von Quadt, Albrecht

On December 29, 2013, an isolated vulcanian-type eruption occurred at Chaparrastique volcano (El Salvador) after 12 years of inactivity. The eruption was classified as VEI 2 and produced an ash plume with a maximum height of ~9 km. Textural and compositional data fromphenocrysts fromthe erupted products have been inte- grated with geochemical and isotopic information frombulk rocks to elucidate the magmatic processes respon- sible for the reawakening of volcanic activity. Phenocrysts consist of Fo-rich poikilitic olivines hosting high-Mg titanomagnetites, and Fo-poor olivines coexisting with low-Mg titanomagnetites.Mineral-melt equilibria suggest an origin for the distinct phenocryst populations by mixing between a high-T (~1130–1150 °C), basaltic magma with fO2 (NNO buffer) typical of the lower crust inarc systems and a low-T (~1060–1080 °C), basaltic andesiticmagma with fO2 (NNO+1 buffer) commonly encountered in shallower, more oxidized crustal reservoirs. Thermobarometry based on Fe-Mg ex- change between orthopyroxene and clinopyroxene constrains the crystallization before eruption at relative low-P (~150–250MPa) and low-T (~1000–1050 °C).Mixing between twochemically distinctmagmas is also ev- idenced by the occurrence of reverse zoned plagioclase phenocrysts with resorbed sodic cores and re-growth of sieve-textured calcicmantles. Conversely, plagioclase rims exhibit disequilibriumcompositions addressed to de- compression kinetics (~10−3 MPa/s) driven by rapidmagma ascent to the surface (~0.03 m/s). Major and trace element modelling excludes fractional crystallization as the primarymechanismcontrolling the bulk rock variability,whereas geochemical data align along amixing trend between two end-members represen- tative of the primitive basalt and the differentiated basaltic andesite. Trace element and isotope data indicate that the primary source of magmatism is an enriched MORB-like mantle with the contribution of fluxes ofmetaso- matic fluids and/ormelts produced by the subducted slab. The roleplayed by slab-fluid inputs of carbonate origin and slab-melts fromthe hemipelagic sediments seems to beminimal. Assimilation/contamination processes of magmas by crustal rocks are also negligible. In contrast, the geochemical signature of magmas is greatly influ- enced by slab-derived aqueous fluids produced prevalently by progressive dehydration of marine sediments and altered basaltic crust

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