Deep carbon through time: Earth’s diamond record and its implications for carbon cycling and fluid speciation in the mantle

Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earth’s history. Diamonds’ inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as ∼3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ∼3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total δ13C data range from −17.1 to −1.9 ‰ (P = −8.4 to −1.9 ‰; E = −17.1 to −2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in δ13C of peridotitic diamonds has been at −5 (±2) ‰ since the earliest diamond growth ∼3.5 Ga, and this mode is also observed in the eclogitic diamond record since ∼3 Ga. The skewness of eclogitic diamonds’ δ13C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent. No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal δ13C mode of −5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga

Exhumation of lower mantle inclusions in diamond: ATEM investigation of retrograde phase transitions, reactions and exsolution

A multiphase inclusion (KK-83) in a diamond from the Kankan deposits in Guinea, resulting from complex reactions between primary lower mantle phases, was examined in detail using analytical transmission electron microscopy. The inclusion consists of a large (300 μm) diopside crystal with a small symplectitic intergrowth in one corner, comprising olivine and tetragonal almandine pyrope phase (TAPP). The olivine part of the symplectite contains small exsolutions of Mg-Al-chromite and rare Ca-carbonate. A second inclusion of ferropericlase observed in the same diamond indicates a primary origin within the lower mantle. The clinopyroxene formed through a series of reactions at the expense of touching inclusions of CaSi- and MgSi-perovskite during convective ascent of the diamond through the transition zone. The proposed reaction sequence is: MgSiPvk+CaSiPvk→MgSiIlm+CaSiPvk→ringwoodite+stishovite+CaSiPvk→clinopyroxene+relict ringwoodite. The high Al-content of the bulk symplectite indicates ringwoodite as precursor phase coexisting with clinopyroxene. A slight deficiency of SiO2 during the diopside forming reaction leads to a small amount of relict ringwoodite (<0.1 vol% of the total inclusion is occupied by ringwoodite) coexisting with diopside. The ringwoodite captures the observed high amount of Al. Subsequent breakdown of ringwoodite to wadsleyite+TAPP produces the observed symplectitic intergrowth. During the phase transformation of wadsleyite to olivine, exsolution of Mg-Al-chromite occurs further decreasing the original solubility of Al, Cr and Ti, in accordance with experimental data on the element partitioning between wadsleyite and olivine. Based on these observations, TAPP can form as a retrograde phase within the transition zone of the Earth’s mantle and is not restricted to the upper part of the lower mantle. High Fe3+-contents may favour its formation

On the formation of peridotite-derived Os-rich PGE alloys

Osmium-rich Pt group element (PGE) alloys occur worldwide in association with chromite in ultramafic (peridotite) complexes. It has been suggested that these Os-rich alloys formed under extreme P-T conditions in the lowermost mantle, before the metallic core of the Earth formed, or later, in the outer core, and have been transported to the upper mantle as xenoliths in deep-rooted mantle plumes

High energy synchrotron X-ray fluorescence trace element study of a millimeter-sized asteroidal particle in preparation for the Hayabusa2 return sample analyses

The trace element content and distribution including rare Earth elements (REEs) measured in mm-sized asteroidal samples returned by JAXA's Hayabusa2 mission are important chemical parameters to decipher asteroid Ryugu's chronology of formation linked to early Solar System processes. In order to identify and analyze ancient Solar Nebula components, such as calcium-aluminum-rich inclusions (CAIs), by their trace element and REE content, a synchrotron beam with an incident energy of 90 keV is used to optimize the XRF signal and increase the information depth in the sample. The application of a (sub-)microscopic X-ray beam with such a high excitation energy not only allows for the detection of a wide range of heavy elements, but also to study their 3 dimensional distribution in mm-sized samples by means of X-ray fluorescence computed tomography (XRF-CT). The experiment was performed in anticipation of the initial analysis of the Hayabusa2 return samples at beamline ID15a of the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The samples were analyzed with a focused beam of 0.5 x 0.5 mu m2, achieving limit of detection values as low as 0.5 ppm with an acquisition time of 1 s. Here we present results of scanning XRF(-CT) analysis of a mm-sized sample of the Murchison meteorite, a Mighei type chondrite (CM2), wherein a 9.6 x 11.5 x 8.2 mu m(3) CAI phase was detected and analyzed. The CAI grain is shown to be detectable throughout the entire sample volume (~700 mu m) during an XRF-CT scan over a full 360 & nbsp; angular range, thus proving the applicability of the method to study the microscopic distribution of high-Z elements at trace level concentration within millimeter-sized asteroidal particles

Detection of a Ca-rich lithology in the Earth's deep (>300 km) convecting mantle

Earth's deep convecting upper mantle is believed to represent a rather homogenous geochemical reservoir of spinel or garnet lherzolite with primitive major element and moderately depleted trace element composition. Only where subduction occurs is this homogeneity disrupted by a suite of rocks ranging from eclogites/garnet pyroxenites (former oceanic crust) to residual harzburgites. In addition to these well documented peridotitic and metabasaltic rocks we have now discovered the presence of a chemically distinct reservoir in the deep convecting upper mantle. In situ structural analyses (micro X-ray diffraction and micro Raman spectroscopy) and three-dimensional trace element mapping (confocal micro X-ray fluorescence imaging) of polyphase inclusions in a diamond from Guinea that formed at about 300–360 km depth reveal the existence of a deep Ca-rich source, in the absence of several common mantle minerals, like olivine, garnet and low-Ca pyroxene. This reservoir may represent metasomatized oceanic lithosphere (rodingites, ophicarbonates) or metamorphosed carbonaceous sediments