Physics
Scientific paper
Aug 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49.1813w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 49, Issue 8, pp.1813-1823
Physics
15
Scientific paper
Diffusion coefficients for Sm, Sr, and Pb in fluorapatite at 900°-1250°C were obtained by measuring experimentally-induced diffusional uptake profiles of these elements in the margins of gem-quality apatite crystals. The crystals were immersed in synthetic melts enriched in the trace elements of interest and presaturated in apatite, and the resulting diffusion gradients were characterized by electron microprobe analysis. Except in the case of Pb, the diffusivities define good Arrhenius lines for the respective elements: D Sm = 2.3 × 10 -6 exp (-52,200/ RT ) D Sr = 412 exp (-100,000/ RT ). (Diffusion perpendicular to and parallel to c is measurably different in the case of Sr; the Arrhenius equation given above is an average for the two directions). Results on Pb were erratic, probably because extremely Pb-rich melts were used for some of the experiments. Data believed to be reliable define the following Arrhenius line: D Pb = 0.035 exp (-70,000/ RT ). Constraints based on closure of natural apatites with respect to Pb suggest that the experimental data can be extrapolated, with sizeable uncertainty, to temperatures as low as 550°C. When applied to the question of isotopic and trace-element equilibration of residual or entrained apatites with crustal melts, the measured diffusivities indicate that 0.05-cm crystals will rarely preserve the original Pb-isotope characteristics of the source; the same is not true, however, of Sr (and, under some conditions, the REE), which may be unaffected at crystal cores during typical melting events.
Bruce Watson E.
Harrison Mark T.
Ryerson Frederick J.
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