Essay Example on Fluids play a critical role in the subduction Environment









Fluids play a critical role in the subduction environment The major fluid phases include H2O and CO2 released by metamorphic reactions in the subducting oceanic plate may alter the bulk rock geochemical characters of the overlying mantle wedge Iwamori and Nakakuki 2013 Peacock 1990 From the Th Yb vs Nb Yb Fig and Nb Th vs Zr Nb Fig relations it is evident that a subduction modified hydrous mantle was involved in the genesis of SGB rocks Thus for a better comprehension on the slab derived contributions to the geochemical budget of TH 1 and TH 2 suite of rocks HREE abundances were used as key tracers to identify the melting condition due to their low mobility in subduction zones and moderately incompatible behavior Iwamori and Nakakuki 2013 McDade et al 2003a As the HREE budget of the samples are considered as unaffected by the subduction processes we confine our calculations particularly to more incompatible LREE HFSE and LILE for better results Relatively depleted samples with low REE abundances 

TH 1 sample TH 2 sample were taken as the starting compositions and partition coefficients of hydrous lherzolite from McDade et al 2003 was used for the calculations The trace element compositions of the modified mantle wedge were calculated using bulk partition coefficients of each element for hydrous melting by assuming 20 of partial melting F 0 2 to generate TH 1 and TH 2 suites which is considered as typical values of most of the arc magmas Tatsumi and Eggins 1997 For the trace element compositions of the modified mantle wedge Cmw modal batch melting was assumed and representative samples Csample TH 1 sample TH 2 sample were modeled using bulk partition coefficients of hydrous peridotite D McDade et al 2003 according to the following equation Cmw Csample F 1 F D For the IAT rocks TH 1 the bulk partition coefficients were calculated for each selected trace element using the mineral mode of a hydrous lherzolite with 56 5 olivine 28 7 orthopyroxene 14 1 clinopyroxene 4 spinel McDade et al 2003a For the boninitic rocks TH 2 a harzburgitic source with 71 olivine 23 orthopyroxene 3 clinopyroxene 3 spinel McDade et al 2003b approximate to the point of clinopyroxene exhaustion and respective mineral melt partition coefficients were deployed McDade et al 2003b assumed H2O has a negligible effect on the olivine melt partition coefficients Thus we adopted opx melt and cpx melt partition coefficients for hydrous melting from McDade et al 2003b and olivine melt partition coefficients from McDade et al 2003a 

The spinel melt partition coefficients were assumed to be insignificant for all the trace elements used for modelling Further the degree of partial melting F was assumed to be 0 2 as it is considered as the general range of most of the arc magmas see Tatsumi and Eggins 1995 The pre subduction mantle wedge Cps compositions are considered as the mantle source for the IATs TH 1 and boninites TH 2 before any subduction related inputs We presumed that the trace element budget of the HREE is generally not affected by the subduction process and considered that the HREE budget of the pre subduction mantle wedge and the modified mantle wedge are same see McDade et al 2003a The trace element compositions of the pre subduction mantle wedge Cpmw were estimated from the primitive mantle composition Sun and McDonough 1989 by assuming it as the starting composition and a non modal batch melting model has been deployed according to the following equation Cps Cpmw D D F 1 P Mineral melt partition coefficients for the orthopyroxene clinopyroxene olivine and spinel were taken from McDade et al 2003a and 2003b 

Bulk partition coefficients for the partial melting were calculated for the following partial melting reaction 0 82cpx 0 40opx 0 08sp 0 30ol 1 0 melt Robinson et al 1998 The degree of partial melting involved in the generation of pre subduction mantle wedge is determined by matching the HREE concentration of the pre subduction mantle wedge with the HREE concentration of the modified mantle wedge Further the relative contributions of the subduction related components were calculated by subtracting the trace element abundances in the modified mantle wedge and the pre subduction wedge and are converted as percentage of subduction component of each trace element relative to the modified mantle wedge abundance Table As illustrated in Fig the slab derived components to the total element budget from IAT to boninite related rocks exhibit similar variations However the magnitude of slab components varies significantly for each element group e g LREE LILE and HFSE The variations in the subduction components reflect variable amount of fluid input and heterogenous metasomatism of the mantle wedge The maximum subduction modification is observed in the LILE group upto 99 in TH 1 and TH 2 Among the HFSE group the Zr Hf is less affected by the slab contribution The Nb Ta relations are also indicate slab fluids were significantly contributed to their geochemical variation TH 1 TH 2 Our calculations demonstrate that LILE budget of both TH 1 and TH 2 significantly controlled by the subduction components However the imprints of subduction modifications were also significant in the HFSE and LREE budget of SGB rocks

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