Petrography and mineral chemistry of mantle xenoliths in a carbonate-rich melilititic tuff from Mt. Vulture volcano, southern Italy

Abstract

We present petrographic and mineralogical data for 21 mantle xenoliths (12 lherzolites, 8 wehrlites and 1 composite) selected from a suite of more than 70 samples collected from the Monticchio Formation, Mt. Vulture volcano, southern Italy. The xenoliths are rounded, coarse- to porphyroclastic-textured, and very fresh, with the following equilibrated mineral assemblages; olivine (Fo90–92), orthopyroxene (∼En89, Wo2.0), clinopyroxene (Mg90–92, 3–6% Al2O3, 1–1.5% Cr2O3), and chrome-spinel (14–20% MgO, ∼30–40% Cr2O3). Many xenoliths contain partial melt glasses and accessory sulphide (pentlandite) Some contain primary mica (phlogopite with ∼4% FeO, 1.8% Cr2O3, 1.4–2.8% TiO2) with slightly zoned rims (Fe-, Ti-, Al-enriched). One contains relics of garnet (pyrope; Mg84). Secondary veins in several xenoliths contain carbonate with significant Sr levels (∼0.5–1.0% SrO), occasional apatite and scarce melanite, all typical of carbonatites and presumably related to the host magma (melilitite/carbonatite). Although amphibole is a common megacryst in the same volcanic units, no primary amphibole was found in the xenoliths themselves. Calculated pressures and temperatures using a range of geothermometers/barometers give values of 14–22 kbar and 1050–1150°C. In particular, the En-Sp and Di-Sp thermo/barometers (Mercier, 1980) show a good positive correlation between P and T. The Monticchio xenoliths lie on the high-T side of an ‘oceanic’ geotherm. The xenolith geotherm is hotter than general heat flow values in this region at the current day (50 mWm−2) but it compares well with the high-pressure end of a typical alkaline continental rift.

Nyerereite from carbonatite rocks at Vulture volcano: implications for mantle metasomatism and petrogenesis of alkali carbonate melts Research Article

“…Extrusive carbonatites are well preserved only in the Monticchio Formation, while intrusive carbonatite ejecta have been found in the Vulture-San Michele stratovolcano and seem to be absent in the early phonolite ignimbrite [10,20,21].…”

“…This accretion and lapilli growth likely occurred during rapid rotation in the mixed phase magma (gas-liquid-solid) while it was ascending in the volcanic conduit [26]. The Vulture lapilli must have formed at various levels in the lithosphere because their cores are mafic-ultramafic plutonic rocks (uncompahgrite, sövite, ijolite) or mantle lherzolite and wehrlite [10]. In general, the larger concentricallyshelled lapilli (>2 cm) have inner layers made of melilitite and outer shells made of agglutinated smaller porphyritic carbonatite lapilli (Figure 2).…”

“…Mantle xenoliths, mostly spinel lherzolites and wehrlite-veined lherzolite, are common in the extrusive carbonatitic rocks (Figure 2). Some xenoliths carry Mg-garnet relics and/or carbonates and are important because they indicate an upper mantle origin for the late Vulture parental magmas [10].…”

Silicate-carbonate-salt liquid immiscibility and origin of the sodalite-haüyne rocks: study of melt inclusions in olivine foidite from Vulture volcano, S. Italy

“…Mantle xenoliths, comprising of mostly spinel lherzolite and wehrlite-veined spinel lherzolites are frequently found [28,29]. Some of the peridotite nodules carry Mg-garnet relicts and/or carbonates and are important because they indicate the upper mantle origin for the Vulture parental magma [30].…”

“…The operating conditions were as follows: primary O 2− beam of 20 μm, a beam current of 2 -4 nA, energy offset of 100 eV and energy slit of 50 eV. Concentrations of elements were determined from the ratios of their isotopes to 30 Si, using calibration curves for standard samples [32]. Low background contents of H 2 O (< 0.03 wt.%) were due to the 24 h high-vacuum exposure of the samples in the mass spectrometer.…”