Magmatic evolution and textural development of the 1739 CE Pietre Cotte lava flow, Vulcano, Italy

Abstract

Textural evidence from occurrences of mingled magmas in lava flows often yields insights into chemical and thermal disequilibrium between multiple magma batches at depth. An understanding of these interactions is key as they can occur on short timescales and may act as eruption triggers, particularly important in very active volcanic settings. This paper focuses on the Pietre Cotte lava flow (Vulcano, Aeolian Islands, Italy), a short (<1 km in length), texturally-heterogeneous rhyolitic extrusion on the northern slope of the active Fossa Cone. The occurrence of (i) multiple magma compositions, (ii) distinct magmatic cumulates (as glomerocrysts) and (iii) mineral resorption textures within glomerocrysts and isolated feldspar phenocrysts in the Pietre Cotte lava flow highlight a complex pre-eruptive magmatic history, including crystal mush remobilisation. Petrographic observations and mineral, bulk rock and glass geochemistry suggest that multiple mingling events occurred during the evolution of the Pietre Cotte magmatic system, evidenced by the recognition of the following components: (1) a remobilised predominantly mafic crystal mush, evident as macrocrysts (crystals >500 µm), which form glomerocrysts within enclaves, (2) a microlitic (<100 µm) trachytic enclave groundmass with microcrysts (100-500 µm), and (3) a rhyolitic glass, which hosts both the enclaves and the glomerocrysts. The macrocrystic mafic assemblage includes clinopyroxene (En 38-47 Wo 45-50 ; Mg# 0.72-0.89), olivine (Fo 49-66) and magnetite (Usp 7-26), with plagioclase (An 40-63 Ab 5-50) and rare alkali feldspar (Or 41-57) also present. Enclaves are comprised of a groundmass of plagioclase (An 43-47) and alkali feldspar (Or 33-57) microlites, with clinopyroxene microcrysts (En 39-42 Wo 47-51 ; Mg# 0.75-0.81) and trachyte groundmass glass. The rhyolitic host is characterised by glass, spherulites, microlites and enclave-derived macrocrysts.

A New Plagioclase-Liquid Hygrometer Specific to Trachytic Systems

“…%, we inferred average crystallization temperature of 1025 ± 13 • C and 1008 ± 12 • C for the latitic and trachytic magmas, respectively. These values are in general agreement with previous estimates based on both thermobarometry and melt inclusion analyses [28][29][30][31]. For the H 2 O estimates, we used literature data from Nicotra et al [49] after testing that the average temperatures of 1025 and 1008 • C effectively minimize the difference (∆An < 0.1 mol.%) between An in plagioclase predicted by the equilibrium model of Namur et al [36] and the natural compositions of phenocrysts.…”

“…The hygrometer has been tested with both experimental and thermodynamically derived melt compositions, providing the best predictions for H 2 O concentrations at temperatures ranging from 850 to 1050 • C. The hygrometer has been also employed to trachyandesitic (latitic) and trachytic products erupted at the La Fossa cone of Vulcano Island. The obtained H 2 O estimates are consistent with the crystallization conditions inferred for the shallow plumbing system of the La Fossa volcano, by means of gas geochemistry [26,27], thermobarometry [28,29] and H 2 O measurements from melt inclusion studies [30,31].…”

“…The presence of products with different degrees of evolution within each single eruptive episode and the widespread occurrence of heterogeneous magmatic enclaves testify to the development of a compositionally and thermally zoned magma storage system [48]. Polybaric and polythermal crystallization phenomena have been also envisaged for the plumbing system of Vulcano island [29,47,49], in concert with assimilation and fractional crystallization (AFC) processes [31,32,50,51]. It is apparent from Equation (2) that the new plagioclase-liquid hygrometer is T-dependent, as for the case of previous feldspar-based hygrometers from the literature (cf.…”

“…%. The clinopyroxene-liquid thermometer was thus applied to clinopyroxene phenocryst, glass and bulk rock analyses from the literature [28,29,49], after a careful selection of mineral-liquid pairs that satisfy the criteria of chemical equilibrium. Indeed, to minimize the error of temperature estimates, only compositional data yielding ∆DiHd values of less than 0.1 were selected.…”

“…This discrepancy is reconciled by considering the melt inclusion study of Gioncada et al [31] that illustrates the decrease in pre-eruptive H 2 O content is caused by an open-system differentiation of alkaline differentiated magmas, in concert with abundant volatile exsolution. Rationally, with respect to basaltic magmas stored at high depths (P > 600 MPa; [29,49]), the crystallization of latitic and trachytic magmas takes place at shallower crustal levels (P < 100 MPa) under H 2 O saturated and/or oversaturated conditions [48]. This is also confirmed by the negative correlation between sulfur content in melt inclusions and the degree of evolution between basalt to trachyte [31], in response to preferential sulfur partitioning into the vapor phase during magma degassing at low-P conditions [54].…”