Origin of High-Pressure Disordered Metastable Phases (Lonsdaleite and Incipiently Amorphized Quartz) in Metamorphic Rocks: Geodynamic Shock Or Crystal-Scale Overpressure?

Gaston Godard, Maria Luce Frezzotti, Rosaria Palmeri, David C. Smith

Research output: Chapter in Book/Report/Conference proceedingChapterpeer-review

11 Scopus citations

Abstract

Two anomalous metastable phases have been recently reported from ultrahigh pressure metamorphic (UHPM) rocks: α-quartz incipiently amorphized under pressure (IAUP quartz) found in eclogites from Antarctica and lonsdaleite, a hexagonal polytype of sp3-bonded carbon, observed in diamond bearing genesis from the Kokchetav Massif, Kazakhstan. Incipiently, amorphized α-quartz and lonsdaleite are anomalous phases that cannot crystallize at equilibrium in metamorphic rocks. They form in place of coesite/stishovite and diamond, respectively, if crystallization of the latter minerals is impeded by kinetics during a shock or at low temperatures. Besides impactites, these metastable phases have been observed thus far in a few UHPM rocks but they could be more common. Indeed, they remain difficult to put in evidence, because they can be identified only by high-resolution in situ techniques, they are preserved in narrow μm-sized zones, and they show a spatial transition toward normal α-quartz and diamond. For these reasons, they have almost completely escaped notice so far.

Original languageEnglish
Title of host publicationUltrahigh-Pressure Metamorphism
PublisherElsevier Inc.
Pages125-148
Number of pages24
ISBN (Print)9780123851444
DOIs
StatePublished - 2011
Externally publishedYes

Keywords

  • Coesite
  • Diamond
  • Lonsdaleite
  • Nonlithostatic pressure
  • Overpressure
  • Pressure-induced amorphization
  • Quartz
  • Shock
  • UHPM

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