Maps of geologically informative Pb isotope parameters
Résumé
The development of large seismic arrays and the large throughput of MC-ICP-MS are providing new impetus to the integration of seismic tomography data (VP, VS, attenuation, shear-wave splitting), geophysical maps (heat flow), and geochemical maps with geology. Synoptic representation of geochemical data started nearly 50 years ago with Hurley and others, who demonstrated that time-integrated parent/daughter ratios (Rb/Sr, U/Pb, Sm/Nd) and apparent crustal residence times inferred from the isotope compositions of radiogenic elements in felsic magmas and metamorphic rocks could be used to identify tectonic provinces. Geochemical parameters derived from such long-lived radioactive isotopic systems are far less noisy than raw trace element ratios in the same rocks: for example, measured uranium concentrations are severely biased by the transit of samples through the water table during erosive exhumation. The U-Th-Pb isotope system is particularly powerful. However, georeferenced geochemical databases are still incomplete. We recently began to map at the continental scale the Pb isotope compositions into axes with geologically informative content: the two-stage Pb model age, which dates the closure of the U-Pb chronometer, and the time-integrated U/Pb (mu) and Th/U (kappa) ratios of the Pb source [1]. Maps of model ages essentially depict the maximum extension of the ~600°C isotherm. Because rocks from granulite facies terranes tend to have Th/U higher than the planetary value of 3.88, maps of kappa ratios track the rise of lower crustal material and its melts. We compiled maps of the above-mentioned Pb parameters from the archeological OXALID database (Western Europe) [2] and localized its samples and from Mamani et al.'s [3] database for the Central Andes. For the Western US, we used NAVDAT ( http://www.navdat.org/ ) for Cenozoic continental felsic igneous rocks and added ore data from the literature (Bouchet et al., this meeting). In all cases, we found that the regional consistency of model ages on the one hand and kappa values on the other hand is very strong and in particular highlights the existence of well-defined Pb isotope provinces. We also found some striking similarity between these provinces and prominent seismological features, such as Moho depth and VP/VS. The understanding of the nature, age, and dynamic role of the units identified by seismic tomography will clearly benefit from incorporating geochemical and geophysical spatial information into surface geology. [1] Albarede F. et al. (2012) Archaeom. 54, 853-867 [2] Stos-Gale, Z.A. and Gale, N. (2009) Archaeol. Anthropol. Sci. (2009) 1, 195-213. [3] Mamani M. et al. (2008) G-cubed 9