© Lloyd, Charles Chandler

 

EARTH MATERIAL LOST FROM LARGE IMPACTS
"During collisions of Moon- to Mars-sized bodies, large fractions of material may be lost. Present theories of planet formation consider neither the fate nor the composition of the 'lost' material."12

"The existence of magmatic iron meteorites, thought to be the cores of planetary embryos, of which specimens from as many as 70 different parent bodies are known, attests to the frequency with which silicate mantles were stripped from differentiated bodies in the early Solar System."12

GEOCHEMISTRY
Measuring the whole-Earth against the solar Fe/Mg ratio indicates "net collisional erosion of approximately 10% silicate relative to metal during the Earth's accretion. If this collisional erosion preferentially removed differentiated crust, the assumption of chondritic ratios among all refractory lithophile elements (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 40Ar, and produces the observed 4He flux through the ocean basins. But the lower radiogenic heat production exacerbates the discrepancy with heat loss."12

"Refractory lithophile elements (RLEs) are defined as those that condense at higher temperatures than the common elements." For the Earth, "it has been an almost axiomatic assumption that the RLEs are present in the BSE in strictly chondritic ratios because these elements had not been observed to fractionate from each other in any type of chondrite". "The RLEs include Ca, Al, rare-earth elements (REEs), the radioactive heat-producing elements U and Th, and many others that are used in geochemical modeling." "The chondritic assumption has gone almost unchallenged for several reasons. Firstly, the assumption dates from well before our present understanding of planet building developed, in particular before the role of high-energy collisions during the later stages of the process was well appreciated." "Secondly, the pattern of chemical fractionations expected from preferential collisional erosion of early formed crust is just that which is expected anyway in the residues of crust formation. The process is therefore difficult to disentangle from the expected effects of the Earth's geological differentiation, except when the geochemical modeling involves summing over the whole planet." "Several strands of contrary evidence... are traditionally explained away by postulating never-sampled hidden reservoirs within the mantle."12

"Accretion of planets is an energetic affair that results in melting, degassing and differentiation rather than homogenization."2

A study finds "that for typical meteoritic volatile contents, degassing is likely to occur discontinuously, with an initial period of negligible degassing and a catastrophic degassing event later during the solidification process. The two key parameters controlling the degassing of magma oceans is the initial volatile content and the overall depth of the magma ocean."6

"Given that the hypothetical primitive mantle would be fertile, producing much more radiogenic heat than the depleted mantle, making it thermally buoyant, the lack of basalts from this hypothetical source is surely significant. The simple explanation is that the depleted mantle comprises the whole mantle, primitive mantle does not exist and the BSE has non-chondritic RLEs due to preferential collisional erosion during its accretion."12