Experimental Petrology of Melilite Nephelinites

Experimental study of natural melilite nephelinite lavas ofintermediate K/Na ratio at low pressure (fo₂ reveals the presenceof a peritectic ‘point’ of distributary type (1090?C)for liquids saturated with leucite, nepheline, and spinel. Withdecreasing temperature on the olivine + melilite cotectic, botholivine and melilite react with such liquids to produce high-calciumpyroxene at the peritectic. Both the olivine + high-calciumpyroxene and melilite + high-calcium pyroxene cotectics arestable at temperatures below the peritectic. Olivines coexistingwith such liquids are much more magnesian than those in comparabletholeiitic liquids. The olivine-liquid Fe-Mg distribution coefficient is a monotonically increasing function of silica activity over the composition range spannedby melilite nephelinite, ugandite, alkali basalt, and tholeiitebasalt liquids. The analogous Fe-Mg distribution coefficientfor melilite and liquid is effectively constant , while that for high-calcium pyroxene and liquidis highly dependent on the chemistry of high-calcium pyroxene(cf., Sack & Carmichael, 1984). Pseudoternary liquidus projectionsof multiply saturated liquids coexisting with nepheline, leucite,and spinel (?olivine?high-calcium pyroxene?melilite) have beenprepared to facilitate graphical analysis of the evolution oflava compositions during hypabyssal cooling. Major element chemicalanalyses and petrographic features of lavas from Mt. Nyiragongo,East Africa and Oahu, Hawaii (e.g., Denaeyer et al., 1965; Wilkinson& Stolz, 1983) confirm the validity of these diagrams andthe systematics established from the experimental data. ^*Reprint requests to R.O. Sack     

Melting relations of some oceanic basalts

The melting relations of a group of oceanic basalts—olivine basalts, tholeiitic to midly alkaline, and high-alumina tholeiites are determined. The liquidi of these rocks follow closely the course of the temperature—iron enrichment relationship of Kilauean thoeliitic basalts, ranging from 1430°C for an olivine enriched tholeiite, to a tholeiite of liquidus temperature of 1190°C, the liquidus phase of the assemblages being either olivine or plagioclase.     

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.     

Parental melts of melilitolite and origin of alkaline carbonatite: evidence from crystallised melt inclusions, Gardiner complex

Perovskite and melilite crystals from melilitolites of the ultramafic alkaline Gardiner complex (East Greenland) contain crystallised melt inclusions derived from: (1) melilitite; (2) low-alkali carbonatite; (3) natrocarbonatite. The melilitite inclusion (1) homogenisation temperature of 1060 °C is similar to liquidus temperatures of experimentally investigated natural melilitites. The compositions are peralkaline, low in MgO (ca.␣5 wt%), Ni and Cr, and they are low-pressure fractionates of more magnesian larnite-normative ultramafic lamprophyre-type melts of primary mantle origin. Low-alkali carbonatite compositions (2) homogenise at 1060–1030 °C and are compositionally similar to immiscible calcite carbonatite dykes derived from the melilitolite magma. Natrocarbonatite inclusions (3) homogenise between 1030 and 900 °C and are compositionally similar to natrocarbonatite lava from Oldoinyo Lengai. Nephelinitic to phonolitic dykes which are related to the calcite carbonatite dykes, are very Zr-rich and agpaitic (molecular Na₂O + K₂O/Al₂O₃ > 1.2) and resemble nephelinites of Oldoinyo Lengai. The petrographic, geochemical and temporal relationships indicate unmixing of carbonatite compositions (ca. 10% alkalies) from evolving melilitite melt and continued fractionation of melilitite to nephelinite. It is suggested that the natrocarbonatite compositions represent degassed supercritical high temperature fluid formed in a cooling body of strongly larnite-normative nephelinite or evolved melilitite. The Gardiner complex and similar melilitolite and carbonatite-bearing ultramafic alkaline complexes are believed to represent subvolcanic complexes formed beneath volcanoes comparable to Oldoinyo Lengai and that the suggested origin of natrocarbonatite may be applied to natrocarbonatites of Oldoinyo Lengai. Received: 18 January 1996 / Accepted: 2 September 1996     

The nephelinitic–phonolitic volcanism of the Trindade Island (South Atlantic Ocean): Review of the stratigraphy,and inferences on the volcanic styles and sources of nephelinites

Trindade Island is located in the South Atlantic Ocean, 1170 km from the Brazilian coast, and represents the eastern end of the E–W Vitória–Trindade Chain. It shows the youngest plume-induced (ca. 3.7 to <0.17 Ma) subaerial volcanism on the South American plate, associated with the Trindade plume activity. Almeida (1961) recognized five volcanogenic successions at Trindade (in decreasing age): the Trindade Complex (TC, >2.4 Ma) and the Desejado (DF, ∼2.4 to 1.5 Ma), Morro Vermelho (MV, <0.17 Ma), Valado (VF, no age) and Paredão (PF, no age) formations, composed of effusive–pyroclastic deposits and subvolcanic intrusions associated with nephelinite–phonolite volcanic episodes. We revised the original Almeida's (1961) stratigraphy with additional field work and petrography to recognize eruptive styles and processes within the nephelinite–phonolite volcanism. Also, available geochemical databases were used to improve the stratigraphic correlation between nephelinites from different units and to characterize their mantle sources.The nephelinitic volcanism may represent Strombolian and Hawaiian–type activity of low viscosity and volatile–rich lavas interlayered with pyroclastic successions (fall–out deposits). Phonolitic deposits record explosive Vulcanian–style episodes of volatile–rich and higher–viscosity lavas interlayered with pyroclastic deposits (mostly pyroclastic flows). Geochemical data allowed the individualization of nephelinites as follows: (1) MV olivine–rich nephelinites and all olivine–free varieties are low K₂O/Na₂O, K₂O/TiO₂ and intermediate CaO/Al₂O₃ that may be derived from N–MORB and HIMU mantle components; (2) the VF olivine–rich nephelinites have high K₂O/Na₂O, K₂O/TiO₂ and CaO/Al₂O₃ that indicates both EM and HIMU mantle sources and; (3) the PF olivine–rich nephelinites show high K₂O/TiO₂ similar to those from VF, and intermediate CaO/Al₂O₃ as nephelinites from MV rocks, suggesting a mixed source with EM + HIMU > N–MORB components.We suggest that the HIMU and EM mantle types resulted from metasomatic episode(s) in the peridotitic mantle beneath the Trindade Island during the Brasiliano Orogeny and later, as previously pointed out by Marques et al. (1999). Thus, the major HIMU component would relate to recycled oceanic crust or lithospheric mantle (mostly CO₂–eclogites) whereas the less important EM component to recycled marine or continental sediments.     

Kalsilite in the lavas of Mt. Nyiragongo (Belgian Congo)

A considerable part of the nephelinite lavas of the volcanoMt. Nyiragongo in the eastern Belgian Congo contains kalsiliteas one of the main constituents. The mineral never occurs asthe only feldspathoid of the rock but is accompanied by nepheline,abundant melilite, and, sometimes, by small to moderate amountsof leucite. Other important constituents of these kalsilite-bearingrocks are clinopyroxene, olivine, perovskite, titanomagnetite,sodalite, &c. The feldspars are lacking. Kalsilite occurs both as complex nepheline-kalsilite phenocrystsin which these phases are strictly co-axial and in the fine-grainedgroundmass as grains separate from those of nepheline. The complex nepheline-kalsilite phenocrysts exhibit a continuousseries of progressing exsolution schematically presented inFig. 5. The series begins with a perthite-like nepheline-kalsilitecore surrounded by a drop-like development of nepheline in themargin of the crystal and ends up with a homogeneous kalsilitecore surrounded by a nepheline margin. The complex phenocrysts occur mostly as aggregates causing atypically glomeroporphyritic texture. Evidence is presentedindicating that, in the very first stages of crystallization,some of the Nyiragongo lavas are able to precipitate small amountsof phenocrysts of approximate composition K₃NaAl₄Si₄O₁₆. Throughcrystal-rise under turbulent currents in the molten lava massthese phenocrysts have been accumulated into aggregates andthus have been preserved until extrusion. Granted sufficientlyslow cooling under static conditions, the phenocrysts wouldhave reacted with the molten lava. The roles of the crystal-riseand of the turbulent currents in lava are illustrated by theoccurrence of the ‘giant’ leucite aggregates foundin the inner walls of the crater and by observations on thelava lake of the mountain. The occurrence of kalsilite in the groundmass is explained bythe existence of a two-phase area in the sub-solidus range inthe nepheline-kalsilite system. The Nepheline Aggregate lavas represent the last extrusionsemitted by the Nyiragongo main crater. The nepheline phenocrystscharacteristic of these lavas range considerably higher in potassiumcontent than the nephelines found in other Nyiragongo flows.The crystals are slightly zoned with a large potassium-richcore coated by a narrow margin with gradually decreasing potassiumcontent. The zoning may be detected only by using special methods.The history of crystallization of the nepheline phenocrystsis considered analogous to that of the complex nepheline-kalsilitephenocrysts with the only difference that the nepheline phenocrystsof the Nepheline Aggregate lavas are less rich in potassiumand, consequently, have not been subjected to exsolution.     

Kalsilite in the lavas of Mt. Nyiragongo (Belgian Congo)

A considerable part of the nephelinite lavas of the volcanoMt. Nyiragongo in the eastern Belgian Congo contains kalsiliteas one of the main constituents. The mineral never occurs asthe only feldspathoid of the rock but is accompanied by nepheline,abundant melilite, and, sometimes, by small to moderate amountsof leucite. Other important constituents of these kalsilite-bearingrocks are clinopyroxene, olivine, perovskite, titanomagnetite,sodalite, &c. The feldspars are lacking. Kalsilite occurs both as complex nepheline-kalsilite phenocrystsin which these phases are strictly co-axial and in the fine-grainedgroundmass as grains separate from those of nephe-line. The complex nepheline-kalsilite phenocrysts exhibit a continuousseries of progressing exsolution schematically presented inFig. 5. The series begins with a perthite-like nepheline-kalsilitecore surrounded by a drop-like development of nepheline in themargin of the crystal and ends up with a homogeneous kalsilitecore surrounded by a nepheline margin. The complex phenocrysts occur mostly as aggregates causing atypically glomeroporphyritic texture. Evidence is presentedindicating that, in the very first stages of crystallization,some of the Nyiragongo lavas are able to precipitate small amountsof phenocrysts of approximate composition K₃NaAl₄Si₄O₁₆. Throughcrystal-rise under turbulent currents in the molten lava massthese phenocrysts have been accumulated into aggregates andthus have been preserved until extrusion. Granted sufficientlyslow cooling under static conditions, the phenocrysts wouldhave reacted with the molten lava. The roles of the crystal-riseand of the turbulent currents in lava are illustrated by theoccurrence of the ‘giant’ leucite aggregates foundin the inner walls of the crater and by observations on thelava lake of the mountain. The occurrence of kalsilite in the groundmass is explained bythe existence of a two-phase area in the sub-solidus range inthe nepheline-kalsilite system. The Nepheline Aggregate lavas represent the last extrusionsemitted by the Nyiragongo main crater. The nepheline phenocrystscharacteristic of these lavas range considerably higher in potassiumcontent than the nephelines found in other Nyiragongo flows.The crystals are slightly zoned with a large potassium-richcore coated by a narrow margin with gradually decreasing potassiumcontent. The zoning may be detected only by using special methods.The history of crystallization of the nepheline phenocrystsis considered analogous to that of the complex nepheline-kalsilitephenocrysts with the only difference that the nephe-line phenocrystsof the Nepheline Aggregate lavas are less rich in potassiumand, consequently, have not been subjected to exsolution.     

Low-pressure fractionation of strongly undersaturated alkaline ultrabasic magma: the olivine-melilite-nephelinite at Moiliili,Oahu, Hawaii

The olivine-melilite-nephelinite at Moiliili, a representative of the Honolulu Volcanic Series, experienced post-eruptive fractional crystallization which resulted in the formation of pegmatoids of restricted volume. The pegmatoids can be equated in composition with melilite-bearing nephelinite (≡melilite-bearing titanaugite melteigite and titanaugite ijolite). Pegmatoid olivine, titaniferous Ca-rich pyroxene and melilite display mild decreases in their 100 Mg/(Mg + Fe) ratios, largely in response to olivine fractionation. Titanomagnetite Fe₂TiO₄ components increased with fractionation, mainly at the expense of Fe₃O₄. Nepheline and sodalite show only restricted compositional variation, although sodalite solid solution is more extensive than in plutonic analogues. Compared with the host olivine-melilite-nephelinite, the pegmatoids are highly depleted in Cr and Ni and enriched in Rb, Sr, Y, Zr, Nb and Ba. The Moiliili fractionation sequence confirms the low-pressure trend olivine-melilite-nephelinite→melilite-bearing nephelinite, previously defined on the basis of relationships in the synthetic system diopside-akermanite-nepheline, but it provides no support for the proposal that olivine-melilite-nephelinite ultimately may fractionate to phonolite. Phonolitic differentiates can be generated by the low-pressure fractionation of alkali feldspar-bearing olivine nephelinites but the relative volumes of the salic derivatives are minor. These volumetric and other constraints inherent in low-pressure fractionation models employing nephelinitic parents suggest that at least some phonolites may be the products of deep crustal or upper mantle anatexis.     

Pyroxenite xenoliths and clinopyroxene megacrysts from the Cenozoic Jbel Saghro Volcanic Field (Anti-Atlas,Morocco): Petrography,mineral chemistry and equilibration conditions

A suite of mafic pyroxenite xenoliths and clinopyroxene megacrysts was brought to the surface by Cenozoic nephelinites of the Jbel Saghro Volcanic Field (Anti-Atlas, Morocco). The large population of samples was subdivided into five groups: (i) clinopyroxenites sensu stricto; (ii) olivine clinopyroxenites; (iii) mica-bearing clinopyroxenites; (iv) kaersutite-bearing clinopyroxenites; (v) clinopyroxene megacrysts. These xenoliths display a cumulate texture (adcumulate, heteradcumulate with poikilitic clinopyroxene including olivine). The clinopyroxenes have the composition of augite and show an appreciable variation of MgO (7.02–14.80 wt.%), TiO₂ (0.58–5.76 wt.%) and Al₂O₃ (2.81–12.38 wt.%) contents in grains. The clinopyroxenes are characterized by convex upward chondrite-normalized REE patterns, they display very similar trace element compositions with low contents of incompatible elements such as Rb (0−0.9 ppm), Ba (0.1–8.3 ppm), Th (0.1−0.3 ppm), U (0.01−0.04 ppm) and Nb (1.3–3.2 ppm). REE contents of the calculated melts in equilibrium with the clinopyroxene megacrysts and clinopyroxene from pyroxenite xenoliths are similar to those of the nephelinites exposed in Jbel Saghro. Crystallization temperatures of pyroxenite xenoliths and clinopyroxene megacrysts range from 950 °C to 1150 °C. Clinopyroxene barometry yielded pressure of crystallization ranging from 0.4 to 0.8 GPa for pyroxenite xenoliths and 0.3 to 0.7 GPa for clinopyroxene megacrysts. This pressure range is in agreement with pyroxenite xenoliths and clinopyroxene megacrysts being crystallized from their parental melts at the lower and upper crust.     

Peralkaline nephelinites

Two fractionation trends in sodic alkaline ultramafic liquids have been predicted from experiments in subsystems of the join Di-Ak-Ne-Lc-Qz. The products of these trends are equated with contrasting suites of peralkaline nephelinites from two nephelinite-carbonatite volcanos of the south Gregory Rift, Shombole (southern Kenya) and Oldoinyo L'engai (northern Tanzania). In both trends, peralkalinity is interpreted to result from fractional crystallization of aluminous clinopyroxene. The Shombole trend has olivine nephelinite as its parental magma, and the differentiation products are mildly peralkaline [(Na+K)/Al1.15] nephelinites. It is the most common lineage observed in nephelinite-carbonatite centres. The Oldoinyo L'engai trend has melilitite or olivine-melilite nephelinite as its parental magma, and produces extremely peralkaline [(Na+K)/Al=1.4–2.3] wollastonite- and combeite- (Na₂ Ca₂Si₃O₉) bearing nephelinites. The presence of a reaction relation between wollastonite and liquid to produce combeite, indicated by corroded wollastonite phenocrysts armoured by combeite in some nephelinites from Oldoinyo L'engai, is confirmed by melting experiments. Combeite nephelinites from Oldoinyo L'engai were erupted simultaneously with natrocarbonatite ash, and are very similar in composition to silicate liquids that have been shown by experiment to be immiscible with natrocarbonatite. Because the L'engai trend is rarely expressed at extrusive centres (combeite has been recorded at only three localities), and combeite nephelinites are highly evolved magmas, it is unlikely that natrocarbonatite is primary to other carbonatite types. It is proposed that carbonatite liquid is exsolved at crustal pressures from a wide range of nephelinitic liquids: Mg-rich carbonatite from primitive, olivine-bearing alkaline ultramafic liquids, Ca-rich carbonatite from olivine-free nephelinites of low peralkalinity, and natrocarbonatite from strongly peralkaline combeite nephelinites.     

The volcanic-subvolcanic rocks of the fernando de noronha archipelago,southern atlantic ocean: Mineral chemistry

Fernando de Noronha archipelago presents an older Remédios Formation with subvolcanic intrusions, belonging to two different alkaline series, the sodic (undersaturated: basanites, tephrites, essexites, tephriphonolites, phonolites), and potassic ones (mildly undersaturated to silicic, with alkali basalts, basaltic trachyandesites, trachyandesites, trachytes), and lamprophyres. The upper Quixaba Formation presents nephelinite flows and basanites. A third minor unit, São José, is constituted by basanites carrying mantle xenoliths. Magnesian olivines occur in the Remédios basanites and alkali basalts, and in nephelinites. Melilites are present as groundmass grains in melilite melanephelinites (MEM). Clinopyroxenes (cpx) are mostly salites to titaniferous salites (Remédios sodic series), grading into aegirines in the differentiated aphyric phonolites. Cpx in the lamprophyres show disequilibrium textures. In the Quixaba flows, cpx are salites, enriched in Mg (especially in MEM). Amphiboles, remarkably, are common in tephriphonolites and phonolites and in basaltic trachyandesites, sometimes with disequilibrum zoning textures, and a conspicuous phase in lamprophyres. Dark micas are present as groundmass plates in MEM, OLM and PYM (olivine and pyroxene melanephelinites), with compositional variety (enriched in Ti, Ba, Sr) depending on the composition of the parent rock; BaO can be as high as 16–19%. Feldspars crystallize as calcic plagioclases, sanidines and anorthoclases, depending on the rock types, as phenocrysts and in groundmass, both in Quixaba and Remédios rocks; they are absent in nephelinites. Nephelines are found in Remédios sodic series types and Quixaba rocks. Haüyne and noseane are rarely observed in Remédios rocks.     

The petrogenesis of basic and ultrabasic alkaline rocks of western Makhtesh Ramon,Israel: geochemistry,melt and fluid inclusion study

The basic and ultrabasic alkaline rocks of western Makhtesh Ramon, Israel crop out in numerous lava flows and subvolcanic bodies. The rock suite is composed of tephrite, basanite, basanitic nephelinite, analcimite, olivine nephelinite, and melilite-olivine nephelinite and in many outcrops is represented by glass-bearing varieties. Melt and fluid inclusions have been studied in olivine, clinopyroxene, and plagioclase phenocrysts. The EP, SIMS and microthermometry methods were used for inclusion study. The geochemical data obtained on glasses of melt inclusions (major, REE, trace elements, volatiles) are compared with the data on whole-rock and groundmass glass compositions. The compositions of melt inclusions reflect the different stages of rock crystallization: the initial products of crystallization are similar to whole-rock compositions whereas final portions of melts are usually enriched in SiO₂, Al₂O₃, and alkalis, and depleted in mafic components. The data on contemporaneous melt and CO₂ inclusions were used for the evaluation of the P–T conditions of rock generation. The following parameters were obtained: tephrite: P = 6.3–7.7 kbar and T = 1,150–1,250°C; basanite: P = 6.6–9.2 kbar and T = 1,150–1,250°C; olivine and analcime-olivine nephelinite: P = 5.6–8.2 kbar and T = 1,150–1,250°C; melilite-olivine nephelinite: 4.0–5.4 kbar and T mainly between 1,150 and 1,200°C. Magma genesis was restricted to P–T conditions of spinel- and plagioclase-lherzolite fields. These data suggest the shallowest depth of magma genesis occurred in Makhtesh Ramon compared to other occurrences of Early Cretaceous magmatism at the Middle East. Differences in the degree of batch partial melting of the same source rocks best explain the diversity of the igneous suite in western Makhtesh Ramon.     

Quantitative Untersuchungen mit der Elektronen-Mikrosonde an Pyroxenen aus Basalten und Peridotit-Einschlüssen

The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis. The composition of the pyroxenes depends on the type of basalt in which they occur. Tholeiitic basalts with normative quartz contain three pyroxenes: orthorombic pyroxenes, pigeonites and augites. All pyroxene phases are zoned and do not show any exsolution. Their Ti and Al contents (Ca-Tschermaks and Ti-augite molecules) are small. All pyroxene phases were formed under disequilibrium with each other and with the melt because of rapid quenching. The sequence of crystallization: orthopyroxene—pigeonite—augite could be established by their Cr content.The alkali olivine basalts undersatured in SiO₂ and the olivine nephelinites are characterized by Ti and Al-rich clinopyroxenes. The distribution of Ti and Al in the pyroxenes of the alkali olivine basalts shows a differentiation trend from the cores of the phenocrysts to their outer zones and to the crystals of the ground mass. Thereby the Ca-Tschermaks molecule is being replaced more and more by the Ti-augite molecule. The Ti content of the pyroxenes of the olivine nephelinites decreases in the last stage of differentiation because simultaneously increasing amounts of titaniferous magnetite crystallize.The pyroxenes of lherzolite peridotite nodules are characterized by high Al and low Ti contents which differ according to the type of basalt (alkali olivine basalt or olivine nephelinite) in which the nodules occur. The homogeneous distribution of the elements within the single grains indicates crystallization under equlibrium conditions. The conditions of their formation are comparable to those of Al-pyroxene peridotites in the upper mantle. The composition of pyroxenes of early accumulates of alkali basaltic melts differ from those of peridotite nodules. Therefore lherzolite nodules can be taken as residues of deeper peridotite masses.     

Ultramafic inclusions and high pressure megacrysts from a nephelinite sill,Nandewar Mountains,north-eastern New South Wales,and their bearing on the origin of certain ultramafic inclusions in alkaline volcanic rocks

Ultramafic inclusions and megacrysts are unusually abundant in a nephelinite sill in the Nandewar Mountains in north-eastern New South Wales. The inclusions are divisible into a Cr-diopside group and a Ti-augite group, the former being dominated by Cr-spinel Iherzolites of restricted modal composition, the latter by olivine and titaniferous Al-rich clinopyroxene assemblages which vary widely in their modal proportions. The principal megacryst species are olivine and black, titaniferous Al-rich clinopyroxene; additional but comparatively rare megacrysts include titanphlogopite, kaersutitic amphibole, and deep green, relatively Fe-rich clinopyroxene. The Cr-spinel Iherzolites conform closely in mineralogy and chemistry with the spinel lherzolites which dominate upper mantle xenolith assemblages in alkaline mafic volcanic rocks from other provinces. Megacrysts and Ti-augite inclusion mineral assemblages are consistently more Fe-rich than analogous phases in the Cr-diopside xenoliths and also display more extensive cryptic variation. The available experimental data on the high pressure liquidus or near-liquidus phases in olivine nephelinite and related compositions indicate that the olivine and black clinopyroxene megacrysts were precipitated at pressures in the vicinity of 15–20 kb. The similarity in the nature and compositions of the principal megacryst species to analogous phases in the Tiaugite group of inclusions indicates that the latter also represent cognate cumulates derived from the olivine nephelinite at broadly comparable pressures. High pressure fractionation of the host olivine nephelinite liquid, controlled mainly by the separation of olivine and aluminous clinopyroxene, produced only comparatively minor compositional changes in the derivative liquid. The hiatus in olivine compositions at approximately Fo_86–88, apparently characteristic of the olivines in coexisting Cr-diopside and Ti-augite inclusions, is assessed in terms of the compositions of olivine in equilibrium with alkali basaltic liquids at high pressures.     

Ultramafic inclusions in basaltic rocks from Hawaii

Ultramafic inclusions and the enclosing basaltic rocks were collected from a number of localities in the Hawaiian Islands; these and other specimens were studied by standard pétrographic techniques and with an electron microprobe. Emphasis was on determination of mineral assemblages, mineral compositions, and variations in composition. Sixty-eight inclusions and thirteen basaltic rocks are described, with partial chemical analyses (Ti, Al, Cr, Fe, Mn, Ni, Mg, Ca, Na, K) of olivines, orthopyroxenes, clinopyroxenes, and some feldspars and other minerals. Inclusions range from dunite to anorthosite, and basaltic hosts range from olivine nephelinite to olivine tholeiite. The inclusions are separable into three categories, which correlate with three groups basaltic hosts: Lherzolite inclusions are relatively poor in Fe, and the component minerals have limited ranges of composition. In Hawaii, lherzolite inclusions occur preferentially in extremely undersaturated hosts (olivine nephelinite, nepheline basanite, and ankaratrite). Other varieties of inclusions (dunite, wehrlite, feldspathic peridotite, pyroxenite) are relatively rich in Fe, and the component minerals have wider ranges of composition. These inclusions, together with gabbro, occur preferentially in hosts which are but moderately undersaturated (alkaline olivine basalt, hawaiite, and ankaramite). The sparse inclusions in nearly-saturated basalt (olivine tholeiite) are petrographically distinct from those in the other two categories. These correlations suggest that the inclusions and the enclosing basaltic rocks are genetically related. As the three suites of inclusions differ chemically, mineralogically, physically, and texturally, more than one origin is probable.     

Entrained Macrocryst Minerals as a Key to the Source Region of Olivine Nephelinites: Humberg, Kaiserstuhl, Germany

Olivine nephelinites commonly contain macrocrysts of olivineand clinopyroxene. Some of these macrocrysts might representfragments of the source region of the host magma transportedto the Earth's surface. If this hypothesis is correct thesefragments can be used to characterize the composition of thesource region and to put constraints on the magma generationprocess. In this study, we investigate the origin of macrocrystsand mineral aggregates from an olivine nephelinite from theKaiserstuhl, Germany. We focus on clinopyroxenes (Cpx), whichcan be divided into three groups. Cpx I is relict Cpx from aggregateswith deformed olivine that is depleted in Ca and characterizedby strong light rare earth element (LREE) fractionation, lowTi/Eu and negative high field strength element (HFSE) anomalies.Its geochemical signature is consistent with formation by carbonatitemetasomatism and with equilibration in the presence of orthopyroxene.Cpx II is Ca-rich Cpx, forming both aggregates with deformedolivine and individual macrocrysts. The LREE, as for Cpx I,are strongly fractionated. Convex REE patterns may be present.The depletion in HFSE is less pronounced. Cpx III is oscillatoryzoned Cpx phenocrysts showing enrichment in Ca, convex REE patternsand no HFSE anomalies. The transition in the trace element abundancesbetween the Cpx of the three groups is gradual. However, CpxI and II did not crystallize from the host magma, as demonstratedby the presence of kink-bands and undulose extinction in theassociated olivine and by the composition of alkali aluminosilicateglass inclusions in Cpx II. Based on the Cpx relationships,we interpret the studied suite of macrocrysts and mineral aggregatesas a mixture of disintegrated fragments of the source regionof the host olivine nephelinite. The process of melt generationwas multi-stage. A primary carbonatite melt ascending from deeperlevels in the mantle, probably from the dolomite–garnetperidotite stability field, reacted with mantle peridotite alongthe solidus ledge in the system lherzolite–CO₂ (< 20–22kbar) and started to crystallize carbonate minerals. Becauseof its low solidus temperature, the resulting carbonate-wehrliteassemblage melted incongruently with the formation of additionalclinopyroxene. The carbonatite melt evolved during crystallizationof carbonate minerals and concomitant incongruent melting ofthe carbonate-wehrlite, accompanied by the segregation of incipientalkali aluminosilicate melts. As a consequence of fast reactionrates in the presence of a carbonatite melt, this process probablytook place under disequilibrium conditions. Further meltingof the assemblage wehrlite + alkali aluminosilicate melt ledto the generation of the olivine nephelinite magma. It entrainedfragments of the wehrlite and brought them to the surface. KEY WORDS: carbonatite; metasomatism; source region; clinopyroxene macrocrysts; wehrlite; olivine nephelinite; Kaiserstuhl     

The system Mg2SiO4-Ca2SiO4-CaAl2O4-NaAlSiO4-SiO2: one atmosphere liquidus equilibria of analogs of alkaline mafic lavas

One atmosphere liquidus relationships in the system Mg₂SiO₄ (Fo)–Ca₂SiO₄ (La)–NaAlSiO₄ (Ne)–CaAl₂O₄ (CA)–SiO₂ (Sil) are presented as analogs for alkaline mafic lavas. Liquidus diagrams are constructed from electron microprobe analyses of quenched liquids and coexisting mineral phases produced in melting experiments and they are depicted in terms of sub-projections within the pseudo-quaternary system Fo–La–[Ne,CA]–Sil. The Ne and CA components are combined to create a normative feldspathoid component defined as Ne#=Ne/[Ne+CA]. Liquidus relations at Ne#=0.5 from this study are compared to relations at Ne#=0.0 and 1.0 from previous studies. In general, liquidus temperatures decrease and positions of liquidus boundaries involving feldspathic phases shift toward the [Ne, CA] component as Ne# increases. The pseudoinvariant points fo+di+pl+mel+l and fo+pl+mel+sp+l exist at Ne#=0.5. These equilibria between forsterite-plagioclase-melilite-liquid are not present in the system when Ne#=1.0 because a boundary curve (fo+di+ne+l) separates the plagioclase and melilite liquidus fields. The liquidus diagrams provide useful analogs for the crystallization sequences of natural primary alkali olivine basalts, basanitoids, basanites, olivine nephelinites, olivine-melilite nephelinites and olivine melilitites.     

Oxygen chemical diffusion in three basaltic liquids at elevated temperatures and pressures

The diffusivity of oxygen has been measured in three basaltic liquids from 1280 to 1450°C and 4 to 21 kilobars using a solid media piston-cylinder apparatus. The measurements were done by monitoring the reduction of ferric iron in previously oxidized spheres of basalt melt. The compositions studied were olivine nephelinite, alkali basalt, and 1921 Kilauea tholeiite.The isobaric temperature dependence of oxygen diffusivity is adequately described by Arrhenius relationships for the three liquids studied. Arrhenius activation energies were determined at 12 kilobars for olivine nephelinite (62± 6 kcal/mole) and tholeiite (51 ± 4 kcal/mole) and at 4, 12, and 20 kilobars for alkali basalt (70 ± 7, 86 ± 6, and 71 ± 14 kcal/mole, respectively). The Arrhenius parameters for the three compositions define a compensation law which is indistinguishable from those for oxygen diffusion in simple silicate melts (DUNN, 1982) and for divalent cation diffusion in basaltic melts (Hofmann, 1980). These results suggest that the principal species contributing to the total diffusivity of oxygen is the oxide anion (O^2?).The isothermal pressure dependence of oxygen diffusion is complex and quite different from that observed for cationic diffusion in silicate melts. All three compositions show a sharp decrease in oxygen diffusivity at approximately the same pressure as the change in the liquidus phase from olivine to pyroxene, but otherwise the pressure dependence can be described by Arrhenius type equations. The equations yield negative activation volumes for the olivine nehpelinite and the alkali basalt. The activation volumes determined for the tholeiite are near zero at low pressure and positive at high pressure. A negative activation volume represents a decrease in the average size of the principal diffusing species.The results of this study are consistent with a melt model which includes both continuous changes in the relative proportions of the various anionic species in the melt with pressure and the occurrence of anionic disproportionation reactions within narrow pressure ranges.     

The Role of Partial Melting in the 15-Ma Geochemical Evolution of Gran Canaria: A Blob Model for the Canary Hotspot

The subaerial portion of Gran Canaria, Canary Islands, was builtby three cycles of volcanism: a Miocene Cycle (8•5–15Ma), a Pliocene Cycle (1•8–6•0 Ma), and a QuaternaryCycle (1•8–0 Ma). Only the Pliocene Cycle is completelyexposed on Gran Canaria; the early stages of the Miocene Cycleare submarine and the Quaternary Cycle is still in its initialstages. During the Miocene, SiO₂ saturation of the mafic volcanicsdecreased systematically from tholeiite to nephelinite. Forthe Pliocene Cycle, SiO₂ saturation increased and then decreasedwith decreasing age from nephelinite to tholeiite to nephelinite.SiO₂ saturation increased from nephelinite to basanite and alkalibasalt during the Quaternary. In each of these cycles, increasingmelt production rates, SiO₂ saturation, and concentrations ofcompatible elements, and decreasing concentrations of some incompatibleelements are consistent with increasing degrees of partial meltingin the sequence melilite nephelinite to tholeiite. The maficvolcanics from all three cycles were derived from CO₂-rich garnetlherzolite sources. Phlogopite, ilmenite, sulfide, and a phasewith high partition coefficients for the light rare earth elements(LREE), U, Th, Pb, Nb, and Zr, possibly zircon, were residualduring melting to form the Miocene nephelinites through tholeiites;phlogopite, ilmenite, and sulfide were residual in the sourceof the Pliocene–Quaternary nephelinites through alkalibasalts. Highly incompatible element ratios (e.g., Nb/U, Pb/Ce,K/U, Nb/Pb, Ba/Rb, Zr/Hf, La/Nb, Ba/Th, Rb/Nb, K/Nb, Zr/Nb,Th/Nb, Th/La, and Ba/La) exhibit extreme variations (in manycases larger than those reported for all other ocean islandbasalts), but these ratios correlate well with degree of melting.Survival of residual phases at higher degrees of melting duringthe Miocene Cycle and differences between major and trace elementconcentrations and melt production rates between the Mioceneand Pliocene tholeiites suggest that the Miocene source wasmore fertile than the Pliocene–Quaternary source(s). We propose a blob model to explain the multi-cycle evolutionof Canary volcanoes and the temporal variations in chemistryand melt production within cycles. Each cycle of volcanism representsdecompression melting of a discrete blob of plume material.Small-degree nephelinitic and basanitic melts are derived fromthe cooler margins of the blobs, whereas the larger-degree tholeiiticand alkali basaltic melts are derived from the hotter centersof the blobs. The symmetrical sequence of mafic volcanism fora cycle, from highly undersaturated to saturated to highly undersaturatedcompositions, reflects melting of the blob during its ascentbeneath an island in the sequence upper margin-corelower margin.Volcanic hiatuses between cycles and within cycles representperiods when residual blob or cooler entrained shallow mantlematerial fill the melting zone beneath an island.