1,721,131 research outputs found
Petrogenetic implications of Ba-sanidine in the Lionato Tuff (Colli Albani Volcanic District, Central Italy)
No full textA vitric fiamme containing BaO-bearing sanidine (BaO = 11 wt.%) was sampled (via di Fioranello, Pome) in the pyroclastic products of the Lower Flow Unit (LFU) belonging to the Villa Senni Eruption Unit in the Colli Albani Volcanic District (Roman Comagmatic Province, Central Italy). The fiamme of the LFU is essentially made up of glass, in which scarce phenocrysts of BaO-bearing sanidine, leucite and Al2O3-bearing Ca-clinopyroxene occur. In this paper the various parameters controlling barium partitioning between alkali feldspar and LFU liquid, are discussed
Indicazioni petrogenetiche dal Ba-sanidino presente nel Tufo lionato (Distretto Vulcanico dei Colli Albani.
No full textCa-Fe-rich exsolution lamellae from olivine in a wehrlitic xenolith, Monti Vulsini Volcanic District, Central Italy
No full textA wehrlitic xenolith containing an assemblage of Cr-spinel, forsterite, Cr-diopside, phlogopite and leucite was sampled in the pyroclastic products of the Monti Vulsini Volcanic District of the Roman Comagmatic Province (Central Italy). The peculiarity of this ultramafic xenolith is the presence of a large number of exsolution lamellae in the olivine crystals. These exsolutions have a peculiar dendritic shape and a Ca-Fe-rich composition due to the microscopic intergrowths of clinopyoxene and magnetite. Melting experiments and mass balance show that the host olivine crystallised from a primitive K-basaltic melt with an uncommon calcium content, before exsolution. Geothermobarometric calculations suggest that the exsolution formed at subsolidus temperature and 6-9 kbar of pressure
Indicazioni petrogenetiche dai granati birifrangenti dei proietti sialici nelle vulcaniti alcalino potassiche dei Monti Sabatini
No full textStrontian fluoro-magnesiohastingsite in Alban Hills lavas (Central Italy): constraints on crystallization conditions
No full textA magmatic calcic amphibole (Ca-M4 greater than or equal to1.50) characterized by large Sr and F contents (SrO >1 wt.%, F >2.5 wt.%) is described. According to the crystal-chemical formula, the amphibole can be classified as a strontian fluoro-magnesiohastingsite and the presence of Sr cations in the A sites is suggested. The amphibole occurs in the groundmass of ultrapotassic lavas from the Alban Hills Volcanic District (Central Italy). This peculiar chemical composition is due to the lavas' groundmass compositional trend occurring under low silica-activity conditions. We suggest that the occurrence of F amphiboles allows us to consider the Alban Hills Volcanic District magmas to be as rich in F as other ultrapotassic magmas
A petrographic and fluid inclusions study of high-grade metamorphic hornfelses (Gennargentu Igneous Complex, sardinia, Italy): preliminary data.
No full textIndicazioni termodinamiche e petrologico sperimentali sull'origine ed evoluzione dei magmi potassici dell'Italia centrale.
No full textOxygen isotope geochemistry of pyroclastic clinopyroxene monitors carbonate contributions to Roman-type ultrapotassic magmas
No full textThe oxygen isotope geochemistry and chemical composition of clinopyroxene crystals from Alban Hills pyroclastic deposits constrain the petrological evolution of ultrapotassic Roman-type rocks. Volcanic eruptions in the 560-35 ka time interval produced thick pyroclastic deposits bearing clinopyroxene phenocrysts with recurrent chemical characteristics. Clinopyroxenes vary from Si-Mg-rich to Al-Fe-rich with no compositional break, indicating that they were derived from a continuous process of crystal fractionation. Based on the δ18O and trace element data no primitive samples were recovered: Monomineralic clinopyroxene cumulates set the oxygen isotope composition of primary magmas in the range of uncontaminated mantle rocks (5.5‰), but their ∑REE composition resulted from extensive crystal fractionation. Departing from these mantle-like δ18OCpx values the effects of crustal contamination of clinopyroxene O-isotope composition were identified and used to monitor chemical variations in the parental magma. δ18O values in Si-Mg-rich clinopyroxene are slightly higher than typical mantle values (5.9-6.2‰), and the low ∑REE contents are representative of early stages of magmatic differentiation. δ18O values as high as 8.2‰ are associated with Al-Fe3+ -rich clinopyroxene showing high ∑REE contents. These δ18O values are characteristic of crystals formed during the late magmatic stages of each main eruptive phase. Geochemical modelling of δ18O values vs. trace element contents indicates that these ultrapotassic magmas were derived from fractional crystallization plus assimilation of limited amounts of carbonate wall rocks starting from a primary melt, and from interaction with CO2 derived from country rocks during crystal fractionation. © Springer-Verlag 2004
Mineral chemistry of pyroclastics erupted during the Tuscolano-Artemisio phase of the Alban Hills volcano (Latium, Italy)
No full textExperimental insights into the origin of crystal-poor phonolitic magmas
No full textCrystal-poor phonolitic magmas have been commonly erupted during the quaternary explosive volcanism of Central Italy. The origin of crystal-poor magmas represents a complex issue of igneous petrology and since the first studies on crystal fractionation by settling, many alternative mechanisms of crystal-melt separation have been proposed [see Bachmann and Bergantz, 2004 and references therein]: i) convective fractionation in a crystallizing boundary layer; ii) gas-driven filter press; iii) thermal gradient responsible for mass transport (thermal migration) resulting in segregation of melt from the mushy, boundary zone of magma chambers; iv) melt migration induced by crystal compaction; v) instability of “solidification front”. In the frame of the highly explosive volcanism of Central Italy, that was fed by differentiated and thermally zoned pre-eruptive systems, not all of the abovementioned mechanisms would operate efficiently. Moreover, these mechanisms are mainly based on theoretical models and natural evidences, but poorly constrained by experiments.
In this work, in order to shed light on the origin of large volumes of highly differentiated, crystal-poor magmas in thermally zoned systems, we have experimentally investigated crystallization, differentiation, and crystal-melt separation in the presence of a thermal gradient. Melt differentiation has been investigated under isothermal conditions (i.e. phase equilibria experiments) as well. As a case study, we have used the Sabatini Volcanic District, one of the main volcanic districts of the Roman Province characterized by several explosive eruptions producing large volumes of crystal-poor phonolitic magma [Masotta et al. 2010]. Phase equilibria experiments constrained the liquid line of descent leading to phonolitic magmas but gave no insights into the origin of crystal-poor textures. On the contrary, thermal gradient experiments produced structures clearly showing crystal-rich textures overlaid by crystal-poor batches of differentiated melt. The structures pictured at the cool top of the charges remind the so-called solidification front [Marsh, 1996].
Thermal gradient experiments, being performed under controlled conditions, allowed us to determine parameters necessary to model the processes responsible for the formation of these batches of differentiated, crystal-poor melt. We advocate that these experiments represent an important tool to model crystal-melt separation in thermally zoned natural systems as well
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