Experimental constraints on the crystallization of natrocarbonatitic lava flows

Abstract

Natrocarbonatitic magmas are characterized by their extremely low viscosities and fast elemental diffusion, and as a consequence of this, their chemistry and crystallinity can change significantly during residence in shallow reservoirs or even due to cooling during lava flow emplacement. Here, we present the results of a series of crystallization experiments conducted at 1-atm confining pressure and in a temperature range between 630°C and 300°C. The experiments were set up to characterize the chemistry and growth processes of the phenocryst phases present in natrocarbonatites. The results are applicable to (1) processes occurring during residence in shallow magma reservoirs and/or (2) during lava flow emplacement. We show that during crystallization of natrocarbonatites at atmospheric pressure, gregoryite is the first mineral to crystallize at 630°C, followed by nyerereite at 595°C. Crystal size distributions of the gregoryites show that the crystals grow rapidly by textural coarsening (i.e., Ostwald ripening). As the crystallization is a continuous process at this pressure, the composition of the residual melt changes in response to the crystallization. However, the experiments also show that individual crystals completely reequilibrate with the changes in melt composition in as little time as <11 min. We therefore conclude that crystallization and diffusion are extremely fast processes in the natrocarbonatitic system and that the measured chemical variations in phenocrysts from Oldoinyo Lengai can be explained by different cooling histories. Finally, we model the rheological control on the emplacement of highly crystallized natrocarbonatitic lavas at Oldoinyo Lengai.

Mineralogical and geochemical characterization of ashes from an early phase of the explosive September 2007 eruption of Oldoinyo Lengai (Tanzania)

“…If a natrocarbonatite magma is allowed sufficiently long residence time in these reservoirs the rapid elemental diffusion will overprint the original compositions inherited from deeper levels of the plumbing system (cf. Mattsson and Caricchi, 2009). Hence it is possible that the two distinct varieties of nyerereite and gregoryite (i.e.…”

“…One possibility may be that they are derived from a deeper-seated natrocarbonatitic reservoir. Mattsson and Caricchi (2009) showed by crystallization experiments at atmospheric pressure that gregoryite crystals in natrocarbonatites will equilibrate rapidly with the melt fraction in response to changes in the chemistry (and the surrounding temperature conditions). Prior to the explosive episode, the OL summit crater held an intricate system of small interconnected magma reservoirs in which a considerable amount of magma could be stored (Kervyn et al, 2008).…”

“…Decomposition of recently emplaced (fresh) natrocarbonatitic lavas could potentially contain both main phenocryst phases (i.e., gregoryite and nyerereite). However, because nyerereite melts at lower temperatures compared to gregoryite (595°C and 630°C, respectively, at atmospheric pressures; Mattsson and Caricchi, 2009), gregoryite should be preferentially preserved. This is not the case in the ashes from OL.…”

The volatile content of hypabyssal kimberlite magmas: some constraints from experiments on natural rock compositions

“…Although the melt fraction could be as low as 40% if the residual melt has a low viscosity approaching that of carbonatite (e.g. Mattsson and Caricchi 2009), it will be higher for silicate melts and in this context any xenocrystic material must be considered as part of the crystal load (Kopylova et al 2007). Extrapolation of the melt versus crystal fraction trends (see Fig.…”

Magnesium isotope fractionation during carbonatite magmatism at Oldoinyo Lengai, Tanzania

“…Stratigraphic relationships and Sr-Nd-Pb isotopic data suggest that the combeite-and wollastonite-bearing nephelinites (CWNs) from the unit Lengai II B are petrogenetically related to natrocarbonatites and may represent differentiation products of the conjugate silicate melts from which the original carbonatite melts exsolved (Kjarsgaard et al, 1995;Bell and Simonetti, 1996;Dawson, 1998;Klaudius and Keller, 2006). By contrast, phonolites from the unit Lengai I, CWNs from the unit Lengai II A and carbCWMNs of the 2007-2008 activity are assumed to represent silicate melts that did not experience liquid immiscibility (Klaudius and Keller, 2006;Keller et al, 2010 (Peterson, 1990;Kjarsgaard et al, 1995;Gittins and Jago, 1998;Mattsson and Caricchi, 2009).…”