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Japan Marine Science and Technology Center Institute for Frontier Research on Earth Evolution (IFREE) Research Program for Geochemical Evolution 1. Research Overview The crust and mantle, which are composed of silicate minerals, represent % of the Earth's mass. Although the crust is volumetrically insignificant in the solid Earth, it contains a large fraction of the elements that preferentially enter the melt when a silicate is melted. High concentrations of these elements in such a small volume mean that the Earth is an extensively differentiated body. While the Earth's mantle is much more homogeneous in compositions than the crust, it has been well established that at least four, compositionally distinct components are required to explain the isotopic compositions of mid-ocean ridge basalts (MORBs) and lavas of ocean islands that are built by the activity of deep-seated hotspots. Understanding the origin of such geochemical endmembers in the crust and mantle is essential to document the evolution of the solid Earth. The Research Program for Geochemical Evolution aims to seek an understanding of the evolutionary processes of the solid Earth based on comprehensive studies of solid earth materials. 2. Some Research Results for Fiscal year 2002 2.1. The role of the subduction factory - the evolution of the Earth's mantle Subduction zones, where oceanic lithosphere is foundering into the Earth's interior, have been working as factories and have contributed significantly to the evolution of the solid Earth. Raw materials, such as pelagic or terrigenous sediment, oceanic crust, and mantle lithosphere, are supplied into the factory (Fig.). In the process of transportation and processing of these raw materials, the factory causes vibrations as earthquakes. The major products of the factory are arc magmas and their solidified materials, continental crust. The waste materials processed in the subduction factory, such as chemically modified lithosphere and delaminated lower continental crust, sink into the deep mantle (Fig.). continental crust mantle wedge residual materials volatiles volcanoes earthquake oceanic materials Raw Materials - oceanic material - mantle material Products - magma/volcanoes - volatiles - continental crust Residues - chemically modified slab - delaminated lower crust Fig.13 The processes occurring in the subduction factory. Raw materials, such as oceanic sediments, oceanic crust, and mantle lithosphere, are fed into the factory and are manufactured into arc magmas and continental crust. The waste materials processed in this factory, such as chemically modified oceanic crust/sediments and delaminated lower continental crust, sink into the deep mantle and are likely to have greatly contributed to the mantle evolution. Dehydration reactions within subducting hydrated basaltic crust occur continuously from very shallow levels to over km depth, but experimental studies on trace element behavior during dehydration are generally limited to those related to the amphiboliteeclogite transformation and element partitioning between aqueous fluids and garnet/clinopyroxene. A notable feature demonstrated by these experiments is that Pb is more preferentially partitioned into H O fluids than U and Th, leaving the residue, after dehydration, with higher U/Pb and Th/Pb than its original composition. It has also been demonstrated that Rb and Nd are released from the subducting crust more readily than Sr and Sm. Thus, residual basaltic crust after the amphibolite-eclogite transformation will have lower Sr/ Sr, higher Nd/ Nd, and higher Pb/ Pb values than hydrated basaltic crust (Fig.). Pb/ Pb ratios of the dehydrated residue 95
JAMSTEC 2002 Annual Report Institute for Frontier Research on Earth Evolution (IFREE) 143Nd/144Nd 207Pb/204Pb 0.5142 0.5134 HIMU 0.5126 2 Ga 0.5118 17.5 17.0 16.5 2 Ga dehydrated MORB 1 Ga DMM OIB 3 Ga 4 Ga EMI 2 Ga fresh MORB dehydrated MORB subducted sediments 2 Ga 1 Ga 0.702 0.706 0.710 0.714 subducted sediments PM EMII 87Sr/86Sr 2 Ga 1 Ga 16.0 EMII 1 Ga PM HIMU 15.5 OIB fresh EMI DMM MORB 15 20 25 30 35 40 206Pb/ 204 Pb Fig.14 Isotopic evolution of subducted oceanic crust and sediment. Variation of isotopic compositions of subducted fresh MORB, dehydrated residue of hydrous MORB and sediments. Ages of subduction are shown with symbols and lines. Compositions of the fresh MORB are calculated with Rb/Sr and Nd/Sm ratios 1% (upper curve) to 10% (lower curve) higher than the MORB source. U/Pb ratios of the fresh MORB are assumed to be same as the source. The MORB source is assumed to be derived from the primitive mantle at 4.0 Ga with parent/daughter ratios which changed continuously from 4 Ga to present. Present isotopic composition of the MORB source is: 87 Sr/ 86 Sr=0.7026, 143 Nd/ 144 Nd=0.5131, 206 Pb/ 204 Pb=17.5, 207 Pb/ 204 Pb=15.4. are likely to be significatly greater than that of the HIMU component, implying that subducted dehydrated basaltic crust may contribute to the genesis of this mantle component. The Sr and Nd isotopic evolution of the dehydrated crust is dependent on Rb-Sr and Sm-Nd ratio changes during partial melting at mid-oceanic ridges and dehydration reactions in subduction zones. Although it is difficult to estimate quantitatively, suitable parent-daughter ratios to produce HIMU-like Sr and Nd isotopic ratios could be explained through the above two processes, including accumulation of both fresh and dehydrated MORB crust (Fig.). The role of subducting sediments in the formation of EMII, one of the enriched geochemical reservoirs in the mantle, has been emphasized by several authors because oceanic sediments generally have high Sr/ Sr and relatively low Nd/ Nd values (e.g., Devey et al., ; Weaver, ). However, oceanic sediments that are subducted into the mantle contain significant amounts of hydrous phases, all of which will decompose to release fluids, ultimately causing significant fractionation of trace elements through fluid migration. Experiments on sediment dehydration have demonstrated that ancient subducted oceanic sediments, while experiencing compositional modification in the subduction factory, may evolve to an enriched component with high Sr/ Sr and Pb/ Pb values. They further indicated that the isotopic signature of the EMII component can be achieved by the addition of small amounts (~wt.%) of dehydrated sediment to DMM-like mantle or primitive mantle (Fig.). Trace element modeling by IFREE suggests that the geochemical characteristics of bulk continental crust can be reasonably explained by mixing of mantlederived basaltic and crust-derived felsic magmas. In order to make an andesitic continental crust, the melting residue after extraction of felsic melts should be removed and delaminated from the initial crust. It is thus of interest to examine the isotopic evolution of a delaminated 'anti-crust' component, based on inferred parent-daughter element concentrations, and to compare such signatures with those of the mantle reservoirs. The results of the calculation are shown in Figure , together with the isotope compositions of the mantle geochemical reservoirs. Quite distinct evolutionary curves with large variations in isotope ratio are obtained, due to differences in the degrees of involve- 96
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