Al-garnet is a common constituent of medium- to high-grade metamorphic rocks of sedimentary and basic to acidic igneous protoliths. Due to its compositional variability (main components: almandine, grossular, pyrope, spessartine), this mineral is very important to decipher the pressure-temperature (P-T) evolution of these rocks. In many cases, garnet occurs as compositionally zoned porphyroblast being the result of prograde metamorphism during which hydrous minerals such as chlorite or lawsonite were decomposed. The relevant temperature interval for corresponding mineral reactions is usually in the range 450-650 °C. Thus, garnet is a nearly perfect mineral to decipher P-T conditions experienced by medium-grade metamorphic rocks. In addition, it preserves minerals which were enclosed during its growth. Therefore, P-T conditions can also be derived for a metamorphic stage before garnet growth based on inclusion minerals in garnet cores.

Prograde metamorphism, characterized by slight heating but significant pressure increase at high temperatures, commonly leads to garnet by breakdown of dry minerals such as cordierite and plagioclase (anorthite component). Corresponding reactions mainly occur between 0.5 and 1.8 GPa. Above this pressure range, garnet cannot be used to precisely determine P-T conditions of rocks, which were subjected to metamorphism in the high-temperature eclogite- and high-pressure granulite-facies, unless melting reactions took place, for example, with participating hydrous minerals such as micas, (clino)zoisite, and amphibole resulting in the formation of peritectic garnet. But the derivation of the P-T conditions of peritectic garnet formation, concerning pressures also below 1.8 GPa, requires complex thermodynamic modelling as various parameters such as H₂O content of the rock and possible melt loss have to be considered. In addition, intracrystalline cation diffusion, particularly of Mg, in garnet complicates this derivation by the perceptible change of the original garnet composition at metamorphic peak temperatures above 750-800 °C. As both complex thermodynamic modelling for peritectic garnet, if applicable at all, and modelling of this cation diffusion were very rarely applied in the past, published P-T paths through the realms of the high-temperature eclogite- and granulite-facies are fairly uncertain and sometimes even wrong.

The annoying intracrystalline cation diffusion, however, can be valuable, for example, in the case of a contact of two garnet generations with different chemical compositions within a single grain. Modelling of this contact feature, which is typical of polymetamorphic rocks, can yield a time interval for early cooling at high temperatures. In such rocks characterized by an early medium-temperature and garnet-free mineral assemblage, initial growth of garnet during burial can be delayed due to an energetic barrier deferring the formation of garnet seeds. This so-called “garnet overstepping” concerns a pressure range up to 1 GPa above the garnet-in curve and leads to garnet porphyroblasts with nearly homogeneous chemical composition. Therefore, this type of garnet was, so far, frequently mistaken for a high-temperature garnet homogenized by intracrystalline cation diffusion. Despite such pitfalls and the aforementioned limitations, garnet is essential to deduce the P-T evolution of metamorphic rocks of deep-seated crustal sections and, thus, to better understand geodynamic processes involving the Earth’s crust.

How to cite: Massonne, H.-J.: Garnet, a marvellous mineral for deriving P-T paths of metamorphic rocks, but what are the pitfalls and limitations?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1857, https://doi.org/10.5194/egusphere-egu23-1857, 2023.

The area of the Slavonian Mountains in Croatia is considered as a natural laboratory for the study of metamorphic processes on polymetamorphic rocks formed during the pre-Variscan, Variscan and Alpine orogenies. Since most lithologies contain garnet, understanding of its growth is an important to unravel the polymetamorphic evolution. Garnet up to 200 µm in size with atoll texture were found in quartz- and mica-rich rocks that have experienced peak metamorphism at intermediate temperatures and pressures in the amphibolite facies. These atoll garnet-bearing rocks occur in the oldest metamorphic complex of the Slavonian Mountains at several localities and are restricted to highly foliated mica-schists and/or paragneisses composed of quartz (~20 vol. %), plagioclase (~40-50 vol. %), biotite (20-30 vol. %), garnet (2-3 vol. %) and opaque minerals.

The observed atoll structures are practically restricted to larger garnet comprising a core, an intermediate cloudy zone composed of garnet and the minerals of the rock matrix (quartz, biotite, plagioclase) and a nearly inclusion-free rim. Smaller garnet with uniform texture and composition, which corresponds to that of the rim of larger garnet, also occurs. The core of the atoll garnet shows higher Ca contents than the rim (12-16 vs. 4-5 mol.% of grossular component), lower Fe contents (68-69 vs. 75-76 mol.% of almandine component) whereas Mg and Mn contents are similar. Furthermore, the three domains of the atoll garnet show almost regular outlines or crystallographic forms. Pseudosection modelling was used to reconstruct a preliminary and simplified P-T path, which is clockwise with maximum pressure conditions at ~1.0-1.2 GPa and temperatures of ~650 °C. However, it is likely that these conditions were followed by a significant pressure decrease accompanied by resorbtion of garnet. Shape of garnet core and cloudy zone, but also mineral phase relations, suggest a second stage of garnet growth, forming the rim, during another pre-Variscan stage of amphibolite-facies metamorphism at P-T conditions of 0.5 GPa and 530 °C. This interpretation resulted from in-situ monazite dating, yielding two mean ages at 522±6 and 473±11 Ma (2σ).

Atoll-shaped garnets are uncommon and have been recognized in contrasting metamorphic environments worldwide. Several models have been proposed to explain their formation, including preferential dissolution of garnet cores by fluid infiltration, polymetamorphism, coalescence of subgrains, and kinetic control associated with rapid growth. Here, the formation of the atoll garnet is interpreted by the following processes: external fluid infiltration into the fine-grained rocks and element exchange between the core of primary small garnet grains and matrix, dissolution of the garnet core and later replacement by a new garnet during a subsequent metamorphic event.

How to cite: Balen, D. and Massonne, H.-J.: Zoned and atoll garnet from the Slavonian Mountains (Croatia) and their significance for the evolution of a complex polymetamorphic terrane, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1991, https://doi.org/10.5194/egusphere-egu23-1991, 2023.

Metarodingites are metasomatized mafic dykes that are embedded within serpentinized mantle rocks and commonly contain garmet. Garnet in this rock type has the potential to preserve compositional and chronological information of the entire metamorphic-metasomatic evolution, from the ocean floor to deep subduction. In this study, we investigate the chemical and chronological record of garnet from chloritized metarodingites and garnet veins, from the Zermatt-Saas unit (Unter Theodulgletscher unit and Pfulwe pass). Compositional major and trace element maps reveal that a first generation of Ti-rich garnet is consumed during chloritization, while a second generation of Ti-poor garnet grows in textural equilibrium with chlorite and titanite. In both metarodingite and garnet vein, garnet rim displays an enrichment in Cr, suggesting that late garnet records the infiltration of fluids equilibrated with serpentinites. Fluids liberated from dehydrating serpentinites infiltrate the metarodingite leading to the first garnet generation dissolution, chloritization and titanite crystallization. To validate this hypothesis, we performed LA-ICPMS U-Pb dating of garnet, titanite and zircon. U-Pb dating of garnet core and rim, returned overlapping ages between ca. 44 and 46 Ma, which coincide with previous estimation of peak metamorphic conditions. Titanite from the metarodingite samples yields an age of ca. 45 Ma, which indicates that fluid release and chloritization occurred indeed at peak conditions. Garnet veins cutting across the foliation of the metarodingite and associated titanite are instead resolvably younger and yield ages of ca. 38-39 Ma and ca. 36 Ma, respectively. Zircon in chloritized mafic dykes from Pfulwe pass consists of a Jurassic magmatic core and a metamorphic rim of ca. 47 Ma, confirming that the major fluid-release event and related metasomatism occurred between 45-47 Ma. Rutile yields a younger age of ca. 34 Ma, probably linked to re-setting during exhumation. In conclusion, we show how garnet from metarodingites preserves the metamorphic-metasomatic history and can be used to gather information on (de)hydration reactions during subduction. The consistency of our multi-mineral geochronological data further indicates that petrochronology of garnet from metarodingites is a robust way to track in time metasomatic events in the subducted oceanic lithosphere.

How to cite: Piccoli, F., Rubatto, D., Millonig, L. J., and Gerdes, A.: Garnet in metarodingites: composition, chronology and link to dehydration/hydration reaction during subduction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4908, https://doi.org/10.5194/egusphere-egu23-4908, 2023.

Mafic–ultramafic lenses embedded in felsic granulites of the Gföhl Unit, Moldanubian Zone, are considered to be mantle fragments incorporated into mid-crustal levels of the Variscan orogenic crust. We investigated a several 100 m sized mafic lens mainly formed by garnet pyroxenite. The primary mineral assemblage comprises calcium-rich garnet (X_Grs = 0.4), kyanite, and sodium-rich clinopyroxene (X_{Na_M2} = 0.29) (± quartz), which indicates pressures above 1.8 GPa and temperatures around 1000 °C. Towards the margins of the mafic lens, the garnet pyroxenites were increasingly overprinted at lower pressures leading to the destabilization of kyanite, Na-rich clinopyroxene, and garnet. A first decompression phase is represented by garnet-hosted sapphirine–spinel–plagioclase symplectites supposedly replacing kyanite and clinopyroxene. A second stage is evident from the partial resorption of garnet by plagioclase and clinopyroxene in the form of a peculiar corrosion tubes penetrating the garnet in a worm-like fashion. Finally, the third stage decompression assemblage is represented by plagioclase–orthopyroxene–spinel symplectites partially replacing garnet. In all cases, garnet shows pronounced secondary compositional zoning towards the decompression products. The secondary zoning is qualitatively similar for the sapphirine–spinel–plagioclase symplectites and the plagioclase–clinopyroxene corrosion tubes and is characterized by a strong decrease of the Grs content accompanied by an increase of the Alm and Prp contents towards the decompression products. For the sapphirine–spinel–plagioclase symplectite, the garnet composition changes from Alm₁₄Prp₄₂Grs₄₄ in the pristine garnet to Alm₂₂Prp₆₃Grs₁₅ at the interface to the symplectite. The compositional change towards the corrosion tubes is from Alm₁₉Prp₄₀Grs₄₁ to Alm₃₀Prp₅₄Grs₁₆. The secondary zoning towards the plagioclase–orthopyroxene–spinel symplectites is characterized by an increase of X_Alm from 0.19 to 0.27 and a concomitant decrease of X_Prp from 0.55 to 0.49 at constant X_Grs of 0.25. In all cases, the compositional changes are gradual suggesting diffusion-mediated re-equilibration of the garnet at decreasing pressures. Time scales for the duration of decompression were estimated by fitting a multicomponent diffusion model to the observed compositional patterns. Depending on the choice of the diffusion coefficients, the time scales vary from several hundreds to hundred thousands of years, whereby the earliest decompression features yield time scales that are five times longer than those obtained from the corrosion tubes and about ten times longer than those obtained from the plagioclase–orthopyroxene–spinel symplectites. These timescales reflect the duration from the onset of the different decompression-induced mineral reactions to the time when the rocks cooled below about 700 °C and the composition patterns of the garnet were effectively frozen. The longest timescales obtained from the early decompression reactions are on the order of 100,000 years and the shortest timescales obtained from the late-stage symplectites are on the order of 1,000 years. Considering the regional metamorphic setting of the Moldanubian Zone, such timescales are remarkably short and suggest rapid transport of the mafic–ultramafic lithologies from mantle depths to the mid-crustal level. Concomitant incorporation into a dominantly felsic environment led to immediate cooling.

How to cite: Asenbaum, R., Racek, M., Zelinková, T., Janoušek, V., Petrishcheva, E., and Abart, R.: Decompression time scales of mantle fragments constrained by secondary chemical zoning of garnet from the Gföhl Unit, Moldanubian Zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14192, https://doi.org/10.5194/egusphere-egu23-14192, 2023.

Knowledge of oxygen diffusion in garnet is crucial for a correct interpretation of oxygen isotopic sig­natures in natural samples. Scicchitano et al. (2022) reported a series of experiments with pyrope and YAG at P-T of 1-atm to 2.5 GPa and 900 °C to 1600 °C, either under nominally-dry or water-saturated conditions, to better constrain the diffusivity of oxygen in garnet. Analysis of ¹⁸O/(¹⁸O+¹⁶O) profiles by Secondary Ion Mass Spectrometry (SIMS) shows that: (i) diffusivities in pyrope and YAG crystals annealed under similar conditions (P = 1 GPa and T = 900 °C) are comparable, suggesting a limited effect of chemical composition on oxygen diffusivity; (ii) diffusivity values calculated for water-saturated experiments at 900 °C fall on the Arrhenius curve described by nominally dry experiments performed at T = 1050-1600 °C; and (iii) several profiles deviate from the Gaussian error function, suggesting complex diffusion behaviour related to diffusion via interstitial (fast) and vacancy (slow) mechanisms. Modelling this process yields oxygen diffusion coefficients, D, that differ by approximately two orders of magnitude between the fast and slow diffusion mechanisms. The new experimental data suggest, however, that the slow mechanism is prevalent in natural garnet compositions and probably controls the retentivity of oxygen isotopic signatures in natural samples. Even though oxygen diffusivity in garnet is comparable to Fe-Mn and Ca diffusivity at high temperature (> 850 °C), oxygen diffusivity is slower than cation diffusivity at P-T conditions typical of crustal metamorphism due to its larger activation energy. Original oxygen isotopic signatures therefore can be retained in garnet showing zoning partially re-equilibrated by the diffusion of other major elements.                 

References

Scicchitano et al. (2022), American Mineralogist, 107, 1425-1441.

How to cite: Scicchitano, M. R., Jollands, M. C., Williams, I. S., Hermann, J., Rubatto, D., Kita, N. T., Nachlas, W. O., Valley, J. W., Escrig, S., and Meibom, A.: Experimental calibration of oxygen diffusion in garnet and implications for retention of primary oxygen isotopic signatures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1226, https://doi.org/10.5194/egusphere-egu23-1226, 2023.

Directed garnet growth in a peraluminous pegmatoid from the Moldanubian zone in the Bohemian Massif (AT) formed almandine-spessartine garnet that crystallized at the transition from melt to subsolidus stage. One side of the garnet crystal was in contact with a solidified matrix, the other with melt, resulting in asymmetric microstructure, crystal morphology and major element compositional zoning.

Based on major element composition, three garnet growth stages are inferred: (i) magmatic stage during which garnet grew as part of the coarse-grained assemblage Pl + Grt + Ky + Bt. This growth zone has the highest Mn content and P concentrations of 0.4 – 0.5 wt%, (ii) intermediate stage with decreasing Mn, and increasing Fe, Mg and Ca contents and Mg# (=Mg/(Mg+Fe)), (iii) subsolidus stage forming garnet reaction rims with highest Mg and Ca contents and Mg#.

Growth stage (i) involves the development of sector zoning, which is defined by the colour of garnet and the presence of < 1 micrometer sized inclusions observed in optical light microscope (OM). These are dominated by phosphates in Grt{110} growth sectors and by rutile in Grt{112} sectors. Additionally, nanoinclusions of 20 – 50 nm size were identified by scanning transmission electron microscopy of garnet even for zones that appear inclusion-free in OM.

Chemical compositions obtained from electron microprobe analyses integrate over garnet and the inclusions and thus also reflect the different nanoinclusion-contents of the two garnet growth sectors. Compared to the Grt{112} sector, the Grt{110} sector is c. 0.1 wt% higher in P, 100 – 150 ppm higher in Na, and 100 – 200 ppm lower in Ti, which is in line with the prevalence of Na-containing phosphates and the comparatively lower abundance of rutile in this sector.

The following scenarios are considered for the genesis of the nanoinclusions: As sector zoning has developed during magmatic conditions of growth stage (i), overgrowth of pre-existing accessory phases is implausible, as no mechanism for selective incorporation of different phases at different facets is known. Instead, facet specific minor and trace element partitioning during garnet growth and subsequent exsolution, or alternatively, sector specific nucleation and co-growth of accessory rutile and phosphates are both reasonable explanations for the observed distribution of inclusions in specific garnet sectors. These considerations indicate facet-selective processes likely related to the different crystal structure features exposed at the interfaces in contact with the melt.

We conclude that facet specific formation of nanoinclusions is an important factor controlling the trace element composition of pegmatoid garnet apart from bulk melt composition and pT-conditions. When interpreting P, Na and Ti contents in garnet, the potential presence of nanoinclusions that are invisible in the optical light microscope needs to be accounted for, as they may be more widespread in pegmatoid garnet than expected.

The study was funded by the Austrian Science Fund (FWF): I4285-N37 and the Slovenian Research Agency (ARRS): N1-0115.

How to cite: Kohn, V., Alifirova, T., Daneu, N., Griffiths, T., Abart, R., and Habler, G.: Nanoinclusions in apparently inclusion-free sector-zoned pegmatoid garnet – their impact on P, Ti and Na concentrations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4836, https://doi.org/10.5194/egusphere-egu23-4836, 2023.

Metamorphic garnet can be the ultimate source of information about geodynamic and geochemical processes in the Earth’s crust. Compositionally zoned garnet porphyroblasts preserve records of the host rock’s reaction path, its element transport properties, the fluid-rock interaction during metamorphism as well as absolute ages and rates of these processes. Especially variations of rare earth element (REE) concentrations in garnet are reflecting many of these geodynamic and geochemical processes. In order to extract this information, the thermodynamic equilibrium and kinetic contributions of the REE uptake in garnet must be distinguished and quantified, for which high resolution major- and trace element mappings together with numerical growth simulations are an indispensable tool.   

Utilizing high resolution trace element and µ-Raman mapping together with combined thermodynamic-geochemical-diffusion models we discriminate the equilibrium and kinetic aspects of the heavy (H) REE uptake in a garnet porphyroblast from a high-pressure/low temperature blueschist from the Dominican Republic. Like many metamorphic garnets from different rock types and tectonic settings, the HREE zoning in the investigated garnet comprises an inner, bell-shaped part with a pronounced central peak and an overall decrease of all HREE from core to inner rim. The central peak in the garnet core decreases in intensity with decreasing atomic number of the REE. This trend is followed by a concentric zone of HREE enrichment and a drastic HREE decrease towards the outermost rim. Superimposed on this common trend is a concentric pattern of minor recurring fluctuations in the HREE concentrations with regularly spaced sets of peaks and troughs along the entire garnet radius.

Comparison of the trace element mappings and thermodynamic-geochemical models show that the inner, bell-shaped part results from fractional garnet growth in an unchanged mineral assemblage. The model results further show that the width of the central peak is controlled by the bulk permeability of the interconnected transport matrix and the fraction of matrix minerals that the garnet equilibrates with. The correlation of high resolution µ-Raman mappings of the inclusion suite and the trace element mappings indicate that the REE enrichment zone is caused by HREE redistribution during the titanite-rutile transition.

The superimposed REE fluctuations result from changing element transport properties of the host rock and mark recurring changes from equilibrium REE uptake to transport-limited REE uptake in garnet. Such fluctuating element transport properties can be best explained by pulse-like fluid fluxes that rhythmically change the interconnectivity of the intercrystalline transport matrix. Increasing numbers of published spatially highly resolved REE analyses show that such trace element fluctuations are common in metamorphic garnet indicating that recurring changes in rock permeabilities due to pulsed fluid fluxes are a common phenomenon during metamorphism.

How to cite: Konrad-Schmolke, M., Halama, R., Chew, D., Heuzé, C., and De Hoog, J.: Towards a comprehensive use of garnet as an indicator of geodynamic-geochemical processes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11106, https://doi.org/10.5194/egusphere-egu23-11106, 2023.

The global onset and evolution of subduction-driven plate tectonics is one of the most debated topics in Earth sciences. Although very diverging views are hold, most observations indicate a transition from a stagnant- to a mobile-lid regime in the late Archean (e.g. Palin et al. 2020). Since then, geothermal gradients (T/P) of metamorphic rocks decreased, interpreted as an evolution from warm and shallow to cold and steep subduction (e.g. Brown et al. 2006), which may explain the oldest unequivocal evidence of UHP metamorphism at ~620 Ma (Jahn et al. 2001). By contrast, extreme UHP conditions of ~7 GPa at ~975 °C have been supposed for ~1.8 Ga crustal rocks in Western Greenland, mainly based on carbonaceous inclusions in garnet interpreted as diamond partially replaced by graphite as well as oriented inclusions of a hydrous phase interpreted as orthopyroxene exsolution from a majoritic precursor (Glassley et al. 2014).

In order to find mineralogical evidence for UHP metamorphism, like coesite, we used a detrital approach that has been demonstrated to be very powerful (Schönig et al. 2018, 2019, 2020; Baldwin et al. 2021). Modern sands from eight catchments draining the proposed UHP terrane in Western Greenland were extensively screened by semi-automated Raman heavy-mineral analysis (n = 52,130 grains) and electron microprobe analysis of garnet major-element chemistry as well as hyperspectral Raman imaging (>680 million spectra) of mineral-inclusion assemblages (n = 2,674 garnets). In all samples, amphibole, garnet, orthopyroxene, augitic clinopyroxene, and sillimanite represent the majority of heavy minerals, reflecting erosional material sourced from amphibolite- to granulite-facies rocks. Garnet chemistry and mineral inclusion assemblages, particularly the common co-existence of sillimanite and rutile inclusions, indicate a major garnet growth stage at MP to HP granulite-facies conditions. Though, lower garnet X_Mg and/or higher X_Mn and/or X_Ca as well as a more frequent occurrence of hydrous mineral inclusions (amphibole and phlogopite-biotite) of a smaller proportion (~20 %) imply garnet growth at lower temperature conditions, interpreted as relicts of prograde metamorphism. Garnets predicted to be grown at the highest P conditions (~6 %) commonly host inclusions of augitic clinopyroxene, amphibole, plagioclase, and quartz, mainly indicating HP amphibolite facies conditions that in maximum may have reached the transition zone between amphibolite- and eclogite-facies conditions. Furthermore, we show that neither the reported existence of diamond nor the interpretation of a majoritic precursor hold against a critical re-assessment. Overall, the total absence of minerals indicating UHP conditions (like coesite and diamond) and even HP conditions (like omphacite or glaucophane) in our large detrital dataset as well as alternative interpretations for reported UHP indicators strongly challenge the existence of a Paleoproterozoic UHP terrane in Western Greenland.

How to cite: Schönig, J., Benner, C., Meinhold, G., von Eynatten, H., and Lünsdorf, K.: Detrital garnet petrology challenges Paleoproterozoic UHP metamorphism in Western Greenland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12621, https://doi.org/10.5194/egusphere-egu23-12621, 2023.

The wish to obtain age data from garnet became reality in 1980 when Griffin and Brueckner¹ published the first-ever Sm-Nd garnet ages. The Silurian Sm-Nd ages that they obtained for eclogites, which until then had been considered Precambrian in age, showed the tremendous potential of this technique. Following analytical developments and constraints on chronometer systematics², the technique became widely used in various metamorphic systems and settings, including in rocks from the upper mantle. Following the first successful attempt at Lu-Hf garnet dating³ and calibration of the ¹⁷⁶Lu decay constant^4,5, the Lu-Hf system was established as an alternative to Sm-Nd in garnet chronology, with possible advantages, including higher P/D, shorter half-life and lower daughter-element diffusivity.

Like other radiometric techniques that use isotope dilution, these techniques have the advantage of high precision, but are time-intensive and provide grain-averaged ages. Dating of individual garnet zones with either technique is possible, but only in specific cases^6-8. Novel approaches to in-situ garnet chronology by combining laser ablation micro-sampling with U-Pb analysis by MC-ICP-MS⁹ or Lu-Hf analysis by ICP-MS/MS¹⁰ provide an exciting addition to garnet chronology, with advantages and disadvantages inverted compared to convention techniques: lower precision, but rapid throughput and high spatial resolution.

The field of garnet chronology is now at an exciting point, where applications of the technique have greatly diversified, new techniques are emerging, and improvements are ongoing – in sample-size requirements for conventional techniques, and accuracy and precision for in-situ techniques. How do these techniques compare, and which approach is best, or "good enough", in a given case? This presentation will focus on the status-quo anno 2023, and will explore frontier applications of Lu-Hf garnet dating in the crust and mantle.

¹ Griffin, W.L., Brueckner, H.K. (1980) Nature 285, 319-321.
² Mezger, K., Essene, E.J., Halliday, A.N. (1992) Earth Planet. Sci. Lett. 113, 397-409.
³ Duchêne, S., Blichert-Toft, J., Luais, B., Télouk, P., Lardeaux, J.-M., Albarède, F. (1997) Nature 387, 586-589.
⁴ Scherer, E.E., Münker, C., Mezger, K. (2001) Science 293, 683-687.
⁵ Söderlund, U., Patchett, P.J., Vervoort, J.D., Isachsen, C.E. (2004) Earth Planet. Sci. Lett. 219, 311-324.
⁶ Pollington, A.D., Baxter, E.F., (2010) Earth Planet. Sci. Lett. 293, 63-71.
⁷ Dragovic, B., Baxter, E.F., Caddick, M.J. (2015) Earth Planet. Sci. Lett. 413, 111-122.
⁸ Tual, L., Smit, M.A., Cutts, J.A., Kooijman, E., Kielman-Schmitt, M., Majka, J., Foulds, I. (2022) Chem. Geol. 607, 121003.
⁹ Millonig, L.J., Albert, R., Gerdes, A., Avigad, D., Dietsch, C. (2020) Earth Planet. Sci. Lett. 552, 116589.
¹⁰ Simpson, A., Gilbert, S., Tamblyn, R., Hand, M., Spandler, C., Gillespie, J., Nixon, A., Glorie, S. (2021) Chem. Geol. 577, 120299.

How to cite: Smit, M.: Garnet chronology: status quo, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10218, https://doi.org/10.5194/egusphere-egu23-10218, 2023.

Garnet in the lower crust may grow during sub-solidus and hyper-solidus heating, and in some cases during cooling. Equilibrium phase diagram sections for mafic orthogneiss predict that the greatest volume of garnet should grow over a narrow temperature range with heating above the solidus. This peritectic garnet can preserve a record of multiple magma injections into complex magma mushes that form plutons in continental magmatic arcs. The exhumed lower crustal section from Fiordland New Zealand provides an ideal laboratory for using garnet to track processes in the continental magmatic arc within the Gondwana margin during the Cretaceous.

The Western Fiordland Orthogneiss (WFO) dominates the inboard lower crustal section exposed in Fiordland. The three major plutons in the WFO preserve a range of igneous and metamorphic fabrics, with some of the oldest igneous foliations and magmatic contacts obvious in the central Misty Pluton and the youngest sub-solidus fabrics observed in the southernmost Malaspina Pluton which intruded c. 117 Ma based on zircon U-Pb ages. In the Malaspina Pluton, garnet Sm-Nd ages for ≥1 cm diameter peritectic garnet grains along cross-cutting trondhjemite veins range from 117.0±4.1 and 116.2±2.0 Ma for garnet cores (N=3), and from 115.8±2.6 to 108.0±2.0 Ma for bulk grains and rims (N=10). In contrast, garnet Sm-Nd ages for ≤0.5 cm diameter garnet grains from 3 locations In the Malaspina are 104.1±1.8, 106.2±2.1, and 103.6±2.2 Ma. Major and trace element zoning is compatible with insignificant volume diffusion in the large grains and significant compositional changes in the small grains. The Bloch et al. (2020) diffusion coefficients for Sm and Nd indicate that significant diffusion would cease at c. 700°C in small grains from the Malaspina Pluton; therefore, we calculate cooling rates of c. 21 (northeast) to c. 10°C (southwest) per m.y. The cooling ages combined with phase diagram sections and mineral thermobarometry indicate exhumation rates of 0.2 to 0.4 kbar per m.y. for the southwestern part of the Malaspina Pluton.

Our results indicate that initial garnet growth ranged from near-synchronous to c. 4 m.y. after construction of the Malaspina Pluton by injection of magma sheets. We speculate that faster cooling in the northeastern Malaspina may have been driven by extension during growth of sub-solidus L-S tectonites which are best developed in this area. Combining these data with structural analysis and detailed evaluation of magma source compositions for individual sheets in WFO plutons allows us to evaluate magma plumbing systems for the Fiordland Arc.

How to cite: Stowell, H., Bollen, E., Schwartz, J., and Klepeis, K.: Tracking magma intrusion and cooling in the lower crust with garnet Sm-Nd geochronology and phase diagram sections, Fiordland New Zealand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16751, https://doi.org/10.5194/egusphere-egu23-16751, 2023.