Experimental constraints on amphibole stability in primitive alkaline and calc-alkaline magmas

Equilibrium crystallization experiments were carried out on two primitive basaltic rocks (APR16: Na2O+K2O=4.40 wt%; CM42: Na2O+K2O=2.59 wt%) with the aim to investigate the amphibole stability in the differentiation processes at deep crustal level, of primitive alkaline (APR16) and calc-alkaline (CM42) magmas. The experiments were performed with different initial H2O contents (0-5 wt%), at pressure of 800 MPa, in the temperature range of 975-1225 °C. For the explored conditions, amphibole crystallization occurs in both compositions at H2O in the melt >7wt% while the temperature of their occurrence is lower in the alkaline composition (<1050 °C in APR16 and ≥1050 °C in CM42). Moreover, amphibole crystallization seems to be influenced by the Na2O/K2O ratio rather than the absolute Na2O content in the melt. This is evident when experimental results on the APR16 and CM42 are compared with experimental data obtained from a primitive ultrapotassic composition (leucite-basanite: Na2O+K2O=4.58 wt%) and with thermodynamic modelling by the Rhyolite-MELTS algorithm. The comparison shows that amphibole never saturates the leucite-basanite at any of the investigated/modelled conditions, even when an extended crystallization increases the Na2O of melts up to contents like those of calc-alkaline experimental glasses. We conclude that, at pressure of 800 MPa and hydrous conditions, only primitive liquids with Na2O/K2O ratio ≥0.9 are more prone to crystallize amphibole

Clues on the origin and differentiation of primitive arc magmas: the coarse grained, high-Mg basaltic enclaves of Capo Marargiu (Sardinia, Italy)

Recent models depict thermally mature magmatic systems as transcrustal crystal mushes where, from the base to the roof, primary magmas evolve towards silicic end-members through crystal fractionation. In these cumulitic piles, igneous distillation is variably overprinted by open system dynamics, with the result that plutonic and volcanic rocks show puzzling textures and hybrid bulk compositions. The first evolutionary stage of primary arc magmas occurs at the mantle-crust transition, where high pressure conditions may prevent water loss, thus promoting the fractionation of pyroxenitic to hornblenditic cumulates. Intriguingly, the average composition of continental crust is too felsic to be in equilibrium with the mantle, implying that (i) preserved mafic cumulates are too scarce than predicted according to mass balance principles, and (ii) high pressure cumulitic rocks and inclusions eventually exposed at Earth surface provide precious insights on the lower crustal evolution of arc basalts and picrites. The goal of this PhD project is to give a contribution to the scientific debate about arc magma evolution by investigating the basaltic to andesitic rocks from Capo Marargiu Volcanic District (i.e., CMVD). This is an Oligo-Miocene calc-alkaline complex located in north-western Sardinia (Italy) and characterised by the widespread occurrence of basaltic andesitic domes and dikes. One of these domes hosts abundant crystal-rich enclaves containing ~50% of millimetre- to centimetre-sized clinopyroxene and amphibole crystals with intriguing textures. Based on textural, geochemical, and experimental data, it is demonstrated that the CMVD plumbing system is a complex polybaric structure developed from the base of Sardinian crust to hypabyssal depths, where hydrous high-Mg basalts produce basaltic andesitic magmas by fractional crystallisation of high-pressure assemblages and assimilation of the Hercynian basement. Buoyant basaltic andesites undergo a second stage of differentiation in the shallow crust, leading the more evolved andesitic melts intruded as dikes. Enclave mineral texture and chemistry constrain the hybrid nature of these rocks due to the entrainment of lower crustal clinopyroxene + amphibole cumulates in a basaltic andesitic magma ascending towards shallow depths

Petrological constrains on the coarse-grained, high-Mg basaltic enclaves of Capo Marargiu (Sardinia, Italy)

We present results from a textural and geochemical study conducted on calc-alkaline volcanic and hypoabyssal rocks from the Oligo-Miocene Capo Marargiu Volcanic District (CMVD; Sardinia, Italy). Stratigraphic units of CMVD consist of lava domes, a pyroclastic breccia interbedded with lava flows, and dikes. The pyroclastic breccia is in lateral contact with a low crystallinity (∼15 vol.% phenocrysts), massive hypoabyssal body hosting decimetre-sized, coarse-grained enclaves with porphyritic textures (∼50 vol.% phenocrysts). These crystal-rich enclaves and the host rock exhibit phase assemblages of clinopyroxene + plagioclase + amphibole + olivine + magnetite + low-Ca pyroxene, and plagioclase + clinopyroxene + magnetite + low-Ca pyroxene, respectively. Clinopyroxene phenocrysts (≤5 mm in size) in the crystal-rich enclaves show two compositionally distinct populations: type 1 clinopyroxenes are diopsides (Mg#85-90), whereas type 2 clinopyroxenes are augites (Mg#74-82). Plagioclase phenocrysts (≤1 mm in size) from crystal-rich enclaves and the host rock are normally zoned with An75-96 cores grading to An50-75 rims. The composition of amphibole phenocrysts (≤20 mm in size) is Mg-hastingsite. The mineral texture is poikilitic suggesting early crystallization of amphibole with respect to plagioclase. In fact, the primitive (∼Mg#76), high-T amphiboles include clinopyroxene, whereas the more evolved (∼Mg#65), low-T phenocrysts host plagioclase. Amphiboles are also surrounded by characteristic reaction coronas, consisting of tiny microlites (&lt;5 μm in size) of clinopyroxene, low-Ca pyroxene, plagioclase, magnetite and ilmenite. Olivine occurs as phenocrysts (≤1 mm in size) with Fo79-87 cores surrounded by Fo66-79 rims. The bulk-rocks of crystal-rich enclaves are high-Mg basalts (i.e., 10 wt.% MgO), whereas the host rock is a more differentiated basaltic andesite (i.e., 5 wt.% MgO). Major oxides and compatible trace element modelling suggest that the basaltic andesitic magma originates by crystal fractionation of olivine + clinopyroxene from a high-Mg basalt [1]. In turn, compatible trace elements in the high-Mg basalt are low (330 ppm Cr, 130 ppm Ni) relative to picritic arc magmas, as the result of crystal fractionation of olivine ± Cr-spinel from a primary magma at depth [1; 2]. Thermobarometric calculations on phenocrysts from the crystal-rich enclaves in equilibrium with the high-Mg basalt yield pressures (600-400 MPa) and temperatures (1200-950 °C) consistent with phase diagrams derived by experiments conducted on primitive arc liquids [3]. Nevertheless, (i) the breakdown of the opacitic amphibole rim [4], (ii) the late appearance of plagioclase, and (iii) the correspondence between ∼Mg#65 natural amphiboles and mineral compositions experimentally-derived at 200 MPa [5], indicate that the crystal-rich enclaves experienced a decompression path started at higher crustal depths. In this view, the high-Mg basalt and the basaltic andesite represent two different regions of a chemically zoned magma chamber formed by crystal fractionation of a primary magma ponding at ∼500 MPa. Subsequently, buoyancy forces associated with density gradients caused upward migration of the basaltic andesite carrying portions of the adjacent high-Mg basalt to shallower crustal levels. References: [1] Yamamoto M (1988) Contrib Mineral Petrol 99:352-259 [2] Eggins SM (1993) Contrib Mineral Petrol 114:79-100 [3] Foden JD and Green DH (1992) Contrib Mineral Petrol 109:479-493 [4] Reagan MK et al. (1983) Bull Volcanol 49:415-434 [5] Sisson TW and Grove TL (1993) Contrib Mineral Petrol 113:143-16

Amphibole growth from a primitive alkaline basalt at 0.8 GPa: Time-dependent compositional evolution, growth rate and competition with clinopyroxene

Amphibole growth rates were experimentally determined at hydrous (3.3-4.2 wt% H2O), isobaric (0.8 GPa) conditions, variable temperature (1030 and 1080 °C) and dwell time (0.25, 3, 6, and 9 h), using as starting material a primitive alkaline basalt from Procida island (Campi Flegrei Volcanic District, south Italy). Amphibole growth rates decrease from 1.5·10-7 to 2.9·10-8 cm·s-1 as the duration of our experiments increase from 0.25 to 9 hours. Moreover, increasing both temperature and water content leads to similar growth rate increase at constant dwell time. The comparison between amphibole and clinopyroxene growth rates determined at the same experimental conditions reveals for amphibole a faster growth relatively to the coexisting clinopyroxene, regardless of the dwell time. Furthermore, the experimental time appears to be a critical parameter for the composition of synthetic amphiboles; specifically, edenite is the dominant composition in short experiments (≤3 h), particularly, at low temperature (1030 °C), whereas the magnesiohastingsitic amphibole becomes progressively more important shifting towards longer duration and higher temperature run conditions. The magnesiohastingsite, on the basis of the amphibole-liquid Fe-Mg exchange coefficient values, results to be the compositional term representative of amphibole-melt equilibrium at the investigated P-T-H2O conditions. Finally, experimental growth rates from this study have been used to investigate the crystallization time of natural amphiboles and clinopyroxenes from the Oligo-Miocene cumulates of north-western Sardinia (i.e. Capo Marargiu Volcanic District, Italy), yielding crystallization times of 1.46-3.12 yr

Experimental time constraints on the kinetic and chemistry of amphibole at deep crustal levels

Aiming to improve the current knowledge about amphibole growth kinetics at deep crustal levels, new amphibole growth rate data are provided. Our findings, indeed, may be useful to correctly interpret the textural features of amphibole-bearing mafic cumulates and rocks, and for a better constraining of the timescales of magmatic processes at upper mantle-lower crustal depths. Experiments were performed to determine the amphibole growth rates in a primitive alkaline basalt from Procida island (Campi Flegrei Volcanic District, southern Italy) at the following conditions: temperature of 1030 and 1080 °C, pressure of 0.8 GPa, water content in the range 3.3-4.2 wt%, and variable dwell time from 0.25 to 9 h. Amphibole growth rates range from 1.5·10-7 to 2.9·10-8 cm·s-1 with increasing the duration of the experiments. It is observed that, keeping a constant dwell time, an increase of the experimental temperature or of the water content results in comparable growth rate increase. Coexisting synthetic amphibole and clinopyroxene show a faster growth rates in favour of amphibole regardless of the dwell time, since the chemical and structural similarities of these minerals cause kinetic competition. Moreover, the chemical composition of amphibole is influenced mainly by the experimental time; in detail, in the shortest (≤3 h) and low temperature runs edenite is the prevailing composition whereas the magnesiohastingsitic term becomes dominant at higher temperature and longer run duration. Based on the interpretation of the Fe-Mg exchange coefficient values between amphibole and coexisting liquid, the magnesiohastingsitic amphibole is considered to be the stable term at the investigated run conditions. Finally, the resulting growth rates have been applied to constrain the crystallization time of natural amphiboles and clinopyroxenes from the Oligo-Miocene cumulates of north-western Sardinia (i.e., Capo Marargiu Volcanic District, Italy), yielding crystallization times in the range 1.46-3.12 yr

Petrological constraints on the high-Mg basalts from Capo Marargiu (Sardinia, Italy). Evidence of cryptic amphibole fractionation in polybaric environments

This study deals with the textural and compositional characteristics of the calc-alkaline stratigraphic sequence fromCapoMarargiu Volcanic District (CMVD; Sardinia island, Italy). The area is dominated by basaltic to intermediate hypabyssal (dikes and sills) and volcanic rocks (lava flows and pyroclastic deposits) emplaced during the Oligo-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 amphibole crystals. This mineral assemblage is in equilibrium with a high-Mg basalt recognised as the parental magma of the entire stratigraphic succession at CMVD. Analogously, centimetre-sized clots of medium- and coarsegrained amphibole+plagioclase crystals are entrapped in andesitic dikes that ultimately intrude the stratigraphic sequence. Amphibole-plagioclase cosaturation occurs at equilibrium with a differentiated basaltic andesite. Major and trace element modelling indicates that the evolutionary path of magma is controlled by a two-step process driven by early olivine + clinopyroxene and late amphibole + plagioclase fractionation. In this context, enclaves represent parts of a cumulate horizon segregated at the early stage of differentiation of the precursory high-Mg basalt. 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) reaction minerals found in amphibole coronas formed at the interfacewith more differentiated melts infiltratingwithin the cumulate horizon, and carrying the crystal-rich material with themupon eruption. Coherently, the mineral chemistry and phase relations of enclaves indicate crystallisation in a high-temperature, high-pressure environment under water-rich conditions. On the other hand, the upward migration and subsequent fractionation of the residual basaltic andesite in a shallower, colder, and hydrous region of the CMVD plumbing system lead to the formation of the amphibole-plagioclase crystal clots finally entrained by the andesitic dikes. Indeed, phenocrysts from thesemore evolved products record the final crystallisation path ofmagma during ascent towards the surface. Magma decompression and volatile loss cause the formation of amphibole reaction coronas and the crystallisation of a more sodic plagioclase in equilibrium with basaltic andesitic to andesitic melts. The bulk-rock geochemical signature of these products testifies to open-system, polybaric magma dynamics, accounting for variable degrees of crustal assimilation of the Hercynian basement of Sardinia

Snapshots of primitive arc magma evolution recorded by clinopyroxene textural and compositional variations. The case of hybrid crystal-rich enclaves from Capo Marargiu Volcanic District (Sardinia, Italy)

Capo Marargiu Volcanic District (CMVD) is an Oligo-Miocene calc-alkaline complex located in northwestern 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 that 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