The evolution of excess argon in alpine biotites — A40Ar-39Ar analysis

Alpine biotites containing excess⁴⁰Ar have been analysed by step-heating argon analysis of both neutron irradiated and unirradiated samples. In addition to age spectra the data are discussed in terms of the thermal release of⁴⁰Ar,³⁹Ar,³⁷Ar and³⁶Ar and also displayed on a correlation plot of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar which is used to interpret the data and present a model of isotopic evolution during metamorphic cooling. This diagram overcomes misleading complications of isochron plots. The samples exhibit the following argon systematics: (1) flat age spectra for 80–90%³⁹Ar release with anomalously old ages but early gas fractions that approximate the accepted cooling ages; (2) each sample shows decreasing³⁶Ar/⁴⁰Ar with increasing temperature of heating step with three samples having a negative correlation of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar and one a positive correlation; (3) there appear to be two³⁶Ar components, one released at high temperatures and correlated with radiogenic⁴⁰Ar and one released at low temperatures which is not correlated with radiogenic⁴⁰Ar; and (4) there is no significant effect of neutron irradiation on the release of⁴⁰Ar and³⁶Ar.Interpretation suggests that these biotites contain a record of the evolution and isotopic composition of ambient argon retained within the metamorphic host rocks during cooling. After incorporation of argon of high⁴⁰Ar/³⁶Ar another argon component, of atmospheric composition, was retained at lower temperature and argon partial pressures.     

Understanding the textures and origin of shock melt pockets in Martian meteorites from petrographic studies,comparisons with terrestrial mantle xenoliths,and experimental studies

Abstract— We present a textural comparison of localized shock melt pockets in Martian meteorites and glass pockets in terrestrial, mantle‐derived peridotites. Specific textures such as the development of sieve texture on spinel and pyroxene, and melt migration and reaction with the host rock are identical between these two apparently disparate sample sets. Based on petrographic and compositional observations it is concluded that void collapse/variable shock impedance is able to account for the occurrence of pre‐terrestrial sulfate‐bearing secondary minerals in the melts, high gas emplacement efficiencies, and S, Al, Ca, and Na enrichments and Fe and Mg depletion of shock melt compositions compared to the host rock; previously used as arguments against such a formation mechanism. Recent experimental studies of xenoliths are also reviewed to show how these data further our understanding of texture development and can be used to shed light on the petrogenesis of shock melts in Martian meteorites.     

Geo-botanical evidence of late quaternary mass wasting in block field areas of Virginia

Studies of block fields at Massanutten Mountain, Virginia, document and provide information on the magnitude and frequency of mass movement on these coarse-grained slopes. Block fields, which traditionally have been considered relicts of Pleistocene climate, occur on and downslope from resistant Massanutten Sandstone in Passage Creek basin. Slopes are strewn with boulders and vegetative cover on the open block fields is absent to sparse. Several block fields were investigated for evidence of recent movement using the degree of lichen growth, differences in block weathering, and rock-fall damaged trees. Temporal and spatial aspects of geomorphic processes can be inferred through dendrogeochronology. Corrasion scars and stem ages infer a minimum age for a geomorphic or hydrologic event. High flows on Passage Creek coincided with tree-ring determined dates of block-field movement. Although, Pleistocene periglacial climate may have facilitated original formation of block fields, some block fields now continue to spread downslope during intense runoff events. Present block-field mass wasting may be the principal erosional process in these areas of resistant rock.     

Sm-Nd and U-Pb dating of Early Proterozoic mafic-felsic volcanism in Northernmost Sweden

A Sm-Nd mineral isochron of 1932 ± 45 Ma on a Kiruna greenstone is defined by greenschist facies metamorphic minerals, apparently older than the regional Svecokarelian metamorphism (< 1880 Ma). However, this age agrees within error with U-Pb zircon ages of 1909 ± ₁₆¹⁷ Ma on quartz-bearing prophyries which overlie but are, in part, intercalated with basic volcanics. If these basic volcanics, following previous studies, are considered to be contemporary with the Kiruna greenstones, the present data suggest that eruption of both basic and acid volcanics and associated hydrothermal metamorphism took place over a short time interval close to 1.9 Ga.The greenstone sample is depleted in light rare-earths and has high initial ¹⁴⁵Nd/¹⁴⁴Nd indicating a source in which such depletion has existed for a long time. In contrast, an albite diabase sample with a somewhat younger mineral age (1.80 ± 0.08 Ga) is light rare earth enriched and has a correspondingly low initial Nd isotopic composition.     

A Tertiary age for Alpine high-pressure metamorphism in the Gran Paradiso massif, Western Alps: a Rb–Sr microsampling study

The timing of high‐pressure (HP) metamorphism in the internal basement massifs of the Western Alps has been contentious. In the Gran Paradiso massif silvery micaschists, thought to have developed from granitic precursors, contain assemblages indicative of pressures in excess of 18 kbar at 500–550 °C. This paper presents unique geochronological data for the paragenesis of the silvery micaschist HP assemblage. Rb–Sr microsampling of an apatite–phengite pair thought to have remained closed to Rb–Sr exchange since the HP paragenesis formed has yielded an age of 43.0 ± 0.5 Ma. Greenschist retrogression occurred after 36.3 ± 0.4 Ma, probably in the interval 36–34 Ma. The localised disturbance of the Rb–Sr system in phengite, apatite and allanite during retrogression means that only in situ microsampling could obtain meaningful ages from these rocks. The new data indicating a Tertiary age for HP metamorphism in the Gran Paradiso massif agree with recent data for other internal basement massifs in the Western Alps. A model fitting the Gran Paradiso massif into the Western Alpine framework is presented.     

Origin of the Tongbai-Dabie-Sulu Neoproterozoic low-δ 18O igneous province, east-central China

Zircons from 71 diverse rocks from the Qinling-Tongbai-Dabie-Sulu orogenic belt in east-central China and, for comparison, eight from adjoining areas in the South China and North China blocks, have been analyzed for in situ ¹⁸O/¹⁶O ratio and/or U–Pb age to further constrain the spatial distribution and genesis of Neoproterozoic low-δ ¹⁸O magmas, that is, δ ¹⁸O(zircon) ≤4 ‰ VSMOW. In many metaigneous rock samples from Tongbai-Dabie-Sulu, including high-pressure and ultrahigh-pressure eclogites and associated granitic orthogneisses, average δ ¹⁸O values for Neoproterozoic “igneous” zircon cores (i.e., 800–600 Ma) vary from ?0.9 to 6.9 ‰, and from ?9.9 to 6.8 ‰ for Triassic metamorphic rims (i.e., 245–200 Ma). The former extend to values lower than zircons in primitive magmas from the Earth’s mantle (ca. 5–6 ‰). The average Δ ¹⁸O (metamorphic zircon ? “igneous” zircon) values vary from ?11.6 to 0.9 ‰. The large volume of Neoproterozoic low-δ ¹⁸O igneous protoliths at Tongbai-Dabie-Sulu is matched only by the felsic volcanic rocks of the Snake River Plain hotspot track, which terminates at the Yellowstone Plateau. Hence, the low-δ ¹⁸O values at Tongbai-Dabie-Sulu are proposed to result from shallow subcaldera processes by comparison with Yellowstone, where repeated caldera-forming magmatism and hydrothermal alteration created similar low-δ ¹⁸O magmas. However, the possibility of involvement of meltwaters from local continental glaciations, rather than global Neoproterozoic glaciations, cannot be precluded. Our data indicate that Neoproterozoic low-δ ¹⁸O magmas that are either subduction- or rift-related are present locally along the western margin of the South China Block (e.g., Baoxing Complex). It appears that Neoproterozoic ¹⁸O-depletion events in the South China Block as the result of hydrothermal alteration and magmatism affected a much larger area than was previously recognized.     

The role of magma mixing in the petrogenesis of mafic alkaline lavas,Rockeskyllerkopf Volcanic Complex,West Eifel,Germany

The quaternary Rockeskyllerkopf Volcanic Complex (RVC) comprises three spatially and temporally distinct volcanic centers that can also be distinguished on the basis of their geochemical signatures. All the volcanic products in the complex are olivine basanites whose major and trace element compositions span almost the entire range defined for the West Eifel field as a whole. The RVC lavas have lower Al₂O₃, Na₂O and Y contents and higher TiO₂, CaO, K₂O, Sc, V, Co, Rb, and Ba than the Tertiary lavas in nearby Hocheifel volcanic field. Within the complex, the oldest South East Lammersdorf Center (SEL) comprises primitive lavas with an average MgO content of ～11 wt.% and La_N/Yb_N of 29?±?2. The second center, Mäuseberg, has similar MgO to SEL but is distinct in its much higher La_N/Yb_N of 42?±?2. The Rockeskyllerkopf Center, which was erupted after a break in activity, comprises lavas similar in composition to the SEL Center but with distinctly higher Al₂O₃ and lower MgO contents. Given the lack of evidence for significant fractionation or assimilation in the RVC lavas, we attribute the compositional variations within and between the centers of the RVC to be due to variations in the composition of the source region in combination with magma mixing. Our preferred model involves 1–5% partial melting of LREE-enriched mantle in the garnet stability field, likely within the thermal boundary layer at the base of the lithospheric mantle. These melts mixed to variable degrees with 2–4% partial melts of phlogopite-spinel peridotite formed at higher levels in the modally metasomatised lithospheric mantle.     

Strontium isotopic equilibration during metamorphism of tillites from the Ogcheon Belt,South Korea

The NE/SW trending Ogcheon Belt formed during the Mesozoic by deformation and metamorphism of a sedimentary sequence which includes tillite formations of late Precambrian or Lower Palaeozoic age. In this study Rb-Sr isotopic data are reported from cm-scale sampling of clasts and matrix in these tillites, which underwent metamorphism at grades ranging from the biotite zone to the kyanite zone. Sr-isotopic disequilibrium between clasts and matrix persists up to the highest grade but in four out of five rocks the matrix samples approximate to isochrons (MSWD's<12). The mechanism for this selective isotopic equilibration is considered to be related to the metamorphic reactions in the matrix, which were probably accompanied by a substantial fluid flow. The matrix isochrons for three of the rocks, together with separated white micas from associated schists, suggest that metamorphism took place in the Trias, approximately 200 Myr ago. Metamorphism was followed by slow cooling and biotite closure temperatures were not reached until 190-150 Myr. — These results suggest that where high fluid/rock ratios can be demonstrated on independent petrological grounds, the analysis of small-scale whole-rock samples may allow effective Rb/Sr dating of metamorphism.     

A swash–backwash model of the single epidemic wave

While there is a large literature on the form of epidemic waves in the time domain, models of their structure and shape in the spatial domain remain poorly developed. This paper concentrates on the changing spatial distribution of an epidemic wave over time and presents a simple method for identifying the leading and trailing edges of the spatial advance and retreat of such waves. Analysis of edge characteristics is used to (a) disaggregate waves into ‘swash’ and ‘backwash’ stages, (b) measure the phase transitions of areas from susceptible, S, through infective, I, to recovered, R, status (S → I → R) as dimensionless integrals and (c) estimate a spatial version of the basic reproduction number, R ₀. The methods used are illustrated by application to measles waves in Iceland over a 60-year period from 1915 to 1974. Extensions of the methods for use with more complex waves are possible through modifying the threshold values used to define the start and end points of an event.