--> ABSTRACT: Single Grain Age Provenance with the <SUP>40</SUP>Ar/<SUP>39</SUP>Ar Laser Probe, by Paul R. Renne, Tim A. Becker; #91003 (1990).

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ABSTRACT: Single Grain Age Provenance with the 40Ar/39Ar Laser Probe

Paul R. Renne, Tim A. Becker

A 40Ar/39Ar laser probe system is capable of dating individual crystals of sand-size potassic mineral grains (e.g., feldspars, micas, hornblende) as young as Quaternary in age with high precision. Step-heating with the laser allows assessment of possible argon loss or excess argon, as with conventional bulk 40Ar/39Ar analysis. Thus the argon retention ages of detrital minerals can be used as criteria for inferring the specific provenance of sandstones. In addition, single crystal argon retention ages can provide a wealth of information about both the predepositional and postdepositional histories of detrital grains, including clarification of source region tectonics, sediment transport, and diagenetic processes. An example is gi en from the middle Eocene arkosic Montgomery Creek Formation of northern California, whose provenance was previously ambiguous. Laser probe dating of more than 100 grains of biotite and muscovite indicate derivation from the Idaho batholith region rather than the adjacent Klamath Mountains or nearby Sierra Nevada. Rapid unroofing, erosion, and sedimentation of muscovite grains dated as young as 47 Ma require minimum uplift rates of about 2 mm/yr for the Idaho Batholith. The age distribution of Montgomery Creek muscovite grains is consistent with a single fluviatile system transporting these as well as sediments bound for the much larger Tyee basin of the Oregon Coast Ranges. Biotite grains are variably vermiculitized, and the age of this postdepositional alteration inferred from analysis of composite grains is between 4 and 10 Ma.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990