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Terrestrial and Lunar Magmatism: An Evolutionary Overview
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
E.V. Sharkov, O.A. Bogatikov, V.I. Kovalenko
The continental crust, 70–80% of which is composed of Archaean rocks (Taylor and McLennan, 1985), presently accounts for less than 1% of the mantle mass. The extraction of this quantity of present-day upper crust must have caused minimal changes in upper-mantle composition. However, study of the degree of crustal depletion in micro-elements (Cs, Rb, K, Ba, U, Th, La and Ce) relative to the primitive mantle suggests that reworking of large volumes of mantle was necessary to form the crust. According to different assessments, the extraction of crust rich in LILE required 30–50% of the primitive mantle (or one-third, according to Jackobsen and Wasserburg (1979)) to have been reworked (Ringwood, 1978; O’Nions et al., 1979; Allegre 1987; etc.). In fact, it means that all of the upper mantle, down to depths of about 670 km, must have been involved in reworking to a certain degree, in order to produce the observed ratio of incompatible elements and the observed isotopic characteristics of the rocks. In this case, the crust is enriched in lithophile elements to the extent that their radii and valencies differ from those of typical mantle cations (Mg and Fe) in six-fold co-ordination. This regularity signifies that crystal-melt fractionation was a leading cause of the generation of crustal material from the mantle (Taylor and Mclennan, 1985). Based on these data, the depth of magmatic ocean can be evaluated as approximately 700 km.
Basaltic dykes and their xenoliths from the Gerroa–Kiama region, southern Sydney Basin, New South Wales: evidence for multiple intrusive episodes
Published in Australian Journal of Earth Sciences, 2022
S. Abu-Shamma, I. T. Graham, P. Lennox, G. Bann, A. Greig
As the dykes are considerably altered, the whole-rock geochemical analyses were performed using the much lower-cost fpXRF method, and thus the interpretation of the geochemical data has relied upon the high-field-strength elements Ti, Y, Zr and Nb, as these are immobile under most situations (Pearce, 2014). The light lithophile elements such as K, Na, Ca, Ba and Sr are moderately to highly mobile during low-temperature alteration, metasomatism and metamorphism (Hughes, 1972), and thus can be depleted or enriched during these processes, resulting in different concentrations than in the original rock. The fpXRF is not reliable for SiO2 and, to a lesser extent, Al2O3.