Physics
Scientific paper
Jun 1970
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1970gecoa..34..713r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 34, Issue 6, pp.713-725
Physics
7
Scientific paper
Rb-Sr isotopic analyses on ten total-rock samples from the Harney granite, southern Black Hills, South Dakota yield a total-rock isochron granite age of 1744 ± 12 m.y. and an initial Sr 87 /Sr 86 ratio of 0.7143 ± 0.0006. Four muscovite fractions from the granite give ages that are younger than this value; their age deficit can be correlated with rubidium concentration. Rb-Sr analyses on 25 mineral specimens from three zoned pegmatites associated with the granite yield discordant ages. Ages slightly younger than noted in the granite are observed in all micas, microcline and pollucite. Albite and the phosphate mineral assemblages are highly enriched in excess Sr 87 and give absurd ages. Spodumene analyses yield ages both higher and lower than the granite age. Speculations on possible mechanisms which produced the observed ages include Sr 87 leakage from layer lattice silicates and the more basic problem of the development of pegmatite zonation An idealized model which can result in discordant mineral ages and significant enrichment of Sr 87 in residual fluid phases is discussed in terms of a hypothesized slow cooling of zoned pegmatites.
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