Retrograde Reactions Research Articles

Applications of Diels-Alder Chemistry in Biomaterials and Drug Delivery.

Recent studies, leveraging click chemistry reactions, have significantly advanced the fields of biomaterials and drug delivery. Of these click reactions, the Diels-Alder cycloaddition is exceptionally valuable for synthetic organic chemistry and biomaterial design, as it occurs under mild reaction conditions and can undergo a retrograde reaction, under physiologically relevant conditions, to yield the initial reactants. In this review, potential applications of the Diels-Alder reaction are explored within the nexus of biomaterials and drug delivery. This includes an emphasis on key platforms such as polymers, nanoparticles, and hydrogels which utilize Diels-Alder for drug delivery, functionalized surfaces, bioconjugation, and other diverse applications. Specifically, this review will focus on the use of Diels-Alder biomaterials in applications of tissue engineering and cancer therapies, while providing a discussion of the advantages, platforms, and applications of Diels-Alder click chemistry.

Thermodynamic and kinetic controls on phase stability and incorporation of water in larnite (β-Ca2SiO4): implications for calcium silicate inclusions in diamonds

Larnite (β-Ca2SiO4) has previously been reported as an inclusion in sub-lithospheric diamonds and is generally interpreted as a retrograde reaction product of calcium silicate perovskite. In this study, we review the controls on the stability of the Ca2SiO4 polymorphs and show that phosphorus is likely essential for the preservation of β-Ca2SiO4. We also report a detailed study of the solubility of water and its incorporation mechanisms in γ-Ca2SiO4 and phosphorus-doped β-Ca2SiO4 using FTIR spectroscopy on high-pressure experiments quenched from 4–9.5 GPa and 1000–1200 °C combined with ab initio calculations. The experimentally determined water solubilities are in the range of 107–178 ppm. Our FTIR spectra and ab initio calculations indicate that for phosphorus-free γ-Ca2SiO4 the incorporation mechanism involves protonated Si and Ca1 vacancies. For phosphorus-bearing β-Ca2SiO4, our preferred incorporation mechanism involves one Si4+ ion replaced by one P5+ ion with a single protonated Ca2 vacancy. The low water solubility observed here for larnite implies that if primary calcium silicate perovskite inclusions trap high water concentrations during diamond growth from a volatile-rich fluid, measurements of the concentration of water in larnite will not provide a useful record of the initial volatile concentration. Instead, water would be hosted in other retrograde reaction products, possibly including exsolved fluids.

The mineralogy and alteration history of the Yamato-type (CY) carbonaceous chondrites

The CY chondrites are a group of thermally metamorphosed carbonaceous chondrites. Although they share similarities with the CM and CI chondrites, their primary properties argue for a distinct classification. Previous studies have highlighted their isotopically heavy bulk compositions (δ17O = 10 ‰, δ18O = 21 ‰, Δ17O = 0 ‰) and exceptionally high sulphide abundances (10–30 vol%). In this work we explore their petrography and alteration history. The CYs accreted low abundances of chondrules (15–20 area%) with average apparent diameters slightly larger (∼320–340 µm) than the CM chondrites. In contrast to the CMs, the CYs record an early episode of brecciation prior to the main window of aqueous alteration. Subsequent fluid activity produced a range of alteration extents with both CY2 and CY1 chondrites documented. Phyllosilicate minerals in the CYs were a mix of serpentine and saponite (including occurrences of Na-saponite) with minor quantities of chlorite (within chondrules). An initial generation of Fe-sulphides formed by sulfidation of metal, and by precipitation from S-rich fluids. Three generations of carbonates are recognized, an early generation that infilled voids left by brecciation and co-precipitated with sulphide, a later generation that co-precipitated with magnetite and a final Fe-Mg-bearing generation which formed large (>100 µm) clasts. Only the first-generation carbonates are found in the CY2s, while the CY1s preserve all three generations. Phosphates occur as Ca-apatite or rarely as Mg-bearing apatite and have hydroxylapatite compositions, indicating low halogen activities in the alteration fluids. Refractory oxides (ilmenite and Cr-spinel) occur as precipitates adhering to the margins of phyllosilicates. They formed late in the alteration sequence and attest to oxidizing conditions. During the late-stages of aqueous alteration Fe-sulphides were replaced by magnetite. Thermal metamorphism (Stage II-IV: ∼300–750 °C) overprinted aqueous alteration leading to dehydration and recrystallization of the phyllosilicate matrix and the decomposition of some carbonate phases. Most Fe-sulphide grains survived heating without decomposition as initial partial decomposition from pyrrhotite to troilite under closed system conditions led to elevated ƒS2 gas and resulted in a stabilizing effect. Retrograde reactions between trapped S2 gas and metal/magnetite formed a final generation of Fe-sulphides. The survival of Fe-sulphides and their stochiometric troilite compositions are evidence for near-closed system heating. Analysis of organic matter by Raman spectroscopy supports an interpretation of short-duration heating (on the scale of minutes to days), at peak temperatures between 750 and 900 °C. Thus, an impact event was the most likely cause of metamorphic heating.

Formation of low-pressure reaction textures during near-isothermal exhumation of hot orogenic crust (Bohemian Massif, Austria).

Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet-biotite-sillimanite-plagioclase-K-feldspar-quartz-granitic melt ±kyanite ±ilmenite ±rutile, recording peak metamorphic conditions of 0.9-1.1 GPa and 780-820°Cestimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite-spinel and cordierite-quartz reaction textures at former garnet-sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching 0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite-spinel and cordierite-quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4-0.5 GPa and 750-800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP-UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at 340Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of 335Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.

Skarns at a peridotite contact, Neyriz, Iran: High-temperature melting and metamorphism in a forearc setting

In the Neyriz area of southern Iran unusual skarns are found above serpentinised peridotites at the contact with crystalline limestones. They have been interpreted as the high-temperature product of intrusion by hot peridotite into limestones, as low-temperature rodingites (the product of calcium metasomatism associated with serpentinisation), or a fortuitous juxtaposition of unrelated rocks. Their age is not known. The skarns are wollastonite-pyroxene-calcite rocks in which dark green pyroxenes are fassaites with high Al, Fe and Ti with high Ca-Tschermak's components. The field relations, textures, mineral assemblages and compositions, and melt inclusions in wollastonite and fassaite indicate the skarns formed by melting at the contact between peridotites and limestones with retrograde reactions during cooling forming garnet and anorthite. There are uncertainties in temperature estimates since pressure, XCO2 and other compositional variables are unknown, but melting temperatures were likely to have been close to 1100°C with garnet formation at approximately 900°C. Later alteration of some skarns and formation of rodingites close to the limestone-peridotite contacts occurred during low-temperature Ca metasomatism, probably after emplacement of the ophiolite during Zagros collision. A hot intrusive origin for the skarns appears incompatible with an arc-related supra-subduction origin of the ophiolite inferred from geochemical studies, but recent work in eastern Indonesia shows that during late Neogene subduction rollback, melts formed above hot mantle that intruded highly extended continental crust in a forearc setting. The scale, timing and temperatures of melting and metamorphism are very similar to those of the Late Cretaceous Neyriz ophiolite.

Stable isotope and geochemical evidence for genesis of secondary copper deposits at Girilambone, New South Wales, Australia

AbstractThe Girilambone copper deposit of New South Wales, Australia, serves as an end-member model for development of a classic oxidation-zone profile in an environment virtually free of both the effects of active tectonics and significant fluctuation in the local water table. The oxidation zone of other copper deposits may be interpreted for history of recently active tectonics, palaeoclimate, and water-table stability by comparison to the Girilambone deposit. Unlike the oxidation profiles of porphyry copper deposits of western North America, which have been overprinted by many water-table fluctuations produced by active tectonics, the Girilambone deposit appears to have little modification to the original oxidation profile. Oxidation of primary sulfides at Girilambone was an exothermic process facilitated by chemolithotrophic bacteria, recorded by malachite oxygen isotope thermometry estimates of up to 52°C, and very light malachite carbon isotope values. The bacteria generated CO2 which migrated upwards to react with copper rich meteoric fluids of the vadose zone to precipitate malachite. Unlike porphyry copper deposits of western North America which experienced recent tectonic activity, the secondary minerals (clays, iron oxides and copper carbonates) at Girilambone were not repeatedly fractured and offset during oxidation to re-establish permeability. This reduced permeability of the oxidation zone and slowed the release of CO2 from the system, producing significantly elevated partial pressure of CO2, sufficient for azurite formation to dominate. Azurite oxygen and carbon isotope values indicate formation at lower temperatures up to 32°C, and with less bacterial activity than for malachite. The sulfide-digesting bacteria at Girilambone were relatively free of seasonal swings in population, as the deposit does not have the interbanded azurite and malachite typical of episodic bacterial populations triggered by seasonal precipitation. Thus the absence of significant Cu-carbonate banding at Girilambone serves as a palaeoclimate indicator. Deeper in the Girilambone oxidation zone, native copper and cuprite dominate, whereas chalcocite formed an enrichment blanket just above and at the modern water table. Oxygen and carbon isotope values for pseudomorphs of malachite after azurite indicate that these were generated as a retrograde reaction when CO2 production from bacterial digestion of sulfides waned and temperatures in the oxidation zone were near ambient. In the post mining environment, chloride-rich groundwater seeps actively precipitate atacamite, while exposed remnants of sulfide masses form an outer rind of porous malachite. Exceptions to this oxidation zone sequence occur due to localised fluid channelisation and perched water-table lenses that generated mineralogical overprints.

The influence of microscale lithological layering and fluid availability on the metamorphic development of garnet and zircon: insights into dissolution–reprecipitation processes

AbstractThe response of garnet and zircon to prograde amphibolite-facies metamorphism in late Proterozoic mica schists from the Scottish Highlands has been investigated. Spatial analysis of zircon populations using scanning electron microscopy was undertaken in Dalradian Schists that have undergone a sequence of prograde garnet growth and localised breakdown reactions involving coupled dissolution–reprecipitation. Fluid availability and matrix permeability strongly control this metamorphic response and different generations of garnet contain radically different populations of metamorphic micro-zircon and associated changes in the detrital zircon population. Micro-zircon abundance increases during garnet growth, whereas that of detrital zircon decreases. The mineralogy of the matrix influences zircon abundance in porphyroblast phases, where garnet overgrows a micaceous matrix zircon-rich garnet forms and where it overgrows a quartzofeldspathic matrix the result is zircon-poor garnet. Following garnet growth, micro-zircon abundance decreases at each stage of the prograde reaction history, with sillimanite-zone schists containing the lowest abundance, suggesting micro-zircons are texturally less stable at staurolite- and sillimanite-grade metamorphism. Micro-zircons are distributed evenly across host minerals in the matrix, with the exception of retrograde chlorite where micro-zircons are absent due to fluids removing Zr before new zircon can precipitate. There is an overall decrease in the mode of zircon at each stage of the reaction history, indicating that zircon is a highly reactive phase during amphibolite-facies metamorphism and is very sensitive to individual prograde and retrograde reactions.

Lower crustal resistivity signature of an orogenic gold system

Orogenic gold deposits provide a significant source of the world’s gold and form along faults over a wide range of crustal depths spanning sub-greenschist to granulite grade faces, but the source depths of the gold remains poorly understood. In this paper we compiled thirty years of long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and south-eastern South Australia that have sensitivity to the electrical resistivity of the crust and mantle, which in turn depend on past thermal and fluid processes. This region contains one of the world’s foremost and largest Phanerozoic (440 Ma) orogenic gold provinces that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the long-period MT and GDS data shows a remarkable correlation between orogenic gold deposits with > 1 t production and a < 20 Ω m low-resistivity region at crustal depths > 20 km. This low-resistivity region is consistent with seismically-imaged tectonically thickened marine sediments in the Lachlan Orogen that contain organic carbon (C), sulphides such as pyrite (FeS2) and colloidal gold (Au). Additional heat sources at 440 Ma due to slab break-off after subduction have been suggested to rapidly increase the temperature of the marine sediments at mid to lower crustal depth, releasing HS− ligands for Au, and CO2. We argue that the low electrical resistivity signature of the lower crust we see today is from a combination of flake graphite produced in situ from the amphibolite grade metamorphism of organic-carbon in the marine sediments, and precipitated graphite through retrograde hydration reactions of CO2 released during the rapid heating of the sediments. Thus, these geophysical data image a fossil source and pathway zone for one of the world’s richest orogenic gold provinces.

Partial melting and P-T evolution of eclogite-facies metapelitic migmatites from the Egere terrane (Central Hoggar, South Algeria)

Abstract The Egéré terrane (Central Hoggar, South Algeria) includes mafic eclogite lenses boudinaged in metapelitic rocks with high-pressure relicts. These metapelites show textural records of partial melting, mainly primary melt inclusions enclosed in garnet crystals and later crystallized as “nanogranitoids.” Garnet porphyroblasts also contain inclusions of quartz, kyanite, phengite, biotite, staurolite, and rutile and show a smoothed prograde zoning with a Mn bell-shaped profile. The peak high-pressure metamorphic assemblage consists of garnet, kyanite, phengite (Si up to 6.36), quartz, rutile, ±ilmenite, ±feldspars, and melt. Phengite has partially transformed into fine-grained aggregates of biotite, plagioclase, and K-feldspar, a microstructure interpreted as resulting from a dehydration melting during exhumation. Phengite breakdown, along with other retrograde reactions, produced a late paragenesis with biotite, plagioclase, K-feldspar, quartz, almandine-rich garnet, ±sillimanite, ±staurolite, ±muscovite, and ilmenite. The thermodynamic modeling of P-T pseudosections allows us to constrain various steps of the metamorphic history: beginning of the garnet growth at 4.0 kbar and ~600 °C during prograde metamorphism; pressure peak at 14–20 kbar; temperature peak at 800–820 °C; formation of the last assemblage at 6.0–5.5 kbar and 725–685 °C. Partial melting likely started during the prograde path when crossing the H2O-saturated solidus, at T ≥ 650–670 °C and P ≥ 10 kbar, continued upon heating, up to the peak conditions, as well as during decompression. This evolution is interpreted in terms of subduction of the continental crust to mantle depths, followed by an exhumation through a clockwise P-T path during the Pan-African orogeny. The Egéré metapelites are relatively well-preserved eclogite-facies rocks, contain inclusions of “nanogranitoids” hitherto very little known in eclogite-facies metamorphic rocks, and represent an unusual trace of subduction within a Neoproterozoic orogen.

The connection between molecular switches and mechanophores

Dr. Hai Qian earned his BS and MS in chemistry at Nankai University, where he worked with Prof. Yu Liu on supramolecular polymers. In 2012, he moved to the US and worked under the guidance of Prof. Ivan Aprahamian at Dartmouth College for his PhD, during which he was focused on the development of hydrazone-based switches, fluorophores, and adaptive materials. He is currently a postdoctoral research associate in Prof. Jeffrey Moore's lab, where he develops retrograde reactions for sustainable energy storage and also investigates force as a means to trigger a variety of productive mechanochemical transformations.

Metabasic rocks from the Variscan Schwarzwald (SW Germany): metamorphic evolution and igneous protoliths

In the Variscan Schwarzwald metabasic rocks form small bodies included within anatectic plagioclase-biotite gneisses. Many metabasites first underwent an eclogite-facies metamorphism at about 2.0 GPa and 670–700 °C, resulting in the assemblage garnet + omphacite + rutile + quartz ± epidote ± amphibole ± kyanite. Since these eclogites are nearly free of an OH-bearing phase, they underwent almost complete dehydration during subduction, suggesting formation along an average to warm top-of-the-slab geotherm of 10–13 °C/km. The age of the Variscan high-P/high-T metamorphism is > 333 Ma. After partial exhumation from ~ 65 to ~ 15 km depth, the eclogites were overprinted under increasing activity of H2O by a number of retrograde reactions. The degree of this overprint under amphibolite-facies conditions (0.4–0.5 GPa/675–690 °C) was very different. Up to now, only retrograde eclogites have been found, but some samples still contain omphacite. Kyanite is at least partially transformed to aggregates of plagioclase + spinel ± corundum ± sapphirine. On the other hand, there are amphibolites that are extensively recrystallized and show the assemblage amphibole + plagioclase + ilmenite/titanite ± biotite ± quartz ± sulphides. The last relic phase that can be found in such otherwise completely recrystallized amphibolites is rutile. After the amphibolite-facies metamorphism at ~ 333 Ma, the metabasites underwent a number of low-temperature transformations, such as sericitization of plagioclase, chloritization of amphibole, and formation of prehnite. The intimate association of metabasite bodies with gneisses of dominantly meta-greywacke compositions suggests derivation from an active plate margin. This view is corroborated by bulk-rock geochemical data. Excluding elements that were mobile during metamorphism (Cs, Rb, Ba, K, Pb, Sr, U), the concentrations of the remaining elements in most of the metabasites are compatible with a derivation from island-arc tholeiites, back-arc basin basalts or calc-alkaline basalts. Only some samples have MORB precursor rocks.

Mineral Chemistry and Reaction Textures of Calc-silicate Rocks of the Lunavada Region, SAMB, NE Gujarat

Abstract Calc-silicate rocks occurring in and around Lunavada town belong to the Kadana Formation of the Lunavada Group of the Aravalli Supergroup. These rocks are embedded within pelitic schists in the form of sporadically distributed lensoidal bodies and are surrounded by quartzitic ridges. Rocks of the Kadana Formation and its surrounding area have experienced intrusive event named as ‘Godhra granite’ which occupy the SW part of this area. Some prominent field characteristics shown by these calc-silicates include fine to medium grained size, dark grey colour, unoriented/star shaped amphibole needles and maculose structure. These rocks have contact metamorphic textural features and the typical mineral assemblage viz., Act +Di + Qtz + Ttn+ Cal ± Mc ± Bt ± Pl ± Ep ± Scp ± Chl with minor proportion of apatite, zircon and opaques. EPMA studies revealed that the Ca-amphibole composition of these rocks ranges from magnesio-hornblende to actinolite whereas the clinopyroxene is salitic to diopsidic and the mica is found to be phlogopitic biotite. Certain prograde and retrograde reactions textures present within these rocks have been interpreted, for e.g. the development of diopside (salite) from ankeritic composition which is then retrogressed and actinolite appeared, prograde reaction leading to the formation of titanite from ilmenite and calcite and breakdown of scapolite into calcite and quartz with plagioclase lacking co-existence with these scapolites, indicating retrogression. Mineral assemblage and mineral chemistry data interpretation points towards the calcareous sandstone or marl as a probable protolith having impure calcareous composition, moreover, field characteristics and reaction textures observed give indication that the protolith might had passed through the contact metamorphic event to give rise to the present day calc-silicates.

Deformation-Induced and Reaction-Enhanced Permeability in Metabasic Gneisses, Iona, Scotland: Controls and Scales of Retrograde Fluid Movement

The spatial distribution of greenschist-facies retrograde reaction products in metabasic gneisses from Iona, western Scotland, has been investigated. The retrograde products may be broadly accounted for by a single reaction, but their different spatial and temporal development indicates that a series of reactions occur with significantly different scales of metasomatic transfer. After initial fluid influx linked to deformation-induced high permeability, reaction-enhanced permeability, coupled to cycling of fluid pressure during faulting, strongly controls the pervasive retrogression. Ca-plagioclase and pyroxene in the gneisses are replaced by albite and chlorite in pseudomorphic textures, and this is followed by localized epidotization of the albite. Two main generations of epidote are formed in the gneisses. Epidosite formation is associated with prominent zones of cataclasite indicating a strong link between faulting and fluid influx. In contrast, complete alteration of albite to epidote in the host metabasic gneisses is spatially complex, and areas of pervasive alteration may be constrained by both epidote-rich veins and cataclasites. In other instances, reaction fronts are unrelated to structural features. Volume changes associated with individual stages of the reaction history strongly control the localized distribution of epidote and the earlier more widespread development of chlorite and albite. Such behaviour contrasts with adjacent granitic gneisses where epidotization is restricted to local structural conduits. Many small-scale mineralized fractures with evidence of having previously contained fluids do not enhance the pervasive retrogression of the metabasic gneisses and represent conduits of fluid removal. Retrogression of these basement gneisses is dominated by a complex combination of reaction-enhanced and reaction-restricted permeability, kinetic controls on the nucleation of reaction products, changes in fluid composition buffered by the reactions, and periodic local migration of fluids associated with fault movements. This combination generates spatially complex patterns of epidotization that are limited by cation supply rather than fluid availability and alternations between focused and pervasive types of retrogression.

Low temperature grain-scale retrograde alteration of detrital minerals in Ajali formation from Benin Flank Of Anambra Basin, Nigeria

Petrographical study of detrital and authigenic minerals from the Ajali Formation in the Benin flank of Anambra Basin was carried out with the aim of interpreting the textural characteristics and alteration patterns. Samples were collected from Ayowgiri sand quarry site and along Fugar-Agenebode road where good exposures of the Ajali Sandstone occurred. Heavy minerals were concentrated using bromoform according to standard procedures. The heavy mineral concentrates were mounted on glass slides with araldite glue. In addition, thin sections of sandstone were prepared. The samples were studied with the aid of polarizing microscopes. The textural features and relationships among the heavy minerals were used in the interpretations. The results indicate that the detrital heavy minerals (Fe-Ti oxides and garnet) have undergone a very low-temperatureretrograde changes rather than ordinary in-situ dissolution by weathering and burial diagenesis. The alteration of Fe-Ti oxides resulted into the formation of leucoxene and chlorite while that of garnet resulted only into the formation of chlorite. Detrital magnetite was partially dissolved with skeletal remains while the authigenic grains showed well preserved structures. The re-equilibration of these minerals and partial dissolution were probably due to rise in temperature and change in the chemistry of the formation water. Reducing and acidic conditions at elevated temperature probably favoured dissolution and alteration of the detrital Fe-oxides while the formation of authigenic hematite was aided by oxidizing condition. Temperature increase was probably induced by either underneath thin lithospheric plate or igneous activities at the center of the basin. The conversion of the detrital minerals to authigenic chlorite and partial replacement structures represent a typical case of retrograde reaction which could be described as evidence of a very low-temperature anchi-metamorphism at mineral grain level.&#x0D; Key words: Authigenic, chlorite, Fe-oxides, garnet, retrograde, anchi-metamorphism.

Sources and Effects of Fluids in Continental Retrograde Shear Zones: Insights from the Kuckaus Mylonite Zone, Namibia

Midcrustal rocks in retrograde metamorphic settings are typically H2O-undersaturated and fluid-absent and have low permeability. Exhumed continental retrograde faults, nonetheless, show evidence for the operation of fluid-mediated weakening mechanisms during deformation at midcrustal conditions. To explore the origin and effects of fluids in retrograde faults, we study the Kuckaus Mylonite Zone (KMZ), an exhumed crustal-scale, strike-slip shear zone in the southern Namibian Namaqua Metamorphic Complex. The KMZ deformed quartzofeldspathic migmatised gneisses at midcrustal retrograde conditions (450-480°C, 270-420 MPa) in the Mesoproterozoic, 40 Ma after granulite facies peak metamorphism at 825°C and 550 MPa. The mylonites contain fully hydrated retrograde mineral assemblages, predominantly adjacent to anastomosing high-strain zones, providing evidence of local H2O saturation and fluid presence during deformation. Whole rock and quartz veinδ18O values suggest that at least some of the fluids were meteoric in origin. The rocks across the shear zone retain the effect of different protoliths, implying little effect of fluid-rock interaction on whole rock major element chemistry. Together with a general scarcity of quartz veins, this suggests that fluid/rock ratios remained low in the KMZ. However, even small amounts of H2O allowed reaction weakening and diffusion-precipitation, followed by growth and alignment of phyllosilicates. In the ultramylonites, a fine grain size in the presence of fluids allowed for grain size sensitive creep. We conclude that the influx of even small volumes of fluids into retrograde shear zones can induce drastic weakening by facilitating grain size sensitive creep and retrograde reactions. In retrograde settings, these reactions consume fluids, and therefore elevated fluid pressures will only be possible after considerable weakening has already occurred. Our findings imply that the range of seismic styles recently documented at active retrograde transform faults may not require high fluid pressures but could also arise from other local weakening mechanisms.

Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria

Abstract In this article, we report the mineral chemistry and petrographic features of charnockitic exposure of Iboropa within Precambrian Basement Complex of Nigeria. The mineral assemblages are pyroxene, plagioclase, biotite, hornblende, alkali feldspars, microperthite, quartz and ilmenite, with apatite occurring as accessory mineral. Apatite occurs in abundance as euhedral crystals. Orthopyroxene observed is strongly pleochroic and has numerous microfractures, and it is hypersthene (En45Fs54Wo1) with low TiO2 and MnO, having extremely low percentage of CaO. Hypersthene is mantled by a complex corona of amphibole, and the amphibole is hornblende with a chemical formula: (K,Na)(Ca,Fe)2 (Fe,Mg,Al,Ti)5(Al,Si)8O22(OH)2. Plagioclase occurs as inclusions in both pyroxene and biotite. Biotite has high concentration of TiO2 and extremely low CaO. The opaque mineral observed is ilmenite and it is concentrated around hypersthene and amphibole. Rare earth element (REE) displays negative Eu anomaly with enrichment of light REE over heavy REE. Amphiboles surrounding orthopyroxene are evidences of retrograde reactions and are formed at the expense of orthopyroxene reacting with plagioclase and quartz in the presence of fluid. The relationship between the mineral assemblages suggests the retrogression of the gneiss that might be as a result of rehydration process, and it is a transition from granulite facies to amphibolite facies during a retrogressive form of metamorphism.

Role of MHC-I Expression on Spinal Motoneuron Survival and Glial Reactions Following Ventral Root Crush in Mice.

Lesions to the CNS/PNS interface are especially severe, leading to elevated neuronal degeneration. In the present work, we establish the ventral root crush model for mice, and demonstrate the potential of such an approach, by analyzing injury evoked motoneuron loss, changes of synaptic coverage and concomitant glial responses in β2-microglobulin knockout mice (β2m KO). Young adult (8–12 weeks old) C57BL/6J (WT) and β2m KO mice were submitted to a L4–L6 ventral roots crush. Neuronal survival revealed a time-dependent motoneuron-like cell loss, both in WT and β2m KO mice. Along with neuronal loss, astrogliosis increased in WT mice, which was not observed in β2m KO mice. Microglial responses were more pronounced during the acute phase after lesion and decreased over time, in WT and KO mice. At 7 days after lesion β2m KO mice showed stronger Iba-1+ cell reaction. The synaptic inputs were reduced over time, but in β2m KO, the synaptic loss was more prominent between 7 and 28 days after lesion. Taken together, the results herein demonstrate that ventral root crushing in mice provides robust data regarding neuronal loss and glial reaction. The retrograde reactions after injury were altered in the absence of functional MHC-I surface expression.

Monazite as a monitor for melt‐rock interaction during cooling and exhumation

AbstractGranulite facies cordierite–garnet–biotite gneisses from the southeastern Reynolds Range, central Australia, contain both orthopyroxene‐bearing and orthopyroxene‐free quartzofeldspathic leucosomes. Mineral reaction microstructures at the interface of gneiss and leucosome observed in outcrop and petrographically, reflect melt‐rock interaction during crystallization. Accessory monazite, susceptible to fluid alteration, dissolution and recrystallization at high temperature, is tested for its applicability to constrain the chemical and P–T–time evolution of melt‐rock reactions during crystallization upon cooling. Bulk rock geochemistry and phase equilibria modelling constrain peak pressure and temperature conditions to 6.5–7.5 kbar and ~850°C, and U–Pb geochronology constrains the timing of monazite crystallization to 1.55 Ga, coeval with the Chewings Orogeny. Modelling predicts the presence of up to 15 vol.% melt at peak metamorphic conditions. Upon cooling below 800°C, melt extraction and in situ crystallization of melt decrease the melt volume to less than 7%, at which time it becomes entrapped and melt pockets induce replacement reactions in the adjacent host rock. Replacement reactions of garnet, orthopyroxene and K‐feldspar liberate Y, REE, Eu and U in addition to Mg, Fe, Al, Si and K.We demonstrate that distinguishing between monazite varieties solely on the basis of U–Pb ages cannot solve the chronological order of events in this study, nor does it tie monazite to the evolution of melt or stability of rock‐forming minerals. Rather, we argue that analyses of various internal monazite textures, their composition and overprinting relations allow us to identify the chronology of events following the metamorphic peak. We infer that retrograde reactions involving garnet, orthopyroxene and K‐feldspar can be attributed to melt‐rock interaction subsequent to partial melting, which is reflected in the development of compositionally distinct monazite textural domains. Internal monazite textures and their composition are consistent with dissolution and precipitation reactions induced by a high‐T melt. Monazite rims enriched in Y, HREE, Eu and U indicate an increased availability of these elements, consistent with the breakdown of orthopyroxene, garnet and K‐feldspar observed petrographically. Our study indicates that compositional and textural analysis of monazite in relation to major rock‐forming minerals can be used to infer the post‐peak chemical evolution of partial melts during high‐ to ultrahigh‐temperature metamorphism.

Discovery of Se-rich canfieldite, Ag8Sn(S,Se)6, from the Shuangjianzishan Ag–Pb–Zn deposit, NE China: A multimethodic chemical and structural study

AbstractDuring a study of the ore minerals belonging to the recently discovered Shuangjianzishan Ag–Pb–Zn deposit in NE China, we have discovered exceptional selenium enrichment in canfieldite (up to 11.6 wt.% of Se). Incorporation of Se into canfieldite has been investigated by an integrated approach using field emission scanning electron microscopy, electron microprobe and single-crystal X-ray diffraction. Canfieldite has been identified as one of the dominant Ag-bearing ore minerals in the studied deposit, which occurs mostly in slate-hosted vein type Ag–Pb–Zn ore bodies. Selenium is either homogeneously or, remarkably, heterogeneously distributed in the different canfieldite fragments studied. Chemical variations of Se are mostly attributable to a series of retrograde reactions resulting in diverse decomposition and exsolution of primary phases during cooling, or alternatively, related to influxes of Se-rich fluids during the formation of canfieldite. To evaluate the effects of the Se-for-S substitution in the structure, a crystal of Se-rich canfieldite [Ag7.98Sn1.02(S4.19Se1.81)Σ6.00] was investigated. The unit-cell parameters are:a= 10.8145(8) Å andV= 1264.8(3) Å3. The structure was refined in the space groupF$\bar{4}$3mtoR1= 0.0315 for 194 independent reflections, with 20 parameters. The crystal structure of Se-rich canfieldite was found to be topologically identical to that of pure canfieldite. If the short Ag–Ag contacts are ignored (due to the disorder), the two Ag atoms in the structure can be considered as three-fold (Ag1) and four-fold (Ag2) coordinated. Tin adopts a regular tetrahedral coordination. As in the case of Te-rich canfieldite, the refinement of the site-occupancy factor indicates that Se is disordered over the three anion positions.

Prograde metamorphic zircon formation in gabbroic rocks: The tale of microtextures

AbstractU–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.