Characterizing the fluid films surrounding inclusions in lithospheric diamond: further clues to the environment of diamond formation

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The formation of diamond is a process fundamental to our understanding of the deep carbon cycle of the Earth. The occurrence of mineral inclusions provides important information about the conditions under which the enveloping diamond formed. Although diamond is considered to be able to form in a variety of ways, a fluid or melt phase is usually implicated as the carbon source. The nature of this mobile agent is of extreme interest since it may also cause metasomatic overprinting of mantle rocks in addition to providing clues about the process of diamond formation. The presence of fluid inclusions is generally restricted to specific types of diamond (flbrous, cloudy or coated) and are very rare in gem-quality monocrystalline diamond, which make up the majority of recovered diamonds. However, a thin fluid layer surrounding a variety of mineral inclusions in numerous diamonds has recently been identified by Nimis et al. (2016) and is considered to represent a mobile phase trapped during diamond formation. Aside from Raman spectra indicating the presence of OH and Si, nothing is known about the composition of this phase. The goal of this project is to analyze the composition of the fluid layer surrounding mineral inclusions in a set of diamonds provided by Prof. F. Nestola (University of Padua, Italy). As a first step, the samples will be thoroughly characterized to establish the presence of a fluid layer and the depth of the inclusion from the diamond surface using Raman spectroscopy and a high-resolution digital microscope (this has already been done for 4 samples). With this information we will then employ time-resolved laser ablation ICPMS using a newly installed "fast funnel" ablation system in combination with the elimination of spectral skew by alignment of laser and mass spectrometer. This system provides a much improved time resolution, allowing for a "cleaner" distinction of the signal from the fluid phase, with less mixing from the adjacent inclusion. As the laser continues to ablate into the inclusion, compositional information for the mineral will be obtained, which can be used via two-component mixing systematics to help deconvolve the fluid signal as well. At a minimum, reliable quantitative elemental concentration ratios will be obtained, although semiquantitative concentration estimates may also be possible for the fluid, thus helping to constrain the composition of this mobile medium responsible for crystallization of monocrystalline gem-quality diamonds in the deep lithospheric mantle.