Computer Science
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.126..457c&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 126, no. 4, p. 457-472
Computer Science
67
Chemical Elements, Earth Mantle, Isotopes, Isotopic Enrichment, Melting, Peridotite, Wyoming, Chemical Composition, Cratons, Igneous Rocks, Sedimentary Rocks
Scientific paper
Elemental and isotopic compositions of spinel peridotite, pyroxenite and glimmerite xenoliths in Eocene minette dikes from the Highwood Mountains and Eagle Buttes, Montana, U.S.A., reveal a prolonged, yet episodic, history of melt removal and addition within the shallow lithospheric mantle of the Archean Wyoming Craton or its modified margin. Ancient, but highly variable, enrichment in incompatible elements is indicated by extreme Sr, Nd and Pb isotopic compositions (Sr-87/Sr-86 = 0.705 to 1.02; epsilon(sub Nd) = -9 to -43; Pb-206/Pb-204 = 15.8 to 23.2). Very low Os-187/Os-188 (0.110 or less), corresponding to Re depletion model ages (T(sub RD) of 2.7 to 2.9 Ga in some of the peridotites reflects melt removal during the Archean. At least one product of Archean melt migrating through this mantle section is preserved as a websterite xenolith that gives 2.7 Ga model ages for both the Sm-Nd and Re-Os isotopic systems. The majority of xenoliths, however, define Pb-Pb and Sm-Nd isochrons of mid-Proterozoic age and have Re-Os T(sub RD) ages of 2 Ga or less. The mid-Proterozoic age could reflect either the time of formation of these peridotites in the shallow mantle or a time of severe overprinting of the incompatible element budget of pre-existing material by interaction with migrating fluids and/or melts. Glimmerite veins within one harzburgite sample yield 1.8 Ga Monazite U-Pb ages and probably represent the products of crystallization of the fluid/melt responsible for the incompatible element enrichment. The material introduced in the Proterozoic was derived from much older, presumably Archean, crustal materials as shown by marked negative Eu anomalies in many samples and highly evolved initial Sr and Nd isotopic compositions. The data highlight the complex chemical evolution experienced by mantle lithosphere and suggest a coupling between the timing of processes affecting the lithospheric mantle and those recorded in the overlying crustal section.
Carlson Richard W.
Irving Anthony J.
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