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New kids on the block grow up
The radiogenic isotopic decay systems of Lu-Hf, W-Hf, and Re-Os continue to mature. These systems, among the newest on the geochemical scene, are providing insights into the early history of the Earth and the dynamics of the crust-mantle-core system. Measurements made by the new generation of plasma-source mass spectrometers are leading the charge. A reevaluation of the decay rate of 176Lu to 176Hf (T1/2 ~ 37 Ga) by Bizzarro et al. (Nature, v. 421, p. 931) eliminates the need for isotopically enriched sources, most likely felsic crust, at the very earliest stages of the earth's development and helps to reconcile Hf isotopes with the more established Nd-isotope record. New W-Hf analyses from a range of meteorites also suggest that development of the Moon and the Earth's core occurred within the first 30 million years of the life of the solar system (Kleine et al., Nature, v. 418, p. 952; Yin et al., Nature, v. 418, p. 949). Evidence emerged from the Os isotope composition of natural osmiridium grains, measured by ion probe, of possible chemical fractionations accompanying inner core formation that bear on the contribution of the core to some mantle plumes (Meibom and Frei, Science, v. 296, p. 516). The recognition that the element Re may be lost during volcanic degassing (Lassiter, Eos, Transactions of the American Geophysical Union, v. 83, p. 1447) has also led to suggestions that continental crust may have significantly higher Re contents than previously thought (Sun et al., Nature, v. 422, p. 295), which has implications for long-term cycling of the mantle and crust. Finally, Schaefer et al. (Nature, v. 420, p. 304) showed that some hotspot-related lavas from the Azores have extremely low 187Os/188Os ratios, providing convincing evidence that ancient (< 2.5 Ga) oceanic lithosphere may be recycled to the surface within the mantle plume that feeds the hotspot.
Where the Mountains Come down to the Sea
The crossover between geochemistry and the life sciences also continues to bear fruit. From the geochemistry of hydrothermal systems, clues are emerging about energy sources for life in extreme environments. These clues are relevant to both study of the Earth and the search for extraterrestrial life (Reysenbach and Shock, Science, v. 296, p. 1077). Chapelle et al. (Nature, v. 415, p. 312) documented a hydrogen-based microbial community existing in hydrothermal waters circulating within deeply buried rocks in Idaho. This community may be an analogue for hydrogen-based ecosystems on other planets. Suitable mineral hosts for preserving ancient biomarkers are also being increasingly recognized. Schieber (Geology, v. 30, p. 531) documented microbial features and geochemical signatures in ancient diagenetic pyrite that will help to characterize sulfate-reducing communities from the geological record. Fish et al. (Nature, v. 417, p. 423) recovered ribosomal RNA from samples of evaporite halite formed between 11 and 425 million years ago, which suggests that these samples preserved traces of complex microbial communities.
Oldies but Goodies?
Geochemical evidence was also at the forefront of controversies regarding some of the Earth's oldest biomarkers. Questions arose over the sedimentary origin of unusual banded rocks from Akilia in west Greenland. These rocks, once thought to have formed in Earth's early ocean, are over 3850 Ma in age and contain carbon with an unusual isotopic signature that may relate to earliest life. However, new geochemical data and interpretations from Fedo and Whitehouse (Science, v. 296, p. 1448) suggest instead that these rocks began life deep within the Earth as a hot magma-a very unhealthy environment for life-and were subsequently altered by interaction with high-temperature fluids. This uncertainty regarding parentage calls the origin of carbon isotope anomalies into question, although the topic remains a hot one (Mojzsis, Science, v. 298, p. 917). In another twist for early life studies, Schopf et al. (Nature, v. 416, p. 73) used novel micro-Raman analysis and imaging techniques to show correlations between carbonaceous matter and cell-like microstructures in the approximately 3.5-giga-annum Apex cherts from western Australia (see figure). These have long been regarded as some of the earliest preserved bacterial and cyanobacterial microfossils. However, this was not the last word on the subject. In the same journal issue, Brasier et al. (Nature, v. 416, p. 76) used similar image analysis and micro-Raman techniques to directly question the biogenic origin of these structures. Undoubtedly, more will be heard on this important issue.
Geochemistry is a diverse and active science, central to understanding the
complex environment in which we live. As the engines of research continue to
move forward at high speed, exciting results are sure to follow.
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