© Lloyd, Charles Chandler

 

ACCRETION

"Early differentiation of the Earth involved the separation of Fe-rich metal from silicate material to form the core and mantle. High temperatures were necessary and differentiation likely occurred in response to large-scale melting, induced by high-energy impacts that resulted in one or more deep magma oceans."14

Kinetic energy from these impacts caused the melting.4

"Earth's accretion history was dominated by tens of high-energy collisions with Moon- to Mars-sized bodies."14

"A series of magma oceans is likely if the crystallization time for the bulk of a deep magma ocean is short, i.e. on the order of 10^3 years."14

"Our model assumes Earth accretion and core differentiation occur simultaneously and the accretion history consists of a finite number of impacts distributed over 30 million years time, each impact large enough to create a regional or possibly global magma ocean." However, "The likelihood that the entire Earth was repeatedly liquified by impacts seems remote." "20 impacts were chosen because similar results were found in cases with more impacts."10

"The current dominant paradigm in mantle geochemistry is that the continental crust is complementary to the depleted mantle, which is the source of ocean floor basalt. However, attempts to mass-balance the continental crust with the depleted mantle assuming chondritic RLE ratios for the BSE show that the amount of depleted mantle that is complementary to the crust comprises only a fraction of the whole BSE... or approximately 50% from highly incompatible trace elements such as Rb. The remaining 70 to 50% is assumed to be primitive mantle, which has not had crust extracted from it, and should thus preserve chondritic proportions of RLEs, give rise to [chondritic] basalts... and contain a large fraction of the BSE's heat-producing elements (U, Th, and K). But it has become apparent that this paradigm is beset by several problems, one of the most obvious being that there is no empirical evidence from any rocks that primitive mantle exists, anywhere."12

"There is no single type of chondritic meteorite that would represent a suitable parental material for the Earth, neither with regard to chemistry nor isotopic composition."14

Towards the end of accretion, "geochemically enriched crust is preferentially lost" from large impacts.12