Primary Liquidus Phase Research Articles

Origin of magmatic harzburgite as a result of boninite magma evolution – An illustration using layered harzburgite-dunite cumulate from the Troodos ophiolite complex

Olivine (Ol) and orthopyroxene (Opx) are the primary liquidus phases of boninite in modern subduction settings and in many ophiolite complexes. It is thus straightforward to expect the formation of harzburgite cumulate resulting from boninite magma evolution. However, such magmatic harzburgite has been rarely studied. Here, we report the results of our study on such harzburgite from the Troodos ophiolite complex.The Troodos cumulate harzburgite (locally lherzolite) is characteristically interlayered with dunite, showing varying thickness on millimeter to decimeter scales, as the result of volumetrically varying multiple pulses of melt injection into the evolving magma chamber. We illustrate the development of the interlayered cumulate by phase equilibrium analysis. The parental melt of each pulse begins to crystallize olivine to form a dunite layer before reaching the Ol-Opx cotectic, along which Ol and Opx coprecipitate to form a harzburgite layer. Periodical replenishment will result in dunite-harzburgite interlayered cumulate. In cases when replenishment may be delayed, the melt along the Ol-Opx cotectic can evolve to the Ol-Opx-clinopyroxene (Cpx) eutectic to form harzburgite with some Cpx or lherzolite. The calculated melts in equilibrium with spinels in the cumulate are characteristic of boninite in major element compositional spaces. The calculated melts in equilibrium with Cpx and Opx in the cumulate share the same as, or identical to, trace element patterns of the Troodos boninite (both glasses and bulk-rock compositions). Petrological modeling of the boninite magma evolution shows a crystallization order of Ol, Opx, Cpx, plagioclase. Our study also emphasizes the importance in considering dunite-harzburgite/lherzolite cumulate when interpreting seismic structure of the crust in subduction settings, especially in rock sequences associated with subduction initiation thought to be indicated by boninite magmatism.

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

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

Cuspidine-sodalite natrocarbonatite from Oldoinyo Lengai, Tanzania: a novel hybrid carbonatite formed by assimilation of ijolite

AbstractA unique hybrid natrocarbonatite, collected from the new ash cone of the volcano Oldoinyo Lengai. Tanzania in July 2008, consists of phenocrysts of nyerereite and gregoryite together with xenocrysts of clinopyroxene, nepheline and Ti-andradite set in a groundmass of cuspidine, sodalite, ferroan manganoan monticellite, K-Fe sulphide and manganoan titanian magnetite and gregoryite. The xenocrysts were not in equilibrium with the melt which formed their current host, as clinopyroxenes and Ti-andradite are mantled by cuspidine, and nepheline by sodalite and phlogopite—potassian kinoshitalite solid solutions. A microxenolith of ijolite exhibits similar reaction phenomena. The minerals of the xenocryst suite have similar compositions to plutonic ijolites found at Oldoinyo Lengai, and are thus considered to be derived by the fragmentation of such material in a previously contaminated natrocarbonatite melt. The latter, which has cuspidine, sodalite and monticellite as primary liquidus phases, is considered to have been formed by the complete assimilation of ijolitic material in a natrocarbonatite magma at depth in the volcano conduit. The occurrence of trace amounts of cuspidine, Fe-Mn-monticellite, K-Fe sulphide and Mn-Ti-spinel in recently erupted natrocarbonatites is ascribed to similar, but less extensive, assimilation of silicate material prior to their eruption.

EXPERIMENTAL STUDIES OF THE SYSTEM Na2Ca(CO3)2 - NaCl - KCl AT 0.1 GPa: IMPLICATIONS FOR THE DIFFERENTIATION AND LOW-TEMPERATURE CRYSTALLIZATION OF NATROCARBONATITE

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

Predicting the Spinel–Nepheline Liquidus for Application to Nuclear Waste Glass Processing. Part II: Quasicrystalline Freezing Point Depression Model

The crystal‐melt equilibria in complex 15‐component‐simulated high‐level waste (HLW) glass melts are modeled based on quasicrystalline concepts that are discussed in Part I. A pseudobinary phase diagram between a transition metal ferrite spinel (an incongruent melt product of a transition metal iron‐rich acmite) and nepheline is defined based on calculation of the quasicrystalline melt precursors calculated by a freezing point depression approach. The pseudobinary lies within the Al2O3–Fe2O3–Na2O–SiO2 quaternary system that defines the crystallization of basalt glass melts. The modeling provides the partitioning of species between the melt and the primary liquidus phases. The medium‐range order of the melt and the melt‐crystal exchange equilibria are defined based on a constrained mathematical treatment that considers the crystallochemical coordination of the elemental species in acmite and nepheline. The liquidus phases that form are shown to be governed by the melt polymerization (Part II) and the thermodynamic octahedral site preference energies (Part I). The liquidus model developed based on these concepts has been used to prevent unwanted crystallization in the world's largest HLW melter for the past 4 years while allowing &gt;10 wt% higher waste loadings to be processed.

Solubility of niobium in the system CaCO3?CaF2?NaNbO3 at 0.1�GPa pressure: implications for the crystallization of pyrochlore from carbonatite magma

Experimental data are presented for the solubility of NaNbO3 in the ternary system CaCO3–CaF2–NaNbO3 (or calcite–fluorite–lueshite) over the temperature range 500–1,000°C at 0.1 GPa pressure. Liquidus to solidus phase relationships are given for the pseudo-binary join ([CaCO3]60[CaF2]40)100-x–(NaNbO3) x (0<x<60 wt%). These data show that the maximum solubility of NaNbO3 in these liquids is about 17 wt% (or 13.8 wt% Nb2O5) at approximately 930°C, and is represented by the appearance of pyrochlore as the primary liquidus phase. The sub-liquidus assemblages with decreasing temperature for NaNbO3 contents of 20–50 wt% are: pyrochlore + liquid; pyrochlore + CaF2 + liquid; pyrochlore + CaF2 + CaCO3 + liquid. The solidus assemblage is pyrochlore + CaF2 + CaCO3 at temperatures of approximately 700°C (20 wt% NaNbO3) and 600°C (40 wt% NaNbO3). NaNbO3 is present only in sub-solidus assemblages. These data show that in this fluorine-bearing anhydrous system pyrochlore is the principal Nb-hosting supra-solidus phase, in contrast to fluorine-free hydrous melts from which perovskite-structured compounds crystallize. The crystallization of pyrochlore and/or perovskite-structured compounds from haplocarbonatite liquids is thus considered to be dependent upon the F/OH ratio of the melt.

Solubility of niobium in the system CaCO3–Ca(OH)2–NaNbO3 at 0.1 GPa pressure

Experimental data are presented for the solubility of NaNbO3 in the ternary system CaCO3–Ca(OH)2–NaNbO3 (or calcite–portlandite–lueshite) over the temperature range 500–950 °C at 0.1 GPa pressure. Near-liquidus phase relationships are given for the pseudo-binary join ([CaCO3]45[Ca(OH)2]55)100–x–(NaNbO3)x; (0 <x <70 wt%). These data show the presence of a large field of calcite plus liquid and the absence of NaNbO3 as a primary liquidus phase until the melts contain greater than 60 wt% NaNbO3. The primary crystallization fields of calcite and NaNbO3 are separated by a steep thermal valley located at about 55 wt% NaNbO3. Quenched liquids contain calcite, portlandite, sodium carbonates and perovskite-structured calcium niobates. The maximum solubility of Nb2O5 in this system is estimated to be on the order of 48 wt% at 650 °C at 0.1 GPa pressure. The experiments show that perovskite-structured compounds rather than pyrochlore crystallize from these fluorine-free, water-rich melts.

The geology, petrology, geochronology and source region character of the layered gabbronoritic Oranjekom Complex in the Kibaran Namaqua mobile belt, South Africa

The Oranjekom Complex, a small mafic to anorthositic layered intrusion situated on the eastern margin of the Namaqua Mobile Belt, forms part of a number of such intrusions that were intruded early in the evolution of the belt. These have subsequently been deformed and isochemically metamorphosed to amphibole-bearing rocks. Petrographic and geochemical evidence indicates that the magmas were alumina-rich and that plagioclase was the primary liquidus phase, but in situ differentiation trends are shown mainly by the mafic phases. This implies that plagioclase crystals, although present in the magma, remained partially in suspension, whereas the mafic minerals accumulated. This is supported by both major and trace element character of the rocks. The RbSr and SmNd isotopic systems indicate that the Oranjekom Complex was intruded and metamorphosed at ca. 1100 Ma. This was followed by further metamorphism at ca. 944 Ma. The isotopic character indicates a depleted mantle source, that gives an age slightly older but within error of the estimated time of intrusion. Further data is required to precisely constrain the age of intrusion of these important markers in the evolution of the Namaqua Orogen. The isotope data indicate that there was limited crustal interaction indicating a thin (possibly juvenile) crust at that time.

Preliminary studies of the solubility and stability of perovskite group compounds in the synthetic carbonatite system calcite-portlandite

Perovskites are common accessory minerals in carbonatites and are one of the major hosts for the rare earth elements (REE) and Nb. In an attempt to understand the conditions under which perovskites of diverse composition crystallise, preliminary hydrothermal experiments on the solubility and stability of perovskite-group compounds in the synthetic carbonatite system calcite-portlandite (CCCH) have been undertaken at a pressure of 1 kbar and temperatures ranging from 550–750°C. Experiments were conducted along the pseudoternary joins: CC 45CH 55X; CC 65CH 35X; CC 35CH 65X, where X = CaTiO 3 (perovskite), NaLaTi 2O 6 (loparite), SrTiO 3 (tausonite), NaNbO 3 (lueshite). The experiments showed that perovskite is highly soluble (up to 30wt%) in haplocarbonatite liquids and crystallises as a “groundmass” phase. In contrast other perovskite group compounds are unstable and break down to a variety of NaLaSr carbonates, lanthanian pervoskite, calcian loparite and calcian niobate. Lueshite and tausonite are not found as primary liquidus phases. These data suggest that lueshite and tausonite will not crystallise from low temperature carbonatite melts and that Ti, REE and Nb will be preferentially concentrated in residual liquids from which calcian loparite and diverse niobates crystallise.

Carbonates of the magnesite–siderite series from four carbonatite complexes

AbstractCarbonates of the magnesite-siderite series have been found and analysed in carbonatites from the Lueshe, Newania, Kangankunde, and Chipman Lake complexes. This series has been represented until now only by a few X-ray identifications of magnesite and three published analyses of siderite and breunnerite (magnesian siderite). Most of the siderite identified in carbonatites in the past has proved to be ankerite, but the new data define the complete solid-solution series from magnesite to siderite. They occur together with dolomite and ankerite and in one rock with calcite. The magnesites, ferroan magnesites and some magnesian siderites may be metasomatic/hydrothermal in origin but magnesian siderite from Chipman Lake appears to have crystallized in the two-phase calcite + siderite field in the subsolidus CaCO3-MgCO3-FeCO3 system. Textural evidence in Newania carbonatites indicates that ferroan magnesite, which co-exists with ankerite, is a primary liquidus phase and it is proposed that the Newania carbonatite evolved directly from a Ca-poor, Mg-rich carbonatitic liquid generated by partial melting of phlogopite-carbonate peridotite in the mantle at pressures &gt;32 kbar.

The Peralkaline Nepheline Syenites of the Junguni Intrusion, Chilwa Province, Malawi

AbstractThe mineralogy of the highly peralkaline Junguni nepheline syenite intrusion of the Chilwa alkaline province has been investigated. The rocks comprise alkali feldspar, very abundant nepheline, locally exceptionally abundant sodalite, sodic pyroxenes, scarce biotite, rare amphibole, and an extensive range of accessory minerals. Electron microprobe analyses indicate that the pyroxenes define an evolutionary trend from salite through aegirine-augite to aegirine, which is unusual in its broadness and ill-definition. This is explained by a series of overlapping trends produced by fluctuating Fe3+/Fe2+ ratios caused by variations in alkali content of the magma, probably produced by periodic alkali loss by surface de-gassing. Much of the sodalite occurs as a primary liquidus phase, but the paragenesis of certain ramifying sodalite veins is more problematical. It is possible that such ramifying masses are the product of coupled migration of alkalis and volatiles by diffusion in a gravitation field under pressure gradients generated by eruptive events, or by the formation of immiscible Na- and Cl-rich liquids. Both early calcic and late mangan-fluor eckermannitic/arfvedsonitic amphiboles occur. The micas vary from biotites with Mg: Fe ratios &lt; 0·5 to almost pure annites; they are fluor-micas and characterized by high Mn contents. Analyses of niobian rutile, mangan ilmenite, ferroan pyrophanite, manganese-rich eucolite, låvenite, mangan-titan låtvenite, rosenbuschite, wöhlerite, pyrochlore, eudialyte and what is believed to be only the second occurrence of kupletskite are given. Many of these minerals are rich in Zr, Nb, Na and Mn and thus typical of the assemblages found in extreme agpaitic complexes such as Ilimaussaq and Lovozero.

Water-saturated melting relations to 5 kilobars of three Leucite Hills lavas

The water-saturated phase relations of three Leucite Hills lavas have been determined at pressures up to 5 kb. Phlogopite is the major primary liquidus phase in orendite and wyomingite at pressures, > 1 kb, but clinopyroxene and olivine are the major primary liquidus phases in madupite at pressures up to 5 kb. Leucite is a liquidus phase in all three magmas at pressures <0.5kb. The experimental results are in reasonable agreement with the inferred crystallization sequences for the Leucite Hills lavas and have applications to the crystallization and differentiation of potassium-rich magmas within the crust.

New experimental data for the system CaO-MgO-SiO 2-H 2O and a synthesis of inferred phase relations

Subsolidus and vapor-saturated liquidus phase relations for a portion of the system CaO-MgO-SiO 2-H 2O, as inferred from experimental data for the composition regions CaMgSi 2O 6-Mg 2SiO 4-SiO 2-H 2O and CaMgSi 2O 6-Mg 2SiO 4-Ca 3MgSi 2O 8 (merwinite)-H 2O, 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 CaMgSi 2O 6-Mg 2SiO 4-Ca 3MgSi 2O 8-H 2O, 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.