Physics
Scientific paper
Apr 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981e%26psl..53..189d&link_type=abstract
Earth and Planetary Science Letters, Volume 53, Issue 2, p. 189-202.
Physics
533
Scientific paper
Equations describing trace element and isotopic evolution in a magma chamber affected simultaneously by fractional crystallization and wallrock assimilation are presented for a model where the mass assimilation rate (M&sun;a) is an arbitrary fraction (r) of the fractional crystallization rate (M&sun;c). The equations also apply to recharge of a crystallizing magma. Relatively simple analytical expressions are obtained for both radiogenic isotope variations (Nd, Sr, Pb) and stable isotopes (O, H) including the effects of mass-dependent fractionation. For r = 1 a modified zone refining equation is obtained for trace element concentrations, but for r < 1 behavior is a combination of zone refining and fractional crystallization. As r --> ∞, simple binary mixing is approached. The isotopic and trace element ``mixing'' trends generated can be much different from binary mixing, especially for r < 1. The model provides the basis for a more general approach to the problem of wallrock assimilation, and shows that binary mixing models are insufficient to rule out crustal assimilation as a cause of some of the isotopic variations observed in igneous rocks, including cases where clustering of isotopic values occurs partway between presumed endmember values. The coupled assimilation-fractional crystallization model provides an explanation for certain trace element and isotopic properties of continental margin orogenic magmas (e.g. Sr concentration versus 87Sr/86Sr) which had previously been interpreted as evidence against assimilation. So-called ``pseudoisochrons'' can be understood as artifacts of contamination using this model. A significant correlation exists between country rock age and low 143Nd/144Nd ratios in continental igneous rocks, clearly suggestive that crustal contamination is generally important.
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