New shock microstructures in titanite (CaTiSiO5) from the peak ring of the Chicxulub impact structure, Mexico
Nicholas E. TimmsMark A. PearceTimmons M. EricksonAaron J. CavosieAuriol S. P. RaeJohn WheelerA. WittmannL. FerrièreM. H. PoelchauNaotaka TomiokaG. S. CollinsS. P. S. GulickCornelia RasmussenJoanna Morgan
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Keywords:
Titanite
Baddeleyite
Hypervelocity
Shock metamorphism
Coesite
Allanite
Abstract The High Field Strength Elements (HFSE) Ti, Nb and rare earth elements (REE) are commonly regarded as immobile during hydrothermal activity and metamorphism, making them important tracers of geological processes. Here, we report a Ti-REE-Nb-As mineralisation recently discovered in quartz, feldspar, muscovite ± biotite, fluorapatite, hematite, epidote and dravite-schorl veins hosted in quartz ± feldspar ± muscovite ± biotite gneisses from the Monte Leone nappe (Switzerland/Italy). The veins formed during prograde metamorphism and were boudinaged and/or folded during peak metamorphism under lower amphibolite facies. The mineralisation consists of megacrysts (≫ 2 cm) of allanite-(Ce) and Nb-REE-rich titanite-(I). Titanite-(I) displays prominent primary oscillatory- and sector-zonings in Y+REE and Nb. Allanite-(Ce) and titanite are also present as metamorphic minerals disseminated in the host-rock. The vein-hosted megacrysts and their host rocks have identical Nd isotope systematics, indicating that the HFSE mineralisation results from small-scale remobilisation of host-rock components. Localised, fluid-assisted dissolution of vein-hosted allanite-(Ce), epidote, and dravite-schorl during retrograde alpine deuteric alteration resulted in cavities lined with chlorite, muscovite, hematite, and diverse REE minerals. The same fluids caused titanite-(I) to break down into a porous assemblage of acicular niobian rutile with lamellae of crichtonite-group minerals and/or hematite and a suite of REE-Nb-Ti micro-minerals. A few titanite (titanite-II) crystals preserve an intermediate stage of the dissolution-reprecipitation process. Unlike titanite-(I), they display a patchwork-like micro-texture (100 μm-sized subgrains with inhomogeneous Nb concentrations); they host lamellae of crichtonite-group minerals within cleavage planes of the parent titanite, as well as secondary Y+Nb+REE oxides and calcite along subgrain boundaries. Calcite indicates that CO2-enriched fluids promoted the destabilisation of titanite-(I). Highly localised fluid flow accounts for the common occurrence of fresh and altered allanite-(Ce) and titanite in close proximity. The HFSE-enriched veins reveal a complex history of mobility of minor elements (Ti, Nb, REE, As ± B, Be) together with major components (Si, Al, K, Na, Fe) from the host rock, resulting in their early (prograde) concentration within the veins, and their remobilisation upon the action of oxidised CO2-bearing fluids during retrograde metamorphism. In general, crystallisation of enriched phases during prograde metamorphism may be an important step in determining the fertility of a source rock for hydrothermal HFSE deposits.
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Muscovite
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Titanite
Allanite
Metamictization
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Titanite
Allanite
Geochronology
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Northern Norrbotten, Sweden is a key part of Baltic Shield and provides a record of magmatic, tectonic and related, superimposed, Fe oxide–apatite and iron oxide–copper–gold (IOCG) mineralization, during the Svecokarelian orogeny. Titanite and allanite from a range of mineral deposits in the area have been analysed for U–Pb isotope systematics and trace element chemistry using laser ablation quadrupole inductively coupled plasma-mass spectrometry (LA-ICP-MS). Analyses of a single sample from the regional scapolite–albite alteration give an age of 1903 ± 8 Ma (2σ) and may be contemporaneous with the early stages of Fe mineralization (1890–1870 Ma). Analyses of titanite and allanite from undeformed IOCG deposits indicate initial alteration at 1862 ± 16 Ma. In many deposits subsequent metamorphic effects reset titanite isotope systematics from 1790 to 1800 Ma, resulting in a spread of U–Pb isotope analyses along concordia. In some instances core regions may record evidence of early thermal events at around 2050 Ma. Titanite and allanite from deformed IOCG deposits on major shear zones record ages from 1785 ± 21 Ma to 1777 ± 20 Ma, corresponding to deformation, metamorphism and secondary hydrothermal alteration as a result of late orogenic movements. The lack of intracrystalline variations in titanite and allanite trace element chemistry suggests that hydrothermal fluid chemistry and metal source were the main controls on mineral trace element chemistry. Titanite from undeformed Fe oxide–apatite and IOCG deposits is typically light rare earth element (LREE) enriched, and shows low U/Th ratios and low Ni in both intermediate to acid and basic volcanic-hosted deposits. This is consistent with a granitic source for metals. Minor variations in trace element patterns are consistent with the influence of aqueous complex formation on relative REE solubility. Deposits related to the Nautanen Deformation Zone have relatively heavy REE (HREE)-enriched titanite, and LREE-depleted allanite, with high U/Th ratios and elevated Ni contents, consistent with leaching of metals from the local basic volcanic rocks. All hydrothermal titanites are high field strength element enriched (Nb, Ta, Zr) indicating their transport as a result of either high salinities or high F contents, or both. The data overall support models of IOCG-type mineralization as a result of regional circulation of saline hydrothermal fluids in association with major crustal structures, with at least some metallic components derived from the granitoid rocks of the area. All the deposits here show evidence of subsequent metamorphism, although penetrative fabrics are restricted to regional-scale deformation zones.
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Abstract Allanite-(La) (containing up to 7.80 wt.% V 2 O 3 and with La/Σ REE and La/Ce atomic ratios up to 0.54 and 1.45, respectively) and allanite-(Ce) (up to 8.46 wt.% V 2 O 3 ) occur in close association with vanadian muscovite, barian tomichite and vanadian titanite in the main ore zone of the Hemlo gold deposit, Ontario, Canada. Allanite-(Ce) generally occurs as a minor constituent in cross-cutting veins or along foliation planes, whereas allanite-(La) invariably occurs in direct contact with titanite. The high V concentrations in the allanites from Hemlo are readily attributable to an adequate local source of this element, and are most likely controlled mainly by a simple substitution of V for Al in octahedral coordination. Vanadian allanite-(La) and vanadian allanite-(Ce), without any systematic differences in other constituents, are clearly distinct in REE composition, in respect to both the relative concentrations of La and Ce and abundances of other REE . The formation of both allanites (Ce- and La-rich) indicates very localised remobilisation and concentration of REE during a late hydrothermal alteration. The unusual REE composition of vanadian allanite-(La) directly reflects partitioning of REE with coexisting titanite, and the formation of this unusual phase may be attributable to replacement of earlier titanite with redistribution of REE in the solid state.
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Allanite-(La) (containing up to 7.80 wt.% V203 and with La/ZREE and La/Ce atomic ratios up to 0.54 and 1.45, respectively) and allanite-(Ce) (up to 8.46 wt.% V203) occur in close association with vanadian muscovite, barian tomichite and vanadian titanite in the main ore zone of the Hemlo gold deposit, Ontario, Canada. Allanite-(Ce) generally occurs as a minor constituent in cross-cutting veins or along foliation planes, whereas allanite-(La) invariably occurs in direct contact with titanite. The high V concentrations in the allanites from Hemlo are readily attributable to an adequate local source of this element, and are most likely controlled mainly by a simple substitution of V for A1 in octahedral coordination. Vanadian allanite-(La) and vanadian allanite-(Ce), without any systematic differences in other constituents, are clearly distinct in REE composition, in respect to both the relative concentrations of La and Ce and abundances of other REE. The formation of both allanites (Ce- and La-rich) indicates very localised remobilisation and concentration of REE during a late hydrothermal alteration. The unusual REE composition of vanadian allanite-(La) directly reflects partitioning of REE with coexisting titanite, and the formation of this unusual phase may be attributable to replacement of earlier titanite with redistribution of REE in the solid state. KEYWOROS: allanite-(La), allanite-(Ce), vanadian allanite, REE mobility, titanite.
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Titanite
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Coesite exsolved from supersilicic titanite was discovered in an impure calcite marble at Kumdykol in the Kokchetav UHP (ultrahigh-pressure) metamorphic terrane, northern Kazakhstan. This impure marble consists mainly of calcite, K-feldspar, diopside, and symplectites of diopside + zoisite, with minor amount of titanite, phengite, and garnet. No diamond was found in the marble. Coesite and quartz, which have needle or platy shapes measuring about 20-60 mm in length, occur as major exsolved phases in the cores and mantles of titanite crystals with minor calcite and apatite. The strongest Raman band for the coesite needles and plates was confirmed at about 524 cm-1 with a weak band at about 271 cm-1. To estimate the initial composition of the titanite before coesite exsolution, exsolved phases were reintegrated by measuring their area fractions on digital images. The highest excess Si in titanite was thus determined to be 0.145 atoms per formula unit (apfu). This composition requires a pressure higher than 6 GPa on the basis of phase relations in the system CaTiSiO5-CaSi2O5. This pressure is consistent with other evidence of high pressure in the same marble, such as 1.4-1.8 wt% K2O and over 1000 ppm H2O in diopside. Supersilicic titanite and coesite exsolution also indicate that SiO2 exsolution occurred in the coesite stability field during exhumation of the UHP metamorphic unit.
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