Pb isotope data from late Proterozoic subduction-related rocks: Implications for crust-mantle evolution

Abstract

Assessment of the role of subduction in the generation of the continental crust and the creation of heterogeneities within the upper mantle is hindered by a lack of knowledge of the associated chemical fluxes. Here we evaluate further the relative fractionation of Th, U and Pb within subduction zones. Pb isotope ratios have been measured in a group of late Proterozoic subduction-related volcanics and intrusions from Saudi Arabia, and in feldspars from selected samples. Feldspar-whole rock pairs are used to calculate time-integrated μ, ω and κ ratios. Similar calculations are offered for the fine-grained rocks from which feldspar could not be separated, by assuming initial ratios based upon the feldspar data from other samples, and models of crust-mantle evolution. These data are used to estimate the U/Pb, Th/Pb and hence Th/U fractionation associated with subduction, and the most striking feature is the low-Th/U signature, which is similar to that measured in many recent island arc basalts. Thus, the data confirm the low-Th/U flux from mantle to crust above modern subduction zones. This contrasts sharply with the high Th/U of average continental crust, and with granites of similar age in the Damara belt of Namibia, which are largely derived from pre-existing continental crust, and have Pb isotope ratios that reflect higher U/Pb, Th/Pb and Th/U. It is concluded that these elevated ratios result from intracrustal differentiation processes. However, no clear correlation is observed between Th/U and Rb/Sr in a range of basalts to granites from the Central Andes, suggesting that variations in these ratios result, at least in part, from different processes. It has recently become popular to invoke subduction as a mechanism for the generation of isotope heterogeneity in mantle remote from present-day subduction zones. We explore this possibility for the Mesozoic flood basalts of Gondwana, which are characterised by high ϵ Sr and low ϵ Nd; a signature apparently derived largely from continental mantle lithosphere. They may be divided into low-Ti and high-Ti groups, and each has distinctive Pb and Sr isotope ratios. None suggest subduction-modified source regions that have evolved in response to a low-Th/U enrichment. However, the low-Ti basalts do have Sr-Pb isotope characteristics that are consistent with addition of subducted sediment to their mantle sources. The high-Ti basalts indicate source regions with high time-integrated Th/U ratios, similar to those of many Dupal OIB. Thus, there is further evidence for a link between the Dupal signature and processes thought to occur in the subcontinental lithospheric mantle sources of some flood basalts.