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Microprobe analysis and dating of monazite from the Potsdam Formation, New York; a progressive record of chemical reaction and fluid interaction

Julien Allaz, Bruce Selleck, Michael L. Williams and Michael J. Jercinovic
Microprobe analysis and dating of monazite from the Potsdam Formation, New York; a progressive record of chemical reaction and fluid interaction
American Mineralogist (July 2013) 98 (7): 1106-1119

Abstract

It has been recognized for several decades that REE-phosphates (monazite and xenotime) can grow during diagenesis and low-grade metamorphism. Growth of REE-bearing accessory phases at low-grade conditions commonly involves pervasive fluid-rock interaction, dissolution of detrital grains, transportation, and precipitation of REEs, typically facilitated by an increase in temperature. The occurrence of low-grade REE-phosphate offers a rare opportunity to date crystallization/mineralization and possibly fluid percolation. We report here the results of in situ dating by electron microprobe of Paleozoic authigenic and low-grade monazite and xenotime overgrowths on detrital monazite and zircon, respectively. Samples are from the Potsdam Formation, a basal sandstone deposited uncomformably on Proterozoic basement of the Adirondack Mountains of New York State. This study also focuses on the textural and chemical relationships of these REE-bearing accessory phases. Textures include rounded and fractured detrital monazite and zircon, which contrast with new sub-euhedral REE-phosphate overgrowths. Monazite overgrowths are enriched in LREE and depleted in HREE compared to detrital cores. The U and Th concentrations are low, typical of low-grade metamorphic conditions. Monazite core ages yield Proterozoic ages between 1.17 and 0.90 Ga (Shawinigan and Ottawan orogeny). Monazite overgrowth and xenotime ages indicate four to five major overgrowth events between ca. 500 Ma (shortly after the time of deposition) and ca. 200 Ma. As these ages are relatively young and the actinide content is low (Sigma < 2 wt%), the radiogenic Pb content of monazite overgrowths and xenotime is low (<400 ppm). Therefore, EPMA dates have relatively large uncertainties. Nevertheless, the ages determined broadly correlate with major Paleozoic orogenic events recorded in the Appalachian Orogen to the East (Taconic, Salinic, Acadian, Neo-Acadian, and Alleghanian). Fluid percolation, driven by orogenic loading, may induce dissolution of detrital monazite and zircon. Subsequent precipitation of new monazite and xenotime probably results from changes in fluids or metamorphic conditions. This study demonstrates the power of the EMPA technique to resolve the fluid-related growth history of REE-phosphates in low-grade metasediments.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 98
Serial Issue: 7
Title: Microprobe analysis and dating of monazite from the Potsdam Formation, New York; a progressive record of chemical reaction and fluid interaction
Affiliation: University of Massachusetts at Amherst, Department of Geosciences, Amherst, MA, United States
Pages: 1106-1119
Published: 201307
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 79
Accession Number: 2013-061383
Categories: Mineralogy of non-silicatesGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
N43°00'00" - N44°30'00", W75°00'00" - W73°30'00"
Secondary Affiliation: Colgate University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201337
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