Ali Pourmand

Hf–W–Th evidence for rapid growth of Mars and its status as a planetary embryo

Terrestrial planets are thought to have formed through collisions between large planetary embryos of diameter ∼1,000–5,000 km. For Earth, the last of these collisions involved an impact by a Mars-size embryo that formed the Moon 50–150 million years (Myr) after the birth of the Solar System. Although model simulations of the growth of terrestrial planets can reproduce the mass and dynamical parameters of the Earth and Venus, they fall short of explaining the small size of Mars. One possibility is that Mars was a planetary embryo that escaped collision and merging with other embryos. To assess this idea, it is crucial to know Mars’ accretion timescale, which can be investigated using the 182Hf–182W decay system in shergottite-nakhlite-chassignite meteorites. Nevertheless, this timescale remains poorly constrained owing to a large uncertainty associated with the Hf/W ratio of the Martian mantle and as a result, contradicting timescales have been reported that range between 0 and 15 Myr (refs 6–10). Here we show that Mars accreted very rapidly and reached about half of its present size in only Myr or less, which is consistent with a stranded planetary embryo origin. We have found a well-defined correlation between the Th/Hf and 176Hf/177Hf ratios in chondrites that reflects remobilization of Lu and Th during parent-body processes. Using this relationship, we estimate the Hf/W ratio in Mars’ mantle to be 3.51 ± 0.45. This value is much more precise than previous estimates, which ranged between 2.6 and 5.0 (ref. 6), and lifts the large uncertainty that plagued previous estimates of the age of Mars. Our results also demonstrate that Mars grew before dissipation of the nebular gas when ∼100-km planetesimals, such as the parent bodies of chondrites, were still being formed. Mars’ accretion occurred early enough to allow establishment of a magma ocean powered by decay of 26Al.

IRON 60 EVIDENCE FOR EARLY INJECTION AND EFFICIENT MIXING OF STELLAR DEBRIS IN THE PROTOSOLAR NEBULA

Among extinct radioactivities present in meteorites, 60 Fe (t1/2 = 1.49 Myr) plays a key role as a high-resolution chronometer, a heat source in planetesimals, and a fingerprint of the astrophysical setting of solar system formation. A critical issue with 60 Fe is that it could have been heterogeneously distributed in the protoplanetary disk, calling into question the efficiency of mixing in the solar nebula or the timing of 60 Fe injection relative to planetesimal formation. If this were the case, one would expect meteorites that did not incorporate 60 Fe (either because of late injection or incomplete mixing) to show 60 Ni deficits (from lack of 60 Fe decay) and collateral effects on other neutron-rich isotopes of Fe and Ni (coproduced with 60 Fe in core-collapse supernovae and AGB-stars). Here, we show that measured iron meteorites and chondrites have Fe and Ni isotopic compositions identical to Earth. This demonstrates that 60 Fe must have been injected into the protosolar nebula and mixed to less than 10 % heterogeneity before formation of planetary bodies. Subject headings: solar system: formation — nuclear reactions, nucleosynthesis, abundances — methods: analytical — supernovae: general

Distribution coefficients of 60 elements on TODGA resin: application to Ca, Lu, Hf, U and Th isotope geochemistry.

Batch equilibration experiments are conducted to measure the distribution coefficients (K(d)) of a large number of elements in nitric, nitric plus hydrofluoric, and hydrochloric acids on Eichrom TODGA extraction chromatography resin. The K(d)s are used to devise a multi-element extraction scheme for high-precision elemental and isotopic analyses of Ca, Hf, Lu, Th and U in geological materials, using high-purity lithium metaborate (LiBO(2)) flux fusion that allows rapid digestion of even the most refractory materials. The fusion melt, dissolved in nitric acid, is directly loaded to a TODGA cartridge on a vacuum chamber for elemental separation. An Ln-Spec cartridge is used in tandem with TODGA for Lu purification. The entire procedure, from flux digestion to preparation for isotopic analysis, can be completed in a day. The accuracy of the proposed technique is tested by measuring the concentrations of Ca (standard bracketing), Hf, Lu, Th and U (isotope dilution), and the isotopic composition of Hf in geostandards (USNM3529, BCR-2, BHVO-1, AGV-1 and AGV-2). All measurements are in excellent agreement with recommended literature values, demonstrating the effectiveness of the proposed analytical procedure and the versatility of TODGA resin.

Geochemical fingerprinting of trans-Atlantic African dust based on radiogenic Sr-Nd-Hf isotopes and rare earth element anomalies

Mineral dust is an important component of Earth’s climate system and biogeochemical cycles on a global scale. In order to understand the relationship between climate processes in the source areas and the properties of aerosols at distant receptor sites, we must be able to identify the source provenance of dust. Here we present a multiproxy study that characterizes the temporal variability in the geochemical composition of long-range African dust (LRAD) collected between 2003 and 2011 in the trade winds on the Caribbean island of Barbados. We find systematic differences between Sr-Nd-Hf isotopic composition and rare earth element anomalies of individual dust events and evidence of seasonal shifts in dust source activity and transport. These results indicate that coherent geochemical source signatures of LRAD can be preserved even after transport across thousands of kilometers. We investigated the possibility of identifying the potential source areas through comparisons with literature data. However, these data are almost entirely based on measurements of soil and sediment samples; this could lead to biases because of soil-aerosol particle size and composition differences. Nonetheless, our data suggest that many samples are linked to sources in Mali and sub-Saharan regions. Radiogenic Nd-Hf composition of aerosols can potentially be a useful proxy to study the proximity of mineral dust sources to depositional sites. In order to establish firmer links between LRAD and dust source areas, however, we require much more data on the geochemical composition of aerosols from potential source areas in North Africa.

A new method for MC-ICPMS measurement of titanium isotopic composition: Identification of correlated isotope anomalies in meteorites

A new protocol is presented for precise measurements of titanium isotopes in natural samples. Titanium was separated via ion-exchange and extraction chromatography in two stages. Tests on Ti standard solutions show that isobaric interferences from Ca, V, and Cr can be adequately corrected, as long as these elements are present at atomic ratios of Ca/Ti < 20, V/Ti < 2, and Cr/Ti < 0.1. Furthermore, Zr2+ and Mo2+ have no influences on Ti+ signals when atomic ratios of Zr/Ti < 0.002 and Mo/Ti < 0.04. Compared with these correction limits, the purified solutions have corresponding ratios several orders of magnitude lower, indicating that the chemical separation technique is effective. This newly developed method has been successfully applied to geostandards and a wide variety of bulk meteorites. Our results are in good agreement with the data from Trinquier et al. (Science, 2009, 324, 374–376)1 and reveal a linear correlation between isotope anomalies of two Ti nuclides in bulk meteorites. The correlation reflects incomplete mixing of the carrier phases for Ti isotope anomalies before bulk meteorite formation.

Thulium anomalies and rare earth element patterns in meteorites and Earth: Nebular fractionation and the nugget effect

This study reports the bulk rare earth element (REEs, La–Lu) compositions of 41 chondrites, including 32 falls and 9 finds from carbonaceous (CI, CM, CO and CV), enstatite (EH and EL) and ordinary (H, L and LL) groups, as well as 2 enstatite achondrites (aubrite). The measurements were done in dynamic mode using multi-collector inductively coupled plasma mass spectrometers (MC-ICPMS), allowing precise quantification of mono-isotopic REEs (Pr, Tb, Ho and Tm). The CI-chondrite-normalized REE patterns (LaN/LuN; a proxy for fractionation of light vs. heavy REEs) and Eu anomalies in ordinary and enstatite chondrites show more scatter in more metamorphosed (petrologic types 4–6) than in unequilibrated (types 1–3) chondrites. This is due to parent-body redistribution of the REEs in various carrier phases during metamorphism. A model is presented that predicts the dispersion of elemental and isotopic ratios due to the nugget effect when the analyzed sample mass is limited and elements are concentrated in minor grains. The dispersion in REE patterns of equilibrated ordinary chondrites is reproduced well by this model, considering that REEs are concentrated in 200 lm-size phosphates, which have high LaN/LuN ratios and negative Eu anomalies. Terrestrial rocks and samples from ordinary and enstatite chondrites display negative Tm anomalies of �� 4.5% relative to CI chondrites. In contrast, CM, CO and CV (except Allende) show no significant Tm anomalies. Allende CV chondrite shows large excess Tm (� +10%). These anomalies are similar to those found in group II refractory inclusions in meteorites but of much smaller magnitude. The presence of Tm anomalies in meteorites and terrestrial rocks suggests that either (i) the material in the inner part of the solar system was formed from a gas reservoir that had been depleted in refractory dust and carried positive Tm anomalies or (ii) CI chondrites are enriched in refractory dust and are not representative of solar composition for refractory elements. A new reference composition relevant to inner solar system bodies (CI * ) is calculated by subtracting 0.15% of group II refractory inclusions to CI. The observed Tm anomalies in ordinary and enstatite chondrites and terrestrial rocks, relative to carbonaceous chondrites, indicate that material akin to carbonaceous chondrites must have represented a small fraction of the constituents of the Earth. Tm anomalies may be correlated with Ca isotopic fractionation in bulk planetary materials as they are both controlled by addition or removal of refractory material akin to fine-grained group II refractory inclusions.

Plasmonic optical imaging of gold nanorods localization in small animals

Gold nanoparticles (GNP) have been intensively investigated for applications in cancer imaging and therapy. Most imaging studies focused on microscopic imaging. Their potential as optical imaging probes for whole body small animal imaging has rarely been explored. Taking advantage of their surface plasmon resonance (SPR) properties, we aim to develop a noninvasive diffuse optical imaging method to map the distribution of a special type of GNP, gold nanorods (GNR), in small animals. We developed an integrated dual-modality imaging system capable of both x-ray computed tomography (XCT) and diffuse optical tomography (DOT). XCT provides the animal anatomy and contour required for DOT; DOT maps the distribution of GNR in the animal. This SPR enhanced optical imaging (SPROI) technique was investigated using simulation, phantom and mouse experiments. The distribution of GNR at various concentrations (0.1–100 nM, or 3.5 ug/g–3.5 mg/g) was successfully reconstructed from centimeter-scaled volumes. SPROI detected GNR at 18 μg/g concentration in the mouse breast tumor, and is 3 orders more sensitive than x-ray imaging. This study demonstrated the high sensitivity of SPROI in mapping GNR distributions in small animals. It does not require additional imaging tags other than GNR themselves. SPROI can be used to detect tumors targeted by GNR via passive targeting based on enhanced permeability and retention or via active targeting using biologically conjugated ligands.

The Vanishing of Urmia Lake: A Geolimnological Perspective on the Hydrological Imbalance of the World’s Second Largest Hypersaline Lake

Urmia Lake was the second largest hypersaline lake in the world and the largest terminal lake in West Asia prior to catastrophically losing about 90% of its surface area over the last few decades as a consequence of anthropogenic disruptions. Urmia Lake fills in a tectonic depression in northwest Iran and draws water from an average catchment area of 52,000 km2 with 13 perennial and seasonal tributaries. The salinity of the lake varies from 140 to more than 220 g L−1 and is a function of the imbalance between water inflow and evaporation. Urmia Lake’s water supply has diminished significantly over the last three decades, leading to a drastic reduction in the lake’s surface area and has driven the salinity upwards of 380 g L−1. Through geochemical profiles of water and sediment samples from Urmia Lake, we examine the role of regional geology in the lake’s water chemistry and sedimentary evolution. Grain-size analysis of the siliciclastic fractions from core top samples reveals that the majority of the Urmia Lake bottom sediments consist of silt and clay-silt particles. This distribution transitions to sand and silty sand in the NW part of the lake, where felsic intrusive and metamorphic rocks are present. This distribution pattern, which is also manifested as changes in the lithogenic, conservative elements and the chemical index of alteration (CIA), is attributed to weathering resistance of heavy minerals. The distribution pattern of conservative elements (e.g., Ti, Zr, Si, Al), as well as redox-sensitive and mobile elements (e.g., Fe, K, Rb, Sr), in the clastic fraction of the lake’s sediments signifies the role of regional geology in defining the chemical signature of the Urmia Lake sediment. In addition, the total organic matter content shows significant correlation with the distribution pattern of clay in the sediment, potentially indicating stabilization of organic matter on fine-grained particles. The concentrations of selected heavy metals (Ni, Cr, V, and Hg) in the surficial sediments indicate that the middle and southeastern part of the lake were moderately contaminated with mercury at the time of the analysis. A significant finding is that prior to the catastrophic loss of water accelerated in the last three decades, the Urmia Lake brine was classified as Na + K-Cl-Mg-SO4 during the 1977–1997 period. The ionic signature of the Urmia brine has drastically shifted to Na + K-SO4-Mg-Cl by 2010 followed by a doubling of the total dissolved solid (TDS) content.

ERRATUM: “IRON-60 EVIDENCE FOR EARLY INJECTION AND EFFICIENT MIXING OF STELLAR DEBRIS IN THE PROTOSOLAR NEBULA” (2008, ApJ, 686, 560)

Due to an error in processing the data, some values of 60 Ni corrected for instrumental mass fractionation using 61Ni/58Ni ratios are incorrect. The corrected Table 2 is appended below. Nevertheless, the shifts that result from this correction are all within error bars and do not affect the conclusions of the paper. The figures are also unaffected.

Erratum: Iron-60 evidence for early injection and efficient mixing of stellar debris in the protosolar nebula (Astrophysical Journal (2008) 686 (560))

Due to an error in processing the data, some values of 60 Ni corrected for instrumental mass fractionation using 61Ni/58Ni ratios are incorrect. The corrected Table 2 is appended below. Nevertheless, the shifts that result from this correction are all within error bars and do not affect the conclusions of the paper. The figures are also unaffected.