Current applications using key mineral phases in igneous and metamorphic geology: perspectives for the future

The study of magmatic and metamorphic processes is challenged by geological complexities like geochemical variations, geochronological uncertainties and the presence/absence of fluids and/or melts. However, by integrating petrographic and microstructural studies with geochronology, geochemistry and phase equilibrium diagram investigations of different key mineral phases, it is possible to reconstruct insightful pressure–temperature–deformation–time histories. Using multiple geochronometers in a rock can provide a detailed temporal account of its evolution, as these geological clocks have different closure temperatures. Given the continuous improvement of existing and new in situ analytical techniques, this contribution provides an overview of frequently utilized petrochronometers such as garnet, zircon, titanite, allanite, rutile, monazite/xenotime and apatite, by describing the geological record that each mineral can retain and explaining how to retrieve this information. These key minerals were chosen as they provide reliable age information in a variety of rock types and, when coupled with their trace element (TE) composition, form powerful tools to investigate crustal processes at different scales. This review recommends best applications for each petrochronometer, highlights limitations to be aware of and discusses future perspectives. Finally, this contribution underscores the importance of integrating information retrieved by multi-petrochronometer studies to gain an in-depth understanding of complex thermal and deformation crustal processes

Tectono-Thermal History of the Neoarchean Balehonnur Shear Zone, Western Dharwar Craton (Southern India)

AbstractA widely spaced Neoarchean shear zone network traverses the granite-greenstone terrains of the Western Dharwar craton (WDC). The NNW-SSE trending Balehonnur shear zone traverses the largest part of the preserved tilted Archean crustal ensemble in the Western Dharwar craton (WDC) from the amphibolite-granulite transition in the south to greenschist facies in the north and eventually concealed under Deccan lava flows. Published tectonic fabrics data and kinematic analysis, with our data reveal a sinistral sense of shearing that effectuate greenstone sequences, Tonalite-Trondhjemite-Granodiorite Gneisses (TTG), and Koppa granite as reflected in variable deformation and strain localization. A profound increase of strain towards the core of the shear zone in the ca. 2610 Ma Koppa granite is marked by a transition from weak foliation outside the shear zone through the development of C-S structures and C-prime fabrics, mylonite to ultramylonite. The mineral assemblages in the Koppa granite and adjoining greenstone indicate near peak P-T conditions of 1.2 Gpa, 775-800°C following a slow cooling path of 1.0 GPa and 650°C. Field-based tectonic fabrics data together with U-Pb zircon ages reveal that the Koppa granite emplaced along the contact zone of Shimoga-Bababudan basin ca. 2610 Ma, coinciding with the emplacement of ca. 2600 Ma Arsikere-Banavara, Pandavpura, and Chitradurga granites further east which mark the stabilization of WDC. Significant variation in major element oxide (SiO2 = 56-69 wt.%) together with high content of incompatible elements (REE, Nb, Zr, and Y) and high zircon crystallization temperatures (~1000°C) of Koppa granite suggests derivation by partial melting of composite sources involving enriched uppermost mantle and lower crust. The development of widely spaced shear zones is probably linked to the assembly of eastern and western blocks through westward convergence of hot oceanic lithosphere against already cratonized thick colder western block leading to the development of strain heterogeneities between greenstone and TTGs due to their different mineral assemblages leading to rheological contrast in the cratonic lithologies

Understanding VNIR Plagioclase Signatures on Mars Through Petrographic, Geochemical, and Spectral Characterization of Terrestrial Feldspar‐Bearing Igneous Rocks

Abstract Plagioclase feldspar is a common mineral in terrestrial rocks and has recently been detected on Mars surface with visible near‐infrared (VNIR) spectroscopy. The presence of plagioclase using this method is determined through the identification of an absorption band centered around 1.3 μm on reflectance spectra, which requires the incorporation of Fe 2+ in plagioclase lattice. Previous laboratory studies of powder mixtures showed that plagioclase should only be detectable if present in amounts >90% as its absorption band can be masked by those of mafic minerals. Plagioclase composition, but also the size of the grains and the associated minerals in a rock, influence the spectral signature of plagioclase feldspars. Thus, the analysis of whole, uncrushed rocks appears to be relevant for comparisons with Mars remote sensing observations that have shown plagioclase‐like signatures. In the present work, we performed laboratory measurements on five feldspar‐bearing terrestrial rocks of various nature chosen because they reflect the first order terrestrial magmatic variability. The mineralogical composition of these samples, the chemical composition of feldspar crystals, and total rock spectra as well as spectra of each mineral were determined using microscopy, electron microprobe, and VNIR spectroscopy. Our study shows that plagioclase signature is visible on the spectra of macroscopic rocks containing between 30% and 80% plagioclase of different compositions (An 25 ‐An 67 ). Our findings have strong implications for the interpretation of feldspar signatures on Mars, which can belong to a range of feldspar‐bearing rocks and could thus provide information about the formation of Mars' crust

Mt. Etna plumbing system revealed by combined textural, compositional, and thermobarometric studies in clinopyroxenes

International audienceCoupled textural and in situ geochemical studies of clinopyroxene (cpx) phenocrysts, from both historical and recent eruptions of Mt. Etna volcano, provide a means to investigate the processes occurring in the deepest portion of the feeding system (>10 km depth). Five distinct textures were recognized: (1) normal oscillatory zoning, (2) normal zoning with Fe-rich rim, (3) sieve-textured core, (4) reverse oscillatory zoning, and (5) dusty rim. Electron microprobe analyses indicate an almost constant diopside–augite composition, with a slight enrichment in the enstatite for more recent erupted cpx. Core-to-rim compositional profiles, performed along the cpx, reveal distinct compositional characteristics. Normal oscillatory zoning is often characterized by a sharp increase in FeO (Δ ~ 2 wt%) accompanied by a drop in Al2O3 on the outermost 30 μm. Reverse oscillatory zoning, by contrast, exhibits a drop in FeO, Al2O3 (Δ ~ 2 wt%), and a remarkable crystal rim increase in MgO (up to 5 wt%). Similar compositional changes are evident in dusty-textured rims, which are characterized by dissolution edges and overgrowth containing glass pockets and channels. No significant compositional variations have been observed across crystals with sieve-textured cores. Trace element concentrations show enrichments in Sr, La, Zr, and REE, together with a decreasing La/Yb ratio (from ~7 to ~4) in rims of normally zoned crystals. Cpx with reverse zoning and dusty rims has low Sr, La, Zr, and REE contents toward crystal rims. Thermometers and barometers, based on equilibrium cpx-melt pairs, suggest that cpx cores start nucleating at 720 MPa, with the majority of them forming between 600 and 400 MPa but continuing to crystallize until very shallow depths (<100 MPa). Normal oscillatory-zoned phenocrysts surrounded by rims form at pressures shallower than 400 MPa, while reverse zoning and dusty rims occur between 400 and 500 MPa. Coupled petrologic and thermobarometric studies on both clinopyroxenes and plagioclases, associated with detailed textural and in situ geochemical analyses, are promising tools to reconstruct the entire magma ascent path beneath open-system volcanoes. At Mt. Etna, two distinct processes could account for the observed textures: Fe-rich rims in normal oscillatory-zoned crystals can be related to decompression-induced crystallization, while reverse zoning and dusty rims can be produced by mixing with a more basic magma at 400–500 MPa (i.e., ~10 km). Textural features are not restricted to a particular evolutionary phase of the volcano, which suggest that the deep feeding system has not changed significantly since the first alkaline volcanic phase

Origin of the oldest (3600–3200 Ma) cratonic core in the Western Dharwar Craton, Southern India: Implications for evolving tectonics of the Archean Earth

This contribution presents a comprehensive synthesis on the origin of oldest (3600–3200 Ma) cratonic core in the Western Dharwar Craton (WDC), Southern India based our new field, geochronologic, elemental and Nd-Hf isotopes data on the Holenarsipur greenstone belt and adjacent granitoids combined with published record on the stratigraphically equivalent greenstone belts in the same region. Our study shows that the oldest cratoniccore in the WDC formed through assembly of different tectonic units close to 3200 Ma. These tectonic units include microcontinental remnants with oceanic plateaus, oceanic island arcs and preserved oceanic crust similar to modern oceanic crustal section close to a spreading center. Isotopic age data for these greenstone sequences (detrital zircons -ca.3600–3230 Ma; volcanics 3384–3200 Ma) and adjoining granitoids (3430–3400 and 3350–3270 Ma with remnants of older 3600–3500 Ma gneisses) indicate that a large part of the old TTG-type granitoids in the WDC is either coeval to or slightly younger than associated greenstone units. The final stage of assembly of these tectonic elements into cratonic framework through horizontal motion of intervening oceanic crust and eventual slab breakoff is marked by the formation of ca. 3200 Ma trondhjemite magmas, emplacement of which into the lower crust caused partial convective overturn of the crust, thereby leading to the development of dome and keel patterns followed by metamorphism and cratonization of the WDC crust at 3100 Ma. Our study strongly suggests that the tectonic environments in which Archean cratons formed require some sort of horizontal motion but not necessarily modern plate tectonics. However, vertical addition of juvenile crust in hotspotenvironments associated with mantle plumes played a major role in building of early crustal nuclei. The model proposed for the formation of the cratonic core in the western Dharwar craton is compatible with observations made globally in most of the Paleoarchean craton

Trace elements in apatite and titanite : a new proxy to discriminate magma evolution ?

International audienceThe granitoid record has evolved through geological time from tonalite-trondhjemite-granodiorite (TTG) in the Archean toward typical granodiorite and granite with arc magma compositions in the present-day. However, many fundamental questions remain regarding the geodynamics of the early Earth and the timing of the transition to modern subduction-driven tectonics. The Archean rock record is often hampered by metamorphic overprinting and focusing on accessory minerals that may be impervious to metamorphism is a potential promising approach to gain insights into early Earth processes.. Here we present new geochemical data on two accessory phases (titanite, apatite) from a compilation of granitoids that cover a large span of the geological record from the Archean to the Phanerozoic. We demonstrate that trace element analysis and detailed petrographic work can give direct information about the petrogenesis of the host magmas even when these granitoids have been affected by metamorphism. Our results show that the chemical signature of accessory minerals allow discrimination of the different magma types generated during crustal evolution and give insights into their source characteristics. Since these accessory minerals often survive sedimentary processes, the Archean sedimentary record may offer an untapped archive of critical information

Prolonged interovulatory interval and hormonal changes in mares following the use of ovuplant™ to hasten ovulation

In its first year of commercial availability in the United States, reports from the field indicated that Ovuplant™ (a deslorelin-containing slow-release implant for hastening ovulation in mares) was associated with a delayed return to estrus in mares not becoming pregnant. Supposedly this effect was particularly prevalent in mares subsequently administered PGF2α to cause luteal regression after embryo collection. The present experiment was conducted 1) to determine if the field observations were repeatable under controlled experimental conditions, and 2) to gather endocrine data that might yield information on the underlying cause(s) of this observation. Twenty-five light horse mares were used. Ovaries of each mare were examined by transrectal ultrasonography daily during estrus until ovulation. Once a follicle \u3e30 mm was detected, the mare received either Ovuplant (treated group; N = 13) at the recommended dosage or a sham injection (controls; N = 12); treatments were administered in a manner to ensure that they were unknown to personnel involved with data collection. On day 7 after ovulation, each mare received a luteolytic injection of PGF2∂. Mares were examined every other day until return to estrus or development of a 30 mm follicle, at which time daily examination was performed until ovulation. Jugular blood samples were collected daily. Two mares receiving Ovuplant did not return to estrus within 30 days and their data were not included in the statistical analyses; in contrast, no control mare exhibited such an extended interovulatory interval. For all other mares receiving Ovuplant, the interval between the first and second ovulations was longer (P = .0001) than that of control mares by an average of 6.2 days. In addition, plasma LH concentrations were lower (P \u3c .05) in the treated mares on days 0 through 4, 9, 11, 18, and 19 after the first ovulation. Plasma FSH concentrations were also lower (P = .017) in treated mares from days 4 to 11 and on days 6 and 5 prior to the second ovulation (P = .005). Differences in progesterone and estradiol were observed but were less consistent than for LH and FSH. Mares receiving Ovuplant had fewer small (P =.026), medium (P = .003) and large (P = .045) follicles prior to the second ovulation. In conclusion, Ovuplant treatment at the recommended dosage decreased follicular activity after ovulation and increased the interovulatory interval in mares short-cycled with PGF2∂. These effects appear to be mediated by a hyposecretion of LH and(or) FSH

Minor Minerals, Major Implications: Using Key Mineral Phases to Unravel the Formation and Evolution of Earth's Crust

International audienceKey minerals, including zircon, apatite, titanite, rutile, monazite, xenotime, allanite and garnet, can retain critical information about petrogenetic and geodynamic processes. This Special Publication showcases snapshots of the latest developments using key minerals in igneous, metamorphic and detrital rocks through current-state reviews, contributions focused on case studies and newly developed techniques