Mineralogical and geochemical changes of natrocarbonatites due to fumarolic activity

at Oldoinyo Lengai volcano, Tanzania

Zaitsev A.N.*, Keller J.**, Jones G.***, Grassineau N.****

* Department of Mineralogy, St. Petersburg State University, St. Petersburg, Russia; ** Institute für Mineralogie, Universität Freiburg, Freiburg, Germany; *** Department of Mineralogy, Natural History Museum, London, UK; **** Department of Geology, Royal Holloway University of London, London, UK.


Modern Oldoinyo Lengai natrocarbonatites from the Gregory Rift in northern Tanzania. are well known for their instability under normal atmospheric conditions (Dawson et al 1987, Keller & Krafft 1990). Fresh rocks can be observed for a short time only after eruption and after a few days of exposure to the atmosphere the natrocarbonatite lavas change color to gray or brown and contain abundant subsolidus minerals such as the hydrous carbonates thermonatrite, trona and nahcolite. Extended periods of low-temperature alteration of the natrocarbonatites leads to the formation of rocks, which contain pirssonite, calcite, and rarely shortite in various proportions (Zaitsev & Keller 2006, Zaitsev et al 2008).

Surprisingly, fumarolic activity at Oldoinyo Lengai has not received much attention. Measurements of fumarolic gas temperatures vary from 49 to a maximum of 312 C (Dawson, 1962; Keller and Krafft, 1990; Koepenick et al., 1996). The composition of gas from fumaroles at Oldoinyo Lengai has been studied by Javoy et al. (1989) and Koepenick et al. (1996) who reported CO2-H2O gases containing 64-74 mol.% CO2, 24-34% H2O (48.7% and 49.2%, respectively, in Javoy et al., 1989) with traces of H2, CO, H2S, HCl, HF and CH4. Study of gases by Oppenheimer et al. (2002) and Burnard et al (2006) revealed a much higher water content of about 75-77 mol.%, however, the observed high water content may be attributed to a mixing of magmatic and meteoric water or contamination by old hydrated natrocarbonatites. Data on abundances and isotopic composition of the gases (N, He, Ne, Ar, Xe) are reported by Teague et al (2008) and Fischer et al (2006).

Subsolidus minerals in altered natrocarbonatites were studied from four active fumarolic fields: (1) in the north-western part of the crater near hornito T49, (2) in the central part of the crater, hornito T46, (3) in the northern area of the crater near the same hornito T49 and (4) in the southern area of the crater near hornito T30. The temperature of the gases from second locality was measured as 54.5 C (at the surface), 73.8 74.7 C (25 cm down the crack) and 141.1 C (1 m down the crack). The measured pH and Eh of natrocarbonatite were pH = 6.1 and Eh = 87 mV for a sample from a 1 cm surface layer and pH = 8.6 and Eh = 240 mV for a sample 20 cm down from the surface (for pH and Eh measurement both samples were cooled to a temperature of 20 C).

Sulphur is the only mineral previously reported from fumaroles (Keller and Krafft, 1990; Koepenick et al., 1996). The mineral typically occurs as prismatic crystals up to 3 mm in size and covers the surface of natrocarbonatite in contact with fumarolic gases. The natrocarbonatites are soft (they can be easily kneaded by hand), porous, water-saturated and are characterised by various shades of white, grey, fawn, black or even red.

Two mineral assemblages were distinguished in the samples studied by XRD and SEM/EDS with reference to published studies of fresh and altered natrocarbonatite. The first assemblage, represents relicts of primary magmatic natrocarbonatite minerals and the second assemblage, represents the subsolidus minerals formed during alteration of natrocarbonatite. In addition to sulphur, calcite and gypsum are major subsolidus minerals in altered natrocarbonatites.

The natrocarbonatites in fields of fumarolic activity undergo as well substantial chemical transformation. The rocks contain between 0.1 and 23.9 wt.% Snative, 0-32.9 wt.% SO3, 0.8-30.7 wt.% CO2 and 0.6-12.9 wt.% H2O. Compared to fresh natrocarbonatites they are strongly depleted in K (0.1-2.4 wt.% K2O), Na (0.2-6.8 wt.% Na2O) and Cl (<0.5 wt.%) and show enrichment in Ca (21.0-41.7 wt.% CaO) and F (1.7-14.8 wt.%). Sr and Ba are also high with up to 4.7 wt.% SrO and 9.2 wt.% BaO.

Three samples of native sulphur, collected in 1999, 2000 and 2005, have been analysed for S isotopic composition the d34S values are 5.3 (1999), +0.1 (2000) and +2.2 CDT (2005).

Conclusions. Fumarolic activity in the northern active crater produce previously unrecognized mineral assemblage of subsolidus minerals at Oldoinyo Lengai that consist of native sulphur, gypsum, calcite, anhydrite, barite and celestine in various proportions. Hot H2O-rich fumarolic gases emitting from numerous vents and cracks cause complete dissolution of primary natrocarbonatite minerals gregoryite, nyerereite and sylvite. Fluorite and khanneshite are stable minerals during alteration.

The mineralogy and bulk rock geochemistry show nearly complete removal of Na, K and Cl. In contracts Ba and Sr, leached from gregoryite and nyerereite, were immobile and involved in formation of barite and celestine. High content of fluorine in studied rocks is attributed to relative enrichment of residual in fluorite. Enrichments of some altered natrocarbonatites in sulphate component, up to 32.9 wt% SO3, may indicate oxidation of native sulphur, deposited around the vents, to SO42- and sulphate involvement in formation of gypsum.

Isotopic composition of native sulphur, d34S = -5.3 to +2.2 CDT, indicate deep-seated, mantle source of the element.

This research was supported by a Marie Curie International Fellowship within the 6th European Community Framework Programme, Alexander von Humboldt-Stiftung and German Research Foundation (DFG).


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