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.

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 ([1], [2]) 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 [3] 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.

[1] Tusch et al. (2022), PNAS 119

[2] Hasenstab-Dübeler et al. (2022), Chem. Geo. 614

[3] König et al. (2011), Geochim. Cosmochim. Acta 75

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 [3]. In conclusion, our results demonstrate that mantle-derived rocks from the Yilgarn Craton carry isotope signatures directly referring to Hadean processes.

[1] Hasenstab-Dübeler et al. (2022) Chem. Geol. 614, 121-141

[2] Tusch et al. (2019) GCA 257, 284-310

[3] Fischer-Gödde et al. (2021) Goldschmidt Abstract 4362

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.