Siderophile Element Systematics of the Early Archean Mantle: Evidence from 3.3 Ga Ruth Well Komatiites, Pilbara Craton, Western Australia

The main goal of this study is to place constraints on the absolute HSE abundances, W isotopic composition, and time-integrated Re/Os ratio of the early-mid Archean mantle using 3.3 Ga Ruth Well komatiites from the Pilbara Craton in Western Australia. While the heterogeneous nature of the mantle in terms of W and Os isotopic composition and highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, Pd) abundances is well established based on komatiite research of the past two decades [e.g., 1,2,3], the origin, magnitude, and longevity of this heterogeneity are still little understood. This is particularly true for the earlier Archean mantle, where the available komatiite record is limited.

Major-, lithophile trace element-, and HSE abundance, as well as Re-Os isotopic data are reported from a set of drill core samples. Regression of the lithophile trace element and HSE abundances against the MgO content indicate limited element mobility during post-magmatic processes. The komatiites are depleted in both light and heavy REE [(La/Sm)_N = 0.70 ± 0.03, (Gd/Yb)_N = 1.37 ± 0.05]. In the set of samples analyzed, Os isotopic compositions exhibit poor correlation with the Re/Os ratios, likely due to limited Re mobility long after lava emplacement. The calculated total Pt and Pd abundances in the Ruth Well komatiite source are 68±9% of those in the estimates for the modern Bulk Silicate Earth. This estimate is similar to the source characteristics of several other early Archean komatiite systems, such as the Komati and Weltevreden [4]. This deficit in HSE is likely due to the Ruth Well source not receiving the full complement of late accreted component; however, the exact mechanism and timing for creating this heterogeneity is yet to be established based on the pending ¹⁸²W and additional Os isotopic and HSE abundance data.

[1] Maier et al. 2009, Nature 7255: 620-623; [2] Puchtel et al. 2016, G-cubed 17: 2168-2193; [3] Puchtel et al. 2018, GCA 228: 1-26; [4] Puchtel et al. 2014, GCA 125: 394-413.