Mineral inclusions in rutile: A novel recorder of HP-UHP metamorphism

The ability to accurately constrain the secular record of high- and ultra-high pressure metamorphism on Earth is potentially hampered as these rocks are metastable and prone to retrogression, particularly during exhumation. Rutile is among the most widespread and best preserved minerals in high- and ultra-high pressure rocks and a hitherto untested approach is to use mineral inclusions within rutile to record such conditions. In this study, rutiles from three different high- and ultrahigh-pressure massifs have been investigated for inclusions. Rutile is shown to contain inclusions of high-pressure minerals such as omphacite, garnet and high silica phengite, as well as diagnostic ultrahigh-pressure minerals, including the first reported occurrence of exceptionally preserved monomineralic coesite in rutile from the Dora–Maira massif. Chemical comparison of inclusion and matrix phases show that inclusions generally represent peak metamorphic assemblages; although rare prograde phases such as titanite, omphacite and corundum have also been identified implying that rutile grows continuously during prograde burial and traps mineralogic evidence of this evolution. Pressure estimates obtained from mineral inclusions, when used in conjunction with Zr-in-rutile thermometry, can provide additional constraints on the metamorphic conditions of the host rock. This study demonstrates that rutile is an excellent repository for high- and ultra-high pressure minerals and that the study of mineral inclusions in rutile may profoundly change the way we investigate and recover evidence of such events in both detrital populations and partially retrogressed samples

Spatial and temporal control of Archean tectonomagmatic regimes

Secular trends in plutonic whole-rock geochemistry pose critical, although non-unique, constraints to early Earth tectonics. Here, we present a large whole-rock geochemical (879 collated samples) dataset for granitoids from the Pilbara Craton, Western Australia, applying it to test the link between secular trends and proposed tectonic mechanisms. We show that the spatio-temporal distribution of granitoid trace element geochemistry is constrained within discrete lithotectonic blocks supporting the reconstruction of its tectonomagmatic evolution. Time-sliced geochemical contour mapping of key petrogenetic ratios indicates the craton underwent rifting ∼3.2 Ga (billion years ago), marking a transition from predominantly sodic magmatism to a broader magmatic compositional spectrum. Our results demonstrate that rift-assisted breakup of proto-cratons is a viable craton growth mechanism. We identify a possible evolutionary sequence beginning with drips and upwellings below a Paleoarchean mafic plateau, which is subsequently dismembered by rifting. These plateau fragments form rigid blocks in the Mesoarchean, between which weaker, thinner crust accommodates minor convergence and divergence manifested as short-lived mobile lid-like features before stabilization. We conclude that these features do not require an active lid, plate tectonic regime

The stability of cratons is controlled by lithospheric thickness, as evidenced by Rb-Sr overprint ages in granitoids

The ancient cores of modern continents, cratons, are the oldest blocks of “stable” lithosphere on Earth. Their long-term survival relies on the resistance of their underlying thick, strong, and buoyant mantle keels to subsequent recycling. However, the effect of substantial geographical variations in keel thickness on the post-assembly behaviour and mass movement within these continental cores remains unknown. Here, we demonstrate that the spatial distribution of fluid-reset in-situ Rb-Sr ages for Paleo-Mesoarchean (3.6–2.8 billion years ago; Ga) granitoids of the Pilbara Craton, Australia shows remarkable correlation with independently-constrained lithospheric thickness models. Without craton-wide heating/magmatic events, these anomalously young Rb-Sr ages document episodes of fluid infiltration into granitoid complexes as a response to lithospheric reactivation by far-field stresses. This correlation implies that craton-wide fluid mobilization triggered by extra-cratonic Neoarchean to Mesoproterozoic (2.8–1.0 Ga) tectonic events is facilitated by variations in lithospheric strength and thickness. Compared to areas of older overprints, the two-thirds of the craton comprised of younger reset ages is underlain by comparatively thin lithosphere with higher susceptibility to reactivation-assisted fluid flow. We propose that even the strongest, most pristine cratons are less stable and impermeable than previously thought, as demonstrated by the role of granitoid complexes and cratons as selective lithospheric “sponges” in response to minor tectonic forces. Therefore, variations in lithospheric thickness, likely attained before cratonization, exert a crucial control on billions of years of fluid movement, elemental redistribution and mineralization within ancient continental nuclei

Can the amnesia of the Earth's continental crust be treated by the resilience of accessory minerals ?

International audienceDelineating the evolution of the Earth’s dynamics and interactions between its different silicate reservoirs (ocean crust, continental crust, mantle) is key to understanding planetary differentiation and the conditions of surface habitability. Today, plate tectonic processes play a major role in creating and destroying the Earth’s crust, and modifying its silicate mantle. Reconstructing its long-term evolution is, however, extremely difficult since the Hadean record is essentially missing and most Archean rocks have experienced reworking and overprinting of their original signatures. An interesting source of information can be found in robust accessory minerals (e.g. zircon, apatite, monazite, titanite...). They can resist secondary processes and erosion, can be dated and incorporate important trace elements sensitive to magma conditions (temperature, SiO2 content, fO2). The properties of these tiny minerals are giving the opportunity to recover informations about igneous and metamorphic rocks at the grain scale using few microns spot sizes. These minerals are particularly interesting for Precambrian terranes that have been affected by secondary processes such as multiple metamorphic events, fluid circulation, alteration or erosion. In this contribution, I will present (i) recent development on our understanding of the chemical behaviour of accessory minerals in different magma types through geological time, (ii) new data on apatite inclusions armoured within zircons on Archean terrane and (iii) will discuss the advantages and limitations of using accessory minerals in Archean terranes. Altogether, these new petro-geochemical tools can help to better reconstruct continental crust through time and can be valuable for future work on provenance studie

Moderne Petrologie – was ist das?

International audienceDer Petrologe entschlüsselt mit physikalisch-chemischen Methoden die in einem Gestein enthaltenen Hinweise auf dessen Entstehungsgeschichte und liefert so dem Geologen, aber auch dem Lagerstättenkundler und dem Ingenieurgeologen wichtige Hinweise für deren Arbeit in einem weiteren Zusammenhang bzw. mit anderer Zielrichtung. Dazu verwendet er heute einerseits modernste analytische Geräte und andererseits immer ausgefeiltere thermodynamische und kinetische Berechnungs-und Darstellungsmethoden, wie etwa Pseudoschnitte, auf deren Aussagekraft näher eingegangen wird. Als Berufsfelder findet ein Absolvent des Petrologiestudiums nicht nur die Grundlagenwissenschaft selbst vor, sondern sämtliche Bereiche der Grundstoffindustrie (Bergbau und Verarbeitung) bis hin zu den auf denselben physikalisch-chemischen Grundlagen operierenden High-Tech-Unternehmen im Bereich moderner Werkstoffe (Keramiken, Halbleiter, Glas, etc.)

An apatite for progress: Inclusions in zircon and titanite constrain petrogenesis and provenance

International audienceApatite has recently gained considerable attention as a mineral with many uses within the Earth and planetary sciences. Apatite chemistry has recently given new insight into a wide range of geological processes and tools, for example, magmatism, metasomatism, planetary geochemistry, and geochronology. We expand the utility of apatite here by presenting a novel way to fingerprint magma chemistry and petrogenesis using apatite inclusions within robust titanite and zircon. We present trace element data from apatite mineral inclusions shielded within magmatic zircon and titanite. Importantly, apa-tite inclusion and host titanite chemistries detailed in this study allow estimation of the whole-rock Sr and SiO 2. We show how these data can be used to assess the degree of fractionation of the host magma and to calculate key trace element abundances and ratios. We demonstrate that the inclusions can be linked to discrete periods in the crystalliza-tion history of the host phases, thus providing insight into petrogene-sis. The results highlight that apatite compositions might discriminate modern granitoids (younger than 2.5 Ga) from Archean-Proterozoic transitional granitoid compositions (sanukitoid signatures). Development of such a petrological tool has important potential for interpretation of provenance and a better understanding of the secular evolution of the continental crust, including that of early Earth

Accessory phases (titanite, apatite, zircon) behaviour in late Caledonian high Ba-Sr plutons, Scotland: petrogenetic and source implications

International audienc