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Postcumulus processes in oceanic-type olivine-rich cumulates: The role of trapped melt crystallization versus melt/rock interaction
No full textCombined microstructural and geochemical investigations on MORB-type primitive olivine-rich cumulates intruded in the Erro-Tobbio (ET) mantle peridotites (Voltri Massif, Ligurian Alps, Italy) revealed that significant chemical changes in minerals were caused by postcumulus crystallization. This is indicated by the occurrence of accessory interstitial minerals (Ti-pargasite, orthopyroxene and Fe-Ti oxides) and by systematic chemical zoning in intercumulus clinopyroxene, resulting in marked trace element (e.g. REE, Ti and Zr) enrichment at constant high Mg-numbers (0.88-0.91) and LREE depletion. Geochemical modelling shows that low trapped melt amounts (<5%) are sufficient to produce the observed trace element enrichments. Chemical zoning in large (mm-size) clinopyroxenes was dominantly caused by in situ fractional crystallization of trapped interstitial liquid rather then porous flow migration of externally derived evolved melts. Zr enrichment relative to REEs in vermicular clinopyroxene and pargasitic amphibole point to small-scale migration and interaction between residual evolved low melt fractions and the olivine cumulus matrix at final stage of crystallization
Melt migration and intrusion in the Erro-Tobbio peridotites (Ligurian Alps, Italy): insinghts on magmatic processes in extending lithospheric mantle
No full textThe Alpine/Apennine ophiolites are largely thought to represent lithosphere sectors mostly formed at an ocean/continent transition and allow the direct observation of petrologic and geodynamic processes in extensional systems evolving from continental rifting to ultraslow spreading. The Alpine/Apennine peridotites experienced multiple melt/ rock interaction and melt intrusion events occurred at different lithospheric depths, thus providing insights on mantle dynamics and lithosphere-asthenosphere interactions during progressive lithosphere extension. Here we present an overview of this multi-stage melt migration and intrusion history, as recorded in the Erro-Tobbio (ET) peridotites (Ligurian Alps, Italy). In the ET spinel peridotites, the oldest intrusion event is documented by the diffuse occurrence of cm-scale folded pyroxenite bands. They display variably fractionated REE spectra, marked by LREE depletion and absent EUN anomaly. Unusual trace element signature (high Sc,V contents and low MREE/HREE ratios) in clinopyroxenes from one pyroxenite layer is witness of a precursor garnet-bearing magmatic assemblage. Spinel pyroxenites likely originated as high-P (> 15-20 kbar) intrusions that preceded the extension-related peridotite exhumation. In the spinel peridotites, field, textural and chemical evidence (e.g. olivine embayment replacing pyroxene porphyroclasts, increasing modal olivine at constant bulk Mg values), points that they experienced open-system melt migration by reactive porous flow, subsequent to pyroxenite intrusion and folding. Melt/rock interaction (causing olivine crystallization and pyroxene dissolution) occurred at high melt volumes at deep lithospheric levels. At shallower lithospheric depths, the ET peridotites were impregnated by melts causing significant plagioclase enrichment and crystallization of poikilitic orthopyroxene replacing mantle olivine and clinopyroxene. Reacted clinopyroxenes preserve strong LREE depletion, indicating that impregnating melts originated as depleted melt fractions. After impregnation, peridotites underwent multiple gabbroic intrusion events. Structural and geochemical features of melt impregnation and melt intrusion products point to a progressive change in melt composition and dynamics. Peridotite impregnation was caused by diffuse migration of opx-saturated depleted melts, and is consistent with cooling and crystallization of migrating melts when the peridotites, due to lithosphere extension and thinning, became part of shallower and colder lithospheric environments. The subsequent intrusion events originated by MORB-type aggregated magmas that had not experienced significant compositional modifications during ascent. The transition from porous flow melt migration to emplacement of magmas in fractures reflects progressive change of the lithospheric mantle rheology, across the ductile to brittle transition, during extension-related uplift and cooling of the ET mantle
Experimental and natural contraints on the spinel-plagioclase subsolidus transition in mantle peridotite
No full textTHE SUBSOLIDUS SPINEL TO PLAGIOCLASE TRANSITION IN MANTLE PERIDOTITES: NATURAL AND EXPERIMENTAL CONSTRAINTS
No full textExperimentally-derived Ca-Na partitioning between plagioclase and clinopyroxene : a new geobarometer for mantle rocks
No full textExperimentally-derived Ca-Na partitioning between plagioclase and clinopyroxene : a new geobarometer for mantle rocks
No full textThe crystallization of plagioclase-bearing equilibrium assemblages in mantle peridotites is commonly considered witness of lithospheric mantle exhumation at shallow depth (e.g. Rampone et al., 1993; Ozawa & Takahashi, 1995; Newmann et al., 1999). Experimental works have pointed out that at subsolidus conditions plagioclase lherzolite assemblages are stable up to 1 GPa. Subsolidus experimental data in the complex chemical system TiO2-Cr2O3-Na2O-FeO-CaO-MgO-Al2O3-SiO2 on depleted and fertile lherzolite compositions (Borghini et al., 2010) have documented systematic compositional variations in coexisting minerals at decreasing pressure within the plagioclase stability field, similar to what observed in natural equilibrated plagioclase peridotites. In these experiments, Ca-Na partitioning between plagioclase and clinopyroxene is strictly dependent on pressure, with plagioclase composition varying significantly (An=59-83) in a narrow range of pressure. In a recent paper, Borghini et al. (2011) have defined the anorthite isopleths in plagioclase as a function of P and T in a specific fertile lherzolite composition, and they have demonstrated that plagioclase-facies equilibrium pressures can be estimated by comparing experimental data on a fertile lherzolite and microstructural-chemical observations in natural samples with the same bulk composition. This method represents a first-order empirical tool to trace the decompressional evolution of the lithospheric mantle up to very low-pressure and it is potentially applicable to natural plagioclase peridotites of different origin. Nevertheless, the composition of plagioclase at variable bulk Na2O/CaO ratios has been not fully discerned yet, making this approach directly applicable to a limited bulk composition range. Here, we present the results of new experiments on a Na-enriched lherzolite (bulk Na2O/CaO ratio = 0.13), representative of many refertilized lithospheric peridotites at extensional settings, thus enlarging significantly our investigated compositional spectrum (Na2O/CaO = 0.08 - 0.13). The goal of this work is to provide a formulation for a geobarometer analysis based on Ca-Na plagioclase-clinopyroxene partitioning on a larger experimental data set which takes into account the effect of bulk Na2O/CaO ratio and is consequently applicable to a wider compositional range of mantle rocks. References: Borghini, G., Fumagalli, P., Rampone, E. (2010): J. Petrol., 51, 229-254; Borghini, G., Fumagalli, P., Rampone, E. (2011): Lithos, in press; Newman, J., Lamb, W.M., Drury, M.R. & Vissers, R.L.M. (1999): Tect., 303, 193-222; Ozawa, K. & Takahashi, N. (1995): Contrib, Min. Pet., 120, 223-248; [5] Rampone, E., Piccardo, G.B., Vannucci, R., Bottazzi, P., Ottolini, L. (1993): Contrib, Min. Pet., 115, 1-17. AGU Fall meeting, San Francisco, Calif., 5-9 Dec., 2011. Section: Volcanology, Geochemistry, Petrology. Session: Mineralogical Signals as Keys to Understanding Petrological Processes II Posters. Abstract V11A-251
Grain growth in spinel and plagioclase peridotites : a time-resolved experimental study on textural evolution
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The spinel-plagioclase transition in the shallow upper mantle : subsolidus experiments on fertile and depleted lherzolite
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