Years Of Solar System History Research Articles

Early mantle dynamics inferred from 142Nd variations in Archean rocks from southwest Greenland

The composition and evolution of the silicate Earth during Hadean/Eoarchean times are widely debated and largely unknown due to the sparse geological record preserved from Earthʼs infancy. The short-lived 146Sm–142Nd chronometer applied to 3.8–3.7 Ga old mantle-derived amphibolites from the Isua Supracrustal Belt (ISB) in southwest Greenland has revealed ubiquitous 142Nd excesses in these rocks compared to modern samples and terrestrial Nd standards. Because the parent isotope, 146Sm, was extant only during the first few hundred million years of Solar System history, this implies derivation of the Greenland samples from a source formed in the Hadean. This mantle source is the oldest yet identified on Earth and therefore provides key information about the nature and evolution of early-differentiated reservoirs. In contrast, modern mantle-derived rocks from around the world do not have 142Nd anomalies, suggesting that the primordial heterogeneities detected in Earthʼs early mantle have been erased over time. In order to better constrain the rate at which early mantle heterogeneities have been re-homogenized, we produced new 146Sm–142Nd data for both 3.8 and 3.3 Ga old mafic rocks from different tectonic domains of the ISB, accompanied by their corresponding 147Sm–143Nd and 176Lu–176Hf systematics. The 3.8 Ga suite yields 142Nd excesses comparable to those detected previously in 3.7 Ga old ISB amphibolites, indicating that Eoarchean mafic ISB lavas originated from sources with similar differentiation histories despite being from different juxtaposed tectonic segments. Conversely, 3.3 Ga old amphibolites from the ISB do not show resolvable 142Nd anomalies compared to terrestrial Nd standards. Since Rizo et al. (2012) reported 142Nd anomalies in 3.4 Ga old ISB samples, the present data suggest that the primordial 142Nd heterogeneities in the Isua mantle disappeared between 3.4 and 3.3 Ga. The present data set consists of samples from a unique location where 500 million years of history of the early terrestrial mantle have been preserved, hence offering an exceptional opportunity to gain new insight into the compositional evolution and dynamic workings of Earthʼs primordial mantle.

Ubiquitous late veneer

Iron-loving elements are thought to have been added to Mars, Earth and the Moon after core formation. An analysis of meteorites formed in the first two to three million years of Solar System history suggests that a similar late veneer was added elsewhere too.

Chronometry of Meteorites and the Formation of the Earth and Moon

The planets of the Solar System grew by collisions, starting with the aggregation of tiny dust particles within the solar nebula and culminating in giant collisions between large planetary bodies. These giant impacts occasionally caused the formation of satellites such as the Earth's Moon. Our understanding of planet formation is based on information from various sources, including meteorites - leftovers from the earliest stages of planet formation - and samples from the Earth and Moon. By combining results from isotopic dating of these materials with dynamic modelling of the solar nebula and planet formation, researchers can reconstruct the accretion and early evolution of planetary bodies during the first [~]100 million years of Solar System history.

HED Meteorites and Their Relationship to the Geology of Vesta and the Dawn Mission

Howardite-eucrite-diogenite (HED) meteorites, thought to be derived from 4 Vesta, provide the best sampling available for any differentiated asteroid. However, deviations in oxygen isotopic composition from a common mass-fractionation line suggest that a few eucrite-like meteorites are from other bodies, or that Vesta was not completely homogenized during differentiation. The petrology and geochemistry of HEDs provide insights into igneous processes that produced a crust composed of basalts, gabbros, and ultramafic cumulate rocks. Although most HED magmas were fractionated, it is unresolved whether some eucrites may have been primary melts. The geochemistry of HEDs indicates that bulk Vesta is depleted in volatile elements and is relatively reduced, but has chondritic refractory element abundances. The compositions of HEDs may favor a magma ocean model, but inconsistencies remain. Geochronology indicates that Vesta accreted and differentiated within the first several million years of solar system history, that magmatism continued over a span of ∼10 Myr, and that its thermal history extended for perhaps 100 Myr. The protracted cooling history is probably responsible for thermal metamorphism of most HEDs. Impact chronology indicates that Vesta experienced many significant collisions, including during the late heavy bombardment. The age of the huge south pole crater is controversial, but it probably ejected Vestoids and many HEDs. Continued impacts produced a regolith composed of eucrite and diogenite fragments containing only minor exotic materials. HED meteorites serve as ground truth for orbital spectroscopic and chemical analyses by the Dawn spacecraft, and their properties are critical for instrument calibration and interpretation of Vesta’s geologic history.

Short-lived radionuclides as monitors of early crust–mantle differentiation on the terrestrial planets

The kinetic energy from large impacts, the gravitational energy released by core formation, and the heat provided by the decay of short-lived radioactive isotopes all drive extensive melting and chemical differentiation of silicate planets/planetesimals during and shortly after their formation. This early differentiation is best preserved on small objects such as the parent bodies of the eucrite and angrite meteorites where silicate melts were produced within 3 million years of solar system formation. The W isotopic composition of some iron meteorites testifies to core segregation on small planetesimals within as little as one million years or less of solar system formation. On larger objects, such as the Moon, Mars and Earth, the evidence for early differentiation provided by long-lived radioisotope systems has been variably overprinted by the continuing differentiation of these objects, but a clear signature of extensive early planet-scale differentiation is preserved in a variety of short-lived radioisotope systems, particularly, I–Pu–Xe, Hf–W and 146Sm– 142Nd. All these systems suggest that global differentiation of planetesimals and the terrestrial planets occurred during the first hundred million years of solar system history. This early processing of the Moon, Mars and Earth, may have fundamentally affected the evolution of these planets and their current internal compositional structure.

Extensive impact melting on the H-chondrite parent asteroid during the cataclysmic bombardment of the early solar system: Evidence from the achondritic meteorite Dar al Gani 896

DaG 896 is an olivine-rich microporphyritic rock of komatiitic composition. Both the olivine composition (Fa 17.5±2.1, [Mn/Mg] = 0.0061) and the bulk oxygen isotopic composition (δ 17 O = +2.55, δ 18 O = +3.50) indicate that DaG 896 is a sample of the H-chondrite parent body. The bulk chemistry shows an H-chondritic distribution of lithophile elements, whereas chalcophile and siderophile elements are strongly depleted, indicating formation through whole-rock melting (or nearly so) of H-chondrite material, nearly complete loss of the metal plus sulfide component, and crystallization without significant igneous fractionation. Superheated, severely shocked chondritic relics (∼10 vol%), typically in the form of corroded lithic fragments <100 μm in size intimately distributed within the igneous lithology, indicate that melting was triggered by a highly energetic impact, which possibly induced shock pressures of ∼80–100 GPa. The relatively young 3.704 ± 0.035 Ga 40 Ar- 39 Ar crystallization age is consistent with the impact melting origin, as magmatism in the asteroid belt was active only in the first hundred million years of solar system history. Based on textural data and thermodynamic crystallization modelling, we infer that DaG 896 crystallized from a liquidus temperature of ∼1630°C under relatively slow cooling rates (∼10°C h −1) to ∼1300°C, before quenching. The two-stage cooling history indicates that a reasonable formation environment might be a dike intruding cooler basement below a crater floor. Metal-silicate fractionation may have been accomplished, at least at the centimeter-scale of the studied meteorite sample, through differential acceleration of immiscible liquids of different density during the intense flow regimes associated with the excavation and modification stages of the cratering mechanism. Alternatively, DaG 896 may represent a surface sample of a differentiated melt body at the floor of an impact crater, as gravitational settling appears to be an effective process at the surface of a chondritic parent asteroid: for metal particles 1 to 10 mm in size, typically observed in partially differentiated impact melt rocks, Stokes’ Law indicates a settling velocity >1 m h −1 during the first few hours of crystallization on asteroidal bodies of >25 km radius. The ∼3.7 Ga age of DaG 896 nearly overlaps with the slightly older resetting ages of H-chondrites (all impact melts) available from the literature, indicating that the H-chondrite parent asteroid underwent extensive impact melting at the enduring of the cataclysmic bombardment of the early solar system. Such an age overlap may also indicate early disruption of the initial H-chondrite parent asteroid. The close similarity between the bulk composition and degassing age of DaG 896 and silicate inclusions in IIE iron meteorites is further evidence in support of a common origin by impact melting and metal-silicate segregation on the H-chondrite parent asteroid. Our new high-precision oxygen isotopic measurements of H-chondrites (Δ 17 O = 0.77 ± 0.04) should be extended to IIEs to verify this possible petrogenetic link.

Panel proposes solar system missions

A proposed probe to the Kuiper Belt and Pluto and another to Europa are among the priority Solar System exploration missions that should be pursued by NASA over the next decade, according to an 11 July report by a steering group of the Space Studies Board of the U.S. National Research Council (NRC).The report, “New Frontiers in the Solar System: An Integrated Exploration Strategy,” was requested by NASA, and proposes a set of new missions and facilities to respond to key questions in four cross‐cutting themes. The themes, which the report indicates form the basis for an integrated space exploration strategy are: the first billion years of Solar System history; volatiles and organics: the stuff of life; the origin and evolution of habitable worlds; and processes: how planets work.

Growth of the continental crust: a planetary-mantle perspective

This paper addresses the net growth through time of the total continental crust (CC), not to be confused with sporadic growth of individual continental masses (e.g., by accretion of island arcs), a process that does not necessarily require net growth. Geochemists tend to emphasize differences between CC and oceanic crust (OC), but from a whole-planet perspective CC and OC are remarkably similar in composition and density. The CC/OC distinction is canonically defined in terms of elevation. However, the Earth's bimodal elevation pattern, clearly reflects a genetic distinction. The OC participates directly in mantle convection, whereas the CC is the remaining, relatively stable portion of the crust. The Earth's advective cooling mechanism requires massive subduction of lithosphere, which due to the inherent buoyancy of crust tends to require a low crust/total lithosphere thickness ratio. Given relatively constant lithosphere thickness (except near thermally buoyant mid-ocean ridges), the Earth responds by heaping most of its crust into a few thick continents, allowing subduction to freely operate over a large area of relatively thin crust. This crustal thickness bimodality engenders the bimodal elevation pattern. Prevailing wisdom tends to hold that the rate of CC growth has been slow and ongoing; typical models show CC volume roughly proportional to 1/ h, where h is the total heat generation by the Earth's K, Th and U. However, such models are generally vague as to the mechanism that supposedly holds back growth, even after 4.5 Ga. The strong T-dependence of mantle rheology probably ensured that the mantle took only a few million years to cool within a few hundred degrees of its modern T. Gradual compositional evolution of the CC from a plausible initial composition (e.g., basaltic, anorthositic) might have slightly lowered the mean CC density, but this effect is of second-order importance in comparison to the general increase in crust/total lithosphere thickness ratio (for any given crustal thickness) engendered by temporal cooling of the mantle. Intense cratering, occurring coevally with early intense crust/mantle recycling, probably reconstituted virtually all of the CC extant prior to ~ 3.9 Ga, but CC was probably not recycled into the mantle any faster than it was regenerated. Lunar samples and meteorites provide strong circumstantial evidence that in primordial times the outer few hundred kilometers of all terrestrial planets were largely molten. Outcomes of primordial magmatism must have been strongly dependent on the pressure regimes within the planetary interiors. In general, the far greater dT/ dP of silicate melting curves vs. adiabats probably tended to foster a layered magmasphere, with a partially molten or paracumulate layer, constrained by dT/ dP relationships to a P range of roughly 3 GPa, surmounted by fully molten “magma ocean” layer, also limited in thickness by efficient convective cooling. In the Earth, P-stabilized garnet and Al-rich pyroxene were key factors limiting ultimate yield of crustal feldspar. This effect has been examined with quantitative models of high-P fractional crystallization (including, in some models, periodic replenishment of the magma ocean from below). Results indicate that a plausibly deep (~ 300–400 km) magma ocean would produce a crust comparable in both composition and thickness to that of the modern Earth. Mildly worrisome discrepancies involve CaO (for which high-P pyroxene compositions are poorly constrained) and SiO 2. The crust probably evolved to its present SiO 2-rich composition through differentiation catalyzed by H 2O, which presumably was not as abundant in the magma ocean as in the crust soon afterward. Nominal model results for mean crustal thickness are typically ~ 1.2–1.3 × the modern global mean, but for this parameter the model is designed to give conservatively high results, so the discrepancy is probably not significant. After the chaotic first few million years of solar system history, I suggest that planetary size and composition, for more than time, govern crustal volume.