Università di Roma La sapienza

The geochemical characteristics of volcanic products in a variety of tectonic settings demonstrate that incorporation of crustal material into magmas is a relatively common process. Contamination of magmas by crustal components, in turn, can have a significant effect on magma composition and rheology. Despite this, the mechanism by which contamination occurs is still not well established and its efficacy is denied by some. In this study we focus on magma^ carbonate interaction and on the rock shells (cumulates and skarns) formed at the contact between a magma chamber and its wall-rocks.We deduce that previous, unsuccessful attempts at carbonate assimilation^fractional crystallization (AFC) modelling can be related to the paucity of information about the cumulate zone in contact with skarns.We use one of the best examples of a magmatic plumbing system emplaced within a thick carbonate substratum (the Colli AlbaniVolcanic District in Central Italy) to demonstrate that a ‘skarn environment’can act as a source of CaO-rich silicate melts, and that the assimilation of these melts into the primitive magma is the main process responsible for magma contamination, rather than the ingestion of solid carbonate wall-rocks. In particular, by means of microtextural observations, mineral chemistry, whole-rock geochemical data and MELTS simulations we highlight the effect of high Ca-Tschermaks (CaAl2SiO6) activity in the melt on the stability of Cr-spinel, olivine, and clinopyroxene in cumulate rocks, define a reaction-cumulate zone where clinopyroxene crystallization is favoured, and model the magmatic differentiation processes active in this zone.

We report on the newly discovered lava flow that erupted in the Colli Albani Volcanic District, which is the most recent and, geochemically the most peculiar effusive event recognised in the entire ultrapotassic Roman Province (Central Italy). This lava flow is associated with the Monte Due Torri scoria cone, located approximately 5 km south of the Albano hydromagmatic centre (69–36 ka). TheMonte Due Torri scoria cone displays well-preserved morphological characteristics and the 40±7 ka age determined for the associated lava flow indicates that its activity was nearly contemporaneous to the most recent, explosive activity that occurred at the Albano centre from 41 to 36 ka. By comparing chemical and petrological features of the Monte Due Torri lava flow, Albano products, and older products (N69 ka), we show that the youngest Colli Albani eruptions were fed by two new batches of parental magmas that originated in a phlogopite-bearing metasomatised mantle, each one feeding one of the two youngest eruptive cycles (at 69 ka and 41–36 ka). The trace element signature, e.g., very low Pb content, of primitive (MgON3 wt.%) magmas feeding the initiation of the hydromagmatic activity at Albano (69 ka) and the subsequent effusive activity at Monte Due Torri (40 ka) indicates that a magma chamber located in the deep anhydrite-bearing dolomite formation was tapped. However, the polygenic activity, the changes in magma composition, and the variable thermometamorphic clasts occurring in the hydromagmatic deposits (recording variable substrata) suggest, particularly for the Albano eruptive centre, a more complex plumbing system consisting of at least two more magma chambers at a shallower depth, i.e., in the Mesozoic limestone and Pliocene pelite formations. The large amount of stratigraphic, volcanological, and geochemical data collected for the Colli Albani Volcanic District, one of the main districts in the ultrapotassic Roman Province, enable us to contribute insights into the still open debate regarding the temporal variation of the metasomatised mantle source of the Italian potassic magmas. Based on our data, i.e., variation of radiogenic and trace elements over time, we suggest that the observed variation in the mantle source of the ultrapotassic magmas can be related to progressive consumption of the phlogopite component in the metasomatised source rather than the transition from lithosphere- to asthenosphere-derived magmatism and/or the transition from orogenic to anorogenic magmatism.

The geochemical characteristics of volcanic products in a variety of tectonic settings demonstrate that incorporation of crustal material into magmas is a relatively common process. Contamination of magmas by crustal components, in turn, can have a significant effect on magma composition and rheology. Despite this, the mechanism by which contamination occurs is still not well established and its efficacy is denied by some. In this study we focus on magma-carbonate interaction and on the rock shells (cumulates and skarns) formed at the contact between a magma chamber and its wall-rocks. We deduce that previous, unsuccessful attempts at carbonate assimilation-fractional crystallization (AFC) modelling can be related to the paucity of information about the cumulate zone in contact with skarns. We use one of the best examples of a magmatic plumbing system emplaced within a thick carbonate substratum (the Colli Albani Volcanic District in Central Italy) to demonstrate that a “skarn environment” can act as a source of CaO-rich silicate melts, and that the assimilation of these melts into the primitive magma is the main process responsible for magma contamination, rather than the ingestion of solid carbonate wall-rocks. In particular, by means of microtextural observations, mineral chemistry, whole-rock geochemical data and MELTS simulations we highlight the effect of high Ca-Tschermaks (CaAl2SiO6) activity in the melt on the stability of Cr-spinel, olivine, and clinopyroxene in cumulate rocks, define a reaction-cumulate zone where clinopyroxene crystallization is favoured, and model the magmatic differentiation processes active in this zone.

Wehave investigated lava flows representative of thewhole eruptive history of the Colli Albani ultrapotassic volcanic district (Central Italy). One of the most intriguing features concerning some of these lava flows is the occurrence of primary, magmatic calcite in the groundmass. The primary, magmatic nature of calcite has been inferred by microtextural investigations showing that it typically occurs i) interstitially, associated with clinopyroxene, nepheline and phlogopite, ii) in spherical ocelli, associated with nepheline, fluorite and tangentially arranged clinopyroxene, and iii) in corona-like reaction zones around K-feldspar xenocrysts. These microtextural features distinctly indicate that calcite crystallized froma carbonate melt in a partially molten groundmass, implying that the temperature of the system was above the solidus of the hosted lava flow (N850 °C). Geochemical features of calcite crystals (i.e., stable isotope values and trace element patterns) corroborate their primary nature and give insights into the origin of the parental carbonate melt. The trace element patterns testify to a high-temperature crystallization process (not hydrothermal) involving a carbonate melt coexisting with a silicate melt. The high δ18O (around 27‰SMOW) andwide δ13C (−18 to+5‰PDB) values measured in the calcites preclude a mantle origin, but are consistent with an origin in the crust. In this framework, the crystallization of calcite can be linked to the interaction between magmas and carbonate-bearing wall rocks and, in particular, to the entrapment of solid and/or molten carbonate in the silicate magma. The stability of carbonate melt at lowpressure and the consequent crystallization of calcite in the lava flow groundmass are ensured by the documented, high activity of fluorine in the studied system and by the limited ability of silicate and carbonate melts to mix at syn-eruptive time scales.

The main effect of magma-carbonate interaction on magma differentiation is the formation of a silica-undersaturated, alkali-rich residual melt. Such a desilication process was explained as the progressive dissolution of CaCO3 in melt by consumption of SiO2 and MgO to form diopside sensu stricto. Magma chambers emplaced in carbonate substrata, however, are generally associated with magmatic skarns containing clinopyroxene with a high Ca-Tschermak activity in their paragenesis. Data are presented from magma-carbonate interaction experiments, demonstrating that carbonate assimilation is a complex process involving more components than so far assumed. Experimental results show that, during carbonate assimilation, a diopside-hedenbergite-Ca-Tschermak clinopyroxene solid solution is formed and that Ca-Tschermak/diopside and hedenbergite/diopside ratios increase as a function of the progressive carbonate assimilation. Accordingly, carbonate assimilation reaction should be written as follows, taking into account all the involved magmatic components: CaCO3solid+SiO2melt+MgOmelt+FeOmelt+Al2O3melt → (Di-Hd-CaTs)sssolid+CO2fluid The texture of experimental products demonstrates that carbonate assimilation produces three-phases (solid, melt, and fluid) whose main products are: i) diopside-hedenbergite-Ca-Tschermak clinopyroxene solid solution; ii) silica-undersaturated CaO-rich melt; and iii) C-O-H fluid phase. The silica undersaturation of the melt and, more importantly, the occurrence of a CO2-rich fluid phase, must be taken into account as they significantly affect partition coefficients and the redox state of carbonated systems, respectively.

The major processes that control the genesis of potassic volcanic rocks, like the timing of multi-stage mantle metasomatism, remain largely unclear. In an attempt to clarify the timing of the metasomatic process, a detailed geochronologic and geochemical study has been conducted on the ultrapotassic rocks of the Colli Albani Volcanic District (Central Italy). New 40Ar/39Ar data coupled with literature and newly performed 87Sr/86Sr, 143Nd/144Nd and chemical data allow us to precisely delineate the time-dependent geochemical variations of the magmas erupted at the Colli Albani Volcanic District and to better define mantle source processes responsible for their genesis. The temporal geochemical variations observed in the Colli Albani magmas indicate that: i) the ultrapotassic magmas originated from a metasomatized mantle source in which phlogopite is the potassium-bearing phase; ii) the partial melting of the mantle source involved mainly phlogopite and clinopyroxene (±olivine), whereas the role of accessory phases was less significant; and iii) the metasomatic process that led to the formation of the phlogopite in the mantle can be reasonably related to events that have occurred during the Paleozoic Era

Crystal-rich lithic clasts occurring in volcanic deposits are key tools to understand processes of storage, cooling, and fractionation of magmas in pre-eruptive volcanic systems. These clasts, indeed, represent snapshots of the magma-chamber/host-rock interface before eruptions and provide information on crystallization, differentiation, and degrees of interaction between magma and wall-rocks. In this study, with the aim to shed light on magma-carbonate interaction and CO2 emission in volcanic areas, we focused on the petrology of cumulate and skarn rocks by using as case study a suite of mafic and calcite-bearing lithic clasts from the Colli Albani Volcanic District. By means of phase relations, bulk rock chemistry, phase compositions, and stable isotope data we have recognized different types of cumulates and skarns. Cumulates containing either clinopyroxene±olivine associated with Cr-bearing spinel or glass+phlogopite have been divided in primitive and differentiated, respectively. Primitive cumulates originate at the interface between a relatively primitive magma and carbonate-bearing rocks and show evidences of olivine instability (i.e. heteradcumulate texture) due to carbonate assimilation. Differentiated cumulates, characterized by Ca-rich olivines, phlogopite, and glass containing calcite, form from a differentiated magma in a system open to CaO-contamination. Skarns has been divided in exoskarns, characterized by xenomorphic texture and abundant calcite, and endoskarns, characterized by hypidiomorphic texture, Ca-Tschermak-rich mineral phases, and interstitial glass. Exoskarns formed by means of solid state reactions in a dolostone protolith whereas endoskarns crystallized at subliquidus temperature from a silicate melt that experienced exoskarns assimilation. Our study evidences that magma-carbonate interaction can not be considered a one step process exhausting just after the formation of skarn shells. Magma and carbonate rocks, when in contact, continuously interact leading to the formation of exoskarns, endoskarns, cumulates (primitive and differentiated ones), and differentiated melts. Finally, by means of oxygen and carbon isotope compositions of calcite in equilibrium with skarns, we demonstrate that carbonate assimilation represents a source of massive CO2 degassing mechanism due to the consumption of calcite and removing of CO2 during the decarbonation process.