Journal ArticleDOI
Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas
TLDR
A comparative study of Archean and post-Archengitic granitic rocks shows significant changes with time as mentioned in this paper, which is a direct consequence of the cooling of Earth, and the high rare-earth element fractionation and the low Yb content of the Archean granitoids indicate the major role of garnet and hornblende, whereas these two minerals do not play a prominent part in the genesis of modern granitic rock.Abstract:
The comparative study of Archean and post-Archean granitic rocks shows significant changes with time. The high rare-earth element fractionation and the low Yb content of the Archean granitoids indicate the major role of garnet and hornblende, whereas these two minerals do not play a prominent part in the genesis of modern granitic rocks. This difference is a direct consequence of the cooling of Earth. In Archean time the subducted oceanic crust was young and warm, so it reached the conditions of melting before dehydration had occurred, leaving a garnet- and hornblende-bearing residue. In contrast, the modern subducted oceanic slab is generally old and cold, so it is dehydrated before it reaches the melting conditions of hydrous tholeiite; therefore, in the absence of a hydrous phase, it cannot melt at shallow depth. The fluids produced by dehydration reactions of modern crust rehydrate the overlying mantle wedge, which, in consequence, can undergo partial melting and give rise to calc-alkaline magmas; in this case, olivine and pyroxene are the most important residual phases. The location of calc-alkaline magma genesis in subduction-zone environments has migrated over time from the subducted Archean oceanic crust to the mantle wedge, a migration attributed to the progressive cooling of Earth.read more
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Journal ArticleDOI
The geochemical evolution of the continental crust
TL;DR: A survey of the dimensions and composition of the present continental crust is given in this paper, where it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons).
Journal ArticleDOI
Derivation of some modern arc magmas by melting of young subducted lithosphere
Marc J. Defant,Mark S. Drummond +1 more
TL;DR: In this paper, the trace-element geochemical properties of the adakites (termed "adakites") of modern island and continental arcs are shown to be consistent with a derivation by partial melting of the subducted slab, and in particular that subducting lithosphere younger than 25 Myr seems to be required for slab melting to occur.
Journal ArticleDOI
An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution
TL;DR: In this article, an analysis of an extensive adakite geochemical database identifies two distinct compositional groups: high-SiO2 adakites (HSA) which represent subducted basaltic slab-melts that have reacted with peridotite during ascent through mantle wedge and low-Si O 2 adakitic mantle wedge.
Journal ArticleDOI
Chemical composition and evolution of the upper continental crust: Contrasting results from surface samples and shales
TL;DR: The average chemical composition of the upper continental crust (UC) as a function of age is estimated from chemical analyses, geologic maps, stratigraphic sections and isotopic ages as discussed by the authors.
Journal ArticleDOI
Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation
Max W. Schmidt,Stefano Poli +1 more
TL;DR: In this article, phase diagrams of hydrous mid-ocean ridge (MOR) basalts to 330 km depth and hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones.
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