2:00pm - 2:30pm Invited Session KeynoteTopics: 2.03 Setting the stage for a habitable planet: Solid Earth processes through time
A new tool to trace the redox-state of the upper mantle in the Archaean
1University of Portsmouth, United Kingdom; 2Géosciences Montpellier, Université de Montpellier, CNRS, France
The redox state of the upper mantle in the Archaean through to the Proterozoic is a key parameter as it would have buffered atmospheric composition and interacted with the ocean-atmosphere system. There have been multiple approaches using geochemical proxies, such as V-Sc and redox sensitive stable isotopes (e.g. Fe) applied to mantle-derived rocks to investigate this problem. As whole rock samples are prone to overprinting (alteration, metamorphism) and as mafic rocks in particular are difficult to date, a technique using a robust U-bearing accessory mineral might allow better and more trustworthy temporal constraints to be measured. Recent work developing an oxybarometer based on S in apatite using µ-XANES has shown great promise as apatite can seamlessly incorporate reduced and oxidised S species and directly reflect the fugacity of host magmas. Nonetheless, apatite crystals in a matrix rock are prone to alteration and recrystallisation, but apatite inclusions trapped in zircon during magmatic crystallisation are robust, with the advantage that the enclosing zircon can be dated and the mantle source traced via Lu-Hf and O isotopes. To demonstrate that this approach works, we have studied 2.35 Ga TTGs and 2.13 Ga sanukitoids from the Mineiro Belt, Brazil. These rocks temporally straddle the Great Oxidation Event. Apatite inclusions in zircons from this TTG-sanukitoid transitional magmatic record reveal a change from reduced to more oxidised conditions from pre- to post-GOE. We then discuss how this approach has and can be further applied to Archaean rocks as a tracing tool for earlier oxygenation events.
2:30pm - 2:45pmTopics: 2.03 Setting the stage for a habitable planet: Solid Earth processes through time
Early mantle processes inferred from high-precision 182W-142Nd isotope systematics of igneous rocks from the Singhbhum Craton, India
1Institut für Geologie und Mineralogie, Universität zu Köln, Germany; 2Department of Geology, University of Johannesburg, South Africa; 3German University of Technology in Oman, Oman
The scarcity of well-preserved exposed Precambrian rocks as well as post-emplacement metamorphism and alteration hamper a detailed understanding of mantle differentiation processes on the early Earth. This issue can be overcome by powerful tools such as the short-lived isotope decay series such as 182Hf-182W and 146Sm-142Nd isotope systems that only record radiogenic ingrowth during the Hadean Eon.
This study focuses on high-precision measurements of 182W/184W and142Nd/144Nd isotope compositions using MC-ICP-MS (, ) of ultramafic, mafic and felsic rocks from the Singhbhum Craton (India) that range in their crystallization age from 3.5 to 1.6 Ga. Due to the susceptibility of W to secondary fluid-rock interactions, the rock samples that were chosen were initially tested for mobility of W by a comparison of W with other equally incompatible elements such as Th, Ta, U  that were obtained by ICP-MS with a focus on precise measurements of High Field Strength Element concentrations (<6% uncertainty). Preliminary 182W isotope data that ranges from µ182W= -1.6 to -0.7 (±2-3 ppm; 95% CI) shows a slight tendency to negative values that are unresolvable from the modern mantle value. This data, in combination with robust 142Nd isotope constraints can give fresh insights into early mantle differentiation of the Singhbhum Craton and access the timescales of homogenization of the ambient mantle with late accreted material.
 Tusch et al. (2022), PNAS 119
 Hasenstab-Dübeler et al. (2022), Chem. Geo. 614
 König et al. (2011), Geochim. Cosmochim. Acta 75
2:45pm - 3:00pmTopics: 2.03 Setting the stage for a habitable planet: Solid Earth processes through time
142Nd and 182W systematics of Neoarchean rocks from the Yilgarn Craton, W-Australia
1Unversity of Cologne, Germany; 2Vrije Universiteit Brussel; 3Geological Survey of Western Australia; 4University of New South Wales
The short-lived isotope systems 146Sm-142Nd and 182Hf-182W were active during the first ca. 500 Ma and 50 Ma after solar system formation. As a result of recent analytical advances, it is now possible to detect small 142Nd-182W variations (≤ 3ppm) within terrestrial samples providing unprecedented information on Earth’s accretion, early differentiation, as well as mantle mixing and homogenization rates.
Here, we present high precision 142Nd/144Nd and 182W/184W data for Neoarchean samples from the Yilgarn Craton, W-Australia, using previously published MC ICP-MS protocols [1,2]. We report µ142Nd deficits as low as -4.2 ± 1.4 for 2.7 Ga mafic-ultramafic samples from the Kalgoorlie Terrane. A contemporaneous mafic-ultramafic suite from the Kambalda area displays small µ142Nd values between +0.4 ± 1.2 to -1.5 ± 0.9 that seem to correlate positively with ε143Nd. If interpreted to represent a differentiation model age, this event could not have happened earlier than 4.13 Ga. This suite reveals a correlation of long-lived ε143Nd-ε176Hf isotope systematics, suggestive of a pristine mantle source. We further suggest that µ182W excesses from the Kalgoorlie and Kambalda suites (+5.3 ± 3.6 and +4.5 ± 1.6) demonstrate a missing late veneer component in the mantle source, in line with previously reported ε100Ru excesses found in the same samples . In conclusion, our results demonstrate that mantle-derived rocks from the Yilgarn Craton carry isotope signatures directly referring to Hadean processes.
 Hasenstab-Dübeler et al. (2022) Chem. Geol. 614, 121-141
 Tusch et al. (2019) GCA 257, 284-310
 Fischer-Gödde et al. (2021) Goldschmidt Abstract 4362
3:00pm - 3:15pmTopics: 2.03 Setting the stage for a habitable planet: Solid Earth processes through time
A 182W isotope perspective on the sources of Paleoarchean TTGs from the Eastern Kaapvaal Craton, southern Africa
1Freie Universität Berlin, Germany; 2Universität zu Köln
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.