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Geochemistry
Adam Kent

Geochemistry is a diverse field defined by the elements of the periodic table, their constitutive isotopes and the myriad ways in which these are distributed through natural materials. Thus geochemists must also be diverse in their approaches. A full understanding of natural systems requires integration of evidence from geochemical studies with that from other fields — seismology, petrology and hydrology to name but a few. Recent research highlights this fact.

What goes down must come up

Nowhere is the linkage between geochemistry and other disciplines more critical than in understanding the mysterious inner workings of Earth. One persistently contentious issue is how geochemical evidence for layered convection of Earth's mantle jibes poorly with seismic tomographic images that show subducted lithosphere happily descending into the deep mantle.

David Bercovici and Shun-Ichiro Karato of Yale University, writing in Nature in September 2003, suggested a way around this impasse. They argued that dehydration melting of slowly upwelling mantle at produces a thin (less than 10 kilometers) layer of melt at approximately 410 kilometers depth. This layer filters out incompatible elements — those which prefer to enter a melt phase — from the slowly rising lower mantle material, thus allowing the upper mantle to remain in a depleted state. Does the model work? As Al Hofmann of the Max Planck Institute for Chemistry in Mainz, Germany, notes in an accompanying discussion, the jury is still out, but the idea does at least go some way towards reconciling the current standoff between the geochemical and geophysical viewpoints.

The concept of mantle plumes — narrow upwelling zones rising from within the earth's mantle — also underwent increased scrutiny in 2003, with conferences in Iceland and England held to debate conflicting points of view. Some of the strongest evidence for mantle plumes tapping deep and previously undegassed mantle is from the high ratios of primordial helium-3 to helium-4 (the latter produced by uranium and thorium decay) in some oceanic basalt lavas. In March 2003 in Earth and Planetary Science Letters, Andreas Meibom of Stanford University and colleagues suggested that melting processes in the shallow upper mantle could also produce high helium-3 to helium-4 ratios. Questions remain, and such an idea is controversial.

And, to add a little bit more spice to the pot, Fin Stuart, from the Scottish Universities Environmental Research Centre and colleagues reported last July in Science the highest yet measured magmatic helium-3 to helium-4 ratios. These came from primitive lavas from Baffin Island in far northeastern Canada. The lavas relate to initial outpourings of the mantle plume currently centered beneath Iceland, and from this perspective the high helium-3 to helium-4 ratios are no real surprise.

The devil however, is in the details. Strontium and neodymium isotope compositions of the same rocks show that the high helium-3 to helium-4 signature correlates with input from the depleted upper mantle, which should have relatively low amounts of helium-3 (not as expected from the deeper primitive mantle). Transfer of helium between the lower and the upper mantle can resolve this paradox, but it is by no means clear how such transfer could occur. Expect spirited debate on this and other plume-related matters in the future.

Attack of the lightweights

Helium was also in the news over the past year for other reasons. Bernard Marty of CRPG in France and colleagues presented helium isotope and other geochemical data in Nature in September suggesting that the Paris Basin's Dogger aquifer has remained efficiently isolated from underlying aquifers for several million years. Isolation on such geologic timescales is unprecedented and may only be broken when faulting or other earth movements allow contact between previously isolated permeable layers.

In another study, with important ramifications for mass and heat flow within the ocean crust, Henrietta Edmonds from the University of Texas and colleagues reported in Nature in January 2003 geochemical and other data, which suggest that ultra-slow spreading mid-ocean ridge segments can be far more hydrothermally active than previously thought. The data, collected during the recent AMORE expedition to the ultra-slow spreading Gakkel Ridge located under the Arctic Ocean, shows that concentrations of manganese in seawater above the ridge are elevated in many locations. Together with temperature anomalies and changes in seawater particle density, the data show that seafloor hydrothermal systems are highly active along this ridge.

Other unexpected results came from this expedition. Peter Michael from the University of Tulsa and colleagues showed in June 2003 in Nature that the chemistry of basalt magmas erupted along some portions of the Gakkel Ridge require a much greater degree of mantle melting than expected for a slow spreading ridge, suggesting that spreading rate may be less important than mantle composition or mantle temperature in controlling magma production in these environments.

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Kent is an assistant professor at Oregon State University in Corvallis. E-mail: adam.kent@geo.oregonstate.edu.

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