Experimental crystallization of undercooled felsic liquids: Generation of pegmatitic texture

Mona Liza C. Sirbescu, Christian Schmidt, Ilya V. Veksler, Alan G. Whittington, Max Wilke

Research output: Contribution to journalArticlepeer-review

31 Scopus citations

Abstract

The crystallization kinetics of silicate liquids were studied experimentally in the system haplogranite- B-Li-H2O, at variable degrees of undercooling and variable water concentration. We investigated the kinetics of nucleation and crystallization of unseeded synthetic hydrous haplogranite with 1wt % Li2O+2.3wt % B2O3 added (composition C1) and 2wt % Li2O+4.6wt % B2O3 added (composition C2). Compositions C1 and C2 are simplified representative bulk compositions of Lirich pegmatites and their highly differentiated cores, respectively. Starting water contents varied between 3 and 9wt %. With few exceptions, the system remained water-undersaturated. About 86 isothermal runs of 1-60 days duration, grouped in 25 time series of constant temperature and initial H2O content, were carried out at temperatures from 400 to 700°C at 300 MPa, corresponding to variable degrees of undercooling between the liquidus and glass transition. Viscosity measurements indicate that the glass transition for both compositions is below 400°C for 3wt % water and below 300°C for 6.5wt % water. The melts remained virtually crystal free at 400°C, about 100°C and 120°C above the glass transition for compositions C1 and C2, respectively, in experiments up to 30 days long. This result is consistent with the existence of low-temperature, undercooled melts in the crust. At lower values of undercooling the runs crystallized partially, up to about 70% volume fraction. Undercooling and the amount of water are the main factors controlling nucleation and growth rates, and therefore textures. Minerals nucleate and grow sequentially according to mineralspecific nucleation delays. The mineral assemblage started with Li-Al stuffed quartz (in C1) and virgilite (in C2), solid-solutions between quartz and γ-spodumene. The quartz-like phases were typically followed by spherulitic alkali feldspar-quartz intergrowths, euhedral petalite, and fine-grained muscovite. Nearly pure quartz formed as rims and replacement of metastable virgilite and stuffed quartz, in particular at the boron- and water-rich crystallization front of large feldspar or petalite. With the exception of muscovite, all minerals nucleated heterogeneously, on the capsule wall or on pre-existing minerals, and grew inwards, towards the capsule center. Experimental textures resembled the textures of zoned pegmatites, including skeletal, graphic, unidirectional, radiating, spherulitic, massive, and replacement textures. In some cases, when fluid saturation was reached, miarolitic cavities developed containing euhedral crystals. Although unidirectional growth rates appeared to slow down in time, volumetric rates for stable graphic alkali-feldspar quartz intergrowths and petalite remained constant for up to 60 days and 70% crystallization. Metastable stuffed quartz and virgilite diminished in their growth rates in runs of 30 days or longer, were resorbed in the melt, and were partially replaced by second-generation quartz. Unobstructed, self-sustained crystal growth in conditions of very low nucleation density appears to be the dominant mechanism to form giant pegmatitic crystals, although experimental growth rates are much slower than predicted in nature based on conductive-cooling models.

Original languageEnglish
Article numberegx027
Pages (from-to)539-568
Number of pages30
JournalJournal of Petrology
Volume58
Issue number3
DOIs
StatePublished - Mar 1 2017

Keywords

  • Crystal growth rate
  • Crystal nucleation
  • Experimental petrology
  • Igneous texture
  • Lithium pegmatites
  • Viscosity

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