Elastic-thermobarometry: methods and applications to ultrahigh pressure metamorphic rocks

Ultra-High Pressure (UHP) metamorphic rocks represent the evidence we have for the detailed reconstruction of tectonic processes, which are the source of major natural hazards like deep-focus earthquakes and volcanic eruptions. Mineral inclusions are often the only proof of ultrahigh pressure metamorphism and their study provides insights into the mechanisms of subduction and subsequent exhumation of metamorphic rocks. The most frequently used approach for this purpose is the application of equilibrium thermodynamics assuming a linear relationship between the inferred Pressure (P) and the depth of formation of metamorphic mineral assemblages. For instance, coesite or diamond-bearing systems suggest that rocks can be exhumed from depths even greater than 100 km. However, a major current controversy is whether high-pressure minerals actually indicate such great depths of subduction or whether they are the result of tectonic overpressure during subduction. If tectonic overpressure are present, coesite- and diamond-bearing rocks could come from shallower domains of the lithosphere and a revaluation of the current knowledge of plate-tectonics would be necessary. So far, there are no available techniques to constrain the amount of deviatoric paleo-stress present during metamorphic processes. A first attempt has been recently proposed combining mineral physics and petrology: the elastic-thermobarometry. The advantage of this technique is that it is not based on the equilibrium thermodynamic assumption but, rather, on the contrast in the elastic properties of two crystals that are constrained within a confined space such as a mineral inclusion and its surrounding host. The analysis of solid inclusions that are fully buried within their hosts by non-destructive techniques, such as Raman spectroscopy or single crystal X-ray diffraction, reveal pressures that can considerably deviate from the external (ambient) one. This is the so-called residual pressure and it arises as a response to the contrast in the thermo-elastic properties between the host and the inclusion if, for example, the entrapment of the inclusion occurred at high P-T conditions. Importantly, the amount of residual pressure is linked to the entrapment pressure and knowing the physical properties of the two crystals (i.e. their equations of state), using theoretical models it is possible to back-calculate the P-T conditions of inclusion entrapment. The current theoretical models for interpreting the residual pressure are based on simplified assumptions and ideal host-inclusion systems (e.g. isotropic elasticity for both the host and the inclusion crystals, shape of the inclusions are spherical and infinite in size for the host crystal). In this regard, this PhD Thesis has two main objectives: (i) to understand, from an experimental point of view, how much the deviations from the ideal host-inclusion system can actually influence the thermobarometric estimates and (ii) to apply the new theoretical and experimental developments of the elastic thermobarometry to a natural case of study. The first point focuses on the use of Raman spectroscopy to measure and determine the strain state of natural (i.e. non-ideal) mineral inclusions while the second one develops the application of this technique to the famous UHP metamorphic rocks of the Dora Maira Massif (Western Alps). This study allowed the development of experimental protocols devoted in selecting reliable mineral inclusions for elastic-thermobarometric purposes. Backbone of this work are zircon inclusions because they represent one of the most common accessory minerals in metamorphic rocks and, furthermore, can give also age information on the metamorphic processes. Finally, the application of the elastic thermobarometric method to a natural case of study, shows the possibility in considering the presence of deviatoric stresses during inclusion entrapment starting from experimental measurements of stress field in host-inclusion mineral systems. Although this last point remains currently difficult to confirm, the aim of this work is also to give some future perspectives that, eventually, can be used to start in describing metamorphic processes to a higher level that has never been envisaged before

Fossil subduction recorded by quartz from the coesite stability field

Metamorphic rocks are the records of plate tectonic processes whose reconstruction relies on correct estimates of the pressures and temperatures (P-T) experienced by these rocks through time. Unlike chemical geothermobarometry, elastic geobarometry does not rely on chemical equilibrium between minerals, so it has the potential to provide information on overstepping of reaction boundaries and to identify other examples of non-equilibrium behavior in rocks. Here we introduce a method that exploits the anisotropy in elastic properties of minerals to determine the unique P and T of entrapment from a single inclusion in a mineral host. We apply it to preserved quartz inclusions in garnet from eclogite xenoliths hosted in Yakutian kimberlites (Russia). Our results demonstrate that quartz trapped in garnet can be preserved when the rock reaches the stability field of coesite (the high-pressure and hightemperature polymorph of quartz) at 3 GPa and 850 \ub0C. This supports a metamorphic origin for these xenoliths and sheds light on the mechanisms of craton accretion from a subducted crustal protolith. Furthermore, we show that interpreting P and T conditions reached by a rock from the simple phase identification of key inclusion minerals can be misleading

Systematic review and critique of circulating miRNAs as biomarkers of stage I-II non-small cell lung cancer

Selected circulating microRNAs (miRNAs) have been suggested for non-invasive screening of non-small cell lung cancer (NSCLC), however the numerous proposed miRNA signatures are inconsistent. Aiming to identify miRNAs suitable specifically for stage I-II NSCLC screening in serum/plasma samples, we searched the databases \u201cPubmed\u201d, \u201cMedline\u201d, \u201cScopus\u201d, \u201cEmbase\u201d and \u201cWOS\u201d and systematically reviewed the publications reporting quantitative data on the efficacy [sensitivity, specificity and/or area under the curve (AUC)] of circulating miRNAs as biomarkers of NSCLC stage I and/or II. The 20 studies fulfilling the search criteria included 1110 NSCLC patients and 1009 controls, and were of medium quality according to Quality Assessment of Diagnostic Accuracy Studies checklist. In these studies, the patient cohorts as well as the control groups were heterogeneous for demographics and clinicopathological characteristics; moreover, numerous pre-analytical and analytical variables likely influenced miRNA determinations, and potential bias of hemolysis was often underestimated. We identified four circulating miRNAs scarcely influenced by hemolysis, each featuring high sensitivity (> 80%) and AUC (> 0.80) as biomarkers of stage I-II NSCLC: miR- 223, miR-20a, miR-448 and miR-145; four other miRNAs showed high specificity (> 90%): miR-628-3p, miR-29c, miR-210 and miR-1244. In a model of two-step screening for stage I-II NSCLC using first the above panel of serum miRNAs with high sensitivity and high AUC, and subsequently the panel with high specificity, the estimated overall sensitivity is 91.6% and overall specificity is 93.4%. These and other circulating miRNAs suggested for stage I-II NSCLC screening require validation in multiple independent studies before they can be proposed for clinical application

Polychromatic polarization: Boosting the capabilities of the good old petrographic microscope

Polychromatic polarizing microscopy (PPM) is a new optical technique that allows for the inspection of materials with low birefringence, which produces retardance between 1 nm and 300 nm. In this region, where minerals display interference colors in the near-black to gray scale and where observations by conventional microscopy are limited or hampered, PPM produces a full spectrum color palette in which the hue depends on orientation of the slow axis. We applied PPM to ordinary 30 μm rock thin sections, with particular interest in the subtle birefringence of garnet due both to non-isotropic growth or to strain induced by external stresses or inclusions. The PPM produces striking, colorful images that highlight various types of microstructures that are virtually undetectable by conventional polarizing microscopy. PPM opens new avenues for microstructural analysis of geological materials. The direct detection and imaging of microstructures will provide a fast, non-destructive, and inexpensive alternative (or complement) to time-consuming and more costly scanning electron microscope–based analyses such as electron backscatter diffraction. This powerful imaging method provides a quick and better texturally constrained basis for locating targets for cutting-edge applications such as focused ion beam-transmission electron microscopy or atom probe tomography

Supplemental Material: Resetting of zircon inclusions in garnet: Implications for elastic thermobarometry

 Additional data and analytical details. </p

Establishing a protocol for the selection of zircon inclusions in garnet for Raman thermobarometry

he structural and chemical properties of zircon inclusions in garnet megablasts from the Dora Maira Massif (Western Alps, Italy) were characterized in detail using charge contrast imaging, Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The aim of this work is to determine to what extent the degree of metamictization, metamorphic recrystallization, inherent structural heterogeneity, chemical composition, and zoning, along with the elastic stress imposed by the host mineral, can influence the Raman peak position of the zircon inclusion and hence, the residual pressure estimated via Raman geo-thermobarometry. We show and confirm that metamictization and inherent structural heterogeneity have a major influence in the Raman spectra of zircon in terms of peak position and peak width. We suggest that, for spectral resolution of 2 cm-1, the peak width of the B1g mode near 1008 cm-1 of reliable grains must be smaller than 5 cm-1. The method can be applied to both inherited igneous and newly formed Alpine metamorphic crystals. By coupling structural and chemical information, we demonstrate that there are no significant diferences between the Raman spectra of zircon with oscillatory-zoned texture, formed during magmatic crystallization, and those formed by fluid-induced Alpine (re)crystallization. The discrimination between magmatic and metamorphic zircon based only on micro-textural constraints is not robust. Finally, our results allow establishing a protocol devoted to the selection of reliable buried zircon inclusions, relying only on Raman spectroscopic measurements, to use for elastic thermobarometry applications

Metasomatic horizon sealing serpentinite-metasediments pair in the Zermatt-Saas metaophiolite (Northwestern Alps): record of a channel for focussed fluid flow during subduction

A metasomatic horizon (MH) occurs between the metaophiolite (serpentinite and metaophicarbonates) basement and metasedimentary sequence (chaotic rocks and calcschists) of the Lake Miserin Ophiolite, in the high pressure Zermatt-Saas Zone of the Northwestern Alps. Macro- and microstructural analyses combined with petrological and geochemical investigations of the MH and surrounding lithologies unravelled a polyphase blastesis-deformation history, which led to the formation of a complex fabric and minero-chemical alteration of the serpentinite basement-metasediments interface. Dehydration, decarbonation and carbonation interplayed from early Alpine subduction up to HP-LT metamorphic peak (T=550-630 °C, P=1.8-2.5 GPa), to produce a distinctive, pervasive amphibole (tremolite/actinolite) replacement both in carbonate-rich and serpentinite-rich domains pertaining to the MH protoliths, i.e. serpentinite and carbonate-bearing metabreccia of the chaotic rock unit. This characteristic amphibole metasomatism is more pronounced toward the contact with the metaophicarbonates, and the average δ18OVSMOW and δ13CVPDB values of dolomite within the MH (+14.4‰ and +0.7‰ respectively) lie between those of the metaophicarbonates and of calcschist. These results suggest that Mg- H2O-rich fluids from the dehydrating slab, CO2 released by decarbonation and SiO2-rich fluids evolved in calcschists mixed together and circulated mostly along the metaophiolite basement/metasediments interface, where the MH developed and recorded a preferential channel for mixed metamorphic fluid flow. These findings highlight and confirm that the study of metasomatic rocks in convergent systems is crucial to comprehend the behaviour of different fluids circulating, mixing and interacting with lithologies along slab-parallel discontinuities, which act as major fluid conduits for deep volatile recycling

Carbonation of subduction-zone serpentinite (high-pressure ophicarbonate; Ligurian Western Alps) and implications for the deep carbon cycling

Much of the long-term carbon cycle in solid earth occurs in subduction zones, where processes of devolatilization, partial melting of carbonated rocks, and dissolution of carbonate minerals lead to the return of CO2 to the atmosphere via volcanic degassing. Release of COH fluids from hydrous and carbonate minerals influences C recycling and magmatism at subduction zones. Contradictory interpretations exist regarding the retention/storage of C in subducting plates and in the forearc to subarc mantle. Several lines of evidence indicate mobility of C, of uncertain magnitude, in forearcs. A poorly constrained fraction of the 40-115 Mt/yr of C initially subducted is released into fluids (by decarbonation and/or carbonate dissolution) and 18-43 Mt/yr is returned at arc volcanoes. Current estimates suggest the amount of C released into subduction fluids is greater than that degassed at arc volcanoes: the imbalance could reflect C subduction into the deeper mantle, beyond subarc regions, or storage of C in forearc/subarc reservoirs.We examine the fate of C in plate-interface ultramafic rocks, and by analogy serpentinized mantle wedge, via study of fluid-rock evolution of marble and variably carbonated serpentinite in the Ligurian Alps. Based on petrography, major and trace element concentrations, and carbonate C and O isotope compositions, we demonstrate that serpentinite dehydration at 2-2.5 GPa, 550 \ub0C released aqueous fluids triggering breakdown of dolomite in nearby marbles, thus releasing C into fluids. Carbonate + olivine veins document flow of COH fluids and that the interaction of these COH fluids with serpentinite led to the formation of high-P carbonated ultramafic-rock domains (high-P ophicarbonates). We estimate that this could result in the retention of ~0.5-2.0 Mt C/yr in such rocks along subduction interfaces. As another means of C storage, 1 to 3 km-thick layers of serpentinized forearc mantle wedge containing 50 modal % dolomite could sequester 1.62 to 4.85 Mt C/yr.We stress that lithologically complex interfaces could contain sites of both C release and C addition, further confounding estimates of net C loss at forearc and subarc depths. Sites of C retention, also including carbonate veins and graphite as reduced carbonate, could influence the transfer of slab C to at least the depths beneath volcanic fronts