Metamorphic microdiamond formation is controlled by water activity, phase transitions and temperature

Abstract

Metamorphic diamonds hosted by major and accessory phases in ultrahigh-pressure (UHP) metamorphic terranes represent important indicators of deep subduction and exhumation of continental crust at convergent plate boundaries. However, their nucleation and growth mechanisms are not well understood due to their small size and diversity. The Bohemian microdiamond samples represent a unique occurrence of monocrystalline octahedral and polycrystalline cubo-octahedral microdiamonds in two different metasedimentary rock types. By combining new and published data on microdiamonds (morphology, resorption, associated phases, carbon isotope composition) with P–T constraints from their host rocks, we demonstrate that the peak P–T conditions for the diamond-bearing UHP rocks cluster along water activity-related phase transitions that determine the microdiamond features. With increasing temperature, the diamond-forming medium changes from aqueous fluid to hydrous melt, and diamond morphology evolves from cubo-octahedral to octahedral. The latter is restricted to the UHP-UHT rocks exceeding 1100 °C, which is above the incongruent melting of phengite, where microdiamonds nucleate along a prograde P–T path in silicate-carbonate hydrous melt. The observed effect of temperature on diamond morphology supports experimental data on diamond growth and can be used for examining growth conditions of cratonic diamonds from kimberlites, which are dominated by octahedra and their resorbed forms.

Morphology and Raman spectral parameters of Bohemian microdiamonds: implications to elastic geothermobarometry

“…The phase boundary between diamond and kyanite host is straight and closed, without any associated phases or pore spaces except from a sub-micron layer of graphite and a low-pressure assemblage of ferrite, quartz and white mica, and a single cavity with relics of a quench phase (Fig. 3b, see also Kotková et al 2021). The grains are homogeneous, without any core-rim structures in CL, or subgrains in FIB-TEM, and lack inclusions in their interior.…”

“…2a). Dissolution features are rare and limited to a few single negative trigons on octahedral faces (Kotková et al 2021). Octahedral macles also rarely occur (Fig.…”

“…The interface between diamond and host garnet and zircon is straight, with steps (Fig. 3d), or curved, and it can be closed or partially opened, with elongated gaps and steps with (111) orientation creating a zig-zag surface (Figs 3h, i) containing amorphous quench material (see also Kotková et al 2021). In addition, a 0.5 to 1 µm thick layer of graphite and amorphous carbon, with a straight boundary with diamond not indicating any replacement, occurs at the diamond interface with garnet (Fig.…”

“…Fourth, the presence of diamonds in Ti-rich zircon domains (up to 190 ppm) allows constraining their entrapment at ≥ 1100 °C and 4.5-5.0 GPa, P-T conditions similar to those determined for the associated garnet peridotites (Medaris et al 2015;Haifler and Kotková 2016). The rocks were exhumed along a near-to-isothermal (ITD) adiabatic exhumation path, characteristic of diamondbearing ultrahigh-pressure terranes worldwide (Haifler and Kotková 2016;Kotková et al 2021). U-Pb ages of zircon and rutile overlap close to 340 Ma, suggesting a rapid exhumation (Kotková et al 1996).…”

“…First, they are enclosed in garnet with cubic, high crystallographic symmetry which will therefore develop isotropic strain under hydrostatic stress (Mazzucchelli et al 2019) as well as kyanite and zircon (elastically anisotropic). Second, they are well-preserved, without graphitization features, and they form individual grains in the host phases -unlike other microdiamonds in the world that can show graphite coatings and make part of the multiphase solid inclusions (see Kotková et al 2021 and references therein). Third, they include both perfect octahedral crystals and imperfect cuboid forms.…”