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A 182W isotope perspective on the sources of Paleoarchean TTGs from the Eastern Kaapvaal Craton, southern Africa

182W deficits in terrestrial rocks are currently strongly debated since their origin can be ascribed to different processes. These include (1) core-mantle interaction, (2) grainy late accretion, and (3) early silicate differentiation. Mantle-derived rocks from the eastern Kaapvaal Craton yield variably negative µ182W values that are systematically correlated with initial values of the long-lived Hf-Nd-Ce isotope systems. These have been interpreted to reflect incorporation of an early Hadean crustal restite either in the deep mantle sources of Archean mantle plumes or within the upper mantle or lower lithosphere of the Kaapvaal Craton. Interestingly, granitoids from the Kaapvaal Craton are either overlapping with the modern 182W isotope composition or carry a strongly negative µ182W of down to -10, overlapping with Mesoarchean diamictites from the Kaapvaal Craton. Deviation of some granitoids from the Kaapvaal 182W-176Hf and 182W-143Nd array are likely caused by disturbance of the whole-rock Hf-Nd data or by fluid mobility of W. Here we will further explore the 182W isotope composition of the Paleoarchean Ngwane Gneiss suite from the Ancient Gneiss Complex (Eswatini) and TTG plutons from the Barberton Mountain Land, that reveal a time-integrated increase of initial epsHf values in magmatic zircon. Our results provide further constraints on the origin of granitoids that plot at the upper end of the µ182W-epsHf array. We will present first 182W data produced on the NEOMA MC-ICPMS in Berlin, which have a high level of accuracy as revealed by replicate measurments from samples previously measured at University of Cologne.


J. Elis Hoffmann1, Jonas Tusch2, Pauline Sandor1, Guillaume Florin1
1Freie Universität Berlin, Germany; 2Universität zu Köln
GeoBerlin 2023