GMPV8.1

The knowledge of the viscosity of magmas as a function of melt composition and temperature is central in the numerical modeling of the magmatic dynamics and eruptive scenarios. Recent studies suggest that the measure of the viscosity of volcanic melts is affected by the occurrence of nano-scale modification (i.e., crystallization) of the melt structure during the measurement. This challenges the possibility of being able to quantify the crystal-free melt phase contribution to the measured viscosity. Herein we provide a new strategy that allows an accurate estimation of the melt viscosity at eruptive conditions starting from the Brillouin and Raman scattering of parental glasses. Our results show that the ratio between bulk and shear moduli and the boson peak position of glass embed the melt fragility. However, at the moment there are no known reasons why some of these quantities are linked together in such multicomponent systems. At this effort, we also provided a mean to evaluate whether the local structure of glasses may justify this correlation by testing the well-known prototypical binary system Na₂O-SiO₂. Concluding, our strategy allows the estimation of the melt viscosity as a function of temperature avoiding its direct measurement, ridding also the measurement from unwished crystallization.

How to cite: Cassetta, M., Di Genova, D., Zanatta, M., Boffa Ballaran, T., Kurnosov, A., Giarola, M., Biesuz, M., Sorarù, G. D., Giannetta, B., Zaccone, C., Daldosso, N., and Mariotto, G.: Estimating the viscosity of volcanic and binary melts from the vibrational properties of their parental glasses, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-559, https://doi.org/10.5194/egusphere-egu22-559, 2022.

Volcanic arc lavas often carry mushy rocks from deeper levels of magma plumbing systems, termed “plutonic xenoliths”, to the surface. These xenoliths offer the opportunity to study magma evolution in the crust. Here we focus on the Lesser Antilles island arc, known for its abundance of plutonic xenoliths, and describe a new suite of plutonic xenoliths from St Vincent. 

The whole rock chemical compositions of erupted lavas from St Vincent are consistent with closed system fractionation as the dominant magma evolution process [1,2].  However, the chemical compositions of crystals in plutonic xenoliths, which record magmatic processes and the composition of melts present during crystal growth in mushes at depth in the crust, are commonly more complex. Thus, plutonic xenoliths provide a more detailed picture of magma plumbing system processes and geochemical diversity than is available from the chemical compositions of erupted lavas alone.

The new St Vincent plutonic xenoliths show intriguing textural features such as centimetre scale mineralogically distinct bands and amphibole replacement rims on clinopyroxene. Similar textures in plutonic xenoliths elsewhere have been attributed to open system processes such as reactive porous melt flow [3,4].   

We use the major and trace element compositions of clinopyroxene and amphibole from St Vincent plutonic xenoliths to assess evidence for reactive flow and open system processes in the St Vincent magma plumbing system. A distinctive hornblende-olivine gabbro xenolith contains two clinopyroxene populations (Mg# 82-89 and Mg# 77-79, < 1.5 mm) and pervasive poikilitic amphibole texturally associated with clinopyroxene. Both the lower Mg# (77-79) clinopyroxene and amphibole are enriched in LREE, Zr, Nb (amphibole), Ni and Cr. Modelling of closed system fractionation of primitive St Vincent basalt fails to reproduce these enrichments. Instead, the chemical data and textural characteristics suggest that the lower Mg# (77-79) clinopyroxene and amphibole formed via reactive melt flow. In contrast with the closed system fractionation trends displayed by whole rock lava compositions, this study provides strong evidence that open system reactive melt flow operates in the St Vincent magma plumbing system, modifying mineral assemblages and compositions. Thus, reactive melt flow may (cryptically) contribute to magma chemical evolution on St Vincent.

[1] Heath, E., Macdonald, R., Belkin, H., Hawkesworth, C., & Sigurdsson, H. (1998). Journal of Petrology, 39(10).
[2] Fedele, L., Cole, P.D., Scarpati, C., & Robertson, R.E. (2021). Lithos, 392, 106150.
[3] Sanfilippo, A., MacLeod, C.J., Tribuzio, R., Lissenberg, C.J., & Zanetti, A. (2020). Frontiers in Earth Science, 8, 473.
[4] Cooper, G. F., Davidson, J. P., & Blundy, J. D. (2016). Contributions to Mineralogy and Petrology, 171(10).

How to cite: Brown, J., Prytulak, J., Cooper, G., Humphreys, M., and Iveson, A.: Plutonic xenoliths reveal open system magma evolution processes beneath St Vincent, Lesser Antilles arc, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6072, https://doi.org/10.5194/egusphere-egu22-6072, 2022.

The fractionation of garnet from arc magmas is hypothesized to play an important role in a wide range of geologic processes including the formation of continental crust, the oxidation of arc magmas and the development of porphyry copper deposits. However, garnet is only stable in mafic to intermediate hydrous arc magmas at pressures of at least 0.8-1 GPa and is extremely rare in erupted arc magmas. It is therefore difficult to directly document and study garnet fractionation in the field. Instead, garnet fractionation is frequently inferred based on trace element proxies such as La/Yb, Dy/Yb and Sr/Y. As garnet stability is strongly pressure sensitive, these ratios are also commonly used as proxies for fractionation pressure and crustal thickness. However, this approach is problematic as these ratios span a wide range of values in primary mantle melts independent of crustal thickness, and can also be modified within the crust by amphibole fractionation and plagioclase accumulation.

We show here that Mn/Mg ratios provide an attractive alternative method for inferring garnet fractionation in erupted lavas. Primary mantle melts have highly restricted Mn/Mg ratios that are consistent with melt in equilibrium with mantle olivine. Therefore, this ratio does not appear to keep a record of subducted slab contributions, unlike most trace element proxies. Using a large compilation of experimental data and new high-precision analyses of Mn partitioning in existing garnet-bearing experiments, we show that all common cumulate silicate phases except garnet have Mn/Mg K_D values below 0.5, while the garnet K_D is greater than 1, and thus garnet fractionation produces derivative magmas with distinctly lower Mn/Mg ratios.  Using the compiled experimental data, we parameterized an empirical model of Mn partitioning in garnet as a function of pressure and temperature. This model allows for the rigorous investigation of the role of garnet fractionation at both modern and ancient subduction zones. We find clear evidence for garnet fractionation in most arcs with seismically estimated crustal thicknesses greater than ~45 km. This garnet fractionation signature is observable at relatively unevolved melt compositions (≤54 wt. % SiO₂). At these melt compositions garnet is likely only stable at pressures of at least 1.5 GPa, suggesting that garnet fractionation initiates at or below the Moho.

How to cite: Klein, B. and Müntener, O.: Mn/Mg ratios of arc lavas show that early garnet fractionation occurs near the Moho of thick continental arcs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10034, https://doi.org/10.5194/egusphere-egu22-10034, 2022.

The processes driving the chemical differentiation of mantle-derived magmas emplaced at deep levels of the continental crust are mostly unknown. Melt compositions may be modified through interplay of fractional crystallization, magma mixing, crustal assimilation and melt-rock and melt-crystal mush reactions. These processes may play a major role in dictating the compositions of evolved magmas rising at shallow crustal levels. The Ivrea Mafic Complex from Italian Alps is a km-scale gabbronorite-diorite body intruding the lower continental crust during the post-Variscan transtensional tectonics, and includes several ultramafic bodies of inferred cumulate origin at its deepest levels. To shed light on the magmatic processes occurring in the lowermost continental crust, we have carried out new petrographic and petrological investigations of a cumulus ultramafic lens exposed near the Balmuccia mantle massif.

The studied ultramafic lens consists of tens of meters thick dunites mantled by pyroxenites along the contacts with the enclosing gabbronorites. The pyroxenites have highly variable thickness, which never exceeds 10 meters. Near the contact with the dunites, the pyroxenites include irregularly shaped, cm-thick dunite lenses elongated subparallel to the dunite-pyroxenite contact. The contact between the pyroxenites and the enclosing gabbronorites is characterized by irregular alternations, cm- to tens of cm-scale in thickness and in most cases folded, of pyroxenites, melagabbronorites and gabbronorites. The dunites have olivine with low forsterite proportion (81-82 mol%) and spinel with low Cr# (10-14). The pyroxenites are olivine- and plagioclase-bearing near and away from the contact with the dunites, respectively. Pyroxenites also occur as subparallel cm-scale thick veins within the dunites. Taken as a whole, the pyroxenites have up to 30 vol% amphibole (titanian pargasite) and include accessory amounts of ilmenite, Al-spinel and Fe-Cu sulfides. Along mm-scale transects from the dunites to the included pyroxenite veins, we observed a gradual decrease in Mg# and Cr# in pyroxenes, amphibole and spinel. The enclosing gabbronorites are amphibole-free and ilmenite- and quartz-bearing. They have markedly lower Mg# than the pyroxenites and nearly flat chondrite-normalized REE patterns with positive Eu anomaly. The REE pattern of the pyroxenites is distinct in the marked LREE depletion and negative Eu anomaly.

We propose that the pyroxenites developed by reaction between dunites and infiltrating melts, relatively rich in SiO2, which had already undergone plagioclase fractionation. The compositions of these reacting melts cannot be reconciled with those of the melts forming the enclosing gabbronorites. The gabbronorite parental melts presumably interacted with the pyroxenites to give rise to the melagabbronorites. We speculate that pyroxenitization of the dunites occurred during the early stages of the building of the Ivrea Mafic Complex.

How to cite: Mariani, D., Renna, M. R., and Tribuzio, R.: Pyroxenitization of dunites in the lower continental crust: evidence from the Ivrea Mafic Complex (Italian Alps), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-554, https://doi.org/10.5194/egusphere-egu22-554, 2022.

Keywords: Central Alps, magmatic system, thermodynamic modelling, crystallization, crustal contamination

Understanding which processes are active and quantifying their relative influence during the differentiation of intracontinental magmatic systems remains a major challenge, as these processes can either (1) involve magmas and their crystallization products (fractional crystallization, reactive melt flow...) and/or (2) crustal contamination through various vectors (bulk assimilation, reactive assimilation, host-rock partial melting…). Whereas the influence of some of these processes can be inferred from field evidence, it needs to be constrained and quantified. This question can be addressed in the Central Alps (N Italy, SE Switzerland), where a complete, crustal-scale post-Variscan (Permian) magmatic system has been documented from lower crustal (Braccia gabbro, Malenco unit) and mid-crustal intrusives (Sondalo gabbro, Campo unit) to upper crustal intrusives and extrusives (Bernina unit). We present preliminary results, combining field work to petrological and geochemical characterization and modelling.

Petrological investigations on major element bulk-rock composition shows a complete differentiation trend from the less differentiated lower crust intrusive mafic rocks (Ol-gabbro, gabbro: 40-50 wt.% SiO₂, Mg# 45-75, 0.1-0.8 wt.% K₂O; and diorite: 45-60 wt.% SiO₂, Mg# 45-55, 0.15-0.5 wt.% K₂O), to upper crust felsic rocks (granite/rhyolite: 55-85 wt. % SiO₂, Mg# 5-50, 1-6 wt.% K₂O). By contrast, middle crust intrusive rocks encompass the full compositional range from Ol-gabbro and gabbro (45-50 wt.% SiO₂, Mg# 35-90, 0-3 wt.% K₂O), to alkali-rich diorite (50-60 wt.% SiO₂, Mg#: 40-55, 0.5-2 wt.% K₂O) and granite (50-85 wt.% SiO₂, Mg#: 5-50, 1-6 wt.% K₂O). To test the role of equilibrium and fractional crystallization, thermodynamic models were run using Rhyolite-MELTS software, and compared to experimental results in the 0-1 GPa pressure range from the literature. Some correlations between our samples compositions and the models (e.g., for CaO contents and Mg#) can be seen, but the latter fails at reproducing SiO₂ and K₂O differentiation trends.

Bulk-rock compositions indicate that magmas follow a composite differentiation trend between tholeiitic and calc-alkaline series, and the low abundance of olivine, even for the most primitive rocks indicates that before reaching the lower crust, magma was already fractionated during it ascent through the mantle. However, major differentiation does not seem to occur in the lower crust, being set fertile by previous tectono-metamorphic events. Instead, most of differentiation occurs in the fertile middle crust, since a wide major elements compositional range is observed. Both experimental and modelling results show that the observed diversity of composition cannot be attributed to fractional crystallization solely, notably by the high K₂O content at high Mg#. This suggest a potential role on crustal contamination; although evidence for contamination can be documented in the field (e.g., garnet, cordierite-bearing gabbro surrounding xenoliths), the extent of this contamination and its vectors remains to be constrained.

How to cite: Morin, M., Petri, B., and Ulrich, M.: Petrological evolution of Permian magmatism in Central Alps (SE Switzerland, N Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4411, https://doi.org/10.5194/egusphere-egu22-4411, 2022.

How to cite: Lustrino, M., Pistocchi, L., Ronca, S., Innocenzi, F., and Agostini, S.: Carbonate assimilation of ultrabasic magma: The Pleistocene Cupaello kamafugitic volcano (central Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5673, https://doi.org/10.5194/egusphere-egu22-5673, 2022.

The closure of Neotethys induced from calcalkaline through shoshonite to alkaline felsic and mafic intrusive within the Central Anatolia Crystalline Complex (CACC) during the Late Cretaceous-Early Paleogene. Despite of the genesis of alkaline felsic products are well understood, there is lack of data and petrogenetic explanation and nature of the alkaline felsic products in the eastern part of the Central Anatolia region and their relation between their equvalents in the western parts of the regions. Özvatan is the area where these late felsic magmatism is observed at the most eastern part of Central Anatolia. In accordance with the nature of these foid-bearing igneous rocks, we have carried out detailed petrographic, whole rock geochemical and Sr-Nd-O isotopic study in order to unravel all of these and compared with the equivalents within the Central Anatolia Region. Foid-bearing igneous rocks, which outcrop near Özvatan town in the vicinity of Kayseri city, intruded into the Paleozoic schist and marble as the basement of the region. Özvatan foid-bearing igneous rocks are mostly syenite and minor urtite in composition. The foid bearing igneous rocks have holocrystalline hipidiomorph texture and have nepheline, alkali feldspar, plagioclase, mica, amphibole, clinopyroxene, garnet, cancrinite, sodalite with rare amount of sphene, zircon, apatite, fluorite and opaque minerals. The unit is differentiated to six subgroups according to their mineralogical composition and texture features. Each subgroup has similar mineral compositions with different mineral proportions. These are biotite nepheline syenite, biotite sodalite melanite nepheline syenite, melanite cancrinite biotite nepheline syenite, cancrinite biotite amphibole melanite nepheline syenite, cancrinite melanite pyroxene amphibole nepheline syenite and urtite. The foid-bearing syenites have magma segregation and MME which are nepheline monzogabbro and diorite in composition. The mineral chemistry reveal that the alkali feldspars are orthoclase, plagioclases are bytownite, the pyroxenes are mostly diopsite, amphiboles are hastingsite, cancrinite are vishnevite, garnets are melanite (andradite), micas are annite and phlogopite in compositions. The detail mineralogical, petrographical and the mineral chemistry studies reveal that the melt from which Özvatan foid-bearing syenites may derived from a depth over 60 km. On the other hand, the mineral chemistry of the mica reveals that Özvatan foid-bearing syenites are generated from the crust-mantle mixed source magma. The high ⁸⁷Sr/⁸⁶Sr (0.707822-0.710544) and low ¹⁴³Nd/¹⁴⁴Nd (0.512300-0.512379) of Özvatan foid-bearing syenites are indicative of mantle sources with large continental crustal components. Nepheline oxygen isotope data from Özvatan foid-bearing syenites have a range of δ¹⁸O values +9.4 - +10.5‰ and are compatible with the values for mixed (mantle-crustal) origin. All isotope data suggest that these intrusive rocks have experienced fractional crystallisation coupled with the crustal assimilation with enriched mantle source. Özvatan foid-bearing syenites compared with the equivalents in the Central Anatolia Region, the Özvatan foid-bearing syenites are derived from a source which is richer in volatile components with less crustal contamination and comes from a deeper source than the western edge around Kırsehir city.

 Keywords: Sr-Nd-O Isotope Geochemistry.

Acknowledgement: This study was supported by Ankara University Department of Scientific Research Projects (17B0443003) and TUBITAK 116Y240 project.

How to cite: Deniz, K., Kadioglu, Y. K., Gullu, B., and Koralay, T.: Nature of the Özvatan (Kayseri) Foid-bearing Rocks from the Central Anatolia (Turkey): Sr-Nd-O Isotope Geochemical Approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12940, https://doi.org/10.5194/egusphere-egu22-12940, 2022.