Hf isotope composition of the late Cenozoic basaltic rocks from northwestern Colorado, USA: New constraints on mantle enrichment processes

Details Hf isotope composition of the late Cenozoic basaltic rocks from northwestern Colorado, USA: New constraints on mantle enrichment processes Hf isotope composition of the late Cenozoic basaltic rocks from northwestern Colorado, USA: New constraints on mantle enrichment processes

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.119..495b&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 119, no. 4, p. 495-509

Computer Science

11

Basalt, Cenozoic Era, Colorado, Earth Mantle, Geochemistry, Hafnium Isotopes, Lutetium, Neodymium, Composition (Property), Earth Crust, Isotope Ratios, Lava, Lithosphere

Scientific paper

Late cenozoic basaltic lavas from northwestern Colorado and the northern Rio Grande rift have higher than expected epsilon(sub Hf) values relative to the epsilon(sub Hf)-epsilon(sub Nd) correlation of oceanic island basalts (OIB). In contrast, samples from the central and southern Rio Grande rift have lower than expected epsilon(sub Hf) values, relative to the epsilon(sub Hf)-epsilon(sub Nd) correlation of OIB. The variation in epsilon(sub Hf) and epsilon(sub Nd) values with geographic position is interpreted to be a result of sampling a predominantly asthenospheric source for lavas from the south Rio Grande rift and adjacent Basin and Range, and a predominantly lithospheric source for lavas from northwestern Colorado. The epsilon(sub Hf) and epsilon(sub Nd) values of the depleted asthenospheric mantle are likely to have formed from ancient depletion of spinel or garnet peridotite, which, respectively, produced anomalously low and high Hf-176/Hf-177 relative to the OIB array. Although a variety of enrichment processes for formation of the lithospheric mantle may be invisioned, silicate metasomatism caused by basaltic liquids crystallizing in the mantle best explains the Hf-Nd relationships in Colorado: the ancient metasomatizing liquids could not have been derived from melting garnet periodite but instead must have been derived through melting spinel peridotite to produce the anomalously high epsilon(sub Hf) values relative to epsilon(sub Nd) values. The inference that Proterozoic lithosphere was stabilized in the spinel stability field contrasts strongly with the proposed lithospheric source regions of recent magmatism. Low Lu/Hf ratios of all of the samples from the Rio Grande rift and northwestern Colorado, compared to the calculated Lu/Hf ratio of the source based on Hf isotopic compositions, suggests that all of the lavas were derived from a garnet-bearing source. This implies that the subcontinental lithospheric mantle has stabilized garnet at the expense of spinel, perhaps through cooling of the lithosphere associated with formation of a stable craton and crustal thickening.

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