Oldoinyo Lengai Research Articles

Tinderet volcano, Kenya: an altered natrocarbonatite locality?

AbstractThe Tinderet volcano (19.9 to 5.5 Ma), located within the Kavirondo rift in Kenya, contains blocks of carbonatite lavas with calcite, minor apatite, fluorite, spinel-group minerals, accessory perovskite and 'plumbopyrochlore'; nyerereite is present as inclusions in the perovskite. At least four types of calcite are present in the carbonatite lavas; they differ in morphology, composition and origin. The dominant variety is secondary type-II calcite, which is enriched in sodium (up to 1.1 wt.% Na2O) and strontium (up to 1.3 wt.% SrO). The spinel-group minerals are manganese-bearing and include Mn-rich magnetite, magnesioferrite and jacobsite. Oxygen isotope data for bulk carbonatite samples (δ18O = +16.2 % to +22.6 % VSMOW) support a low crystallization temperature for the secondary calcite. Petrographic, mineralogical and isotopic data indicate that the Tinderet carbonatites are similar to natrocarbonatites from the Oldoinyo Lengai and Kerimasi volcanoes that have altered and recrystallized to form calcite carbonatites. These data support the hypothesis that some of the Tinderet carbonatites were originally alkali-rich rocks which contained primary nyerereite.

ASTER spectral analysis of ultramafic lamprophyres (carbonatites and aillikites) within the Batain Nappe, northeastern margin of Oman: a proposal developed for spectral absorption

We developed a scientific proposal on spectral absorption in remote sensing and a new image-processing method that is purely based on multispectral satellite image spectra to map ultramafic lamprophyre and carbonatite occurrences. The proposed method provides a simple, yet efficient, tool that will help exploration geologists. In this proposal, in which the spectral absorption is applicable to all satellite images obtained in visible, reflected infrared, and thermal infrared spectral wavelength regions, we found that the carbonatites appear white in colour on a greyscale or RGB thermal infrared image obtained in the thermal infrared wavelength region (3–15 μm) due to molecular emission of thermal energy by such carbonate content, particularly the wavelength recorded by the sensor and that the variation of absorption in spectral bands of an outcrop is due to the differences in percentage of carbonate content or the spectral, spatial, radiometric, or temporal resolution of satellite data or the occurrences of carbonatites to incident energy. The results were confirmed by studying the spectral absorption characteristics of carbonatites in selected world occurrences including parts of Batain Nappe, Oman; Fuerteventura (Canary Islands), Spain; Mount Homa, Kenya; Ol Doinyo Lengai, Tanzania; Mount Weld region, (Laverton), Australia, and Phalaborwa region, South Africa, using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat Thematic Mapper (TM) satellite data. A subsequent study of visible near-infrared (VNIR) and shortwave infrared (SWIR) ASTER spectral bands of Early Cretaceous alkaline ultramafic rocks of Batain Nappe, along the northeastern margin of Oman to map for the occurrences of carbonatite and aillikite (ultramafic lamprophyres) dikes and plugs, showed their detection mainly by the diagnostic CO3 absorption (2.31–2.33 μm) in ASTER SWIR band 8. The results of image interpretations were verified and confirmed in the field and were validated through the study of laboratory analyses. A few more carbonatite dike occurrences were interpreted directly over the greyscale image of ASTER bands and true-colour interpretations of a Google Earth image along this margin. The carbonatites and aillikite occurrences of the area are rich in apatite, iron oxide, phlogopite, and REE-rich minerals and warrant new exploration projects.

Volatile-rich silicate melts from Oldoinyo Lengai volcano (Tanzania): Implications for carbonatite genesis and eruptive behavior

This study presents volatile, trace, and major element compositions of silicate glasses (nepheline-hosted melt inclusions and matrix glass) from the 2007–2008 explosive eruption at Oldoinyo Lengai volcano, Tanzania. The bulk compositions of the heterogeneous ash erupted in 2007–2008 are consistent with physical mixing between juvenile nephelinite magma and natrocarbonatite emplaced during the preceding ∼25 years of effusive carbonatite eruption. The melt inclusions and matrix glasses span a wide range of silica-undersaturated compositions, from ∼46wt% SiO2 and (Na+K)/Al∼3 in the least evolved melt inclusions to 38wt% SiO2 and (Na+K)/Al up to 12 in the matrix glass. The depletion in SiO2 between melt inclusions and matrix glass is accompanied by strong enrichment in all of the incompatible trace elements measured (Ba, Nb, La, Ce, Sr, Zr, Y), which is consistent with fractional crystallization of a bulk mineral assemblage with SiO2 higher than that of the melt inclusions but inconsistent with silicate melt evolution by assimilation of carbonatite. The melt inclusions are volatile-rich with 2.7wt% to 8.7wt% CO2 and 0.7wt% to 10.1wt% H2O, indicating that Oldoinyo Lengai is a hydrous system. This is contrary to the long-held assumption that Oldoinyo Lengai is relatively anhydrous, which is based on the observation that natrocarbonatite lavas are water-poor. We argue that natrocarbonatites are derived from hydrous carbonate liquid that degas H2O at low pressure. The silicate glass data show that H2O concentration is negatively correlated with incompatible element enrichment, which we attribute to crystallization of the melt in response to decompression degassing of H2O. The eruptive cycle at Oldoinyo Lengai reflects changes in bulk silicate magma viscosity due to extensive H2O-driven crystallization and explosive eruptions occur when volatiles (i.e. H2O>CO2 gas, and carbonate liquid) cannot separate from the crystal-rich nephelinite magma. Melt H2O content decreases as a function of pressure; however CO2 concentration in the melt inclusions is buffered by the presence of immiscible carbonate liquid. CO2 content increases with melt evolution parameters (e.g. increasing (Na+K)/Al) due to enhanced solubility with alkali enrichment and SiO2 depletion in the melt. The matrix glasses and evolved melt inclusions, on the other hand, experienced low pressure (<50MPa) CO2 degassing and were not buffered by a coexisting carbonate liquid. Whereas the melt inclusions are the most CO2-rich yet identified, their CO2/Nb ratios are without exception lower than that in MORB, indicating that a volatile-rich mantle source is not required for Oldoinyo Lengai.

Parental carbonatitic melt of the Koala kimberlite (Canada): Constraints from melt inclusions in olivine and Cr-spinel, and groundmass carbonate

This study of the hypabyssal kimberlite with particularly fresh olivine from the Palaeogene Koala pipe (Ekati cluster, Slave craton, Canada) is focused on reconstruction of the kimberlite parental melt. The Koala kimberlite sample is a typical ultramafic rock (archetypal or Group-I kimberlite) containing large abundance of forsteritic olivine set in a carbonate-rich matrix. The euhedral olivine-II shows significant zoning in terms of forsterite content and trace elements. Only the rims of olivine-II are considered to be magmatic — they are uniform in terms of Fo (91.5±0.2mol%) and host primary crystal (apatite, Cr-spinel) and melt inclusions. The magmatic spinel phenocrysts are strongly zoned from titanian magnesian chromite to pleonaste, with the cores having the same composition as spinel inclusions in olivine. The primary and pseudo-secondary melt inclusions in both magmatic Cr-spinel and olivine comprise a complex assemblage of daughter phases, dominated by carbonate minerals (dolomite, eitelite (Na2Mg(CO3)2), shortite Na2Ca2(CO3)3, nyerereite Na2Ca(CO3)2, zemkorite (Na,K)2Ca(CO3)2, and fairchildite (K2Ca(CO3)2)). In addition to the dominant carbonates, the melt inclusions contain phosphates, chlorides and sulphates (bradleyite (Na3Mg(PO4)(CO3), halite, sylvite, aphthitalite (Na0.25K0.75)2SO4)), but the silicate component is represented by tetraferriphlogopite only. The melt inclusions in olivine studied in heating experiments show immiscibility of two carbonate liquids on heating and cooling, and homogenise at low temperature<700°C. The liquid immiscibility texture is also characteristic of the calcitic groundmass, and represented by irregularly shaped calcite segregations and near-spherical globules. The calcite in the globules is unusually enriched in Na2O, SrO and BaO (in wt.%, 0.37–0.65, 1.17–2.40, 2.37–4.18, respectively). Such compositions are compared to secondary calcite replacing nyerereite and shortite in the natrocarbonatite lavas of the modern Oldoinyo Lengai volcano. The results on mineralogy, composition and homogenisation temperature of melt inclusions and the groundmass of the Koala kimberlite imply that the parental kimberlite liquid was a Si-poor Ca–Mg carbonate, enriched in alkali elements, lithophile trace elements, phosphorus, chlorine and sulphur.

Reprint of “Geochemistry and petrogenetic significance of natrocarbonatites at Oldoinyo Lengai, Tanzania: Composition of lavas from 1988 to 2007”

The natrocarbonatites of Oldoinyo Lengai, Tanzania, are unique in magmatic petrology. The historical activity of Oldoinyo Lengai has seen changes from nephelinitic to natrocarbonatitic character of the emitted magmas. Since 1983 the activity was characterized by the effusion of fluid natrocarbonatite lava from which we have collected and analyzed fresh samples in the summit crater from 1988 to 2007. The available compositional data set forms the basis for presenting and discussing the typical composition and variation of natrocarbonatites and their relationship to the silicate magmas of Oldoinyo Lengai. The “type” natrocarbonatite major and trace element composition is derived for an average of 25 samples with low standard deviation. Oldoinyo Lengai carbonatites are unique in almost all aspects of their petrological and geochemical characteristics and are characterized as extremely alkali-rich, with Na2O+K2O generally about 40wt.%, and with high CaO contents of 14–18wt.%. This composition results from the presence of phenocrysts of nyerereite (Na,K)2Ca(CO3)2 and gregoryite (Na,K,Cax)2−x(CO3) dominating the highly porphyritic natrocarbonatite lavas, with sylvite and fluorite as main groundmass minerals. The significance of particular trace element concentrations and ratios of equally incompatible elements (REE, Ba, Sr, Th/U, Nb/Ta, Zr/Hf) is evaluated for models of liquid–liquid separation. In defining a “type” natrocarbonatite composition, we also distinguish special variations in chemical and/or mineralogical compositions as follows: (1) silicate-bearing natrocarbonatites, characterized by the occurrence of nephelinite spheroids, as in the 1993 and 2006 lavas; (2) residual melt compositions as described from the 1988 eruptive period as represented by the aphyric, filter-pressed interstitial melt of solidifying porphyritic lavas; (3) an interlude during 2000 when natrocarbonatites with sylvite and fluorite microcrysts were emitted. After 25years of mostly mild activity characterized by effusion and spattering of fluid natrocarbonatite lava, the paroxysmal ash eruptions of September 4, 2007, changed dramatically the crater morphology, eruptive dynamics and magma composition of Oldoinyo Lengai. Fresh natrocarbonatites — if present at the bottom of the deep crater pit formed during the 2007–2008 explosive activity — will possibly remain inaccessible for decades.

Helium and carbon isotope systematics of cold “mazuku” CO2 vents and hydrothermal gases and fluids from Rungwe Volcanic Province, southern Tanzania

We report new helium and carbon isotope (3He/4He and δ13C) and relative abundance (CO2/3He) characteristics of a suite of 20 gases and fluids (cold mazuku-like CO2 vents, bubbling mud-pots, hot and cold springs) from 11 different localities in Rungwe Volcanic Province (RVP), southern Tanzania and from 3 additional localities in northern Tanzania (Oldoinyo Lengai Volcano and Lake Natron). At RVP, fluids and gases are characterized by a large range in He-isotope compositions (3He/4He) from 0.97 RA to 7.18 RA (where RA = air 3He/4He), a narrow range in δ13C ratios from −2.8 to −6.5‰ (versus VPDB), and a large range in CO2/3He values spanning nearly four orders of magnitude (4×109 to 3.2×1013). Oldoinyo Lengai possesses upper‐mantle-like He–CO2 characteristics, as reported previously (Fischer et al., 2009), whereas hot springs at Lake Natron have low 3He/4He (~0.6 RA), CO2/3He (~5–15×108) and intermediate δ13C (~−3.7 to −4.9 ‰). At RVP, fluid phase samples have been modified by the complicating effects of hydrothermal phase-separation, producing CO2/3He and δ13C values higher than postulated starting compositions. In contrast, gas-phase samples have not been similarly affected and thus retain more mantle-like CO2/3He and δ13C values. However, the addition of crustal volatiles, particularly radiogenic helium from 4He-rich reservoir rocks, has modified 3He/4He values at all but the three cold CO2 gas vent (i.e., mazuku) localities (Ikama Village, Kibila Cold Vent and Kiejo Cold Vent) which retain pristine upper-mantle He-isotope (~7 RA) and He–CO2 characteristics. The extent of crustal contamination is controlled by the degree of interaction within the hydrothermal system, which increases with distance from each major volcanic center. In contrast, we propose that pristine cold CO2 mazuku gases collected at stratigraphic contacts on the flanks of RVP volcanoes may potentially tap isolated gas pockets, which formed during previous eruptive events and have remained decoupled from the local hydrothermal system. Furthermore, by identifying and utilizing unmodified gas samples, we determine mantle versus crustal provenance of the CO2, which we use to estimate mantle-derived CO2 fluxes at both Rungwe and Lake Natron. Finally, we investigate the origin of the apparent discrepancy in He isotopes between fluids/gases and mafic phenocrysts at RVP (from Hilton et al., 2011), and discuss the tectonic (i.e., rift zone dynamics) and petrogenic conditions that distinguish RVP from other plume-related subaerial rift zones.

Geochemistry and petrogenetic significance of natrocarbonatites at Oldoinyo Lengai, Tanzania: Composition of lavas from 1988 to 2007

The natrocarbonatites of Oldoinyo Lengai, Tanzania, are unique in magmatic petrology. The historical activity of Oldoinyo Lengai has seen changes from nephelinitic to natrocarbonatitic character of the emitted magmas. Since 1983 the activity was characterized by the effusion of fluid natrocarbonatite lava from which we have collected and analyzed fresh samples in the summit crater from 1988 to 2007. The available compositional data set forms the basis for presenting and discussing the typical composition and variation of natrocarbonatites and their relationship to the silicate magmas of Oldoinyo Lengai. The “type” natrocarbonatite major and trace element composition is derived for an average of 25 samples with low standard deviation. Oldoinyo Lengai carbonatites are unique in almost all aspects of their petrological and geochemical characteristics and are characterized as extremely alkali-rich, with Na2O+K2O generally about 40wt.%, and with high CaO contents of 14–18wt.%. This composition results from the presence of phenocrysts of nyerereite (Na,K)2Ca(CO3)2 and gregoryite (Na,K,Cax)2−x(CO3) dominating the highly porphyritic natrocarbonatite lavas, with sylvite and fluorite as main groundmass minerals. The significance of particular trace element concentrations and ratios of equally incompatible elements (REE, Ba, Sr, Th/U, Nb/Ta, Zr/Hf) is evaluated for models of liquid–liquid separation. In defining a “type” natrocarbonatite composition, we also distinguish special variations in chemical and/or mineralogical compositions as follows: (1) silicate-bearing natrocarbonatites, characterized by the occurrence of nephelinite spheroids, as in the 1993 and 2006 lavas; (2) residual melt compositions as described from the 1988 eruptive period as represented by the aphyric, filter-pressed interstitial melt of solidifying porphyritic lavas; (3) an interlude during 2000 when natrocarbonatites with sylvite and fluorite microcrysts were emitted. After 25years of mostly mild activity characterized by effusion and spattering of fluid natrocarbonatite lava, the paroxysmal ash eruptions of September 4, 2007, changed dramatically the crater morphology, eruptive dynamics and magma composition of Oldoinyo Lengai. Fresh natrocarbonatites — if present at the bottom of the deep crater pit formed during the 2007–2008 explosive activity — will possibly remain inaccessible for decades.

Continuing the Carbonatite Controversy Preface

Carbonatites have always been controversial (Mitchell, 2005). Their magmatic origin was disputed in the early days of the last century, regardless of the fact that experiments clearly demonstrated the crystallization of magmatic calcite (Wyllie and Tuttle, 1960). The observation of the eruption of natrocarbonatite lava in Oldoinyo Lengai (Dawson 1962) finally convinced petrologists that they were dealing with the products of magmatic carbonate liquids. Since that time, further controversies have emerged, especially regarding the ultimate origin of carbonatite magmas, for which there are two ‘endmember’ hypotheses. The generally accepted hypothesis is based on isotopic evidence and suggests that carbonatites are from deep asthenospheric sources, such as mantle plumes (Bell, 2001; Bell et al., 2004). This fails to explain why carbonatites are essentially confined to the continental lithosphere and are extremely rare in the ocean basins (Woolley and Kjarsgaard, 2008; Woolley and Bailey, this issue), and leads to the alternative hypothesis of lithosphere-generated carbonatitic magmatism. It may be that this is simply because we have not yet understood how to identify carbonatites in oceanic regions (Bailey and Kearns, this volume), or there may be some more profound reason why carbonatites cannot form within or erupt through oceanic lithosphere.

Mineralogy, geochemistry and petrology of the phonolitic to nephelinitic Sadiman volcano, Crater Highlands, Tanzania

Sadiman volcano is located in the Crater Highlands area of northern Tanzania, which lies next to the western escarpment of the Gregory rift—a part of the eastern branch of the East African Rift system. It consists of interlayered phonolitic tuffs, tuff breccias (with blocks of nephelinites) and nephelinitic lava flows. Rare xenoliths of phonolite lava and ijolite were observed within the nephelinite lavas with ijolite blocks occurring in phonolitic tuffs. No evidence for the presence of melilite-bearing and/or carbonatitic rocks was found during this study. On the basis of petrography, mineralogy and geochemistry the nephelinites are divided into highly porphyritic nephelinite, wollastonite nephelinite and phonolitic nephelinite, the latter of which is the dominant variety at Sadiman. Nepheline+clinopyroxene+titanite±perovskite±andradite–schorlomite±wollastonite±sanidine±sodalite are the principle pheno- and microphenocryst phases. The nephelinites are highly evolved (Mg#=0.17–0.26) alkaline to peralkaline (AI=0.88–1.21) rocks enriched in incompatible elements such as Rb, Ba, Th, U, Nb, Pb, Ta, Sr and light REEs, and strongly depleted in P and Ti. This suggests derivation from an enriched mantle source and fractionation of apatite and Ti-rich mineral(s). Primary melt inclusions in nepheline phenocrysts (Thomogenization=860–1100°C) indicate enrichment of volatile components in the melts, particularly of fluorine (up to 1.8wt.% in silicate glass) resulting in the formation of daughter fluorite in partly and complete crystallized inclusions. The Sadiman nephelinites crystallized under relatively oxidizing conditions (above the FMQ buffer), which differ from the reducing conditions reported for trachytic and pantelleritic rocks from other parts of the Gregory rift. Similar rock types and relatively oxidizing conditions are known from Oldoinyo Lengai and other localities, all of which are closely associated with carbonatites. By analogy, we conclude that andradite–schorlomite-rich nephelinites may indicate a pre-stage on the evolutionary path towards carbonatitic magmatism.

Trace element geochemistry of nyerereite and gregoryite phenocrysts from natrocarbonatite lava, Oldoinyo Lengai, Tanzania: Implications for magma mixing

The abundances of Li, P, Cl, V, Mn, Rb, Sr, Y, Cs, Ba, Pb, Th, U and REE, within and between, phenocrysts of nyerereite and gregoryite occurring in natrocarbonatite lavas erupted from the active volcano Oldoinyo Lengai (Tanzania) have been determined by electron microprobe, LA-ICP-MS and SIMS. These data show that, in general, nyerereite is enriched in Rb (71–137ppm), Sr (14,485–23,240ppm), Y (2.0–8.9ppm), Cs (1.6–5.3ppm), Ba (4000–11,510ppm), but poorer in Li (21–91ppm), P (820–1900ppm) and V (5.1–47ppm) relative to gregoryite (Rb=43–106; Sr=4255–7275; Y=0.3–4.0; Cs=0.6–5.1; Ba=1125–7052; Li 84–489; P=6790–15,860; V=33–155ppm). Nyerereite is highly enriched in REE (La=236–973; Ce=395–1044ppm) relative to gregoryite (La=59–309; Ce=59–301ppm). Chondrite normalized REE distribution patterns for nyerereite and gregoryite are parallel and linear with no Eu anomalies. They show extreme enrichment in light REE and depletion in heavy REE (nyerereite La/YbCN=1759–7079; gregoryite La/YbCN=1051–10,247). Significant differences exist in the abundances of trace elements within and between coexisting crystals occurring in diverse natrocarbonatite flows, although there do not appear to be any significant secular variations in phenocryst compositions in lavas erupted from a given vent. It is concluded that both major, minor and trace element compositional data for nyerereite and gregoryite phenocrysts occurring in natrocarbonatite lavas are derived by the crystallization of several different batches of magma in a continuously replenished fractionating magma chamber. Natrocarbonatite lavas are considered to be hybrids formed by the mixing of both crystals and melts formed from several batches of natrocarbonatite magma; thus bulk rock compositions cannot represent the compositions of the primary magma composition before the onset of fractionation. Differentiation of natrocarbonatite melts leads to enrichment of residua in Ba and Mg.

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

Primary silicate–melt and carbonate–salt inclusions occur in the phenocrysts (nepheline, fluorapatite, wollastonite, clinopyroxene) in the 1917 eruption combeite–wollastonite nephelinite at Oldoinyo Lengai. Silicate–melt inclusions in nepheline clearly show liquid immiscibility phenomena expressed in the presence of carbonate globules in silicate glass. The coexistence of inclusions with markedly different proportions of silicate glass+vapor-carbonate globule in the core of nepheline phenocrysts, the presence of carbonate–salt inclusions in fluorapatite and our heating experiments strongly suggest that their entrapment began at temperatures higher than 1130°C in an intermediate chamber when initial carbonated nephelinite melt was heterogeneous and represented a mixture of immiscible liquids. Silicate–natrocarbonatite melt immiscibility took place at high temperature and immiscible nephelinite and carbonatite liquids coexisted over a wide temperature range from ≥1130°C to 600°C. Homogenization of a carbonate globule (dissolution of the gas bubble in carbonate melt) at 900–940°C indicates that after separation from silicate magma the natrocarbonatite represented homogeneous liquid in the 900–1130°C temperature range, whereas below these temperatures immiscible melts of different composition and fluid phase have separated from it. The bulk composition of homogeneous natrocarbonatite melt may be estimated as ≈20% CaF2, 40–60% (Na,K)2CO3 and 20–40% CaCO3 based on the coexistence of nyerereite, calcite and fluorite and the rapid phase transition (carbonate aggregate→carbonate liquid) at 550–570°C observed in vapor-carbonate globules of nepheline-hosted silicate–melt inclusions and on the Na2CO3–CaCO3–CaF2 phase diagram. Silicate glasses of nepheline-hosted immiscible inclusions drastically differ from host nephelinite in the abundance of major and trace elements. They are high peralkaline ((Na+K)/Al — up to 9.5) and virtually free of water (H2O<0.6wt.%). Their very high Zr/Hf and Nb/Ta ratios and Li contents indicate that these silicate glasses represent the most evolved compositions at Oldoinyo Lengai. The peralkaline character of nephelinite melt is expressed in the composition of the daughter mineral assemblage within silicate–melt inclusions in nepheline (delhayelite, leucite, mica, clinopyroxene). These minerals show strong deficiency in Al and enrichment in Fe3+ that is also common to the groundmass of the Oldoinyo Lengai combeite–wollastonite nephelinites.

Carbonate–silicate immiscibility and extremely peralkaline silicate glasses from Nasira cone and recent eruptions at Oldoinyo Lengai Volcano, Tanzania

Phenocrysts of garnet, pyroxene and nepheline in peralkaline nephelinite from the Nasira parasitic cones at Oldoinyo Lengai contain quenched immiscible silicate (peralkalinity=2–13) and Na–Ca-carbonate melts. Their bulk compositions further define the limits of liquid immiscibility for peralkaline carbonated nephelinite magmas and confirm this process was operative at Oldoinyo Lengai during older stages of activity. Groundmass glasses in Nasira nephelinites are peralkaline (peralkalinity=5.5–9.5) but less evolved than melt inclusion glasses (peralkalinity=8–13) in nepheline phenocrysts, implying that these magmas are hybrids formed by magma mixing. Groundmass glass in diverse peralkaline combeite nephelinite ash clasts with and without melilite and/or wollastonite formed in the January–June 2008 eruptions of Oldoinyo Lengai are also exceptionally peralkaline. Two trends in their compositions are evident: (1) increasing peralkalinity from 6 to 10 with SiO2 decreasing from 42 to 33wt.%; (2) increasing peralkalinity from 6 to 16 with SiO2 decreasing from 45 to 40wt.%. All recent glasses are considered to be more evolved than groundmass glass in Nasira combeite nephelinite. These data indicate that several varieties of nephelinite exist at Oldoinyo Lengai. Their parental magmas are considered to have been initially enriched in alkalis during partial melting of their metasomatized asthenospheric sources and further by subsequent assimilation, or re-solution, of previously exsolved natrocarbonatite melt in the magma chamber(s) underlying Oldoinyo Lengai. On this basis, none of the bulk compositions of peralkaline stage II lavas at Oldoinyo Lengai, including Nasira, are considered to represent those of liquids as their compositions are determined by rheological factors (phenocryst accumulation; cumulate disruption) and assimilation processes. The formation of combeite is considered to be a consequence of natrocarbonatite melt assimilation.

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

Liquid–liquid partitioning of 42 elements between synthetic silicate melts and immiscible fluoride, chloride, carbonate, phosphate and sulfate liquids was studied at temperatures of 650–1100°C, pressures 72–100MPa, with 0–11wt.% H2O. One series of experiments was performed in a rotating internally-heated autoclave where separation of the immiscible liquids was assisted by centrifugal forces. An analogous series of experiments was done in static rapid-quench cold-seal pressure vessels. The experimentally determined liquid–liquid distribution coefficients (D’s) vary over several orders of magnitude, as a result of variable Coulombic interactions between cations and anions. For alkaline, alkaline earth and rare earth elements ther is a strong and systematic dependence of the liquid/liquid D values on the ionic potential Z/r for all the examined systems. In contrast, highly charged cations (e.g., HFSE) show no systematic relationships between the D’s and Z/r.New experimental constraints on the carbonate/silicate liquid–liquid D values presented here confirm that rare metals such as Nb, Zr, REE, Th and U concentrate in silicate liquids, and therefore carbonatites that carry economical rare metal mineralization are not likely to have formed by liquid immiscibility. The comparison between experimentally-determined carbonate–silicate liquid–liquid D values and bulk-rock natrocarbonatite vs. nephelinite compositions at the Oldoinyo Lengai in Tanzania reveals significant discrepancies for Cs, Rb, Ba, Be, Zn, heavy REE, Ti, Mo and W, thus rendering a simple, one-stage immiscibility model for Oldoinyo Lengai questionable.

The Ngorongoro Crater as the biggest geotouristic attraction of the Gregory Rift (Northern Tanzania, Africa) – geotouristic valorization, touristic development and hazard

The Ngorongoro Crater, as the largest unflooded and not destroyed volcanic caldera on Earth, is one of the major geotouristic attractions of East Africa. Unique on the global scale richness of wildlife, exotic cultures of indigenous people and specific position of the Crater within the East African Rift System, each year attracts thousands of tourists eager for an unforgetable experience. Their number continues to grow from year to year, reaching the value of nearly half a million visitors within the last few years. Constantly developing tourism industry, besides many advantages, also causes a number of risks, both for the world of living nature and inanimate nature objects, as well as for the local population. In 1959, recognizing the unique and special touristic and geoeducative values of this location, the Ngorongoro Conservation Area (NCA) was established. Almost twenty years later, in 1978, the area was included in the UNESCO World Cultural and Natural Heritage List. Furthermore, within the Ngorongoro Conservation Area as well as in its immediate neighborhood there are many objects that also deserve to be called geotouristic attractions, such as: Olduvai Gorge, Crater Olmoti, Crater Empakai and Oldoinyo Lengai volcano. In a relatively short distance from the Crater also the highest mountain in Africa – the Kilimanjaro volcano and the biggest active volcano of this continent, Meru, are located.

Sr-Na-bearing åkermanite and nepheline in nephelinite from Nagahama, Hamada, Shimane Prefecture, Southwest Japan

Akermanite and nepheline found within melilite-olivine nephelinite in Nagahama Plateau, Hamada city, Shimane Prefecture, Southwest Japan, were investigated to characterize their occurrence and chemical compositions. Samples of massive and dense blue-gray melilite-olivine nephelinite were collected from a Naka-Uchida tunnel construction site. Their bulk composition is characterized by an extremely low SiO2 content of 35.75 wt% and high Sr and Ba contents, reaching 2091 ppm and 958 ppm, respectively. Forsterite, akermanite, augite, ulvospinel and apatite occur as microphenocrysts. Nepheline occurs as an interstitial mineral between these microphenocrysts. Euhedral nepheline, zeolite-group minerals, diopside and calcite occur in druses. Akermanite occurs as prismatic to anhedral microphenocrysts. The rims of the akermanite microphenocrysts are dissolved and overgrown by augite. Akermanite has an average composition of (K0.011Na0.390Ba0.004Sr0.031Ca1.566)Σ2.002(Mn0.007Fe2+0.088Mg0.457Ni0.001Fe3+0.134Cr3+0.001Ti0.003Al0.295)Σ0.986(Al0.074Si1.926)Σ2.000O7, and contains up to 4.62 wt% Na2O, 1.68 wt% SrO, and 0.64 wt% BaO. Calculated Fe2O3 and FeO based on charge balance are 5.36 wt% and 3.11 wt%, respectively. Fe2+/(Fe2+ + Fe3+) ratios estimated from the FeLβ/FeLα intensity ratios range from 0.47 to 0.79, indicating that a considerable amount of Fe3+ is contained within akermanite. The chemical composition of nepheline in the groundmass is (Na0.655K0.304)Σ0.959(Fe3+0.025Al0.978)Σ1.003(Al0.012Si0.988)Σ1.000O4 on average. No excess SiO2 nor anorthite components were detected. The bulk composition of the Hamada melilite-olivine nephelinite and the compositions of akermanite and nepheline are similar to those of equivalents from the carbonatite-nephelinite-phonolite Oldoinyo Lengai volcano in Tanzania. This similarity may support the proposed genetic model that the Hamada nephelinite magma was derived from a source material formed by carbonate metasomatism of a lherzolite upper mantle.

Experimental Studies of the System Na2CO3-CaCO3-MgF2 at 0{middle dot}1 GPa: Implications for the Differentiation and Low-temperature Crystallization of Natrocarbonatite

The system [Na2Ca(CO3)2 ]100–x – (50 wt.% NaCl + 50 wt.% KCl)x (10 < x < 80 wt.%), a join in the quaternary system Na2CO3 – CaCO3 – NaCl – KCl, has been investigated at temperatures ranging from 450° to 900°C at 0.1 GPa. Liquidus temperatures are on the order of 825°C, with a depression on the liquidus to 750°C at about 23 wt.% halide corresponding to a piercing point. Liquids that are poor in halides relative to the composition of the piercing point have a nyerereite-like (Na,Ca,K) carbonate as the primary liquidus phase; it coexists with a one-phase halide-rich liquid. Liquids with ~ 30–70 wt.% halide initially precipitate calcite as the primary liquidus phase. Below 700°C, calcite reacts with liquid to form nyerereite-like (Na,Ca,K) carbonate. From 600° to 550°C, this carbonate coexists with two immiscible halide liquids. Below the solidus at ~550°C, discrete exsolved inclusions of (Na,K)Clss occur in (K,Na)Clss and of (K,Na)Clss in (Na,K)Clss. These data are relevant to the differentiation and crystallization of the natrocarbonatitic melt at Oldoinyo Lengai under hypabyssal and plutonic conditions; they predict the formation of cumulate rocks composed of nyerereite, gregoryite, halite and sylvite. Disaggregation of such assemblages could lead to formation of sylvite–fluorite macrocrystal natrocarbonatite.

Precise thermal ionization mass spectrometric measurements of 142Nd/ 144Nd and 143Nd/ 144Nd isotopic ratios of Nd separated from geological standards by chromatographic methods

A chromatographic separation technique for 142Nd/ 144Nd and 143Nd/ 144Nd isotope ratio measurements is established and applied to the analyses of geological standards of basaltic compositions (BCR-2, BIR-1) using Isoprobe-T TIMS. The instrument was tested for reliability and reproducibility to measure Nd isotope composition using the synthetic standard JNdi-1. The techniques were also applied to a carbonatite lava sample, OL-6, Oldoinyo Lengai, to check the validity of method for carbonatite matrix. The isotope ratios of 143Nd/ 144Nd for synthetic Nd standard JNdi-1, geological standards BCR-2, BIR-1, and carbonatite lava sample OL-6 obtained by these methods are in good agreement with previously published data. The 143Nd/ 144Nd values for JNdi-1 and BCR-2 have an external precision of ±13 ppm and ±15 ppm (2 σ), respectively. The JNdi-1 and BCR-2 data for 142Nd/ 144Nd has an external precision of ±12 ppm and ±8 ppm (2 σ), respectively. The 142Nd/ 144Nd composition of the two geological standards BCR-2 and BIR-1 are indistinguishable from synthetic mono-element standard JNdi-1, and they all fall within the 12 ppm (2 σ) envelope of external precision. The external reproducibility is sufficient to distinguish and resolve 20 ppm anomalies in 142Nd/ 144Nd values.

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

Ashes from Oldoinyo Lengai were collected four days after the onset of the recent explosive episode (i.e., on September 7th 2007). The ash is composed of poorly-vesicular natrocarbonatite droplets, vesicular microcrystalline nephelinite shards, and a mixed variety containing both silicate and carbonate minerals in variable proportions. Simple mixing calculations show that the whole-rock composition of the ashes can be explained by mixing natrocarbonatite and nephelinite magmas with a ratio of 4:1. The dominant silicate minerals are clinopyroxene, nepheline, Ti-andradite, wollastonite and alumoåkermanite. Ti-magnetite is the most common oxide mineral. This mineral assemblage is similar to that present in the 1966 eruption products. In contrast to the 1966–1967 explosive eruption where clinopyroxene is resorbed and corroded, the ashfall from September 7th contains a large amount of euhedral clinopyroxene crystals, suggesting that magma mixing was heterogeneous and incomplete in this initial stage of the eruption. This is also supported by the petrography of the ashes. The composition of the dominant carbonate minerals (i.e., gregoryite and nyerereite) and the fluidal textures of the natrocarbonatite droplets suggest mixing of higher-viscosity nephelinite and low-viscosity natrocarbonatite magmas. Characteristic carbonate minerals produced by alteration cannot be found in the ashes. This suggests limited interaction with the older, pre-existing, natrocarbonatites inside the summit crater of the volcano. The carbonate minerals show textural evidence of being partially resorbed into the hotter nephelinitic magma. At least part of this decomposition of carbonate phases (releasing CO 2 and contributing to increased explosivity) must have occurred within the volcanic edifice such that the released gas is allowed to expand during decompression.

The Lava sequence of the East African Rift escarpment in the Oldoinyo Lengai – Lake Natron sector, Tanzania

A 500 m sequence of horizontal lava flows forms the Gregory rift escarpment of the western rift shoulder between Lake Natron and Oldoinyo Lengai. A detailed volcanic stratigraphy of this >1.2 Ma evolution of the EAR in Northern Tanzania is presented. The sequence is formed by several distinct rock suites, with increasing alkalinity from base to top. Alkali olivine basalts of the Waterfall Sequence at the base are followed by a basanite series, and by a range of evolved nephelinites forming the upper part of the escarpment. Numerous dykes and Strombolian scoria deposits indicate local fissure eruptions as opposed to or in addition to more distant sources. Primitive compositions within each of the series indicate variable candidates for primary magmas. The composition of the basanite suite ranges from primitive mantle melts (high Mg#, Cr, Ni) to more evolved rocks, in particular hawaiites, generated by fractionation of olivine, pyroxene and magnetite. Inter-bedded within the basanite suite, one single olivine melilitite flow with high Mg# and abundant olivine and pyroxene megacrysts is the only primitive candidate for the nephelinite suite. However, in view of the large compositional gap and marked differences in incompatible element ratios, a relation between this flow and the nephelinites remains hypothetical. The variation within the evolved nephelinite series can be partly explained by fractionation of pyroxene, apatite, perovskite (and some nepheline), while magma mixing is indicated by zonation patterns of pyroxene. The most evolved nephelinite, however, differs significantly from all other nephelinites in major and trace elements. Thus the entire sequence is petrologically not a coherent evolution, rather the result of different mantle melts fractionating under variable conditions. Carved into the rift scarp of the study area west of Engare Sero is a young explosion crater, the Sekenge Crater. Sekenge Tuffs are olivine melilitites, similar to other craters and maars of the “Younger Extrusives” on the rift valley floor surrounding Oldoinyo Lengai. Further, still younger alkaline tuffs are found on the top of the rift shoulder.

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

With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917,1940-1941, and 1966-1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007-2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-(a) over circle kermanite, Ti-andradite, resorbed pyroxene and Fe-Ti oxides, and a Na-Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite- wollastonite-melilite nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25-30 wt.% SiO2, with 7-11 wt.% CO2, high alkalies (Na2O 15-19%, K2O 4-5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and nephelinite, with about one third natrocarbonatite and two thirds nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite-wollastonite nephelinite (CWN) of OL by enrichment of CO2 and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition.