Newly detected shock-induced high-pressure phases formed in amphibolite clasts of the suevite breccia (Ries impact crater, Germany): Liebermannite, kokchetavite, and other ultrahigh-pressure phases

Abstract

Amphibolite clasts in the suevite of the Ries impact crater contain shock-induced melt veins (SMVs) with high-pressure phases such as majoritic garnet, jadeitic clinopyroxene and others. In addition, heat conduction from hot SMVs into adjacent rock portions locally produced further high P–T melt pools. These melts were preferentially generated in rock domains, where the SMVs cross older (‘pre-Ries’) veinlets with analcime or prehnite and larger grains of sericitized plagioclase. Melting of such chemically different local bulk systems (Na-, Ca-, Ca-Na- and K-Na-rich) was facilitated by low solidus temperatures of the original secondary OH-bearing phases. From the resulting shock-induced melts, liebermannite, kokchetavite, jadeite, nonstoichiometric and albitic jadeite, grossular, vuagnatite, lawsonite + coesite, and clinozoisite crystallized during pressure release. Vuagnatite is now proven to be a genuine high-pressure phase. Its ubiquitous distance of 20–35 μm from the hot shock veins suggests a temperature sensitivity typical for an OH-bearing phase. In local Na-rich melts albitic jadeite appears instead of the assemblage jadeite + SiO2. Liebermannite, a dense polymorph of K-feldspar was identified by Raman spectroscopy. After stishovite, liebermannite constitutes the second known high-pressure phase in the Ries that contains silicon exclusively in six-fold coordination. The KAlSi3O8-polymorph kokchetavite was formed in alkali-rich melt glasses. Pressure and temperature values in the range of about 8–11 GPa and ~ 800–1100 °C were estimated from the chemical compositions of locally occurring majoritic garnets (Si = 3.21–3.32 and 3.06–3.10 apfu), respectively, and the presence of fine-grained aggregates of lawsonite and coesite. Generally, the neighboring areas of the veins are characterized by a sequence of variable high-pressure phases documenting strongly falling P–T conditions with increasing distance from the vein. These novel features enlighten the dynamic event during passage of a shock wave.

Raman spectroscopic study of the transformation of nitrogen‐bearing K‐cymrite during heating experiments: Origin of kokchetavite in high‐pressure metamorphic rocks

“…New findings of kokchetavite in polyphase inclusions are being reported from a growing number of localities 7–9,14 . Surprisingly, almost all of these findings occur in rocks with low bulk potassium content.…”

“…Surprisingly, almost all of these findings occur in rocks with low bulk potassium content. The common interpretation for the origin of kokchetavite is that it comprises a part of the crystallization products of the melt captured at high or even ultrahigh‐pressure conditions 4–6,8,13–16 . This raises the question of the possible origin of this unusual phase in rocks with relatively low potassium concentration 3,8,9 .…”

“…New findings of kokchetavite in polyphase inclusions are being reported from a growing number of localities. [7][8][9]14 Surprisingly, almost all of these findings occur in rocks with low bulk potassium content. The common interpretation for the origin of kokchetavite is that it comprises a part of the crystallization products of the melt captured at high or even ultrahigh-pressure conditions.…”

“…The common interpretation for the origin of kokchetavite is that it comprises a part of the crystallization products of the melt captured at high or even ultrahigh-pressure conditions. [4][5][6]8,[13][14][15][16] This raises the question of the possible origin of this unusual phase in rocks with relatively low potassium concentration. 3,8,9 So far, no kokchetavite has been detected in UHPM metapelitic rocks from the Kokchetav massif [17][18][19][20][21][22] or the Erzgebirge.…”

Inclusions in magmatic zircon from Slavonian mountains (eastern Croatia): anatase, kumdykolite and kokchetavite and implications for the magmatic evolution

“…More findings of kumdykolite and/or kokchetavite have been reported recently, such as multiphase inclusions with kokchetavite in detrital garnet from UHP metamorphic terrane in eastern Papua New Guinea (Baldwin et al, 2021) and kumdykolite and kokchetavite in multiphase inclusions (together with cristobalite) in garnets from UHP terrains in the central Saxonian Erzgebirge in the northwestern Bohemian Massif in Germany (Schönig et al, 2020). Stähle et al (2022) reported the finding of kokchetavite, together with some other UHP phases, in alkali-rich melt glass (shock-induced melt veins) in weakly to moderately shocked amphibolite clasts from the young Ries impact crater in southern Germany (age of structure: 14.8 Ma). Here kokchetavite is found to be in an association with other UHP minerals (liebermannite, albitic jadeite and the immediate vicinity of majoritic garnet), which may indicate crystallisation pressures of nearly 10 GPa.…”