Tecchiato, Vanni

This study deals with the textural and compositional characteristics of the calc-alkaline stratigraphic sequencefrom Capo MarargiuVolcanicDistrict (CMVD;Sardiniaisland,Italy). The areaisdominatedbybasalticto interme-diate hypabyssal (dikes and sills) and volcanic rocks (lavaflows and pyroclastic deposits) emplaced during theOligo-Miocene orogenic magmatism of Sardinia. Interestingly, a basaltic andesitic dome hosts dark-grey,crystal-rich enclaves containing up ~50% of millimetre- to centimetre-sized clinopyroxene and amphibolecrystals. This mineral assemblage is in equilibrium with a high-Mg basalt recognised as the parental magma ofthe entire stratigraphic succession at CMVD. Analogously, centimetre-sized clots of medium- and coarse-grained amphibole + plagioclase crystals are entrapped in andesitic dikes that ultimately intrude the stratigraphicsequence. Amphibole-plagioclase cosaturation occurs at equilibrium with a differentiated basaltic andesite. Majorand trace element modelling indicates that the evolutionary path of magma is controlled by a two-step processdriven by early olivine + clinopyroxene and late amphibole + plagioclase fractionation. In this context, enclavesrepresent parts of a cumulate horizon segregated at the early stage of differentiation of the precursory high-Mg ba-salt. This is denoted by i) resorption effects and sharp transitions between Mg-rich and Mg-poor clinopyroxenes,indicative of pervasive dissolution phenomena followed by crystal re-equilibration and overgrowth, and ii) reac-tion minerals found in amphibole coronas formed at the interface with more differentiated melts infiltrating with-in the cumulate horizon, and carrying the crystal-rich material with them upon eruption.....

Capo Marargiu Volcanic District (CMVD) is an Oligo-Miocene calc-alkaline complex located in north-western Sardinia (Italy) and characterized by the widespread occurrence of basaltic to andesitic domes. One of these domes hosts abundant crystal-rich enclaves with millimeter-to-centimeter-sized clinopyroxenes showing intriguing textural features as a result of complex magma dynamics. To better understand the mechanisms governing the early evolution of the CMVD magmatic system, such clinopyroxene phenocrysts have been investigated in terms of their major, trace element and isotopic compositions. Three distinct clinopyroxene populations have been identified, i.e., Type 1, Type 2, and Type 3. Type 1 appears as the sub-rounded cores of diopsidic clinopyroxenes with overgrowth textures corresponding to Type 2 and Type 3. These latter populations may also occur as single isolated crystals. Type 2 diopsidic pyroxene exhibits oscillatory zoning and spongy cellular textures with Type 3 overgrowths, whereas Type 3 are polycrystalline augitic glomerocrysts with occasional Type 2 overgrowths. The crystal overgrowths are striking evidence of magma recharge dynamics. Type 1 (cpxMg#83-92), Type 2 (cpxMg#75-82) and Type 3 (cpxMg#72-79) are, respectively, in equilibrium with Sardinian mantle-derived high-Mg basalts (HMB with meltMg#56-73), least differentiated basaltic andesites (BA with meltMg#45-56) and evolved basaltic andesites (EBA with meltMg#41-50). Type 1 and Type 2 are diopsidic phenocrysts which have evolved along a similar geochemical path (i.e., linear increase of Al, Ti, La, and Hf contents, as well as negligible Eu-anomaly) controlled by olivine + clinopyroxene + amphibole fractionation. This differentiation path is related to phenocryst crystallization from hydrous HMB and BA magmas stalling at moderate crustal pressures. The occurrence of globular sulfides within Type 1 suggests saturation of the HMB magma with a sulfide liquid under relatively low redox conditions. Moreover, Type 1 clinopyroxenes show variable 87Sr/86Sr ratios ascribable either to assimilation of crustal material by HMB magma or a mantle source variably contaminated by crustal components. In contrast, Type 3 augitic phenocrysts recorded the effect of plagioclase and titanomagnetite fractionation (i.e., low Al and Ti contents associated with high La and Hf concentrations, as well as important Eu-anomaly) from more degassed EBA magmas ponding at shallow depths. Rare titanite associated to Type 3 and titanomagnetite crystals point to high oxidizing conditions for EBA magmas. The 87Sr/86Sr ratios of both Type 2 and Type 3 are almost constant, suggesting a limited interaction of BA and EBA magmas with the country rock. The overall textural and compositional features of Type 1, Type 2 and Type 3 clinopyroxene phenocrysts lead to the conclusion that CMVD was characterized by a polybaric plumbing system where geochemically distinct magmas crystallized and mixed under variable environmental conditions

It is widely accepted that, for the correct interpretation of bulk rock compositions, degassing process controlling both loss of magmatic volatiles and significant changes in the contents of volatile chemical species must be considered. The continuous degassing experiments presented in this study attempt to determine the absolute and relative change in abundances of volatile components in the melt at shallow levels, simulating what might occur during slow and fast ascent of magma from depth without crystallization. We performed disequilibrium decompression experiments using as starting melt a bubblefree but volatile-bearing trachybasalt. The charges were isothermally decompressed at 1,150 °C from 400 MPa down to 50 MPa at rates of 0.01 MPa/s and 1 MPa/s. Results demonstrate that degassing of 1 wt% H2O of initial volatile content in the melt is not enough to induce melt compositional changes as well as H2O supersaturation in the trachybasaltic melt. In contrast, the minimum H2O threshold to observe Cl, B, and Li devolatilization corresponds to 3 wt% H2O and volatile supersaturation is attained at the fast decompression rate of 1 MPa/s. An increase of CO2 up to 0.3 wt% do not change the partitioning behaviour of these chemical species between vapor and trachybasaltic melt. Moreover, CO2 degassing is less efficient with respect to H2O transfer from the melt into the vapor phase. As a consequence, the trachybasaltic melt is preferentially supersaturated in CO2 with decreasing pressure. Disequilibrium degassing does not change the bulk oxidation state of the melt.

Rhyolite and felsite cuttings were collected at Krafla volcano during the perforation of the Iceland Deep Drilling Project Well 1 (IDDP-1). The perforation was stopped at a depth of 2100m due to intersection with a rhyolite magma that intruded the felsite host rock. Rhyolite cuttings are vitrophiric (glass ~95%, RHL) and exhibit a mineral assemblage made of plagioclase+augite+pigeonite+titanomagnetite. Felsite cuttings display evidences of partial melting, responding to variable degrees of quartz+plagioclase+alkali feldspar+augite+ titanomagnetite dissolution. The interstitial glass analyzed close to (i.e., FLS1) and far from (i.e., FLS2) the reaction surface of pyroxene from felsite cuttings shows continuous changes between the two end-members. FLS1 is compositionally similar to RHL, showing Na2O+K2O+REE depletions, counterbalanced by MgO+CaO enrichments. Conversely, FLS2 exhibits opposite chemical features. REE-exchange thermobarometric calculations reveal that plagioclase and augite cores from rhyolite and felsite formed under identical conditions, along a thermal path of 940–960 °C. However, in terms of major and trace element concentrations, plagioclase and augite crystal cores are not in equilibrium with the rhyolite magma, suggesting the incorporation of these minerals directly from the host felsite. To better understand the petrogenetic relationship between rhyolite and felsite, two sets of crystallization and partial melting experiments have been carried out at P=150 MPa and T=700–950 °C. Rhyolite crystallization experiments (RCE) reproduce the two-pyroxene assemblage of IDDP-1 rhyolite cuttings only at T≤800 °C, when the crystal content (≥19%) is higher than that observed in the natural rhyolite (~5%). Under such conditions, the RCE glass is much more differentiated (i.e., marked CaO depletion and Eu anomaly) than RHL. On the other hand, felsite partial melting (FPM) experiments show interstitial glass with a bimodal composition (i.e., FPM1 and FPM2) comparable to FLS1 (≈RHL) and to FLS2, only at T=950 °C. This effect has been quantified by fractional crystallization and batch melting modeling, denoting that FLS1 (≈RHL) and FLS2 reflect high (≥70%) and low (≤8%) degrees of felsite partialmelting, respectively. In contrast, modeling RHL by crystal fractionation requires the removal of an amount (~22%) of solid material that is inconsistent with the low crystal content of the natural IDDP-1 rhyolite. It is therefore concluded that natural rhyolite and felsite represent, respectively, the near-liquidus and sub-solidus states of a virtually identical silicic magma, either feeding aphyric to subaphiric rhyolitic eruptions, or solidifying at depth as phaneritic quartzofeldspathic rocks. Felsite lenses from the Krafla substrate may explore variable degrees of remelting and remobilization processes. The intrusion into felsite of a fresh silicic magma from depth may lead to low degrees of partial melting, whereas the persistent heat release from intense basaltic intrusive events at Krafla may be the source of high degrees of felsite partial melting and consequent rejuvenation of the previously solidified silicic magma.

This study documents the compositional variations of phenocrysts from a basaltic trachyandesitic sill emplaced in the Valle del Bove at Mt. Etna volcano (Sicily, Italy). The physicochemical conditions driving the crystallization and emplacement of the sill magma have been reconstructed by barometers, oxygen barometers, thermometers and hygrometers based on clinopyroxene, feldspar (plagioclase + K-feldspar) and titanomagnetite. Clinopyroxene is the liquidus phase, recording decompression and cooling paths decreasing from 200 to 0.1 MPa and from 1050 to 940 °C, respectively. Plagioclase and K-feldspar cosaturate the melt in a lower temperature interval of ~1000–870 °C. Cation exchanges in clinopyroxene (Mg-Fe) and feldspar (Ca-Na) indicate that magma ascent is accompanied by progressive H2O exsolution (up to ~2.2 wt. %) under more oxidizing conditions (up to ΔNNO + 0.5). Geospeedometric constraints provided by Ti–Al–Mg cation redistributions in titanomagnetite indicate that the travel time (up to 23 h) and ascent velocity of magma (up to 0.78 m/s) are consistent with those inferred for other eruptions at Mt. Etna. These kinetic effects are ascribed to a degassing-induced undercooling path caused principally by H2O loss at shallow crustal conditions. Rare earth element (REE) modeling based on the lattice strain theory supports the hypothesis that the sill magma formed from primitive basaltic compositions after clinopyroxene (≤41%) and plagioclase (≤12%) fractionation. Early formation of clinopyroxene at depth is the main controlling factor for the REE signature, whereas subsequent degassing at low pressure conditions enlarges the stability field of plagioclase causing trace element enrichments during eruption towards the surface.

A correct description and quantification of the geochemical behaviour of REE+Y (rare earth elements and Y) and HFSE (high field strength elements) is a key requirement for modeling petrological and volcanological aspects of magma dynamics. In this context, mafic alkaline magmas (MAM) are characterized by the ubiquitous stability of clinopyroxene from mantle depths to shallow crustal levels. On one hand, clinopyroxene incorporates REE+Y and HFSE at concentration levels that are much higher than those measured for olivine, plagioclase, and magnetite. On the other hand, the composition of clinopyroxene is highly sensitive to variations in pressure,temperature, and melt-water content, according to exchange-equilibria between jadeite and melt, and between jadeite/Ca-Tschermak and diopside-hedenbergite. As a consequence, the dependence of the partition coefficient on the physicochemical state of the system results in a variety of DREE+Y and DHFSE values that are sensitive to the magmatic conditions at which clinopyroxenes nucleate and grow. In order to better explore magma dynamics using clinopyroxene chemical changes, an integrated P-T-H2Olattice strain model specific to MAM compositions has been developed. The model combines a set of refined clinopyroxene-based barometric, thermometric and hygrometric equations with thermodynamically-derived expressions for the lattice strain parameters, i.e., the strain-free partition coefficient (D0), the site radius (r0), andthe effective elastic modulus (E). Through this approach, it is found that the incorporation of REE+Y and HFSE into M2 and M1 octahedral sites of clinopyroxene is determined by a variety of physicochemical variables that may or may not change simultaneously during magma differentiation. The applicability of the P-T-H2O-lattice strain model to natural environments has been verified using clinopyroxene-melt pairs from a great number of volcanic eruptions at Mt. Etna volcano (Sicily, Italy). DREE+Y and DHFSE values recovered by the model have been used as input data to quantify fractional crystallization processes in natural MAM compositions. Results from calculation illustrate that the concentration of REE+Y and HFSE in the magma is primary controlled by the geochemical evolution of clinopyroxene in terms of major cation exchange-equilibria and trace cation lattice strain properties.