Date: 16:00 – 18:00, Friday, Fabruary, 17, 2023
Title：A review of noble gas studies on natural diamonds
Diamond is thermodynamically more unstable than graphite at room temperature and pressure, so naturally occurring diamonds had been formed in the high-temperature and high-pressure environment in the Earth’s mantle, which is deeper than 150 km, and were quickly transported to the surface by a kimberlite magma. Because diamonds are hard and chemically stable, they preserve the information they acquired during their formation to the present day. Besides nitrogen and hydrogen incorporated into the crystal lattice, diamonds also contain impurities on a larger scale, such as mineral and fluid inclusions that were introduced from the surrounding environment when they were formed. In addition to the isotopic ratio of the carbon and nitrogen, information obtained from such inclusions is key to knowing the time (age) and environment in which the diamond was formed.
Based on the noble gas isotope ratios of both Siberian diamonds and kimberlites, we have revealed that (1) the kimberlites originated in an upwelling from the deep mantle (mantle plume) and (2) the diamonds formed in a very short period of time (on a geological timescale) between the plume’s ascent to the shallow mantle and the kimberlite magma formation. In addition, the noble gas isotope ratios of diamonds in ultrahigh-pressure metamorphic rocks, which are pieces of continental crust that were subducted into the mantle and returned to the surface in the past, suggest that volatiles supplied by mantle plumes may play an important role in the circulation process of crustal materials that were subducted into the mantle. In the seminar, I would like to introduce these studies on natural diamonds.
Title: The primordial superdeep mantle effectively replaced by subduction-related components
The Cretaceous Juína Kimberlite Province, located on the southwestern margin of the Amazonian Craton is well-known as a region with abundant occurrence of diamonds, in which the majority of them (>98 %) has superdeep (>300 km) origin. Here we determined all noble gas isotopes for twenty-one selected diamonds, totalizing forty-six measurements. The measurements were performed by the crushing extraction method applying at least two strokes (10 and 20 MPa). We confirmed the superdeep origin of the selected diamonds through the identification of high-pressure mineral phases by Raman spectroscopy. These phases are mainly represented by wüstite (ferropericlase), but also by larnite, breyite, maohokite, and ringwoodite. The helium isotopic ratios are predominantly radiogenic with low 3He/4He ratios (0.01-4.94 RA). In less extent, we obtained high and primordial 3He/4He isotopic ratios (10.00-27.74 RA). Notably, two diamond samples record the existence of a primordial component based on both helium and neon isotopic ratios. The diamond that has an inclusion of ringwoodite show strictly primordial 3He/4He isotopic ratios (20.49 and 27.74 RA), whereas the other noble gases containing high blank contribution. The 40Ar/36Ar isotopic ratios of Juína diamonds indicate a mixing between the air (296) and a mantle component (909) that was strong affected by the former. It is important to observe that some argon results are slightly lower than the air ratio, which would suggest mass fractionation of the light noble gases. Thus, we need to confirm if the neon isotopic ratios are not related to this process. As commonly observed in mantle-derived minerals, the Kr and Xe isotopes show atmospheric composition, and they have strong blank contribution. Therefore, we suggest that an ancient subduction zone (probably with Cretaceous age) was responsible for overprinted the primordial noble gas composition of the superdeep mantle section beneath Juína through the effective recycling of a subducted-related component.