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The minerals of Oldoinyo Lengai natrocarbonatite lavas are unstable under atmospheric conditions. Subsolidus mineral assemblages in natrocarbonatites were studied in 105 samples from contemporary eruptions ranging from present day to about 100 years old. The subsolidus minerals in natrocarbonatites were formed (i) along cracks on the lava surface from hot gases escaping during cooling, (ii) as atmospheric alteration by solution of water-soluble minerals, in particular halides and gregoryite, and by hydration of nyerereite under the influence of meteoric water and (iii) by reaction with fumarole gases. After solidification, the lavas were cut by a network of thin cracks, the edges of which are covered by polymineralic encrustations. Samples collected 2–24 h after eruption contain nahcolite, trona, sylvite, and halite with accessory kalicinite and villiaumite. Atmospheric humidity results immediately (≥ 2 h after eruption) in alteration of black lavas that is marked by the appearance of white powdery thermonatrite with nahcolite on the lava surface. Subsequent reaction (weeks, months, years) of natrocarbonatite with meteoric water and the atmosphere results in the formation of pirssonite, gaylussite, shortite, trona, thermonatrite, nahcolite and calcite. Generally, the first important step is the formation of pirssonite and the end-members are calcite carbonate rocks or loose aggregates. Fumarolic activity is common for the active northern crater of the volcano. Reaction of hot (54–141 °C) fumarolic gases with natrocarbonatite leads to the formation of sulphur, gypsum, calcite, anhydrite, monohydrocalcite, barite and celestine. Changes in mineralogy of the natrocarbonatite lead to substantial chemical transformation. The most obvious chemical changes in this process are the loss of Na, K, Cl and S, combined with an increase in H2O, Ca, Sr, Ba, F and Mn. The oxygen and carbon isotopic composition of altered natrocarbonatites shows a significant shift from the primary “Lengai Box” to high values of δ18O and δ13C. Calcite exhibits δ13C values between − 2‰ and − 4‰ PDB and δ18O values of + 23‰ to + 26‰ SMOW. The observed assemblages of secondary minerals formed by reaction with atmosphere and meteoric water, the changes in chemical composition of the natrocarbonatite and field observations suggest that alteration of natrocarbonatite is an open-system low-temperature process. It takes place at temperatures between 8 and 43 °C with the addition of H2O to the system and the removal of Na, K, Cl and S from the carbonatites. Low-temperature thermodynamic models developed for alkali carbonate systems can be used for the interpretation of Oldoinyo Lengai subsolidus mineralization. 相似文献
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Oldoinyo Lengai in the Northern Tanzania rift is the only active nephelinite–carbonatite stratovolcano. We report the discovery of thermonatrite, aphthitalite, halite and sylvite fumarole deposits on recent natrocarbonatite lava flows erupted in the summit crater during the wet season. These salt deposits occur as delicate, concave fringes or tubes that line the cooling cracks in the lava flows and consist of intergrowths of euhedral crystals. The presence of a dark altered zone, depleted in halides and alkalies, adjacent to cooling cracks and observations of steam fumaroles emanating from the fractures suggest that the salts are formed by sublimation from saturated vapours generated by the extrusion of lavas over meteoric water. The crystallisation sequence recorded in the salts suggests that mixing between meteoric steam and magmatic CO2 and H2S occurs at high temperatures resulting in the sublimation of carbonates and sulphates. At lower temperatures the vapours are dominated by meteoric steam and sublimate halides. The high solubility of the fumarole salts within meteoric water and their formation only during the wet season implies that these are ephemeral deposits that are unlikely to be preserved in the geological record. 相似文献
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Fumarolic encrustations and natrocarbonatite lava from the active crater of Oldoinyo Lengai volcano, Tanzania, were sampled and analysed. Two types of encrustation were distinguished on the basis of their REE content, enriched (~ 2800–5600 × [REEchondrite]) and depleted (~ 100–200 × [REEchondrite]) relative to natrocarbonatite (1700–1900 × [REEchondrite]. REE-enriched encrustations line the walls of actively degassing fumaroles, whereas REE-depleted encrustations occur mainly along cracks in and as crusts on cooling natrocarbonatite lava flows; one of the low REE encrustation samples was a stalactite from the wall of a possible fumarole. The encrustations are interpreted to have different origins, the former precipitating from volcanic gas and the latter from meteoric/ground water converted to steam by the heat of the overlying lava flow(s). REE-profiles of encrustations and natrocarbonatite are parallel, suggesting that there was no preferential mobilization of specific REE by either volcanic vapour or meteoric water vapour. The elevated REE-content of the first group of encrustations suggests that direct REE-transport from natrocarbonatite to volcanic vapour is possible. The REE trends observed in samples precipitating directly from the volcanic vapour cannot be explained by dry volatility based on the available data as there is no evidence in the encrustation compositions of the greatly enhanced volatility predicted for Yb and Eu. The observed extreme REE-fractionation with steep La/Sm slopes parallel to those of the natrocarbonatite reflects solvation and complexation reactions in the vapour phase that did not discriminate amongst the different REE or similar transport of REE in both the natrocarbonatite magma and its exsolving vapour. The low concentrations of REE in the encrustations produced by meteoric vapour suggest that the temperature was too low or that this vapour did not contain the ligands necessary to permit significant mobilization of the REE. 相似文献
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5.
The most voluminous eruption of natrocarbonatite lava hitherto recorded on Earth occurred at Oldoinyo Lengai in March–April
2006. The lava flows produced in this eruption range from blocky 'a'a type to smooth-surfaced inflated pahoehoe. We measured
lava inflation features (i.e. one tumulus and three pressure ridges) that formed in the various pahoehoe flows emplaced in
this event. The inflation features within the main crater of Oldoinyo Lengai are relatively small-scale, measuring 1-5 m in
width, 2.5–24.4 m in length and with inflation clefts less than 0.4 m deep. Their small sizes are in contrast to a tumulus
that formed on the northwestern slope of the volcano (situated ~1140 m below the crater floor). The tumulus is roughly circular,
measures 17.5 × 16.0 m, and is cut by a 4.4 m deep axial inflation cleft exposing two separate flow units. We measured the
elastic properties (i.e. shear- and bulk moduli) of natrocarbonatitic crust and find that these are similar to those reported
for basaltic crust, and that there is no direct correlation between magmastatic head and pressure required to form tumuli.
All inflated flows in the 2006 event were confined by lateral barriers (main crater, erosional channel or erosional gully)
suggesting that the two most important factors for endogenous growth in natrocarbonatitic lava flows are (1) lateral barriers
that prevent widening of the flow, and (2) influx of new material beneath the viscoelastic and brittle crust. 相似文献
6.
Oldoinyo Lengai is the world's only active carbonatite volcano and considerable debate still surrounds the genesis of its magmas. Gases were collected from two fumaroles discharging close to the then active vent in October 2003. Measured fumarole temperatures were ≤ 195°C, despite the nearby, vigorous eruptive activity. Gases were sampled and analysed for noble gas isotopes. Freshly erupted natrocarbonatite lavas, a 1917 nephelinite and a sub-recent wollastonite bearing rock were also collected and analysed for noble gas isotopes using vacuum crushing techniques. In all the lava samples the neon, argon, krypton and xenon isotope ratio data are indistinguishable from air implying atmospheric contamination of the hygroscopic rocks. In the fumaroles, measured 3He/4He ratios are between 4 and 7 R/Ra. This range is similar to published values for silicate xenoliths of the East African Rift implying a local lithospheric mantle source for the volatile component of the Lengai magmas. It is still unclear if the natrocarbonatites themselves come from this region, or if fractionation and/or liquid immiscibility generate the carbonate magmas from a silicate melt within the crust itself. 相似文献
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Matthieu Kervyn Gerald G. J. Ernst Jurgis Klaudius Jörg Keller François Kervyn Hannes B. Mattsson Frederic Belton Evelyne Mbede Patric Jacobs 《Bulletin of Volcanology》2008,70(9):1069-1086
The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to
April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the
crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed
by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO2). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer
(MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity.
The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 105 m3. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion
from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this
unique natrocarbonatite volcano. 相似文献
8.
Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania 总被引：1，自引：0，他引：1
A detailed study of Oldoinyo Lengai has led to the recognition of two major cone-building stages. An early, predominantly phonolitic stage, Lengai I, forms the southern cone. The recent nephelinitic Lengai II developed following a major sector collapse event over Lengai I. Petrography of Lengai II lavas show that nephelinite is combeite- and wollastonite-bearing. All Oldoinyo Lengai lavas are peralkaline and highly evolved in terms of low Mg#, Ni and Cr values. Within the unique Lengai II combeite–wollastonite–nephelinite (CWN) peralkalinity increases with time to extreme values (Na + K)/Al = 2.36. Mineralogical expression of peralkalinity is the presence of combeite and Na-rich clinopyroxene. In addition, exceptionally high Fe2O3 (up to 10.28 wt.%) in nepheline is an indicator for alumina deficiency. Combeite also shows high Fe3+. Phonolite and CWN of Lengai I and Lengai II show similarly enriched LILE and LREE values and generally parallel patterns in PM normalized and REE plots. 相似文献
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The paper describes olivine melilitites at Oldoinyo Lengai, Tanzania, and from tuff cones from the Tanzanian rift valley in the vicinity of Oldoinyo Lengai. Oldoinyo Lengai is the only active carbonatite volcano and is distinguished by its alkali-rich natrocarbonatites. Lengai is also unique for its extreme peralkaline silicate lavas related directly to the natrocarbonatites. Primitive olivine melilitites are, according to their Mg# and Ni, Cr contents, the only candidates in the Lengai area for primary melt compositions. Incompatible trace elements, including REE, constrain the melting process in their sub-lithospheric sources to very low degrees of partial melting in the garnet stability field. The strong peralkaline trend at Oldoinyo Lengai is already recognisable in these primary or near-primary melts. More evolved olivine melilitites, with Mg# < 60 allow the fractionation line in its major and trace element expressions to be followed. Nevertheless, a large compositional gap separates the olivine melilitites and olivine-poorer melilitites from the phonolites and nephelinites that form the bulk of the Lengai cone. These silicate lavas show a high degree of peralkalinity and are highly evolved with very low Mg, Ni and Cr. Prominent examples of the recent evolution are the combeite–wollastonite nephelinites that are unique for Lengai. In their Sr, Nd, and Pb isotope relationships the olivine melilitites define a distinct group with the most depleted Sr and Nd ratios and the most radiogenic Pb isotopes. They are closest to a supposed HIMU end member of the Lengai evolution, which is characterised by an extreme spread in isotopic ratios, explained as a mixing line between HIMU and EM1-like mantle components. 相似文献
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