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

J. Kotková,Y. Fedortchouk,R. Wirth,M. J. Whitehouse

doi:10.1038/s41598-021-87272-1

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.

Highlights

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
Microdiamonds of UHP metamorphic terranes are found in metasedimentary rocks with continental crustal affinities that have been subducted to mantle depths in collisional orogens and provide key information about deep subduction processes
Pioneering investigations[4] using Transmission Electron Microscopy (TEM) with focused ion beam sample preparation (FIB) and Fourier Transform Infrared Spectroscopy (FTIR) documented both solid and fluid nanoinclusions in microdiamonds whose diverse composition largely reflects that of the host rock and C–O–H fluid[1,5]

Summary

Introduction

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 Their nucleation and growth mechanisms are not well understood due to their small size and diversity. Microdiamonds of UHP metamorphic terranes are found in metasedimentary rocks with continental crustal affinities that have been subducted to mantle depths in collisional orogens and provide key information about deep subduction processes These microdiamonds (several tens to, rarely, hundreds of micrometers in size) mostly occur as inclusions within the rock-forming and accessory phases (garnet, clinopyroxene, kyanite and zircon)[1]. We integrate a study of Bohemian microdiamonds with available data on confirmed microdiamond occurrences in metasedimentary rocks (gneisses) worldwide to discover the origin of UHP diamonds and to constrain the factors controlling their nucleation and growth features. We propose a new model for UHP microdiamond crystallization worldwide, making a substantial step forward in the presently accepted concept of microdiamond formation from a supercritical C–O–H fluid/melt[1]

Methods

Conclusion