mantle metasomatism; arc volcanism; subduction zone; experimental petrology; adakite
Skora Susanne, Freymuth Heye, Blundy Jon, Elliott Tim, Guillong Marcel (2017), An experimental study of the behaviour of cerium/molybdenum ratios during subduction: Implications for tracing the slab component in the Lesser Antilles and Mariana Arc, in Geochimica et Cosmochimica Acta
, 212, 133-155.
Kimura Jun-Ichi, Gill James B., van Keken Peter E., Kawabata Hiroshi, Skora Susanne (2017), Origin of geochemical mantle components: Role of spreading ridges and thermal evolution of mantleTHERMOCHEMICAL EVOLUTION OF EARTH'S MANTLE, in Geochemistry, Geophysics, Geosystems
, 18(2), 697-734.
Freymuth Heye, Elliott Tim, van Soest Matthijs, Skora Susanne (2016), Tracing subducted black shales in the Lesser Antilles arc using molybdenum isotope ratios, in Geology
, 44(12), 987-990.
Kimura Jun-Ichi, Gill James B., Skora Susanne, van Keken Peter E., Kawabata Hiroshi (2016), Origin of geochemical mantle components: Role of subduction filterORIGIN OF EARTH'S GEOCHEMICAL COMPONENTS, in Geochemistry, Geophysics, Geosystems
, 17(8), 3289-3325.
Carter L. B., Skora S., Blundy J. D., De Hoog J. C. M., Elliott T. (2015), An Experimental Study of Trace Element Fluxes from Subducted Oceanic Crust, in Journal of Petrology
, 56(8), 1585-1606.
Skora S., Mahlen N. J., Johnson C. M., Baumgartner L. P., Lapen T. J., Beard B. L., Szilvagyi E. T. (2015), Evidence for protracted prograde metamorphism followed by rapid exhumation of the Zermatt-Saas Fee ophiolite, in Journal of Metamorphic Geology
, 33(7), 711-734.
Skora Susanne, Blundy Jon D., Brooker Richard A., Green Eleanor C. R., de Hoog Jan C. M., Connolly James A. D. (2015), Hydrous Phase Relations and Trace Element Partitioning Behaviour in Calcareous Sediments at Subduction-Zone Conditions, in Journal of Petrology
, 56(5), 953-980.
Kimura Jun-Ichi, Origin of geochemical mantle components: Role of subdcution filter, in Geochemistry, Geophysics, Geosystems
Subduction zones provide the major geotectonic setting where surficial crust can enter the earth deep interior, as two tectonic plates collide. One process that is ultimately related to subduction zones is the formation of volcanic arcs. Hydrous fluids and melts are released from the subducting lithospheric slab as it gets heated up while sinking through earth's mantle. These chemically buoyant fluids and melts interact with the overlying column of mantle, eventually triggering melting by lowering its melting point. Water-bearing basaltic magmas so-produced ascend into the crust, differentiate to more silicic (andesitic) compositions and eventually give rise to volcanism on the over-riding plate.Adakites represent a very specific rock type that can be found in arcs, differing chemically from common arc andesites, especially in their trace element signature. It has been recognized early on that these particular arc lavas resemble the partial melt that is generated in the subducted basaltic crust at sub-arc depth. This has hence led to the interpretation that arc adakites represent the primary slab melt component. Early studies have found that adakites are restricted to young subduction zones, and the idea was born that a particularly hot temperature regime is required, in which the subducted basaltic crust can melt. Since the discovery of adakites in young and hot subduction zone environments, however, an increasing amount of studies have identified adakites in arcs where slabs considered being too old (and cold) are subducting. This has prompted the need of alternative models. As of yet, there appears no real consensus throughout the literature on what process eventually leads to adakite formation, other than garnet (and/or clinopyroxene, and/or amphibole) must be present in the melting source. In particular, not much is known on the interaction of slab fluids with mantel wedge peridotite, despite a) interpretation of adakites as representing the slab melt, and b) knowledge that adding melts that are in equilibrium with subducting basalt to wedge peridotites is driving a set of metasomatic reactions, because this combination is out of equilibrium. These metasomatic reactions, in turn, have the potential to significantly modify the slab fluid signal. I now suggest using an experimental approach to investigate the interaction of slab fluids with wedge peridotite in great detail.The first experimentally testable hypothesis is that the different temperature profiles that a slab fluid can follow exert important control on the bulk chemistry of the fluids as they reactively travel upwards. Recent advances in thermal modelling of the slab have suggested that, overall, slab top temperatures in the majority of arcs are well above the solidus temperatures of H2O-saturated basalt. These observations challenge the classic view that the slab temperatures provide the sole control on adakite formation. The link between the majority of adakites being located in arcs where young and hot slabs are subducting, on the other hand, remains striking. The effect that kinetics (e.g. incomplete equilibration of slab fluids upon ascent) has on reactive melt transport is also poorly constraint and only very few experimental studies exist at present (with applications to different tectonic settings). These studies, however, have highlighted the importance of investigating such processes in more detail as they exert important control on the chemical evolution of a fluid phase during their reactive ascent. The third experimentally testable hypothesis is that oxidising conditions in the mantle wedge are key for the genesis of adakites. This hypothesis follows from studies on Cu-Au±Mo deposits that are directly associated with the presence of adakites, suggesting oxidising conditions in the melting source of > FMQ+2 (in contrast to the sub-arc mantle above which common arc andesites are erupting (FMQ±1)). In fact, it is to expect that the prevailing oxygen fugacity will be an important parameter for the presence or absence of garnet (and/or clinopyroxene, and/or amphibole), as all these minerals are well known for being able to accommodate significant amounts of Fe2+ as well as Fe3+. A systematic experimental study on the effect of different oxygen fugacities on mantle peridotite phase assemblages, however, needs yet to be done. The proposed experimental and analytical study will provide much needed data on one of the least understood processes of the subduction zone cycle: the slab fluid-mantle interactions. Such data will be crucial step to better understand how ascending slab fluids interact with wedge peridotite, and the effect that these processes have on slab fluid and melt chemistries. The results of this study will also have major implications, for example, for thermal models of subduction zones by placing petrological constraints on wedge temperatures, as well as redox conditions.