Melt inclusions in emerald from the Habachtal/Austria after re-homogenization at 700°C and two kbar

With homogenization experiment (700°C, 2 kbar) on polished thin sections of emeralds from the Habachtal we demonstrate that the primary crystallization of emerald started at significantly


Introduction
demonstrated for the first time melt inclusions in the emerald of the Habachtal/Austria.In that paper, the authors used well-ordered graphite and extremely fluid-rich melt inclusions for an estimation of the trapping conditions.According to that study, we demonstrated that the emerald mineralization in the Habachtal was primarily from an extremely fluid-rich pegmatite-like aluminosilicate melt under supercritical conditions, at high temperatures and moderate pressures (~700°C, 5 kbar).These results contradict all previous work (e.g., Nwe and Grundmann, 1990).
Melt inclusions which show the characteristic phase composition are mostly small and rare and were not noticed by the previous scientists.By careful analyses of the melt inclusions, a preliminary pseudo-binary solvus could be deduced.For the solvus crest, a temperature of 698°C and a water concentration of 27.7 % was estimated.Furthermore, we could show that MgCO3 in the melt inclusions correlate very well with the bulk water concentration of the corresponding melt inclusions.MgCO3 vs. H2O show an excellent Lorentzian distribution, which is a strong proof for the supercritical conditions and show a significant water loss by counter diffusion (see Thomas et al. in preparation).

Sample and methods
For the study of melt inclusions in emerald we have used double-polished thick sections, about 300 µm thick, from a crystal (about 15 x 4 mm) in biotite schist from the gallery D of the Habachtal mine/Austria (Fig. 1 in Thomas et al. 2020).

Homogenization experiment
For the homogenization of the melt inclusions we used the hydrothermal rapid-quench pressure vessel technic (see Thomas et al. 2009).A double-polished plate of emerald were placed into an Au-capsule (5 x 30 mm) with a fixed amount of pure water.The inclusions were re-melted at 700°C and 2 kbar at a run time of 20 hours.The used temperature of 700°C was chosen from the estimated solvus temperature in Thomas et al. 2020.

Raman spectroscopy
Raman spectra were recorded with the EnSpectr Raman microscope RamMics M532 in the spectral range of 80 -4000 cm -1 using a 50 mW single mode 532 nm laser, an entrance aperture of 20 µm, a holographic grating of 1800 g/mm, and a spectral resolution of 4 -6 cm -1 .

Results
In order to validate of our results presented in Thomas et al. 2020, we have performed a hydrothermal rapid quench experiment (700°C, 2 kbar).From this experiment result a critical temperature of 700°C and a water concentration of 31.6 ± 2.4 %.That is an excellent confirmation of our prediction.Figure 1 shows a typical water-rich melt inclusion in emerald from the Habachtal after re-homogenization and quenching.The phase composition of this melt inclusions is depicted for clarification (G -glass, L -solution, V -vapor).In addition to the typical melt inclusions in emerald shown in Figs. 1 and 2 we have found several large glassy melt inclusions (Fig. 3) containing a water-bearing glass and a large vapor bubble.In these inclusions we found nahcolite crystals [NaHCO3].Given the relatively high concentration of nahcolite (about 6.5 % (vol/vol)), demonstrated by strong Raman bands at 225.4 and 1045, medium bands at 1266, and 1432 cm -1 ) the trapped melt was originally alkali carbonate-rich, which we know from the MgCO3-daughter crystals in the unheated melt inclusions at roomtemperature.However, as already discussed in Thomas et al. 2020, additional MgCO3 also formed as a secondary phase, because of the pervasive presence of Mg 2+ ions.
The Raman spectra of the inclusion glass are difficult to interpret, because no relevant crystallized Be-mineral phase in the glass could be found.A very strong and polarizable Raman band at 568 cm -1 is cautiously interpreted as Na-Li-Be tetrafluoride in relatively high concentration in the glass.According to Quist et al. (1972) and Toth et al. (1973) the Raman band at 568 cm -1 can be assigned to ν1(BeF4 2-) species.The bands at 354, 436, and 467 cm -1 represent Be(OH)2 components.Strong and broad bands between 800 and 1000 cm -1 977 cm -1 are possible evidence of the presence of boric acid (880 cm -1 ) and alkali-borates.Figure 4 gives the Raman spectra of the inclusion glass in comparison with the inclusion host (emerald).The steeply rising background in the low frequency range is typical for all the studied glassy melt inclusions.
Between the glass-vapor phase boundary of some large melt inclusions we also detected small zircon crystals with clear indications that this crystals were grown in the melt and not trapped accidentally, showing that melt can solve compounds like zircons at about 700°C.

Figure 2
Figure 2 show typical melt inclusions trapped near the solvus crest of about 700°C.According to

Figure 1
Figure 1Near-critical melt inclusion in emerald from the Habachtal: G -glass, L -

Figure 3 A
Figure 3A large melt inclusion containing a water-rich glass (G), alkali carbonate-rich

Figure 4
Figure 4Raman spectra of the melt inclusion glass of a re-homogenized melt inclusion of Fig.1 Fig. 4a