Carbon forms: paths and processes in the Earth is a thematic set of six papers arising from the lectures presented at the Lake Como School, held at Villa del Grumello, Como, Italy (15–20 October 2017), and organized by the Graduate School of Milano Bicocca. This collection of lecture notes focusses on the structure of carbon allotropes, the geodynamics of deep Earth's carbon transport and fixation at mantle conditions, carbon degassing by ascending magmas, and the vast tectonic carbon degassing at the Earth's surface.
New science is emerging for carbon, one of the fundamental elements for life, energy, and global climate change on our planet. Carbon is cycled between surface, atmosphere, and oceans. Most of the recyclable carbon is stored deep in the Earth. Ingassing from the atmo-hydrosphere into the mantle occurs by subduction; in the mantle carbon resides in solids such as diamonds and is transported by fluids and melts that migrate upward. Outgassing into the atmosphere is effected by volcanism and tectono-metamorphic soil emissions. The forms and processes of carbon ingassing and outgassing are still poorly constrained. There is as yet no consensus on how and when carbon transforms from solid to fluid …
In the Italian Alps, the Ivrea-Verbano Zone (IVZ) is known as one of the best preserved, i.e., not re-equilibrated during Alpine metamorphism, Variscan Units and, from SW to NE, it extends from Ivrea to Locarno. The sub-units constituting the IVZ are the Kinzigite Formation (supracrustal metapelites intercalated with marbles and metamafic rocks), the Peridotitic Massifs (Baldissero, Balmuccia and Finero) and the gabbroic Mafic Complex. The well-studied lithologies from Val Strona di Omegna and Val Sesia provide evidence of a nearly completed section from the lower crust (e.g., mafic granulites, metamafic migmatites, migmatitic metapelites) to the middle crust (e.g., amphibolites, marbles, calc-silicates and minor quartzites). In the present work, we focus on the less-studied area around Ivrea town to provide further insights into the P-T-X evolution of the IVZ.Our field work shows that the main attribute of the Ivrea outcrops is the presence of metamafic rocks intercalated with enderbitic granulites and minor high-grade metapelites (stronalites). The lithologies and their field relationships are compatible with the metamafic septa intercalated with migmatitic meta-sediments of pelitic to psammitic composition and calc-silicates of the Kinzigite Formation described in the Val Strona di Omegna and Val Sesia areas.The metabasites (orthopyroxene + clinopyroxene + plagioclase + amphibole + spinel + magnetite + ilmenite) are two-pyroxene granulites devoid of garnet. They are characterized by the widespread presence of brown amphibole, whose volume may locally exceed 20% of the rock (point-counting data). The enderbitic granulites consist of orthopyroxene + plagioclase + ilmenite + magnetite, relict calcic plagioclase + K-feldspar ± quartz, and minor retrograde amphibole and biotite. The stronalites are metapelites consisting of garnet + plagioclase + sillimanite + quartz + rutile + relict biotite. Despite the very simple mineralogy, more than one generations of the same mineral assemblage have been identified in the studied rocks by both textural relationships and mineral chemistry. These data suggest a complex metamorphic evolution of the studied area.Preliminary P-T estimates (winTWQ software) have been obtained for each mineral assemblage of the two-pyroxene granulites. The results suggest a prograde-to-peak evolution under amphibolite- to granulite-facies conditions. Pressure is not higher than 5,5 – 6,5 kbar, in agreement with the absence of garnet. The temperature varies depending on the considered mineral assemblage.Our data suggest that the Ivrea area belongs to the Kinzigite Formation and corresponds to a lower crust at the transition with a middle crust. The peculiar presence of the enderbitic granulites suggests a more complex evolution of this area with respect to the Val Strona di Omegna and Val Sesia areas.
Magmatic CO2 emissions can affect the atmosphere composition, thereby driving long term global climate changes. Early Cenozoic climate trends are generally associated with changes in global silicate weathering related to the India-Asia convergence and collision, whereas changes in degassing from Neo-Tethyan magmatic arcs and their likely climatic effects are largely dismissed. Here, we characterize the petrography and measure the volatile content (e.g. CO2, H2O, F, Cl and S) of glassy, bubble-bearing and reheated melt inclusions within quartz, feldspar and pyroxene crystals from Early Cenozoic basalts, andesites and rhyolites from Ladakh (India), Tibet and Iran. Integrating our unprecedented measurements with modeling of the Neo-Tethyan geodynamics, we quantitatively assess the history of magmatic emissions from the Neo-Tethyan arcs and their contribution to Early Cenozoic climate changes. Assessing the Neo-Tethyan magmatic forcing of Early Cenozoic climate has major implications for our understanding of global volatile cycling on geological timescales.
<p>The Ventura Espiritu Santo Volcanic Field (VESVF) and the Sierra Chichinautzin (SCN) are two monogenetic volcanic fields originated in different tectonic environments in the central portion of Mexico (continental rift and subduction). The VESVF is located 35 km NE of the city of San Luis Potos&#237; in the south of the Basin and Range extensional province. This volcanic field was formed by the eruption of alkaline magmas of mafic composition transporting mantle xenoliths described as spinel lherzolites and pyroxenites (Luhr et al., 1989; Aranda -G&#243;mez and Luhr, 1996). The SCN is a Quaternary volcanic field located in the Trans-Mexican Volcanic Belt (TMVB) between two Quaternary arc-volcanoes (Popocatepetl and Nevado de Toluca[AR1]&#160;). Some authors believe that its origin has been related to the subduction of the Cocos plate beneath the North American plate (Marquez et al., 1999; Meriggi et al., 2008); however, the basalts present in the SCN are geochemically similar to OIBs.</p><p>New isotopic data of noble gases and CO<sub>2</sub> in fluid inclusions from the VESVF and SCN are presented in this work, since these two areas offer a great opportunity to study the local lithospheric mantle features and related processes (e.g., metasomatism, partial melting) occurring beneath Mexico. Twelve fresh xenoliths from the VESVF and two aliquots of olivine phenocrysts of andesites from SCN were selected. Based on the petrographic analysis, it was determined that the set of xenoliths exhibit same paragenesis: Ol> Opx>> Cpx> Spinel; all samples are plagioclase-free and are classified as spinel-lherzolites and harzburgites. Both the boundaries and the fractures of the crystals develop veins composed of yellowish glass and tiny crystals of carbonates. Lavas from SCVF were previously described as olivine andesites mainly aphanitic and porphyritic with few (<10%) phenocrysts of olivine and orthopyroxene (Marquez et al., 1999; Straub et al., 2011).</p><p>The mantle xenoliths and the olivine phenocrysts have comparable Rc/Ra values (where Rc/Ra is the <sup>3</sup>He/<sup>4</sup>He corrected for air contamination and normalized to air He). We find Rc/Ra compositions of 6.9-7.7 and 7.2-7.3, respectively, which are within the MORB-like upper-mantle range (Graham, 2002). The highest CO<sub>2</sub> concentrations are observed in olivine phenocrysts from SCN (9.2&#183;10<sup>-7</sup> mol/g and 1.3&#183;10<sup>-6</sup> mol/g), while the xenoliths cover a wide range of concentrations with values as high as 3.9&#183;10<sup>-7</sup> mol/g in Cpx. The isotopic composition of CO<sub>2</sub> (d<sup>13</sup>C vs PDB) in the olivine phenocrysts is around -6.2&#8240; with CO<sub>2</sub>/<sup>3</sup>He ratios of 3.3&#183;10<sup>9</sup>, which are comparable to MORB-like range (-8&#8240;<d<sup>13</sup>C<-4&#8240;); the mantle xenoliths in contrast, although displaying similar CO<sub>2</sub>/<sup>3</sup>He ratios (2.8&#183;10<sup>9</sup>), exhibit more positive d<sup>13</sup>C signature between -1.0 and -2.7%. We propose that these differences testify for isotopic heterogeneity in the mantle beneath the two areas, with and reflect mantle metasomatism underneath VESVF driven by interaction with carbonate rich-melts (likely consequence of carbonate recycling during the subduction process), as also evidenced by the petrographic analysis.</p>
