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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS2015

Evidence of Metasomatism in the Lowest Petrographic Types Inferred from A Na(-), K, Rich Rim Around A LEW 86018 (L3.1) Chondrule

Ordinary chondrites (OCs) represent the most abundant extraterrestrial materials and also record the widest range of alteration of primary, pristine minerals of early Solar system material available for study. Relatively few investigations, however, address: (1) the role of fluid alteration, and (2) the relationship between thermal metamorphism and metasomatism in OCs, issues that have been extensively studied in many other meteorite groups e.g., CV, CO, CR, and enstatite chondrites. Detailed elemental abundances profiles across individual chondrules, and mineralogical studies of Lewis Hills (LEW) 86018 (L3.1), an unequilibrated ordinary chondrite (UOC) of low petrographic type of 3.1 returned from Antarctica, provide evidence of extensive alteration of primary minerals. Some chondrules have Na(-), K(-), rich rims surrounded by nepheline, albite, and sodalite-like Na(-), Cl(-), Al-rich secondary minerals in the near vicinity within the matrices. Although, limited evidences of low temperature (approximately 250 C) fluid-assisted alteration of primary minerals to phyllosilicates, ferroanolivine, magnetite, and scapolite have been reported in the lowest grades (less than 3.2) Semarkona (LL3.00) and Bishunpur (LL3.10), alkali-rich secondary mineralization has previously only been seen in higher grade greater than 3.4 UOCs. This preliminary result suggests highly localized metamorphism in UOCs and widens the range of alteration in UOCs and complicates classification of petrographic type and extent of thermal metamorphism or metasomatism. The work in progress will document the micro-textures, geochemistry (Ba, Ca, REE), and isotopic composition (oxygen, Al(-)- 26 Mg-26) of mineral phases in chondrules and adjoining objects to help us understand the formation scenario and delineate possible modes of metamorphism in UOCs.

Mishra, Ritesh Kumar↗
NASA NTRS2019

Unravelling Complex Geologic Histories Using U–Pb and Trace Element Systematics of Titanite

Unravelling the spatio-temporal evolution of orogenic terranes requires a comprehensive understanding of the duration and extent of metamorphic events and hydrothermal alteration. Commonly used minerals such as zircon and monazite may not fully record geological histories in complex tectonic settings because their elemental constituents do not react under many metamorphic and metasomatic conditions. Here, we complement the current geochronological record of the Capricorn Orogen, Western Australia, with titanite U–Pb geochronology and geochemistry of felsic intrusive rocks to draw conclusions about the use of titanite in understanding the evolution of orogenic terranes. Because titanite usually incorporates common-Pb and may be variably reset by multiple metamorphic and hydrothermal events, a workflow is provided here for the systematic and robust interpretation of titanite U-Pb data. The addition of trace element data in titanite is particularly effective for differentiating whether a grain is igneous, recrystallized or metamorphic. We have developed several petrogenetic indices to differentiate these three types of titanite using Zr-in-titanite temperature, Th/U, Th/Pb, Al/(Al + Fe), light to heavy rare earth element ratio, and Eu anomalies. The addition of trace element geochemistry can also highlight anomalously radiogenic (sup 207)Pb/(sup 206)Pb reservoirs. Utilization of our workflow in the Capricorn Orogen reveals that titanite ages from the same samples as published zircon U–Pb data range from coeval to several hundreds of Myr of age difference between the two minerals. Titanite geochronology and trace element geochemistry indicates ~20 Myr of previously unrecognized prolonged cooling for the Capricorn Orogeny to ca. 1750 Ma. The spatial extent of the ca. 1210–1170 Ma part of the Mutherbukin Tectonic Event is also broadened significantly farther north and south than previously recognized. Incorporating titanite geochronology and trace geochemistry with more commonly used techniques (e.g., zircon and monazite petrochronology) extends our ability to resolve the complete history of large-scale orogenic terranes.

Hugo K. H. Olierook↗
NASA NTRSDate not supplied

Understanding Olivine and Pyroxene in Chondritic IDPs

There is a widespread depletion of Cr from the FeO-rich olivine in Wild-2, relative to type IIA chondrules(Frank et al., 2014)-the degree of this depletion is akin to that in unequilibrated chondrites such as Krymka (LL3.2) or Rainbow (CO3.2).Since Cr is highly mobile under even mild thermal metamorphism(as low as 200ºC), it is a sensitive indicator of such an event, showing greater depletion in smaller grains (i.e., matrix vs. chondrules) (Grossman and Brearley, 2005). Grossman and Brearley (2005) found that the distribution of Cr in FeO-rich olivine systematically changes as metamorphism increases between type 3.0 and type 3.2. Thus, Wild-2 olivine appears to carry evidence of mild thermal metamorphism. It is critical to determine whether there is any evidence for this from samples of other comets. For this we can examine an hydrous chondritic IDPs. Unfortunately, the data for olivine and pyroxene in IDPs is sparse, mainly decades old and lacks useful information on minor elements. We have begun to collect new compositional data for olivine and pyroxene in chondritic interplanetary dust particles (filling a major gap in our knowledge, as approximately half of these grains derive from comets), to determine whether the results obtained for Wild-2 are typical for comets in general, or whether Wild-2 (or its accretion components) has experienced an atypical geological history. A determination that a comet, or its accretion components, had experienced significant thermal metamorphism would greatly alter models of early solar system history.

Michael Zolensky↗
DOE OSTI2020

InGaSb Defect Filter Layer to Improve Performance of GaSb Solar Cells Grown on GaAs Substrates

The reduction of the threading dislocation density in metamorphic GaSb grown on GaAs substrates through the use of InGaSb defect filter layers has been investigated. More specifically, we study the effects of strain and thickness on the ability of a InGaSb defect filter layer to reduce threading dislocations in GaSb solar cells grown on GaAs substrates. The strain between the GaSb metamorphic layer on GaAs substrate (99.5% relaxed) and the InGaSb defect filter layer is varied by changing the indium composition in the InGaSb layer. Here, it is demonstrated that an InGaSb defect filter layer with 0.6% strain is more effective for blocking threading dislocations compared with higher-strain layers, resulting in improved short-circuit current (J sc ) and open-circuit voltage (V oc ) for the metamorphic GaSb solar cell. The optimization of the defect filter layer involves varying the thickness of the layer to achieve the lowest possible threading dislocation density. This also takes into account the critical thickness of the InGaSb layer on GaSb to avoid generation of threading dislocations from the InGaSb layer itself. It is shown that adding an In 0.11 Ga 0.89 Sb defect filter layer with thickness of 250 nm and 0.6% strain beneath a GaSb solar cell grown on a GaAs substrate improves V oc from 0.1 V to 0.16 V and J sc from 19.7 mA/cm 2 to 24.7 mA/cm 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2021

Magmatic, hydrothermal and ore element transfer processes of the southeastern Archean Superior Province implied from electrical resistivity structure

Although magmatic- and metamorphic- derived fluids are widely recognized mineralizing agents, the role of crustal architecture in defining source and sink zones within the middle to lower crust and upper mantle of ancient orogens remains enigmatic. The globally largest and best-preserved Archean greenstone belts lie in the Superior Province, Canada. They provide an ideal location to investigate the influence of igneous construction and subsequent syn-deformational plutonism and metamorphism on the localization of metal-rich melts and fluids throughout the crustal column. Integration of three-dimensional magnetotelluric modelling and seismic reflection sections across the Abitibi subprovince reveals details of a 'whole-of-crust' magmatic and hydrothermal system. East-west low resistivity structures broadly underlie the surface traces of the major deformation zones that are host to significant gold endowment (>200 Moz), while mid-crustal cross trends suggest mineralized fluids flowed along 'pipes' within fault planes. Most low resistivity structures are inferred to represent domains containing interconnected zones of graphite and/or sulfide. Furthermore, these delineate relict mantle source/transit domains and crustal pathways enriched by the flow of magmas or metamorphic fluids genetically related to a late-stage pulse of ore-bearing magmatism, possibly as a result of slab break-off or delamination. Using the combined MT and seismic data, we develop a 3-D crustal-scale model which highlights how evolving orogenic architecture-controlled mass transfer and metallogenic processes developed.

58 GEOSCIENCES↗
DOE OSTI2022

Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide insights into deep (~20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist-to-eclogite facies oceanic- and continental-affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P-T-D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn-subduction return flow along similar P-T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top-to-the-S-SW shear (prograde-to-peak subduction), top-to-the-NE shear (blueschist facies exhumation), and then E-W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi-mineral Rb-Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ~50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ~50–40 and ~43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene-Oligocene subduction channel proceeded at rates of ~1.5–5 mm/yr and accommodated ~80% of the total exhumation of this HP/LT complex.

58 GEOSCIENCES↗
NASA NTRS1973

Pyroxene poikiloblastic rocks from the lunar highlands

The results of detailed petrographic, X-ray, electron microprobe, ion probe, and Ar-40/Ar-39 age studies of pyroxene poikiloblastic breccias, an important lunar highlands lithology, are interpreted to indicate that high grade metamorphic recrystallization occurred over wide regions of the moon at about 4.0 G.y. This metamorphism was probably related to a period of high meteorite influx at that time. The temperatures achieved were highly variable but in some cases were sufficiently intense to cause varying degrees of partial melting of the precursor highlands breccias. A complete spectrum of metamorphic grades from only slight recrystallization to virtually complete melting would be expected in such a model. Such a spectrum is observed in the Apollo 16 rocks.

Bence, A. E.↗
NASA NTRS1974

Isotopic evidence for a terminal lunar cataclysm

Most highland total rock samples define a single U-Pb isochron which corresponds to a metamorphism age of about 3.9 aeons. This age is also obtained for internal U-Pb isochrons for some of these samples. The data on 18 rock samples range from concordant samples with U-238/Pb-206 ratio of about 1.2 to discordant ones with U-238/Pb-206 ratio of about 0.02. This feature, coupled with a correlated pattern of U-238/Pb-204 ratios, indicates that Pb was extensively mobilized at about 3.9 aeons. The observed Pb-U fractionation is essentially due to Pb volatilization during the metamorphic events. Volatile Pb transport is not accompanied by similar effects in Rb and must therefore be attributed to a specific process. Rb-Sr internal isochrons for the same rocks determine distinct metamorphic events in the interval 3.85 to 4.00 aeons. It is concluded that highland samples from widely separated areas bear the imprint of an event or series of events in a narrow time interval which can be identified with a cataclysmic impacting rate of the moon at about 3.9 aeons, although diffrentiation by internal magma generation cannot be excluded.

Tera, F.↗
NASA NTRS1974

Ferromagnetic resonance studies of thermal effects on lunar metallic Fe phases

Ferromagnetic resonance results of annealing experiments are discussed which illustrate the thermal effects on lunar metallic iron phases already present in a lunar fines sample. Spectral features of ferromagnetic resonance produced in the sample by heat treatments at temperatures between 600 and 1025 C are described which resemble those detected in lunar breccias and crystalline rocks. A correlation is shown to exist between these features and the degree of thermal metamorphism. It is noted that this correlation can be used as a built-in geothermometer or probe to investigate the thermal history and degree of metamorphism of a lunar sample containing metallic iron phases. The thermal history of a metaclastic rock is analyzed in this way, and it is shown that thermal metamorphism is an effective process for increasing relaxation times or the stability of the natural remanent magnetization carried by single-domain metallic iron particles.

Tsay, F.-D.↗
NASA NTRS1986

Thermoluminescence and Antarctic meteorites

The level of natural thermoluminescence (TL) in meteorites is the result of competition between build-up, due to exposure to cosmic radiation, and thermal decay. Antarctic meteorites tend to have lower natural TL than non-Antarctic meteorites because of their generally larger terrestrial ages. However, since a few observed falls have low TL due to a recent heating event, such as passage within approximately 0.7 astronomical units of the Sun, this could also be the case for some Antarctic meteorites. Dose rate variations due to shielding, heating during atmospheric passage, and anomalous fading also cause natural TL variations, but the effects are either relatively small, occur infrequently, or can be experimentally circumvented. The TL sensitivity of meteorites reflects the abundance and nature of the feldspar. Thus intense shock, which destroys feldspar, causes the TL sensitivity to decrease by 1 to 2 orders of magnitude, while metamorphism, which generates feldspar through the devitrification of glass, causes TL sensitivity to increase by a factor of approximately 10000. The TL-metamorphism relationship is particularly strong for the lowest levels of metamorphism. The order-disorder transformation in feldspar also affect the TL emission characteristics and thus TL provides a means of paleothermometry.

Sears, D. W. G.↗
NASA NTRS1988

Late-Archaean crustal growth in the Lewisian Complex of Northwest Scotland: Diachroneity in magmatic accretion and implications for models of crustal growth

The Lewisian Complex of northwestern Scotland is an Archaean basement terrain that was reworked during the Proterozoic. Previous geochronological studies have established that the complex represents an entirely new crustal addition during late-Archaean times, and the difference between a Sm-Nd accretion date of 2920 + or - 50 Ma and a Pb/Pb metamorphic date of 2680 + or - 70 Ma was interpreted as indicating a crustal accretion-differentiation superevent (CADS) lasting 240 + or - 110 Ma. A combined Rb-Sr, Pb/Pb, and Sm-Nd isotopic study reported was applied to the Lewisian CADS to attempt to address the problem of whether this comprised a short regional scale magmatic accretion event followed approximately 200 Ma later by regional high-grade metamorphism, or a long period of episodic or semi-continuous magmatic accretion prior to high-grade metamorphism.

Whitehouse, M. J.↗
NASA NTRS1988

Part 1: Aspects of lithospheric evolution on Venus. Part 2: Thermal and collisional histories of chondrite parent bodies

The geological evolution of distinctly different kinds of solar system objects is addressed. Venus has been observed over the past decade by orbital radars on both American and Soviet spacecraft. These surface measurements provide clues to the structure and evolution of the lithosphere. The parent bodies of chondritic meteorites, thought to resemble asteroids, represent the other end of the size spectrum of terrestrial objects. Their early thermal and collisional histories may be constrained by the chemical and textural record preserved in meteorite samples. Impact craters on Venus have been observed by the Soviet Venera 15/16 spacecraft. A formalism is presented by which the size-frequency distribution of impact craters may be used to estimate upper bounds on the mean global rates of volcanic resurfacing and lithospheric recycling on that planet over the past several hundred million years. The impact crater density reported from Venera observations, if valid for the entire Venus surface, indicates a mean volcanic flux no greater than 2 cu km/y, corresponding to a maximum average rate of resurfacing of about 4 km/b.y. For the lowest estimated mean crater retention age of the surface of Venus imaged by Venera 15/16, the rate of lithospheric recycling on Venus does not exceed 1.5 sq km/y. Ordinary chondrite meteorites show textural and chemical patterns indicative of varying intensities of thermal metamorphism. The conventional onion-shell model, which envisions highly metamorphosed material in the core and less intensely heated rocks near the surface, predicts an inverse relation between peak temperature and cooking rate, but none has been observed. A metamorphosed-planetesimal model is devised to explain this discrepancy, whereby heating occurs in planetesimals a few kilometers in radius which then accrete to form 100-km-radius parent bodies. Cooling rates are then randomly controlled by burial depth. Thermal and collisional constraints are examined, and the model is found to be applicable only to highly insulating Al-26-rich planetesimals that remain closely aggregated upon accretion. An alternative model is presented, in which onion-shell parent bodies are collisionally fragmented during metamorphism and then gravitationally reassembled. If reassembly times are short, then cooling rates would be determined by burial depth in the reaccreted parent body. This model, unlike previous ones, can explain both coherent and incoherent cooling of Breccia clasts by collisions during or after metamorphism, respectively.

Grimm, Robert E.↗
NASA NTRS1988

Geological indicators for impact: The anomalous case of the Vredefort structure, South Africa

The Vredefort Dome is located within and almost central to the Witwatersrand basin in its presently known extent. It exposes a central Archean granite core which is surrounded by a collar of supracrustal rocks. These collar rocks outline a strong polygonal geometry. The Archean core is comprised of two concentric zones, the Outer Granite Gneiss (OGG), and the core central Inlandsee Leucogranofels (ILG). The rocks of the inner core display granulite facies metamorphism, while the OGG is in amphibolite facies. The inner core is believed from recent drill hole information to be underlain by mafic and ultramafic gneisses, the extent of which cannot be assessed at present. A fairly broad zone of charnockites separates the OGG and ILG domains. This zone is characterized by a high concentration of pseudotachylite and ductile shearing. Whereas a number of other domical structures are located within or surrounding the Witwatersrand basin, the Vredefort structure is anomalous, in that it has: a partly polygonal geometry; extensive alkali intrusives in the northwestern sector; granophyre dykes (ring-dykes peripheral to the contact collar-basement and NW-SE or NE-SW trending dykes within the Archean basement); contact metamorphism of the collar supracrustal rocks; the overturning of collar supracrustals in the northern sectors; deformation phenomena widely regarded as representing shock metamorphism (pseudotachylite, (sub)planar microdeformation features in quartz, shatter cones and occurrences of high-P quartz polymorphs); a positive 30 mgal gravity anomaly; and high amplitude magnetic anomalies. Recent geophysical, structural and petrological evidence pertinent for the identification of the processes that led to the formation of the Vredefort structure are summarized.

Antoine, L. A. G.↗
NASA NTRS1988

Shocked quartz and more: Impact signatures in K-T boundary clays and claystones

Quartz grains displaying multiple sets of planar features are described from numerous Cretaceous-Tertiary (K-T) boundary clays and claystones at both marine and nonmarine depositional sites around the world. All these sites also show anomalously high amounts of iridium and enrichments of other siderophile elements in cosmic ratios within these boundary units. This combination of mineralogical and geochemical features are used in support of an impact hypothesis for the end-Cretaceous event. Recently, it was suggested that some combination of explosive and nonexplosive volcanism associated with the formation of the Deccan traps in India could be responsible for the mineralogy and geochemistry seen in the K-T boundary units. Besides the obvious contradition of simultaneous explosive and nonexplosive volcanism from one locality during an instant of geologic time, there remains the difficulty of spreading both iridium (and trace elements in cosmic proportions) and quartz grains around the world by volcanic (atmospheric) transport. In addition, the ability of volcanism to produce the type of shock metamorphism seen in minerals at the K-T boundary was not demonstrated. Multiple sets of shock lamellae in quartz are considered characteristic of shock metamorphism in rocks at the sites of known impact craters and are the type of deformation seen in quartz from K-T boundary clays and claystones. Single sets of poorly defined lamellae described from rare quartz grains in certain volcanic deposits are characteristic of tectonic deformation and do not correspond to the shock lamellae in quartz from K-T sediments and impact structures. So-called shock mosaicism in quartz and feldspar grains described from volcanic deposits can result from many processes other than shock metamorphism, and therefore is not considered to be an effect characteristic solely of shock. The mineralogy of shock-metamorphosed grains at the K-T boundary also argues against a volcanic origin.

Bohor, Bruce F.↗
NASA NTRS1988

Significance of the late Archaean granulite facies terrain boundaries, Southern West Greenland

Three distinct episodes and occurrences of granulite metamorphism in West Greenland are described: (1) the oldest fragmentary granulites occur within the 3.6-Ga Amitsoq gneisses and appear to have formed 200 Ma after the continental crust in which they lie (Spatially associated rapakivi granites have zircon cores as old as 3.8 Ga, but Rb-Sr, whole-rock Pb-Pb, and all other systems give 3.6 Ga, so these granulites apparently represent a later metamorphic event); (2) 3.0-Ga granulites of the Nordlandet Peninsula NW of Godthaab, developed immediately after crustal formation in hot, dry conditions, are carbonate-free, associated with voluminous tonalite, and formed at peak metamorphic conditions of 800 C and 7 to 8 kbar (Synmetamorphic trondhjemite abounds and the activity of H2O has been indicated by Pilar to have varied greatly); and (3) 2.8-Ga granulites south of Godthaab, lie to the south of retrogressed amphibolite terranes. Prograde amphibolite-granulite transitions are clearly preserved only locally at the southern end of this block, near Bjornesund, south of Fiskenaesset. Progressively deeper parts of the crust are exposed from south to north as a major thrust fault is approached. Characteristic big hornblende pegmatites, which outcrop close to the thrust in the east, have been formed by replacement of orthopyroxene. Comparable features were not seen in South Indian granulites. It was concluded that no one mechanism accounts for the origin of all granulites in West Greenland. Various processes have interacted in different ways, and what happened in individual areas must be worked out by considering all possible processes.

Friend, C. R. L.↗
NASA NTRS1990

Disentangling nebular and asteroidal features of CO3 carbonaceous chondrite meteorites

Ten carbonaceous CO3 chondrites (including four chondrites from Antarctica and the Colony, Isna, Kainsaz, Lance , Ornans, and Warrenton chondrites) were analyzed with respect to mean compositions of olivines and low-Ca pyroxenes in order to distinguish the primary nebular features of these chondrites from the secondary (asteroidal or nebular) features. In three of the Antarctic chondrites, matrices and metal grains were analyzed in order to investigate the origin of mineralogical trends in the CO3 sequence. Based on these results, the CO3 chondrites were classified into subtypes 3.0-3.7. In the silicates of chondrites ALH A77307 and Colony, classified as type 3.0, metamorphic effects appear to be absent. Chemical and mineralogical studies suggest that the type 3.1 to 3.7 CO chondrites represent a metamorphic sequence that formed from material closely resembling type 3.0 CO chondrites by metamorphism in one or more planetesimals or asteroids, not by interactions between chondritic ingredients in the solar nebula.

Scott, Edward R. D.↗
NASA NTRS1992

Redox effects in ordinary chondrites and implications for asteroid spectrophotometry

The sensitivity of reflectance spectra to mean ferrous iron content and olivine and pyroxene proportion enhancements in the course of metamorphic oxidation is presently used to examine whether metamorphically-induced ranges in mineralogy, and corresponding spectral parameters, may explain the observed variations in S-asteroid rotational spectra. The predicted spectral variations within any one chondrite class are, however, insufficient to account for S-asteroid rotational spectra, and predicted spectral-range slopes have a sign opposite to the rotational measurements. Metamorphic oxidation is found unable to account for S-asteroid rotational spectra.

Mcsween, Harry Y., Jr.↗
NASA NTRS1992

Sudbury project (University of Muenster-Ontario Geological Survey): Sr-Nd in heterolithic breccias and gabbroic dikes

One major objective of our Sudbury project was to define origin and age of the huge breccia units below and above the Sudbury Igneous Complex (SIC). The heterolithic Footwall Breccia (FB) represents a part of the uplifted crater floor. It contains subrounded fragments up to several meters in size and lithic fragments with shock features (greater than 10 GPa) embedded into a fine- to medium-grained matrix. Epsilon(sub Nd)-epsilon(sub Sr) relationships point to almost exclusively parautochthonous precursor lithologies. The different textures of the matrix reflect the metamorphic history of the breccia layer; thermal annealing by the overlying hot impact melt sheet (SIC) at temperatures greater than 1000 C resulted in melting of the fine crushed material, followed by an episode of metasomatic K-feldspar growth and, finally, formation of low-grade minerals such as actinolite and chlorite. Isotope relationships in the Onaping breccias (Gray and Green Member) are much more complex. All attempts to date the breccia formation failed: Zircons are entirely derived from country rocks and lack the pronounced Pb loss caused by the heat of the slowly cooling impact melt sheet (SIC). Rb-Sr techniques using either lithic fragments of different shock stages or the thin slab method, set time limits for the apparently pervasive alkali mobility in these suevitic breccias. The data array and the intercept in the plots point to a major Rb-Sr fractionation around 1.54 Ga ago. This model age is in the same range as the age obtained for the metasomatic matrix of the FB. Rb-Sr dating of a shock event in impact-related breccias seems to be possible only if their matrix had suffered total melting by the hot melt sheet (FB) or if they contain a high fraction of impact melt (suevitic Onaping breccias), whereas the degree of shock metamorphism in rock or lithic fragments plays a minor role. In the Sudbury case, however, the impact melt in the seuvitic breccias is devitrified and recrystallized, which changed Rb/Sr ratios quite drastically. Therefore, the Onaping breccias give only age limits for alteration and low-grade metamorphism. The Sm-Nd system was not reset during the Sudbury event; clasts as well as the matrix in the FB and in the Onaping breccias show preimpact 'Archean' Nd isotope signatures.

Buhl, D.↗