What governs the origin of the slab component in subducting

1. 3 1 2 3 1 2 60 50-2 55 50-13 50 45 52-1 CHM30b 77-5 Jd (in mol%) 40 35 CPX2 CHM30a 30 55-4 25 20 CPX1 75 80 85 90 95 100 Mg# CPX2 P in kbar CPX2 accretionary wedge CPX2 mantle wedge low-T altered crust + sediments sheeted dikes + gabbros 20 kbar After Schmidt & Poli (1998) Catalina Island Trescolmen Monviso a) deformed eclogite metabasaltic b) massive eclogite metagabbroic CPX AMP KYA a)PHE, b) ZOI GRT QZ 10 52-1 55-3 50-2 1 59-1 50-13 55-4 6 CHM30 16 0.1 55-3 Sm (AMP) in ppm C b 1000 a 5 52-1 12 800 T CHM30b Ad25 T 0.01 4 600 50-2 MORB field 8 Cr (ppm) 77-5 MgO (wt%) 3 400 Y (AMP) in ppm 0.001 4 59-1 50-13 200 0.001 0.01 0.1 1 10 2 Ad25 Sm (CPX) in ppm CHM30 0 0 0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 2.5 3 3.5 CHM30b 1 TiO (wt%) TiO (wt%) 55-4 2 2 77-5 16 0 58 Z c d 0 0.5 1 1.5 Z 54 Y (CPX) in ppm 12 50 46 8 (wt%) 42 CaO (wt%) 2 SiO 38 4 200 34 52-1 30 0 0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 2.5 3 3.5 50-2 TiO (wt%) TiO (wt%) 150 2 2 16 6 e f P 100 50-13 Sr (AMP) in ppm 12 P 59-1 P 4 55-4 200 8 CHM30 50 FeO total (wt%) P 55-3 O (wt%) 2 2 Na 4 CHM30b 77-5 Ad25 0 0 0 50 100 150 0 0 1 2 3 4 0 0.5 1 1.5 2 2.5 3 3.5 Sr (CPX) in ppm TiO (wt%) TiO (wt%) 2 2 What governs the origin of the slab component in subducting oceanic crust? Insights from trace element distribution in deformed and massive eclogites Thomas Zack1,2, Toby Rivers2 & Steven F. Foley1 1 Mineralogisch-Petrologisches Institut, Universität Göttingen, Goldschmidtstr.1, 37077 Göttingen, Germany 2 Dept. Earth Sciences, Memorial University of Newfoundland, St. John‘s, Newfoundland A1B 3X5, Canada 1. Aim of this study Trace element enrichments of island arc magmas are supposed to be derived from water liberated from dehydrating slabs during prograde metamorphic reactions. In this study we want to focus on this high pressure metamorphic system by investigating trace element mineral concentrations in close natural analogues – orogenic eclogites. The tools used to unravel the complexities of orogenic eclogites include a modern electron microprobe at Universität Göttingen, Germany (phase mapping, high contrast compositional imaging) and a state-of-the-art laser ablation ICP-MS (LAM) facility at Memorial University, Canada. Theoretically it is possible to calculate the trace element concentration of fluids coming out of the subducting slab if I) the chemical composition and the modal abundance of all phases is known, II) equilibrium is reached during fluid liberation and III) partition coefficients for relevant trace elements between these phases and fluid are known. Even though the chemical composition in a subducting slab varies significantly, variations of e.g. MORB are known rather precisely and modal abundances can be calculated with existing thermodynamic data (Schmidt & Poli 1998; EPSL 163: 361-379). The requirements II) and III) are more problematic and will be discussed in this poster. 3. Chemical heterogeneity vs homogeneity of omphacite Spot analyses of omphacite in all samples reveal that in some samples the major element composition varies widely, while in others the composition is restricted (Fig. 3a and b). One key sample shows the transition between heterogeneous and homogeneous omphacites (Fig. 3c). Here it can be observed that large, homogeneous omphacites grew at the expense of small, heterogeneous omphacites. Our favoured explanation is a dynamic recrystallization, where mass transfer processes (facilitated by infiltrating fluids) dominate over dislocation creep (Godard & van Roermund 1995; J. Struc. Geol. 17: 1425-1443). 5. The role of deformation in subduction zones In a subsuite of eclogites from Trescolmen we have found a close approach towards equilibrium on a major and trace element basis. This was probably reached by a combination of fluid infiltration and dynamic recrystallization during eclogite-facies conditions. However, other samples completely failed to reach equilibrium. The question we would like to raise here is: If and where does deformation and fluid mobility occur in the subducting slab? We might speculate that shearing and significant fluid quantities exist in a zone near the slab-mantle boundary and that both deformation and fluid availability decrease towards deeper sections in the oceanic crust. Since the uppermost oceanic crust and overlying sediments comprise the dominant amount of subducted water and important trace elements (e.g. B, As, Cs, Rb, Ba, Pb), this sections has to be most thoroughly analyzed with respect to the degree of equilibrium if numerical models like Rayleigh fractionation is applied to calculate fluid compositions from dehydrating metamorphic rocks. Trescolmen might serve here as a good analogue to conditions near the slab-mantle boundary (Fig. 5), with variably deformed metabasalts enclosed in pervasively deformed micaschists. 2. Distinction of Trescolmen eclogites Samples in this study are from Trescolmen, Swiss Alps. Eclogites at this locality occur in numerous meter- to decameter-large bodies enclosed in garnet micaschists, both lithologies being deformed under the same eclogite-facies conditions. This locality has a rich inventory of eclogite varieties, which can be distinguished on textural (deformed vs massive eclogites; Fig. 1) and chemical grounds (metagabbroic vs metabasaltic; Fig. 2). Generally metagabbroic eclogites are coarser grained and less deformed than metabasaltic (see Fig. 1), but exceptions occur. Fig. 3 Omphacite compositions as a criteria for degree of equilibrium. a) 7 samples show restricted major element composition, whereas b) 4 samples scatter strongly in composition. c) A transition between an old generation of heterogeneous CPX1 and younger, homogeneous CPX2 can be observed in this backscattered image (scale bar- 200 µm; two laser ablation holes drilled in CPX2). Fig. 5 Cross section through subduction zone with proposed natural analogues for different sections in subarc environments. Catalina Island is characterized by pervasive deformation, the Trescolmen area (this study) shows variably deformed metabasalts enclosed in pervasively deformed metapelites, whereas Monviso has massive metagabbros with even some metastable gabbros. 4. Trace element systematics For the purpose of evaluating equilibrium vs disequilibrium processes in our eclogite, we order our samples with respect to the probability of approaching equilibrium. The best criteria are a thorough foliation, since deformation enhances metamorphic processes, and a restricted chemical variability, best documented by omphacite major element composition. By evaluating each sample, we found four samples which show a thorough foliation (defined by aligned omphacite, phengite and amphibole) and have a restricted omphacite variability. These preferred samples also demonstrate ideal behaviour in terms of their trace element partitioning characteristics. They show consistent partitioning values for various elements (demonstrated for Dclinopyroxen/amphibole in Fig. 4). Instead, partitioning values scatter widely for the remaining samples, best documented in Fig. 4 for Sr. 6. Preferred D-values for subduction zone models As we have demonstrated that a close approach towards equilibrium has been reached at least in some investigated eclogites and that similar conditions might exist in the most important parts in the subducting slab (with respect to water and fluid-mobile trace elements), fluid compositions might be calculated by means of simple numerical models that use partition coefficients between minerals and fluid. However, these mostly experimentally derived partition coefficients only exist for a limited amount of minerals (cpx, grt, rut, amp; see e.g. Brenan et al. 1998, GCA 62: 3337-3347). It is now possible from our preferred samples to derive an extensive partition coefficient set between hydrous minerals and fluid by combining the data from Table 1 with experimentally derived DCpx/Fluid. Even taking low modal abundances (e.g. 1%) of hydrous phases into account, it becomes clear that the existance of zoiste will control the budget for LREE (e.g. Ce, Sm, Nd), Pb, U and Th, while phengite dominates LILE (e.g. Ba). On the other hand, elements like Li, Be and B are controlled by major phases, since no hydrous phase shows extreme preferences for these elements. Fig. 1 Phase maps of two samples derived from whole thin section element mapping (290000 points in 6 h each) Fig. 2 Similarity of Trescolmen samples with MORB. Three samples with highest MgO, Cr and lowest TiO2 are most likely metagabbros (circles), remaining samples metabasaltic (squares and diamond). Please notice relationship between accessory phases and chemistry (C- chromite, T- talc, Z- zoisite, P- paragonite). Table 1 Summary of average partition coefficients between various coexisting hydrous phases and clinopyroxene calculated from our preferred samples. Fig. 4 Distribution diagrams for trace elements between clinopyroxene and amphibole. Preferred samples are indicated by solid squares, remaining samples by open diamonds. Solid lines mark average DAmp/Cpx values for our preferred samples, extrapolation of the lines to the origin demonstrates Henry‘s Law behaviour.