Ternary Feldspar System Research Articles

Magmatic processes revealed by textural and compositional trends in Merapi dome lavas

Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallization of groundmass phases. We have investigated this process using textural and compositional analysis of dome material from Merapi volcano, Central Java, Indonesia. Samples included dome lavas from the 1986–88, 1992–93, 1994 and 1995 effusive periods as well as pyroclastic material deposited by the November 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts+microlites), the liquids present in Merapi andesites are highly evolved (rhyolitic) at the time of eruption. Feldspar microlites in dome rocks consist of plagioclase cores (Ab 63An 29Or 8) surrounded by alkali feldspar rims (Ab 53An 5Or 42), compositional pairs which are not in equilibrium. A change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock. As decompression drives volatile exsolution, rates of degassing and resultant microlite crystallization may be governed by magma ascent rate. Microlite crystallinity is nearly identical among the 1995 dome samples, an indication that similar microlite growth conditions ( P H 2O and temperature) were achieved throughout this extrusive period. However, microlite number density varied by more than a factor of four in these samples, and generally increased with distance from the vent. Low vent-ward microlite number densities and greater microlite concentrations down-flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density, suggesting that extrusion slowed during 1995. Crystal textures and compositions in the 1992–93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rates of undercooling determined by effusion rate) are cyclic.

TEM investigation of potassium-calcium feldspar inclusions in Boggild plagioclase

Potassium-calcium feldspar inclusions in a Boggild plagioclase were investigated by analytical high-resolution transmission electron microscopy (HRTEM), and the formation process of these inclusions is considered based on the Al-Si ordering scheme in ternary feldspar system. Nanometer-scale chemical analyses and selected-area electron diffraction (SAED) patterns show that the rims of the inclusions consist of lamellar intergrowths of potassium feldspar (C symmetry) and anorthite (P symmetry), whereas the core is un¯¯ 11 exsolved potassium feldspar (C symmetry). Potassium feldspar and anorthite in the in¯ 1 clusions share the crystallographic orientation of the host Boggild plagioclase. High-resolution lattice images indicate coherent interfaces between the inclusions and the host Boggild plagioclase and the phases in the inclusions. The results indicate a unique formation process of potassium-calcium feldspar inclusions in a ternary feldspar: crystallization of host labradorite → precipitation of potassium feldspar → decomposition of Boggild exsolution lamellae → growth of anorthite on the rims of potassium feldspar inclusions. Potassium-calcium feldspar inclusions may be found in Boggild and other plagioclase, and they have the potential to give constraints on thermal histories of K-rich plagioclase.

Fractional crystallization and zoning in igneous feldspars: ideal water-buffered liquid fractionation lines and feldspar zoning paths

Diagrams giving plagioclase and sanidine fractionation paths and liquid fractionation lines under conditions of ideal water-buffered fractional crystallization in the ternary feldspar system were constructed graphically using topological reasoning, and experimental data and calculated phase relationships from the literature. The liquidus lines and solidus or solvus paths are unique at constant P and aH2O. The composition of a liquid evolves with time and moves along a fractionation line by removal of successive crystal fractions, whereas the compositions of each of the crystal fractions lie on and define a solidus or solvus path. Most but not all such water-buffered lines and paths differ only slightly from those in which water is free to build up during crystallization and aH2O to increase, as in many rocks. Liquid compositions lying along liquidus fractionation lines are not normally preserved, unless erupted as aphyric lavas. The solidus or solvus paths may be preserved either as overgrowth zones in crystals (zoning paths) or as a series of crystal fractions in layered intrusions. The topologies of the lines and paths depend mainly on the nature of the two-feldspar boundary line separating the plagioclase and sanidine fields which is a function of PH2O or aH2O at constant P; increases in either progressively lower the liquidi and solidi and cause larger intersections of the solidi with the solvus. One-feldspar solidus paths at high P and aH2O are simple, whereas they are complex and may bend back on themselves at low PH2O or low aH2O at high P. Two-feldspar paths may be simultaneous (cotectic) or sequential (peritectic). The former are simple and do not meet at high P and aH2O, the critical solution line lying in the gap; they are complex and may bend back or overlap at low PH2O or low aH2O at high P, the position of the critical solution line being hard to determine. Liquids which have simultaneously fractionated two feldspars may fractionate only one towards the end, crystallization changing from subsolvus to hypersolvus. Sequential paths may involve overgrowth of an early feldspar by a later one, usually sanidine overgrowths on plagioclase, but plagioclase overgrowths on sanidine occur. These complexities explain in part the difficulties of unravelling the textural and compositional relationships of ternary feldspars in water-poor felsic igneous rocks (even in the absence of alteration or complex magma dynamics) and of trying to deduce phase relationships from natural occurrences of feldspars.

An exsolution silica-pump model for the origin of myrmekite

Myrmekite, as defined here, is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase (calcic albite-oligoclase), intimately associated with potassium feldspar in plutonic rocks of granitic composition. Hypotheses previously invoked in explanation of myrmekite include: (1) direct crystallization; (2) replacement; (3) exsolution. The occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement. However, several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases. Exsolution of potassium feldspar system projected from [AlSi2O8] involves the exchange CaAlK-1Si-1, in which the AlSi-1 tetrahedral couple is resistant to intracrystalline diffusion. By contrast, diffusion of octahedral K proceeds relatively easily where it remains uncoupled to the tetrahedral exchange. We suggest here that where the ternary feldspar system is open to excess silica, the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net-transfer reaction K+Si=Orthoclase, leaving behind indigenous Si that reports as modal quartz in the evolving plagioclase as the CaAl component is concomitantly incorporated in this same phase. Thus silica is “pumped” into the reaction volume from a “silica reservoir”, a process that enhances redistribution of both Si and Al through the exsolving ternary feldspar.

A TEM and microprobe study of a two-perthite alkali gabbro: Implications for the ternary feldspar system

An unusual magmatic three-feldspar syenogabbro occurs 3 m inside the contact of the Klokken gabbro-syenite stock. It contains plagioclase, mesoperthite and cryptoperthite, together with a low temperature symplectite intergrowth. Textural relationships have been investigated by cathodoluminescence, bulk chemistry by microprobe, and exsolution microtextures and intracrystalline boundaries by TEM. The mesoperthite has a bulk composition around Or26Ab52An22, well outside accepted limits of ternary feldspar solid solution. It is a mainly coherent, lamellar intergrowth of sodic andesine and orthoclase with incipient Mtwinning. The cryptoperthite, bulk composition around Or61Ab33An6, is a coherent lamellar intergrowth of orthoclase and sodic low oligoclase. The compositions of the exsolved phases have been estimated. The meso- and cryptoperthite crystals have sharp boundaries with each other.

Towards a more practical two-feldspar geothermometer

The thermodynamic basis of several recent attempts to formulate a simple two-feldspar geothermometer is discussed, together with a review of earlier empirical geothermometers and ones based on experimental studies in the ternary feldspar system. It is shown that double-binary thermometers which involve the combination of regular solution mixing models for the binary alkali feldspar system with ideal mixing in plagioclases do not give a satisfactory representation of two-feldspar relations, especially for albite-rich compositions where a critical point exists. Thermometers based on mixing parameters for ordered alkali feldspar frameworks are even more unjustified both because low-plagioclases are certainly non-ideal, and because of uncertainty in knowing the degree of Al-Si order in the alkali feldspar when exchange equilibrium was achieved. A ‘thermodynamic’ thermometer requires knowledge of ternary activities which are at present unknown. Experimental determinations of relationships in the ternary feldspar system are reviewed and the correct general form of the thermometer constructed using mainly the experimental data of Seck (1971a) and Smith and Parsons (1974). Chemographic tests for equilibrium between feldspar pairs are suggested and petrographie features discussed. In an appendix new values are given of Margules parameters calculated for binary disordered alkali feldspars from recent solvus data up to 15 kbars, and their physico-chemical basis examined. We suggest that accurate representations of the mixing properties of disordered alkali feldspars using Margules parameters are at present premature.

Ternary feldspars: Kinetics and possible equilibria at 800� C

The ternary feldspar system KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8 was reinvestigated at 650 ° C and 800 ° C (P H2O = 1 kb) using mixtures of crystalline plagioclases and alkali feldspars as starting materials. The compositions of plagioclases and alkali feldspars of the run products were determined by X-ray means. The Or-content of the feldspar phases was determined by measuring the position of the (201) X-ray peak of the unexchanged feldspars, whereas the An-content was determined by measuring the same X-ray peak of the K-exchanged feldspars. The reaction rate of a reaction leading to a more An-rich plagioclase (type II reaction) is much faster than a reaction producing a more Ab-rich plagioclase (type I). In a type II reaction run times of approximately 20 days are needed to reach new constant plagioclase and alkali feldspar compositions at 650 ° C, and 10 days are needed to reach constant compositions at 800 ° C. In a reaction of type I only the outer zone of the plagioclases reacts to more Abrich compositions. A diffuse zone with a wide range of compositions was observed in 650 ° C runs. Equilibrium could not be reached in these experiments within 45 days. At 800 ° C a new zone having a specific composition develops in 42 days. This new zone is believed to be in equilibrium with the coexisting alkali feldspar. The depth of reaction is calculated as 0.03 μm after 42 days (800 ° C, P f= 1 kb). The reaction between the two feldspar phases could be reversed at 800 ° C. The following compositions are considered to represent equilibrium data at 800 ° C and P t = 1 kb: Recent data obtained with gels of ternary feldspar composition as starting materials do not agree with the results presented in this paper. Gels obviously crystallize spontaneously forming coexisting feldspars of non - equilibrium composition - alkali feldspars too rich in Ab and plagioclases too rich in An.

Segregation in ternary solutions.

The spinodal surface and the stable binodal surface must touch tangentially at any extremum/saddle point and along any consolute line. Because of this, the most convenient way to study immiscibility in a ternary is to calculate the spinodal surface. Other authors have accomplished this in a binary regular and subregular solution and in a ternary regular solution but the derived equations are not applicable to a ternary subregular solution or to a Kohler solution. The equation for the spinodal in a ternary Kohler solution is herein derived so that now it is possible to study immiscibility using the spinodal in any ternary Kohler solution, including a subregular ternary solution. MEIJIRING used the spinodal equation to derive relations which characterise the form of segregation in a ternary regular solution. When the “effective” regular solution parameter is substituted into these relations, they can be used as a rough approximation to characterise segregation in a subregular ternary solution; the number of peaks and saddles in the ternary solvus can be predicted but the temperature/composition of them cannot. This rough scheme is used with a graph which shows that the composition of the critical point on a subregular binary solvus is only a function of the ratio of the two subregular parameters. The temperature dependence of subregular solution parameters is capable of exerting a strong control on the unique features of a ternary spinodal, so that a more exact study of immiscibility must involve the calculation of the spinodal surface using temperature dependent solution parameters. A model spinodal for the ternary feldspar system at l kb is calculated using temperature dependent subregular solution parameters.

Origin of the younger granites of northern Nigeria

The similarity of the lead isotopic composition in older granites, pegmatites and the younger granites, together with the distribution of tin and the absence of tourmaline in the Mesozoic ring-complexes, are utilised as evidence to establish that the Nigerian younger granites were derived from an initial peralkaline melt by fusion of local ‘basement’ rocks. It is also suggested that the associated metaluminous trend is caused simply by more complete melting, and that peraluminous granites developed during subsequent cooling. When feldspar normative compositions of younger granites are plotted in the quartz-saturated ternary feldspar system (James and Hamilton, 1969), they support a cyclic event of fusion and cooling to account for the variation in rock-type. The actual source material has not been established but it could be similar to bauchite, a quartz-fayalite monzonite, which was melted by a linear zone of high heat flow from the mantle during the disruption of Gondwanaland.

Petrochemistry and feldspar crystallisation in the silicic volcanic rocks, central North Island, New Zealand

Chemical analyses of various andesitic, dacitic, and especially rhyolitic volcanic rocks are presented, including groundmass and phenocrystic feldspar components separated from a number of these rocks. The rhyolites are characterised by the early separation of phenocrystic plagioclase (An 25–46 ), a second potash-rich feldspar (Or 63–68 ) occurring only rarely. It is shown that in terms of the Ab−Or−An system, the rhyolitic total rock compositions all project into the plagioclase field, and that only a few of the groundmass compositions have reached the feldspar cotectic surface, thus explaining the low frequency of occurrence of sanidine. Analyses of 5 groundmass compositions which coexist with phenocrystic plagioclase, quartz, and sanidine provide information on the projection (within the ternary feldspar system) of the curve defining the intersection of the feldspar cotectic and quartz surfaces. These analyses plot close to the position of this curve as delineated by Carmichael (1963) . An apparent correlation is also noted between the compositions (in terms of the normative feldspar components) of coexisting plagioclasegroundmass pairs and the nature of their coexisting phenocrystic ferromagnesian assemblage i.e. hypersthene; hypersthene+hornblende; or biotite+hornblende±hypersthene.

Petrochemistry and feldspar crystallisation in the silicic volcanic rocks, Central North Island, New Zealand

Chemical analyses of various andesitic, dacitic, and especially rhyolitic volcanic rocks are presented, including groundmass and phenocrystic feldspar components separated from a number of these rocks. The rhyolites are characterised by the early separation of phenocrystic plagioclase (An 25–46 ), a second potash-rich feldspar (Or 65–68 ) occurring only rarely. It is shown that in terms of the Ab-Or-An system, the rhyolitic total rock compositions all project into the plagioclase field, and that only a few of the groundmass compositions have reached the feldspar cotectic surface, thus explaining the low frequency of occurrence of sanidine. Analyses of 5 groundmass compositions which coexist with phenocrystic plagioclase, quartz, and sanidine provide information on the projection (within the ternary feldspar system) of the curve defining the intersection of the feldspar cotectic and quartz surfaces. These analyses plot close to the position of this curve as delineated by Carmichael (1963) . An apparent correlation is also noted between the compositions (in terms of the normative feldspar components) of coexisting plagioclasegroundmass pairs and the nature of their coexisting phenocrystic ferromagnesian assemblage i.e. hypersthene; hypersthene+hornblende; or biotite+hornblende±hypersthene.