Mathematics – Logic
Scientific paper
Jul 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51.1833m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 51, Issue 7, pp.1833-1842
Mathematics
Logic
5
Scientific paper
Lead isotopic ratios and concentrations have been measured in lavas from the Aleutian volcanic centers of Adak (12) and Atka (12). Whereas lavas from Atka have very small Pb-isotopic ranges ( 206 Pb / 204 Pb = 18.78-18.86; 207 Pb / 204 Pb = 15.55-15.62; 208 Pb / 204 Pb = 38.31-38.52) those from Adak have ranges nearly twice as large ( 206 Pb / 204 Pb = 18.34-18.86; 207 Pb / 204 Pb = 15.47-15.63; 208 Pb / 204 Pb = 37.81-38.56). On lead-lead isotopic diagrams, Atka data form tight clusters whereas those from Adaak define a well-developed linear trend extending from the Atka field toward less radiogenic values. At a given 206 Pb / 204 Pb , the lavas from both islands plot in the MORB trend for 208 Pb / 204 Pb but Atka lavas plot above the MORB 207 Pb / 204 Pb - 206 Pb/ 204 Pb trend and those from Adak fall both within and above the trend. Lead contents in lavas from Atka increase four-fold (5-21 ppm) over the compositional range of the volcanic suite (49-68% SiO 2 ). In contrast, Actak lavas (49-65%) have concentration levels of 0.6-13 ppm and display no simple correlation with SiO 2 . The lead isotopic data alone can be explained by three different processes. Model 1 assigns lead isotopic differences to original magma source heterogeneity. According to Model 2, the isotopic ratios of a primary, non-radiogenic component from the mantle are elevated by the addition of an isotopically enriched slab-derived component. In contrast, Model 3 assumes a primary radiogenic magma produced by melting of the slab is contaminated by a non-radiogenic lithospheric component during conduit formation. Because these models all adequately explain the lead isotopic data, supplementary geologic, pétrographic, geochemical and isotopic data must be used to select the most likely model. Careful consideration of the evidence suggests Model 3 best explains our extensive lead isotopic data as well as other characteristics of Aleutian lavas. Our study suggests detailed isotopic studies of individual volcanic centers (arc or otherwise) can be extremely useful in understanding the complex processes of magma generation, extraction, ascent and evolution.
Marsh Bruce D.
Myers James D.
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