In search of lost planets – the paleocosmochemistry of the inner solar system

Halliday, Alex N.; Porcelli, D.

doi:10.1016/s0012-821x(01)00479-4

Cited by 104 publications

(59 citation statements)

References 59 publications

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“…Rb/Sr ratios and Sr isotope systematics, which constrain the timing of the volatile loss, support these conclusions (e.g. Halliday & Porcelli 2001).…”

supporting

confidence: 52%

Collisional erosion and the non-chondritic composition of the terrestrial planets

O’Neill

Palme

2008

Phil. Trans. R. Soc. A.

245

188

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The compositional variations among the chondrites inform us about cosmochemical fractionation processes during condensation and aggregation of solid matter from the solar nebula. These fractionations include: (i) variable Mg-Si-RLE ratios (RLE: refractory lithophile element), (ii) depletions in elements more volatile than Mg, (iii) a cosmochemical metal-silicate fractionation, and (iv) variations in oxidation state. Moon-to Mars-sized planetary bodies, formed by rapid accretion of chondrite-like planetesimals in local feeding zones within 10 6 years, may exhibit some of these chemical variations. However, the next stage of planetary accretion is the growth of the terrestrial planets from approximately 10 2 embryos sourced across wide heliocentric distances, involving energetic collisions, in which material may be lost from a growing planet as well as gained. While this may result in averaging out of the 'chondritic' fractionations, it introduces two non-chondritic chemical fractionation processes: post-nebular volatilization and preferential collisional erosion. In the latter, geochemically enriched crust formed previously is preferentially lost. That post-nebular volatilization was widespread is demonstrated by the non-chondritic Mn/Na ratio in all the small, differentiated, rocky bodies for which we have basaltic samples, including the Moon and Mars. The bulk silicate Earth (BSE) has chondritic Mn/Na, but shows several other compositional features in its pattern of depletion of volatile elements suggestive of non-chondritic fractionation. The whole-Earth Fe/Mg ratio is 2.1G0.1, significantly greater than the solar ratio of 1.9G0.1, implying net collisional erosion of approximately 10 per cent silicate relative to metal during the Earth's accretion. If this collisional erosion preferentially removed differentiated crust, the assumption of chondritic ratios among all RLEs in the BSE would not be valid, with the BSE depleted in elements according to their geochemical incompatibility. In the extreme case, the Earth would only have half the chondritic abundances of the highly incompatible, heat-producing elements Th, U and K. Such an Earth model resolves several geochemical paradoxes: the depleted mantle occupies the whole mantle, is completely outgassed in 40 Ar and produces the observed 4 He flux through the ocean basins. But the lower radiogenic heat production exacerbates the discrepancy with heat loss.

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“…Rb/Sr ratios and Sr isotope systematics, which constrain the timing of the volatile loss, support these conclusions (e.g. Halliday & Porcelli 2001).…”

supporting

confidence: 52%

Collisional erosion and the non-chondritic composition of the terrestrial planets

O’Neill

Palme

2008

Phil. Trans. R. Soc. A.

245

188

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“…Taken as a whole, the data indicate that moderately volatile element depletion is ubiquitous among mare basalts, through a large-rather than a small-scale process, or by hydrodynamic escape [83]. Given the differences in moderately volatile element abundances between the Moon and the Earth, it is not likely that this variation solely reflects depletion prior to planet formation [24,31]. A depletion process prior to the formation of chondrules, chondrites and planet formation in the hot, dusty solar nebula can explain the systematic depletion of volatile elements in chondrites [19], which are proxy materials for the building blocks of planets, but other depletion and/or replenishment events are also required to explain the differing volatile budgets among differentiated planetary bodies.…”

Section: Mechanisms For Generating Moderately Volatile Element Depletmentioning

confidence: 84%

Evaporative fractionation of volatile stable isotopes and their bearing on the origin of the Moon

Day

Moynier

2014

Phil. Trans. R. Soc. A.

104

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The Moon is depleted in volatile elements relative to the Earth and Mars. Low abundances of volatile elements, fractionated stable isotope ratios of S, Cl, K and Zn, high μ ( 238 U/ 204 Pb) and long-term Rb/Sr depletion are distinguishing features of the Moon, relative to the Earth. These geochemical characteristics indicate both inheritance of volatile-depleted materials that formed the Moon and planets and subsequent evaporative loss of volatile elements that occurred during lunar formation and differentiation. Models of volatile loss through localized eruptive degassing are not consistent with the available S, Cl, Zn and K isotopes and abundance data for the Moon. The most probable cause of volatile depletion is global-scale evaporation resulting from a giant impact or a magma ocean phase where inefficient volatile loss during magmatic convection led to the present distribution of volatile elements within mantle and crustal reservoirs. Problems exist for models of planetary volatile depletion following giant impact. Most critically, in this model, the volatile loss requires preferential delivery and retention of late-accreted volatiles to the Earth compared with the Moon. Different proportions of late-accreted mass are computed to explain present-day distributions of volatile and moderately volatile elements (e.g. Pb, Zn; 5 to >10%) relative to highly siderophile elements (approx. 0.5%) for the Earth. Models of early magma ocean phases may be more effective in explaining the volatile loss. Basaltic materials (e.g. eucrites and angrites) from highly differentiated airless asteroids are volatile-depleted, like the Moon, whereas the Earth and Mars have proportionally greater volatile contents. Parent-body size and the existence of early atmospheres are therefore likely to represent fundamental controls on planetary volatile retention or loss.

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“…The specific choice of the formation age of the Moon, however, does not greatly change the slopes of the tie lines connecting the bulk lunar initial with the initial 87 Sr/ 86 Sr ratios of the samples at the time of their crystallization ( Figure 8A) (Anders and Grevesse, 1989), and thus indicates formation of the Moon from precursor materials that were already volatile depleted (Halliday and Porcelli, 2001).…”

Section: Sr-nd Isotope Mixing Modelsmentioning

confidence: 96%

A Rb–Sr and Sm–Nd isotope geochronology and trace element study of lunar meteorite LaPaz Icefield 02205

Rankenburg

Brandon

Norman

2007

Geochimica et Cosmochimica Acta

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In search of lost planets – the paleocosmochemistry of the inner solar system

Cited by 104 publications

References 59 publications

Collisional erosion and the non-chondritic composition of the terrestrial planets

Collisional erosion and the non-chondritic composition of the terrestrial planets

Evaporative fractionation of volatile stable isotopes and their bearing on the origin of the Moon

A Rb–Sr and Sm–Nd isotope geochronology and trace element study of lunar meteorite LaPaz Icefield 02205

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