Mathematics – Logic
Scientific paper
Mar 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53..717w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 53, Issue 3, pp.717-724
Mathematics
Logic
7
Scientific paper
Two end-member types of U-Pb fractionation behaviour are recognised: type I in which Pb-isotopic homogenisation occurs, and type II, in which Pb-isotopic homogenisation is incomplete or absent. Type II can be further subdivided into three categories depending upon the relationship of the post-fractionation 3 ( 238 U / 204 Pb ) value to prefractionation 2 . In type IIa, 3 is unrelated to 2 ; in type IIb, 3 = k 2 , where k is a constant; in type IIc, 3 values are uniform. Each of these types of behaviour results in a predictable effect upon observed uranogenic Pb-isotope systematics which may lead to an incorrect geochronological interpretation. An example of the relationship between these specific types of U-Pb behaviour and metamorphic grade is provided from the late-Archaean Lewisian complex of N.W. Scotland. This complex was metamorphosed during the ~2660 Ma Badcallian event. Three suites of tonalitic-granodioritic grey gneisses have been analysed; pyroxene-granulite facies from Scourie; hornblende-granulite facies from Gruinard Bay; and amphibolite facies from the northern region. The present-day Pb-isotopic composition of these suites can be interpreted as representing a transition from type lib behaviour in the lowest grade rocks resulting in a rotated isochron, through type Ha in the hornblende-granulites which yield a scatter of data points, to type I in the pyroxene-granulites which yield an isochron corresponding to the Badcallian event. This transition presumably reflects increasing U-depletion with metamorphic grade and ultimately, at the highest grade, Pb mobility and/or depletion. In addition, a consideration of the thorogenic Pb-isotopic data reveals a systematic variation during prograde metamorphism. An attempt to test the model of 2660 Ma initial ratio inhomogeneity by analysis of K-feldspars from the amphibolite facies gneiss suite reveals ~1700 Ma Laxfordian resetting of their Pb-isotope systematics. This study has implications for Pb / Pb geochronological data particularly in lower grade metamorphic terrains. It suggests that reliable isochrons may only be generated by type I behaviour, or by the imposition of high 3 values during type IIa behaviour at higher grades of metamorphism.
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