In search of late-stage planetary building blocks

Walker, Richard J.; Bermingham, K. R.; Liu, Jingao; Puchtel, I. S.; Touboul, M.; Worsham, E. A.

doi:10.1016/j.chemgeo.2015.06.028

Cited by 65 publications

(61 citation statements)

References 177 publications

(248 reference statements)

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“…This is to avoid disagreement with the isotopic compositions of the highly siderophile elements in the Earth's mantle and because the estimated mass of material accreted after the Moon-forming impact, approximately 0.5 wt% in the so-called Late Veneer, is too small (e.g. [54]). Relative to the chondrites, N and Xe are notably depleted (figure 2) in estimates of the bulk Earth [47,55].…”

Section: Introductionmentioning

confidence: 99%

The origin of inner Solar System water

Alexander¹

2017

Phil. Trans. R. Soc. A.

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Of the potential volatile sources for the terrestrial planets, the CI and CM carbonaceous chondrites are closest to the planets' bulk H and N isotopic compositions. For the Earth, the addition of approximately 2–4 wt% of CI/CM material to a volatile-depleted proto-Earth can explain the abundances of many of the most volatile elements, although some solar-like material is also required. Two dynamical models of terrestrial planet formation predict that the carbonaceous chondrites formed either in the asteroid belt (‘classical’ model) or in the outer Solar System (5–15 AU in the Grand Tack model). To test these models, at present the H isotopes of water are the most promising indicators of formation location because they should have become increasingly D-rich with distance from the Sun. The estimated initial H isotopic compositions of water accreted by the CI, CM, CR and Tagish Lake carbonaceous chondrites were much more D-poor than measured outer Solar System objects. A similar pattern is seen for N isotopes. The D-poor compositions reflect incomplete re-equilibration with H 2 in the inner Solar System, which is also consistent with the O isotopes of chondritic water. On balance, it seems that the carbonaceous chondrites and their water did not form very far out in the disc, almost certainly not beyond the orbit of Saturn when its moons formed (approx. 3–7 AU in the Grand Tack model) and possibly close to where they are found today. This article is part of the themed issue ‘The origin, history and role of water in the evolution of the inner Solar System’.

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Section: Introductionmentioning

confidence: 99%

The origin of inner Solar System water

Alexander¹

2017

Phil. Trans. R. Soc. A.

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“…(CI is a class of meteorites; ''C'' stands for ''carbonaceous'' and ''I'' for ''Ivuna'' where the type specimen fell in Tanzania.) This estimate is likely quite excessive; CI chondrites have inappropriate (nonterrestrial) 187 Os/ 188 Os and Pd/Ir ratio, making them unlikely sources of carbon in the silicate Earth (Walker et al, 2015). Other classes of meteorites have lower C contents.…”

Section: Survival Of Life In the Hadean And Early Archean With Concenmentioning

confidence: 99%

“…The time-averaged flux of Fe(0) for (R8) and (R9) was large in the Hadean. The abundance of Pt-group elements constrains a veneer mass of asteroid material that arrived after the Moon-forming impact to <0.3-0.7% of the mass of the mantle 4 · 10 24 kg Marchi et al, 2012Marchi et al, , 2013Marchi et al, , 2014Raymond et al, 2013;Walker et al, 2015). I use 0.5% of the mantle mass or 2 · 10 22 kg for an example calculation.…”

Section: Survival Of Life In the Hadean And Early Archean With Concenmentioning

confidence: 99%

Geological and Geochemical Constraints on the Origin and Evolution of Life

Sleep

2018

Astrobiology

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The traditional tree of life from molecular biology with last universal common ancestor (LUCA) branching into bacteria and archaea (though fuzzy) is likely formally valid enough to be a basis for discussion of geological processes on the early Earth. Biologists infer likely properties of nodal organisms within the tree and, hence, the environment they inhabited. Geologists both vet tenuous trees and putative origin of life scenarios for geological and ecological reasonability and conversely infer geological information from trees. The latter approach is valuable as geologists have only weakly constrained the time when the Earth became habitable and the later time when life actually existed to the long interval between ∼4.5 and ∼3.85 Ga where no intact surface rocks are known. With regard to vetting, origin and early evolution hypotheses from molecular biology have recently centered on serpentinite settings in marine and alternatively land settings that are exposed to ultraviolet sunlight. The existence of these niches on the Hadean Earth is virtually certain. With regard to inferring geological environment from genomics, nodes on the tree of life can arise from true bottlenecks implied by the marine serpentinite origin scenario and by asteroid impact. Innovation of a very useful trait through a threshold allows the successful organism to quickly become very abundant and later root a large clade. The origin of life itself, that is, the initial Darwinian ancestor, the bacterial and archaeal roots as free-living cellular organisms that independently escaped hydrothermal chimneys above marine serpentinite or alternatively from shallow pore-water environments on land, the Selabacteria root with anoxygenic photosynthesis, and the Terrabacteria root colonizing land are attractive examples that predate the geological record. Conversely, geological reasoning presents likely events for appraisal by biologists. Asteroid impacts may have produced bottlenecks by decimating life. Thermophile roots of bacteria and archaea as well as a thermophile LUCA are attractive.

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“…This is in contrast to the expectations from rather different and very large metal/silicate NM partition coefficients. The chondritic NM relative abundances also follow from 190 Pt-187 187 Os isotopic systematics [Meisel et al, 2001;Walker, 2015]. This feature of the mantle NM abundances is known since long [e.g., Wanke et al, 1984] and is repeatedly discussed in the literature [Holzheid et al, 2000;Kramers, 1998;Maier et al, 2009;Walker, 2016].…”

Section: Noble Metal Abundances In the Mantlementioning

confidence: 90%

“…Absolute chondritic contents of NM were "diluted" via mixing within the mantle, which had already lost NM (during the main stage of core segregation), but the relative chondritic concentration pattern was preserved. To supply NM into the mantle, a contribution of the chondritic material ≈ 0.5% of the Earth mass is required [Walker, 2015]. The relative abundances of NM seen in Figure 1b imply enstatite-like chondrites to be carriers of these species and thus a principal component of the late veneer.…”

Section: Noble Metal Abundances In the Mantlementioning

confidence: 99%

The late Earth's accretion: Processes and materials

Tolstikhin¹

2018

Russ. J. Earth Sci.

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In accord with the standard Earth accretion scenario, the late accretion supervened the last collision with a massive proto-planet, segregation of the core, and (partial) solidification of the magma ocean. These processes took place ≈ 40 Ma after Sun formation or somewhat later. Traces of the processes and respective materials have been preserved as specific elemental and isotopic abundances in the earth's mantle. Pu-238 U-129 I-Xe systematics trace the accreting materials and the rate of mantle mixing and degassing. Recently proposed interpretations of this last systematics appear to be precarious and are particularly discussed in this contribution. During the late accretion a terrestrial regolith, including chondritic and solar-wind-irradiated materials, was rapidly accumulating on the surface of the early thick basaltic crust, enriched in incompatible elements. This early crust had not been preserved. Its overturn(s) into the mantle during several 100th Ma after Sun formation and (partial) isolation from the mantle convection allow all principal observations, related to the informative systematics mentioned above, to be satisfied, providing the transfer of the crust®olith "cake" was not accompanying by fractionation and degassing, in contrast to present-day slab subduction.

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In search of late-stage planetary building blocks

Cited by 65 publications

References 177 publications

The origin of inner Solar System water

The origin of inner Solar System water

Geological and Geochemical Constraints on the Origin and Evolution of Life

The late Earth's accretion: Processes and materials

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