Inês Pereira

To understand the onset and evolution of cold subduction on Earth, detrital sedimentary rocks of Precambrian age, potentially derived from exposed high-P low-T metamorphic rocks can be investigated. This requires the estimation of peak metamorphic pressure and temperature, time of formation, and source lithology (P-T-t-X) of detrital single grains. Rutile is a common accessory mineral in subducted oceanic crust and one of the most likely minerals from subducted rocks to survive sedimentation processes. As single grain T-t-X estimates on rutile are possible, it is a prime candidate for the investigation of subduction processes through time.We developed a method to identify rutile formed in modern cold subduction conditions, by combining in-situ polarised Fourier Transform Infrared Spectroscopy (FTIR) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).Our study reveals a pressure-dependent variation in hydrogen content within metamorphic rutile, ranging from less than 10 to 2500 μg/g H2O. Higher peak metamorphic pressures correspond to elevated H2O contents, particularly noticeable in mafic low-temperature eclogite facies rutile, suggesting H-in-rutile can be used as a pressure indicator. Using Zr in rutile as a temperature indicator, H2O/Zr ratios act as proxies for thermal gradients (P/T) in metamorphic rutile. When combined with low Nb, W, and Sn contents, typical of mafic protoliths, it is possible to identify modern-style cold subduction of mafic crust using trace element signatures in detrital rutile.Therefore, detrital rutile can serve as a tracer for subduction conditions over time, as modern-style cold subduction signatures are preserved in rutile during weathering and sedimentary processes. In this study, we test our novel approach on detrital rutile grains of sandstones and arkoses from the Torridon and Ardvreck Groups, Hebridean in NW Scotland. Our analysis reveals that some grains of the Torridon Group of late Proterozoic age (detrital ages ranging from 1.0 to 1.9 Ga; Pereira et al., 2020) exhibit high H2O/Zr ratios and low total Nb+W+Sn contents, typical of low-T eclogite facies rutile. This implies that low-T eclogites which formed during cold subduction were likely exposed and eroded in the catchment of the sedimentary basins, indicating modern-style cold subduction during the Mesoproterozoic. We propose that the combined measurement of H2O and trace elements in detrital rutile is a powerful tool to search for remnants of cold subduction through the Earth’s history. Pereira, I., Storey, C.D., Strachan, R.A., Bento dos Santos, T., Darling, J.R., 2020. Detrital rutile ages can deduce the tectonic setting of sedimentary basins. Earth Planet. Sci. Lett. 537, 116193. https://doi.org/10.1016/j.epsl.2020.116193

This work investigates the relationships between partial melting, melt extraction, pluton growth and silicic volcanism in garnet-bearing felsic volcanic rocks that were extruded in the Iberian Pyrite Belt, at ca. 345 Ma. The garnets are of peritectic origin, displaying textural and chemical features of disequilibrium crystallization during partial melting reactions involving biotite at high temperatures (up to 870 °C) in the middle-lower crust. Major element composition suggests compositional equilibrium with the entrained and pinitized peritectic cordierite, but reveals some subsequent homogenization by diffusion. Trace element maps and spot analyses of garnet show, nonetheless, significant trace element variations, reflecting biotite and Y-REE-P-rich accessory phase breakdown during partial melting reactions. Peritectic garnet and cordierite growth resulted in the preservation of Th- and Y-rich prograde suprasolidus monazite, which constrains the timing of partial melting of the metapelitic protolith at ca. 356.8 ± 2.4 Ma. The zircon cargo further shows that a significant amount of zircon crystals from previously crystallized felsic melts were also remobilized and erupted. These were likely stored in an upper crustal pluton that grew episodically since ca. 390 Ma during voluminous melt generation periods within the middle to lower crust, which also resulted in voluminous volcanism. The geochemical trends of the felsic volcanic rocks reflect the entrainment of xenoliths of peritectic garnet, cordierite and feldspar, and as such, the garnet-bearing felsic volcanic rocks represent an erupted mixture of a lower-temperature (ca. 770 °C) silicic melt and autocrysts, and peritectic phases and zircon crystals from previously crystallized and stored felsic melts.

Rutile, titanite, and ilmenite are the most common Ti-bearing minerals in metamorphic rocks. Experimental constraints have shown that titanite is stable at low-grade metamorphic conditions, rutile at HP, and ilmenite at HT-LP conditions. Yet, petrological evidence suggests that titanite can also be stable at LT-HP. This implies that both titanite and rutile can be used to develop proxies to track HP metamorphism, which can have interesting applications. In this study, we have investigated the natural occurrence and chemistry of Ti-bearing minerals in gabbroic rocks that record different degrees of metamorphism, including LP amphibole-bearing gabbros from the ocean floor (Mid-Atlantic and Indian ridge IODP LEGs) and from an obducted ophiolite (Chenaillet) and HP Alpine metagabbros including blueschist and eclogite facies rocks from the Western Alps and Corsica. We have performed detailed petrography and Raman spectroscopy and analysed major and trace elements mineral chemistry using EPMA and LA-ICP-MS. We found that rutile is stable at low pressure (< 2 kbar) in ocean-floor amphibole-bearing gabbros, lower than experimental constraints had previously suggested. Rutile is also found in eclogitic metagabbros from the Western Alps and can be chemically distinguished from LP rutile. Blueschist metagabbros from the Western Alps and eclogitic metabasalts from Corsica have titanite stable instead of rutile. While the titanite to rutile transition is pressure and temperature dependent, we demonstrate how small variations in bulk-rock Ti/Ca and Ca/Al values within the NCKFMASHTO chemical system may shift their stabilities. High-pressure titanite from these metamafic rocks exhibits La depletion and low La/Sm_N values in comparison to titanite from amphibolite-facies mafic rocks. La/Sm_N or Nb together with Yb and V can be used to distinguish HP titanite from titanite formed under other P-T settings. These new systematics can be useful in studies using detrital Ti-bearing minerals to probe the HP metamorphic record through time.

The investigation of key minerals including zircon, apatite, titanite, rutile, monazite, xenotime, allanite, baddeleyite and garnet can retain critical information about petrogenetic and geodynamic processes and may be utilized to understand complex geological histories and the dynamic evolution of the continental crust. They act as small but often robust petrochronological capsules and provide information about crustal evolution, from local processes to plate tectonics and supercontinent cycles. They offer us insights into processes of magmatism, sedimentation, metamorphism and alteration, even when the original protolith is not preserved. In situ techniques have enabled a more in-depth understanding of trace element behaviour in these minerals within their textural context. This has led to more meaningful ages for many stages of geological events. New developments of analytical procedures have further allowed us to expand our petrochronological toolbox while improving precision and accuracy. Combining multiple proxies with multiple minerals has contributed to new interpretations of the crustal history of our planet.

Low concentrations of Na, Ca, K, Fe, Mg and Al in quartzite commonly prevent the crystallization of index metamorphic minerals, inhibiting the obtainability of thermobarometric calculations. Quartzite typically contains quartz, zircon and rutile; therefore, single-element thermometers, such as Zr-in-rutile, may be applied. We investigate changes in trace-element composition of rutile from quartzite through increasing metamorphic conditions. Studied samples derive from a quartzite package (Luminárias Nappe, Minas Gerais, Brazil) where previous thermobarometric constraints on metapelites showed an increasing metamorphic grade southwards, from high-pressure lower amphibolite facies (580°C; 0.9 GPa) to eclogite facies (630°C; 1.4 GPa). Rutile from the lower-grade facies samples show a large spread in Zr concentrations, with the highest values corresponding to temperature estimates higher than metamorphic conditions affecting those units, and thus interpreted as inherited detrital signatures. A narrower spread in Zr concentration is observed in rutile grains from the higher-grade facies, and estimated Zr-in-rutile temperatures agree with previous thermobarometric constraints. Therefore, we show that at 630°C, Zr contents in detrital rutile from quartzites re-equilibrate. The comparison between the quartzite- and metapelite-hosting rutile grains from the same area shows that the resetting of the geothermometer in the latter seems to occur at slightly lower temperatures (∼50°C lower). Supplementary material: A full dataset of analized trace elements in rutile is available at https://doi.org/10.6084/m9.figshare.c.6793887

Detrital heavy minerals have helped address geologically complex issues such as the nature and origin of the early terrestrial crust, the growth and evolution of the continental crust, and the onset of plate tectonics, together with palaeogeographic and supercontinent cycles reconstructions. With the advent of in situ analytical techniques and a more complete understanding of trace element behaviour in rock-forming and accessory minerals, we have now at our disposal a powerful suite of tools that we can apply to multiple proxies found as detrital minerals. These can be in situ dating, trace element or isotopic tracing applied to both mineral hosts and their inclusions. We opted to showcase minerals that occur as primary minerals in a wide range of rock compositions and that can provide reliable age information. Additionally, over recent decades their chemistries have been tested as proxies to understand crustal processes. These are zircon, garnet, apatite, monazite, rutile and titanite. We include an overview and provide some approaches to overcome common biases that specifically affect these minerals. This review brings together petrological, sedimentological and geochemical considerations related to the application of these detrital minerals in crustal evolution studies, highlighting their strengths, limitations and possible future developments.

The Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex (FCR-LAC) located in the Central Iberian Zone of the European Variscan Belt is delimited by two shear zones, which have been interpreted as constraining its exhumation. New U–Pb zircon and apatite ages for the migmatites allowed us to obtain a peak metamorphic age of the anatectic complex (316.8 ± 2.0 Ma), and to estimate fast cooling (7–54°C Ma ^–1 ) and fast exhumation (0.2–1.3 mm a ⁻¹ ) after a vertical displacement of c. 6–9 km. Our results show a similar zircon age distribution between migmatites and the Excomungada Formation from the host Ediacaran–Cambrian Douro–Beiras Supergroup. Although the existence of an inherited zircon population lasting c. 500 Ma, absent in the metasedimentary units, suggests the contribution of Ollo de Sapo magmatic rocks during the Lower–Middle Pennsylvanian melting events. Detrital zircon ages of the Pinhão Formation (Douro Group) have contrasting sources to units from the Beiras Group, which may indicate the presence of a barrier that prevented older detrital components from reaching the Douro basin. However, at odds with what has been postulated before, such a barrier must have been placed farther south of the FRC-LAC, and thus not coincident with the Juzbado–Penalva do Castelo shear zone. Supplementary material: A full description of the stratigraphy and lithologies (Supplementary 1 and 2, respectively); the analytical conditons of the LA-ICP-MS analyses (Supplementary Material 3); the U–Pb zircon and apatite data of the FCR-LAC migmatites and the U–Pb detrital zircon data of the DBSG (Supplementary Material 4) are available at https://doi.org/10.6084/m9.figshare.c.6002006

Rutile, titanite and ilmenite are the most common Ti-bearing minerals found in metamorphic rocks of variable grades. Rutile and titanite, in particular, are extremely useful minerals as they can be dated using U-Pb, and Zr concentrations are calibrated as geothermometers for both minerals, making them valuable petrochronometers. Previous experimental studies on MORB composition [1] established that titanite is more stable at LT-LP, rutile at HP (> 12 kbar), while ilmenite at HT-LP metamorphic conditions. Despite these phase stabilities, the natural occurrence of rutile at LP (< 12 kbar) and titanite at HP (> 20 kbar) and ilmenite at both HP and LP indicates strong uncertainties on our current understanding about their stabilities. [2] demonstrated a non-trivial compositional effect mainly driven by CaO content, on the titanite-out reaction for granitoid compositions (2-4 kbar). For MORB compositions, experimental constraints are currently lacking in the 400-600 ºC temperature range.Here we present the results of a set of experiments run in a piston-cylinder apparatus using a gold capsule with a NNO oxygen fugacity buffer. We tested multiple starting materials, with different Ti/Ca values, including: 1) a pulverised eclogite (MORB composition) powder with titanite and rutile as well as a few initial eclogitic silicate mineral seeds, promoting nuclei for mineral overgrowth, 2) the same eclogite, glassed and pulverised in the lab, with fewer product seeds, and some of these with added Ti powder; 3) a different MORB powder with crushed titanite and kaersutite seeds. More than 30 experiments were conducted, with pressure ranging between 12 and 23 kbar, and temperature between 400 and 750 ºC in water-saturated conditions and using a cold pressure-seal capsule technique. Due to the challenging LT experiments, equilibrium is not attained, but dissolution and precipitation features are often observable. Epidote is one of the first minerals to nucleate and grow when the initial water content is > 10 wt%, and crystallisation is followed by amphibole. We show that when Ti/Ca is high, rutile is stable even at lower pressures, and when Ti/Ca is low, titanite seeds appear metastable even at higher pressures (19 kbar) and low temperatures. This is in agreement with petrological observations (i.e. peak titanite reported in blueschist rocks). At lower water saturation conditions (10 wt%), reactions are more sluggish, but successful experimental assemblies show that at 600 ºC and 14 kbar titanite seeds become unstable and start reacting with the basalt bulk rock powder to form ilmenite. We found that H2O content, as well as Ti/Ca ratios appear to influence the stability of these Ti-phases in a mafic system. These results can be used to constrain the stabilities of rutile, titanite and ilmenite, which in turn elucidate the P-T-X conditions that these accessory minerals are able to record.[1] Liou, et al. (1998). Schweiz. Mineral. Petrog. Mitt., 78, 317-335. [2] Angiboust, S., & Harlov, D. (2017). Am. Min., 102, 1696-1708.