Super-reducing conditions in ancient and modern volcanic systems: sources and behaviour of carbon-rich fluids in the lithospheric mantle

Carmeltazite, ZrAl2Ti4O11, a New Mineral Trapped in Corundum from Volcanic Rocks of Mt Carmel, Northern Israel

“…The mineralogical assemblage at Mt Carmel shows several analogies with those observed in calcium-aluminum inclusions (CAIs) in carbonaceous chondrites (CCs). Besides the recently described tistarite [2], hibonite [11] and krotite [1], the new mineral described here resembles the Zr-bearing phases found in CC, e.g., panguite [13], kangite [14] and allendeite [15]. Furthermore, although the inferred conditions of the Mt Carmel assemblages are similar to those of the CAIs in terms of temperature and f O 2 , crystallization appears to have formed at higher pressures, ca.…”

“…The corundum aggregates in which the carmeltazite occurs appear to have formed near the crust-mantle boundary (ca. 30 km depth [11]), in the presence of excess volatiles. The abundance of carbon in the system (SiC, TiC and amorphous C as common phases) and the low f O 2 required by the observed assemblages (∆IW −6 to −10 [12]) suggests that the volatiles were dominated by mantle-derived CH 4 + H 2 , which reduced a volume of mafic to ultramafic melt.…”

Dellagiustaite: A Novel Natural Spinel Containing V2+

“…The rocks in Israel contain super-reduced mineral assemblages, which crystallized from high-T melts trapped in corundum aggregates (micro-xenoliths) ejected from Cretaceous volcanoes of picritic tholeiite composition [9,10]. In these rocks, highly reduced minerals have been described, such as moissanite (up to 4 mm long; larger than those reported in [33]), tistarite, khamrabaevite, and gupeiite along with native vanadium, and have been interpreted as the result of interaction between deep-seated magmas and CH 4 ± H 2 fluids in volcanic plumbing systems, producing local ultra-reducing conditions [9][10][11][12]. The Argentinian rocks containing dellagiustaite are very similar to type-DF pockets in Mt Carmel, although, in the dellagiustaite-bearing rocks, the vanadium alloy is included in both hibonite and grossite and has tubular-cylindrical shapes.…”

“…Recently, dellagiustaite has also been found in late-stage pyroclastic ejecta of small Cretaceous basaltic volcanoes exposed on Mt Carmel (Israel); aggregates of hopper-formed corundum crystals (Carmel Sapphire TM ) are common in the tuffs of these volcanoes and in associated alluvial deposits, mainly in the Kishon River [9][10][11][12]. Later stage ejecta show the crystallization sequence: corundum + Liq → (low-REE) hibonite → grossite + spinel ± krotite → Ca 2 Al 3 O 6 F + fluorite.…”

Unraveling the Effects of Melt–Mantle Interactions on the Gold Fertility of Magmas

“…In contrast, relatively little attention has been paid to melt-rock interaction processes occurring in the subcontinental lithospheric mantle (SCLM) that might lead to changes in the oxidation state and composition of ascending magmas after partial melting in the asthenosphere and before they reach crustal levels. For example, Chin et al (2014) and Griffin et al (2018) explored the effects of the SCLM on the composition of primary asthenosphere-derived melts during their ascent and concluded that metasomatism might be an important process that modifies the composition and f O 2 of ascending melts and fluids. More recently, Tollan and Hermann (2019) also found that arc magmas oxidize during ascent and reaction with surrounding peridotite before reaching crustal levels.…”