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Daughter-parent isotope systematics in U-Th-bearing igneous accessory mineral aseemblages as potential indices of metamorphic history: A discussion of the concept

Silver, Leon T. (1991) Daughter-parent isotope systematics in U-Th-bearing igneous accessory mineral aseemblages as potential indices of metamorphic history: A discussion of the concept. In: Stable isotope geochemistry : a tribute to Samuel Epstein. Special Publication - Geochemical Society. No.3. Geochemical Society , San Antonio, TX, pp. 391-407. ISBN 0-941809-02-1.

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It is proposed that the patterns of isotopic disturbance in Pb-U-Th systems for the assemblage of radioactive minerals in any granite or equivalent orthogneiss may have value not only as geochronological tools for determining the time constants of metamorphic events but also may have significant potential as indices of the nature and intensity of metamorphism (including thermal, hydrothermal, deformational, weathering, and other environments of change). One of the special qualities of radiogenic Pb-U-Th geochemistry is the coexistence of three daughter-parent (D/P) systems: ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U, and ²⁰⁸Pb/²³²Th. The uranogenic pair of D/P systems are coupled chemically so that they lead to rigorously linked isotope ratio variations during disturbance. The thorogenic D/P system, because of the contrasting Th and U chemistry, may behave quite independently, providing special insights into the mobility of Pb, U, and Th. Accessory mineral assemblages in most rocks contain from three to eight or more radioactive minerals in which U and Th commonly are quite fractionated. Each mineral and its three contained D/P systems appear to have distinctive responses to metamorphism and show differential sensitivity to different metamorphic variables. The responses are conditioned by accumulated radiation damage at the time of metamorphism. Each mineral species has a unique structural response to influence, and commonly shows large inter- and intra-grain variations in accumulated disorder. The rate-competitive roles of structural annealing and metamorphic disturbance are believed to determine the net effect on the D/P systems. Any individual sample may be treated as an isogradic collection of responses at a given position in a metamorphic gradient. The number of minerals X 3 D/P systems provides a large matrix of sensitive, precisely measured parameters for characterizing that position in the gradient. Several properly selected samples can characterize the entire gradient. This then offers a general basis for extracting quantitative indices of the nature and intensity of the metamorphism. Ultimately, this may permit calibration of metamorphic effects derived from several important variables neglected in some current thermochronological models. Hypothetical models representing possible assemblage D/P responses derived from different metamorphic disturbances are offered. Some examples from southern California of natural responses to metamorphism are provided: (1) The effects of mylonitization as a modifier of the radioactive mineral assemblages are clearly demonstrated in titanite, apatite, and allanite in a tonalite pluton cut by the Eastern Peninsular Ranges mylonite zone. (2) The correlation of radiation damage with isotopic disturbance in zircons is documented in a Cretaceous granodiorite that was hydrothermally altered during the Miocene.

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Additional Information:© 1991 Geochemical Society. The observations leading to the proposals in this paper have been collected with the assistance of many colleagues and students over a period of nearly 40 years. Their discussions and labors are deeply appreciated. Geraldine Silver has been an unflagging partner in this effort for most of that time. During all of that period I have enjoyed the powerful intellectual companionship of Samuel Epstein. I want to thank him for the rich flavors he has contributed to my working environment and for his warm friendship. Parts of the work represented in this paper have been supported at length by the Department of Energy and by a National Science Foundation grant #EAR-851927. Division of Geological and Planetary Sciences, California Institute of Technology, Contribution No. 5045.
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Caltech Division of Geological and Planetary Sciences5045
Series Name:Special Publication - Geochemical Society
Issue or Number:3
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ID Code:115934
Deposited By: Tony Diaz
Deposited On:28 Jul 2022 22:15
Last Modified:15 Aug 2022 22:19

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