Connecting Slabs to Volcanic Arcs and the Mantle: Recent Advances in the Petrology and Geochemistry of Subduction-Related Rocks
Presiding: J Ryan, University of South Florida/National Science Foundation; I Savov, National Museum of Natural History, Smithsonian Institution
V51B-01 INVITED 08:30h
Element Transfer from Slab to Wedge; the Volcanic Arc Perspective
Volcanic arc magmas are characterised, as is well known, by geochemical patterns which feature the selective enrichment of some elements (the subduction-mobile or non-conservative elements) relative to others (the subduction-immobile or conservative elements). More detailed evaluation of these patterns highlights three groups of subduction-mobile elements: a low-temperature group (e.g., As, Ba, Pb, Rb) comprising elements released at temperatures of diagenesis and above; an intermediate-temperature group (e.g. Th, LREE) comprising elements released mainly at temperatures of amphibolite facies and above; and a high-temperature group (e.g., (e.g., Nb, Ta, Hf, Zr) comprising elements released mainly at melting temperatures and above. Typically, the fluxes of these three groups of elements vary according not only to input flux but also to the thermal and geodynamic state of the subduction system. Using element ratios such as Ba/Th and Nb/Th as proxies for these three groups, it is possible to demonstrate that arc segments characterised by shallow convecting mantle exhibit the greatest selective addition of the low-temperature group, while segments characterised by collisions, ridge subduction, rifting and edge flow exhibit the greatest addition of the high-temperature group. The HREE and the relatively compatible elements appear to be effectively conservative in all cases. A particular outcome of the arc-based mobility studies is the relationship between slab temperature and lava chemistry. Leeman and co-workers have established that B provides a potential proxy for slab temperature because of the temperature-controlled release of B at shallow depth. This work demonstrates that Hf and Zr also provide a proxy for slab temperature based on the temperature-controlled dissolution of zircon in fluids and melts at greater depths. By quantifying the chemistry-temperature link using Hf isotope and element-based ratios, it is possible to confirm the inference from geochemical patterns that the mobility of this element is largely restricted to unusual subduction events such as the beginning and end of the subduction cycle and episodes of ridge subduction and rifting. Thus, the transfer of elements from slab to wedge may vary considerably within the subduction cycle and this may explain, in part, why studies of arc lavas and subduction complexes do not always produce the same conclusions.
V51B-02 INVITED 08:45h
Element Transfer from Slab to Wedge: the Subducted Plate Perspective
Element release from subducted materials can be investigated using thermodynamics, experiment and observation. Thermodynamic calculations quantify free energy changes for reactions in which an element is converted from a chosen solid state to a chosen dissolved state. Such calculations illustrate the dependency of element mobility on, in particular, ionic potential and complexing agents, but have restricted application because of our limited knowledge of thermodynamic parameters for subduction pressures and temperatures. The experimental approach takes real subducted materials and subjects them to subduction pressures and temperatures in the laboratory. This provides data on element mobility under the conditions of the experiment, but is limited by the fact that experiments cannot easily simulate the real world of open systems, complex fluid compositions, and progressive chemical change during subduction. Analysis of subducted materials that have experienced known metamorphic conditions give a good indication of element addition provided the protolith composition can be estimated. However, whereas it is straightforward to use immobile elements to fingerprint the composition of the fresh protolith and then to estimate chemical gains and losses relative to that composition, it is not always clear whether these gains and losses took place on the ocean floor before subduction, during the early stages of subduction, at the pressure and temperature of the peak metamorphic event, or during exhumation. Despite these caveats and gaps in our knowledge, the existing database essentially supports the general concept of selected element enrichment prior to subduction followed by progressive release of these and other elements during subduction. Of the three main types of subducted material, hydrated mantle is enriched in H, B, U and other elements and releases an important proportion of its H (as water) - and perhaps other elements - below 100km. Metabasic rocks become selectively enriched in elements of low ionic potential, predominantly in brownstone and zeolite facies prior to subduction. They then release these during progressive subduction, while elements of higher ionic potential are probably released only if eclogite undergoes melting or if fluid-rock ratios are particularly high. Subducted sediments often provide the greatest budget of elements of high ionic potential (such as Th and LREE) and probably release these at depth in supercritical fluids and melts. Behaviour of the high field strength elements remains ambiguous, with most experiments indicating transport in slab-derived melts, but with few eclogites (even those recycled through the deep Earth) exhibiting HFSE depletion.
V51B-03 09:00h
Influence of Varied Extents of Slab Dehydration on Melting Conditions and Primary Mantle-derived Magmas, Northern Cascadia Subduction System
The northern Cascadia margin of North America represents the classic example of a "hot" subduction system. The downgoing Juan de Fuca plate ranges in age from about 10 m.y. along the coast of central Washington to less than 5 m.y. off central Vancouver Island. The Garibaldi volcanic belt (GVB) is a trench parallel, NNW-trending chain of small composite volcanoes, subglacial volcanic domes, and lava fields that lies 250 km east of the convergen margin. Beneath the GVB, the oceanic lithosphere attains an age that decreases northward from about 22 m.y to13 m.y. Primitive mafic lavas, which occur trenchward of the GVB volcanic front, range northward from high-alumina olivine tholeiites, magnesian andesites and LILE- and LREE-enriched calc-alkaline basalts at Glacier Peak, northern Washington, through transitional basalts in southern British Columbia to alkali olivine basalts and trachybasalts at Meager Mountain and Bridge River at the northern end of the volcanic belt. Concentrations of fluid-mobile-elements (FMEs) in the basalts suggest a significant interrelationship between forearc metamorphic dehydration/devolatization processes in the subducted plate and melting conditions within the mantle wedge beneath the GVB volcanic front. The forearc GVB mafic lavas generally possess an exceptionally low and more restricted range of boron abundances (0.9-5.7 ppm) compared to lavas of other subduction systems, including those of the High Cascades. The basalts also are characterized by low abundances of other FMEs (e.g., 1.5-7.9 ppm Pb, 5-11 ppm Rb, 0.10-0.36 ppm Cs, 0.7-1.7 ppm As, and 0.1-0.4 ppm Sb). Concentrations of B and FMEs in the lavas, as well as FME/LREE (e.g. Cs/La, Rb/La, and Pb/Ce) and 87Sr/86Sr ratios, correlate positively with the inferred age of the subducted slab immediately beneath the volcanic front. Such variations are consistent with increased metamorphic devolatilization of the Juan de Fuca lithosphere within the Cascadia seismogenic zone as the downgoing plate becomes younger and hotter northward along the convergent margin. Correlations between FME and major-element, REE and HFSE abundances suggest that more extensive dehydration of the slab beneath the northernmost basaltic centers resulted in lower degrees of wedge melting (5-10%) at higher P-T than conditions responsible for their more southerly counterparts (10-22%). The GVB mafic lavas therefore record a close relationship between the forearc metamorphic conditions and the degree of mantle melting beneath the volcanic front (i.e., the nature of primary arc magmas).
V51B-04 09:15h
Is there a link between the shallow forearc devolatilization processes, island arc magma generation and deeply subducted metamorphic slabs?
Shallow slab devolatilization is not only witnessed through fluid expulsion at accretionary prisms, but is also evidenced by the occurrence of serpentinized mantle domains and/or mud volcanism in the forearc regions of the Izu-Bonin, Mariana and South Sandwich intraoceanic arc-basin systems. The protoliths of these forearc serpentinites are mantle harzburgites that have suffered large volume melt extraction (up to 25 %) prior to interactions with fluids released from downgoing plates.These forearc serpentinites all show U-shaped REE patterns and very low REE abundances (0.001-0.1 X chondrites). Relative to global depleted mantle values these rocks typically have one to two orders of magnitude lower HFSE, REE, Th and U contents. Interestingly, all forearc rocks thus far examined show extreme enrichments of fluid mobile elements (FME: B, As, Sb, Cs). Because the 11/10B, 7/6Li and 87/86Sr isotope systematics in forearc serpentinites point to non-seawater-related processes, studies of elemental excesses and anomalous isotopic signatures recorded in the forearc serpentinites allow assessment of how much of the subducted inventory is lost between 10 and 40 km depths. Based on similar but substantial enrichments of FME in the Mariana forearc serpentinites recovered at ODP Legs 125 and 195, we report large slab inventory depletions of B (75%); Cs (25%); As (15%); Li (15%) and Sb (8%); surprisingly low (generally less than 2%) depletions of Rb, Ba, Pb, U, Sr, and no depletions in REE and the HFSE. Such slab-metasomatized mantle wedge materials may be dragged to depths of arc magma generation, as proposed by Tatsumi (1986) and Straub and Layne (2001), and thus represent an unexplored class of mantle material, different in its origins and geochemical fingerprint from mantle rocks such as may be represented by arc xenoliths, or the basal ultramafic sections of high-pressure ophiolites.
V51B-05 09:30h
Talc and Chlorite 'Hybrid' Rocks in Subduction Melanges; Their Role in Fluid and Element Recycling Through Subduction Zones
The fluxing of H2O, CO2 and trace elements through subduction zones is one of the most important geochemical cycles on Earth. Volatile components released during progressive metamorphism of subducting oceanic crust are thought to cause melting of the overlying mantle wedge, producing arc magmas. The petrological evolution and devolatilisation of the principle components of subducting slabs (MORB, sediments, serpentinite) are now well constrained. However, volatile and trace element flux through subduction zones remains highly contentious. The eclogite-facies rocks of northern New Caledonia represents oceanic crust that was previously subducted to depths of ~60 km. Within the terrane are high-pressure melange zones consisting of a range of mafic, metasedimentary and ultramafic lithologies that have been strongly sheared and intermixed. These melange zones have been observed on sub-metre to kilometre scales and comprise a significant proportion of the terrane. The melanges contain hybrid rock-types, such as carbonate-rich talc and chlorite schists, that are not equivalents of any typical protolith found at the Earth's surface. Using field relations, bulk-rock geochemistry and oxygen isotopes, we suggest that these hybrid rocks formed during subduction by metasomatism and mixing of serpentinites, mafic rocks and metasediments. Subducted mafic rocks and serpentinites may deliver H2O to sub-arc depths, but fluid release occurs at temperatures too low (>650 >|C) to dissolve significant amounts of the slab-sourced elements that are found in arc magmas. Hybrid rock-types may comprise a significant proportion of subducting slabs and have the potential to transport large amounts of volatiles to sub-arc depths. Talc and chlorite-rich hybrid rocks will undergo almost complete dehydration at relatively high temperatures (~ 800 >|C) allowing for elevated element solubility in fluids or partial melting of adjacent pelitic and mafic rocks. By contrast, carbonates in these rocks are stable to very great depths and may be important for recycling C into the deep mantle. Therefore, hybrid rock-types in subducting slabs may be critical for element and fluid recycling through subduction zones and the evolution of arc magmas.
V51B-06 09:45h
Li and B Insights into Subduction Signatures in the Mantle
Boron and lithium are powerful tracers of subducted materials in forearc and sub-arc mantle regions. Their isotopic systematics (Δ11B and Δ7Li) may be useful in identifying slab-mantle exchange processes and quantifying slab outfluxes. Early results for Li and B isotopes in basaltic lavas and mantle rocks suggest complexities in the transit of Li and B from slabs into different mantle domains. B contents and Δ11B in forearc mantle rocks suggest early release of B from slabs and preferential removal of 11B, leading to low Δ11B on deep or hot slabs, seen in some arc settings (i.e. Rose et al 2001; Bebout and Nakamura 2001; Leeman et al 2004). However, many arcs record Δ 11B and B contents too high to explain via shallow removal, necessitating inputs of forearc mantle to arc sources (i.e., Straub and Layne, 2001). Data for intraplate sources are equivocal, with some results on OIBs positing low Δ 11B (Chaussidon and Marty 1995), and others suggesting greater complexity. Intraplate basalts all show B depletions relative to MORBs, suggesting pervasive subduction-induced B redistribution in the Earth. While arc lavas and forearc rocks show Li enrichments, Δ 7Li in these samples are distinct. Basalts are uniform in Δ 7Li, varying ±2‰ from the mean value for MORBs. Mantle samples are, by contrast, diverse, ranging from +10‰ to -17‰. As Li partitions strongly into serpentine and other hydrated magnesian minerals, and solid-fluid exchanges result in strong Li isotopic fractionations, it is possible to generate rocks that are heterogeneous in Li content and Δ 7Li via progressive fluid-rock exchange at relatively low temperatures. Magmas sample the Δ 7Li of a large region of mantle, so fine-scale heterogeneity may be averaged out. The limited range of Δ 7Li in lavas may also indicate that subducted materials do not transport a fractionated Li isotopic signature into the mantle.