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Differentiation and storage of magmas at crustal-mantle boundary depth: linking experiments, models and field observations

The timescale of magma formation, storage and ascent beneath active volcanoes is crucial to constrain the magma chamber dynamics and provide the basis for volcanic hazard assessment. The interpretation of rock textures is the key step of the back-analysis studies targeted at the reconstruction of the magma chamber dynamics and depends on our understandings of the crystallization kinetic processes in silicate melts. Beside crystal formation, dissolution and reaction of pre-existing crystals, are processes that commonly affect the kinetics of magma crystallization and that, however, are broadly constrained from the theoretical and the experimental point of view.
Experimental studies can shed a light on subterranean processes generating eruptions, constraining pre- and syn-eruptive physical and chemical key magmatic variables such as pressure, temperature, volatile and crystal content, melt composition and viscosity. A major goal of modern experimental studies is to constrain the physical and chemical parameters of deep magmatic reservoirs, such as those located at Moho-lower crust boundary.
This session invites contributions dealing with the nucleation and growth of minerals, the “fate of pre-existing crystals”, the effects of crystallization kinetics on mineral chemistry as well as the partitioning of trace elements between crystals and melts during high-pressure differentiation and storage of magmas. We particularly encourage submissions on experimental petrology, thermodynamic and geochemical modelling, as well as on field studies.

Convener: Barbara BonechiECSECS | Co-conveners: Cristina Perinelli, Mario Gaeta, Alessandro Fabbrizio
Presentations
| Wed, 25 May, 13:20–14:50 (CEST)
 
Room -2.16

Wed, 25 May, 13:20–14:50

Chairperson: Barbara Bonechi

13:20–13:23
Introduction

13:23–13:30
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EGU22-559
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ECS
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On-site presentation
Michele Cassetta et al.

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.

13:30–13:37
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EGU22-7251
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ECS
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On-site presentation
Franziska Keller et al.

Caldera-forming eruptions are among the most hazardous natural events on Earth and pose a significant risk for global consequences in the future. Recent petrological re-evaluations of caldera and intercaldera deposits in several volcanic systems worldwide suggest a cyclic behavior in the evolution of these subvolcanic reservoirs, comprising distinct maturation, fermentation and recovery phases. Here, we test the application of this caldera cycle framework on the Aso system in Central Kyushu (Japan) by evaluating pre-caldera activity of the Aso-4 caldera-forming event.

The Aso system is an archetypical example of a multi-cyclic caldera-forming volcanic edifice, which was built by four catastrophic caldera-forming events between 266 ka (Aso-1) and 86.4 ka (Aso-4). These caldera-forming eruptions are separated by extensive post- and pre-caldera activity from numerous vents inside and outside the caldera areas. The deposits produced from these vents range from basalts to rhyolites in bulk-rock compositions and are dominated by a mineral assemblage consisting of plagioclase, ortho- and clinopyroxene, Fe-Ti oxides and apatite.

Plagioclase and orthopyroxene are the most abundant minerals in the Aso system. However, their often wide compositional ranges, recording multiple stages of magma evolution, make it difficult to pinpoint chemical differences between pre- or post-caldera eruptions; geochemical analyses of orthopyroxene and plagioclase give overlapping ranges for pre-Aso-4 and Aso-4 caldera-forming events with Mg# between 71 to 75 and An% between 35 to 80, respectively. On the other hand, accessory mineral phases, such as titanomagnetite and apatite, chemically re-equilibrate fast with the melt and hence predominantly inherit conditions prevailing just prior to eruption. Average MnO contents in titanomagnetite record a distinct increase from ~0.8 wt% to ~0.9 wt% in pre-Aso-4 crystals to 1.2 wt% in the silicic cap of the Aso-4 system, indicating the progressive evolution of the system towards more differentiated compositions. Similarly, the F-Cl-OH record of apatite in the pre-Aso-4 and the Aso-4 system indicate the transition from water-undersaturated conditions during magmatic evolution in the pre-Aso-4 system to water-saturated conditions in the silicic cap of the Aso-4 system. Concomitant with these variations, changes in dissolved water contents in melts and storage temperatures are observed, with temperatures progressively decreasing from the pre-Aso-4 units (> 900 °C) to the silicic portion of the Aso-4 deposits (~860-880 °C). Dissolved water contents in the melt, in turn, increase from ~3-4 wt% in the pre-caldera system to ≥ 4.6 wt% in the Aso-4 silicic cap, close to or even at volatile saturation considering ~100-400 ppm CO2 in the melt at storage depths (~1.5 - 2 kbar). These findings are in good agreement with the caldera cycle framework and suggest the progressive differentiation of the Aso system during a maturation phase through volatile exsolution in the fermentation stage to the catastrophic Aso-4 caldera-forming eruption.

How to cite: Keller, F., Popa, R.-G., Geshi, N., Miyakawa, A., and Bachmann, O.: Tracking caldera cycles in the Aso-4 magmatic system, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7251, https://doi.org/10.5194/egusphere-egu22-7251, 2022.

13:37–13:44
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EGU22-8042
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Virtual presentation
Barbara Cserép et al.

By investigating fossil eruption products, we can better understand the behaviour of volcanoes and the processes occurring well beneath the volcanic edifice, leading to eruptions. In a felsic, crystal-rich (> 50 volume%), long-dormant mushy system, one of the most critical rejuvenation factors is the addition of hot mafic magma. In the case of Ciomadul volcano (Southeast Carpathians), a dominantly explosive eruption phase occurred at 56-50 ka following a ca. 40 kyr long dormancy. Three eruption units were studied in detail: Băile Tuşnad (Ee5/1tf), the Covasna–Harghita frontier (Ee5/1kh) and Mohoş roadcut (Ee5/1mo). Pumices from these locations have relatively high-Mg values (avg. of 0.56-0.62 mol%) and similar Sr/Y (147-157, but 225 in the Ee5/1tf) and Dy/Yb (1.71-1.73, but 1.89 in Ee5/1tf) ratios akin to the adakite-like rocks. Variation of these bulk pumice as well as the titanite trace element ratios indicates early garnet fractionation or residual garnet in the mantle source region. Trace element signature of the pumices shows strong enrichment of Ba and Sr and a depletion in heavy REE suggesting that the primary magmas originated by partial melting of strongly metasomatized lithospheric mantle.

Mineralogy of the pumices is plagioclase, amphibole and biotite phenocrysts and apatite, titanite, zircon and FeTi oxide accessories. Mafic crystal clots of orthopyroxene, clinopyroxene surrounded by amphibole occur occasionally. Among the phenocrysts, amphibole shows a complex compositional zoning. One of the most particular features is the appearance of high-Mg and low-Al cores, found very rarely, if any, within amphibole, worldwide. Trace element composition of amphiboles shows also considerable variation and suggest crystallization at various magmatic environments. The high-Mg and low-Al amphibole xenocrysts have also peculiar rare earth element patterns: they have relatively low total REE content and a strong depletion in heavy REE. This REE pattern is akin to the amphiboles found in metasomatized spinel harzburgites and xenoliths showing melt-solid reaction. We assume that these amphiboles could have been a near-liquidus phase consistent with an ultrahydrous equilibrium melt. Trace element composition of these early formed amphiboles is thought to reflect the composition of the amphiboles at the source region, i.e. shows an inherited nature. The strongly hydrous nature of the primary magmas is reflected also by the incongruent transition from pyroxenes to amphiboles as shown by the mafic clot textures.

Petrological features of the pumices suggest that eruption took place by reactivation of a relatively cold (T=700-775 °C), dacitic crystal mush. Rejuvenation was triggered by recharge of hydrous, less viscous mafic magma carrying the early formed crystal assemblage. Reheating and volatile flux initiated rapid remelting, magma ascent and eruption as indicated by thin overgrowth in amphibole and plagioclase and the lack of reaction zone around amphibole. This eruption scenario might have an implication for rapid reactivation after long dormancy in case of apparently inactive volcanoes.

This research was supported by the Hungarian National Research, Development and Innovation Fund (NKFIH) within K135179 project and the ÚNKP-21-3 New National Excellence Program of the Ministry for Innovation and Technology from the source of the NKFIH.

How to cite: Cserép, B., Harangi, S., Erdmann, S., Kovács, Z., and Lukács, R.: High-Mg amphibole and bulk-rock composition from Ciomadul dacitic pumices suggest rapid eruption trigger by strongly hydrous mafic magma recharge, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8042, https://doi.org/10.5194/egusphere-egu22-8042, 2022.

13:44–13:51
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EGU22-6072
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ECS
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On-site presentation
Joshua Brown et al.

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.

13:51–13:58
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EGU22-10428
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ECS
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Virtual presentation
New insights into H2O wt.% content in a residual melt during hornblendite crystallization in the Fuegian Batholith (Patagonia) 
(withdrawn)
Maria Fernanda Torres Garcia et al.
13:58–14:05
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EGU22-10034
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ECS
Benjamin Klein and Othmar Müntener

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 KD values below 0.5, while the garnet KD 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.

14:05–14:12
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EGU22-446
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ECS
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Virtual presentation
Mahesh Halder et al.

Close spatial association of plutonic and volcanic rocks in Large Igneous Provinces is very rare. Occurrence of various mafic rocks (syeno-diorite, gabbro, basalt and mafic dyke) and silicic rocks in the Girnar volcano-plutonic complex of Deccan Traps provides a unique opportunity to understand complex petrogenetic processes. Alkaline rocks (syeno-diorite and mafic dyke) display an enriched LREE pattern (La/SmN ~5.0 to 7.8), whereas the tholeiitic gabbro and basalt show a relatively flat LREE pattern (La/SmN ~0.8 to 1.8) with a positive Eu anomaly (1.1 to 1.3). Trace elements modelling of alkaline rock compositions are consistent with their origin by 5–9% partial melting of a spinel lherzolite source, with the melt experiencing 74–88% fractional crystallization. Tholeiitic gabbro and basalt could be generated from 20% and ~5% melting of a depleted MORB source, respectively. Distinct isotopic composition of silicic rocks (87Sr/86Sri = 0.7204–0.7275, εNd(i) = −6.8 to −7.3, 206Pb/204Pbi = 18.74–19.02, 207Pb/204Pbi = 15.76–15.79, 208Pb/204Pbi = 39.63–40.03, εHf(i) = −6.0 to −7.3) indicate an origin from melting of a crustal source contaminated with (≤5%) mantle-derived mafic melts. The presence of shattered clinopyroxene crystals in silicic rocks and variation in Ti content of quartz are attributed to a sudden release of pressure during magma ascent. The Girnar Complex is emplaced in a reactivated fault lineament where mafic rocks (both alkaline and tholeiitic) were uplifted by several kilometers after its formation in a shallow magma chamber. Silicic rocks were emplaced along the concentric marginal fault of the Girnar Complex, synchronously or just after this uplift event.

How to cite: Halder, M., Paul, D., and Stracke, A.: Magmatic evolution of Girnar volcano-plutonic complex of Deccan Traps, India: Sr-Nd-Pb-Hf isotopic evidence of multiple sources, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-446, https://doi.org/10.5194/egusphere-egu22-446, 2022.

14:12–14:19
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EGU22-554
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ECS
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Virtual presentation
Davide Mariani et al.

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.

14:19–14:26
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EGU22-319
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ECS
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Virtual presentation
Monika Chaubey and A. Krishnakanta Singh

The Platinum Group Element (PGE) systematics and whole-rock geochemistry of mafic and ultramafic rocks from the ophiolites of Indo-Myanmar Orogenic Belt, northeast India were studied to comprehend their source characteristics and the fractionation behavior of PGE during magmatic evolution. Geochemical studies of mafic rocks reflect Mid-Oceanic Ridge (MOR) Basalt  to Ocean Island Basalt (OIB) affinities while ultramafic rocks have both MOR and Supra Subduction Zone (SSZ) setting signatures. The basaltic rocks show flat REE patterns and slightly depleted LREE [(La/Sm)N = 0.97-1.01], showing MOR-type basalt whereas gabbro shows enriched LREE [(La/Sm)N = 2.85-4.24; (Sm/Yb)N = 2.50-2.88], and characterized by OIB-type mafic rock. Conversely,  pyroxenites exhibit depleted LREE (LaN/SmN = 0.54-1.16) but flat MREE (SmN/YbN = 2.78–4.02) reflect spoon-shaped pattern, whereas harzburgite and dunite show U shaped [(LREE and HREE enrichment (La/Sm)N= 2.55-3.61, (Tb/Yb)N = 0.51-0.86, respectively] REE patterns which indicate formation in a forearc environment. The PGE contents in gabbro (ΣPGE =8.8-16.0 ppb) and basalt (ΣPGE =5.6-15.3 ppb) are lower than PGE abundances of harzburgite (ΣPGE =125.6-142.8ppb), dunite (ΣPGE =248-360 ppb) and pyroxenites (ΣPGE = 159.7-1156.8 ppb). The rocks show strongly enriched PPGEs over the IPGEs which indicates co-precipitation with early sulfide fractionation. In all samples (except pyroxenite) pronounced Rh and Pd enhancements relative to Pt suggest its removal during fractional crystallization. Pyroxenites mark the transition from sulfide-undersaturation displayed by harzburgite and dunite to sulfide-saturation displayed by basalt and gabbro. It is, therefore, substantiated that PGE distribution in mafic and ultramafic rocks of Indo-Mayanmar Ophiolites was controlled by sulphide saturation in parental magma and have not only been affected by partial melting processes but also affected by crystal fractionation process during their generation in diverse tectonic environments such as MOR, OIB (plume-type), and SSZ.

How to cite: Chaubey, M. and Singh, A. K.: Platinum-group element geochemistry and whole-rock systematics of mafic-ultramafic rocks from the Indo-Myanmar Orogenic Belt Ophiolites, NE India: Implications on mantle processes and tectonic settings, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-319, https://doi.org/10.5194/egusphere-egu22-319, 2022.

14:26–14:33
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EGU22-4411
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ECS
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On-site presentation
Mérédith Morin et al.

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.

14:33–14:40
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EGU22-5673
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On-site presentation
Michele Lustrino et al.

The Pleistocene Intra-Apennine Province (IAP) in central Italy contains several small volume eruptive centres, characterized by basic-ultrabasic lithologies, often characterized also by ultracalcic and ultrapotassic compositions. They are emplaced above the thick carbonate and evaporite sedimentary sequence of the Apennine Chain. The small monogenetic volcano of Cupaello (~640 ka) offers the chance to investigate exotic volcanic rocks such as ultracalcic kamafugites, known in literature as coppaelite. This volcano also offers the opportunity to study the interaction of ultrabasic melts with sedimentary carbonates (Maiolica Formation). Indeed, assimilation of local carbonate-rich sediments has been already documented as a likely process at least in the neighbouring IAP volcanoes of San Venanzo [1] and Polino [2] volcanoes. On the other hand, alternative views considers the CaO-rich (CaO up to 38.8 wt%) and SiO2-poor (SiO2 down to 14.2 wt%) composition of some Cupaello pyroclastic rocks a reflection of a carbonatitic component in their mantle source [3].

 Cupaello lavas are ultrabasic (silica = 42.6-44.1 wt%) and ultrapotassic (K2O = 5.2-7.6 wt%; K2O/Na2O = 18.0-33.9) rocks, characterized by euhedral to subhedral phenocrysts of clinopyroxene and phlogopite set in a hypohyaline-hypocrystalline groundmass made of melilite, kalsilite, phlogopite, olivine, calcite and glass. Perovskite, opaques, wollastonite, monticellite and apatite represent the accessory phases.

The trace element signatures of Cupaello kamafugites, such as high LILE (e.g., Rb = 482-673 ppm), high LILE/HFSE ratios (Ba/Nb = 71-82), negative Eu anomalies (Eu/Eu* = 0.68-0.72), as well as the presence of negative anomalies for Nb, Ta, P and Ti coupled with peaks for Pb in primitive mantle-normalized diagrams are compatible with the derivation from a subduction-modified source [4]. Isotopic ratios confirm this hypothesis, with the presence of strongly radiogenic 87Sr/86Sr (0.71123-0.71125), unradiogenic 143Nd/144Nd (0.51200-0.51207) and 206Pb/204Pb isotopic ratios buffered to 18.76.

The negative correlation of major oxides and trace elements with CaO, pointing toward an end-member represented by the Apennines limestone lithologies, offers an alternative hypothesis to the widely accepted presence of a carbonatitic component. The very small volume of emplaced magma, as well as the thick carbonate succession to be pierced to reach the surface renders unavoidable strong crustal assimilation by the original magma. The high Fo (89.5-90.2) in Cupaello olivine, as well as the thin border of monticellite around the rare olivine could be explained by assimilation of limestone wall rock, as demonstrated experimentally [5]. The CaO-richest (CaO up to 38.8 wt%) and SiO2-poorest (SiO2 down to 14.2 wt%) compositions are found in pyroclastic/epiclastic deposits (no longer available for sampling). We believe that these whole-rock compositions reflect the presence of abundant, almost completely sterile, secondary calcite.

Bibliography

 

[1] Lustrino et al., 2020, Earth-Sci. Rev., 208, 103256.

[2] Lustrino et al., 2019, Sci. Rep., 9, 1-14.

[3] Stoppa and Cundari, 1995, Contrib. Mineral. Petrol., 122, 275-288.

[4] Carminati et al., 2012, Tectonophysics., 579, 173-192.

[5] Lustrino et al., 2022, Geology, https://doi.org/10.1130/G49621.1

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.

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EGU22-12940
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Virtual presentation
Kiymet Deniz et al.

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.

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Conclusions