Anders Meibom

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

Isotopic Compositions of Cometary Matter Returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single 17O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is 16O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.

Young chondrules in CB chondrites from a giant impact in the early Solar System

Chondrules, which are the major constituent of chondritic meteorites, are believed to have formed during brief, localized, repetitive melting of dust (probably caused by shock waves) in the protoplanetary disk around the early Sun. The ages of primitive chondrules in chondritic meteorites indicate that their formation started shortly after that of the calcium-aluminium-rich inclusions (4,567.2 ± 0.7 Myr ago) and lasted for about 3 Myr, which is consistent with the dissipation timescale for protoplanetary disks around young solar-mass stars. Here we report the 207Pb–206Pb ages of chondrules in the metal-rich CB (Bencubbin-like) carbonaceous chondrites Gujba (4,562.7 ± 0.5 Myr) and Hammadah al Hamra 237 (4,562.8 ± 0.9 Myr), which formed during a single-stage, highly energetic event. Both the relatively young ages and the single-stage formation of the CB chondrules are inconsistent with formation during a nebular shock wave. We conclude that chondrules and metal grains in the CB chondrites formed from a vapour–melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated. These findings therefore provide evidence for planet-sized objects in the earliest asteroid belt, as required by current numerical simulations of planet formation in the inner Solar System.

Distribution of magnesium in coral skeleton

Ion micro-probe imaging of the aragonite skeleton of Pavona clavus, a massive reef-building coral, shows that magnesium and strontium are distributed very differently. In contrast to strontium, the distribution of magnesium is strongly correlated with the fine-scale structure of the skeleton and corresponds to the layered organization of aragonite fibers surrounding the centers of calcification, which have up to ten times higher magnesium concentration. This indicates a strong biological control over the magnesium composition of all structural components within the skeleton. Magnesium may be used by the coral to actively control the growth of the different skeletal crystal components.

The Statistical Upper Mantle Assemblage

A fundamental challenge in modern mantle geochemistry is to link geochemical data with geological and geophysical observations. Most of the early geochemical models involved a layered mantle and the concept of geochemical reservoirs. Indeed, the two layer mantle model has been implicit in almost all geochemical literature and the provenance of oceanic island basalt (OIB) and mid-ocean ridge basalt (MORB) [van Keken et al., Annu. Rev. Earth Planet. Sci. 30 (2002) 493–525]. Large-scale regions in the mantle, such as the ‘convective’ (i.e. well-stirred, homogeneous) upper mantle, sub-continental lithosphere, and the lower mantle were treated as distinct and accessible geochemical reservoirs. Here we discuss evidence for a ubiquitous distribution of small- to moderate-scale (i.e. 10^2–10^5 m) heterogeneity in the upper mantle, which we refer to as the statistical upper mantle assemblage (SUMA). This heterogeneity forms as the result of long-term plate tectonic recycling of sedimentary and crustal components. The SUMA model does not require a convectively homogenized MORB mantle reservoir, which has become a frequently used concept in geochemistry. Recently, Kellogg et al. [Earth Planet. Sci. Lett. 204 (2002) 183–202] modeled MORB and OIB Sr and Nd isotopic compositions as local mantle averages of random distributions of depleted residues and recycled continental crustal material. In this model, homogenization of the MORB source region is achieved by convective stirring and mixing. In contrast, in the SUMA model, the isotopic compositions of MORB and OIB are the outcome of homogenization during sampling, by partial melting and magma mixing (e.g. [Helffrich and Wood, Nature 412 (2001) 501–507]), of a distribution of small- to moderate-scale upper mantle heterogeneity, as predicted by the central limit theorem. Thus, the ‘SUMA’ acronym also captures what we consider the primary homogenization process: sampling upon melting and averaging. SUMA does not require the participation of distinct (e.g. lower mantle) reservoirs to explain OIB compositions. The complementary methods outlined in this work and in e.g. Kellogg et al. and Helffrich and Wood, provide a strong link between geochemical data and potential geophysical observables, such as the degree of partial melting, size of the melting regime, the characteristic length scales of the chemical heterogeneity and bathymetry, seismic scattering and anisotropy. We support the view that statistical distributions of lithologic components and sampling theory should replace the concept of distinct, isolated geochemical reservoirs, and extensive convective stirring prior to sampling.

Re¿Os isotopic evidence for long-lived heterogeneity and equilibration processes in the Earth's upper mantle

The geochemical composition of the Earths upper mantle is thought to reflect 4.5 billion years of melt extraction, as well as the recycling of crustal materials. The fractionation of rhenium and osmium during partial melting in the upper mantle makes the Re–Os isotopic system well suited for tracing the extraction of melt and recycling of the resulting mid-ocean-ridge basalt. Here we report osmium isotope compositions of more than 700 osmium-rich platinum-group element alloys derived from the upper mantle. The osmium isotopic data form a wide, essentially gaussian distribution, demonstrating that, with respect to Re–Os isotope systematics, the upper mantle is extremely heterogeneous. As depleted and enriched domains can apparently remain unequilibrated on a timescale of billions of years, effective equilibration seems to require high degrees of partial melting, such as occur under mid-ocean ridges or in back-arc settings, where percolating melts enhance the mobility of both osmium and rhenium. We infer that the gaussian shape of the osmium isotope distribution is the signature of a random mixing process between depleted and enriched domains, resulting from a ‘plum pudding’ distribution in the upper mantle, rather than from individual melt depletion events.

The grinding tip of the sea urchin tooth exhibits exquisite control over calcite crystal orientation and Mg distribution

The sea urchin tooth is a remarkable grinding tool. Even though the tooth is composed almost entirely of calcite, it is used to grind holes into a rocky substrate itself often composed of calcite. Here, we use 3 complementary high-resolution tools to probe aspects of the structure of the grinding tip: X-ray photoelectron emission spectromicroscopy (X-PEEM), X-ray microdiffraction, and NanoSIMS. We confirm that the needles and plates are aligned and show here that even the high Mg polycrystalline matrix constituents are aligned with the other 2 structural elements when imaged at 20-nm resolution. Furthermore, we show that the entire tooth is composed of 2 cooriented polycrystalline blocks that differ in their orientations by only a few degrees. A unique feature of the grinding tip is that the structural elements from each coaligned block interdigitate. This interdigitation may influence the fracture process by creating a corrugated grinding surface. We also show that the overall Mg content of the tooth structural elements increases toward the grinding tip. This probably contributes to the increasing hardness of the tooth from the periphery to the tip. Clearly the formation of the tooth, and the tooth tip in particular, is amazingly well controlled. The improved understanding of these structural features could lead to the design of better mechanical grinding and cutting tools.

Extreme Deuterium Excesses in Ultracarbonaceous Micrometeorites from Central Antarctic Snow

Dust to Dust Interplanetary dust particles are thought to sample the most primitive materials in the solar system. Because of their large deuterium enrichments, they are thought to have formed in interstellar molecular clouds—the birthplaces of stars—and to predate the solar system. Duprat et al. (p. 742; see Perspective by Nittler) describe two large interplanetary dust particles collected from Antarctic snow. The particles contain large zones of organic matter with deuterium excesses 10 to 30 times the terrestrial value. Because the organic matter is associated with crystalline silicates similar to those formed within the solar accretion disk, it is expected that the particles themselves formed in the Suns protoplanetary disk, contradicting the idea that all organics with deuterium excesses are of interstellar origin. Interplanetary dust particles recovered from Antarctic snow may provide a sample of the early solar system. Primitive interplanetary dust is expected to contain the earliest solar system components, including minerals and organic matter. We have recovered, from central Antarctic snow, ultracarbonaceous micrometeorites whose organic matter contains extreme deuterium (D) excesses (10 to 30 times terrestrial values), extending over hundreds of square micrometers. We identified crystalline minerals embedded in the micrometeorite organic matter, which suggests that this organic matter reservoir could have formed within the solar system itself rather than having direct interstellar heritage. The high D/H ratios, the high organic matter content, and the associated minerals favor an origin from the cold regions of the protoplanetary disk. The masses of the particles range from a few tenths of a microgram to a few micrograms, exceeding by more than an order of magnitude those of the dust fragments from comet 81P/Wild 2 returned by the Stardust mission.

A single-cell view of ammonium assimilation in coral-dinoflagellate symbiosis

Assimilation of inorganic nitrogen from nutrient-poor tropical seas is an essential challenge for the endosymbiosis between reef-building corals and dinoflagellates. Despite the clear evidence that reef-building corals can use ammonium as inorganic nitrogen source, the dynamics and precise roles of host and symbionts in this fundamental process remain unclear. Here, we combine high spatial resolution ion microprobe imaging (NanoSIMS) and pulse-chase isotopic labeling in order to track the dynamics of ammonium incorporation within the intact symbiosis between the reef-building coral Acropora aspera and its dinoflagellate symbionts. We demonstrate that both dinoflagellate and animal cells have the capacity to rapidly fix nitrogen from seawater enriched in ammonium (in less than one hour). Further, by establishing the relative strengths of the capability to assimilate nitrogen for each cell compartment, we infer that dinoflagellate symbionts can fix 14 to 23 times more nitrogen than their coral host cells in response to a sudden pulse of ammonium-enriched seawater. Given the importance of nitrogen in cell maintenance, growth and functioning, the capability to fix ammonium from seawater into the symbiotic system may be a key component of coral nutrition. Interestingly, this metabolic response appears to be triggered rapidly by episodic nitrogen availability. The methods and results presented in this study open up for the exploration of dynamics and spatial patterns associated with metabolic activities and nutritional interactions in a multitude of organisms that live in symbiotic relationships.