Sphene Emotional: How Titanite Was Shocked When the Dinosaurs Died
Nicholas E. TimmsMark A. PearceTimmons M. EricksonAaron J. CavosieAuriol S. P. RaeJohn WheelerA. WittmannL. FerrièreM. H. PoelchauNaotaka TomiokaG. S. CollinsS. P. S. GulickCornelia RasmussenJ. Morgan
0
Citation
0
Reference
20
Related Paper
Abstract:
Accessory mineral geochronometers such as zircon, monazite, baddeleyite, and xenotime are increasingly being recognized for their ability to preserve diagnostic microstructural evidence of hypervelocity processes. However, little is known about the response of titanite to shock metamorphism, even though it is a widespread accessory phase and U-Pb geochronometer. Here we report two new mechanical twin modes in titanite within shocked granitoids from the Chicxulub impact structure, Mexico. Titanite grains in the newly acquired International Ocean Discovery Program Site expedition 364 M0077A core preserve multiple sets of polysynthetic twins, most commonly with composition planes (K1), = ~{111}, and shear direction (η1) = , and less commonly with the mode K1 = {130}, η1 = ~ . In some grains, {130} deformation bands have formed concurrently with shock twins, indicating dislocation glide with Burgers vector b = [341] can be active at shock conditions. Twinning of titanite in these modes, the presence of planar deformation features in shocked quartz, and lack of diagnostic shock microstructures in zircon in the same samples highlights the utility of titanite as a shock indicator for a shock pressure range between ~12 and ~17 GPa. Given the challenges of identifying ancient impact evidence on Earth and other bodies, microstructural analysis of titanite is here demonstrated to be a new avenue for recognizing impact deformation in materials where other impact evidence may be erased, altered, or did not manifest due to low shock pressure.Keywords:
Titanite
Shock metamorphism
Baddeleyite
Hypervelocity
Coesite
Cite
The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Herrestad, in the eastern part of the 1·0 Ga Sveconorwegian Province, involving progressive metamorphic recrystallization of gabbro to garnet amphibolite and associated behaviour of Zr-bearing minerals. In this case, textures show that baddeleyite is by far the main source of Zr for metamorphic zircon growth. The amount of metamorphic zircon formed was primarily controlled by the degree of metamorphic recrystallization, which in turn was controlled by deformation and the presence of a fluid as a transport medium. Zircon in the Herrestad rocks shows a range of morphologies and internal textures at different degrees of metamorphic recrystallization. Igneous zircon occurs together with baddeleyite in coarse-grained olivine-free facies of the gabbro. Metamorphic polycrystalline zircon rims on baddeleyite and minute (<5 µm) bead-like zircon grains at Fe–Ti oxide boundaries characterize the transition to coronitic metagabbro. With increasing metamorphic recrystallization, polycrystalline zircon rims grow at the expense of baddeleyite and the amount of minute bead-like zircon increases, forming strings of zircon beads with increasing distance from Fe–Ti oxide grains. The progressive breakdown of baddeleyite results in polycrystalline zircon aggregates that become denser and finally form single grains in completely recrystallized garnet amphibolite. Late magmatic zircon crystallized at 1567 ± 5 Ma, whereas metamorphic zircon dates amphibolite-facies metamorphic recrystallization at 970 ± 7 Ma. The Herrestad case illustrates a general rule that the bulk Zr budget in originally baddeleyite-bearing rocks will rapidly become locked into metamorphic zircon during the first event of metamorphic recrystallization, when silica and Zr are released from the igneous minerals. Incomplete metamorphic recrystallization and partial preservation of baddeleyite, however, also allows later stages of zircon formation. Thus, in incompletely reacted rocks the final result may be highly complex with micro-scale zircon of several age generations.
Baddeleyite
Recrystallization (geology)
Cite
Citations (28)
Zircon in greenschist-facies metasedimentary rocks from the Scottish Highlands displays a range of complex textures that reflect low-temperature alteration of original detrital grains. In situ back-scattered electron, cathodoluminescence, electron backscatter diffraction and chemical analyses show that altered zircon is porous, weakly luminescent, enriched in non-formula elements such as Al and Fe, and is associated with fractures within the host zircon. The low-temperature zircon appears to be nano-crystalline and to replace U-rich zircon via modification of whole grains or selective alteration of parts of grains, and is linked to the development of zircon outgrowths. The altered zircon is also associated with epitaxial xenotime outgrowths and inclusions. Low-temperature zircon is abundant in slates and other mica-rich samples and its formation is linked to a dissolution–reprecipitation mechanism. Zircon within quartz-rich host rocks typically shows evidence of deformation and the resulting fractures enhance its dissolution, creating rounded embayed morphologies. In contrast, zircon from phyllosilicate-rich rocks contains more new low-temperature growth. Zircon alters during both prograde and retrograde metamorphic events and its development is controlled by both the progressive accumulation of radiation damage in the host grain and the access of metamorphic fluids to the metamict zircon.
Metamictization
Cite
Citations (136)
Granitic rock
Cite
Citations (6)
Titanite
Cite
Citations (13)
Abstract The growth and dissolution behaviour of accessory phases (and especially those of geochronological interest) in metamorphosed pelites depends on, among others, the bulk composition, the prograde metamorphic evolution and the cooling path. Monazite and zircon are arguably the most commonly used geochronometers for dating felsic metamorphic rocks, yet crystal growth mechanisms as a function of rock composition, pressure and temperature are still incompletely understood. Ages of different growth zones in zircon and monazite in a garnet‐bearing anatectic metapelite from the Greater Himalayan Sequence in NW Bhutan were investigated via a combination of thermodynamic modelling, microtextural data and interpretation of trace‐element chemical ‘fingerprint’ indicators in order to link them to the metamorphic stage at which they crystallized. Differences in the trace‐element composition ( HREE , Y, Eu N /Eu* N ) of different phases were used to track the growth/dissolution of major (e.g. plagioclase, garnet) and accessory phases (e.g. monazite, zircon, xenotime, allanite). Taken together, these data constrain multiple pressure–temperature–time ( P–T–t ) points from low temperature (<550 °C) to upper amphibolite facies (partial melting, >700 °C) conditions. The results suggest that the metapelite experienced a cryptic early metamorphic stage at c . 38 Ma at <550 °C, ≥0.85 GPa during which plagioclase was probably absent. This was followed by a prolonged high‐ T , medium‐pressure (~600 °C, 0.55 GPa) evolution at 35–29 Ma during which the garnet grew, and subsequent partial melting at >690 °C and >18 Ma. Our data confirm that both geochronometers can crystallize independently at different times along the same P–T path and that neither monazite nor zircon necessarily provides timing constraints on ‘peak’ metamorphism. Therefore, collecting monazite and zircon ages as well as major and trace‐element data from major and accessory phases in the same sample is essential for reconstructing the most coherent metamorphic P–T–t evolution and thus for robustly constraining the rates and timescales of metamorphic cycles.
Allanite
Felsic
Anatexis
Trace element
Cite
Citations (39)
Massif
Metamictization
Cite
Citations (75)
Abstract U–Pb zircon dates of metagabbroic rocks, such as eclogite, mafic granulite, and garnet amphibolite, are used to constrain the timing of tectonometamorphic evolution in orogens worldwide. For such interpretation, however, it is imperative to define at which stage of the P–T evolution that zircon crystallization took place: the prograde, peak, or retrograde stage. In order to accurately interpret metamorphic zircon ages, it is necessary to assess how the zircon crystallized or recrystallized, as zircon can dissolve or grow under different metamorphic conditions. Zircon is robust to retrograde isotopic resetting under most crustal conditions, but equilibrium Zr mass balance models have suggested that zircon is largely produced during retrograde metamorphism. This study takes a textural approach and identifies and reviews zircon‐forming textures and reactions in gabbro and metagabbro at different metamorphic grades, ranging from subgreenschist to upper amphibolite‐ and eclogite‐facies, and at different stages of metamorphic recrystallization. The textural relationships demonstrate that, in metagabbro, metamorphic zircon grows during the early stage of metamorphic recrystallization, independent of pressure and temperature. The mode of zircon formation is remarkably similar throughout different stages of metamorphic recrystallization, and the most significant source of Zr is igneous baddeleyite. Hence, in contrast to the equilibrium mass balance model, most zircon in metagabbro forms by prograde metamorphic reactions that consume igneous phases, and not by late retrograde reactions, and the onset of zircon forming reactions is governed primarily by the introduction of a hydrous fluid, commonly accompanied by ductile deformation.
Recrystallization (geology)
Baddeleyite
Cite
Citations (17)
Sillimanite
Cordierite
Cite
Citations (41)
Ages retrieved from accessory minerals in high-grade metamorphic rocks place important constraints on the timing of events and the rates of tectonometamorphic processes operating in the deep crust. In suprasolidus rocks, the dissolution and growth of zircon and monazite are strongly dependent on the P–T conditions of metamorphism and the chemistry and quantity of anatectic melt present. Along a clockwise P–T path, prograde heating above the solidus leads to episodic melt loss and changes in melt chemistry that have important implications for the dissolution and growth of zircon and monazite. In this study, phase equilibria modelling of open-system melting is coupled with experimental data on zircon and monazite solubility to evaluate the stability of these minerals at suprasolidus conditions along several schematic clockwise P–T paths. In migmatite melanosomes and residual granulites, some zircon is expected to survive heating to peak temperature and subsequent isothermal decompression, whereas monazite may be completely consumed, consistent with the observation that inherited cores are less common in monazite than in zircon. After decompression, during cooling to the solidus, new zircon and monazite growth from melt trapped along grain boundaries in melanosomes and residual granulites is expected to be limited. By contrast, leucosomes in migmatites and anatectic granites are predicted to contain mostly newly formed zircon and monazite with minimal inherited components, unless significant entrainment of these minerals from the source occurs. The preservation of cores inside newly formed zircon, as observed in many anatectic granites, demonstrates that segregation, ascent and emplacement is commonly fast enough to limit dissolution of these inherited grains.
Cite
Citations (259)
Shock metamorphism
Texture (cosmology)
Cite
Citations (122)