Ma and 3,870 Ma, respectively. So far there continues to be no convincing evidence in the lunar record for impact melts older than about 3.9 Ga.« less
  • The extensional Bitterroot mylonite zone defines the eastern and southern border of the Bitterroot metamorphic core complex and is generally interpreted to be the major structure which accommodated unroofing of the metamorphic core. The most commonly cited evidence for the age of mylonitization are [sup 40]Ar/[sup 39]Ar ages for hornblend, muscovite, biotite, and potassium feldspar from the southern Bitterroot mylonite zone that indicate rapid cooling of the core rocks between 45.5 and 43.5 Ma. More recently, an [sup 40]Ar/[sup 39]Ar K-feldspar age of 46.4 [+-] 0.8 Ma for an undeformed rhyolite dike that cuts across the mylonitic fabric places amore » minimum age constraint on the southern part of the shear zone. The authors have obtained new [sup 40]Ar/[sup 39]Ar data for metapelitic rocks and amphibolites from the northeast border of the Bitterroot metamorphic core complex near an area where mylonitized granitoid rocks yielding 48--52 Ma U-Pb zircon crystallization ages constrain the maximum age of mylonitization. Isochran ages of 47.9 [+-] 0.9 and 49 [+-] 1 Ma for hornblende separated from deformed amphibolite pods in the northeast border zone are within analytical uncertainty of the younger mylonitized granitoid crystallization ages and indicate rapid post-crystallization cooling through temperatures of [approximately]780--800 K. They attribute this cooling to denudation related to shear zone development. Muscovite and biotite isochron ages from metapelitic rocks within the shear zone are significantly younger, between 42 and 44 Ms., and generally agree with mica ages obtained by Garmezy and Sutter for the southern part of the shear zone. However, all mica ages from the Bitterroot shear zone are younger than the minimum age of the shear zone deduced from the age of cross-cutting rhyolite dikes.« less
  • /sup 40/Ar//sup 39/Ar age spectra analyses of detrital potassium feldspars from the North Sea demonstrate that the technique can be used to date the provenance of sandstones and to reconstruct the thermal history of sedimentary basins. Potassium feldspar separates from four Mesozoic deltaic sandstones from the southern Viking Graben all yield plateau ages that cluster around 400 Ma, suggesting derivation from nearby Caledonian-ages crystalline basement. Potassium feldspars from a Paleocene turbidite, also from the southern Viking Graben, yield a minimum age of Late Precambrian. Facies relationships suggest that the Paleocene turbidite was sourced from the Precambrian Shetland platform. Two samplesmore » of the Permian Rotliegende sandstone from the Sole Pit basin were also analyzed. Potassium feldspar separates from both samples yield complex age spectra indicating components from both Precambrian and Caledonian-aged crystalline rocks. The nearby London-Brabant Massif is a likely source of the Precambrian feldspars. Recent /sup 40/Ar rad loss (2 to 4%) was detected in one sample from the southern Viking Graben. Following the methods of Harrison and McDougall (1982), an activation energy of 36 kcal/mol was calculated. With this activation energy, the observed amount of Ar loss should have occurred in only 3 or 4 Ma at the present-day temperature of 115/sup 0/C. A more geologically reasonable temperature history that satisfies the /sup 40/Ar//sup 39/Ar systematics requires that the present-day temperature has been maintained for less than 2 Ma, and that the temperature rose above 90/sup 0/C within the past 5 Ma.« less
  • Lunar breccia 67975 is a feldspathic fragmental breccia from North Ray crater, Apollo 16. It contains clasts of alkali gabbronorite and ultra-KREEPy mafic fragment-laden melt breccias, which are unique among Apollo 16 samples. Both are alkali- and iron-rich rocks with moderate to high REE concentrations. They more strongly resemble Apollo 14 gabbronorites and alkali anorthosites and KREEP-rich rocks than they do other Apollo 16 samples. The other clasts in 67975 are the ferroan anorthosites, feldspathic melt rocks, and magnesian granulites, which are typical of other feldspathic fragmental breccias. Examination of bulk and mineral compositions of other breccias and melt rocksmore » suggests that alkali gabbronorite may be a minor component in other North Ray crater breccias and feldspathic melt rocks. This implies that alkali gabbronorite was a fairly early (4.0 b.y.) crustal component in the North Ray crater region.« less