Ma Of Solar System History Research Articles

Age and origin of IIE iron meteorites inferred from Hf-W chronology

Abstract Non-magmatic iron meteorites, including the IIE group, can provide important insights into the history of metal-silicate differentiation and collisions on planetesimals. To better constrain the evolution of metal segregation and impacts on the IIE parent body, W isotopic data are reported for 10 IIE iron meteorites and a metal vein from the Portales Valley H6 chondrite. In addition, Pt isotopic data were obtained to quantify cosmic ray-induced neutron capture effects on W isotopes. After correction for these effects, the IIE iron meteorites exhibit variable pre-exposure 182W compositions, translating into Hf-W model age clusters of ∼4–5 million years (Ma), ∼10 Ma, ∼15 Ma, and ∼27 Ma after CAI formation. These distinct 182W clusters likely represent samples from several discrete metallic melt pools on the IIE parent asteroid. The earliest metal segregation event at ∼4–5 Ma was likely facilitated by 26Al decay as an internal heat source. By contrast, the younger Hf-W model ages may not be chronologically meaningful, and probably reflect the effects of secondary mixing and re-equilibration of metal and silicates, likely facilitated by impacts on the IIE parent body. Thus, contrary to prior work, the Hf-W systematics of IIE iron meteorites do not require a protracted history of metal-silicate separation on the IIE parent body. Instead the results of this study are fully consistent with a single partial metal-silicate differentiation event driven by endogenic heating at ∼4–5 Ma, followed by one or multiple impact events causing mixing and re-equilibration of metal and silicates at a later stage. The exact timing of these impact event(s) remains poorly constrained, but they most likely occurred in the first few tens of Ma of Solar System history.

Temporal evolution of primordial tungsten-182 and 3He/4He signatures in the Iceland mantle plume

Studies of short-lived radiogenic isotope systems and noble gas isotopic compositions of plume-derived rocks suggest the existence of primordial domains in Earth's present-day mantle. Tungsten-182 anomalies together with high 3He/4He in Phanerozoic rocks from large igneous provinces and ocean island basalts demonstrate the preservation of early-formed (within the first 60 Ma of solar system history) mantle domains tapped by modern mantle plumes. It has proven difficult to link the evidence for primordial domains with geochemical evidence for more recent processes, such as recycling. The Greenland-Iceland plume system, starting with eruptions of the Paleocene North Atlantic Igneous Province, is later manifested in the mid-Miocene to modern volcanic products of Iceland. Here, we report Pb isotopic compositions, μ182W (deviations in 182W/184W of a sample from a laboratory reference standard in parts per million), and 3He/4He, as well as highly siderophile element concentrations and Re-Os isotopic systematics of basaltic samples erupted at different times during the ~60 Ma history of the Greenland-Iceland plume. Paleocene samples from Greenland, representing the early stage of the mantle plume, are characterized by variable 3He/4He ranging from 7 to 48 R/RA (measured 3He/4He normalized to the atmospheric ratio) and an average μ182W of −4.0 ± 3.6 (2SD), within modern upper mantle-like values of 0 ± 4.5. The basalts from Iceland can be divided into two groups based on their Pb isotope compositions. One group, consisting mostly of Miocene basalts, is characterized by 206Pb/204Pb ranging from ~18.4 to 18.5, 3He/4He ranging from 17.8 to 40.2 R/RA, and μ182W values ranging from +1.7 to −9.1 ± 4.5. The other group, consisting mainly of Pleistocene and Holocene basalts, is characterized by higher 206Pb/204Pb, ranging from ~18.7 to 19.2, 3He/4He ranging from 7.9 to 25.7 R/RA, and μ182W values ranging from −0.6 to −11.7 ± 4.5. Collectively, the Greenland-Iceland suite examined requires mixing between a minimum of three mantle source domains characterized by distinct Pb-He-W isotopic compositions, in order to account for this range of isotopic data. The temporal changes in the isotopic data for these rocks appear to track the dominant contributing plume components as the system evolved. One of the domains is indistinguishable from the ambient upper oceanic mantle and contributed substantial material throughout the time progression. The other two domains are most likely primordial reservoirs that underwent limited de-gassing. Given the negative μ182W values in some rocks, one of these domains likely formed within the first 60 Ma of solar system history and is a major contributor to the youngest basalts. The isotopic characteristics of Greenland-Iceland plume-derived rocks reveal episodic changes in the source component proportions.

Tungsten-182 in the upper continental crust: Evidence from glacial diamictites

Study of igneous and sedimentary rocks that may be representative of the chemical and isotopic characteristics of portions of the upper continental crust (UCC) has provided important insights to the origin and growth of the continents, as well as the large-scale chemical evolution of the silicate Earth. For example, changes in the major and trace element compositions, as well as long-lived radiogenic isotope systematics of (meta)sedimentary rocks through time have led to the conclusion that at least some portions of the Archean UCC were enriched in mafic-ultramafic components compared to UCC with younger provenance (Taylor and McLennan, 1985; Condie, 1993; Gaschnig et al., 2016; Chen et al., 2016; Tang et al., 2016; Garçon et al., 2017). Short-lived radiogenic isotope systems are an additional means of tracing the contributions of diverse mantle-derived components to the UCC. Tungsten-182 anomalies have been observed in both ancient and modern rocks. Although these anomalies were ultimately created by processes that occurred while 182Hf was extant during the first ~60 Ma of Solar System history, the causes of their incorporation and preservation in the rock record, as well as the frequency and distribution of rocks in the UCC with isotopic anomalies remain poorly understood. Here, μ182W values (deviations in 182W/184W from laboratory reference standard in parts per million) are reported for previously well-characterized, fine-grained glacial diamictites deposited between ~3.0 and ~0.3 Ga. Glacial diamictites deposited during the Mesoarchean as part of the Witwatersrand Supergroup in southern Africa are characterized by an average μ182W value of −12.5 ± 5.0 (2SD), yet a diamictite of similar age from the spatially associated Pongola Supergroup (Mozaan) is not isotopically anomalous. The isotopically anomalous diamictites are also characterized by comparatively high Ni and highly siderophile element (HSE) abundances, indicative of significant contributions from ultramafic, most likely komatiitic components. No resolvable 182W anomalies were found in glacial diamictites with lower Ni and HSE, deposited after 2.3 Ga. This in turn suggests that W isotopic anomalies in Archean UCC, at least in part, reflect contributions from deep mantle upwellings that produced some of the komatiites. The new results provide further evidence that the isotopic composition of W in the Archean crust was highly variable.

The coupled 182W‐142Nd record of early terrestrial mantle differentiation

AbstractNew Sm‐Nd, Lu‐Hf, Hf‐W, and Re‐Os isotope data, in combination with highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, and Pd) and W abundances, are reported for the 3.55 Ga Schapenburg komatiites, South Africa. The Schapenburg komatiites define a Re‐Os isochron with an age of 3550 ± 87 Ma and initial γ187Os = +3.7 ± 0.2 (2SD). The absolute HSE abundances in the mantle source of the Schapenburg komatiite system are estimated to be only 29 ± 5% of those in the present‐day bulk silicate Earth (BSE). The komatiites were derived from mantle enriched in the decay products of the long‐lived 147Sm and 176Lu nuclides (initial ɛ143Nd = +2.4 ± 0.1, ɛ176Hf = +5.7 ± 0.3, 2SD). By contrast, the komatiites are depleted, relative to the modern mantle, in 142Nd and 182W (μ182W = −8.4 ± 4.5, μ142Nd = −4.9 ± 2.8, 2SD). These results constitute the first observation in terrestrial rocks of coupled depletions in 142Nd and 182W. Such isotopic depletions require derivation of the komatiites from a mantle domain that formed within the first ∼30 Ma of Solar System history and was initially geochemically enriched in highly incompatible trace elements as a result of crystal‐liquid fractionation in an early magma ocean. This mantle domain further must have experienced subsequent melt depletion, after 182Hf had gone extinct, to account for the observed initial excesses in 143Nd and 176Hf. The survival of early‐formed 182W and 142Nd anomalies in the mantle until at least 3.55 Ga indicates that the products of early planetary differentiation survived both later planetary accretion and convective mantle mixing during the Hadean. This work moreover renders unlikely that variable late accretion, by itself, can account for all of the observed W isotope variations in Archean rocks.

Evidence for an early nitrogen isotopic evolution in the solar nebula from volatile analyses of a CAI from the CV3 chondrite NWA 8616

Nitrogen and noble gas (Ne–Ar) abundances and isotope ratios, determined by CO2 laser extraction static mass spectrometry analysis, as well as Al–Mg and O isotope data from secondary ion mass spectrometry (SIMS) analyses, are reported for a type B calcium–aluminum-rich inclusion (CAI) from the CV3 chondrite NWA 8616. The high (26Al/27Al)i ratio of (5.06±0.50)×10−5 dates the last melting event of the CAI at 39-99+109ka after “time zero”, limiting the period during which high-temperature exchanges between the CAI and the nebular gas could have occurred to a very short time interval. Partial isotopic exchange with a 16O-poor reservoir resulted in Δ17O>−5‰ for melilite and anorthite, whereas spinel and Al–Ti-pyroxene retain the inferred original 16O-rich signature of the solar nebula (Δ17O⩽−20‰). The low 20Ne/22Ne (⩽0.83) and 36Ar/38Ar (⩽0.75) ratios of the CAI rule out the presence of any trapped planetary or solar noble gases. Cosmogenic 21Ne and 38Ar abundances are consistent with a cosmic ray exposure (CRE) age of ∼14 to 20Ma, assuming CR fluxes similar to modern ones, without any evidence for pre-irradiation of the CAI before incorporation into the meteorite parent body. Strikingly, the CAI contains 1.4–3.4ppmN with a δ15N value of +8‰ to +30‰. Even after correcting the measured δ15N values for cosmogenic 15N produced in situ, the CAI is highly enriched in 15N compared to the protosolar nebula (δ15NPSN=−383±8‰; Marty et al., 2011), implying that the CAI-forming region was contaminated by 15N-rich material within the first 0.15Ma of Solar System history, or, alternatively, that the CAI was ejected into the outer Solar System where it interacted with a 15N-rich reservoir.

40 Ar/ 39 Ar ages of impacts involving ordinary chondrite meteorites

Abstract Impact ages have now been determined for c. 100 ordinary chondrites, providing information about impacts in the asteroid belt throughout the history of the Solar System. Most notably, there is evidence for (a) impact events in the first 100 Ma of Solar System history, during the accretion of material; (b) an increase in impact ages produced between 3500 and 4100 Ma, presumably related to the suspected ‘lunar cataclysm’; (c) many L chondrites being degassed during the disruption of their parent body slightly less than 500 Ma ago; and (d) at least one younger event in the H chondrites. There are no 40 Ar/ 39 Ar ages that clearly match the cosmic ray exposure ages of any meteorites, which means that the collisions that liberated the fragments we now have as meteorites did not produce time–temperature histories capable of degassing radiogenic 40 Ar. Since there is no way to know a priori whether an ordinary chondrite impact melt is the result of a recent or ancient event, much of the progress will continue to come from reconnaissance studies of shocked ordinary chondrites.

Non-basaltic asteroidal magmatism during the earliest stages of solar system evolution: A view from Antarctic achondrites Graves Nunatak 06128 and 06129

The recently recovered paired Antarctic achondrites Graves Nunatak 06128 and 06129 (GRA) are meteorites that represent unique high-temperature asteroidal processes that are identified in only a few other meteorites. The GRA meteorites contain high abundances of sodic plagioclase, relatively Fe-rich pyroxenes and olivine, abundant phosphates, and low temperature alteration. They represent products of very early planetesimal melting (4565.9 ± 0.3 Ma) of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. The magmatism represented by these meteorites is contrary to the commonly held belief that the earliest stages of melting on all planetary bodies during the first 2–30 Ma of solar system history were fundamentally basaltic in nature. These sodic plagioclase-rich rocks represent a series of early asteroidal high-temperature processes: (stage 1) melting and partial extraction of a low-temperature Fe–Ni–S melt, (stage 2) small degrees of disequilibrium partial melting of a sodium- or alkali-rich chondritic parent body with additional incorporation of Fe–Ni–S melt that was not fully extracted during stage 1, (stage 3) volatile-enhanced rapid extraction and emplacement of the Na-rich, high-normative plagioclase melt, (stage 4) final emplacement and accumulation of plagioclase and phosphates, (stage 5) subsolidus reequilibration of lithology between 962 and 600 °C at an fO 2 of IW to IW + 1.1, and (stage 6) replacement of merrillite and pyroxene by Cl-apatite resulting from the interaction between magmatic minerals and a Cl-rich fluid/residuum melt. The subsolidus events started as early as 4561.1 Ma and may have continued for upwards of 144 million years. The existence of assemblages similar to GRA on several other planetary bodies with different geochemical characteristics (ureilite, winonaites, IAB irons) implies that this type of early asteroidal melting was not rare. Whereas, eucrites and angrites represent extensive melting of a parent body with low concentrations of moderately-volatile elements, GRA represents low-degrees of melting of a parent body with chondritic abundances of moderately volatile elements. The interpretation of the low-temperature mineral assemblage is somewhat ambiguous. Textural features suggest multiple episodes of alteration. The earliest stage follows the interaction of magmatic assemblages with a Cl-rich fluid. The last episode of alteration appears to cross-cut the fusion crust and earlier stages of alteration. Stable isotopic measurements of the alteration can be interpreted as indicating that an extraterrestrial volatile component was preserved in GRA.

40Ar‐39Ar ages of H‐chondrite impact melt breccias

Abstract— 40Ar‐39Ar analyses of a total of 26 samples from eight shock‐darkened impact melt breccias of H‐chondrite affinity (Gao‐Guenie, LAP 02240, LAP 03922, LAP 031125, LAP 031173, LAP 031308, NWA 2058, and Ourique) are reported. These appear to record impacts ranging in time from 303 ± 56 Ma (Gao‐Guenie) to 4360 ± 120 Ma (Ourique) ago. Three record impacts 300–400 Ma ago, while two others record impacts 3900–4000 Ma ago. Combining these with other impact ages from H chondrites in the literature, it appears that H chondrites record impacts in the first 100 Ma of solar system history, during the era of the “lunar cataclysm” and shortly thereafter (3500–4000 Ma ago), one or more impacts ˜300 Ma ago, and perhaps an impact ˜500 Ma ago (near the time of the L chondrite parent body disruption). Records of impacts on the H chondrite parent body are rare or absent between the era of planetary accretion and the “lunar cataclysm” (4400‐4050 Ma), during the long stretch between heavy bombardment and recent breakup events (3500‐1000 Ma), or at the time of final breakup into meteorite‐sized bodies (<50 Ma).

Thermal alteration of asteroids: evidence from meteorites

The world's meteorite collections contain meteorites from at least ∼27 primitive, chondritic and ∼108 partially or totally melted asteroids. Detailed studies of these meteorites have shown that all of their asteroidal parent bodies have been thermally altered by internal heating to some degree. These alterations range from low temperature, aqueous processes (∼0 to <∼300°C), to thermal metamorphism (∼400 to ∼950°C), to partial melting and formation of unfractionated (∼980–1050°C) and fractionated (∼1000 to >1250°C) residues, to partial and complete melting, differentiation and fractional crystallization of asteroids (∼1150 to ⪢1250°C). The most likely heat source was the decay of short-lived radionuclides, notably 26 Al . These thermal alterations took place penecontemporaneously on all asteroids of which we have samples, and in the first few Ma of solar system history. The asteroid 4 Vesta, the likely parent body of the HED meteorites, is a highly differentiated object, may have a metal core, and can be viewed as the smallest of the terrestrial planets. It accreted, was heated, was partially to completely melted, and formed an extrusive basaltic crust, all within a few Ma of formation of CAIs and the dawn of the solar system.

The Missing Psyche Family: Collisionally Eroded or Never Formed?

Asteroid 16 Psyche, the largest M-type asteroid, is widely considered to be the collisionally exposed core of an ∼500-km diameter differentiated parent body which was similar to asteroid 4 Vesta. However, there is no dynamical family associated with Psyche nor are there spectroscopic data for the existence of the mantle or crustal material from the parent body. The usual explanation for the missing material requires that the Psyche parent body was collisionally disrupted early in solar system history, followed by collisional grinding of the family down to below the current observational limit sizes.We test the exposed core hypothesis for the origin of Psyche using a numerical code that simultaneously calculates both the collisional evolution of the asteroid belt and the model family formed by the breakup of the Psyche parent body (PPB). We find that it would take a projectile about 300–350 km in size to thoroughly disrupt a 500-km asteroid (the estimated size of the PPB), and that the probability of such an event occurring in the first 500 Ma of solar system history is only about 1%. While the Psyche model family is found to have been significantly ground down subsequent to its formation, there should be several tens of survivors from the mantle and crust larger than ∼10 km that should be spectroscopically detectable by current technology. Although only a small fraction of the asteroids larger than 10 km have been discovered and observed spectroscopically to date, none have been identified as potential survivors from the PPB (Burbineet al.1996, Meteor. Planet. Sci. 31, 607–620).Given the low probability of the disruption of a Vesta-like body and the lack of either dynamical or observational confirmation of a family or material from the parent body, we think it more likely that Psyche has possibly been shattered by impacts but not catastrophically disrupted. In this case, it would be a plausible candidate parent body for the mesosiderites. The exposed-core scenario more probably applies to other, smaller (diameter ≲100 km) M-type asteroids, which could be the parent bodies of the iron meteorites. However, this interpretation raises the interesting problem of why among the larger asteroids only Vesta and the PPB would have been fully differentiated.

SmNd systematics of a silicate inclusion in the Caddo IAB iron meteorite

We report the first SmNd data from an iron meteorite silicate inclusion. Mineral separates from an inclusion in the Caddo IAB iron define a 147Sm 143Nd age of 4.53±0.02 Ga, and an initial 146Sm 144Sm of 0.0086±0.0021. A plagioclase separate from the inclusion yields the lowest 142Nd 144Nd ratio measured to date, which provides an absolute maximum initial solar sysem ϵ 142 of −2.1 ± 0.4. When the 146Sm 142Nd data are combined with the 147Sm 143Nd data, we obtain our preferred initial solar system ϵ 142 value of −3.2 ϵu, which corresponds to an initial 146Sm 144Sm of 0.009. Rare earth element data from the Caddo silicate inclusion suggest that it was derived by partial melting of a chondritic source prior to incorporation into the metal. Therefore, differentiation of the silicate portion of the parent body must have started approximately within the first 30 Ma of solar system history. The data reported here clearly show that IAB meteorites preserve a record of very early planetary differentiation processes. Modeling of element diffusion indicates that, if the silicate material was incorporated into its FeNi host at a time much later than 4.53 Ga, then the mixing must have taken place at shallow levels (i.e., < 50 m) in the parent body, in order to preserve the SmNd systematics. It is more likely that incorporation of the silicate took place at depths greater than 2 km, and that it occurred within the first ∼ 30 Ma of solar system history. This is consistent with estimates of cooling rates determined for IAB irons.