Andreas Pack

Identification of the giant impactor Theia in lunar rocks

An analysis of motes of the Moon maker How did the Moon form? According to the prevailing hypothesis, a Mars-sized body known as Theia smashed into Earth. Herwartz et al. analyzed fresh basalt samples from three Apollo landing sites and compared them with several samples of Earths mantle. The oxygen isotope values measured in these lunar rocks differ significantly from the terrestrial material, supporting the giant-impact hypothesis. Science, this issue p. 1146 Isotopic oxygen measurements suggest that the Moon comprises material distinct from Earth’s mantle. The Moon was probably formed by a catastrophic collision of the proto-Earth with a planetesimal named Theia. Most numerical models of this collision imply a higher portion of Theia in the Moon than in Earth. Because of the isotope heterogeneity among solar system bodies, the isotopic composition of Earth and the Moon should thus be distinct. So far, however, all attempts to identify the isotopic component of Theia in lunar rocks have failed. Our triple oxygen isotope data reveal a 12 ± 3 parts per million difference in Δ17O between Earth and the Moon, which supports the giant impact hypothesis of Moon formation. We also show that enstatite chondrites and Earth have different Δ17O values, and we speculate on an enstatite chondrite–like composition of Theia. The observed small compositional difference could alternatively be explained by a carbonaceous chondrite–dominated late veneer.

Iron-60 Heterogeneity and Incomplete Isotope Mixing in the Early Solar System

Short-lived radionuclides (e.g., 26Al, 53Mn, 60Fe, 182Hf) are widely used to refine the chronology of the early solar system. They provide chronological information, however, only if they were homogeneously distributed in the source region of the objects under scrutiny at the time of their formation. With the high level of precision now achieved on isotopic measurements, very short time intervals can in principle be resolved and a precise evaluation of the initial homogeneity degree becomes increasingly crucial. High-precision nickel isotope data for differentiated meteorites (angrites, ureilites) and chondritic (CB) components allow us to test the initial distribution of radioactive 60Fe and stable Ni isotopes. Although these meteorites appear to have formed nearly contemporaneously, they yield variable initial 60Fe/56Fe ratios. Besides, the CB metal nodules and ureilite silicates show nucleosynthetic anomalies. The new data presented here do not confirm the recently inferred late injection of 60Fe into the protoplanetary disk. Instead, live 60Fe was present, but heterogeneously distributed, from the start of the solar system, revealing an incomplete mixing of material from various nucleosynthetic sources and restricting the use of the 60Fe-60Ni system as a chronometer.

Oxygen and Carbon Isotope Variations in a Modern Rodent Community – Implications for Palaeoenvironmental Reconstructions

Background The oxygen (δ18O) and carbon (δ13C) isotope compositions of bioapatite from skeletal remains of fossil mammals are well-established proxies for the reconstruction of palaeoenvironmental and palaeoclimatic conditions. Stable isotope studies of modern analogues are an important prerequisite for such reconstructions from fossil mammal remains. While numerous studies have investigated modern large- and medium-sized mammals, comparable studies are rare for small mammals. Due to their high abundance in terrestrial ecosystems, short life spans and small habitat size, small mammals are good recorders of local environments. Methodology/Findings The δ18O and δ13C values of teeth and bones of seven sympatric modern rodent species collected from owl pellets at a single locality were measured, and the inter-specific, intra-specific and intra-individual variations were evaluated. Minimum sample sizes to obtain reproducible population δ18O means within one standard deviation were determined. These parameters are comparable to existing data from large mammals. Additionally, the fractionation between coexisting carbonate (δ18OCO3) and phosphate (δ18OPO4) in rodent bioapatite was determined, and δ18O values were compared to existing calibration equations between the δ18O of rodent bioapatite and local surface water (δ18OLW). Specific calibration equations between δ18OPO4 and δ18OLW may be applicable on a taxonomic level higher than the species. However, a significant bias can occur when bone-based equations are applied to tooth-data and vice versa, which is due to differences in skeletal tissue formation times. δ13C values reflect the rodents’ diet and agree well with field observations of their nutritional behaviour. Conclusions/Significance Rodents have a high potential for the reconstruction of palaeoenvironmental conditions by means of bioapatite δ18O and δ13C analysis. No significant disadvantages compared to larger mammals were observed. However, for refined palaeoenvironmental reconstructions a better understanding of stable isotope signatures in modern analogous communities and potential biases due to seasonality effects, population dynamics and tissue formation rates is necessary.

Technique for High-Precision Analysis of Triple Oxygen Isotope Ratios in Carbon Dioxide

Since the discovery of mass-independent isotope effects in stratospheric and tropospheric gases, the analysis of triple oxygen isotope abundance in carbon dioxide gained in importance. However, precise triple oxygen isotope determination in carbon dioxide is a challenging task due to mass-interference of (17)O and (13)C variations. Here, we present a novel analytical technique that allows us to determine slight deviations of CO(2) from the terrestrial fractionation line [TFL]. Our approach is based on isotopic equilibration between CO(2) gas and CeO(2) powder at 685 degrees C and subsequent mass spectrometric analysis of ceria powder by infrared-laser fluorination. We found that beta(CO2-CeO2), the exponent in the relation alpha(17/16) = (alpha(18/16))(beta), amounts to 0.5240 +/- 0.0011 at 685 degrees C. The oxygen isotope anomaly of CO(2) (Delta(17)O) can be determined for a single analysis of CeO(2) with a precision of +/-0.05 per thousand (1sigma). Our CO(2)-CeO(2) equilibration procedure is performed with an excess of CO(2) so that one analysis of Delta(17)O on CO(2) requires at least 3.5 mmol of CO(2) gas. Our new technique allows accurate and precise determination of Delta(17)O in CO(2) and opens up a new field for investigating triple oxygen isotope abundance in various types of natural CO(2).

Trachyandesitic volcanism in the early Solar System

Significance Volcanism is a fundamental geological process on planets and was substantial during crustal growth on planetary bodies in the early Solar System, as witnessed by ubiquitous rocks of basaltic composition, e.g., on Earth, Moon, Mars, and asteroids. Besides basaltic volcanism, trachyandesite lavas are generated on Earth. The first occurrence of a trachyandesite lava in the meteorite collections demonstrates that trachyandesitic, alkali-, and silica-rich volcanism takes place not only on Earth today but already occurred on a small planetesimal ∼4.56 billion years ago. It sets new constraints on mechanisms and styles of early Solar System volcanism. Volcanism is a substantial process during crustal growth on planetary bodies and well documented to have occurred in the early Solar System from the recognition of numerous basaltic meteorites. Considering the ureilite parent body (UPB), the compositions of magmas that formed a potential UPB crust and were complementary to the ultramafic ureilite mantle rocks are poorly constrained. Among the Almahata Sitta meteorites, a unique trachyandesite lava (with an oxygen isotope composition identical to that of common ureilites) documents the presence of volatile- and SiO2-rich magmas on the UPB. The magma was extracted at low degrees of disequilibrium partial melting of the UPB mantle. This trachyandesite extends the range of known ancient volcanic, crust-forming rocks and documents that volcanic rocks, similar in composition to trachyandesites on Earth, also formed on small planetary bodies ∼4.56 billion years ago. It also extends the volcanic activity on the UPB by ∼1 million years (Ma) and thus constrains the time of disruption of the body to later than 6.5 Ma after the formation of Ca–Al-rich inclusions.

Revealing the climate of snowball Earth from Δ17O systematics of hydrothermal rocks

Significance The snowball Earth hypothesis predicts that the entire Earth was covered with ice. Snowball Earth events were suggested to have occurred several times during the Precambrian. Classic paleo-thermometers (e.g., 18O/16O in marine carbonates) are not available from snowball Earth episodes, and only a few reconstructions of 18O/16O in ancient meteoric water exist. Here we present a novel approach to reconstruct the 18O/16O composition of ancient meteoric waters using the triple oxygen isotopic composition (17O/16O and 18O/16O) of hydrothermally altered rocks. The inferred 18O/16O for waters that precipitated at (sub)tropical paleo-latitudes on a Paleoproterozoic (∼2.4 gigayears ago) snowball Earth are extremely low. Today, similar compositions are observed only in central Antarctica. The oxygen isotopic composition of hydrothermally altered rocks partly originates from the interacting fluid. We use the triple oxygen isotope composition (17O/16O, 18O/16O) of Proterozoic rocks to reconstruct the 18O/16O ratio of ancient meteoric waters. Some of these waters have originated from snowball Earth glaciers and thus give insight into the climate and hydrology of these critical intervals in Earth history. For a Paleoproterozoic [∼2.3–2.4 gigayears ago (Ga)] snowball Earth, δ18O = −43 ± 3‰ is estimated for pristine meteoric waters that precipitated at low paleo-latitudes (≤35°N). Today, such low 18O/16O values are only observed in central Antarctica, where long distillation trajectories in combination with low condensation temperatures promote extreme 18O depletion. For a Neoproterozoic (∼0.6–0.7 Ga) snowball Earth, higher meltwater δ18O estimates of −21 ± 3‰ imply less extreme climate conditions at similar paleo-latitudes (≤35°N). Both estimates are single snapshots of ancient water samples and may not represent peak snowball Earth conditions. We demonstrate how 17O/16O measurements provide information beyond traditional 18O/16O measurements, even though all fractionation processes are purely mass dependent.

THE KAKOPETROS AND RAVDOUCHA IRON-OXIDE DEPOSITS, WESTERN CRETE, GREECE: FLUID TRANSPORT AND MINERALIZATION WITHIN A DETACHMENT ZONE

Small iron deposits at Kakopetros and Ravdoucha in western Crete are hosted by an extensional detachment zone at the roof of the high-pressure–low-temperature metamorphic core complex known as the Phyllite-Quartzite unit. The iron oxides occur in a brecciated layer of phyllite, quartzite, and marble up to tens of meters thick. They fill fractures and vugs in the breccia and partly impregnate the marble. The iron oxides, which were formerly mined in open pits, are predominantly composed of goethite and subordinate oxyhydroxides of the manganomelane group. The field relationships and microstructures indicate that precipitation of the iron-oxide minerals was related to fluid flow focussed along the detachment fault. δ 18 O values of goethite indicate crystallization at low temperatures (31°–40°C) and at a shallow depth of about 1 km. Microscopic investigations show that the deposition of iron oxides was syntectonic and occurred during deformation in the uppermost crust. Similar iron oxides are reported from low-angle brittle detachment horizons in the Cordilleran metamorphic core complexes of North America and suggest that small iron- and manganese-oxide deposits of this type may be a characteristic feature of detachment zones.

Description of an aerodynamic levitation apparatus with applications in Earth sciences.

BackgroundIn aerodynamic levitation, solids and liquids are floated in a vertical gas stream. In combination with CO2-laser heating, containerless melting at high temperature of oxides and silicates is possible. We apply aerodynamic levitation to bulk rocks in preparation for microchemical analyses, and for evaporation and reduction experiments.ResultsLiquid silicate droplets (~2 mm) were maintained stable in levitation using a nozzle with a 0.8 mm bore and an opening angle of 60°. The gas flow was ~250 ml min-1. Rock powders were melted and homogenized for microchemcial analyses. Laser melting produced chemically homogeneous glass spheres. Only highly (e.g. H2O) and moderately volatile components (Na, K) were partially lost. The composition of evaporated materials was determined by directly combining levitation and inductively coupled plasma mass spectrometry. It is shown that the evaporated material is composed of Na > K >> Si. Levitation of metal oxide-rich material in a mixture of H2 and Ar resulted in the exsolution of liquid metal.ConclusionsLevitation melting is a rapid technique or for the preparation of bulk rock powders for major, minor and trace element analysis. With exception of moderately volatile elements Na and K, bulk rock analyses can be performed with an uncertainty of ± 5% relative. The technique has great potential for the quantitative determination of evaporated materials from silicate melts. Reduction of oxides to metal is a means for the extraction and analysis of siderophile elements from silicates and can be used to better understand the origin of chondritic metal.

Temperature and atmospheric CO2 concentration estimates through the PETM using triple oxygen isotope analysis of mammalian bioapatite

Significance Our data suggest that the sudden rise in atmospheric temperature during the Paleocene–Eocene transition was not accompanied by highly elevated carbon dioxide concentrations >∼2,500 ppm. Instead, the low 13C/12C isotope ratios during the Paleocene–Eocene Thermal Maximum were most likely caused by a significant contribution of methane to the atmosphere. We present data applying a newly developed partial pressure of CO2 proxy. The Paleocene–Eocene Thermal Maximum (PETM) is a remarkable climatic and environmental event that occurred 56 Ma ago and has importance for understanding possible future climate change. The Paleocene–Eocene transition is marked by a rapid temperature rise contemporaneous with a large negative carbon isotope excursion (CIE). Both the temperature and the isotopic excursion are well-documented by terrestrial and marine proxies. The CIE was the result of a massive release of carbon into the atmosphere. However, the carbon source and quantities of CO2 and CH4 greenhouse gases that contributed to global warming are poorly constrained and highly debated. Here we combine an established oxygen isotope paleothermometer with a newly developed triple oxygen isotope paleo-CO2 barometer. We attempt to quantify the source of greenhouse gases released during the Paleocene–Eocene transition by analyzing bioapatite of terrestrial mammals. Our results are consistent with previous estimates of PETM temperature change and suggest that not only CO2 but also massive release of seabed methane was the driver for CIE and PETM.

Tracing the oxygen isotope composition of the upper Earth's atmosphere using cosmic spherules

Molten I-type cosmic spherules formed by heating, oxidation and melting of extraterrestrial Fe,Ni metal alloys. The entire oxygen in these spherules sources from the atmosphere. Therefore, I-type cosmic spherules are suitable tracers for the isotopic composition of the upper atmosphere at altitudes between 80 and 115 km. Here we present data on I-type cosmic spherules collected in Antarctica. Their composition is compared with the composition of tropospheric O2. Our data suggest that the Earths atmospheric O2 is isotopically homogenous up to the thermosphere. This makes fossil I-type micrometeorites ideal proxies for ancient atmospheric CO2 levels.