ALBERT

Description: Komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean. The high degree of partial melting ( F ) and pressure ( P ) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate compositional variations in Archean komatiites, a global selection of 38 Archean komatiites spanning five cratons (Kaapvaal, Zimbabwe, Yilgarn, Pilbara, Superior) were analysed for their major and trace element contents. Included are the Aluminium-Depleted (ADK, Barberton-Type) and Aluminium-Undepleted (AUK, Munro-Type) petrogenetic types that have been equated with high P /moderate F and moderate P /high F , respectively, on the basis of their Al/Ti and Gd/Yb ratios. Following calculation of the primary magma composition of each suite, we show that the absolute Al content at a specified MgO proves a more sensitive indicator of P than either of the above two ratios and hence we introduce a new classification using Al. The Mg# is a reliable proxy for F , independent of the two endmember melting styles, fractional and batch. We demonstrate that most komatiites form by batch melting, ceding to fractional melting with decreasing pressure as the density contrast between the liquid and solid grows. The Munro AUKs are the only suite to show evidence of fractional melting, with melt extraction occurring at the lowest F and P , 25% melting at 5 GPa (mantle potential temperature, T P = 1750°C) whereas the ADKs of Barberton segregated at the highest F and P (40%, 9 GPa, T P = 1950°C). The petrogenetic type is a combination of P and F , where, at a given pressure, higher F will produce AUKs over ADKs as majorite is consumed in the source. Through numerical simulations, it is shown that both types can occur within the same mantle plume, with ADKs forming in its cooler, distal fringes whereas AUKs occur along its axis. Furthermore, and contrary to previous views, there is no temporal distinction between the two komatiite types, with both AUKs and ADKs occurring throughout the Archean. By contrast, younger, 2·7 Ga komatiites tend to have sources that are more depleted than those of older, 3·5 Ga komatiites. Komatiites are invaluable records of the mantle’s chemical and physical evolution during the Archean.

Description: The Ediacaran period began with the deglaciation of the ca. 635 Ma Marinoan snowball Earth and the deposition of cap dolostones on continental shelves worldwide during post-glacial sea-level rise. These carbonates sharply overlie glacial sediments deposited at low paleolatitudes and preserve negative carbon isotope excursions. The snowball Earth hypothesis invokes an almost complete cessation of primary productivity in the surface ocean. Because assimilatory uptake of Zn appears to fractionate its isotopes, Zn isotope ratios measured in carbonate precipitated in the surface ocean should track fluctuations in primary productivity. Here we report the first Zn isotopic data, together with carbon and oxygen isotopic profiles from a Neoproterozoic cap dolostone, the Nuccaleena Formation in the Flinders Ranges, South Australia. We interpret the Zn isotopic data in terms of a two-stage evolution of the deglacial ocean. Slightly 66 Zn-enriched values at the base of the cap dolostone indicate immediate resumption of the biological pump upon melting of the surface ocean, but this signal was diluted by intense surface runoff that drove 66 Zn ( 66 Zn/ 64 Zn, versus the JMC Lyon reference) values down to the composition of continentally derived Zn. A subsequent rise in 66 Zn records a vigorous increase in primary production and export from a nutrient-laden surface ocean.

Description: Geophysical observations suggest sub-arc convective flow transports melt-exhausted and metasomatized wedge mantle into deeper mantle regions. Reciprocally, asthenospheric, fertile mantle may supply back-arc ridges distal to the trench by shallow, lateral mantle ingress, insinuating initial wedge mantle depletion in its back-arc region. Here we show that light Fe isotope compositions of the Central Lau Spreading Centre located in the Lau back-arc basin on the farside of the Tonga-Kermadec arc are indicative for derivation from a modified arc-front mantle with elemental and Nd-isotopic memory of former slab fluid addition. We propose that this shallow wedge material has been transported from the sub-arc mantle to the back-arc either convectively or in a buoyant diapir. This implies that melt-depleted mantle in subduction zones is, at least in parts, recycled in a resurfacing loop. This can explain the depletion in back-arc regions, and the progressively depleted nature of island arc sources in maturing arc systems.

Description: Highlights • Tungsten isotopes reveal vertical structure in mantle plumes. • Plume heads and tails each sample a distinct ancient mantle reservoir. • ULVZs contribute material to OIBs. • LLSVPs may contribute to both OIBs and LIPs. • Chemical heterogeneity in the mantle may be restricted to the base of LLSVPs. Abstract The lowermost mantle is driven to Earth's surface by mantle plumes, providing a volcanic record of its structure and composition. Plumes comprise a head and tail, which melt to form large igneous provinces (LIPs) and ocean island basalts (OIBs), respectively. Recent analyses have shown that LIPs and OIBs exhibit tungsten (W) isotope heterogeneity that was created in the first ∼60 million years of our solar system's evolution. Moreover, the isotopic signature found in LIPs differs to that found in OIBs, revealing that the melt products of plume heads must be dominated by a different ancient mantle reservoir to that of plume tails. However, existing geodynamical studies suggest that plume heads and tails sample the same deep-mantle source region and, therefore, cannot account for any systematic differences in composition. Here, we present a suite of numerical simulations of thermo-chemical plumes and an isotopic model for W sources in the mantle. Our results demonstrate that the W isotope systematics of LIPs and OIBs can, under certain conditions, arise as a dynamical consequence of plumes forming in a heterogeneous, thermo-chemical boundary layer. We find that ultra low-velocity zones (ULVZs), which sit on the core–mantle boundary (CMB), likely contribute to the chemical diversity observed in OIBs but not LIPs, while any dense components residing inside large low shear-wave velocity provinces (LLSVPs) may contribute to both. This study places geochemical observations from Earth's surface in a geodynamically consistent framework and illuminates their relationship with seismically imaged features of the deep mantle.