A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab

Abstract

Two distinct types of eclogites from the Raspas Complex (Ecuador), which can be distinguished based on petrography and trace element geochemistry, were analyzed for their stable (Li, O) and radiogenic (Sr, Nd) isotope signature to constrain metasomatic changes due to fluid-overprinting in metabasaltic rocks at high-pressure conditions and to identify fluid sources. MORB-type eclogites are characterized by a relative LREE depletion similar to MORB. High-pressure (HP) minerals from this type of eclogite have highly variable oxygen isotope compositions (garnet: +4.1 to +9.

Tracing the effects of high-pressure metasomatic fluids and seawater alteration in blueschist-facies overprinted eclogites: Implications for subduction channel processes

“…Therefore, elevated  18 O values may be related to inherited effects of low-temperature seawater alteration from the precursor rock, or to the metasomatic fluid itself (Putlitz et al, 2000;Halama et al, 2011). Both possibilities will be discussed in turn.…”

“…It has also been demonstrated that fluids in HP metamorphic rocks play a key role in promoting equilibration of isotopic systems used as geochronometers (Glodny et al, 2003(Glodny et al, , 2008. In turn, some isotopic systems may still record the effects of previous seafloor alteration or the isotopic signature of the precursor rock (Halama et al, 2010(Halama et al, , 2011John et al, 2011;Miller et al, 1988;Putlitz et al, 2000).…”

“…Thus, details about the fluid composition and fluid-induced metasomatic processes can be obtained from metasomatically transformed eclogites (Halama et al, 2011;Penniston-Dorland et al, 2010;van der Straaten et al, 2008).…”

“…Increasing attention has been directed towards metasomatic changes in subducted high-pressure (HP) rocks caused by fluids released from the subducted slab (e.g., Halama et al, 2011;Marschall et al, 2009;Miller et al, 2009;Penniston-Dorland et al, 2010) and by fluid-rock interaction in the subduction channel (Bebout and Barton, 1993;Horodyskyj et al, 2009;van der Straaten et al, 2008). The subduction channel is defined as the zone of partially serpentinized mantle peridotite between the down-going slab and the dry mantle wedge and develops by reaction of slab fluids with the mantle wedge peridotite (Cloos and Shreve, 1988;Gerya et al, 2002).…”

“…These metasomatized rocks may constitute new bulk compositions that can influence the redistribution of major and trace elements (e.g., Bebout, 2007;King et al, 2006;Putnis and John, 2010;Scambelluri et al, 2006), which may be transported over short distances (redistribution on cm scale; e.g., Hermann, 2002;Spandler et al, 2003; or long distances (removal from a system on outcrop scale; e.g., Gao et al, 2007;Zack and John, 2007;John et al, 2008;Spandler et al, 2011). Radiogenic and stable isotope studies in fluid-overprinted metamorphic rocks can provide information about the extent of fluid-rock interactions (e.g., Baker et al, 1997;Yui et al, 1997;Zheng et al, 1999) and the identification of potential fluid sources (e.g., Bebout and Barton, 1993;Halama et al, 2010Halama et al, , 2011King et al, 2006;Scambelluri and Phillipot, 2001;Sadofsky and Bebout, 2004). In principle, fluids in the subduction zone may originate from (I) the partly serpentinised, subducted mantle peridotite underneath the oceanic crust (Hyndman and Peacock, 2003;Ranero et al, 2003), (II) the altered oceanic crust (AOC; e.g.…”