SOIL FORMATION BENEATH THE EARTH'S OLDEST KNOWN (3.46 GA) UNCONFORMITY?

The Earth's oldest known unconformity (~3.5 Ga) was intersected 174 m below the modern surface in Australia's Pilbara craton, during diamond core drilling (Astrobiology Drilling Project leg 8). The target was drilled to determine whether a paleosol related to this unconformity is preserved at depth in the absence of modern weathering, between undeformed, low-grade metamorphic rocks of the Coonterunah Group (>3.5 Ga) and overlying Strelly Pool Chert (~3.46 Ga). Our investigation shows that Coonterunah Group volcanic rocks are progressively altered toward the unconformity. Approximately twenty meters below the unconformity, the parental rock color gradually changes from green to gray/white. Optical microscopy reveals that the altered rock underwent isovolumetric mineral transformation, whereby original plagioclase and biotite were completely replaced by quartz and sericite. Alteration patterns approaching the unconformity include 1) an increase in the degree of sericitization and silicification, 2) a change from chloritization to sericitization, 3) a decrease in the volume of carbonate-replaced minerals. Organic matter was identified with Raman spectroscopy and found only within the alteration zone. In the altered interval, TiO₂ normalized ratios of Al₂O₃, MgO, CaO, FeO, Fe₂O₃, K₂O, Na₂O, C, Zr, Nb, Th, and U show distribution patterns similar to those of well-characterized Archean paleosols. However, elevated concentration of S is also recognized within the alteration interval at three separate zones (175.55, 181.6, and 183.7 m), suggesting a hydrothermal input. TiO₂ normalized SiO₂, P₂O₅, Zn, and Cu ratios are elevated at 174.9 m and 182.1 m, coincident with elevated chondrite normalized light-REE's. The anomalous behavior of L-REE's together with SiO₂ and P₂O₅ might have resulted from silcrete formation during multiple soil formation events, or through ground water fluctuation. Alternately, elevated S related to a hydrothermal event in the alteration profile may have resulted in the enrichment of L-REE's. Our preliminary work suggests: 1) the existence of world's probable oldest paleosol, 2) the evidence of world's oldest terrestrial organic matter, 3) an alteration profile resulted from the weathering of terrestrial organic matter, and 4) an Achaean ground water regime similar to the present.