Trace- and rare-earth-element geochemistry of the June 1993 natrocarbonatite lavas, Oldoinyo Lengai (Tanzania): Implications for the origin of carbonatite magmas

Silicate–natrocarbonatite liquid immiscibility in 1917 eruption combeite–wollastonite nephelinite, Oldoinyo Lengai Volcano, Tanzania: Melt inclusion study

“…A liquid immiscibility relationship has been suggested between peralkaline silicic rocks and natrocarbonatite at Lengai based on volcanological, geochemical and isotopic data (Bell and Dawson, 1995;Church and Jones, 1995;Dawson, 1989Dawson, , 1998Dawson et al, 1996;Halama et al, 2007;Keller and Spettel, 1995;Keller et al, 2010;Mitchell, 2009;Peterson, 1989aPeterson, , b, 1990Simonetti et al, 1997;Williams et al, 1986). Numerous experiments simulating the Lengai natrocarbonatite and related silicate compositions also support immiscibility (Brooker and Kjarsgaard, 2011;Cooper et al, 1975;Hamilton et al, 1979;Jago, 1991;Jago and Gittins, 1991;Kjarsgaard and Peterson, 1991;Kjarsgaard et al, 1995;Wyllie, 1994, 1998;Kjarsgaard, 2008, 2011;Veksler et al, 2012).…”

“…This spectrum indicates high abundance of most trace elements in the globule with respect to the host nepheline and also shows approximately equal contents of Rb and Cs in both globule and host mineral. The content of Rb in CWN nepheline is 160 ppm (SIMS, Table 6) and in fresh natrocarbonatites is 130-285 ppm (Simonetti et al, 1997;Zaitsev et al, 2008Zaitsev et al, , 2009). …”

Partitioning of elements between silicate melt and immiscible fluoride, chloride, carbonate, phosphate and sulfate melts, with implications to the origin of natrocarbonatite

“…8 are averages of all experiments using starting mixture OL (six centrifuge and two rapid-quench experiments in total). Bulk rock element ratios were calculated from average major and trace element concentrations taken from published analyses of natrocarbonatite (Church and Jones, 1995;Dawson et al, 1995;Keller and Spettel, 1995;Simonetti et al, 1997) and wollastonite-combeite nephelinite (Donaldson et al, 1987;Peterson, 1989;Keller and Spettel, 1995;Klaudius and Keller, 2006). The error bars represent standard deviations of the averages and reflect the overall variability of the D values in the experiments and of the concentration data in the rock samples.…”

Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania

“…However, the preservation and identification of alkali carbonates in older deposits is rare and difficult Zaitsev et al 2008a). On two occasions, June 1993 (Dawson et al 1994;Church and Jones 1995;Simonetti et al 1997) and March 2006(Kervyn et al 2008a, the erupted NaC lava had a higher SiO 2 content (∼3-6 wt.%) than the usual NaC composition (<1 wt.%). This silicate component was due to centimeter-sized spherical nephelinite blebs and disintegrated crystals floating in an otherwise "normal" nyerereite-gregoryite-phenocrystic natrocarbonatite.…”

“…The solvus of the two-liquid field narrows considerably with further decreasing pressure and depending on the exact composition in the system (e.g., Koster van Groos and Wyllie 1968; Macdonald et al 1993;Mitchell 2009). Also plotted are the averages for natrocarbonatite and Lengai II CWN and the latest nephelinite composition (CO 2 -poor CWN) from April 2008 Simonetti et al 1997;Veksler et al 1998), and in melt inclusions (Mitchell 2009), a plumbing system with a peralkaline nephelinite as the major reservoir can be envisaged. Fractional crystallization of this nephelinite involving dominantly pyroxenes, garnet, and Timagnetite Peterson and Kjarsgaard 1995;Dawson 1998) with enrichment of volatiles, in particular CO 2 , and alkalis leads to compositions ready for liquid immiscible separation.…”