Direct geochemical evidence for a very early (>3.85 billion year old) oxidised terrestrial mantle

Vickie Bennett, Allen Nutman and Marc Norman

The complex and likely significant role of the mantle in the chemical evolution of the atmosphere, hydrosphere and lithosphere, is being increasingly recognised, with for example, some studies linking the development of an oxygen rich atmosphere directly to changes in mantle redox chemistry (e.g. 1, 2; figure 1). To define conditions in the early (>3.8 Ga) mantle, we measured major and trace element concentrations, including the redox sensitive element vanadium (3,4), from rare, well-preserved, temporally and geographically distinct suites of spinel periodites from early Archean terranes of southwest Greenland (Figure 2). The sample ages are determined by U-Pb zircon dating of cross cutting and/or intrusive tonalites and by their primitive 187 Os/ 188 Os isotopic compositions (5), and include > 3.81 Ga and recently identified > 3.85 Ga peridotites. The chemical affinities of these samples such as their Si-Al-Mg proportions, olivine/orthopyroxene ratios and mineral compositions, are more similar to modern abyssal peridotites than to cratonic mid-Archean lithospheric mantle represented by xenoliths from southern Africa and Siberia ; the peridotites are interpreted as early Archean lithospheric mantle that was trapped within ancient sialic crust during its formation.

 

 

Figure 1. The mantle is a significant component of the global gas cycle. Were changes in the composition of gases released from the mantle related to the rise of an oxygen rich atmosphere?

Trace element compositions determined by solution ICP-MS are compared with suites of post-Archean peridotites of known tectonic setting, measured by identical methods, hus eliminating problems of interlaboratory bias and enabling a relative fO 2 resolution between sample suites of better than 0.5 log units. The 3.81 Ga and 3.85 Ga peridotites are indistinguishable from modern peridotites on V-MgO (figure 3) and V-Al 2 O 3 arrays, indicating melt extraction in the early Archean mantle at oxygen fugacities between FMQ-3 and FMQ, that is, identical to the present day. All early Archean peridotites fall within 0.5 log units of average post Archean mantle spinel peridotites.

 

Figure 2. The oldest (> 3,850 Ma) samples of the mantle are preserved in southwest Greenland and provide an important resource for testing models of early Earth evolution. Note student for scale.

Thus, the V systematics of these peridotites provide no evidence for evolving redox conditions in the mantle from > 3.85 Ga to the present day and suggest the composition of volcanic gases was similar throughout Earth history. There is no evidence for a secular change in mantle redox state from 3.85 Ga to the present day; establishment of favourable environments for the origin of life and the rise of an oxygen rich atmosphere cannot be directly linked to changes in mantle chemistry. The data also require the change from a highly reducing upper mantle, in equilibrium with metallic Fe during core formation, to an oxidised mantle must have occurred within 700 myr of core formation, that is between 4.56 Ga and 3.85 Ga.

References: 1. Kasting, J., et al., 1993, J. Geol. 101, 245-257. 2. Kump, L., et al., 2001 G-cubed 2, 2000GC000114. 3. Canil, D. 2002, Earth Planet. Sci. Lett. 111, 83-95. 4. Lee et al, 2003, Geochim. Acta 67, 3045-3064. 5. Bennett et al. 2002, Geochim. Acta 66, 2615-2730.

Figure 3. The compositions of the redox sensitive element V are the same in ancient and modern mantle samples demonstrating that there has not been a signficant change in mantle redox chemistry over the last 3,850 million years. Model curves are from Lee et al. (2003).