Tungsten isotopes are the ideal tracers of core-mantle chemical interaction. Given that W is moderately siderophile, it preferentially partitioned into the Earth's core during its segregation, leaving the mantle depleted in this element. In contrast, Hf is lithophile, and its short-lived radioactive isotope 182 Hf decayed entirely to 182 W in the mantle after metal-silicate segregation. Therefore, the 182 W isotopic composition of the Earth's mantle and its core are expected to differ by about 200 ppm. Here, we report new high precision W isotope data for mantle-derived rock samples from the Paleoarchean Pilbara Craton, and the Réunion Island and the Kerguelen Archipelago hotspots. Together with other available data, they reveal a temporal shift in the 182 W isotopic composition of the mantle that is best explained by core-mantle chemical interaction. Core-mantle exchange might be facilitated by diffusive isotope exchange at the core-mantle boundary, or the exsolution of W-rich, Si-Mg-Fe oxides from the core into the mantle. Tungsten-182 isotope compositions of mantle-derived magmas are similar from 4.3 to 2.7 Ga and decrease afterwards. This change could be related to the onset of the crystallisation of the inner core or to the initiation of post-Archean deep slab subduction that more efficiently mixed the mantle.

No abstract

No abstract

No abstract

No abstract

No abstract

The first terrestrial Pb-isotope paradox refers to the fact that on terrestrial differentiation processes. It is particularly useful average, rocks from the Earth's surface (i.e. the accessible Earth) for tracing the extraction history of continental crust plot significantly to the right of the meteorite isochron in a common from the mantle. Uranium and Th are strongly lithophile Pb-isotope diagram. The Earth as a whole, however, should plot refractory elements and were concentrated in the bulk close to the meteorite isochron, implying the existence of at least one Earth during accretion whereas the moderately volatile terrestrial reservoir that plots to the left of the meteorite isochron. daughter element Pb is believed to be depleted. There The core and the lower continental crust are the two candidates is the added possibility that because of its chalcophile that have been widely discussed in the past. Here we propose that character, appreciable amounts of Pb are concentrated subducted oceanic crust and associated continental sediment stored in the core. Thus, the U/Pb and Th/Pb ratios of the as garnetite slabs in the mantle Transition Zone or mid-lower mantle various silicate Earth reservoirs are high and, with the are an additional potential reservoir that requires consideration. We exception of some ancient samples, all terrestrial rocks present evidence from the literature that indicates that neither the contain a very significant proportion of radiogenic Pb. core nor the lower crust contains sufficient unradiogenic Pb to Silicate Earth Pb-isotope systematics reflect the fact that balance the accessible Earth. Of all mantle magmas, only rare U and Th are highly incompatible and very strongly alkaline melts plot significantly to the left of the meteorite isochron. concentrated in the continental crust, yet only U is redox-We interpret these melts to be derived from the missing mantle sensitive. Lead, which is less incompatible than its parent reservoir that plots to the left of the meteorite isochron but, significantly, elements, is highly mobile in fluids and as a result, overabove the mid-ocean ridge basalt (MORB)-source mantle evolution abundant in the continental crust relative to elements line. Our solution to the paradox predicts the bulk silicate Earth with similar partition coefficient for anhydrous mantle to be more radiogenic in 207 Pb/ 204 Pb than present-day MORBmelting (Miller et al., 1994; Brenan et al., 1995). source mantle, which opens the possibility that undegassed primitive The U-Pb isotope system is complex not only because mantle might be the source of certain ocean island basalts (OIB). of the contrasting chemistry of U and Pb but also because Further implications for mantle dynamics and oceanic magmatism there are two U isotopes that decay to two different are discussed based on a previously justified proposal that lamproites isotopes of Pb. There is a near order of magnitude and associated rocks could derive from the Transition Zone. difference in half-life between the two U isotopes...

The Kalkarindji continental flood basalt province of northern Australia erupted in the mid Cambrian . It now consists of scattered basaltic lava fields, the most extensive being the Antrim Plateau Volcanics (APV) -a semi-continuous outcrop (c. 50,000 km 2 ) reaching a maximum thickness of 1.1 km. Cropping out predominately in the SW of the APV, close to the top of the basalt succession, lies the Blackfella Rockhole Member (BRM). Originally described as 'basaltic agglomerate' the BRM has, in recent years, been assumed to be explosive tephra of phreatomagmatic origin, thus providing a potent vehicle for volatile release to the upper atmosphere. Our detailed field investigations reveal that this basaltic agglomerate is, in reality, giant rubble collections (15 -20 m thick) forming the upper crusts of rubbly pāhoehoe lava units 25 -40 m thick; covering 18,000 -72,000 km 2 and an estimated volume of 1,500 -19,200 km 3 . These flows, rheologically but not chemically, distinct from the majority of Kalkarindji lavas, indicate a fundamental change in eruption dynamics. A low volatile content, induced high amounts of pre-eruptive degassing causing super-cooling and an increase in crystal nucleation and viscosity. A more viscous lava and a consistently faster rate of effusion (analogous to that of Laki, Iceland) created the flow dynamics necessary to disturb the lava crust to the extent seen in the BRM. Volatile release is estimated at 1.65 × 10 4 -2.11 × 10 5 Tg total CO2 at a rate of 867 Tg a -1 and 9.07 × 10 3 -1.16 × 10 5 Tg SO2 at 476.50 Tg a -1 . These masses accounted for 0.5% of Cambrian atmospheric conditions whilst limiting factors reduced the effect of volatile delivery to the atmosphere, thus any potential global impact caused by these flows alone was minimal.