Strange polymorphs and where to find them: a melt inclusion story

Abstract

<p>Small portions of pristine melt with diameters of 2 to 50µm are increasingly recognized as a rather common occurrence in high grade metamorphic terranes which experienced melting. Their study delivers crucial chemical information on partial melts at depth. But they are also unique "natural experimental charges" where the behaviour of the silicate melt can be investigated, directly in the natural rocks, under P-T-t conditions which cannot be completely reproduced in the laboratory.</p><p>Each nanogranitoid case study has consistently shown H<sub>2</sub>O-bearing, silica and alkali-rich melt. However, rather than a classic granitoid assemblage consisting mainly of quartz and feldspar(s), on cooling these isolated melt droplets produce a plethora of mineral phases identified via microRaman spectroscopy that are rarely –or never- observed as rock-forming minerals. Cristobalite (tetragonal) and tridymite (orthorhombic) are often present as SiO<sub>2</sub> polymorphs, and hexagonal kokchetavite as a polymorph of KAlSi<sub>3</sub>O<sub>8</sub>. NaAlSi<sub>3</sub>O<sub>8</sub> occurs as orthorhombic kumdykolite, whereas CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8 </sub>may occur either as monoclinic svyatoslavite or trigonal dmisteinbergite. Two presently unidentified phases have been also recognized via Raman and analysed via electron microprobe. One has the main peak at 426-430 cm<sup>-1</sup> and has the composition of a granitic glass, whereas the second has a main peak at 412 cm<sup>-1</sup> and a variable composition depending on the inclusion in which it occurs. As their main peaks occur in the same region of most tectosilicates, it is likely that they are two new polymorphs of feldspar, to the best of our knowledge never reported before. These polymorphs have been so far identified in inclusions mainly hosted in garnet, zircon and, in one case, sapphirine and trapped under an extremely variable range of metamorphic conditions (from low P migmatites to UHP eclogites) in very different rock types (metagranitoids, metasediments, mafic and ultramafic rocks).</p><p>Microstructures confirm that all of these phases crystallize directly from the trapped melt on cooling, independently of the internal P of the inclusions or the original conditions of melt entrapment. They appear to be the result of metastability in the inclusions, possibly during rapid crystallization of a melt, not caused by rapid cooling but by the peculiar undercooled and supersaturated conditions achieved on cooling by a melt confined in a small cavity (Ferrero & Angel, 2018). According to this possibility, these polymorphs can be regarded as kinetically stabilized, yet possibly thermodynamically metastable, phases as recently proposed by Zolotarev et al. (2019) for dmisteinbergite. A preliminary crystallization experiment on a haplogranitic melt at undercooled conditions however failed to reproduce such phases. Another possibility is that under natural cooling the confined inclusions experience underpressurization, and the system (i.e. the trapped melt) reacts crystallizing phases, i.e. the polymorphs, less dense than their common counterparts. This would result in the decreasing of the P gradient between inclusions and surrounding rock, equivalent to reducing the free energy of the system.</p><p>References</p><p>Ferrero, S. & Angel, R. 2018. JPet 59, 1671–1700.</p><p>Zolotarev, A.A. et al. 2019. Minerals  9, 570.</p>

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

“…7b). The positions of the first two bands are clearly different from those of K-cymrite at ~ 115 cm -1 and 381 cm -1 (Kanzaki et al 2012;Mikhno et al 2013;Ferrero et al 2016;Romanenko et al 2021). In our sample ZLN128/128a nanometer-sized grains of kokchetavite are often surrounded with albitic jadeite (Fig.…”

“…Therefore, the reaction K-cymrite = kokchetavite + H 2 O cannot any longer be regarded as a second-order reaction and Kok does not necessarily seem to be a metastable phase. We should, however, not forget that this mineral is a low-density phase (~ 2.45 g cm -3 ) and was observed to be stable at < 3 GPa in high-pressure granulites of the Bohemian Massif (Ferrero et al 2016).…”

“…Note a single band at 760 cm µm-sized inclusions in clinopyroxene and garnet in the Kokchetav ultrahigh-pressure complex (Hwang et al 2004). It was erroneously assumed that both phases are characterized by the same space group (P6/mmm) and that Kok is only formed by continuous dehydration of K-cymrite (Mikhno et al 2013;Ferrero et al 2016). Moreover, Romanenko et al (2021) experimentally produced K-cymrite and dehydrated this phase to Kok.…”

Remarkable petrographic features of Sri Lankan granulites; with special regard to unexpected occurrence of “felsite-nanogranite inclusions” in garnet

“…The ‗isobaric cooling of trapped melts in garnet' of stage ① in Hiroi et al (2014) should be changed to ② ‗cooling of trapped melts with substantial overpressure caused by the large difference in compressibility between melt inclusions and host garnet' (Angel et al, 2014(Angel et al, , 2015Ferrero et al, 2016Ferrero et al, , 2018Hiroi et al, 2019). The cracking of host garnet of stage ③ may have been caused by deformation of host rocks during exhumation as well as the deviatoric stresses in the host garnet caused by overpressure (e.g., Angel et al, 2015;Ferrero et al, 2016). The decompression caused by cracking induces instantaneous H 2 O loss from hydrous melt, which imposes effective undercooling of trapped melts resulting in farfrom-equilibrium crystallization to form the characteristic mineral textures of FNIs.…”

Compressibility and pressure-induced structural evolution of kokchetavite, hexagonal polymorph of KAlSi3O8, by single-crystal X-ray diffraction

“…Most of these inclusions are interpreted as 'nanogranitoids' and likely represent melt droplets entrapped during mineral growth at HP/HT conditions. According to Ferrero et al (Ferrero et al 2020) kokchetavite and other phases metastable crystallize of directly from the trapped melt, regardless of the internal pressure or special conditions of melt entrapment. Kokchetavite (including other metastable polymorphs such as cristobalite, kumdykolite, svyatoslavite etc.)…”