Silvio Ferrero | Universitaet Potsdam (original) (raw)

Papers by Silvio Ferrero

Keywords: partial melting carbonatites nanogranites garnet melt inclusions nanocarbonatites Carb... more Keywords: partial melting carbonatites nanogranites garnet melt inclusions nanocarbonatites
Carbonatites are peculiar magmatic rocks with mantle-related genesis, commonly interpreted as the products of melting of CO 2-bearing peridotites, or resulting from the chemical evolution of mantle-derived magmas, either through extreme differentiation or secondary immiscibility. Here we report the first finding of anatectic carbonatites of crustal origin, preserved as calcite-rich polycrystalline inclusions in garnet from low-to-medium pressure migmatites of the Oberpfalz area, SW Bohemian Massif (Central Europe). These inclusions originally trapped a melt of calciocarbonatitic composition with a characteristic enrichment in Ba, Sr and LREE. This interpretation is supported by the results of a detailed microstructural and microchemical investigation, as well as re-melting experiments using a piston cylinder apparatus. Carbonatitic inclusions coexist in the same cluster with crystallized silicate melt inclusions (nanogranites) and COH fluid inclusions, suggesting conditions of primary immiscibility between two melts and a fluid during anatexis. The production of both carbonatitic and granitic melts during the same anatectic event requires a suitable heterogeneous protolith. This may be represented by a sedimentary sequence containing marble lenses of limited extension, similar to the one still visible in the adjacent central Moldanubian Zone. The presence of CO 2-rich fluid inclusions suggests furthermore that high CO 2 activity during anatexis may be required to stabilize a carbonate-rich melt in a silica-dominated system. This natural occurrence displays a remarkable similarity with experiments on carbonate–silicate melt immiscibility, where CO 2 saturation is a condition commonly imposed. In conclusion, this study shows how the investigation of partial melting through melt inclusion studies may unveil unexpected processes whose evidence, while preserved in stiff minerals such as garnet, is completely obliterated in the rest of the rock due to metamorphic re-equilibration. Our results thus provide invaluable new insights into the processes which shape the geochemical evolution of our planet, such as the redistribution of carbon and strategic metals during orogenesis.

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American Mineralogist, 2016

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Contributions to Mineralogy and Petrology, 2015

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Both glassy and crystallized melt inclusions (MI) occur in garnet in metapelitic granulites from ... more Both glassy and crystallized melt inclusions (MI) occur in garnet in metapelitic granulites from the Kerala Khondalite Belt. These rocks were metamorphosed and partially melted at UHT conditions during the Pan-African event, and MI represent droplets of the anatectic melt, originated by dehydration melting of biotite and trapped by garnet growth at supersolidus conditions. An extensive ESEM-BSE mapping study, along with EMPA analysis and re-heating experiments, has been carried out to characterize these anatectic MI. The inclusions range from 4 to 35 μm in diameter and occur as clusters in garnets. In spite of the long time it took for these rocks to cool below 350 °C (at least 60 m.y.), different degrees of crystallization were observed in the same cluster, ranging from totally crystallized to totally glassy. The crystallized MI are referred to as "nanogranites" (Cesare et al., 2009) and always contain quartz, Mg-rich biotite (XFe=0.23) and two feldspars in a fine-grained polycrystalline aggregate. Based on microstructural evidence, biotite crystallized as first phase, preferentially on the walls of the MI, while quartz and feldspars crystallized later, often forming graphic intergrowths and/or melt pseudomorph-like structures (≥ 50 nm) similar to coarser structures (≈ tens of microns scale) observed in the host rocks. The glassy inclusions are rare (about 15% of the total) and smaller in size (≤15 μm in diameter) compared to the crystallized nanogranite MI. Both MI types often show negative crystal-shape and contain trapped crystalline phases that are accessories in the host rock, including rutile, titanite, zircon, apatite and Zn-rich spinel. Partially crystallized MI have been also recognized, containing an amorphous phase identified as a residual melt Where Cl and Ca are preferentially partitioned. Re-heating experiments in a HT hearing stage succeeded in re-homogenizing the nanogranite inclusions. EMP data on 40 re-homogenized MI show an average SiO2=73 wt%, K2O =6.7 wt%, Na2O =1 wt% and CaO <1 wt%. EMP analyses on the primary glassy inclusions in not re-heated samples provide similar compositions, while the differentiated melt in partially crystallized MI has higher CaO content and lower K2O content. The EMP characterization therefore confirms the work hypothesis that the different types of MI had the same original composition, except for the different trapped accessories, and that melt in partially crystallized MI is the result of a differentiation of the original trapped melt via crystal fractionation. The low Na content of melt in inclusions is consistent with the scarcity or absence of plagioclase in the melanosome of the studied samples, and with the UHT conditions at which the rocks melted. In fact, in the Q-Ab-An diagram melts plot far from the haplogranitic minima. Our results show that MI studies represent a powerful novel approach in the petrology of crustal melting and S-type granite genesis, and highlight the potential pitfalls of assuming anatectic melt as having a minimum melt composition. References Cesare, B., Ferrero, S., Salvioli-Mariani, E., Pedron, D., Cavallo, A., 2009. Nanogranite and glassy inclusions: the anatectic melt in migmatites and granulites. Geology 37, 627-630.

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Theoretical considerations suggest that peritectic minerals growing during incongruent melting re... more Theoretical considerations suggest that peritectic minerals growing during incongruent melting reactions may act as hosts for inclusions of anatectic melt. We have recently verified this hypothesis in a granulite from the Kerala Khondalite Belt, India, discovering tiny inclusions within ...

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American Mineralogist, 2016

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Contributions to Mineralogy and Petrology, 2015

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Small volumes (≤ 50µm) of hydrous melt were trapped as primary inclusions in peritectic garnets d... more Small volumes (≤ 50µm) of hydrous melt were trapped as primary inclusions in peritectic garnets during partial melting of metagranitoids from the Orlica-Śnieżnik Dome (Bohemian Massif) at mantle depth [1]. Detailed microstructural/microchemical investigation confirmed the occurrence of a granitic assemblage (biotite+feldspars+quartz) in every investigated inclusion, i.e they are nanogranites [2].
MicroRaman mapping of unexposed inclusions showed the occurrence of residual, H2O-rich glass in interstitial position. Despite the oddity of this finding within a classic regional HP/HT terrain, an incomplete crystallization of the melt inclusions (MI) is consistent with the (relatively) rapid exhumation of the Orlica-Śnieżnik Dome proposed by some authors [e.g. 3]. Moreover glassy and partially crystallized MI have been already reported in lower-P (<1 GPa) migmatites [4]. MicroRaman investigation also showed the possible presence of kumdykolite, a high-temperature polymorph of albite reported in UHP rocks from the Kokchetav Massif as well as the Bohemian massif ([5] and references therein).
Experimental re-homogenization of nanogranites was achieved using a piston cylinder apparatus at 2.7 GPa and 875°C under dry conditions, in order to investigate melt composition and H2O content with in situ techniques. The trapped melt is granitic, hydrous (6 wt% H2O) and metaluminous (ASI=1.03), and it is similar to those produced experimentally from crustal lithologies at mantle conditions.
Re-homogenization conditions are consistent with the results of geothermobarometric calculations on the host rock, suggesting that no H2O loss occurred during exhumation - this would have caused a shift of the inclusion melting T toward higher values. Coupled with the absence of H2O-loss microstructural evidence, e.g. decrepitation cracks and/or vesciculation [4] in re-homogenized nanogranites, this evidence suggests that the nanogranites still preserves the original H2O content of the melt. Our study supports therefore the hypothesis that H2O re-equilibration via diffusion of MI in garnet cannot be implicitly inferred, as already proposed by [5] for lower-P nanogranites, even in case of near-UHP inclusions.
In conclusions, the combined petrological-experimental investigation of near-UHP nanogranites is a novel and fruitful approach, which unlocks the access to deep melt in natural eclogite-facies crustal rocks, improving our understanding of deep melting processes in collisional settings.

References
[1] Walczak, K. (2011), Ph.D. thesis, Krakow, Poland.
[2] Cesare, B. et al. (2009), Geology, 37, 627–630.
[3] Anczkiewicz, R. et al. (2007), Lithos, 95, 363–380.
[4] Ferrero, S. et al. (2012), JMG, 30, 303–322.
[5] Kotková, J. et al. (2014), Am. Min., 99, 1798-1801.
[6] Bartoli, O. et al. (2014), EPSL, 395, 281–290.

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2009 Portland GSA …, Jan 1, 2009

Both glassy and crystallized melt inclusions (MI) occur in peritectic garnets in metapelitic gran... more Both glassy and crystallized melt inclusions (MI) occur in peritectic garnets in metapelitic granulites from the Kerala Khondalite Belt of India. The trapped melt is interpreted to have originated by dehydration melting of biotite. An extensive ESEM-BSE mapping study, along with EMPA ...

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At La Galite Archipelago peraluminous granitoids, cutting through the sedimentary succession, con... more At La Galite Archipelago peraluminous granitoids, cutting through the sedimentary succession, contain abundant entrained material [1, 2]. Enclaves of sedimentary, magmatic and metamorphic origin and xenocrysts of cordierite and garnet were identified. The plutons are related to Miocene anatexis of the crystalline basement buried beneath the Maghrebian Chain [3].

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