Search

Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle (1-4). As a consequence, the sub-arc mantle wedge is widely believed to be oxidized (3,5). The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source (6,7). Here we show that Zn/[Fe.sub.T] (where [Fe.sub.T] = [Fe.sup.2+] + [Fe.sup.3+]) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, [Fe.sup.2+] and Zn behave similarly, but because [Fe.sup.3+] is more incompatible than [Fe.sup.2+], melts generated in oxidized environments have low Zn/[Fe.sub.T]. Primitive arc magmas have identical Zn/[Fe.sub.T] to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/[Fe.sub.T] during early differentiation involving olivine requires that [Fe.sup.3+]/[Fe.sub.T] remains low in the magma. Only after progressive fractionation does [Fe.sub.3+]/[Fe.sub.T] increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood., Oxygen fugacity [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is an intensive parameter describing the chemical activity of [O.sub.2] in a given system (fugacity represents a corrected partial pressure due to [...]

Accretion left the terrestrial planets depleted in volatile components. Here I examine evidence for the hypothesis that the Moon and the Earth were essentially dry immediately after the formation of the Moon--by a giant impact on the proto-Earth--and only much later gained volatiles through accretion of wet material delivered from beyond the asteroid belt. This view is supported by U-Pb and I-Xe chronologies, which show that water delivery peaked ~100 million years after the isolation of the Solar System. Introduction of water into the terrestrial mantle triggered plate tectonics, which may have been crucial for the emergence of life. This mechanism may also have worked for the young Venus, but seems to have failed for Mars., Earth, Mars and Venus are three planets with very different histories. Active plate tectonics, the presence of a liquid ocean, and teeming life characterize the Earth. Mars has a tenuous [...]

Author(s): Christine M. Meyzen (corresponding author) [1]; Janne Blichert-Toft [1]; John N. Ludden [2]; Eric Humler [3]; Catherine Mével [4]; Francis Albarède [1] Radiogenic isotope studies of Sr, Nd and [...]

Author(s): Francis Albarède (corresponding author) [1]; Janne Blichert-Toft [1]; Jeffrey D. Vervoort [2]; James D. Gleason [1]; Minik Rosing [3] Modern basalts have seemingly lost all 'memory' of the primitive [...]

Scientists have long looked at the constituent elements of meteorites to find out how old the Sun and its planets are. The most perfect example of the oldest meteorites--those that [...]

As a young postdoc at the California Institute of Technology (Caltech) in Pasadena I remember glancing through the 1952 logbook of a gas mass spectrometer while the machine readied my [...]

Author(s): Francis Albarède [1] It is thought that Earth formed, along with the rest of the Solar System, from the gravitational collapse of a huge cloud of gas. But what [...]