Using olivine as a probe into early igneous and mantle melting processes: combining in situ oxygen isotopes with minor and trace-element compositions of olivine from Circum-Mediterranean Cenozoic magmatic provinces
Final Report Abstract
In this project, we investigated in situ oxygen isotopes δ18Ovsmow (spot diameter around 8 µm) together with in situ major, minor (spot diameter <= 5 µm), and trace element (spot diameter <50 µm) abundances in five genetically and compositionally different olivine types from Circum-Mediterranean Cenozoic anorogenic and orogenic high-MgO lavas. Our comprehensive sample collection includes geochemically well-characterized olivine crystals, retrieved from geochemically and isotopically well-characterized lava samples. The main findings of our research can be summarized as follows: i. We compare three geochemically different olivine populations (phenocrysts and two types of xenocrysts) that are hosted by Italian kamafugites, with the phenocrystic olivine from Italian lamproites. These two lava types (kamafugites and lamproites) represent the most extreme ultrapotassic volcanics from the Circum-Mediterranean Cenozoic magmatic province. All investigated olivines have extremely high δ18OSMOW up to 25‰. The geochemical diversity in olivine from the kamafugite lava reflects the mineralogical heterogeneity of the shallow mantle below the Apennines resulting from the enrichment by metasomatic agents derived from subduction-induced continental sediment recycling that appears to be a common process during the Alpine/Apennine orogeny. In our view, the joint appearance of melt-related kamafugitic and lamproitic olivines in kamafugite lavas may record a snapshot of a shift from pelitic to carbonate-rich sediment flux recycled within the mantle below the Apennines. ii. Our data of olivine macrocrysts from the Eifel Mg-rich lavas may be grouped as volumetrically dominant igneous olivines or phenocrysts, mantle xenocrysts and low-Ni xenocrysts originated from wherlitic assemblages within the lithospheric mantle. Our estimation of the olivine-liquid equilibria using compositions of the most Mg-rich phenocrysts indicates temperatures considerably lower than 1300 oC, even lower than the MORB olivines and considerably lower than the olivines from Hawaiian and Island plume related lavas. They are considerably enriched in Li, and depleted in Ti. In our view, the Eifel plume doesn’t consist of the material considerably hotter than the ambient mantle, but it needs to be more enriched in fluid-mobile (LILE) elements like Li. This supports the notion that Eifel represents a compositional or wet plume rather then a hot plume. iii. Our comprehensive dataset (EMPA, LA-ICP-MS, SIMS) of olivines (Mg# ~85-95) that are hosted by almost primary, mantle-derived lavas from more than 25 volcanic occurrences spread over the Circum-Mediterranean Cenozoic magmatic province constrain the nature and origin of their mantle reservoirs, as well as the processes of melt formation. We found that the olivines from ‘anorogenic’ and ‘orogenic’ types exhibit contrasting geochemical features with the former showing a range in δ18Ovsmow from ~4.7 to 5.6 ‰, whereas the ‘orogenic’ olivines display δ18Ovsmow values from ~4.7 to 11.0 ‰, some of which are the highest reported so far for olivines hosted by mantle-derived lavas. They also show contrasting behaviour of minor and trace elements the orogenic ones demonstrating fingerprints of recycled continental crust in their mantle source. iv. Our data on San Carlos olivine demonstrate that the different batches of SC olivine have different trace element signatures. Using a newly established QCM MongOl we thoroughly characterized several SC batches, including SC/KA (Köhler and Brey) and the Smithsonian San Carlos batches USNM 111312/44 and 42.
Publications
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(2017). Early igneous and mantle melting processes: Evidence from combined in-situ oxygen isotope, minor and trace element analyses of olivine. DMG-Sektionstreffen mit Posterpräsentation June 2017
Günther, J., Prelević, D., Mertz, D. F.
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(2018). Combined in–situ δ18O, minor & trace element analyses on olivine from the Circum-Mediterranean Cenozoic magmatic province. Goldschmidt 2018, abstracts
Günther, J., Prelević, D., Mertz, D. F., Rocholl, A.
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(2018). Italian Kamafugites Revisited: Fingerprinting Different Metasomatized Mantle Components Using Olivine Geochemistry. EMAW 2018., 3rd European mantle workshop, Pavia - Università Centrale
Günther, J., Prelević, D., Mertz, D. F., Rocholl, A., Mertz-Kraus, R. Conticelli, S.
