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GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 172-16
Presentation Time: 9:00 AM-6:30 PM

DIAMOND FORMATION THROUGH ISOCHEMICAL COOLING OF CHO FLUIDS VS REDOX BUFFERING: EXAMPLES FROM MARANGE PERIDOTITIC AND ZIMMI ECLOGITIC DIAMONDS


SMIT, Karen V., Gemological Institute of America, 50 W 47th Street, New York, NY 10036, STACHEL, Thomas, Earth and Atmospheric Sciences, University of Alberta, 1-24 ESB, Edmonton, AB T6G 2E3, Canada, STERN, Richard A., Canadian Centre for Isotopic Microanalysis, University of Alberta, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB T6G 2E3, Canada, SHIREY, Steven B., Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015, STEELE, Andrew, Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad branch Rd, NW Washington, DC 20015 and WANG, Wuyi, Gemological Institute of America, 50 west 47 Street, New York, NY 10036, wwang@gia.edu

Traditional models for diamond formation within the lithospheric mantle invoke either CO3 reduction or CH4 oxidation. Both mechanisms require O2 exchange with the wall-rock at the site of diamond formation. However, peridotite does not have sufficient buffering capacity to allow for diamond formation via these models and instead peridotitic diamonds may form through isochemical cooling of CHO hydrous fluids1. Marange diamonds provide the first natural confirmation of this new diamond growth model2. Marange diamonds do not contain mineral inclusions, but do contain Ni-N-vacancy complexes (in photoluminescence (PL) spectra) suggesting the source fluids equilibrated in Ni-rich peridotite. They contain abundant micro-inclusions of CH4, the first direct observation of reduced fluids percolating through the mantle2.

Diamond growth from reduced species should show decreasing δ13C from core to rim with decreasing [N], however within CH4-bearing sectors of Marange diamonds, such reduced trends are not observed. Rather, δ13C increases from core to rim2. These contradictory observations can be explained through either mixing between CH4- and CO2-rich hydrous fluids2 or through precipitation from already mixed CH4-CO2 hydrous fluids3. These results demonstrate that Marange diamonds could not precipitate during redox buffering. This non-redox model for diamond formation from cooling hydrous fluids is indicated for many gem-quality peridotitic diamonds3.

By contrast diamond formation in eclogitic assemblages is not well constrained. One Zimmi eclogitic diamond (Sierra Leone), has a core-to-rim trend of decreasing δ13C (-23.4 to -24.5 %) and [N] indicative of reduced fluids. These fluids were likely recycled during Neoproterozoic subduction (Re-Os diamond age of 650 Ma overlaps with timing of subduction4). Unlike CH4-CO2 hydrous fluids, isochemical cooling of such reduced CHO fluids is not efficient at forming diamonds and would not result in measurable carbon isotopic variations (i.e., 1‰ variation seen). Rather, this Zimmi diamond likely formed through redox buffering of reduced subduction-related fluids, infiltrating into more oxidised eclogite.

1. Luth and Stachel, 2014. CMP, 168, 1083 2. Smit et al., 2016. Lithos, 265, 68-81 3. Stachel et al., EPSL 473, 44-51 4. Smit et al., 2016. Precamb Res, 286, 152-166

Session No. 172--Booth# 290

T158. Gemological Research in the 21st Century—Characterization, Exploration, and Geological Significance of Diamonds and other Gem Minerals (Posters)
Monday, 23 October 2017: 9:00 AM-6:30 PM
Halls 4EF (Washington State Convention Center)
Geological Society of America Abstracts with Programs. Vol. 49, No. 6
doi: 10.1130/abs/2017AM-307560
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