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

Garnet, the archetypal cubic mineral, grows tetragonal

Garnet is the archetypal cubic mineral, occurring in a wide variety of rock types in Earth’s crust and upper mantle. Owing to its prevalence, durability and compositional diversity, garnet is used to investigate a broad range of geological processes. Although birefringence is a characteristic feature of rare Ca–Fe3+ garnet and Ca-rich hydrous garnet, the optical anisotropy that has occasionally been documented in common (that is, anhydrous Ca–Fe2+–Mg–Mn) garnet is generally attributed to internal strain of the cubic structure. Here we show that common garnet with a non-cubic (tetragonal) crystal structure is much more widespread than previously thought, occurring in low-temperature, high-pressure metamorphosed basalts (blueschists) from subduction zones and in low-grade metamorphosed mudstones (phyllites and schists) from orogenic belts. Indeed, a non-cubic symmetry appears to be typical of common garnet that forms at low temperatures (<450 °C), where it has a characteristic Fe–Ca-rich composition with very low Mg contents. We propose that, in most cases, garnet does not initially grow cubic. Our discovery indicates that the crystal chemistry and thermodynamic properties of garnet at low-temperature need to be re-assessed, with potential consequences for the application of garnet as an investigative tool in a broad range of geological environments

I was not born cubic, said low-temperature metamorphic garnet

Garnet is the paradigmatic cubic mineral of metamorphic and igneous rocks, and is generally regarded as optically isotropic. Nonetheless, evident birefringence is observed, particularly in the rare Ca-Fe3+ hydrogarnets, which is attributed to the coexistence of two or more cubic phases. A weak birefringence, with rare examples of optical sector zoning, has also been documented in much more common Fe2+-Mg-Mn garnets, but an adequate explanation for its cause is, so far, lacking. Here we show that optically anisotropic garnets are much more widespread than previously thought, both in blueschists and blueschist-facies rocks, as well as in lower greenschist-facies phyllites, but they are frequently overlooked when working with conventional, 30-µm-thick thin sections. Utilizing a multi-technique approach including optical microstructural analysis, BSEM, EMPA, EBSD, FTIR, TEM, EDT and single-crystal XRD, we demonstrate here that the birefringence in these garnets is related to their tetragonal symmetry, that it is not due to strain, and that crystals are twinned according to a merohedral law. We also show that the birefringent garnets from blueschists and phyllites are anhydrous, lacking any hydrogarnet component, and have compositions dominated by almandine (58-79%) and grossular (19-30%) with variable spessartine (0-21%) and very low pyrope (1-7%). Considering the widespread occurrence of optically anisotropic OH-free garnets in blueschists and phyllites, their common low-grade metamorphic origin, and the occurrence of optically isotropic garnets with similar Ca-rich almandine composition in higher-grade rocks, we conclude that garnet does not grow with cubic symmetry in low-temperature rocks (< 400 ◦C). The tetragonal structure appears to be typical of Fe-Ca-rich compositions, with very low Mg contents. Cubic but optically sector-zoned garnet in a lower amphibolite-facies metapelite from the eastern Alps suggests that preservation of tetragonal garnet is favored in rocks which did not progress to T> ≈500 ◦C, where transition to the cubic form, accompanied by change of stable chemical composition, would take place. Our data show that the crystal-chemistry of garnet, its thermodynamics and, in turn, its use in unravelling petrogenetic processes in cold metamorphic environments need to be re-assessed

Inclusions in super-deep diamonds

Super-deep diamonds may originate from a depth of between 300 and 800 km, although their precise depth of origin remains uncertain. When growing, they trap other minerals from their surroundings, which remain unaltered in their diamond capsule on their journey up to the surface of our planet. Through the study of these inclusions it is thus possible to reveal the secrets of deep unseen environments. In this study we aim to determine the formation pressure of super- deep diamonds for the first time by characterising two types of inclusions: CaSiO3-walstromite and ferropericlase. To achieve this goal we investigated CaSiO3-walstromite inclusions by a combination of in situ single-crystal X-ray diffraction, \u201csingle-inclusion elastic barometry\u201d and in situ micro-Raman spectroscopy and we obtained an apparent entrapment pressure of 3c7.1 GPa, corresponding to 3c250 km, at a temperature of 1500 K. In addition, thermodynamic calculations suggested that single inclusions of CaSiO3-walstromite cannot derive from CaSiO3-perovskite. Preliminary X-ray micro-tomography and nuclear resonance scattering data were also collected on ferropericlase-bearing diamonds in order to detect micro-fractures around the inclusions and to determine whether the Fe3+/ 11Fe ratios are in agreement with lower mantle values or not

Trans-oral robotic surgery for the management of oropharyngeal carcinomas: A 9-year institutional experience

Trans-oral robotic surgery (TORS) has changed surgical management of patients with oropharyngeal squamous cell carcinomas (OPSCC). In this study we present surgical and oncologic outcomes of patients with oropharyngeal squamous cell carcinomas, treated using TORS, with and without an adjuvant therapy. Sixty patients with oropharyngeal squamous cell carcinomas treated with TORS between January 2008 and December 2017 have been retrospectively evaluated considering clinicopathologic features, disease characteristics, adjuvant treatments and oncological outcomes. TORS was performed for OPSCC to the base of tongue in 41.7%, tonsils in 46.7%, soft palate and posterior pharyngeal wall in 3.3% and 5%, respectively. Neck dissection was performed in 43.3% of patients. Management strategies included surgery alone in 30%, TORS and adjuvant radiotherapy in 33.3%, and TORS plus adjuvant chemotherapy in 36.7%. The 5-year overall survival of the total group was 77.6%, the 5-year disease-free survival rate was 85.2%, and the 5-year local recurrence-free survival rate was 90.6%. Finally, in selected patients TORS appears to yield similar oncologic outcomes and functional outcomes to traditional techniques and non-operative treatment with a possible benefit on long-term quality of life. The future offers exciting opportunities to combine TORS and radiotherapy in unique ways. However, further research is urgently needed to clarify the indications for adjuvant therapy following TORS resections

Sublithospheric diamond ages and the supercontinent cycle.

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300-700 km depths1,2 and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb-Sr, Sm-Nd, U-Pb and Re-Os) applied to Fe-sulfide and CaSiO3 inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic-Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth-with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust-could have enhanced supercontinent stability

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

Kimberlites reveal 2.5-billion-year evolution of a deep, isolated mantle reservoir

The widely accepted paradigm of Earth's geochemical evolution states that the successive extraction of melts from the mantle over the past 4.5 billion years formed the continental crust, and produced at least one complementary melt-depleted reservoir that is now recognized as the upper-mantle source of mid-ocean-ridge basalts1. However, geochemical modelling and the occurrence of high 3He/4He (that is, primordial) signatures in some volcanic rocks suggest that volumes of relatively undifferentiated mantle may reside in deeper, isolated regions2. Some basalts from large igneous provinces may provide temporally restricted glimpses of the most primitive parts of the mantle3,4, but key questions regarding the longevity of such sources on planetary timescales—and whether any survive today—remain unresolved. Kimberlites, small-volume volcanic rocks that are the source of most diamonds, offer rare insights into aspects of the composition of the Earth’s deep mantle. The radiogenic isotope ratios of kimberlites of different ages enable us to map the evolution of this domain through time. Here we show that globally distributed kimberlites originate from a single homogeneous reservoir with an isotopic composition that is indicative of a uniform and pristine mantle source, which evolved in isolation over at least 2.5 billion years of Earth history—to our knowledge, the only such reservoir that has been identified to date. Around 200 million years ago, extensive volumes of the same source were perturbed, probably as a result of contamination by exogenic material. The distribution of affected kimberlites suggests that this event may be related to subduction along the margin of the Pangaea supercontinent. These results reveal a long-lived and globally extensive mantle reservoir that underwent subsequent disruption, possibly heralding a marked change to large-scale mantle-mixing regimes. These processes may explain why uncontaminated primordial mantle is so difficult to identify in recent mantle-derived melts

Reduction in primary production followed by rapid recovery of plant biomass in response to repeated mid-season droughts in a semiarid shrubland

The frequency and severity of extreme weather events, including droughts, are expected to increase due to the climate change. Climate manipulation field experiments are widely used tools to study the response of key parameters like primary production to the treatments. Our study aimed to detect the effect of drought on the aboveground biomass and primary production both during the treatments as well as during the whole growing seasons in semiarid vegetation. We estimated aboveground green biomass of vascular plants in a Pannonian sand forest-steppe ecosystem in Hungary. We applied non-destructive field remote sensing method in control and drought treatments. Drought treatment was carried out by precipitation exclusion in May and June, and was repeated in each year from 2002. We measured NDVI before the drought treatment, right after the treatment, and at the end of the summer in 2011 and 2013. We found that the yearly biomass peaks, measured in control plots after the treatment periods, were decreased or absent in drought treatment plots, and consequently, the aboveground net primary production was smaller than in the control plots. At the same time, we did not find general drought effects on all biomass data. The studied ecosystem proved resilient, as the biomass in the drought-treated plots recovered by the next drought treatment. We conclude that the effect of drought treatment can be overestimated with only one measurement at the time of the peak biomass, while multiple within-year measurements better describe the response of biomass

Shape evolution and finite deformation pattern in analogue experiments of lithosphere necking

Lithosphere necking evolution determines the 3-D architecture of crustal and upper mantle thinning and related basins, and the heat flow distribution in rifted regions. Despite a large number of studies, lithosphere necking evolution is still a matter of debate. We present the result from lithospheric-scale analog models designed for investigating the necking shape during extension and the vertical distribution of finite deformation in the mechanical lithosphere. In our experiments, lithosphere necking is asymmetric and, in particular, the 3-D distribution of thinning is cylindrical in the crust and very heterogeneous in the mantle. Overall, the evolution of rifting and necking progresses from delocalized to localized deformation