Unusual Carbonatite Lavas from Active Volcano, Oldoinyo Lengai

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Unusual Carbonatite Lavas from Active Volcano, Oldoinyo Lengai     

Carbonatite and highly peralkaline nephelinite melts from Oldoinyo Lengai Volcano,Tanzania: The role of natrite-normative fluid degassing

Oldoinyo Lengai, located in the Gregory Rift in Tanzania, is a world-famous volcano owing to its uniqueness in producing natrocarbonatite melts and because of its extremely high CO₂ flux. The volcano is constructed of highly peralkaline [PI = molar (Na₂O + K₂O)/Al₂O₃ > 2–3] nephelinite and phonolites, both of which likely coexisted with carbonate melt and a CO₂-rich fluid before eruption. Results of a detailed melt inclusion study of the Oldoinyo Lengai nephelinite provide insights into the important role of degassing of CO₂-rich vapor in the formation of natrocarbonatite and highly peralkaline nephelinites. Nepheline phenocrysts trapped primary melt inclusions at 750–800 °C, representing an evolved state of the magmas beneath Oldoinyo Lengai. Raman spectroscopy, heating-quenching experiments, low current EDS and EPMA analyses of quenched melt inclusions suggest that at this temperature, a dominantly natrite_ss-normative, F-rich (7–14 wt%) carbonate melt and an extremely peralkaline (PI = 3.2–7.9), iron-rich nephelinite melt coexisted following degassing of a CO₂ + H₂O-vapor. We furthermore hypothesize that the degassing led to re-equilibration between the melt and liquid phases that remained and involved 1/ mixing between the residual (after degassing) alkali carbonate liquid and an F-rich carbonate melt and 2/ enrichment of the coexisting nephelinite melt in alkalis. We suggest that in the geological past similar processes were responsible for generating highly peralkaline silicate melts in continental rift tectonic settings worldwide.     

Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania

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.     

Thermodynamic Analysis of Secondary Minerals Stability in Altered Carbonatites of the Oldoinyo Lengai Volcano,Northern Tanzania

Carbonatites from the Oldoinyo Lengai volcano, northern Tanzania, are unstable under normal atmospheric conditions. Owing to carbonatite interaction with water, the major minerals—gregoryite Na₂(CO₃), nyerereite Na₂Ca(CO₃)₂, and sylvite KCl—are dissolved and replaced with secondary low-temperature minerals: thermonatrite Na₂(CO₃) · H₂O, trona Na₃(CO₃)(HCO₃) · 2H₂O, nahcolite Na(HCO₃), pirssonite Na₂Ca(CO₃)₂ · 2H₂O, calcite Ca(CO₃), and shortite Na₂Ca₂(CO₃)₃. Thermodynamic calculations show that the formation of secondary minerals in Oldoinyo Lengai carbonatites are controlled by the pH of the pore solution, H₂O and CO₂ fugacity, and the ratio of Ca and Na activity in the Na₂O–CaO–CO₂–H₂O system.     

Mineralogical and chemical transformation of Oldoinyo Lengai natrocarbonatites, Tanzania

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.     

Liquid immiscibility during crystallization of forsterite-phlogopite ijolites at Oldoinyo Lengai Volcano,Tanzania: study of melt inclusions

The paper is concerned with study of melt inclusions in minerals of ijolite xenoliths at Oldoinyo Lengai Volcano. Melt inclusions with different phase compositions occur in forsterite macrocrysts and in diopside, nepheline, fluorapatite, Ti-andradite, and Ti-magnetite crystals. Nepheline contains primary melt inclusions (silicate glass + gas-carbonate globule ± submicron globules ± sulfide globule ± daughter/trapped phases, represented by diopside, fluorapatite, Ti-andradite, and alumoakermanite). The gas-carbonate globule consists of a gas bubble surrounded by a fine-grained aggregate of Na-Ca-carbonates (nyerereite and gregoryite). Fluorapatite contains primary carbonate-rich melt inclusions in the core, which consist of nyerereite, gregoryite, thenardite, witherite, fluorite, villiaumite, and other phases. Their mineral composition is similar to natrocarbonatites. Primary melt inclusions (glass + gas bubble ± daughter phases) are rare in diopside and Ti-andradite. Diopside and forsterite have trails of secondary carbonate-rich inclusions. Besides the above minerals, these inclusions contain halite, sylvite, neighborite, Na-Ca-phosphate, alkali sulfates, and other rare phases. In addition, diopside contains sulfide inclusions (pyrrhotite ± chalcopy- rite ± djerfisherite ± galena ± pentlandite). The chemical compositions of silicate glasses in the melt inclusions vary widely. The glasses are characterized by high Na, K, and Fe contents and low Al contents. They have high total alkali contents (16–23 wt.% Na₂O + K₂O) and peralkalinity index [(Na + K)/Al] ranging from 1.1 to 7.6. The carbonate-rich inclusions in the ijolite minerals are enriched in Na, P, S, and Cl. The data obtained indicate that the parental melt in the intermediate chamber was heterogeneous and contained silicate, natrocarbonate, and sulfide components during the ijolite crystallization. According to heating experiments with melt inclusions, silicate-carbonate liquid immiscibility occurred at temperature over 580 °C.     

Alkalic Pyroxenite Xenoliths from the Lashaine Volcano, Northern Tanzania

The ankaramitic scoria and carbonatite tuffs of the Lashainevolcano, northern Tanzania, contain a suite of alkalic pyroxenitexenoliths, in addition to the previously investigated magnesianlherzolite types. The rocks of the pyroxenite suite, which includemica-dunite and iron-rich lherzolite, consist of varying combinationsof olivine (Fo_86–72), sodic diopside, Ti-pargasite, Ti-phlogopite,ilmenite, chromite, and magnetite. The over-all assemblagesare poorer in alumina than those from other alkalic pyroxenitelocalities. Comparison with the products of experimentally investigatedsystems is difficult because of low alumina, and emphasizesthe need for experimental syntheses on rocks of this type.     

Chemical composition of nyerereite and gregoryite from natrocarbonatites of Oldoinyo Lengai volcano,Tanzania

Alkali carbonates nyerereite, ideally Na₂Ca(CO₃)₂ and gregoryite, ideally Na₂CO₃, are the major minerals in natrocarbonatite lavas from Oldoinyo Lengai volcano, northern Tanzania. They occur as pheno- and microphenocrysts in groundmass consisting of fluorite and sylvite; nyerereite typically forms prismatic crystals and gregoryite occurs as round, oval crystals. Both minerals are characterized by relatively high contents of various minor elements. Raman spectroscopy data indicate the presence of sulfur and phosphorous as (SO₄)²⁻ and (PO₄)³⁻ groups. Microprobe analyses show variable composition of both nyerereite and gregoryite. Nyerereite contains 6.1–8.7 wt % K₂O, with subordinate amounts of SrO (1.7–3.3 wt %), BaO (0.3–1.6 wt %), SO₃ (0.8–1.5 wt %), P₂O₅ (0.2–0.8 wt %) and Cl (0.1–0.35 wt %). Gregoryite contains 5.0–11.9 wt % CaO, 3.4–5.8 wt % SO₃, 1.3–4.6 wt % P₂O₅, 0.6–1.0 wt % SrO, 0.1–0.6 wt % BaO and 0.3–0.7 wt % Cl. The content of F is below detection limits in nyerereite and gregoryite. Laser ablation ICP-MS analyses show that REE, Mn, Mg, Rb and Li are typical trace elements in these minerals. Nyerereite is enriched in REE (up to 1080 ppm) and Rb (up to 140 ppm), while gregoryite contains more Mg (up to 367 ppm) and Li (up to 241 ppm) as compared with nyerereite.     

Ultrabasic Xenoliths and Lava from the Lashaine Volcano, Northern Tanzania

The Lashaine tuff-ring consists of carbonatite tuff and glassyscoria of ankaramitic composition. The pyroclastics encloseejected blocks of country-rock metamorphic rocks and a suiteof ultramafic blocks which are divisible into two groups. Thefirst group, characterized by xenomorphic granular textures,contains rocks comprising varying combinations of pyrope garnet,spinel, magnesian olivine and orthopyroxene, chromiferous diopside,and phlogopite. Analyses are given for garnet lherzolite, lherzolite,harzburgite, and wehrlite and their separate phases. The chemistryof the garnet lherzolite and its phases resembles that of garnetperidotite nodules in kimberlite diatremes, and the A1₂O₂ contentand Ca/Ca+Mg ratio of the clinopyroxenes in the lherzolite andwehrlite indicate more affinities with those in mantle-derivedrocks rather than with peridotites derived by accumulation froma basaltic melt. The phlogopite in a mica garnet lherzolite,that otherwise resembles other mantle garnet peridotites, isan unusual variety containing > 9 per cent TiO₂. The othergroup of ultramafic xenoliths, characterized by cumulate andidiomorphic textures, comprises pyroxenite, with or withoutolivine, mica and amphibole, and mica dunite. Analyses are givenfor a mica dunite and its separate phases. The pressure andtemperature of formation of the various rock-types are estimated,and the relationship of the rocks to each other and to the hostlava is discussed. The chemistry of the host lava is discussedin the light of current experimental data and also in relationto the Northern Tanzania volcanic province. The significanceof the presence of mica in the upper mantle is also discussed.     

Carbonatite Magmatism and Plume Activity: Implications from the Nd, Pb and Sr Isotope Systematics of Oldoinyo Lengai

New Nd (0.51261–0.51268), Pb (²⁰⁶Pb/²⁰⁴Pb: 19.24–19.26),and Sr (0.70437–0.70446) isotopic compositions from tennatrocarbonatite lavas, collected in June 1993 from OldoinyoLengai, the only known active carbonatite volcano, are relativelyuniform, and are similar to data from the 1960 and 1988 flows.Three of the samples contain silicate spheroids, one of whichhas Nd and Sr isotopic ratios similar to host natrocarbonatite,consistent with an origin by liquid immiscibility or the mixingof melts with similar isotopic compositions. Pb isotope datafor two samples of trona are inconsistent with its involvementin the genesis of natrocarbonatite. New Pb isotope data fromsilicate volcanic and plutonic blocks (ijolite, nephelinite,phonolite, syenite) from Oldoinyo Lengai are highly variable(²⁰⁶Pb/²⁰⁴Pb, 17.75–19.34; ²⁰⁷Pb/²⁰⁴Pb, 15.41–15.67;²⁰⁸Pb/²⁰⁴Pb, 37.79–39.67), and define near-linear arraysin Pb-Pb diagrams. The isotopic data for the silicate rocksfrom Oldoinyo Lengai are best explained by invoking discretepartial melting events which generate undersaturated alkalinesilicate magmas with distinct isotopic ratios. Pb isotope ratiosfrom most ijolites and phonolites are predominantly lower andmore variable than from the natrocarbonatites, and are attributedto interaction between silicate melts involving HIMU and EMIsource components and an additional component, such as lower-crustalgranulites, DMM or PREMA (prevalent mantle). Variations in Nd,Pb and Sr isotope ratios from Oldoinyo Lengai, among the largestyet documented from a single volcano, are attributed to mantlesource heterogeneity involving mainly the mixing of HIMU andEMI mantle components. Based on the new isotopic data from OldoinyoLengai and data from other East African carbonatites, and mantlexenoliths, we propose a two-stage model in an attempt to explainthe isotope variations shown by carbonatites in this area. Themodel involves (I) the release of metasomatizing agents withHIMU-like signatures from upwelling mantle (‘plume’)source, which in turn metasomatize the sub-continental (old,isotopically enriched, EMI-like) lithosphere, and (2) variabledegrees and discrete partial melting of the resulting heterogeneous,metasomatized lithosphere. KEY WORDS: carbonatite; isotopes; Oldoinyo Lengai; mantle plumes ^*Telephone: (613) 788–2660, ext. 4419. Fax: (613) 788–4490. e-mail: kbell{at}ccs.carleton.ca     

Metasomatism and Partial Melting in Upper-Mantle Peridotite Xenoliths from the Lashaine Volcano, Northern Tanzania

A group of chrome-spinel peridotite upper-mantle xenoliths fromthe Lashaine volcano, northern Tanzania, differs from otherxenoliths at this locality in containing glassy melt pockets.Modal, mineral chemical and isotopic evidence indicates that,before the melting that was coincident with the xenolith entrainmentand eruption in the Pleistocene, the sub-Tanzanian mantle lithospherehad a complex history. A major element depletion at     

Activity of the carbonatite volcano Oldoinyo Lengai, 1966

From 1960 to August, 1966, the activity of Oldoinyo Lengai took the form of quiet extrusion of carbonatite lava. In August, 1966, the style of activity changed abruptly and violent ash eruptions took place. The activity varied from minor emissions of ash to major Plinian and Vulcanian type eruptions. A new ash-cone built up within the crater and ash was widely distributed on the slopes of the volcano and over the surrounding countryside.The ash consists of sodium carbonate mixed with crystals of nepheline, pyroxene, wollastonite, apatite, melanite and pyrite. Also blocks of ijolite and melteigite were ejected during the activity.

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Zusammenfassung Von 1960 bis zum August 1966 bestand die Tätigkeit des Vulkans Oldoinyo Lengai/Ostafrika in ruhigen Lava-Extrusionen. Im August 1966 änderte sich plötzlich die Art seiner Tätigkeit, und heftige Aschenbrüche fanden statt. Diese Tätigkeit variierte von kleineren Ascheneruptionen bis zu größeren Ausbrüchen plinianischen und vulkanischen Typs. Ein neuer Aschenkegel entstand in dem aktiven Krater, und Asche wurde weithin über die Abhänge des Vulkans und über die Umgebung verteilt.Die Asche besteht aus Natrium-Karbonatit mit einer Beimischung von Kristallen von Nephelin, Pyroxen, Wollastonit, Apatit, Melanit und Pyrit. Während des Ausbruchs wurden auch Ijolith- und Melteigitblöcke ausgeworfen.

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Résumé De 1960 jusqu'en août, 1966, l'activité du volcan Oldoinyo Lengai consistait en coulées tranquilles de lave carbonatitique. En août, 1966, le genre d'activité changea abruptement et de violentes éruptions de cendres se produisirent. L'activité consistait tantôt en de petites émissions de cendres, tantôt en éruptions majeures du genre Plinien et Vulcanien. Un cône neuf de cendres s'amoncelait dans le cratère actif, et les cendres se dispersaient sur le pays environnant.Les cendres se composaient de carbonatite alcaline avec des cristaux de nephéline, pyroxène, wollastonite, melanite et de pyrite. D'ailleurs des blocs d'ijolite et de melteigite furent projetés hors du cratère pendant l'activité.

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday     

Potassium loss during metasomatic alteration of mica pyroxenite from Oldoinyo Lengai,northern Tanzania: contrasts with fenitization

Mica pyroxenite xenoliths, occurring as the cores of nephelinite and ijolite bombs in the pyroclastic deposits of the active volcano Oldoinyo Lengai, have undergone metasomatism in which K was lost and Fe²⁺ and Ti gained. This is unlike the alkali and ferric iron addition that typifies most examples of metastomatism adjacent to peralkaline igneous rocks in carbonatite complexes.     

Eucolite from oldoinyo Lengai,Tanzania

Eucolite occurs in ejected blocks of agpaitic wollastonite-bearing nepheline syenite in the pyroclastics of the active carbonatite volcano Oldoinyo Lengai, northern Tanzania. An analysis of the syenite is given, and chemical, optical and X-ray data for the eucolite are presented. The agpaitic paragenesis at Oldoinyo Lengais is one of the few recorded instances of agpaitic rocks in carbonatite environment.     

Isotopic composition of Sr,Nd, and Pb in pirssonite,shortite and calcite carbonatites from Oldoinyo Lengai volcano,Tanzania

Doklady Earth Sciences -     

Petrology of the Ejected Plutonic Blocks of the Soufriere Volcano, St. Vincent, West Indies

The ejected blocks of the Soufrière volcano consist ofthe mineral phases anorthite (An₉₆-An₈₉; average An₉₃), olivine(Fo₇₉-Fo₆₇; most frequent interval Fo_74-71), salite con taining5–6 per cent Al₂, O₃, hastingsitic amphibole, and magnetitecontaining 6 per cent A1₂O₃, 4 per cent MgO, 7 per cent TiO₂.The minerals occur in various proportions and textures. Theyare virtually unzoned and represent material which has beenejected at, and quenched from, a high temperature. The interstitialscoria present among the mineral grains in the blocks has thecomposition of a saturated sub-alkaline aluminous basalt, andis believed to represent the liquid phase with which the mineralswere in equilibrium at depth. The high-temperature mineralogy,the textures and structures of the rocks support the view thatthe blocks represent crystal cumulates which have crystallizedunder high water-vapor pressures from a fractionating basaltmagma at a depth approximating 6 km. The bulk composition ofthe blocks is such that the resultant liquid fractions are enrichedin silica. Fractional crystallization may be an important factorin the evolution of some calc-alkaline suites.     

Spinel-Lherzolite Xenoliths from the Aritain Volcano,NE-Jordan

Summary Fresh spinel-Lherzolite xenoliths occur within the pyroclastic components of the Aritain volcano, NE-Jordan. The average modal composition of the xenoliths is 67 vol.% olivine, 23 vol. % orthopyroxene, 9 vol. % clinopyroxene and 1.6 vol. % spinel. All xenoliths exhibit a protogranular texture. Selected electron probe analyses of enstatite, diopside, olivine, and spinel are given. Temperatures of their last equilibration range from 925° to 1,025°C. Pressure estimates based on the spinel-Lherzolite stability field restrict the xenolith source to a depth of 37 to 60 km. Up to 18 % X_Cr in spinel indicates an origin from a maximum depth of 60 km.

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Spinell-Lherzolit-Xenolithe aus dem Aritain-Vulkan, NE-Jordanien

Zusammenfassung Die pyroklastischen Gesteine des Aritain-Vulkans, NE-Jordanien, enthalten frische Spinell-Lherzolit-Xenolithe. Der durchschnittliche Mineralbestand der Xenolithe ist 67 Vol. % Olivin, 23 Vol. % Orthopyroxen, 9 Vol. % Klinopyroxen und 1.6 Vol. % Spinell. Alle Xenolithe haben eine grobkörnige Struktur. Repräsentative Analysen von Enstatit, Diopsid, Olivin und Spinell werden mitgeteilt. Die Temperatur ihres letzten Gleichgewichts liegt zwischen 925° und 1.025°C. Druck-Abschätzungen anhand des Stabilitätsfeldes von Spinell-Lherzolit haben ergeben, daß die Xenolithe aus einer Tiefe von 37–60 km kommen. Bis zu 18% X_Cr im Spinell deuten auf eine maximale Tiefe von 60 km hin.

     

Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai'i

A diverse range of crustal xenoliths is hosted in young alkalibasalt lavas and scoria deposits (erupted 3–5 ka) at thesummit of Huallai. Leucocratic xenoliths, including monzodiorites,diorites and syenogabbros, are distinctive among Hawaiian plutonicrocks in having alkali feldspar, apatite, zircon and biotite,and evolved mineral compositions (e.g. albitic feldspar, clinopyroxeneMg-number 67–78). Fine-grained diorites and monzodioritesare plutonic equivalents of mugearite lavas, which are unknownat Huallai. These xenoliths appear to represent melt compositionsfalling along a liquid line of descent leading to trachyte—amagma type which erupted from Huallai as a prodigious lava flowand scoria cone at 114 ka. Inferred fractionating assemblages,MELTS modeling, pyroxene geobarometry and whole-rock norms allpoint to formation of the parent rocks of the leucocratic xenolithsat 3–7 kbar pressure. This depth constraint on xenolithformation, coupled with a demonstrated affinity to hypersthene-normativebasalt and petrologic links between the xenoliths and the trachyte,suggests that the shift from shield to post-shield magmatismat Huallai was accompanied by significant deepening of the activemagma reservoir and a gradual transition from tholeiitic toalkalic magmas. Subsequent differentiation of transitional basaltsby fractional crystallization was apparently both extreme—culminatingin >5·5 km³ of trachyte—and rapid, at 2·75x 10⁶ m³ magma crystallized/year. KEY WORDS: geothermobarometry; magma chamber; xenolith; cumulate; intensive parameters     

Metamorphic Petrology of Xenoliths from Kenya and Northern Tanzania and Implications for Geotherms and Lithospheric Structures

Pyroxenitic and peridotitic xenoliths from the Quaternary volcanicfield of Marsabit (northern Kenya) bear strong evidence of decompressionand cooling. Pyroxenites are mostly garnet (grt) websteritesand grt clinopyroxcnites with some olivine (ol) and amphibole(amph). Grt is mostly rimmed by kelyphitic reaction zones butotherwise appears to have been in stable association with thepyroxenes. Along contacts between grt and rare ol, medium-grainedsymplectites consisting of orthopyroxene (opx), clinopyroxene(cpx), and spinel (spl) occur. Garnets do show significant compositionalvariations from core to rim. Primary pyroxenes are strained,have exsolution lamellae, and are chemically zoned. Integratedcore compositions of pyroxenes and grt compositions yield temperaturesof 1065–950 C and pressures of 28–23 kb (stage1). Pyroxene rims in contact with grt or kelyphite show Ca concentrationssimilar to, but Al concentrations higher than pyroxene rimsremote from garnet. Grt-opx contacts yield pressures of 11.5–9.0kb, and temperatures of 860–770C are obtained from pyroxenerims (stage 2). Peridotites from Marsabit show various stages of transformationfrom the garnet peridotite to the spinel peridotite stabilityfield. On the basis of differences in textures and mineral compositionsthey can be grouped into four types. Type I has a granular textureand contains fine-grained opx-cpx-spl symplectites frequentlysurrounding kelyphite which, in turn, may enclose relict grt.Rare matrix spl has higher Cr/(Cr + Al) ratios (0.25–0.32)than symplectitic spl (0.09). As in grt pyroxenites, matrixpyroxenes are strained, show exsolution lamellae, and have rimcompositions which are dependent on their positions relativeto former garnet. Integrated core compositions of matrix pyroxenessuggest former equilibration temperatures between 1050 and 880Cand pressures between 25 and 19 kb (opx—grt barometryusing composition of relict grt; stage 1). Pyroxene rims yieldsignificantly lower temperatures of 920–785 C (stage2). These P—T estimates and the occurrence of one compositexenolith consisting of type I peridotite and grt pyroxenitepoint to a common P—Tevolution of both grt pyroxenitesand type I peridotites. Granular type II peridotites are characterizedby medium-grained clusters of opx + cpx + spl amph and containmatrix spl, too. Pyroxenes are never strained and are free ofexsolution lamellae. All minerals are homogeneous and thereare no compositional differences between pyroxenes and spinelsof the matrix and those of the spl—opx—cpx clusters.Cr/(Cr+Al) ratios of spl are between 0–07 and 0.11. Two-pyroxenetemperatures are relatively uniform (970–925 C at anassumed pressure of 12 kb; stage 2). Type III peridotites arecoarse-grained granular spl peridotites without any indicationof the former presence of grt. Cr/(Cr + Al) ratios of spl aresimilar to those of peridotite type II. Pyroxenes show minorchemical zoning with Ca increasing in opx but decreasing incpx from core to rim indicating temperatures of 960–900C for pyroxene cores and of up to 1000C