February 23, 2017
Suppose you want to examine the inside of a rock. You can break it, cut it, crush it, grind it, polish it, or employ any number of procedures at cracking your sample open and taking a peek inside. What happens if you exhaust all of these methods and still cannot succeed? Well, you’re probably in the same situation as me and studying the hardest natural substance on Earth – carbonado diamonds.
Carbonados are an enigmatic variety of polycrystalline diamond that have never been found in or associated with typical diamondiferous volcanic rocks (i.e. kimberlites or lamproites). These irregularly-shaped black nodules, found in placer deposits only in Brazil and the Central African Republic, possess a range of peculiar characteristics including high porosity and the ability to grow to large sizes (the largest diamond ever found is a carbonado at 3167 ct.)!
Additionally, carbonado diamonds can be microporphyritic – meaning it’s composed of randomly oriented tiny diamonds sintered together – and possess an unusual suite of inclusions including highly reduced metals and metal-alloys, and euhedral psuedomorphs. Finally, carbonados have isotopically light carbon (δ13C = -24 to -31 ‰) and yield ages of 2.6 to 3.8 Ga based off Pb-Pb in quartz, rutile, and clay found in exterior pores and U-Th-Pb from acid leaching of the secondary minerals (e.g. florencite, kaolinite, orthoclase, and quartz) from the internal pore network.
These are just a few of the unusual characteristics of carbonados, which have led previous researches to develop some “out-of-this-world” models for formation:
By sticking with Occam’s razor, our goal is to prove their terrestrial origin, likely in the mantle, then create a genetic model that can explain all of these strange features and hopefully answer some difficult questions about the origin and kinematics of carbon-saturated fluids within the mantle. However, because of their polycrystalline nature, carbonados cannot be cut with typical diamond saws. Many diamond sawblades and polishing wheels have been damaged and many hours wasted (14 hours in my case) in the futile attempts to cut or polish carbonados.
Fortunately, we have X-ray computed tomography!
The mere presence of pores in diamonds that grow at extreme pressures is unfathomable. Therefore, the pores must shed light on the process that formed these diamonds. We have scanned seven samples (largest is 23.45 ct., but hopefully more are on the way) that are the subject of current research. This is the first three-dimensional, fully-comprehensive, multi-sampled description of the pore network and any other internal textures.
If a fabric or any other systematic texture exists, this too must provide further clues to deciphering the genetic conditions. The alignment of elongated, blebby pores has been noticed by previous researchers in their 2-D surfaces. For the first time, we have been able to map the orientation of the pores in 3-D and identify a preferred fabric within carbonado diamonds.
By continuing to study these diamonds using CT, we hope to finally put the “Carbonado Conundrum” to rest. If our model is accepted, we can further understand the composition, oxygen fugacity, and kinematics of fluids in the mantle, as well as the timing and origin of isotopically light carbon reservoirs in the mantle; this could shed light on the onset of plate tectonics or identify light-carbon-rich primordial reservoirs. Synthesis of carbonado-like diamonds could provide a cheap, light-weight, and near- indestructible material for industrial applications. Carbonado diamonds can provide answers to multiple questions on various scales from planetary evolution to sub-micron diamond growth.
I hope to keep sharing my results on this blog so stay posted for when we give these diamonds a bath in acid.
List of genetic hypotheses adapted from: Haggerty, S. E. (2014). “Carbonado: Physical and chemical properties, a critical evaluation of proposed origins, and a revised genetic model.” Earth-Science Reviews 130: 49-72.
