Alkali Feldspar Phenocrysts Research Articles

Compositional and microtextural zoning in alkali feldspars from the Shap granite and its geochemical implications

Alkali feldspar phenocrysts in the Lower Devonian Shap granite are compositionally and microtexturally zoned. Most of each phenocryst has exsolved to form microperthite with an approximate bulk composition of Or 72 Ab 28 An <1 , but sub-millimetre wide zones within the phenocrysts with an approximate bulk composition Or 69 Ab 30 An 1 , have exsolved to cryptoperthite. The sub-optical scale of exsolution microtextures in the narrow An-rich zones is due to Ca–Al coupling, which slows rates of interdiffusion during exsolution relative to An-poor volumes of feldspar. This form of coupled compositional-microtextural zoning has not been described previously from Or-rich alkali feldspars using electron microscopy but we have seen other examples and predict that it is common. Cryptoperthite zones are of particular importance because they will have trace element and isotopic compositions least altered, subsequent to growth in the magma, of any microtextures in alkali feldspars and so will be the most rewarding targets for chemical analysis.

Residual-liquid origin for a monzonitic intrusion in a mid-Proterozoic anorthosite complex: The Sybille intrusion, Laramie anorthosite complex, Wyoming

The Sybille intrusion (≈100 km 2) is one of three large monzonitic intrusions in the 1.43 Ga Laramie anorthosite complex of southeastern Wyoming. The petrographic, geochemical, isotopic, and geophysical characteristics of Sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates (ferrodiorites) and contamination by Archean wall rocks. The exposed part of the intrusion is composed mainly of coarse-grained monzosyenites with abundant alkali feldspar phenocrysts. The monzosyenites preserve mineralogical evidence for high crystallization temperatures (>1000 °C), mid-crustal emplacement pressures (≈3 kbar), relatively reduced crystallization conditions (2 log units below the fayalite + magnetite + quartz [FMQ] oxygen buffer), and they crystallized in the presence of a CO2-rich fluid phase (Fuhrman et al., 1988; Frost and Touret, 1989). The eastern monzosyenites, those adjacent to contemporaneous anorthosite, are distinguished by an anhydrous mineral assemblage (Fo16-Fo8 olivine, high-Ca pyroxene) lacking modal quartz, silica contents of 60 wt%, and smaller Eu anomalies (Eu/Eu* = 1.2 to 1.3). Abundant xenoliths of Archean wall rocks and anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping emplacement mechanism near the roof of the chamber. We propose that the monzosyenites represent a relatively thin, 0.5-1.0-km-thick, roof to a magma chamber dominated by dense ferrodioritic cumulates at depth. Extensive, open-system fractionation of a ferrodioritic parent magma, residual after crystallization of anorthosite, produced Fe-enriched monzodioritic and/or monzonitic magma in the upper part of the chamber and complementary Fe- and Ti-rich cumulates in the lower levels. We have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance equilibrium-crystallization experiments. The presence of dense ferrodioritic cumulates at depth is consistent with the prominent positive gravity anomaly associated with the Sybille intrusion (Hodge et al., 1973). In the upper parts of the chamber, the fractionated monzodioritic and/or monzonitic magmas eventually became saturated in alkali feldspar. Owing to density contrasts, the alkali feldspar phenocrysts floated to the roof of the chamber, thus producing the exposed porphyritic monzosyenites. In addition, the roof of the chamber was the site of significant melting of Archean gneiss and, locally, metapelite. The Sr and Nd isotopic compositions of the monzosyenites, with Sr isotopic ratios becoming increasingly radiogenic from east ( I Sr = 0.7059 and initial ϵNd = −2.5) to west ( I Sr = 0.7092 and initial ϵNd = −2.6), are consistent with a 5% to 15% addition of Archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics similar to adjacent anorthositic rocks. The stratigraphic and compositional similarity of the Sybille monzosyenites to mangerites in the Bjerkreim-Sokndal intrusion of the Rogaland anorthosite complex, southern Norway, indicates that similar open-system magmatic processes are capable of having produced high-temperature, K-rich monzonitic rocks in other Proterozoic anorthosite complexes.

Microtextural controls of weathering of perthitic alkali feldspars

The relationship between the microtexture and dissolution behaviour of fresh, HF acid-etched and naturally weathered alkali feldspar phenocrysts from the Lower Devonian Shap granite has been investigated by SEM and TEM. A novel resin impregnation technique has revealed the three dimensional shape and interconnectivity of etch pits beneath the weathered crystal surface. Further electron microscope work suggests that Shap phenocrysts are representative of the alkali feldspar in the protolith of many soils. Fresh and unweathered Shap feldspars have a complex microtexture, comprising areas of pristine cryptoperthite and lamellar microperthite cross-cut by volumes of microporous altered feldspar or “patch perthites.” Cryptoperthites are made up of <75 nm wide albite exsolution lamellae (platelets) in tweed orthoclase, whereas lamellar microperthites contain >75 nm wide albite films. The platelets are coherent, but albite films have numerous edge dislocations along their interface with orthoclase; (001) and (010) cleavage surfaces intersect ∼2–3 edge dislocations/μm 2. In three dimensions, these edge dislocations form an orthogonal net in the “Murchison plane” of easy fracture, close to (601). Patch perthites are irregular, semicoherent to incoherent intergrowths of albite and irregular microcline subgrains, with ∼0.65-0.70 sub-μm to μm-sized pores/μm 2. Microporous patch perthites form by dissolution-reprecipitation reactions with magmatic or hydrothermal fluids and pores are present before the alkali feldspars enter the weathering regime. Dissolution of Shap feldspars during natural weathering and laboratory acid etching is controlled by their microtexture, especially by dislocations and exsolution lamellae. The core and strain energy associated with dislocation outcrops on (001) and (010) cleavage surfaces promotes rapid dissolution at those sites and formation of nn-sized etch pits after <30 s of laboratory etching with HF acid vapour. With progressive HF etching, crystallographically controlled differences in the reactivity of etch pit walls cause them to expand more rapidly into orthoclase than albite. Naturally weathered feldspars were collected from the glacial erratic boulders, fine gravels surrounding exposed granite surfaces, and from peat soil overlying the granite. During natural weathering, etch pits on microperthites enlarge almost exclusively by dissolution of albite and resin casts demonstrate that they can penetrate ≥15 μm below the cleavage surface, forming an interconnecting, ladder-like grid of submicrometer wide channels in the Murchison plane. Coherent albite platelets and volumes of albite between dislocations in films dissolve uniformly, but faster than orthoclase. This is probably because the albite lamellae have significant elastic coherency strain, but this is much less, per unit volume of albite, than the core and strain energy associated with edge dislocations. Patch perthites etch in HF vapour and weather rapidly in nature to produce a honeycomb-like texture of interconnecting nanometer- to micrometer-sized pits, which nucleate at preexisting micropores or incoherent subgrain boundaries. The size and density of etch pits on microperthite surfaces, which is determined by rates of growth and coalescence, may be a useful progress variable for natural and experimental dissolution. All alkali feldspars are highly heterogeneous materials whose chemical composition and microtexture can vary on a submicrometer scale. These microtextures are critical variables with regard to the origin of surface roughness of fresh and weathered grains, the controls on absolute dissolution rates and why they commonly change over time, the nonlinear variation of dissolution rate with grain size, the ratio of alkali ions released into solution, and disparities between laboratory dissolution rates and those observed in the field.

Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite

AbstractAlkali feldspar phenocrysts (bulk composition Or75.0Ab24.6An0.4) in the subsolvus Shap granite comprise a fine-scale mixture of subregular pristine crypto- and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Periclinetwinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at ⩽ 590°C by heterogeneous nucleation of thin albite films which coarsened to &gt; 1 µm length. Cryptoperthites developed at &lt; 400°C by homogeneous nucleation of sub-µm long platelets between films. Platelets are coherent, but the coarser microperthite lamellae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite-orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous µm to sub-µm sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lamellae. Albite was selectively dissolved and micropore-rich irregular microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystallization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at &lt; 400°C; the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of crypto-and micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular microcline.

Strong compositional zonation in peralkaline magma: Menengai, Kenya Rift Valley

The second caldera-forming ash-flow tuff erupted from the Menengai volcano is up to 80 m thick and represents ca. 30 km 3 of peralkaline trachytic magma. Activity started with the emplacement of air-fall tuffs, possibly from a central vent, followed by welded ash-flow tuffs and then by unwelded flow material, the proximal facies of which is lag breccia. At least part of the ash-flow deposits was emplaced through the ring fracture. The tuff was erupted from a compositionally zoned magma chamber which showed strong roofward enrichment in Fe, Mn, Na, Nb, Rb, Th, Y, Zr and the REE, and depletion in Al, Mg, Ca, K, Ti, P, and Ba, corresponding to pantelleritic trachyte at the top, grading down through comenditic trachyte to more primitive (Ba-rich) trachyte. The maximum observed enrichment factor is ×4 for Zr. The chamber may also have been zoned isotopically, 87Sr/ 86Sr becoming less radiogenic (from 0.7062 to 0.7048) and Nd more radiogenic ( ϵ from 0 to −2), upwards. Alkali feldspar phenocrysts generally became more sodic, and olivine and clinopyroxene phenocrysts more Ferich, towards the roof. Variable matrix glass compositions and unusually sodic cores of feldspar phenocrysts in some rocks are thought to result from mixing of liquids from more than one compositional layer during magma withdrawal. The partitioning of Ba, Rb, Sr and the REE between alkali feldspar and liquid was apparently dependent on melt peralkalinity. As the eruption proceeded, a range of compositions was progressively tapped. Compositional zonation in the chamber is inferred to have been continuous. The zonation resulted from up to 75% fractional crystallization of an alkali feldspar-olivine-clinopyroxene-titanomagnetite-apatite assemblage from the least-evolved trachytes, accompanied by minor assimilation of wall rocks, especially in the deeper parts of the chamber.

Mineralogical evidence for the derivation of metaluminous, potassic rocks from peralkaline precursors: The Cordon Syenite Complex (Philippines)

Rocks from the Cordon Syenite Complex, located in the Philippine island arc, correspond to miaskitic (metaluminous) potassic lavas from the Roman Region and the Indonesian arc in terms of bulk rock chemistry. However, the mineral chemistry indicates a complex history of multiple equilibrations with liquids of different nature. T-site (Si + Al) occupancy in some clinopyroxenes and Ca contents of alkali feldspar phenocrysts indicate that these phases crystallized under the temperature-pressure subvolcanic regime from host liquids with progressively decreasing (Na + K)/Al ratio. In contrast to current views on the genesis of potassic rocks, it is suggested that differentiated leucite tephrite suites may be derived from peralkaline liquids by alkali loss. Magmas with affinities to lamproites may occur in the Philippine island arc, but a direct genetic relationship between subduction and alkaline/peralkaline magmatism remains questionable.

Strontium, neodymium and lead isotope characteristics of the Granadilla Pumice, Tenerife: a study of the causes of strontium isotope disequilibrium in felsic pyroclastic deposits

Summary The Granadilla Pumice deposit, the product of a single explosive eruption of phonolitic magma, exhibits stratigraphically systematic internal compositional variations interpreted as the result of fractionation of the observed phenocryst assemblage in the pre-eruptive magma body. Whole-rock 87 Sr/ 86 Sr data exhibit a consistent profile, with the most differentiated compositions having the most radiogenic strontium. The resultant 87 Sr/ 86 Sr versus 1/Sr mixing line has a positive slope, suggesting the addition of a relatively radiogenic strontium component to the existing strontium concentration profile. There is no significant systematic variation, however, in neodymium or lead isotopes. Separated alkali feldspar phenocrysts have 87 Sr/ 86 Sr values of 0.7031–0.7032, identical with Tenerife mafic magmas; the strontium isotopic variation is restricted to the complementary glasses. The more radiogenic component in the glasses is only partially removed by acid leaching and has a strontium isotope ratio of about 0.707, deduced from analysis of the leachant. Strong acid leaching, accompanied by up to 25% sample dissolution, results in greater, but not complete, loss of the more radiogenic component, with a good correlation between pre- and post-leaching glass isotopic compositions. These relationships cannot be easily explained by either post-eruptive alteration or contamination of the magma chamber with high- 87 Sr/ 86 Sr country rock. The data are instead consistent with a model of interaction between vesiculating magma and hydrothermal fluids with high 87 Sr/ 86 Sr, immediately before and/or during explosive eruption, involving limited diffusion of strontium from the fluid into bubble walls before quenching of the magma to form glassy pumice. This process may be common during large explosive eruptions, and we suggest that whole-rock strontium isotope variations found in strontium-poor felsic pyroclastic deposits may not reflect purely magmatic processes, even though they closely mimic plausible magmatic variations.

Petrography of orbicular granitoids in the Separation Point

Abstract Eleven occurrences of orbicular fabric in the (mostly) coastal exposures of the Separation Point Granite in the Motueka region, northwest Nelson, are described. Alkali-feldspar phenocrysts, biotite clusters, and orbicules may be concentrated locally. The orbicules, from 3 cm to over 20 cm in diameter, occur as a few to many individuals or as densely packed aggregates (orbiculite) in a granitic matrix similar to the main rock. Orbicules consist of a spheroidal or squarish core either alone or with one to several concentric shells. Cores are essentially plagioclase (often radiating mosaics), whereas shells are either coarse alkali-feldspar (with quartz) or concentrations of magnetite and biotite arranged concentrically or tangentially through the persisting plagioclase, and usually of increasing abundance towards the edges of the orbicules. The orbicules may be elliptically deformed, or even fragmented, indicating a dynamic environment of formation and accumulation in the magma during its consolida...

The petrology of a metamorphosed netveined acid-basic intrusion from the Østfold area, Southeast Norway

A 1325-1015 Ma old minor netveined acid-basic intrusion within the Sveconorwegian basement terrain of SE Norway is described. The intrusion is well preserved in spite of a superimposed Sveconorwegian amphibolite facies metamorphism (around 1015 Ma). In the netveined intrusion the acid and the basic material represent a potassic quartz monzonite of rapakivi type and a highly fractionated tholeiite enriched in FeOx, TiO2, K2O, P2O5 and incompatible trace elements. These two materials have existed side by side as magmas of essentially the same temperature. Interaction between the materials is demonstrated, as there is evidence of: (1) selective element diffusion comprising Sr (and probably K and Rb), (2) phenocryst capturing, where the basic magma well above its solidus temperature at least captured alkali feldspar and quartz phenocrysts from the potassic quartz monzonitic magma, (3) formation of basic hybrids with an emulsion-like structure, and (4) formation of mechanical mixed hybrids. Based on Sr isotope mixing calculations it appears that the two melts existed side by side 1300 Ma ago. It is argued that the potassic quartz-monzonite magma was hot (> 1100°C) and dry (< 0.6% H2O). It was possibly formed by total melting of a material of similar composition which could have been the basic member of the Moss-Filtvet augen gneiss (granite) which outcrops in the neighbourhood, or alternatively by partial melting of older intermediate or basic rocks at a deeper level in the crust. The heat necessary for the melting was presumably introduced by the injection of the tholeiitic magma that also was the source of the basic component of the netveined intrusion. The formation of the netveined structure of the intrusion has presumably been guided by development of curviplanar joints in the basic material, when the potassic quartz monzonite was still very mobile.

Location of a caldera source for the Soldier Meadow Tuff, northwestern Nevada, indicated by gravity and aeromagnetic data: Summary

The Soldier Meadow Tuff in north-western Nevada is part of a sequence of peralkaline and subalkaline rhyolitic rocks of Miocene age that occupies an area of about 1,600 km 2 and has a volume of about 50 km 3 . The formation is mostly welded tuff containing abundant alkali feldspar and quartz phenocrysts in devitrified groundmass containing minute needles of blue-green alkali amphibole. The rhyolite of Badger Mountain is similar in mineralogy and chemistry, but textures suggest that it is a flow rhyolite. The underlying rhyolite of Catnip Mountain and tuff of Trough Mountain are phenocryst-poor but apparently also peralkaline. Gravity and aeromagnetic anomaly maps show 16- by 24-km anomalies near Badger Mountain. An analysis of these maps suggests that observed anomalies could be caused by a concealed caldera of 2,700-m thickness filled with rocks 0.2 g/cm 3 less dense and 0***002 emu/cm 3 lower magnetic susceptibility than the surrounding rocks. Distribution of the Soldier Meadow Tuff and the occurrence of numerous vents in the rhyolite of Badger Mountain suggest that the source for both units is associated with the indicated caldera. Possible alternative sources are other calderas whose locations are suggested by a 7- by 11-km gravity minimum near Rock Spring Table and a 12- by 24-km magnetic minimum west of Soldier Meadow.

Partition coefficients of alkali and alkaline-earth elements between alkali feldspar phenocrysts and their lava matrix

SummaryPlots of partition coefficients (mineral/ matrix) against ionic radius of the alkali and alkaline earth elements in forty-four alkali feldspars from rhyolites and trachytes show two kinds of diagram, one for homogeneous alkali feldspars with a single peak, and the other for cryptoperthites with a wide spread of points, which cannot represent a single peak, but may probably be explicable in terms of two peaks. The difference in behaviour does not seem to be due to the chemical composition, but is apparently related to the crystallo-graphic features of the alkali feldspar structures. In particular the presence of two peaks in the diagrams of the cryptoperthites seems to be due to the existence of lattice positions suited to receive bigger ions (K, Ba, Rb) and smaller ions (Na, Ca, Sr) in the potassic and sodic domains.

Mineralogy of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico

Electron probe microanalyses of minerals in the nine intrusive rock types of the Diablo Plateau document compositional changes accompanying magmatic differentiation. Plagioclase occurs only in the least differentiated rock type. Alkali feldspar phenocrysts show little compositional variation. Groundmass feldspar compositions were modified by exsolution and deuteric alteration, particularly during growth of natrolite. Nepheline compositions in these hypabyssal rocks are intermediate between those of volcanic and plutonic nephelines elsewhere. Mafic minerals show wider compositional variations. Clinopyroxene, the most abundant of these, changed from sodian augite toward hedenbergite, and then toward acmite. The more sodic pyroxenes crystallized later than feldspars and nepheline from residual liquids after emplacement at the now-exposed level. Amphiboles and biotites show progressive decreases in Ti, Mg, and tetrahedral Al/Si. In the least differentiated rock type and in some autoliths, the amphibole is kaersutite; in later formed rocks, hornblendes and sodic amphiboles are widespread. Other prominent minerals showing compositional variation are manganoan fayalite, aenigmatite, and eudialyte. For the nepheline syenites, crystallization temperatures are estimated at 700 to 725 °C, with log ( f O 2 ) = −20 and log ( f H 2 O ) = + 1.6 to 2.1.

Petrology and Rb-Sr age of Precambrian rhyolitic dikes, Llano County, Texas

Rhyolite porphyry (llanite) and iron-rich melarhyolite dikes intrude Precambrian metamorphic rocks in north-central and south-eastern Llano County, respectively. These dikes are chemically and mineralogically similar, except that rhyolite porphyry is richer in quartz and alkali feldspar phenocrysts, whereas melarhyolite contains a greater abundance of biotite and magnetite. There are corresponding enrichments of SiO 2 in rhyolite porphyry and of FeO + Fe 2 O 3 in melarhyolite. The two rock types may have originated as two fractions of a common parent magma enriched in salic and mafic phases, respectively, to produce rhyolite porphyry and melarhyolite. This proposed relation is compatible with major-element chemistry and Rb-Sr systematics. Isotopic analyses of five melarhyolite whole rocks, two rhyolite porphyry whole rocks, groundmass and feldspar phenocrysts from rhyolite porphyry, and a nearby diabase dike yield an isochron age of 1,106 ± 6 m.y. (λ Rb = 1.39 × 10 −11 yr −1 and initial 87 Sr/ 86 Sr of 0.7028 ± 0.0002. Other data, including Zartman9s (1964) analyses of the microcline fraction of alkali feldspar phenocrysts from rhyolite porphyry, indicate subsequent Rb-Sr redistributions. These may be related to devitrification of melarhyolite groundmass and to unmixing of alkali feldspar phenocrysts in rhyolite porphyry.

Element partitioning in alkali feldspars from three intrusive bodies of the central Wasatch Range, Utah

The partitioning of elements in alkali feldspars from the Clayton Peak, Alta, and Little Cottonwood stocks of the central Wasatch Range, Utah, suggests a common magmatic origin for the three intrusive bodies. With decrease in age of intrusion, the alkali feldspars show a successive depletion of Ca, Sr, and Ba and a decrease of the K/Rb ratio. Alkali feldspar phenocrysts from the Little Cottonwood stock contain higher concentrations of Ca, Sr, and Ba and a higher K/Rb ratio than the groundmass alkali feldspars, indicating an earlier period of crystallization in the melt. The phenocrysts are zoned in Ba and have relatively low concentrations in the rims and high concentrations in the centers, suggesting that they crystallized in a melt undergoing progressive depletion of barium.

Tertiary volcanism in the Cape Hillsborough area, North Queensland

Abstract Potassium‐argon ages on alkali feldspar phenocrysts from trachyte lavas and on trachyandesites of the Cape Hillsborough Beds, which crop out north of Mackay, Queensland, yield concordant ages of 32.5 ± 0.4 m.y. This age is Early Oligocene and by correlation provides much firmer control than previously available on the age of the sediments deposited in the Hillsborough Basin. Isotopic ages on an altered biotite and on alkali feldspar from the Mount Jukes Syenite Complex, to the southwest of Cape Hillsborough, are indistinguishable from those measured on the Cape Hillsborough Beds, suggesting a possible genetic relationship between the lavas and the syenite intrusions. Present evidence indicates that Cainozoic volcanism in Queensland occurred in two main eruptive episodes. The earlier episode covered much of the Oligocene and its products are widespread in the southern half of the State. After a long hiatus of little or no volcanism there followed a later episode of volcanism in the Pliocene and Qu...

Leaching of quartz from precambrian hypabyssal rhyolite porphyry, Llano County, Texas

A Precambrian hypabyssal rhyolite porphyry in central Texas has retained its original texture, although alkali feldspar phenocrysts have inverted and unmixed from zoned, single-phase high-temperature feldspar to zoned microperthite with an intermediate microcline host.

Origin of some feldspar phenocrysts and their bearing on petrogenesis

The alkali feldspar phenocrysts of certain granitic rocks include zonally arranged, oriented plagioclase crystals. The oriented plagioclase inclusions are commonly euhedral with oscillating zoning, suggesting a magmatic origin for both the inclusions and the alkali feldspar hosts. In a porphyritic quartz monzonite in the Okanogan Range, Washington, the rims of oriented plagioclase inclusions are systematically more sodic outward from the core of the alkali feldspar hosts. This trend plus other characteristics of the oriented phenocrysts indicate growth in a silicate melt.