EVIDENCE FOR PERVASIVE MELT MIGRATION OF EXTREMELY DEPLETED MELTS IN MANTLE PERIDOTITES FROM SUPRA-SUBDUCTION ZONE: AN EXAMPLE FROM THE IZU-BONIN-MARIANA FOREARC

Abstract

The petrographic, mineralogical and geochemical features of the serpentinites recovered at the top of three seamounts located in the Izu-Bonin-Mariana forearc (South Chamorro, Site 1200, ODP Leg 195; Conical, Site 779A, Leg 125; Torishima, Site 784A, Leg 125) evidence a complex, multistage, evolution of the pristine mantle sequences, involving partial melting, reactive porous flow melt migration, subsolidus metamorphic re-equilibration under decreasing T and open system conditions, and, finally, lowgrade hydrothermal alteration processes. In spite of pervasive serpentinisation, there are areas in which the mantle mineral assemblages are preserved and provide information about the high-T petrologic evolution. The analysed peridotites varied from spinel harzburgite to orthopyroxene-bearing spinel dunite in early modal composition with clinopyroxene modal content mostly in the range of 0 to 2% by volume. Petrologic and geochemical modelling indicates that peridotites experienced up to 25% partial melting. Successively, they were percolated by ascending melts of variable composition. The melt-peridotite interaction is revealed by the following petrochemical features: 1) exsolved and deformed orthopyroxene porphyroclasts are partially replaced by secondary fine-grained assemblage consisting of Ol ± Opx ± Cpx: moreover, secondary clinopyroxene ubiquitously grows in correspondence of fractures across the orthopyroxene porphyroclasts; 2) interstitial clinopyroxene is concentrated locally in clusters or pseudo-veins; 3) the bulk rock major element variation does not match the trends expected for refractory residua after basalt removal; 4) the chemical compositions of the minerals is not correlated with the variation of the modal and chemical composition of bulk rock. Valuable information about the geochemical affinity of the migration melts are furnished by the trace element composition of clinopyroxenes (Cpx). The composition of the Cpx from the Torishima peridotites records the migration of extremely depleted melts with relatively enriched LREE and Sr content. As a whole the trace element fractionation of the putative liquids is consistent with that of boninitic melts. A different story is recorded by the Cpx from the South Chamorro and Conical peridotites. In particular, the geochemical composition of the Cpx from the South Chamorro samples is strongly heterogeneous. Thus, for sake of clarity, three different compositional groups can be recognised: Type1, 2 and 3. Type 1 Cpx are in equilibrium with a calculated liquid characterised by REE composition closely approaching that expected for melt increments produced by 15% fractional melting of spinel-facies depleted mantle. By contrast, Type 3 has Cpx characterised by an exceptional depletion in incompatible trace elements. C1-normalised REE patterns have the maximum at LuN (0.73-1.25). The low REE concentration is accompanied by very low Ti, Zr, Hf, Y, V and Sc concentration (e.g. Ti about 30 ppm). Type 2 Cpx has trace element contents in between those of Type 1 and 3. The REE patterns are characterised by a fractionation in the MREE-HREE region steeper than that expected for liquid produced by fractional melting of spinel facies assemblages (SmN/HoN=0.08-0.14; HoN/YbN = 0.21- 0.46) at the given HREE level (YbN = 2.1-2.7). A stronger MREE-HREE fractionation characterise the REE patterns of the Cpx from the Conical peridotites (SmN/HoN=0.05-0.08; HoN/YbN a 0.55), which show again the YbN in the range of 2.3-2.5. Similar REE fractionations have been documented in Cpx from abyssal peridotites (e.g Johnson, 1998; Hellebrand et al., 2002;), ophiolitic harzburgites (e.g. Kelemen et al., 1995; Dijkstra et al., 2003; Barth et al., 2003)and interstitial Cpx from “replacive” harzurgite-dunite bodies (e.g. Suhr et al., 1998, Suhr, 1999; Zanetti et al., 2005). Geochemical modelling indicates that this compositional character of the percolating melts can be explained by the following three petrologic processes: 1) polybaric evolution of their mantle source, involving an early phase under garnet- facies conditions followed by a second stage under spinel-facies ones; 2) transient composition acquired during the migration of extremely-depleted liquids (e.g. those calculated in equilibrium with Type 3 Cpx through a mantle column with a chemical composition approaching that of residual peridotite after 15% fractional melting. The meltperidotite interaction was mainly controlled by chromatographic- type chemical exchange; 3) transient composition acquired during the migration of less depleted liquids (e.g., those in equilibrium with Type 1 Cpx), through an extremely depleted mantle column. The melt-peridotite interaction involved a significant assimilation of the peridotite orthopyroxene. The model (3) is preferred, because it documents the overall petrochemical features shown by the South Chamorro and Conical peridotites. In conclusion, the strongly refractory characters acquired during the early partial melting episodes affection mantle peridotites from Izu-Bonin-Mariana supra-subduction zone has successively been overprinted by the migration of extremely depleted melts. The boninitic affinity recognised for the melts migrating through the mantle section in the Torishima Seamount area suggests that they were related to island- arc magmatism established in a MOR lithosphere. By contrast, the occurrence of depleted to extremely melts in the South Chamorro and Conical seamounts peridotites is more conceivable with the extinction of the magmatism in a MOR lithosphere.