GMPV7.1

Findings of coesite and diamond in quartzo-feldspathic rocks confirmed the idea that continental crust, despite its buoyancy, can be subducted to ultra-high pressure (UHP) conditions. In addition to these index minerals, UHP conditions can be reflected by specific minor elements incorporated in major minerals, which was well demonstrated in mantle rocks, but poorly explored in continental crust. Here, we investigate garnet with coesite inclusions from subducted metagranites of the Eger Crystalline Complex, Bohemian Massif. The garnet shows chemically distinct concentric domains with minor amounts of P, Na, and Li. From the correlation of these elements, we infer (Na,Li)₁P₁M²⁺_-1Si_-1 substitution, where the Na deficiency is compensated by Li in a 2:1 ratio. This is the first time that such coupled substitution in garnet has been defined and clearly connected to UHP conditions in natural samples, proving itself as a new tool to reveal UHP conditions in garnet. In addition, garnet in subduction zones needs to be considered as an important Li carrier, able to transport significant amounts of Li into the Earth's mantle.

How to cite: Racek, M., Jeřábek, P., Štípská, P., Závada, P., Svojtka, M., Hasalová, P., and Veselovský, F.: (Li,Na)-P substitution in garnet as an indicator of UHP conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7875, https://doi.org/10.5194/egusphere-egu22-7875, 2022.

Documenting ultrahigh-pressure (UHP) metamorphism in the geologic record is a key for understanding the evolution of plate tectonics on Earth. Characteristic UHP minerals like coesite and diamond that form during deep subduction are frequently replaced by their low-pressure polymorphs during exhumation. However, when entrapped as inclusions in resistant host minerals like garnet, coesite and diamond are shielded from external metamorphic fluids and may be preserved. Finding evidence for deep subduction processes in host garnets of large volumes of (partially) re-equilibrated crystalline rocks is challenging, time consuming, and often hampered by poor outcrop conditions due to weathering and soil formation. In contrast, by analyzing detrital garnet, natural processes such as erosion and sedimentary transport can sample garnet grains sourced from fresh as well as altered crystalline rocks located in the drainage area, enabling large crustal volumes to be screened using a comparatively low number of samples. Case-studies from the Western Gneiss Region of Norway (Schönig et al. 2018), the Saxonian Erzgebirge of Germany (Schönig et al. 2019, 2020), and the (U)HP terrane of eastern Papua New Guinea (Baldwin et al. 2021) demonstrate mineral inclusion analysis of detrital garnet to be a complementary and efficient tool in UHP research. This contribution gives a synopsis of the main findings from the three spatially, chronologically, and tectonically distinct UHP terranes studied, putting emphasis on the spatial extent of UHP metamorphism and lithologies involved.

How to cite: Schönig, J., von Eynatten, H., Meinhold, G., Lünsdorf, N. K., and Baldwin, S. L.: Mineral inclusions in detrital garnet – A complementary tool in ultrahigh-pressure research, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8385, https://doi.org/10.5194/egusphere-egu22-8385, 2022.

The textural and chemical characteristics of garnet blasts have been routinely used to provide useful indicators on the rock evolution. However, a more precise 3D characterization of mineral volumes and relations leads to a better understanding of particular metamorphic processes.

The Dinarides, a mountain chain in south-eastern Europe, comprises ultramafic massifs and metamorphic rocks that are relics of ophiolite complexes, which originated along the contact of the European plate and the Adria microplate during the Alpine orogeny. The associated regional geodynamic processes brought in contact hot upper mantle and cold subducted material leading to the formation of high-grade garnet-bearing amphibolites (“metamorphic sole”). A clockwise pressure-temperature path with maximum pressure conditions of 2.1 GPa (ca. 70 km depth) at temperatures around 800 °C was determined for such an amphibolite (Krivaja-Konjuh ultramafic massif (KKUM), Bosnia and Herzegovina) that originated from a N-MORB protolith (Balen & Massonne, 2021). These conditions were followed by a nearly isothermal pressure decrease to 0.4 GPa. Pyrope-almandine garnet, rich in mineral inclusions (plagioclase, amphibole, clinopyroxene, ilmenite, rutile, titanite), is a major mineral in metamorphic sole amphibolites from KKUM. Around garnet, fine-grained symplectites usually form a corona (kelyphite), which consists of Ca-amphibole, plagioclase and opaque phases. A plausible explanation of the observed kelyphite is its formation during rapid decompression caused by the uplift of deep-seated rocks for more than 50 km.

We conducted a micro-scale 3D tomography of mineral blasts in metamorphic sole amphibolites from KKUM using phase-contrast synchrotron radiation computed microtomography (SR μCT) at the SYRMEP beamline (Elettra-Sincrotrone Trieste facility). This X-ray micro-tomography allowed us to retrieve micro-scale morphologic features of minerals, mineral inclusions and symplectites in order to quantify their 3D shapes, dimensions, spatial distribution and orientation. We obtained a full high-precision 3D characterization of the mineral volume and textural description including the 3D morphological features of the smallest components. Statistically relevant data were gathered to study the garnet crystallization and decomposition history and thus the metamorphic evolution of the garnet-bearing amphibolites.

Preliminary SR μCT results indicate garnet grains which vary in size from 150 to 1500 µm in diameter. Their associated reaction rims show a thickness from 5 to 60 µm. Initial measurements indicate the tendency of thicker reaction rims around smaller grains. Further 3D measurements and data treatment to statistically describe the entire garnet and reaction rim populations will follow.

The tomography combined with electron microprobe analyses of minerals, whole-rock chemistry and thermodynamic modelling gave us already insights into the growth stages and resorption of garnet as well as the growth of corona minerals. We will use this information to decipher the amount, nature and morphology of mineral grains formed at different stages of the metamorphic evolution. The 3D approach provides many additional details that can be easily overlooked when only a classical petrological approach to the study of the aforementioned amphibolites is applied.

Balen, D., Massonne, H.-J. (2021). Lithos 394-395, 106184.

How to cite: Balen, D., Kudrna Prašek, M., Pleše, P., and Massonne, H.-J.: 3D characterization of garnet from metamorphic sole amphibolite of ophiolite from the central Dinarides, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4084, https://doi.org/10.5194/egusphere-egu22-4084, 2022.

Garnet is one of the most important minerals that record the dehydration process within the subduction zone. The chemical composition of garnet is usually used to constrain the P-T conditions, but the fluid chemistry and the amounts of fluids related to the garnet formation are not fully understood within the deep subduction zone. For example, previous studies suggested that the mobility of HFSE (Ti, Nb, and Ta) are high in the eclogite-facies conditions compared to the low-grade rocks (Chen et al. 2018). In this study, we report the novel texture of garnet aggregate from the Khungui eclogite in the Zavkhan terrane of western Mongolia. We reveal that the variation of garnet texture and compositional zoning is closely related to the occurrence of the distribution of Ti-bearing minerals (rutile, ilmenite, and titanite).  

The Khungui eclogite consists of garnet, omphacite, quartz, amphibole (barrosite, taramite, hornblende), phengite, plagioclase, epidote, Ti-bearing minerals (rutile, ilmenite, titanite) with minor K-feldspar, zircon and carbonate mineral. Based on the mineral assemblage, three metamorphic stages can be classified: prograde, eclogite (2.1–2.2 GPa, 580–610 °C), and decompression (0.1–0.5 GPa, 575–635 °C). The eclogite stage is presented by garnet + omphacite + barrosite + epidote + phengite + rutile. Based on the garnet microstructure and modal abundances of minerals, the Khungui eclogite is composed of two types of layers: layer I consist of garnet aggregate (GA), quartz and abundant Ti-bearing minerals whereas layer II is composed of single garnet grain (SG) with epidote and omphacite. The major element (Fe, Ca, Mg, and Mn) compositional zoning of the GA shows asymmetric zoning whereas the SG shows symmetric zoning. The EBSD analysis reveals that the GA contains numerous small individual garnet grains that are separated by high angle orientations and the grain boundary of the GA is not controlled by the major element zoning. The GA has inclusions of rutile and shows the close spatial relationship with rutile, ilmenite, and titanite in a matrix which are revealed through an analysis of thin section (Microscopy), element map (EPMA), and core sample (µX-ray CT). In addition, each garnet grains of GA and SG show the concentric zoning of a trace element such as Ti and are increasing concentration core to rim. The GA was often fractured and shows the Mn-rich compositions along the fracture that is close to Ti-bearing minerals which reveals that they formed in order of rutile => ilmenite => titanite at the retrograde stage. These observations suggest that nucleation of garnet to form aggregate could be induced by infiltration Ti-rich aqueous fluid at the eclogite-facies condition, and also later fluid-infiltration caused the modification of garnet to form asymmetric compositional zoning of the GA and Ti-bearing minerals (ilmenite to titanite) at the exhumation stage.

How to cite: Bayarbold, M., Okamoto, A., Dandar, O., Uno, M., and Tsuchiya, N.: Formation of the garnet aggregate of the Khungui eclogite in the Zavkhan Terrane, Western Mongolia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6935, https://doi.org/10.5194/egusphere-egu22-6935, 2022.

The Adirondack Mountains, New York State, USA belongs to the Canadian Grenville Province (Darling and Peck, 2016). The rocks exposed in the Adirondacks are interpreted to be the lower plate of a thrust-system at crustal levels during the Ottawan Orogeny (1090-1050 Ma) of the Grenvillian orogenic cycle. Garnet is abundant throughout the Adirondacks, with the greatest occurrence of megacrystic garnets within central Highlands. In the Gore Mountain area, the Hooper Mine is located 5 kilometers northwest of the Barton Mine and consists of partially melted mafic granulite. The mineral assemblage consists of medium grain size plagioclase, green hornblende and garnet in proportion 60:20:20. We separated the garnets of the Hooper Mine in two categories according to their size, chemical zoning and habitus: (1) Large, euhedral garnet porphyroblasts of diameter > 5 cm (LG), and (2) and small, xenoblastic grains (SG). Both types of garnets contain quartz, rutile and melt inclusions, similar to those observed in Barton Mine (Ferrero et al., 2021). In LG, chemical zoning is weak and inclusions are scattered randomly within the mineral. In SG, zoning coincides with the presence of quartz and melt inclusions in domain of low Ca and Y. Ti-in-quartz and Ti-in-amphibole thermometers in SG give equilibrium temperatures of 800-900 °C at 10 kbar.

Major and trace element analyses on rehomogenised melt inclusions in both types of garnet indicate two types of melts are present in the migmatite – granitic melt in SG and trondhjemitic melt in LG. Stable isotope ratios of oxygen and hydrogen in hornblende (δ²H: -62 to -73 ⁰/₀₀ and δ¹⁸O: 4.7 to 6.7 ⁰/₀₀) indicate that partial melting occurs in a closed isotopic system and records the primary magmatic δ²H signature of the protolith. The range of melt chemistries, combined with the information previously collected in the Barton Mine defines a trend characteristic of primitive TTG melts or TTG embryos. These melts, combined with different proportion of peritectic phases (i.e. garnet, plagioclase and quartz), reproduces the full TTG chemistry range (Moyen, 2011). Therefore, the Mesoproterozoic mafic lower crust might be a perfect laboratory to test early granitoids genesis processes and better understand the link between melt inclusions, plate tectonics and the formation of the continental crust (Nicoli & Ferrero, 2021). 

References:

Ferrero, S. et al. (2021). Embryos of TTGs in Gore Mountain garnet megacrysts from water-fluxed melting of the lower crust. Earth and Planetary Science Letters, 569, 117058.

Darling, R. S. and Peck W.H. (2016). Metamorphic conditions of Adirondack rocks. Adirondack Journal of Environmental Studies, 21(1), 7.

Moyen, J. F. (2011). The composite Archaean grey gneisses: petrological significance, and evidence for a non-unique tectonic setting for Archaean crustal growth. Lithos, 123(1-4), 21-36.

Nicoli, G., & Ferrero, S. (2021). Nanorocks, volatiles and plate tectonics. Geoscience Frontiers, 12(5), 101188.

How to cite: Nicoli, G., Ferrero, S., Darling, R., Yakymchuk, C., Wunder, B., and Tollan, P.: Multiple partial melts trapped in garnets from the Adirondacks lower crust: clues for TTG formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2271, https://doi.org/10.5194/egusphere-egu22-2271, 2022.

Oriented rutile needles in garnet commonly occur in high-temperature / high-pressure rocks such as high-pressure granulites, ultrahigh-pressure rocks, and mantle peridotite. Faceted inclusions of plagioclase and quartz in garnet are also indicators of high – and possibly very high – grade conditions. Both inclusion textures are spectacularly displayed in garnets in sillimanite-bearing gneiss of the North Cascade Range (USA) in rocks that record peak P-T conditions of 1 GPa and 725°C; i.e.  at significantly lower pressure than most other occurrences of rutile needle-bearing garnets and at the low end of the temperature range relative to most other occurrences of faceted (negative crystal) inclusions.

The most dramatic example of faceted inclusions is in sillimanite gneiss containing ~1-2 mm garnets that contain kyanite inclusions and abundant negative crystals of plagioclase. Matrix plagioclase (Pl) is unzoned, but Pl inclusions in garnet are strongly zoned: anorthite content increases by up to 24 mol% from core to rim. Zoned inclusions are surrounded by depletion haloes in Ca and Mg in garnet, documenting inclusion-garnet reaction. Zoning in garnet is most pronounced near Pl inclusions with visible fractures that connect to the garnet rim/matrix. Reaction involving Grt and Pl must involve other phases, such as Qz and kyanite/sillimanite, indicating that inclusions were not completely armored. Inclusion faceting and Grt/Pl zoning indicate that Grt interiors experienced significant modification after entrapment of the inclusions.

Some quartz inclusions are slightly faceted to rounded and are surrounded by Ca-poor regions of garnet. A recent study that applied Qz-in-Grt barometry to isolated, rounded inclusions in these rocks determined lower P (~0.6-0.7 GPa) than previous conventional-barometry results at similar T. These lower-P results are inconsistent with the presence of Ky inclusions in Grt and may reflect the modification of Qz inclusions that is apparent in garnet zoning around Qz and Pl inclusions.

Possible explanations for these observations are that: (1) the estimated P and/or T conditions are significantly lower than the actual conditions and the gneiss therefore experienced previously-unrecognized high-P granulite and/or eclogite facies metamorphism, or (2) rutile needles and faceted inclusions in Grt can form during metamorphism at upper amphibolite facies conditions; in this case, possibly the nature of the P-T-t path and/or role of fluids were important. The first possibility has significant implications for the tectono-metamorphic evolution of the orogen and perhaps other continental arc-related orogens, and the second is important for understanding the metamorphic processes that produce these inclusion textures in garnet. Using element maps and other methods for evaluating garnet and inclusion textures and compositions, we discuss these interpretations and implications.

How to cite: Whitney, D., Buboltz, P., and Hamelin, C.: Do oriented rutile needles and faceted/zoned inclusions in garnet require very high P-T to form?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6781, https://doi.org/10.5194/egusphere-egu22-6781, 2022.

To interpret the remanent pressures, stresses and strains in inclusion phases in garnets as their entrapment conditions by the methods of elastic geobarometry we require accurate and reliable EoS. However, differences between published EoS even for the end-member garnets often prevent meaningful or reliable geological information to be obtained from the stress states of inclusions trapped within them.

We have therefore re-evaluated all published volume and elasticity data for the garnet end members grossular, pyrope, almandine and spessartine for internal consistency. A novel feature of our analysis is that we also developed a method to determine the consistency of these data with experimentally-measured heat capacities. All of the consistent data was then simultaneously fitted by least-squares to determine the parameters of Mie-Grüneisen-Debye thermal-pressure EoS in combination with 3^rd-order Birch-Murnaghan EoS to describe the isothermal compression at 298 K. For grossular and pyrope garnets there is sufficient data to determine that the value of q used to define the volume dependence of the thermal Grueneisen parameter γ as q = d(ln γ)/d(ln V) has a value of q = 0.8(2). For other garnets, the data do not constrain the value of q. We therefore refined a q-compromise version of the Mie-Grüneisen-Debye EoS in which both γ/V and the Debye temperature are held constant at all P and T. For pyrope and grossular the two versions of the Mie-Grüneisen-Debye EoS predict indistinguishable properties over the metamorphic pressure and temperature range, and the same properties as the EoS based on experimental heat capacities. Final refined parameters are listed in the order V₀, K_0T, K’, Debye temperature and γ₀:

Pyrope : 113.13 cm³/mol, 169.3(3) GPa, 4.55(5), 771(28) K, 1.185(12)

Almandine: 115.25 cm³/mol, 174.6(4) GPa, 5.41(13), 862(22) K, 1.16

Spessartine: 117.92 cm³/mol, 177.57(6) GPa, 4.6(3), 860(35) K, 1.18(3)

Grossular: 125.35 cm³/mol, 167.0(2) GPa, 5.07(8), 750(13) K, 1.156(6)

Files containing these EoS for use in the EosFit7 are available at www.rossangel.net and in the EntraPT software for elastic barometry calculations at www.mineralogylab.com.

The biggest change from previously-published EoS is for almandine for which the new EoS predicts geologically reasonable entrapment conditions for zircon inclusions in almandine-rich garnets.

This work was supported by the the ERC-StG TRUE-DEPTHS grant (number 714936) to M. Alvaro.  M. Mazzucchelli is supported by an Alexander von Humboldt research fellowship.         

How to cite: Angel, R., Gilio, M., Mazzzucchelli, M., and Alvaro, M.: Garnet Equations of State: a critical review and synthesis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4283, https://doi.org/10.5194/egusphere-egu22-4283, 2022.

In metamorphic petrology, element–exchange geothermobarometry allows us to retrieve the pressure and temperature (P–T) conditions of (re–)equilibration of a mineral assemblage. These P–T conditions are not necessarily the conditions at which such mineral assemblage formed, nor where the rock reached its peak P and/or T, but those at which there was the last thermodynamic equilibrium (i.e., when the exchange of chemical components among and within phases of the system was stopped). Beyond this point, the system freezes due to slow diffusion kinetics, thus preserving the chemical composition at the reset conditions of equilibration.

The interpretation of P–T estimates from element exchange geothermobarometer is particularly challenging in (U–)HT rocks due to chemical re-equilibration during cooling (Frost and Chacko, 1989; Spear and Florence, 1992). Here we try to overcome the abovementioned problems of determining the UHT conditions of peak metamorphism or of mineral growth by proposing an alternative and viable method. We present and discuss the estimates of equilibration P–T conditions of a crustal garnet–pyroxenite xenolith from the Granatifera tuff, located in the Mercaderes–Rio Mayo area of the southern Colombian Andes, obtained using multiple geothermobarometric methods. This xenolith formed as a residue after extraction of granitic melt, and consists of garnet, clinopyroxene (X_Mg 0.73, Jd₁₆), plagioclase (Ab₇₂An₂₆Or₃), minor pargasitic amphibole (X_Mg 0.87), and accessory rutile and apatite. Garnet is chemically homogeneous (Alm_42–43Pyr_38–41Grs_16–20Sps₁) and often contains inclusions of quartz and zircon within the same crystals, as well as primary melt inclusions. Quartz is present only as inclusion in garnet. The sample has a well–equilibrated granoblastic texture, without evidence of reaction rims pointing to interaction with the host lava during entrapment and magma ascent.

We estimated the pressure and temperature of equilibration using a multi-methodological approach involving intracrystalline geothermometry, elastic geothermobarometry, and classical Fe–Mg exchange between garnet and clinopyroxene. The equilibration temperatures obtained on clinopyroxenes using the intracrystalline geothermometer by Brizi et al. (2000) are around 1150–1250 °C. This estimate is consistent with results of elastic geothermobarometry: the isomekes for quartz– and zircon–in–garnet (Angel et al., 2014; Gilio et al., 2021) indicate equilibration conditions of 1150–1200 °C and 1.7–2.1 GPa. Instead, geothermometry based on Fe–Mg exchange between garnet and clinopyroxene (Nakamura, 2009) gives lower equilibration temperatures of 950–1000 °C, suggesting a re-equilibration during regional cooling at the roots of the magmatic arc. Our results have important implications for the reliability of element–exchange geothermobarometry in UHT rocks. Elastic geothermobarometry gives reliable and independent P–T estimates and it is virtually unaffected by the diffusion­–induced reset during retrogression typical of cation–exchange geothermometry. This new approach solves the long-standing issue of estimating pressure and temperature conditions in HT and UHT rocks and appears to be robust and reliable to temperatures as high as 1200 °C.

References:

Angel et al. (2014) - American Mineralogist 99, 2146-2149. Briziet et al. (2000) - American Mineralogist 85, 1375-1382. Frost & Chacko (1989) - The Journal of Geology 97, 435-450. Gilio et al. (2021) - Journal of Metamorphic Geology. Nakamura (2009) - Journal of Metamorphic Geology 27, 495-508. Spear & Florence (1992) - Precambrian Research 55, 209-241.

How to cite: Gilio, M., Cesare, B., Gianola, O., Ferri, F., Murri, M., Barbaro, A., and Alvaro, M.: Coupled elastic and intracrystalline geothermobarometers to constrain PT conditions of lower arc crust granulites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9342, https://doi.org/10.5194/egusphere-egu22-9342, 2022.

Geochronology provides the time frame for various fields of research in the earth sciences that investigate the geological history from the mineral scale to tectonic plates. The reconstruction of pressure–temperature-time (P–T–t) paths of metamorphic rocks from collisional settings such as orogenic belts, for example, is commonly achieved by linking chronological (U–Pb) and trace element data from U-rich accessory phases such as zircon, monazite, titanite, or rutile, with thermobarometric information derived from rock-forming minerals such as garnet. The ability to extract both types of information from one mineral makes garnet arguably the most versatile and powerful petrochronometer. Garnet has an extensive P–T stability field for a wide variety of rock compositions, and changes in P–T conditions during garnet growth are recorded in compositional changes. Garnet U–Pb ages can thus be linked to compositionally distinct garnet domains, providing a direct link between P–T estimates and chronological data. Although still in its infancy, U–Pb dating of metamorphic garnet by LA–ICPMS is an evolving petrochronological tool with a vast potential and a plethora of possible applications.

This contribution discusses the advantages and limitations of this method, as well as the significance and meaning of garnet ages. The benefits of the method, as compared to conventional isotope dilution techniques, are the ease of sample preparation, rapid data acquisition and processing, high spatial resolution, and relatively low costs. However, the very low (≤1-100 ng/g) and variable U content of regional metamorphic garnet and the frequently ubiquitous presence of inclusions of U-rich accessory minerals are obstacles that affect the precision and accuracy of garnet U–Pb dates, or may render a particular garnet sample undateable altogether. These various aspects of garnet U–Pb dating by LA–ICPMS will be examined by discussing metamorphic garnet from the Alps, which had previously been dated by Sm-Nd chronology (Pollington and Baxter, 2010).

Pollington, A.D., Baxter, E.F., 2010. High resolution Sm–Nd garnet geochronology reveals the uneven pace of tectonometamorphic processes. Earth and Planetary Science Letters 293, 63–71. https://doi.org/10.1016/j.epsl.2010.02.019

How to cite: Millonig, L. J., Beranoaguirre, A., Albert, R., Marschall, H., Baxter, E., and Gerdes, A.: Garnet U–Pb dating by LA–ICPMS: Opportunities, limitations, and applications, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7077, https://doi.org/10.5194/egusphere-egu22-7077, 2022.

Recent advances in analytical techniques and instrumentation allow for the analysis of increasingly smaller sample volumes and lower concentrations. This development significantly expands the possibilities of in-situ geochronology, e.g., LA-MC-ICPMS. Minerals with low U (and Pb) contents such as garnet become the target of in-situ U-Pb geochronology since ages can potentially be obtained from single (sub-)mm-sized garnet grains in thin sections. In this contribution, we explore the current limits of in-situ U-Pb geochronology: what are the minimum concentrations from which an accurate and precise U-Pb age can be obtained?

For that purpose, we have analysed garnets from three different localities that were unsuccessfully analysed in the past using a single-collector sector-field Element XR instrument at FIERCE. These garnets have been re-analysed at FIERCE using a Neptune Plus MC-ICPMS coupled to a RESOLution-LR ArF Excimer laser. The analyses were performed in static mode measuring the masses ²⁰⁶Pb and ²⁰⁷Pb with Secondary Electron Multiplier (SEM) and ²⁰²Hg, ²⁰⁴Pb and ²³⁸U with the Multiple Ion Counters (MIC). With a spot diameter of 193 μm (round) and a fluence of 2 J/cm² at 15 Hz, ca. 18 µm pit depth was ablated in 18s analysis time, resulting in a total of 2 µg of ablated material. This is more than 2,000 times less material compared to conventional isotope dilution analyses and 3,000 times less U than for a typical LA-ICPMS zircon analysis (20 µm spot). Although the analysed garnets typically have U contents below 10 ng/g, about 15–30 spots are commonly sufficient to define a regression line in the Tera-Wasserburg diagram, yielding a precision of typically <3 % for the lower intercept age. Challenges and details of the method will be discussed using samples of metamorphic garnet from Kaapvaal craton granulites and Eastern and Western Variscan eclogites.

How to cite: Beranoaguirre, A., Millonig, L. J., Shu, Q., Albert, R., Marschall, H. R., Gil Ibarguchi, J. I., and Gerdes, A.: Exploring the limits of in-situ U-Pb dating of metamorphic garnet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12133, https://doi.org/10.5194/egusphere-egu22-12133, 2022.

The Eoarchean Isua supracrustal belt (ISB) represents a key supracrustal portion of one of the oldest km-scale regions of Archean crust exposed today. Microstructures and compositional zoning of garnets from the ISB have previously been interpreted to reflect either three or two main metamorphic events. The former interpretation supports the operation of plate tectonics in the early Archean, while the latter suggests Eoarchean non-uniformitarian tectonics. Thus, an in-depth understanding of the evolution of garnet through time is necessary to evaluate these tectonic models.    

A quantitative microstructural and chemical analysis of the garnet porphyroblasts in the ISB and their host rocks shows that variability in garnet characteristics is largely a product of differing degrees of transport-controlled growth mechanisms operating in medium-grade rocks. Sluggish elemental transport is common in samples with low contents of garnet-forming phases and elements, and high abundance of quartz and/or carbonates. Such rocks will develop garnets with high proportions of inclusions, irregular internal morphologies, and localized equilibrium features like patchy zoning. Faster elemental transport in rocks with higher concentration of garnet-forming materials and lower content of non-reactant phases are characterized by higher garnet-to-inclusion ratios and the development of concentric zoning patterns.

The observed diversity of the porphyroblast characteristics across the ISB is thus readily interpreted as a consequence of changes in local bulk compositions, whereas the rocks experienced the same tectonometamorphic evolution. We use garnets showing larger-equilibrium features from two metapelitic rocks from the centre of the belt to decipher the metamorphic evolution of the belt. Here, petrographic observations, garnet trace element compositions, quartz-in-garnet barometry, and geologically-based timing constraints are used to model different P-T-t paths and test the competing models proposed for the belt. Preliminary results suggest that a two-stage garnet growth history can successfully reproduce the observed garnet record. These results highlight that a plate tectonic interpretation of the ISB is not unique and that simple non-uniformitarianism interpretations are viable.

How to cite: Ramírez-Salazar, A., Müller, T., Piazolo, S., Sorger, D., Zuo, J., Webb, A. A. G., Dey, J., and Haproff, P.: Using garnet to the fullest: The tectono-metamorphic evolution of the Eoarchean Isua supracrustal belt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10734, https://doi.org/10.5194/egusphere-egu22-10734, 2022.

Mylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain were investigated for the reconstruction of their metamorphic evolution and P-T history. Micaschists underwent polyphase ductile deformation consisting of an old D1 deformation (~345–340 Ma) associated to shearing and folding and to a penetrative S1 axial plane foliation. The S1 foliation is progressively transposed by the D2 phase, which is associated with upright up to NE verging folds and dextral shear zones. Micaschists are characterized by abundant centimetric garnet crystals with strong compositional zoning. The garnet porphyroblasts (~15 vol%) are associated with plagioclase, quartz, biotite, staurolite, white mica, chloritoid and retrograde chlorite. Ilmenite, rutile, corundum, zircon, monazite, apatite and tourmaline are accessory phases. Garnet presents an iso-orientation of quartz inclusions sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth, discordant respect to the external S2 foliation. The S2 foliation is identified by the preferred orientation of micas and chlorite and by the alternance between quartzo-feldsphatic and micaceous layers. Microstructural investigation with the polarizing microscope reveals different size, number, and distribution of the mineral inclusions in the different garnet portions. The garnet core contains several small inclusions of quartz, rutile, apatite, and minor monazite and zircon. Additional inclusions, observed in the garnet domain around the core are ilmenite, chloritoid, staurolite and white mica. EMPA analyses reveal an even more complex chemical zoning consisting of (i) garnet core, (ii) garnet mantle, and (iii) garnet rim. Pyrope is low and homogeneous in the core (Prp_<2), higher in the mantle (Prp_2-8) and even higher in the rim (Prp_>8). The other components in the garnet core are Grs_21–27; Alm_45–50; Sps_25–30. The garnet mantle is enriched in almandine (Alm_55–85), and depleted in Grs_6–18 and Sps_5–24 as compared to the core. The composition of the rim is characterized by abrupt decrease in Grs_2–3. As regards the P-T path reconstruction, the compositional isopleths that match the composition of the garnet core intersect at about T 460°C, P 1.3-1.5 GPa, whereas the composition of garnet mantle suggests a pressure increase up to 1.8 GPa.  Garnet rim isopleths indicate P-T conditions close to 570°C/0.8-0.9 GPa. The resulting P-T path is clockwise. The systematic finding of new HP rocks in northern Sardinia suggests that the Variscan belt setting is probably more complex than the simple prograde Barrovian sequence described so far.

How to cite: Cruciani, G., Franceschelli, M., Carosi, R., and Montomoli, C.: Garnet zoning in pelitic schist from NE Sardinia, Italy: further evidence of Variscan HP metamorphism, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2453, https://doi.org/10.5194/egusphere-egu22-2453, 2022.

The Zicavo septum in southern Corsica represents one of the Variscan remnants intruded by late Hercynian granitoids. It is made up of the following lithological units cropping out along a NNE-SSW direction with a general NW-SE striking trend: i) an augen orthogneiss; ii) a leptynite-amphibolite series with metapelites and serpentinite bodies, and iii) a metapelitic sequence (Faure et al., 2014). The sequence is polydeformed by two phases of ductile deformation, D1 and D2, the first characterized by top-to-the-SW and the latter by top-to-the-SE sense of shear. The D1 foliation in the amphibolite and metapelite is deformed by SE-verging folds of the D2 phase. Two garnet-bearing micaschist samples have been selected from the metapelitic sequence for detailed microstructural and mineralogical study. The millimetric garnet porphyroblasts (up to 4-5mm in size; ~12-15 vol%) are associated with plagioclase, quartz, biotite, staurolite, white mica, chlorite and chloritoid and minor ilmenite, zircon, monazite, and apatite. The garnet porphyroblasts are subhedral to euhedral, characterized by a high variety of mineral inclusions (Qz+Cld+Mnz+Ilm+Ms+Zrn+Ap; mineral abbreviations after Whitney and Evans, 2010) decreasing in abundance from core to rim. Garnet cores often preserve a relict foliation identified by the alignment of several microinclusions sometimes arranged into a sigmoidal pattern (snowball garnet). In the rock matrix, the pervasive foliation is identified by the preferred orientation of phyllosilicate minerals and by compositional (quartzo-feldsphatic vs. micaceous) layering. Garnet from Sample ZIC10 (mineral assemblage Grt+Bt+Chl+Ms+Qz+Pl+Ilm) show an increase of iron and magnesium contents from core to rim (29.5 - 34.5 wt.% and 1.5 - 2.5 wt.%, respectively) and a corresponding decrease of manganese and calcium (9 - 4.2 wt.% and 3.1 - 2.1 wt.%). Garnet in sample ZIC11 (coexisting with Chl+Cld+Ms+St+Qz+Pl+Ilm) shows a similar compositional trend (from core to rim, Fe = 26.8-33.5 wt.%; Mg = 1.2-2 wt.%; Mn = 6.5-1.6 wt.%; Ca = 6.8-4.1 wt.%). Preliminary estimation of metamorphic P-T conditions by using isochemical phase diagrams and compositional isopleths of garnet components indicates garnet core formation at ca. 1.6 – 1.7 GPa / 500°C. Garnet compositional zoning suggests an increase in temperature accompanied by a decrease in pressure, compatible with a clockwise P-T trajectory. Comparable P-T conditions and P-T path were observed in HP Variscan pelitic schist from northeastern Sardinia.

Faure M. et al. (2014) International Journal of Earth Sciences 103, 1533–1551.

Whitney D.L. and Evans B.W. (2010) American Mineralogist 95, 185–187.

How to cite: Dulcetta, L., Cruciani, G., and Franceschelli, M.: Garnet zoning in Variscan pelitic schist from Zicavo, Corsica (France), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2501, https://doi.org/10.5194/egusphere-egu22-2501, 2022.

Although garnet is an important accessory phase in felsic igenous rocks, its potential for timescales of magmatic processes such as mineral growth rates or ascent rates through the crust is not fully exploited. The origin of garnet in felsic igneous systems range from crystallization from (mostly) peraluminous melts to entrainment of peritectic or xenocrystic garnets originating from country rocks. We investigated garnets from mid-crustal plutonic rocks from the Ivrea-Zone (N-Italy), which contain metapelitic enclaves and composite metamorphic-magmatic xeno-phenocryst garnet. Using microtomography, high resolution EPMA mapping and detailed chemical transects by LA-ICP-MS we identified garnets with metamorphic cores and multiple igneous overgrowth rims. Using independent temperature and pressure constraints such as Zr saturation temperature from zircon-bearing nanogranitoid inclusions and phase equilibrium constraints the crystallization conditions are constrained to 780 to 820°C and ~3-4 kbars, while the garnet core still records lower crustal conditions. To quantify the duration of magmatic overgrowth, we have numerically modeled Cr, Y, REEs and Hf trace element diffusion, as well as multicomponent major divalent cation diffusion within garnet using available experimental diffusion data and Cr diffusion data retrieved from natural garnets. All modelled diffusants conform to a single temperature-time path, in which the temperatures associated with the first and second magmatic overgrowths persisted for 5.4 and 6.3 kyr respectively (Devoir et al. 2021). Rhyolite-MELTS modeling was used to explore various decompression and cooling paths of ascending rhyodacitic magmas and its effects on the density and viscosity evolution. Using a range of H₂O contents and resulting different viscosities for the ascending felsic magma, garnet grain settling velocities of ca. 1 m.yr^-1 were calculated using the Navier Stokes equation. To preserve lower crustal garnet core compositions, maximum time scales of melt extraction of ca. 15 kyr are calculcated. Potential implications for magma ascent rates will be discussed.

Devoir, A., Bloch, E., Müntener, O. (2021) Residence time of igneous garnet in Si-rich magmatic systems: Insights from diffusion modeling of major and trace elements. Earth and Planetary Science Letters 560, 116771

How to cite: Müntener, O., Devoir, A., and Bloch, E.: Garnet as a useful monitor of growth and ascent rates in felsic igneous systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11366, https://doi.org/10.5194/egusphere-egu22-11366, 2022.