A contribution to the pseudoleucite problem

Taylor, Diana; MacKenzie, W. S.

doi:10.1007/bf00376184

Cited by 43 publications

(19 citation statements)

References 9 publications

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“…The first hypothesis is based on textural and field observations (MacKenzie 1915, Pearce 1970, 1993, Peterson et al 1997) and on experimental studies (Peters et al 1966, Liou 1971, Kim & Burley 1971, Roux & Hamilton 1976. The second is equally permissible, based both on observations of leucite par-tially converted into analcime in natural samples (Luhr & Giannetti 1987) and experimental ion-exchange studies (Gupta & Fyfe 1975, Taylor & MacKenzie 1975. The debate has been ongoing since the first detailed study of MacKenzie (1914), who proposed that the analcime is primary.…”

Section: Introductionmentioning

confidence: 99%

Microtextural and Powder-Diffraction Study of Analcime Phenocrysts in Volcanic Rocks of the Crowsnest Formation, Southern Alberta, Canada

Luth

Bowerman

2004

The Canadian Mineralogist

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We present results of powder X-ray-diffraction and microstructural studies of analcime from the volcanic rocks of the Crowsnest Formation, Alberta, Canada. The powder X-ray diffractogram of only one of the analcime samples contains a peak inconsistent with the cubic symmetry that has been attributed to conversion from leucite. We re-evaluate this criterion, and suggest that primary igneous analcime may be cubic as well; caution thus is required in deducing paragenesis from crystal symmetry. A more compelling argument for the primary nature of the analcime is that fresh fracture-surfaces are clean and planar, the characteristics suggested by previous workers to be indicative of crystallization from a melt or fluid. The fracture surfaces lack the porous microstructure previously attributed to the conversion of leucite to analcime. SOMMAIRE Nous présentons les résultats d'études en diffraction X (méthode des poudres) et microstructurales de l'analcime des roches volcaniques de la formation de Crowsnest, en Alberta, Canada. Un des trois échantillons contient une raie en violation de la symétrie cubique, antérieurement attribuée à une transformation de l'analcime à partir de la leucite. Nous ré-évaluons la validité de ce critère, et nous préférons l'hypothèse que la symétrie cubique pourrait aussi caractériser l'analcime ignée primaire. Il est donc nécessaire d'utiliser ce critère paragénétique avec circonspection. Il semble plus sûr de se fier sur la microstructure des phénocristaux. Les surfaces de fractures sont propres et planaires dans le cas de l'analcime fraiche et primaire, indication d'une croissance à partir d'un bain fondu ou bien d'une phase fluide. Les surfaces de fractures sont dépourvues des microstructures poreuses attribuées dans la littérature à la transformation de la leucite en analcime.(Traduit par la Rédaction)

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Section: Introductionmentioning

confidence: 99%

Microtextural and Powder-Diffraction Study of Analcime Phenocrysts in Volcanic Rocks of the Crowsnest Formation, Southern Alberta, Canada

Luth

Bowerman

2004

The Canadian Mineralogist

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“…The Si-undersaturated alkali-feldspar -feldspathoid part of this system is well determined at 1 atm (volatile free, Schairer 1957) and 1, 2 and 5 kbar H 2 O-pressure (Fudali 1963;Taylor and MacKenzie, 1975;Zeng and MacKenzie, 1984). For this study, the most important features of this phase diagram are the granite- (Tuttle andBowen 1958, Luth et al 1964) and phonolite-minima and the alkali-feldspar thermal divide in between.…”

Section: Upper Limit Of Fractionationmentioning

confidence: 79%

Carbonatites in oceanic hotspots

Schmidt¹,

Weidendorfer

2018

Geology

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SUPPLEMENTARY MATERIAL METHODSData were compiled from the pre-sorted georoc database ocean island files (http://georoc.mpchmainz.gwdg.de/georoc/). Hot spots such as Aetna or Samoa with geotectonically complex situations and possible direct interference with subduction were avoided; otherwise we used those with sufficiently large data sets. Each file was filtered for analyses with 95-101.5 wt% totals, samples outside this range or listed as altered were excluded. All bulk rock compositions are then normalized to 100 wt% total on a volatile free basis with all Fe as FeO.The histogram of total alkalis for extrusives shows a clear cut-off to <1.4 wt% K 2 O+Na 2 O. Of e.g. 9'000 extrusives of the Hawaiian hotspot only 24 are below this values, these are essentially from three publications (without geographical coherence), suggesting that these rocks were altered or possibly have an analytical problem; this is corroborated by a K 2 O/Na 2 O ranging from 0.01 to 2 for these 24 samples. We have hence excluded all rocks with <1.4 wt% Na 2 O+K 2 O. Similarly, we did not accept trace element concentrations measured by XRF that are below 5-15 ppm (depending on the element).Within each ocean island, rocks were sorted for (i) Those with X Mg >0.77 and/or Ni>650 ppm, this group is mostly olivine cumulative and was excluded from all plots. (ii) Primitive melts were designated when X Mg =0.77-0.65, which corresponds to equilibrium with olivine of Fo 92-88 at 5-15 mol% Fe 3+ (of Fe tot ), and Ni=150-650 ppm, corresponding to roughly 2000-4500 ppm Ni in mantle olivine (note that the K D olivine/melt of Ni strongly depends on temperature, hence this range is chosen rather large). (iii) To enlarge the dataset, rocks with 0.55100 analyses were retained, the dataset sorted for the above criteria contains 124-198 analyses for Pitcairn, Ascension, St. Helena and Tristan da Cunha; 242-331 for GSA Data Repository 2018136

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“…However, other studies place the phonolite minimum at somewhat higher temperatures, i.e. 750ºC for 100 MPa (Hamilton and MacKenzie, 1965) and 700ºC at 200 MPa (Taylor and MacKenzie, 1975), more consistent with analcime and K-bearing silicate melt becoming stable at 500 MPa and 620À640ºC (Morse, 1969;Roux and Hamilton, 1976).…”

Section: Review Of Previous Studies Of Phase Relations In Neàksàqzmentioning

confidence: 87%

Phase equilibria in NaAlSiO₄–KAlSiO₄–SiO₂–H₂O at 100 MPa pressure: equilibrium leucite composition and the enigma of primary analcime in blairmorites revisited

2014

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Experiments in the system NaAlSiO 4 (Ne)ÀKAlSiO 4 (Ks)ÀSiO 2 (Qz)ÀH 2 O at 100 MPa show that the maximum content of NaAlSi 2 O 6 in leucite is~4 wt.% and that analcime is close to the stoichiometric composition (NaAlSi 2 O 6 .H 2 O). Analcime forms metastably on quenching the higher-temperature experiments; it is secondary after leucite in experiments quenched from 780ºC, while from 850ºC it forms by alteration of leucite, and by devitrification of water-saturated glass. Both processes involve reaction with Na-rich aqueous fluids. Stable analcime forms at 500ºC, well below the solidus, and cannot form as phenocrysts in shallow volcanic systems. New data for natural analcime macrocrysts in blairmorites are presented for the Crowsnest volcanics, Alberta, Canada. Other researchers have suggested that primary analcime occurs as yellow-brown, glassy, analcime phenocrysts. Our microprobe analyses show that such primary analcime is close to stoichiometric, with very low K 2 O (<0.1 wt.%), minor Fe 2 O 3 (0.5À0.8 wt.%) and CaO (~0.5 wt.%). An extrapolation of published experimental data for NeÀKsÀQz at >500 MPa P H 2 O , where Anl + melt coexist, suggests that at >800 MPa two invariant points are present: (1) a reaction point involving Kf + Ab + Anl + melt + vapour; and (2) a eutectic with Kf + Anl + Ne + melt + vapour. We suggest that the nepheline-free equilibrium mineral assemblage for Crowsnest samples is controlled by reaction point (1). In contrast, blairmorites from Lupata Gorge, Mozambique, form at eutectic (2), consistent with the presence of nepheline phenocrysts. Our conclusions, based on high-vs. low-pressure experiments, confirm the suggestion made by other authors, that Crowsnest volcanic rocks must have been erupted explosively to preserve glassy analcime phenocrysts during very rapid, upward transport from deep in the crust (H 2 O pressures >>500 MPa). Only rare examples survived the deuteric and hydrothermal alteration that occurred during and after eruption.

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A contribution to the pseudoleucite problem

Cited by 43 publications

References 9 publications

Microtextural and Powder-Diffraction Study of Analcime Phenocrysts in Volcanic Rocks of the Crowsnest Formation, Southern Alberta, Canada

Microtextural and Powder-Diffraction Study of Analcime Phenocrysts in Volcanic Rocks of the Crowsnest Formation, Southern Alberta, Canada

Carbonatites in oceanic hotspots

Phase equilibria in NaAlSiO₄–KAlSiO₄–SiO₂–H₂O at 100 MPa pressure: equilibrium leucite composition and the enigma of primary analcime in blairmorites revisited

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A contribution to the pseudoleucite problem

Cited by 43 publications

References 9 publications

Microtextural and Powder-Diffraction Study of Analcime Phenocrysts in Volcanic Rocks of the Crowsnest Formation, Southern Alberta, Canada

Microtextural and Powder-Diffraction Study of Analcime Phenocrysts in Volcanic Rocks of the Crowsnest Formation, Southern Alberta, Canada

Carbonatites in oceanic hotspots

Phase equilibria in NaAlSiO4–KAlSiO4–SiO2–H2O at 100 MPa pressure: equilibrium leucite composition and the enigma of primary analcime in blairmorites revisited

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Phase equilibria in NaAlSiO₄–KAlSiO₄–SiO₂–H₂O at 100 MPa pressure: equilibrium leucite composition and the enigma of primary analcime in blairmorites revisited