CaSiO3-walstromite inclusions in super-deep diamonds

Diamonds are considered the unique way to trap and convey real fragments of deep material to the surface of our planet. Over the last thirty years, great strides have been made in understanding of Earth\u2019s lower mantle, mainly thanks to technological and instrumental advances; nevertheless, it is only in the last two decades that a whole range of inclusion parageneses derived from the lower mantle was discovered in diamonds from S\ue3o Luiz (Brazil) (Kaminsky, 2008 and references therein), thereby establishing a \u201cwindow\u201d into the lower mantle. These so-called super-deep diamonds form at depths greater than lithospheric diamonds, more precisely between 300 and 800 km depth, and contain mostly ferropericlase, enstatite (believed to be derived from MgSi-perovskite) and CaSiO3- walstromite (believed to be derived from CaSiO3-perovskite). Even though CaSiO3 not only adopts the perovskite structure with increased pressure and temperature, but also it is considered the dominant Ca-bearing phase in the Earth\u2019s lower mantle (Tamai and Yagi, 1989), at the present day there are no reliable literature data on the pressure at which CaSiO3 crystallizes within diamonds. In order to obtain for the first time a pressure of formation value for CaSiO3-walstromite, several inclusions still trapped in a diamond coming from Juina (Mato Grosso, Brazil) were investigated both by in-situ microRaman spectroscopy and in-situ single-crystal X-ray diffraction. First, we applied \u201csingle-inclusion elastic barometry\u201d as improved by Angel et al. (2014) to determine the pressure of formation of the diamond-inclusion pairs. Starting from the maximum remnant pressure value ever reported (Joswig et al., 2003) and adopting the thermoelastic parameters already present in literature (Swamy and Dubrovinsky, 1997; Liu et al., 2012), we obtained an appar- ent entrapment pressure of 3c7.1 GPa, corresponding to 3c250 km, at 1500 K. The presence of fractures around the inclusions indicates this is a minimum estimate, and it is possible that the entrapment pressure falls at least into the stability field of Ca2SiO4-larnite + CaSi2O5-titanite. In support of this hypothesis we secondly compared our Raman spectra with reference spectra of the same phases obtained from an experimental product of Gasparik et al. (1994). Our preliminary results indicate in at least one inclusion the coexistence of CaSiO3-walstromite + Ca2SiO4-larnite, suggesting that CaSiO3-walstromite forms in sub-lithospheric conditions from the back transfor- mation from CaSiO3-perovskite. Further investigations are in progress in order to find evidence of CaSi2O5-titanite in these inclusions

"EosFit-Pinc: A simple GUI for host-inclusion elastic thermobarometry"—Reply to Zhong et al.

We provide a further algebraic proof that the lines of entrapment conditions for inclusions calculated with the formula of Guiraud and Powell (2006) are not thermodynamic isomekes and therefore do not represent exactly lines of possible entrapment conditions

Depth of Formation of Ferropericlase Included in Super-Deep Diamonds

Super-deep diamonds are believed to have formed at depths of at least 300 km depth (Harte, 2010). A common mineral inclusion in these diamonds is ferropericlase, (Mg,Fe)O (see Kaminsky, 2012 and references therein). Ferropericlase (fPer) is the second most abundant mineral in the lower mantle, comprising approximately 16\u201320 wt% (660 to 2900 km depth), and inclusions of fPer in diamond are often considered to indicate a lower-mantle origin (Harte et al., 1999). Samples from S\ue3o Luiz/Juina, Brazil, are noteworthy for containing nanometer-sized magnesioferrite (Harte et al., 1999; Wirth et al., 2014; Kaminsky et al., 2015; Palot et al., 2016). Based upon a phase diagram valid for 1 atm, such exsolutions would place the origin of this assemblage in the uppermost part of the lower mantle. However, a newly reported phase diagram for magnesioferrite demonstrates that the latter is not stable at such pressures and, thus, it cannot exsolve directly from fPer at lower-mantle conditions (Uenver-Thiele et al., 2017). Here we report the investigation of two fPer inclusions, extracted from a single S\ue3o Luiz diamond, by single-crystal X-ray diffraction and field emission scanning electron microscopy. Both techniques showed micrometer-sized exsolutions of magnesioferrite within the two fPers. We also completed elastic geobarometry (see Angel et al., 2015), which determined an estimate for the depth of entrapment of the two ferropericlase \u2013 diamond pairs. In the temperature range between 1273 and 1773 K, pressures varied between 9.88 and 12.34 GPa (325-410 km depth) for one inclusion and between 10.69 and 13.16 GPa (350-440 km depth) for the other one. These results strengthen the hypothesis that solitary fPer inclusions might not be reliable markers for a lower-mantle provenance

Crystallographic orientations of magnesiochromite inclusions in diamonds: what do they tell us?

We have studied by X-ray diffractometry the crystallographic orientation relationships (CORs) between magnesiochromite (mchr) inclusions and their diamond hosts in gem-quality stones from the mines Udachnaya (Siberian Russia), Damtshaa (Botswana) and Panda (Canada); in total 36 inclusions in 23 diamonds. In nearly half of the cases (n = 17), [111]mchr is parallel within error to [111]diamond, but the angular misorientation for other crystallographic directions is generally significant. This relationship can be described as a case of rotational statistical COR, in which inclusion and host share a single axis (1 df). The remaining mchr\u2013diamond pairs (n = 19) have a random COR (2 df). The presence of a rotational statistical COR indicates that the inclusions have physically interacted with the diamond before their final incorporation. Of all possible physical processes that may have influenced mchr orientation, those driven by surface interactions are not considered likely because of the presence of fluid films around the inclusions. Mechanical interaction between euhedral crystals in a fluid-rich environment is therefore proposed as the most likely mechanism to produce the observed rotational COR. In this scenario, neither a rotational nor a random COR can provide information on the relative timing of growth of mchr and diamond. Some multiple, iso-oriented inclusions within single diamonds, however, indicate that mchr was partially dissolved during diamond growth, suggesting a protogenetic origin of these inclusions

Sound velocities and single-crystal elasticity of hydrous Fo90 olivine to 12 GPa

Nominally anhydrous minerals (NAMs) may contain significant amounts of water and constitute an important reservoir for mantle hydrogen. The colloquial term ‘water’ in NAMs is related to the presence of hydroxyl-bearing (OH-) point defects in their crystal structure, where hydrogen is bonded to lattice oxygen and is charge-balanced by cation vacancies. This hydrous component may therefore have substantial effects on the thermoelastic parameters of NAMs, comparable to other major crystal-chemical substitutions (e.g., Fe, Al). Assessment of water concentrations in natural minerals from mantle xenoliths indicates that olivine commonly stores ~0 – 200 ppm of water. However, the lack of samples originating from depths exceeding ~250 km coupled with the rapid diffusion of hydrogen in olivine at magmatic temperatures makes the determination of the olivine water content in the upper mantle challenging. On the other hand, numerous experimental data show that, at pressures and temperatures corresponding to deep upper mantle conditions, the water storage capacity of olivine increases to 0.2 – 0.5 wt.%. Therefore, determining the elastic properties of olivine samples with more realistic water contents for deep upper mantle conditions may help in interpreting both seismic velocity anomalies in potentially hydrous regions of Earth's mantle as well as the observed seismic velocity and density contrasts across the 410-km discontinuity. Here, we report simultaneous single-crystal X-ray diffraction and Brillouin scattering experiments at room temperature up to 11.96(2) GPa on hydrous (0.20(3) wt.% H2O) Fo90 olivine to assess its full elastic tensor, and complement these results with a careful re-analysis of all the available single-crystal elasticity data from the literature for anhydrous Fo90 olivine. While the bulk (K) and shear (G) moduli of hydrous Fo90 olivine are virtually identical to those of the corresponding anhydrous phase, their pressure derivatives K´ and G´ are slightly larger, although consistent within mutual uncertainties. We then defined linear relations between the water concentration in Fo90 olivine, the elastic moduli and their pressure derivatives, which were then used to compute the sound velocities of Fo90 olivine with higher degrees of hydration. Even for water concentrations as high as 0.5 wt.%, the sound wave velocities of hydrous and anhydrous olivines were found to be identical within uncertainties at pressures corresponding to the base of the upper mantle. Contrary to previous claims, our data suggest that water in olivine is not seismically detectable, at least for contents consistent with deep upper mantle conditions. In addition to that, our data reveal that the hydration of olivine is unlikely to be a key factor in reconciling seismic velocity and density contrasts across the 410-km discontinuity with a pyrolitic mantle

Bobtraillite from Gejiu hyperagpaitic nepheline syenite, southwestern China: new occurrence and crystal structure

Abstract. A second occurrence of bobtraillite is described from the Gejiu nepheline syenite, southwestern China. The extremely rare and complex boron-bearing zirconium silicate is associated with albite, orthoclase, jadeite, fluorite, andradite, titanite, as well as other REE and zirconium-bearing minerals, catapleiite, moxuanxueite, låvenite, eudialyte, britholite-(Ce), and calcioancylite-(La). The EMP and LA-ICP-MS analyses of the studied material give an empirical formula: (Na9.70Li0.42K0.08□1.80)Σ12.00(Sr10.61Ca1.14Fe0.07□0.18)Σ12.00(Zr12.87Ti0.53Nb0.31REE0.08Y0.06U0.02Th0.01□0.12)Σ14.00(Si42.41B5.59Al0.02)Σ48.02O132(OH)12 ⚫ 12H2O. Bobtraillite is trigonal, with space group P3¯c1, a=19.6977(6), c=9.9770(3) Å, V=3352.4(2) Å3, Z=1. Single-crystal structure refinement revealed that all sodium occupies the Na(1) and Na(2) sites; the site occupancy of these two positions is 0.835(18) and 0.15(2), respectively, suggesting that Na(1) site is Na dominant, while Na(2) is a vacancy-dominant site. The [8]-coordinated site has been assigned to Sr and Ca, with free occupancy factors, 0.874(10) and 0.126(10), respectively. These new data indicate that the ideal formula of bobtraillite could be written as (Na,□)12(□,Na)12Sr12Zr14(Si3O9)10[Si2BO7(OH)2]6 ⚫ 12H2O

Water incorporation in synthetic and natural MgAl2O4 spinel

The solubility and incorporation mechanisms of water in synthetic and natural MgAl2O4 spinel have been investigated in a series of high-pressure/temperature annealing experiments. In contrast to most other nominally anhydrous minerals, natural spinel appears to be completely anhydrous. On the other hand, non-stoichiometric Al-rich synthetic (defect) spinel can accommodate several hundred ppm water in the form of structurally-incorporated hydrogen. Infrared (IR) spectra of hydrated defect spinel contain one main O-H stretching band at 3343-3352 cm-1 and a doublet consisting of two distinct O-H bands at 3505-3517 cm-1 and 3557-3566 cm-1. IR spectra and structural refinements based on single-crystal X-ray data are consistent with hydrogen incorporation in defect spinel onto both octahedral and tetrahedral O-O edges. Fine structure of O-H bands in IR spectra can be explained by partial coupling of interstitial hydrogen with cation vacancies, or by the effects of Mg-Al disorder on the tetrahedral site. The concentration of cation vacancies in defect spinel is a major control on hydrogen affinity. The commercial availability of large single crystals of defect spinel coupled with high water solubility and similarities in water incorporation mechanisms between hydrous defect spinel and hydrous ringwoodite (Mg2SiO4) suggests that synthetic defect spinel may be a useful low-pressure analogue material for investigating the causes and consequences of water incorporation in the lower part of Earth's mantle transition zone