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Time-temperature evolution of microtextures and contained fluids in a plutonic alkali feldspar during heating

Research output: Contribution to journalArticle

  • Ian Parsons
  • John D. Fitz Gerald
  • James K. W. Lee
  • Tim Ivanic
  • Ute Golla-Schindler

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Original languageEnglish
Pages (from-to)155-180
Number of pages26
JournalContributions to Mineralogy and Petrology
Issue number2
Publication statusPublished - Aug 2010


Microtextural changes brought about by heating alkali feldspar crystals from the Shap granite, northern England, at atmospheric pressure, have been studied using transmission and scanning electron microscopy. A typical unheated phenocryst from Shap is composed of about 70 vol% of tweed orthoclase with strain-controlled coherent or semicoherent micro- and crypto-perthitic albite lamellae, with maximum lamellar thicknesses < 1 mu m. Semicoherent lamellae are encircled by nanotunnel loops in two orientations and cut by pull-apart cracks. The average bulk composition of this microtexture is Ab(27.6)Or(71.8)An(0.6). The remaining 30 vol% is deuterically coarsened, microporous patch and vein perthite composed of incoherent subgrains of oligoclase, albite and irregular microcline. The largest subgrains are similar to 3 mu m in diameter. Heating times in the laboratory were 12 to 6,792 h and T from 300A degrees C into the melting interval at 1,100A degrees C. Most samples were annealed at constant T but two were heated to simulate an Ar-40/Ar-39 step-heating schedule. Homogenisation of strain-controlled lamellae by Naa dagger"K inter-diffusion was rapid, so that in all run products at > 700A degrees C, and after > 48 h at 700A degrees C, all such regions were essentially compositionally homogeneous, as indicated by X-ray analyses at fine scale in the transmission electron microscope. Changes in lamellar thickness with time at different T point to an activation energy of similar to 350 kJmol(-1). A lamella which homogenised after 6,800 h at 600A degrees C, therefore, would have required only 0.6 s to do so in the melting interval at 1,100A degrees C. Subgrains in patch perthite homogenised more slowly than coherent lamellae and chemical gradients in patches persisted for > 5,000 h at 700A degrees C. Homogenisation T is in agreement with experimentally determined solvi for coherent ordered intergrowths, when a 50-100A degrees C increase in T for An(1) is applied. Homogenisation of lamellae appears to proceed in an unexpected manner: two smooth interfaces, microstructurally sharp, advance from the original interfaces toward the mid-line of each twinned, semicoherent lamella. In places, the homogenisation interfaces have shapes reflecting the local arrangements of nanotunnels or pull-aparts. Analyses confirm that the change in alkali composition is also relatively sharp at these interfaces. Si-Al disordering is far slower than alkali homogenisation so that tweed texture in orthoclase, tartan twinning in irregular microcline, and Albite twins in albite lamellae and patches persisted in all our experiments, including 5,478 h at 700A degrees C, 148 h at 1,000A degrees C and 5 h at 1,100A degrees C, even though the ensemble in each case was chemically homogeneous. Nanotunnels and pull-aparts were modified after only 50 min at 500A degrees C following the simulated Ar-40/Ar-39 step-heating schedule. New features called 'slots' developed away from albite lamellae, often with planar traces linking slots to the closest lamella. Slot arrays were often aligned along ghost-like regions of diffraction contrast which may mark the original edges of lamellae. We suggest that the slot arrays result from healing of pull-aparts containing fluid. At 700A degrees C and above, the dominant defects were subspherical 'bubbles', which evolved from slots or from regions of deuteric coarsening. The small degree of partial melting observed after 5 h at 1,100A degrees C was often in the vicinity of bubbles.

Larger micropores, which formed at subgrai boundaries in patch perthite during deuteric coarsening, retain their shape up to the melting point, as do the subgrain boundaries themselves. It is clear that modification of defects providing potential fast pathways for diffusion in granitic alkali feldspars begins below 500A degrees C and that defect character progressively changes up to, and beyond, the onset of melting.

ID: 2616712