Experimental study of a jotunite (hypersthene monzodiorite): constraints on the parent magma composition and crystallization conditions (P,T, f O 2) of the Bjerkreim-Sokndal layered intrusion (Norway)

The Bjerkreim-Sokndal layered intrusion is part of the Rogaland anorthosite Province of southern Norway and is made of cumulates of the anorthositemangerite-charnockite suite. This study presents experimental phase equilibrium data for one of the finegrained jotunite (Tjörn locality) occurring along its northwestern lobe. These experimental data show that a jotunitic liquid similar in composition to the Tjörn jotunite, but slightly more magnesian and with a higher plagioclase component is the likely parent of macrocyclic units (MCU)III and IV of the intrusion. The limit of the olivine stability field in the experimentally determined phase diagram as well as comparison of the Al₂O₃ content of low-Ca pyroxenes from experiments and cumulates (1.5%) yields a pressure of emplacement 5 kbar. Experimentally determined Fe-Ti oxide equilibria compared to the order of cumulus arrival in the intrusion show that the oxygen fugacity was close to FMQ (fayalite-magnetite-quartz) during the early crystallization. It subsequently decreased relative to this buffer when magnetite disappeared from the cumulus assemblage and then increased until the reentry of this mineral. Calculated densities of experimental liquids show a density increase with fractionation at 7, 10 and 13 kbar due to the predominance of plagioclase in the crystallizing assemblage. At 5 kbar and 1 atm (FMQ-1), where plagioclase is the liquidus phase, density first increases and then drops when olivine (5 kbar) or olivine+ilmenite (1 atm: FMQ-1) precipitate. At 1 atm and NNO (nickel-nickel oxide), the presence of both magnetite and ilmenite as near liquidus phases induces a density decrease. In the Bjerkreim magma chamber, oxides are early cumulus phases and liquid density is then supposed to have decreased during fractionation. This density path implies that new influxes of magma emplaced in the chamber were both hotter and denser than the resident magma. The density contrast inferred between plagioclase and the parent magma shows that this mineral was not able to sink in the magma, suggesting anin situ crystallization process.     

Experimental constraints on the effects of pressure and H2O on the fractional crystallization of high-Mg island arc basalt

H₂O-undersaturated melting experiments of synthesized basalt (SiO₂ = 50.7 wt.%, MgO = 8.3 wt.%, Mg# = 60) were conducted at fO₂ corresponding to NNO+1 and NNO−1 to clarify the effects of pressure (2–7 kbar) and H₂O on fractional crystallization in island arcs. H₂O content was ranged from nominally anhydrous to 4.4 wt.%. Differentiation trends, namely the liquid lines of descent, change sensitively according to pressure-H₂O relations. Tholeiitic differentiation trends are reproduced with H₂O ≤ ∼2 wt.% in primary magma. With such quantities of H₂O, fractional crystallization is controlled by olivine + plagioclase at 2 kbar. Increasing the pressure from 2 to ≥4 kbar induces early crystallization of orthopyroxene instead of olivine and therefore SiO₂ enrichment in the residual melts is suppressed. Increasing H₂O (≥ ∼2 wt.% in primary magma) stabilizes clinopyroxene relative to orthopyroxene and/or magnetite. Although the phase relations and proportions strongly depend on fO₂ and H₂O content, differentiation trends are always calc-alkaline.     

Phenocryst-bulk rock composition relations of abyssal tholeiites and their petrogenetic significance

In order to infer equilibrium phase relations of abyssal tholeiites, olivine, plagioclase, augite, and pigeonite tholeiites from the ocean floor are plotted in terms of the CIPW norm proportions in the tetrahedron olivine-plagioclase-diopside-quartz. The phase relations of abyssal tholeiites have a general similarity in form to those of the experimentally studied relevant systems. Experimental studies on natural basalts allow the pressure of crystallization for abyssal tholeiitic magmas to be evaluated approximately. It appears that the pressure at which the phenocryst-stage crystallization of abyssal tholeiites takes place is as high as 2 or 3 kbar, provided that abyssal tholeiitic magmas are ‘dry’.Abyssal tholeiites could be derived from liquids that are in equilibrium with Ca-poor pyroxene in the pressure range of about 5–8 kbar. Major element chemistry of abyssal tholeiites is incompatible with the view that these tholeiitic basalts are derived from picritic magma by olivine fractionation.     

Crystallochemistry and origin of pyroxenes in komatiites

We present a detailed mineralogical and major- and trace-element study of pyroxenes in two Archean komatiitic flows in Alexo, Canada. The pyroxenes in spinifex-textured lavas commonly are zoned with cores of magnesian pigeonite and rims of augite. Concentrations of incompatible trace elements are low in pigeonite and jump to higher values in the augite mantles, a variation that can be modelled using accepted partition coefficients and assuming crystallization from komatiitic liquids. Crystallization sequences are very different in different parts of both flows. In the flow top, the sequence is olivine followed by augite: deeper in the spinifex sequence, pigeonite crystallizes after olivine, followed by augite; in lower cumulates, orthopyroxene or augite accompany olivine. In spinifex lavas, pigeonite crystallizes sooner than would be predicted on the basis of equilibrium phase relations. We propose that contrasting crystallization sequences depend on the position in the flow and on the conditions of crystal growth. In the flowtop, rapid cooling causes quench crystallization. Deeper in the spinifex layer, constrained growth in a thermal gradient, perhaps augmented by Soret differentiation, accounts for the early crystallization of pigeonite. The cumulus minerals represent a near-equilibrium assemblage. Augites in Al-undepleted Archean komatiites in various localities in Canada and Zimbabwe have high moderate to high Wo contents but their Mg# (Mg/(Mg + Fe) are lower than in augites in komatiites from Barberton, South Africa. We attribute the combination of high Wo and high Mg# in Barberton rocks to the unusually high CaO/Al₂O₃ of these Al-depleted komatiites.     

Formation of shoshonites from calcalkaline basalt magmas: geochemical and experimental constraints from the type locality

Independence volcano, Montana is a major center of the Absaroka volcanic field, from which absarokite, shoshonite, and banakite were originally defined. One magmatic trend at Independence volcano, from high-alumina tholeiitic basalt through shoshonite to high-K dacite, may be modeled by fractional crystallization of observed phenocryst phases (plagioclase, hypersthene, augite, and magnetite). Trace-element and Sr and Nd isotopic compositions of rocks are consistent with this model.Compositions of partial melts from experiments on four rocks at 1 atm and at 10 kbar demonstrate that rock compositions represent a nearly-anhydrous liquid line of descent at a pressure much closer to 10 kbar than to 1 atm. The line of descent involves crystallization of orthopyroxene, not olivine, resulting in strong enrichment in K₂O with little increase in SiO₂. Crystallization at either lower pressures or with water present, involving olivine, results in enrichment in both SiO₂ and K₂O.High-pressure (10 kbar) fractional crystallization of basaltic magma, resulting in formation of shoshonites, may occur at the base of thick crust (e.g., in continental interiors or in very mature arcs). At least a portion of the relationship between K₂O content of arc-related magmas and depth to the Benioff Zone may be attributed to thickening of crust towards the back-arc, resulting in higher pressures of fractionation in Moho-level chambers.     

Mineralogy as a function of depth in the prehistoric Makaopuhi tholeiitic lava lake,Hawaii

The electron probe X-ray microanalyzer has been used to determine the compositional variability of the groundmass minerals and glass in 10 specimens from a complete 225-foot section of the prehistoric tholeiitic lava lake of Makaopuhi Crater, Hawaii. The order of beginning of crystallization was: (1) chromite, (2) olivine, (3) augite, (4) plagioclase, (5) pigeonite, (6) iron-titanium oxides and orthopyroxene, (7) alkali feldspar and apatite, and (8) glass.Although the lake is chemically tholeiitic throughout, the occurrence of ferromagnesian minerals is as though there were a gradation from alkali olivine basalt in the upper chill downwards to olivine tholeiite. Groundmass olivine decreases downwards and disappears at about 20 feet. Pigeonite is absent in the uppermost 5±2 feet, then increases in amount down to 20 feet, below which augite and pigeonite coexist in constant 21 proportions. Strong zoning and metastable compositions characterize the pyroxenes of the chilled zones, but these features gradually disappear towards the interior of the lake to give way to equilibrium pyroxenes. Relatively homogeneous poikilitic orthopyroxene ( Ca₄Mg₇₀Fe₂₆) occurs in the olivine cumulate zone, having formed partly at the expense of pre-existing olivine, augite, and pigeonite ( Ca₈Mg₆₆Fe₂₆). The growth of orthopyroxene is believed to have been facilitated by the slower cooling rate and higher volatile pressure at depth, and by the rise in Mg/Fe ratio of the liquid due to the partial dissolution of settled olivine.Unlike olivine and pyroxene, feldspar is least zoned in the upper and lower chilled regions. The greatest range of compositional zoning in feldspar occurs at 160 to 190 feet, where it extends continuously from Or_1.0Ab₂₂An₇₇ to Or₆₄Ab₃₃An₃. The feldspar fractionation trend in the An-Ab-Or triangle gradually shifts with depth toward more equilibrium trends, even though the zoning becomes more extreme. The variation with depth in the initial (core) composition of the plagioclase suggests the influence of either slow nucleation and growth (undercooling) or slow diffusion in the liquid, relative to the rate of cooling.Idiomorphic opaque inclusions in olivine phenocrysts are chrome-spinels showing continuous variation from 60 percent chromite to 85 percent ulvospinel and to magnetite-rich spinel. A pre-eruption trend of increasing Al with decreasing Cr can be recognized in chromites from the upper chill. Most of the inclusions show a trend of falling Cr and Al, toward an ulvospinelmagnetite solid solution which is progressively poorer in Usp with depth. This trend was produced by solid state alteration of the chromite inclusions during cooling in the lava lake. Ilmenite (average Ilm₉₁Hm₉) coexists with variably oxidized titaniferous magnetite in the basalt groundmass. Estimated oxygen fugacities agree well with other independent determinations in tholeiitic basalt. No sulfide phase has been detected.Fractional crystallization produced a groundmass glass of granitic composition. Average, in percent, is: SiO₂, 75.5; Al₂O₃, 12.5; K₂O, 5.7; Na₂O, 3.1; CaO, 0.3; MgO, 0.05; total FeO, 1.2; and TiO₂, 0.8. Normative Or> Ab. Minor changes in glass composition with depth are consistent with a greater approach towards the granite minimum. Incipient devitrification precluded reliable analysis of glass from the lower half of the section. The SiO₂-phase associated with devitrification contains alkalis and Al and is believed to be cristobalite. Needle-like apatite crystals in the groundmass glass are Siand Fe-bearing fluorapatites containing appreciable rare earths (predominantly Ce) and variable Cl.The grain-size and maximum An content of the cores of plagioclase grains were controlled by cooling rate and are at a maximum at the center of the section. The most homogeneous pyroxene (and olivine, Moore and Evans, 1967), most equilibrium pyroxene trends, most abundant alkali feldspar, and most equilibrium feldspar trends are found at 160 to 190 feet, which is appreciably below that part of the lake which was slowest to crystallize. Volatile pressure, increasing with depth, possibly controlled the degree of attainment of equilibrium more than cooling rate.Since they are dependent on cooling history, some of the modal criteria commonly used for recognizing basalt types, such as the absence of Ca-poor pyroxene, presence of groundmass olivine, and the presence of alkali feldspar, should be applied with caution. Petrographic comparison of basalts from one flow, volcano, or province, with another, should recognize the possible variations due to cooling history alone.Publication authorized by the Director, U.S. Geological Survey     

Experimental study of a low-alkali tholeiite at 1–5 kbar: optimal condition for the crystallization of high-An plagioclase in hydrous arc tholeiite

We conducted melting experiments on a low-alkali tholeiite (SiO₂ ~52 wt%, MgO ~6.5 wt%, CaO/Na₂O~4.4, Al₂O₃/SiO₂ ~0.33) under both H₂O-undersaturated and H₂O-saturated conditions to investigate the effect of H₂O on the Ca–Na partitioning between plagioclase and melt. Experiments were performed in the temperature and pressure ranges of 1,000–1,300°C and 1–5 kbar, respectively, with varying H₂O contents of 0–12wt%. Redox condition was 0–2 log unit above NNO (nickel–nickel oxide) buffer. Temperature-bulk H₂O diagrams for the low-alkali tholeiite are constructed at 1, 2, and 5 kbar, and compositions of near-liquidus plagioclase and coexisting melt are determined. To exclude the effect of melt composition (CaO/Na₂O and Al₂O₃/SiO₂ ratios) on plagioclase composition and to reveal the effect of H₂O on An (=100×Ca/(Ca+Na)) content and (=(Ca/Na)_pl/(Ca/Na)_melt), we focused on the composition of near-liquidus plagioclases which crystallized from melts with nearly constant CaO/Na₂O and Al₂O₃/SiO₂ ratios. Our experimental results show that, at each experimental pressure, An content of the near-liquidus plagioclase and the K_D^Ca-Na almost linearly increases as H₂O content in melt increases. Each of the An content and the variations in a low-alkali tholeiitic system (CaO/Na₂O~4.0–4.5, Al₂O₃/SiO₂ ~0.27–0.33) can be described by one equation using temperature, pressure, and melt H₂O content as parameters. An content and of liquidus plagioclase increases with increasing melt H₂O and with decreasing pressure, elucidating that nearly H₂O-saturated conditions of 2–3 kbar is optimal for the crystallization of the most An-rich plagioclase (>An₈₈). We suggest this pressure condition of 2–3 kbar, corresponding to depth of 7–11 km, plays an important role for the origin of An-rich plagioclase in H₂O-rich low-alkali tholeiite. At pressures more than ca. 4 kbar, crystallization of liquidus Ca-rich clinopyroxene decreases the CaO/Na₂O ratio of liquid, thus prohibiting the crystallization of high-An plagioclase from hydrous tholeiite.     

A petrological study of the younger Tongan andesites and dacites,and the olivine tholeiites of Niua Fo'ou Island,S. W. Pacific

Basaltic andesites are the dominant Tongan magma type, and are characterized by phenocrysts of augite, orthopyroxene (or rarely pigeonite), and calcic plagioclase (modally most abundant phase, and interpreted as the liquidus phase). The plagioclase phenocrysts exhibit slight oscillatory reverse zoning except for abrupt and thin more sodic rims, which are interpreted to develop during eruptive quenching. These rim compositions overlap those of the groundmass plagioclase. The pyroxene phenocrysts also exhibit only slight compositional zoning except for the outermost rim zones; the compositions of these rims, together with the groundmass pyroxenes, vary throughout the compositional range of subcalcic augite to ferroaugite through pigeonite to ferropigeonite, and are interpreted in terms of quench-controlled crystallization. This is supported, for example, by the random distribution of Al solid solution in the groundmass pyroxenes, compared to the more regular behaviour of Al in the phenocryst pyroxenes. The analysed Niua Fo'ou olivine tholeiites are aphyric; groundmass phases are plagioclase (An_17–88), olivine (Fa_18–63), titanomagnetite (usp. _59–73), and augite-ferroaugite which does not extend to subcalcic compositions; this is interpreted to be due to higher quenching temperatures and lower viscosities of these tholeiites compared to the basaltic andesites.Application of various geothermometers to the basaltic andesites suggest initial eruptive quenching temperatures of 1,008–1,124 ° C, plagioclase liquidus temperatures (1 bar) of 1,210–1,277 ° C, and orthopyroxene-clinopyroxene equilibration of 990–1,150 ° C. These calculated temperatures, together with supporting evidence (e.g. absence of olivine and amphibole, liquidus plagioclase, and plagioclase zoning patterns) are interpreted in terms of phenocryst crystallization from magmas that were either strongly water undersaturated, nearly anhydrous, or at best, water saturated at very low pressures (< 0.5 kb). This interpretation implies that these Tongan basaltic andesites did not originate by any of the currently proposed mechanisms involving hydrous melting within or above the Benioff zone.     

Peralkaline nephelinites. I. Comparative petrology of Shombole and Oldoinyo L'engai,East Africa

Shombole, a nephelinite-carbonatite volcano in south Kenya, erupted silicate lavas, carbonatite dikes and tuffs, and pyroclastic rocks similar to those at other East African alkaline centres. Shombole lavas containing cpx + nepheline + accessory minerals range from perovskite-bearing nephelinites (43% SiO₂, volatile-free) to sphene-bearing and phonolitic nephelinites (46–49% SiO₂) and phonolites (49–56% SiO₂) and have low peralkalinity ([Na+K]/Al 1.15) which does not correlate with SiO₂. Early fractionation of olivine and clinopyroxene strongly depleted Ni and Cr concentrations (10 ppm); fractionation of perovskite, melanite, sphene, and apatite produced negative correlations of all REE with SiO₂. Many lavas contain cognate intrusive xenoliths and xenocrysts and oscillatory zoning is a common feature of clinopyroxene, nepheline, and melanite crystals, indicating recycling of intrusive material. Irregular calcite-rich bodies in many samples are interpreted as quenched immiscible Ca-carbonatite liquid, and [Ca-carbonate]-silicate liquid immiscibility is observed in experiments with one nephelinite. Chemical variation in the Shombole suite can be modeled as a combination of crystal fractionation (clinopyroxene and heavy minor phases) and retention of neutral density nepheline derived from disaggregated xenoliths entrained during emplacement of dike swarms. Six newly analyzed lavas from Oldoinyo L'engai, northern Tanzania, are geochemically similar to Shombole nephelinites except that they have relatively high Na₂O+K₂O (average 18% vs 12%) and Zr (average 680 ppm vs 400 ppm). They are extremely peralkaline and are not typical of nephelinites from other centres. Three with [Na+K]/Al1.5 contain euhedral wollastonite phenocrysts; three with [Na+K]/Al2.0 contain combeite (Na₂Ca₂Si₃O₉) phenocrysts and pseudomorphs after wollastonite. Both types contain abundant sodalite phenocrysts (+nepheline+clinopyroxene+melanite+sphene). Seven other wollastonite nephelinite samples from L'engai have been described, but it is a lava type rarely seen in other centres. Combeite has been described from only two other locations (Mt. Shaheru, Zaire; Mayener Feld, Eifel). The hyperalkaline L'engai nephelinites have compositions similar to those of experimental silicate liquids immiscible with natrocarbonatite. Textural evidence for both carbonate-silicate (as carbonate globules) and silicate-silicate (as two optically discrete glasses with distinct compositions) liquid immiscibility is observed in the samples.     

Simulation of the geochemical structure of the Ioko-Dovyren layered intrusion,northwestern Baikal area

The processes of differentiation in the magmatic chamber of the Ioko-Dovyren layered dunite-troctolite-gabbro-gabbronorite massif were simulated using the COMAGMAT-3.5 software package, which is based on the convection-accumulation model for the crystallization of magmatic intrusions. The initial magma composition was assumed to be equal to the weighted mean composition of the rocks composing the intrusion (wt %: 43.92 SiO₂, 9.72 Al₂O₃, 10.53 FeO, 27.88 MgO, 6.99 CaO, 0.59 Na₂O, 0.07 K₂O, and 0.11 TiO₂). The results obtained by simulating the crystallization of this composition within a pressure range of 0–10 kbar indicate that the crystallization sequence determined for the rocks Ol + Chr → Ol+ Pl+ Chr → Ol + Pl+ CPx → ± Ol + Pl+ CPx + LowCaPx in an anhydrous system takes place under pressures of 0–2 kbar. A series of simulations for a system closed with respect to oxygen yielded estimates for the phase and chemical composition of the emplaced magma and the parameters of the optimum model, which reproduces accurately enough the geochemical structure of the Ioko-Dovyren intrusion: the naturally occurring distributions of minerals and components in its vertical section. The correlation coefficients between the concentrations of oxides determined in the rocks and calculated within the model are \(r_{MgO,Al_2 O_3 ,CaO} \) ≥ 0.9 and \(r_{FeO,SiO_2 ,Na_2 O} \) ≥ 0.6. The simulated phase composition of the magma during its emplacement corresponded to melt + olivine (Fo ₈₉). The crystallinity of the parental magma was determined to have been equal to approximately 40 vol % at an assumed cumulus density of 90% near the lower contact and 70% near the upper one. The temperature of the magma during its emplacement was close to 1340°C at a pressure of 1 kbar. In the model, plagioclase and clinopyroxene appear on the liquidus at T?1255°C at T?1210°C, respectively, and the crystallization sequence of cumulus minerals corresponds to that observed in nature. The liquid phase (melt) of the parental magma during its emplacement had the following composition (wt %): 45.95 SiO₂, 15.93 Al₂ O₃, 14.49 MgO, 10.88 FeO, 11.46 CaO, 0.97 Na₂O, 0.11 K₂O, and 0.18 TiO₂. Our results confirm the plausibility of the hypothesis that the inner structure of the Ioko-Dovyren intrusion was formed by the emplacement and differentiation of a single magma portion with no less than 40 vol % crystallinity.     

New experimental data for the system CaO-MgO-SiO2-H2O and a synthesis of inferred phase relations

Subsolidus and vapor-saturated liquidus phase relations for a portion of the system CaO-MgO-SiO₂-H₂O, as inferred from experimental data for the composition regions CaMgSi₂O₆-Mg₂SiO₄-SiO₂-H₂O and CaMgSi₂O₆-Mg₂SiO₄-Ca₃MgSi₂O₈ (merwinite)-H₂O, are presented in pressure-temperature projection. Sixteen invariant points and 39 univariant reactions are defined on the basis of the 1 atm and 10 kbar (vapor-saturated) liquidus diagrams. Lack of experimental control over many of the reactions makes the depicted relations schematic in part.An invariant point involving orthoenstatite, protoenstatite, pigeonite, and diopside (all solid solutions) occurs at low pressure (probably between 1 and 2 kbar). At pressures below this invariant point, orthoenstatite breaks down at high temperature to the assemblage diopside + protoenstatite; with increasing temperature, the latter assemblage reacts to form pigeonite. At pressures above the invariant point, pigeonite forms according to the reaction diopside + orthoenstatite = pigeonite, and the assemblage diopside + protoenstatite is not stable. At 1 atm, both pigeonite and protoenstatite occur as primary liquidus phases, but at pressures above 6–7 kbar orthoenstatite is the only Ca-poor pyroxene polymorph which appears on the vapor-saturated liquidus surface.At pressures above approximately 10.8 kbar, only diopside, forsterite, and merwinite occur as primary liquidus phases in the system CaMgSi₂O₆-Mg₂SiO₄-Ca₃MgSi₂O₈-H₂O, in the presence of an aqueous vapor phase. At pressures between 1 atm and 10.2 kbar, both akermanite and monticellite also occur as primary liquidus phases. Comparison of the 1 atm and 10 kbar vapor-saturated liquidus diagrams suggests that melilite basalt bears a low pressure, or shallow depth, relationship to monticellite-bearing ultrabasites.     

Magma storage conditions and differentiation of the mafic Lower Pollara volcanics,Salina Island,Aeolian Islands,Italy: implications for the formation conditions of shoshonites and potassic rocks

Crystallization experiments of basaltic andesite mafic endmember from the 24 ka Lower Pollara eruption (Salina, Aeolian Islands, Italy) were investigated at 200 MPa, 950–1100 °C, in the H₂O activity (aH₂O) range ~0.3 to 1, and at two ranges of oxygen fugacity (fO₂) between ~FMQ to FMQ+1 and ~FMQ+2 to FMQ+3.3 (log bars, FMQ is fayalite-magnetite-quartz). Comparison of the produced phase assemblages and phase compositions with the natural sample reveals that the storage conditions were ~1050 °C, ~2.8 wt% H₂O in the melt (aH₂O ~0.5), and relatively oxidizing (~FMQ+2.5). The composition of plagioclase in the groundmass indicates a period of cooling to ≤950 °C. The overall differentiation trends of the Salina volcanics can be explained by fractional crystallization close to H₂O saturated conditions (~5 wt% H₂O in the melt at 200 MPa) and most likely by accumulation of plagioclase, i.e., in basaltic andesites, and by various degree of mixing–mingling between the corresponding differentiates. The slightly elevated K₂O contents of the most mafic basaltic andesites that can be found in the lowermost unit of the Lower Pollara pyroclastics reveal earlier processes of moderately hydrous fractional crystallization at higher temperature (>~1050 °C). Fractional crystallization with decreasing influence of H₂O causes a moderate decrease of MgO and a significant increase of K₂O relative to SiO₂ in the residual liquids. It is exemplarily shown that the crystallization of SiO₂-rich phases at high temperature and low aH₂O of only moderately K₂O-rich calc-alkaline basalts can produce shoshonitic and high potassic rocks similar to those of Stromboli and Volcano. This suggests that the observed transition from calc-alkaline to shoshonitic and high potassic volcanism at the Aeolian Arc over time can be initiated by a general increase of magmatic temperatures and a decrease of aH₂O in response to the extensional tectonics and related increase of heat flow and declining influence of slab-derived fluids.     

Die Verteilung der Lanthaniden im Tholeyit von Tholey (Saar) und in Palatiniten,Pseudopegmatiten und Apliten des permischen Vulkanismus im Saar-Nahe-Pfalz-Gebiet

Zusammenfassung Ein Teil der im Unterperm (Wende Unterrotliegendes/Oberrotliegendes) gebildeten magmatischen Gesteine des Saar-Nahe-Pfalz-Gebietes wurde auf Lanthaniden untersucht: Gesteins-Gruppe 1: doleritischer Olivinbasalt, Pigeonit-Tholeyit, Tholeyit (Originalvorkommen) aus dem Schaumberg-Sill bei Tholey (Saar), und Plagiaplit. Gesteins-Gruppe 2: Palatinit, Pseudopegmatit, Aplit.Gesteins-Gruppe 1: Der Tholeyit aus dem Schaumberg-Sill zeigt deutlich höhere absolute Lanthanidengehalte (180 und 230 ppm Y, La-Lu) und eine stärkere Anreicherung der leichteren Lanthaniden La-Eu im Vergleich zu den ozeanischen und kontinentalen tholeiitischen Basalten (100 ppm Y, La-Lu). Das ist ein weiterer Hinweis dafür, daß der Tholeyit vom Locus typicus nicht identisch ist mit dem als tholeiitischer Basalt bezeichneten Gesteinstyp. Im Gegensatz zu den absoluten Lanthanidengehalten bestehen aber praktisch keine Unterschiede bei der relativen Lanthanidenverteilung zwischen dem Liegenden und dem Hangenden des Sill. Mit ansteigendem K₂O/Na₂O-Verhältnis wird in der Reihenfolge: doleritischer Olivinbasalt, Pigeonit-Tholeyit, Tholeyit aus dem Schaumberg-Sill und Plagiaplit eine Zunahme der absoluten Lanthanidengehalte und eine fortschreitende Fraktionierung zwischen den leichteren und den schwereren Lanthaniden festgestellt. Eine aus petrographischen Befunden von Jung (1958) vermutete Differentiation zwischen den genannten Gesteinstypen deutet sich somit auch in der Lanthanidenverteilung an.Gesteins-Gruppe 2: Ähnliche Zusammenhänge bestehen zwischen Palatiniten (180 ppm Y, La-Lu, 3 Proben), Pseudopegmatit (290 ppm Y, La-Lu, 1 Probe) und Apliten (270 ppm Y, La-Lu, 3 Proben), jedoch hier ohne klar erkennbare Abgängigkeit von den K₂O/Na₂O-Verhältnissen.

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Distribution of the lanthanide elements in the tholeyite from tholey (saar, W.-Germany) and in palatinite, pseudopegmatite, and aplite of the permian volcanism in the saar-nahe-pfalz area

Several types of magmatic rocks from the Saar-Nahe-Pfalz area (lower Permian) have been analysed for the 14 lanthanide elements and yttrium: rocks of group 1. doleritic olivine basalt, pigeonite tholeyite, tholeyite (type locality) from the Schaumberg sill near Tholey (Saar), and plagiaplite; rocks of group 2. palatinite, pseudopegmatite, aplite.Rocks of group 1. It is important to note the higher absolute Y and La-Lu contents (180 and 230ppm Y, La-Lu) and the relative enrichment of the lighter lanthanides in the tholeyite from the Schaumberg sill as compared with oceanic and continental tholeiitic basalts (100 ppm Y, La-Lu). This is additional evidence against the assumption that the tholeyite from the type locality has genetic relations to the so-called oceanic and continental tholeiitic basalts. The lanthanide elements in the tholeyite increase in concentration from the lower to the upper part of the Schaumberg sill, but there is little change in relative abundances within the sill. Increasing K₂O/Na₂O ratios are accompanied by an increase of the absolute lanthanide concentrations and by a progressive accumulation of the lighter lanthanides in the sequence: doleritic olivine basalt, pigeonite tholeyite, tholeyite from the Schaumberg sill, plagiaplite. The distribution pattern of the lanthanides in these rocks confirms the explanation of the petrographic observation by Jung (1958) as a magmatic differentiation series.Rocks of group 2. Similar relations were observed between palatinites (180 ppm Y, La-Lu, 3 samples), pseudopegmatite (290 ppm Y, La-Lu, 1 sample), and aplites (270 ppm Y, La-Lu, 3 samples), but there was no clear evidence for a correlation of lanthanide distribution with K₂O/Na₂O ratios.

     

Rare earth element variations resulting from inversion of pigeonite and subsolidus reequilibration in lunar ferroan anorthosites

We present results of a secondary ion mass spectrometry study of the rare earth elements (REEs) in the minerals of two samples of lunar ferroan anorthosite, and the results are applicable to studies of REEs in all igneous rocks, no matter what their planet of origin. Our pyroxene analyses are used to determine solid-solid REE distribution coefficients (D = C_REE in low-Ca pyroxene/C_REE in augite) in orthopyroxene-augite pairs derived by inversion of pigeonite. Our data and predictions from crystal-chemical considerations indicate that as primary pigeonite inverts to orthopyroxene plus augite and subsolidus reequilibration proceeds, the solid-solid Ds for orthopyroxene-augite pairs progressively decrease for all REEs; the decrease is greatest for the LREEs. The REE pattern of solid-solid Ds for inversion-derived pyroxene pairs is close to a straight line for Sm-Lu and turns upward for REEs lighter than Sm; the shape of this pattern is predicted by the shapes of the REE patterns for the individual minerals.Equilibrium liquids calculated for one sample from the compositions of primary phases, using measured or experimentally determined solid-liquid Ds, have chondrite-normalized REE patterns that are very slightly enriched in LREEs. The plagioclase equilibrium liquid is overall less rich in REEs than pyroxene equilibrium liquids, and the discrepancy probably arises because the calculated plagioclase equilibrium liquid represents a liquid earlier in the fractionation sequence than the pyroxene equilibrium liquids. “Equilibrium” liquids calculated from the compositions of inversion-derived pyroxenes or orthopyroxene derived by reaction of olivine are LREE depleted (in some cases substantially) in comparison with equilibrium liquids calculated from the compositions of primary phases. These discrepancies arise because the inversion-derived and reaction-derived pyroxenes did not crystallize directly from liquid, and the use of solid-liquid Ds is inappropriate. The LREE depletion of the calculated liquids is a relic of formation of these phases from primary LREE-depleted minerals. Thus, if one attempts to calculate the compositions of equilibrium liquids from pyroxene compositions, it is important to establish that the pyroxenes are primary. In addition, our data suggest that experimental studies have underestimated solid-liquid Ds for REEs in pigeonite and that REE contents of liquids calculated using these Ds are overestimates.Our results have implications for Sm-Nd age studies. Our work shows that if pigeonite inversion and/or subsolidus reequilibration between augite and orthopyroxene occurred significantly after crystallization, and if pyroxene separates isolated for Sm-Nd studies do not have the bulk composition of the primary pyroxenes, then the Sm-Nd isochron age and ε_Nd will be in error.     

A high pressure experimental study on a magnesian-rich leucite-lamproite from the West Kimberley area,Australia: petrogenetic implications

Liquidus and subliquidus phase relations of a leucite-lamproite (wolgidite) from the West Kimberley area, Australia have been studied experimentally under the volatile conditions of 3.22 wt.% H₂O ( \(X_{CO_2 }\) =0.11) and 13.0 wt.% H₂O ( \(X_{CO_2 }\) =0.03) between 10 to 40 kbar. Under these conditions, liquids are vapour undersaturated. In experiments with 13.0 wt.% H₂O, olivine is the liquidus phase up to 24 kbar and orthopyroxene above 24 kbar. Phlogopite and rutile occur close to the liquidus above 16 kbar. Crystallization temperatures of clinopyroxenes are 50–120° C below the liquidus. Based on these results, wolgidite magma is unlikely to be a partial melt of a garnet- or spinel-lherzolite mantle but could be derived from phlogopite+rutile±olivine±or-thopyroxene assemblages occurring as metasomatized mantle.     

Potassic primary melts of vulsini (Roman Province): evidence from mineralogy and melt inclusions

The origin and the relationships between the high potassic (HKS) and potassic (KS) suites of the Roman Comagmatic Province and the nature of their primary magmas have been intensively debated over the past 35 years. We have addressed these problems by a study of mineralogy (olivine Fo_92-87, Cr-spinel and diopside) and melt inclusions in olivine phenocrysts from a scoria sample of Montefiascone (Vulsini area). This rock is considered as one of the most primitive (MgO=13.5 wt%, NiO=340 ppm; Cr=1275 ppm) in the northern part of the Roman Comagmatic Province. The compositions of both the olivine and their melt inclusions are controlled by two main processes. In the case of the olivine Fo<90.5, fractional crystallization (olivine + diopside + minor spinel) was the principal mechanism of the magma evolution. The olivine (Fo_92-90.5) and the Cr-spinel (Cr#=100. Cr/(Cr+Al)=63-73) represent a near-primary liquidus assemblage and indicate the mantle origin of their parental magmas. The compositions of melt inclusions in these olivine phenocrysts correspond to those of poorly fractionated H₂O-rich ( 1 wt%) primary melts (MgO=8.4-9.7 wt%,FeO^total=6-7.5 wt%). They evidence a wide compositional range (in wt%: SiO₂=46.5-50, K₂O=5.3-2.8, P₂O₅=0.4-0.2, S=0.26-0.12; Cl=0.05-0.03, and CaO/Al₂O₃= 0.8-1.15), with negative correlations between SiO₂ and K₂O, Al₂O₃ and CaO, as well as positive correlations between K₂O, and P₂O₅, S, Cl, with nearly constant ratios between these elements. These results are discussed in terms of segregation of various mantle-derived melts. The high and constant Mg# [100.Mg/(Mg+Fe²⁺)] 73-75 of studied melts and their variable Si, K, P, Ca, Al, S contents could be explained by the melting of a refractory lithospheric mantle source, heterogeneously enriched in phlogopite and clinopyroxene (veined mantle source).     

Petrology and origin of the shergottite meteorites

Shergottites contain cumulus pigeonite and augite, probably without cumulus plagioclase and crystallized under relatively oxidizing conditions. Shergotty and Zagami may differ in the relative proportions of cumulus pyroxenes and crystallized intercumulus liquid, but the compositions of pyroxenes and liquid are similar in both meteorites. Absence of olivine in melting experiments suggests that the shergottites crystallized from fractionated derivatives of primary liquids. Low-Ca pyroxene and augite apparently began to crystallize from these primary liquids prior to plagioclase. Shergottites can be readily distinguished from other achondrite groups by their mineralogies, crystallization sequences and inferred source region compositions. However, the source regions of the shergottites may be related to those of other achondrite types by addition or loss of volatile components.The bulk composition of the Earth's upper mantle overlaps that of permissible shergottite source regions. Shergottites and terrestrial basalts display similarities in oxidation state and concentrations of trace and minor elements with a wide range of cosmochemical and geochemical affinities. Accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body or accretion of the Earth's upper mantle from planetesimals similar to the shergottite parent body may account for many of their similarities. Models of the origin of the Earth's upper mantle which attribute its oxidation state, its siderophile element abundances and its volatile element abundances to uniquely terrestrial processes or conditions, or to factors unique to the origin and differentiation of large bodies, are unattractive in light of the similarities between shergottites and terrestrial basalts.     

High and low pressure phase equilibria of a mildly alkalic lava from the 1965 Surtsey eruption: Experimental results

Melting experiments have been performed on a primitive, mildly alkalic glassy lava (10 wt.% MgO) from the 1965 eruption of the Surtsey volcano located at the tip of the south-eastern propagating rift zone of Iceland. At atmospheric pressure, approximately on the FMQ oxygen buffer, olivine (Fo₈₁) crystallizes from 1240°C, followed by plagioclase (An₇₀) from 1180°C and augite from 1140°C. The experimental glasses coexisting with olivine, plagioclase and augite are ferrobasaltic enriched in FeO (13.6–14.2 wt.%) and TiO₂ (4.0–4.4 wt.%). In high pressure, piston-cylinder, graphite-controlled runs, olivine occurs as the liquidus phase until 14 kbar, above which augite is the liquidus phase. Low-Ca pyroxene is not a liquidus phase at any pressure. The high pressure liquids are, relative to the one atmosphere liquids, significantly enriched in Al₂O₃ and Na₂O and depleted in CaO as a result of changes in the crystallizing assemblages. Furthermore, liquidus augite is dominantly subcalcic and shows significant enrichment in Al and depletion in Ti. Subliquidus plagioclase is enriched in sodium relative to low pressure phase compositions. Evaluated in normative projections, contrasting liquid lines of descent are revealed as a function of pressure. At one atmosphere, the multisaturated liquids are located close to the thermal divide defined by the plane olivine-plagioclase-augite, but appear, with advanced degrees of crystallization, to be moving away from the thermal divide toward normative quartz. The augites crystallizing in the one atmosphere experiments are calcic and slightly nepheline normative. In the 10 and 12.5 kbar experiments, the augites become subcalcic and dominantly hypersthene normative. Because of this shift in augite compositions, transitional basaltic liquids may at high pressure evolve from the tholeiitic side of the olivine-plagioclase-diopside normative divide onto the alkalic side. With increasing pressure above 15 kbar, the liquidus augite compositions move back toward the olivine-plagioclase-diopside normative divide.     

Volatiles H2O,CO2, and Cl in a subduction related basalt

The products of the 1974 eruption of Fuego, a subduction zone volcano in Guatemala, have been investigated through study of silicate melt inclusions in olivine. The melt inclusions sampled liquids in regions where olivine, plagioclase, magnetite, and augite were precipitating. Comparisons of the erupted ash, groundmass, and melt inclusion compositions suggest that the inclusions represent samples of liquids present in a thermal boundary layer of the magma body. The concentrations of H₂O and CO₂ in glass inclusions were determined by a vacuum fusion manometric technique using individual olivine crystals (Fo77 to Fo71) with glass inclusion compositions that ranged from high-alumina basalt to basaltic andesite. Water, Cl, and K₂O concentrations increased by a factor of two as the olivine crystals became more iron-rich (Fo77 to Fo71) and as the glass inclusions increased in SiO₂ from 51 to 54 wt.% SiO₂. The concentration of H₂O in the melt increased from 1.6 wt.% in the least differentiated liquid to about 3.5% in a more differentiated liquid. Carbon dioxide is about an order of magnitude less abundant than H₂O in these inclusions. The gas saturation pressures for pure H₂O in equilibrium with the melt inclusions, which were calculated from the glass inclusion compositions using the solubility model of Burnham (1979), are given approximately by P(H₂O)(Pa)=(SiO₂−48.5 wt.%) × 1.45 × 10⁷. The concentrations of water in the melt and the gas saturation pressures increased from about 1.5% to 3.5% and from 300 to 850 bars, respectively, during pre-eruption crystallization.     

Complex zoning between super-calcic pigeonite and augite from the Graveyard Point sill, Oregon: a record of the interplay between bulk and interstitial liquid fractionation

The 150 m thick late Miocene Graveyard Point sill (GPS) is situated at the Idaho-Oregon border near the southwestern edge of the western Snake River Plain. It records from bottom to top continuous fractional crystallization of a tholeiitic parent magma (lower chilled border, FeO/(FeO+MgO) = 0.59, Ni = 90 ppm) towards granophyres (late pods and dikes, FeO/(FeO+MgO) = 0.98, 78 wt% SiO₂ 3.5 wt% K₂O, <4 ppm Ni) showing a typical trend of Fe and P enrichment. Fractionating minerals are olivine (Fo₇₉-Fo₂), augite (X _Fe = 0.18−0.95), feldspars (An₈₀Or₁-An₁Or₆₂), Fe-Ti oxides (Ti-rich magnetite and ilmenite), apatite and in two samples super-calcic pigeonite (Wo_18–28 Fs_41–54). The granophyres may bear some quartz. Compositionally zoned minerals record a large interval of the fractionation process in every single sample, but this interval changes with stratigraphic height. In super-calcic pigeonite-bearing samples, olivine is scarce or lacking and because super-calcic pigeonite occurs as characteristic overgrowths on augite, its formation is interpreted to be related to the schematic reaction: augite + olivine (component in melt) + SiO₂ (in melt) = pigeonite, that defines the cotectic between augite and pigeonite in olivine-saturated basaltic systems. Line measurements with the electron microprobe reveal that the transition from augite to super-calcic pigeonite is continuous. However, some crystals show an abrupt “reversal” towards augite after super-calcic pigeonite growth. Two processes compete with each other in the GPS: fractional crystallization of the bulk liquid (the bulk melt separates from solids and interstitial liquids in the solidification front) and fractional crystallization of interstitial melt in the solidification front itself. Interplay between those two processes is proposed to account for the observed variations in mineral chemistry and mineral textures. Received: 25 November 1998 / Accepted: 14 June 1999