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

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.