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Volume 9

Bridging the depleted MORB mantle and the continental crust using titanium isotopes

The mechanisms driving the chemical complementarity between depleted MORB mantle (DMM) and continental crust (with an average 'andesitic' composition) remain unclear. By investigating Archean komatiites, and modern enriched (E) and normal (N) MORB samples, we demonstrate that partial melting of the mantle does not fractionate Ti isotopes, whereas intracrustal differentiation causes significant Ti isotopic fractionation between melts and minerals, specifically Fe-Ti oxides. Thus, Ti isotope ratios are tracers of these two magmatic regimes. N-MORB and late Archean (2.9-2.7 Ga) komatiites are depleted in the heavier Ti isotopes compared to E-MORB and middle Archean (3.5-3.3 Ga) komatiites. We show that the depletion in the heavier Ti isotopes of the DMM is due to mantle recycling of the isotopically light residues from the generation of felsic continental crust over 3.5-2.7 Ga. This process must have reached a steady state by ≈ 2.5 Ga, based on the uniform Ti isotopic composition of contemporary N-MORBs and late Archean komatiites. This change is likely due to a decrease in the mantle potential temperature related to the emergence of plate tectonics.

Z. Deng, F. Moynier, P.A. Sossi, M. Chaussidon

Geochem. Persp. Let. (2018) 9, 11–15 | doi: 10.7185/geochemlet.1831 | Published 13 December 2018

An oxygen isotope test for the origin of Archean mantle roots

The origin of the peridotites that form cratonic mantle roots is a central issue in understanding the history and survival of Earth’s oldest continents. A long-standing hypothesis holds that the unusual bulk compositions of some cratonic peridotites stem from their origin as subducted oceanic serpentinite, dehydrated during subduction to form rigid buoyant keels (Schulze, 1986; Canil and Lee, 2009). We present oxygen isotope data from 93 mantle peridotites from five different Archean cratons to evaluate their possible origin as serpentinites. Cratonic mantle peridotite shows remarkably uniform δ18O values, identical to modern MORB-source mantle, that do not vary with bulk rock Si-enrichment or Ca-depletion. These data clearly conflict with any model for cratonic lithosphere that invokes serpentinite as a protolith for cratonic peridotite, and place additional constraints on cratonic mantle origins. We posit that the uniform δ18O was produced by sub-arc and/or MOR depletion processes and that the Si-enriched nature of some samples is unlikely to be related to slab melt infiltration. Instead, we suggest a peridotitic source of Si-enrichment, derived from ascending mantle melts, or a water-fluxed depleted mantle. These variably Si-enriched, cratonic mantle protoliths were then collisionally compressed into the thick cratonic roots that have protected Earth’s oldest continental crust for over 2.5 Gyr.

M.E. Regier, A. Mišković, R.B. Ickert, D.G. Pearson, T. Stachel, R.A. Stern, M. Kopylova

Geochem. Persp. Let. (2018) 9, 6–10 | doi: 10.7185/geochemlet.1830 | Published 7 December 2018

Microplastics contaminate the deepest part of the world’s ocean

Millions of metric tons of plastics are produced annually and transported from land to the oceans. Finding the fate of the plastic debris will help define the impacts of plastic pollution in the ocean. Here, we report the abundances of microplastic in the deepest part of the world’s ocean. We found that microplastic abundances in hadal bottom waters range from 2.06 to 13.51 pieces per litre, several times higher than those in open ocean subsurface water. Moreover, microplastic abundances in hadal sediments of the Mariana Trench vary from 200 to 2200 pieces per litre, distinctly higher than those in most deep sea sediments. These results suggest that manmade plastics have contaminated the most remote and deepest places on the planet. The hadal zone is likely one of the largest sinks for microplastic debris on Earth, with unknown but potentially damaging impacts on this fragile ecosystem.

X. Peng, M. Chen, S. Chen, S. Dasgupta, H. Xu, K. Ta, M. Du, J. Li, Z. Guo, S. Bai

Geochem. Persp. Let. (2018) 9, 1–5 | doi: 10.7185/geochemlet.1829 | Published 27 November 2018