Modification of mineral inclusions in garnet under high-pressure conditions: experimental simulation and application to the carbonate-silicate rocks of Kokchetav massif

Abstract

Samples of poikoblastic garnets from the Escambray (Cuba), Maksyutov (Russia), and Sambagawa (Japan) eclogite complexes were heated to 700–1100 ºC at 3 to 4 GPa (30–40 kbar). Epidote, amphibole, and chlorite inclusions in the garnets underwent dehydration melting over the entire experimental PT range, which is typical of ultrahigh-pressure (UHP) metamorphic complexes. In the presence of aqueous fluids, carbonate minerals in the inclusions began to melt at 800 ºC and 3 GPa. Melting gave rise to new garnet, with the composition controlled by the chemistry of the primary inclusions and by PT run conditions. Garnet either grew directly from the melt or formed by replacement of host garnet walls leaving residual melt at the substitution front in the latter case. Partial melting of inclusions decreased the mechanical strength of the garnet host and led to local shearing. The experimental results were used to interpret observed features in two samples of a diamond-bearing and a diamond-free carbonate-silicate rocks from the Kumdy-Kol deposit in the Kokchetav Massif. Multiphase inclusions in both samples contain newly formed garnet with morphologies and compositions consistent with those produced experimentally under the given PT conditions. Minerals in the inclusions are compositionally similar to those in matrix, thus suggesting that melting may have occurred on a large scale.

Dehydration melting of ultrahigh‐pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet

“…Such MS inclusions may form by one of the following mechanisms: (i) incidental trapped solid phases that were stable during garnet growth; (ii) exsolution of the component KAlSi 2 O 6 from former K‐bearing omphacite in coupling with reaction with quartz, after or during the coesite‐to‐quartz transformation (Yang et al. , 1998); (iii) breakdown products of former UHP phases of coesite + K‐cymrite (Massonne & Nasdala, 2003); (iv) dehydration melting of primary mineral inclusions (one or more phases) in garnet, as experimentally documented by Perchuk et al. (2005, 2008, 2009); (v) crystallization from K‐rich partial melt, which was formed by dehydration melting of K‐bearing minerals in felsic UHP rocks and subsequent infiltration of felsic melts into mafic minerals (Perchuk et al.…”

“…The MS inclusions mainly consisting of Kfs + Qz in garnet and omphacite in eclogites are reported from a number of UHP metamorphic zones, including Dabie-Sulu in east-central China (Enami & Zhang, 1990;Yang et al, 1998;Zeng et al, 2007Zeng et al, , 2009), Qilianshan in west-central China (Song et al, 2003), and Erzgebirge in Germany (Massonne & Nasdala, 2003). Such MS inclusions may form by one of the following mechanisms: (i) incidental trapped solid phases that were stable during garnet growth; (ii) exsolution of the component KAlSi 2 O 6 from former K-bearing omphacite in coupling with reaction with quartz, after or during the coesite-to-quartz transformation (Yang et al, 1998); (iii) breakdown products of former UHP phases of coesite + K-cymrite (Massonne & Nasdala, 2003); (iv) dehydration melting of primary mineral inclusions (one or more phases) in garnet, as experimentally documented by Perchuk et al (2005Perchuk et al ( , 2008Perchuk et al ( , 2009); (v) crystallization from K-rich partial melt, which was formed by dehydration melting of K-bearing minerals in felsic UHP rocks and subsequent infiltration of felsic melts into mafic minerals (Perchuk et al, 2002;Zeng et al, 2009).…”

Oriented multiphase needles in garnet from ultrahigh-temperature granulites, Connecticut, U.S.A.

“…A third possibility is that polyphase mineral inclusions trapped during garnet growth melted during heating (e.g., Perchuk et al 2005Perchuk et al , 2009) and then re-equilibrated with the host garnet structure to form acicular inclusions. While this is a possibility, it seems unlikely.…”

“…While this is a possibility, it seems unlikely. First, it is unusual for melted polyphase inclusions to take on the negative crystal shape of their host (Sobolev and Shimizu 1993;Perchuk et al 2009). Second, the uniform size and phase assemblage of the multiphase needles observed in garnets from multiple localities suggests that they were not the product of melting of trapped mineral inclusions.…”

Partial melting due to breakdown of an epidote‐group mineral during exhumation of ultrahigh‐pressure eclogite: An example from the North‐East Greenland Caledonides

“…Partial melting may have occurred within garnet or kyanite (Perchuk, Burchard, Maresch, & Schertl, 2008;Perchuk et al, 2009), if clinopyroxene, zoisite, and quartz coexisted; but melting in the matrix is more likely due to coexistence of the reacting minerals. Newly-formed garnet II grew on pre-existing garnet and, thus, enclosed the melt along with other minerals.…”