Anatectic melt inclusions in ultra high temperature granulites

Abstract

Partial melting up to ultra high temperature (UHT) conditions is one of the major processes for the geochemical differentiation and reworking of the mid‐ to lower continental crust, with relevant implications on its rheological behaviour. UHT granulites from the Gruf Complex (European Central Alps) display garnet and sapphirine porphyroblasts containing a variety of primary melt inclusions (MI). Typically, MI in garnet occur as glassy and polycrystalline inclusions (i.e. nanogranitoids), the latter commonly organized in mm‐sized clusters associated with primary fluid inclusions (FI). Nanogranitoids are characterized by an elliptical faceted shape, with variable sizes ranging from 2 to 115 µm, while glassy inclusions show negative crystal shapes that usually never exceed 15 µm in diameter and present CO2‐rich shrinkage bubbles. The characteristic mineral assemblage observed in nanogranitoids consists of quartz, biotite, muscovite, plagioclase, K‐feldspar, kokchetavite and rarely aluminosilicates. Glassy and re‐homogenized MI are peraluminous and rhyolitic in composition, with SiO2 = 69 − 80 wt% and Na2O + K2O = 5 − 12 wt%. Commonly, the analysed MI have very high K2O (>6 wt%) and very low Na2O (<2 wt%) contents, indicative for potassic to ultrapotassic melts. Measured H2O contents of the melts range from 2.9 to 8.8 wt%, whereas CO2 concentrations are between 160 and 1738 ppm. Accordingly, calculated viscosities for re‐homogenized MI vary between 104 and 105 Pa·s. Related primary FI mainly contain CO2, with rare occurrence of CO and N2, and are commonly associated with quartz, as well as different carbonates and phyllosilicates. It is assumed that the source for the carbonic fluid was external and probably related to the degassing lithospheric mantle. Consequently, it is argued that anatexis was initially triggered by incongruent dehydration melting reactions involving biotite breakdown and proceeded in the presence of an externally derived COH‐bearing fluid. The coexistence of COH‐bearing fluid and MI indicates that partial melting occurred under conditions of fluid − melt immiscibility. Potassic to ultrapotassic MI in UHT granulites suggests that lower crustal anatexis may play a significant role in the redistribution of heat‐producing elements (such as K2O), potentially influencing the thermal structure of the continental crust.

Mesoarchean melt and fluid inclusions in garnet from the Kangerlussuaq basement, Southeast Greenland

“…This suggests that Grt 1 and Grt 2 contain two different melts, melt 1 and melt 2 respectively. Interestingly, the association nanogranitoids + rutile needles visible exclusively in Grt 1 was recently reported by Ferrero et al (2021 a,b) in case studies of nanogranitoids formed at T ~1000°C, possibly pointing toward a higher formation T for the pair melt 1 /Grt 1 with respect to melt 2 /Grt 2 , as already suggested by the modelling. Further studies involving experimental re-homogenisation using a piston cylinder press (see e.g.…”

“…Moreover, the fact that FI and nanogranitoids occur in the same clusters testifies for the growth of both garnet types to have occurred under conditions of fluid-melt immiscibility (e.g. Ferrero et al 2014;Carvalho et al 2019;Gianola et al 2021), in presence of a COH-fluid.…”

Ultrahigh–temperature metamorphism and melt inclusions from the Sør Rondane Mountains, East Antarctica

“…5a), whereas their chemical compositions are similar in the Qz-Ab-Or space (Fig. 5b; Ferrero et al, 2012;Gianola et al, 2021). This suggests that the effect of reaction between the host garnet is less apparent in the Qz-Ab-Or space.…”

“…In the case of UHT pelitic granulites (Gianola et al, 2021) and UHT khondalites (Ferrero et al, 2012), experimentally re-melted NIs enclosed in garnet do not show the same compositional variation with natural glassy inclusions in the AFM space (orange and blue diamonds versus crosses in Fig. 5c).…”

“…5). Orange and blue symbols are remelted NIs and glassy inclusions from UHT metamorphic rocks (Ferrero et al, 2012;Gianola et al, 2021). These datasets include composition of re-melted NIs showing decrepitation texture.…”