Igneous Processes of the Early Solar System By (original) (raw)
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Ureilite meteorites provide a new model of early planetesimal formation and destruction
Geochemical Perspectives Letters, 2020
Ureilite meteorites are ultramafic rocks derived from parts of the depleted silicate mantle of their parent planetesimal. We used Monte Carlo modelling to explain the observed array of oxygen isotopes and major element chemistry shown by bulk ureilites, after restoration of their missing core and silicate melt components. Despite using a wide range of primitive nebular material, our modelling shows that only a combination of proxy material resembling Allende-type FeO-rich and MgO-rich chondrules, can account for the ureilite oxygen isotope trend and the reconstructed ureilite major element chemistry. Our model predicts formation of a radial gradient in major elements and oxygen isotopes within the planetesimal, with a more Mg-rich silicate interior and a more Fe-rich silicate exterior. Temperatures recorded by ureilites were not high enough to form a magma ocean but were sufficiently high to form a metallic core and silicate melts. The ureilite parent planetesimal was then disrupted by impact. Re-accretion of the outer layers of more Fe-rich silicate material, at the expense of the more MgO-rich material and the core, explains the observed distribution of bulk rock and mineral compositions.
Geochimica Et Cosmochimica Acta, 2010
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.
American Mineralogist, 2008
The recently recovered Antarctic achondrites Graves Nunatak 06128 and 06129 are unique meteorites that represent high-temperature asteroidal processes in the early solar system never before identified in any other meteorite. They represent products of early planetesimal melting (4564.25 ± 0.21 Ma) and subsequent metamorphism of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. This melting event is unlike those predicted by previous experimental or geochemical studies, and indicates either disequilibrium melting of chondritic material or melting of chondritic material under volatile-rich conditions.
Geochimica et Cosmochimica Acta, 2009
We evaluate initial ( 26 Al/ 27 Al) I , ( 53 Mn/ 55 Mn) I , and ( 182 Hf/ 180 Hf) I ratios, together with 207 Pb/ 206 Pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common, closed isotopic system, i.e., the early solar nebula. The relative initial isotopic abundances of 26 Al, 53 Mn, and 182 Hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values, here denoted by I(Al) SS , I(Mn) SS , and I(Hf) SS, respectively. I(Mn) SS and I(Hf) SS = 9.1 ± 1.7 Â 10 À6 and 1.07 ± 0.08 Â 10 À4 , respectively, correspond to ''canonical" I(Al) SS = 5.1 Â 10 À5 . I(Hf) SS so determined is consistent with I(Hf) SS = 9.72 ± 0.44 Â 10 À5 directly determined from an internal Hf-W isochron for CAI minerals. I(Mn) SS is within error of the lowest value directly measured for CAIs. We suggest that erratically higher values measured for CAIs in carbonaceous chondrites may reflect proton irradiation of unaccreted CAIs by the early Sun after other asteroids destined for melting by 26 Al decay had already accreted. The 53 Mn incorporated within such asteroids would have been shielded from further ''local" spallogenic contributions from within the solar system. The relative initial isotopic abundances of the short-lived nuclides are less consistent with the 207 Pb/ 206 Pb ages of the corresponding materials than with one another. The best consistency of shortand long-lived chronometers is obtained for ( 182 Hf/ 180 Hf) I and the 207 Pb/ 206 Pb ages of angrites. ( 182 Hf/ 180 Hf) I decreases with decreasing 207 Pb/ 206 Pb ages at the rate expected from the 8.90 ± 0.09 Ma half-life of 182 Hf. The model solar system age thus determined is T SS,Hf-W = 4568.3 ± 0.7 Ma. ( 26 Al/ 27 Al) I and ( 53 Mn/ 55 Mn) I are less consistent with 207 Pb/ 206 Pb ages of the corresponding meteorites, but yield T SS,Mn-Cr = 4568.2 ± 0.5 Ma relative to I(Al) SS = 5.1 Â 10 À5 and a 207 Pb/ 206 Pb age of 4558.55 ± 0.15 Ma for the LEW86010 angrite. The Mn-Cr method with I(Mn) SS = 9.1 ± 1.7 Â 10 À6 is useful for dating accretion (if identified with chondrule formation), primary igneous events, and secondary mineralization on asteroid parent bodies. All of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies. For I(Mn) SS = 9.1 ± 1.7 Â 10 À6 , parent body differentiation is found to extend at least to 5Mapost−TSS,i.e.,untildifferentiationoftheangriteparentbody5 Ma post-T SS , i.e., until differentiation of the angrite parent body 5Mapost−TSS,i.e.,untildifferentiationoftheangriteparentbody4563.5 Ma ago, or 4564.5Maagousingthedirectlymeasured207Pb/206PbagesoftheD′Orbigny−clanangrites.The4564.5 Ma ago using the directly measured 207 Pb/ 206 Pb ages of the D'Orbigny-clan angrites. The 4564.5Maagousingthedirectlymeasured207Pb/206PbagesoftheD′Orbigny−clanangrites.The1 Ma difference is characteristic of a remaining inconsistency for the D'Orbigny-clan between the Al-Mg and Mn-Cr chronometers on one hand, and the 207 Pb/ 206 Pb chronometer on the other. Differentiation of the IIIAB iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body, and at nearly the same time as one another as shown by the Mn-Cr ages of IIIAB irons and ureilites, respectively. The latest recorded epi-0016-7037/$ -see front matter Published by Elsevier Ltd. Geochimica et Cosmochimica Acta 73 (2009) 5115-5136 sodes of secondary mineralization are for carbonates on the CI carbonaceous chondrite parent body and fayalites on the CV carbonaceous chondrite parent body, both extending to $10 Ma post-T SS . Published by Elsevier Ltd.
Protostars and Planets VI, 2014
Radioisotopic ages for meteorites and their components provide constraints on the evolution of small bodies: timescales of accretion, thermal and aqueous metamorphism, differentiation, cooling and impact metamorphism. Realising that the decay heat of short-lived nuclides (e.g. 26 Al, 60 Fe), was the main heat source driving differentiation and metamorphism, thermal modeling of small bodies is of utmost importance to set individual meteorite age data into the general context of the thermal evolution of their parent bodies, and to derive general conclusions about the nature of planetary building blocks in the early solar system. As a general result, modelling easily explains that iron meteorites are older than chondrites, as early formed planetesimals experienced a higher concentration of short-lived nuclides and more severe heating. However, core formation processes may also extend to 10 Ma after formation of Calcium-Aluminum-rich inclusions (CAIs). A general effect of the porous nature of the starting material is that relatively small bodies (< few km) will also differentiate if they form within 2 Ma after CAIs. A particular interesting feature to be explored is the possibility that some chondrites may derive from the outer undifferentiated layers of asteroids that are differentiated in their interiors. This could explain the presence of remnant magnetization in some chondrites due to a planetary magnetic field.
Geochimica et Cosmochimica Acta, 2005
The 182 Hf-182 W isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the 182 Hf/ 180 Hf and W at the time of CAI formation to (1.07 Ϯ 0.10) ϫ 10 Ϫ4 and Ϫ3.47 Ϯ 0.20, respectively. All magmatic iron meteorites examined here have initial W values that are similar to or slightly lower than the initial value of CAIs. These low W values may in part reflect 182 W-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than ϳ0.3 units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than ϳ1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids.
Geochimica et Cosmochimica Acta, 2009
In order to derive constraints on planetary differentiation processes, and ultimately the formation of the Earth, it is required to study a variety of meteoritic materials and to investigate their melting relations and elemental partitioning at variable pressures, temperatures, and oxygen fugacities (f O2). This study reports the first high pressure (HP) and high temperature (HT) investigation of an enstatite chondrite (Indarch). Four series of experiments exploring various f O2 conditions have been carried out at 1 GPa in a piston-cylinder apparatus using the EH4 chondrite Indarch. We show that temperature and redox conditions have important effects on the phase equilibria of the meteorite: the solidus and liquidus temperatures of the silicate portion increase with decreasing f O2 , and the stability fields of various phases are modified. Olivine and pyroxene are stable around 1.5 log f O2 unit below the iron-wü stite buffer (IWÀ1.5), whereas quartz and pyroxene is the stable assemblage under the most reducing conditions, between IWÀ5.0 and IWÀ4.0, due to reduction of the silicate. While these changes are occurring in the silicate, the metal gains Si from the silicate, (Fe, Mg, Mn, Ca, Cr)-bearing sulfides are observed at f O2 less than IWÀ4, and the partitioning of Ni and Mo are both affected by the presence of Si in Fe-S-C liquids. The f O2 has also a significant effect on the liquid metal-liquid silicate partitioning behavior of Si and S, two possible light elements in planetary cores, and of the slightly siderophile elements Cr and Mn. With decreasing f O2 , S becomes increasingly lithophile, Si becomes increasingly siderophile, and Cr and Mn both become strongly siderophile and chalcophile. The partitioning behavior of these elements places new constraints on models of core segregation for the Earth and other differentiated bodies.