Scapolite phase equilibria and carbon isotope variations in high grade rocks: Tests of the carbon-dioxide-flooding hypothesis of granulite genesis.

Scapolite decarbonation reactions and carbon isotope analysis of CO\sb2 extracted from scapolite are used to determine the presence, composition, and source of fluid components in high grade rocks. Scapolite-plagioclase-garnet-quartz assemblages, common to many lithologies in high grade terranes, monitors CO\sb2 activity ($a$CO\sb2) by the reaction 2 Meionite + Quarts = 5 Anorthite + Grossular + 2 CO\sb2. The P-T-X location of this reaction was calculated using an internally consistent thermodynamic data set for meionite and phases in the CASCH system. Activity-composition relations for meionite in scapolite were calculated from the thermodynamic data set and compositional data on natural scapolite-plagioclase-calcite assemblages. Equilibration pressures of scapolite assemblages were calculated from clinopyroxene-garnet-plagioclass-quartz barometers calibrated for this study. The $a$CO\sb2 was calculated for a variety of high grade gneisses from the southwestern Grenville Province and other terranes. Granulites typically yield low to moderate values of $a$CO\sb2 (less than 0.5). Calc-silicates and meta-anorthosite yield moderate $a$CO\sb2. Deep crustal xenoliths yield a range of $a$CO\sb2. CO\sb2 for carbon isotope analysis was extracted from scapolite in mineral separates and whole rock gneisses with phosphoric acid at 75\sp\circC. Scapolite meta-anorthosite and calcite from adjacent marble have the same carbon isotope composition indicating marble was a local source of fluids leading to \sp{13}C enrichment of the anorthosite. \delta\sp{13}C of granulites and mafic gneisses from the southwestern Grenville range from $-3$ to $-10$ per mil, consistent with a variety of carbon sources and a lack of regional homogenization of carbon in the gneisses via a pervasive fluid. The fluid calculations and isotope data are not consistent with pervasive CO\sb2 flooding in the southwestern Grenville Province. Locally, the effects of interaction of rocks with a CO\sb2 rich vapor phase have been documented and serve as a model to evaluate CO\sb2-rock interaction in other terranes. The calculations are also consistent with the vapor absent granulite facies metamorphism in the Furua Complex of Tanzania, and either vapor absent metamorphism or a mixed CO\sb2-H\sb2O fluid in the Sargur Terrane (southern India), Bergen Arcs (Norway), and many lower crustal xenoliths.Ph.D.Earth SciencesGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/128275/2/8907106.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/128275/5/Moecher-DP_DeepBluepermissions_agreement-CCBY.pdfDescription of Moecher-DP_DeepBluepermissions_agreement-CCBY.pdf : Hidden Deep Blue OA and CC license agreemen

ANALYSIS AND ASSESSMENT OF LETHALITY AND SURVIVABILITY FOR THE MARINE LITTORAL REGIMENT

As the Marine Corps activates the Marine Littoral Regiment (MLR) to serve as the joint force’s reconnaissance and counter-reconnaissance effort, questions abound regarding the MLR’s ability to provide a persistent and lethal presence well inside the reach of our adversaries’ advanced long-range precision fires. In this study, the author uses agent-based combat simulations to inform future force design decisions, live-force experimentation, and tactics. The simulated scenario imagines a future MLR conducting sea control operations in the littorals of the Western Pacific against a peer naval threat. This research investigates the effect that a guard force of autonomous and/or semi-autonomous surface vessels, operating as the guard force of the MLR’s defense in depth, has on the survivability and lethality of the MLR’s land-based anti-ship missile platforms. Summary statistics generated by the simulation indicate that the future battlefield will see high losses on both sides. However, based on the results of 27,200 simulated engagements, this study finds that an MLR using a guard force of armed and unarmed “scouts” as described above can inflict a prohibitively high and unsustainable cost on an enemy naval force.Outstanding ThesisMajor, United States Marine CorpsApproved for public release. Distribution is unlimited

Zircon U-Pb Geochronology of Two Basement Cores (Kentucky, USA): Implications for Late Mesoproterozoic Sedimentation and Tectonics in the Eastern Midcontinent

Basement cores from two wells drilled west and east of the Grenville front consist of feldspathic litharenite and granitic orthogneiss, respectively. Detrital zircon U-Pb ages for the litharenite define a broad dominant U-Pb age mode at ca. 1115 Ma. The dominant mode matches that for the type locality of the Middle Run Formation in the Ohio subsurface and is interpreted to consist of detrital zircons sourced from East Continent Rift volcanic sources (ca. 1100 Ma) and Grenville Shawinigan granites/gneisses (1120–1180 Ma). The youngest detrital zircon ages (ca. 1020 Ma) require a maximum depositional age that is at least 70 My younger than the time of Midcontinent and East Continent rifting and magmatism. We propose that the litharenite is correlative with the Middle Run Formation in Ohio and was deposited in an evolving late Grenville rift/foreland basin adjacent to the exhuming Grenville orogen. Zircon U-Pb secondary-ion mass spectrometry ages from orthogneiss define a discordant array with intercepts of ca. 1500 and 1000 Ma. The oldest concordant dates (ca. 1450 Ma, from oscillatory-zoned cores) are interpreted as the crystallization age of the igneous protolith of the orthogneiss. Metamorphic zircon rims define a weighted mean U-Pb age of 1018 ± 19 Ma (2σ) Ma, interpreted to represent the time of high-grade metamorphism during the late Ottawan phase of the Grenville orogeny. This age pattern matches that of exposed basement in the Central Gneiss Belt of the Grenville Province (Ontario) and similar basement orthogneisses in Ohio and Kentucky that are interpreted to be of Eastern Granite-Rhyolite Province affinity. All age data are consistent with a provenance model of an actively exhuming Grenville orogen at ca. 1000 Ma producing sediment that is mixing with recycled East Continent Rift sediments

Extraction and carbon isotope analysis of CO2 from scapolite in deep crustal granulites and xenoliths

Carbon isotope compositions of scapolite from granulite facies gneisses and lower crustal xenoliths document the composition and constrain the source of carbon in scapolite from the lower crust. CO2 is extracted from scapolite without fractionating carbon isotopes by reaction with phosphoric acid at 25 or 75[deg]C. Thus, partial yields of CO2 from scapolite are sufficient for accurate carbon isotopic analysis. Isotopic compositions of coexisting scapolite and calcite in high-grade calc-silicate gneisses and marbles, and consideration of the crystal chemical environment of CO3 in the scapolite structure, indicate little fractionation of 13C/12C between scapolite and calcite (0.1 +/- 1.2%.) at equilibrium conditions of 650-800[deg]C.The carbon isotope composition of CO2 extracted from scapolite in twenty-nine samples of regional granulite facies gneisses, amphibolites, calc-silicate gneisses, and crustal xenoliths yield values of [delta]13C that range from -10 to -1%. (PDB). High-grade marbles and graphitic paragneisses are precluded as major sources of carbon for scapolite in the high grade rocks analyzed in this study, as the former are isotopically enriched, and the latter isotopically depleted in 13C/12C relative to the range of isotopic compositions determined here. The [delta]13C values for mafic granulites and amphibolites in granulite terranes composed of supracrustal sequences (-10.1 to -4.0%.) may reflect the isotopic composition of diagenetic carbonate present in their basaltic protoliths. The values of [delta]13C for scapolite in mafic xenoliths and some granulite facies orthogneisses (-8.2 to -1.2%.) are consistent with crystallization of the scapolite from a mafic melt or derivation of CO2 from mafic melts emplaced in the lower crust or upper mantle. The values of [delta]13C for scapolites from calc-silicate gneisses and calc-silicate xenoliths (-10.0 to -2.9%.) may result from depletion of 13C/12C as a result of decarbonation of calcite-bearing protoliths during prograde metamorphism.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31945/1/0000898.pd

Source to sink zircon grain shape: Constraints on selective preservation and significance for Western Australian Proterozoic basin provenance

The effect of selective preservation during transportation of zircon grains on the detrital age spectrum is difficult to quantify and could potentially lead to systematic bias in provenance analysis. Here we investigate whether the shape of detrital zircon grains holds provenance information and if the grain shape can assist in understanding preservation. We applied multiple linear regression analysis to identify significant shape properties in detrital zircons from Proterozoic metasediments of the Capricorn and Amadeus basins and their Archean and Proterozoic sources in the Yilgarn Craton and the Musgrave Province in Western Australia. Digital images and isotopic data from 819 SIMS U-Pb dated zircons were examined for correlation between grain shape, age, U and Th content. Out of twelve shape descriptors measured, Minor Axis, the width of zircon grains perpendicular to the crystallographic c-axis, consistently shows the most significant correlation with isotopic age. In the studied population Archean grains are narrower than Proterozoic grains: the probability that grains wider than 75 µm are Archean is less than 30%.Calculations of the proportions of source material in sedimentary rocks relative to the proportions of source material in the overall catchment area (erosion parameter '. K' calculated based on age spectra) produced values typical for mature river systems, with K = 6 for the Yilgarn-Capricorn and K = 5.5 for the Musgrave-Amadeus source-sink system. For the Yilgarn-Capricorn system, we also calculated '. K' based on Minor Axis, to determine whether grain width can be linked to age populations. Results of the shape-based K of 5.3 suggest a similarity between age-based and shape-based '. K' values, demonstrating that zircon grain width may be a useful discriminator of provenance. Contrary to commonly applied qualitative shape classifications, we found no consistent correlations between shape descriptors of magmatic zircons and the composition of their host rock. While metamict zircons were preferentially removed during transport, the similarities in grain shape and age distribution of magmatic and detrital populations suggest that hydraulic sorting did not have a significant effect. We conclude that transport of zircon grains from magmatic source to sedimentary sink affects their width less than their length

Earthquake nucleation in the lower crust by local stress amplification

Deep intracontinental earthquakes are poorly understood, despite their potential to cause significant destruction. Although lower crustal strength is currently a topic of debate, dry lower continental crust may be strong under high-grade conditions. Such strength could enable earthquake slip at high differential stress within a predominantly viscous regime, but requires further documentation in nature. Here, we analyse geological observations of seismic structures in exhumed lower crustal rocks. A granulite facies shear zone network dissects an anorthosite intrusion in Lofoten, northern Norway, and separates relatively undeformed, microcracked blocks of anorthosite. In these blocks, pristine pseudotachylytes decorate fault sets that link adjacent or intersecting shear zones. These fossil seismogenic faults are rarely >15 m in length, yet record single-event displacements of tens of centimetres, a slip/length ratio that implies >1 GPa stress drops. These pseudotachylytes represent direct identification of earthquake nucleation as a transient consequence of ongoing, localised aseismic creep

Sapphirine granulites from Panasapattu, Eastern Ghats belt, India : Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin

AbstractWe report equilibrium sapphirine + quartz assemblage in biotite–orthopyroxene–garnet granulites from a new locality in Panasapattu of Paderu region in the Eastern Ghats granulite belt, which provide new evidence for ultrahigh-temperature (UHT) metamorphism at 1030–1050 °C and 10 kbar in this region. The development of migmatitic texture, stabilization of the garnet–orthopyroxene–plagioclase–K-feldspar association, prograde biotite inclusions within garnet and sapphirine as well as sapphirine and cordierite inclusions within garnet in these granulites indicate that the observed peak assemblages probably formed during prograde dehydration melting of a Bt–Sill–Qtz assemblage, and constrain the prograde stage of the p–T path. The core domains of orthopyroxene porphyroblasts have up to w(Al2O3) 9.6%, which suggest that the temperatures reached up to 1150 °C suggesting extreme crustal metamorphism. These conditions were also confirmed by the garnet–orthopyroxene thermobarometery, which yields a p–T range of 1012–960 °C and 9.4 kbar. The p–T phase topologies computed using isochemical sections calculated in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) for metapelites, garnet-free sapphirine granulites and garnet-bearing sapphirine granulites match the melt-bearing assemblages observed in these rocks. Isochemical sections constructed in the NCKFMASH system for an average sub-aluminous metapelite bulk composition, and contoured for modal proportions of melt and garnet, as well as for the compositional isopleths of garnet, predict phase and reaction relations that are consistent with those observed in the rocks. Garnet and orthopyroxene contain Ti-rich phlogopite inclusions, suggesting formation by prograde melting reactions at the expense of phlogopite during ultrahigh-temperature conditions. These p–T results underestimate 'peak' conditions, in part as a result of the modification of garnet compositions in the domains where some melt was retained. The post-peak evolution is constrained by a succession of melt-present reactions that occur at p < 10 kbar, inferred from micro-structural relations among various minerals. After high-temperature decompression from the metamorphic peak, the p–T path followed a near isobaric cooling stage to T < 900 °C. The UHT rocks investigated in this study occur within a continental collision suture which witnessed prolonged subduction–accretion history prior to the final collision. We correlate the extreme metamorphism and the stabilization of UHT mineral assemblages to heat and volatile input from an upwelled asthenosphere during subduction–collision tectonics in a Proterozoic convergent plate margin

Plio-Pleistocene exhumation of the eastern Himalayan syntaxis and its domal ‘pop-up’

The eastern termination of the Himalayan orogen forms a structural syntaxis that is characterised by young (from 10 to &lt; 1 Ma) mineral growth and cooling ages that document Late Miocene to Pleistocene structural, metamorphic, igneous and exhumation events. This region is a steep antiformal and in part domal structure that folds the suture zone between the Indian and Asian plates. It is dissected by the Yarlung Tsangpo, one of the major rivers of the eastern Himalayan–Tibet region, which becomes the Brahmaputra River in the Indian foreland basin before emptying into the Bay of Bengal. Exceptionally high relief and one of the deepest gorges on Earth have developed where the river's tortuous route crosses the Namche Barwa–Gyala Peri massif (&gt; 7 km in elevation) in the core of the syntaxis. Very high erosion rates documented in sediment downstream of the gorge at the foot of the Himalaya contribute ~ 50% of total detritus to the sediment load of the Brahmaputra. The initiation of very high rates of exhumation has been attributed either to the extreme erosive power of a river flowing across a deforming indentor corner and the associated positive feedback, or to the geometry of the Indian plate indentor, with the corner being thrust beneath the Asian plate resulting in buckling which accommodates shortening; both processes may be important. The northern third of the syntaxis corresponds to a steep domal ‘pop-up’ structure bounded by the India–Asia suture on three sides and a thrust zone to the south. Within the dome, Greater Himalaya rocks equilibrated at ~ 800 °C and 25–30 km depth during the Miocene, with these conditions potentially persisting into the latest Miocene and possibly the Pliocene, with modest decompression prior to ~ 4 Ma. This domal ‘pop-up’ corresponds to the area of youngest bedrock ages on a wide variety of thermochronometers and geochronometers. In this paper we review the extensive scientific literature that has focused on the eastern syntaxis and provide new chronological data on its bedrock and erosion products to constrain the age of inception of the very rapid uplift and erosion. We then discuss its cause, with the ultimate aim to reconstruct the exhumation history of the syntaxis and discuss the tectonic context for its genesis. We use zircon and rutile U–Pb, white mica Ar–Ar and zircon fission track dating methods to extract age data from bedrock, Brahmaputra modern sediments (including an extensive compilation of modern detrital chronometry from the eastern Himalaya) and Neogene palaeo-Brahmaputra deposits of the Surma Basin (Bangladesh). Numerical modelling of heat flow and erosion is also used to model the path of rocks from peak metamorphic conditions of ~ 800 °C to &lt; 250 °C. Our new data include U–Pb bedrock rutile ages as young as 1.4 Ma from the Namche Barwa massif and 0.4 Ma from the river downstream of the syntaxis. Combined with existing data, our new data and heat flow modelling show that: i) the detrital age signature of the modern syntaxis is unique within the eastern Himalayan region; ii) the rocks within the domal pop-up were &gt; 575 ± 75 °C only 1–2 Myr ago; iii) the Neogene Surma Basin does not record evidence of the rise and erosion of the domal pop-up until latest Pliocene–Pleistocene time; iv) Pleistocene exhumation of the north-easternmost part of the syntaxis took place at rates of at least 4 km/Myr, with bedrock erosion of 12–21 km during the last 3 Ma; v) the inception of rapid syntaxial exhumation may have started as early as 7 Ma or as late as 3 Ma; and vi) the Yarlung Tsangpo is antecedent and subsequently distorted by the developing antiform. Together our data and modelling demonstrate that the domal pop-up with its exceptional erosion and topographic relief is likely a Pleistocene feature that overprinted earlier structural and metamorphic events typical of Himalayan evolution. Keywords: Eastern Himalayan syntaxis; Namche Barwa; Surma Basin; Yarlung Tsangpo–Brahmaputra; U–Pb rutile dating; Thermal modellin