Spinels from the "Green Sand" Bed of the 3.3 Ga Mendon Formation: Implications for a Possible new Archean Impact Layer

A distinctive 3-4 m thick unit of greenish sandstone, here termed the "Green Sand" bed, occurs at the top of a 10-12 m thick layer of chert between komatiitic flow units in the 3,334-3,260 Ma Mendon Formation. This sequence has yet to be correlated with other parts of the Mendon Formation because of its structural isolation by two major faults. The chert containing the "Green Sand" also includes a previously unreported impact layer. This unit is unique in our study of Barberton greenstone belt (BGB) zircons in both diversity and antiquity (at least five different zircon populations, including the oldest (>3.7 Ga) zircons found in the Kaapvaal Craton). Much of the data presented here is based on the composition of spinels, which have been shown to have a high potential for preservation in greenschist rocks of the BGB. Spinels in komatiitic volcanic rocks are high-Cr varieties produced by high degrees of partial melting and/or melting of a previously depleted source. Spinel compositions, as with bulk rock and pyroxene compositions (when preserved) accurately reflect variation in komatiitic petrogenesis to yield Type 1 (chondritic alumina, spinels Cr# 75-85), Type 2 (alumina-depleted, spinels Cr# >85) or Type 3 (alumina-enriched, spinels Cr# <75) lavas. Spinels from the "Green Sand" and associated komatiitic lava flows are compositionally distinctive from those in other parts of the Mendon Formation, but closely resemble those of komatiitic rocks in the Weltevreden Formation, previously known only from the northernmost parts of the greenstone belt. Another suite of spinels, associated with meteorite impacts in the BGB, are characterized by high nickel contents, though nickel-rich spinels are yet to be found in the "Green Sand". It is unclear how the "Green Sand" is related to the newly discovered impact layer. Although separated from the impact layer of spherules by 1-2m of chert, it is possible that the unusual components in the "Green Sand" reflect the wide distribution of distantly derived minerals excavated by a large meteorite impact about 3.3 Ga and or the subsequent uplift of near-target crust.