Potassic primary melts of vulsini (Roman Province): evidence from mineralogy and melt inclusions

Abstract

The origin and the relationships between the high potassic (HKS) and potassic (KS) suites of the Roman Comagmatic Province and the nature of their primary magmas have been intensively debated over the past 35 years. We have addressed these problems by a study of mineralogy (olivine Fo92-87, Cr-spinel and diopside) and melt inclusions in olivine phenocrysts from a scoria sample of Montefiascone (Vulsini area). This rock is considered as one of the most primitive (MgO = 13.5 wt%, NiO = 340 ppm; Cr = 1275 ppm) in the northern part of the Roman Comagmatic Province. The compositions of both the olivine and their melt inclusions are controlled by two main processes. In the case of the olivine Fo < 90.5, fractional crystallization (olivine + diopside + minor spinel) was the principal mechanism of the magma evolution. The olivine (Fo92 90.5) and the Cr-spinel (Cr# = 100. Cr/(Cr + A1)--63 73) represent a near-primary liquidus assemblage and indicate the mantle origin of their parental magmas. The compositions of melt inclusions in these olivine phenocrysts correspond to those of poorly fractionated H20-rich ( ~ 1 wt%) primary melts (MgO = 8.4-9.7 wt%, FeO t~ 6 7.5 wt%). They evidence a wide compositional range (in wt%: SiO2 = 46.5 50, K20 = 5.3-2.8, P205 = 0.4-0.2, S=0.26 0.12; C1=0.05 0.03, and CaO/A1203--0.8-1.15), with negative correlations between SiO2 and K20, A1203 and CaO, as well as positive correlations between K20, and P2Os, S, C1, with nearly constant

Limestone assimilation by basaltic magmas: an experimental re-assessment and application to Italian volcanoes

“…San Carlos olivine (Fo 91 ) was added in selected runs (#7 and #8) to adjust the MgO content of the starting bulk silicate composition to the most primitive melt inclusions found in volcanic products of Central-Southern Italy (ca. 9-10 wt %; Marianelli et al 1995;Kamenetsky et al 1995). To investigate the consequence of different degrees of carbonate assimilation on basalt phase relations, variable amounts of Ca-and Mg-carbonate were systematically added to the silicate component.…”

Stromboli volcano (Aeolian Archipelago, Italy): An open window on the deep‐feeding system of a steady state basaltic volcano

“…They are reported at Vulcano (Aeolian islands, Italy), Grenada (Lesser Antilles) and Nicaragua (Central America) as reviewed by Schiano et al [2000]. These CaO-rich, SiO 2 undersaturated primary arc melts would imply the involvement of clinopyroxenite during melting source process, either as clinopyroxenite cumulates under upper mantle conditions [Schiano et al, 2000] or veined mantle [Kamenetsky et al, 1995]. However, the question regarding the origin of Ca-rich melts at Stromboli needs specific geochemical studies to be addressed.…”

Upwelling of deep mantle material through a plate window: Evidence from the geochemistry of Italian basaltic volcanics

“…Basaltic melts can only exhibit an isotopic signature characteristic of crustal material if the major element budget is largely decoupled from the trace element budget. Melting of a metasomatized mantle source or a source sprinkled with continental detritus (e.g., phlogopite peridotite, see references above) is inconsistent with the high K 2 O/H 2 O ratios (>3) of primary melt inclusions analyzed at Vulsini, (RMP [Kamenetsky et al, 1995]). This ratio is on the ''dry'' side of the MORB range in contrast with subduction-zone primary magmas [Sobolev and Chaussidon, 1996].…”