SINGLE MINERAL DATING BY THE PB-PB STEP-LEACHING METHOD - ASSESSING THE MECHANISMS

Stepwise Pb-Pb leaching (PbSL) has been successfully used to date rock -forming silicates directly Linked to metamorphic reactions defining a PT path. The two features of PbSL are an increase of precision and a control on accuracy: the former, by enhancing the Pb-206/Pb-204 and Pb -207/Pb-204 ratios, and the latter, by revealing heterochemical inclus ions via the Pb-208/Pb-206 ratio and checking isotopic equilibrium wit h the host. The question of the need for inclusions as a prerequisite enabling PbSL dating was investigated on a centimenter-sized single cr ystal of museum-quality titanite. We obtained petrographic (optical mi croscope, SEM, electron microprobe, proton microprobe), chemical (ICP- MS), and isotopic (TIMS) data on pristine and increasingly leached spl its of different grain sizes, as well as on leach solutions. The PbSL age of 1.00 Ga is identical to the concordant conventional U-Pb age. B y use of isotopic and elemental correlation diagrams, we were able to resolve three isotopically distinct sources of Pb. Visible inclusions of K-feldspars as micro-crack fillings may contribute to the first 50% of common Pb (Pb-204) release, and visible 5 mu m zircons account, at least in part, for the residue. An additional effect noticed during t he first leach steps is related to surface hydrolysis and consequent w eakening of metal cation-oxygen bonds, which releases structurally bou nd common Pb substituting for Ca in the titanite lattice. The radiogen ic Pb released during most leach steps has uniform Pb-208/Pb-206 ratio s, suggesting that it is derived from a single mineral phase. Further, as different titanite grain sizes produce a different leach trajector y in Pb isotopic space, it can be concluded that this radiogenic Pb is actually derived from the titanite itself. Therefore, PbSL is indeed capable of discriminating common and radiogenic Pb from a single phase . During leaching, a well defined reaction front is seen to advance in to the grains. Inside this front, the titanite is unaltered, and outsi de it, a silica-gel-like rim is formed, in which electron and proton m icroprobe data show retention of high field strength (HFS) elements. W e propose that radiogenic Pb occurs in the tetravalent state (due to r ecoil stripping) so that it behaves as an HFS element and is preferent ially retained in the gel rim during leaching, relative to common diva lent Pb. The mechanism by which a spread in Pb isotope data is obtaine d during PbSL is, therefore, explained by two processes controlling th e release of cations from a mineral grain into a leach solution: (1) a n effective surface dependent hydrolysis of metal cations at the inwar d migrating reaction front whose influence decreases as leaching proce eds and (2) an increasingly dominant remobilization of HFS-elements an d radiogenic Pb from the micro-environment of the leached gel-like lay er which acts as a selective HFS cation absorber. In conclusion, Pb/Pb dating by stepwise leaching can be effective as a dating tool, even i n minerals free from micro-inclusions. Copyright (C) 1997 Elsevier Sci ence Ltd.