Deep drilling at the Siljan Ring impact structure: oxygen-isotope geochemistry of granite

The Siljan Ring is a 362-Ma-old impact structure formed in 1700-Ma-old I-type granites. A 6.8-km-deep borehole provides a vertical profile through granites and isolated horizontal diabase sills. Fluid-inclusion thermometry, and oxygen-isotope compositions of vein quartz, granite, diabase, impact melt, and pseudotachylite, reveal a complex history of fluid activity in the Siljan Ring, much of which can be related to the meteorite impact. In granites from the deep borehole, ¹⁸O values of matrix quartz increase with depth from near 8.0 at the surface to 9.5 at 5760 m depth. In contrast, feldspar ¹⁸O values decrease with depth from near 10 at the surface to 7.1 at 5760 m, forming a pattern opposite to the one defined by quartz isotopic compositions. Values of ¹⁸O for surface granites outside the impact structure are distinct from those in near-surface samples from the deep borehole. In the deep borehole, feldspar coloration varies from brick-red at the surface to white at 5760 m, and the abundances of crack-healing calcite and other secondary minerals decrease over the same interval. Superimposed on the overall decrease in alteration intensity with depth are localized fracture zones at 4662, 5415, and 6044 m depth that contain altered granites, and which provided pathways for deep penetration of surface water. The antithetic variation of quartz and feldspar ¹⁸O values, which can be correlated with mineralogical evidence of alteration, provides evidence for interaction between rocks and impact-heated fluids (100–300° C) in the upper 2 km of the pluton. Penetration of water to depths below 2 km was restricted by a general decrease in impact-fracturing with depth, and by a 60-m-thick diabase sill at 1500 m depth that may have been an aquitard. At depths below 4 km in the pluton, where water/rock ratios were low, oxygen isotopic compositions preserve evidence for limited high-temperature (>500° C) exchange between alkali feldspar and fluids. The high-temperature exchange may have been a post-impact event involving impact-heated fluids, or a post-magmatic event.     

Geochemistry and Intrusive History of the Ashland Pluton, Klamath Mountains, California and Oregon

The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial ⁸⁷Sr/⁸⁶Sr = 0{dot}7044, ¹⁸O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (La_N/Lu_N = 7) and then by a suiteof K₂O- and P₂O₅ rocks of quartz monzodioritic affinity (La_N/Lu_N= 13–21; La_N/Sm_N = 2{dot}4–3{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(La_N/Lu_N= 13–19; La_N/Sm_N = 4{dot}3–6 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately P_total = 2{dot}3kb,P_H2O between 1{dot}5 and 2{dot}2 kb, and f_O2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO₂ togetherwith increasing ¹⁸O and decreasing initial ⁸⁷Sr/⁸⁶Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr₁ and high (¹⁸O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in ¹⁸O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P₂O₅-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P₂O₅-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation.     

Synchrotron cooling and annihilation of ane +-e − plasma: The radiation mechanism for the 5 March, 1979 transient

Positron-electron pair radiation is examined as a mechanism that could be responsible for the impulsive phase emission of the 5 March, 1979 transient. Synchrotron cooling and subsequent annihilation of the pairs can account for the energy spectrum, the very high brightness, and the 0.4 MeV feature observed from this transient, whose source is likely to be a neutron star in the supernova remnant N49 in the Large Magellanic Cloud. In this model, the observed radiation is produced in the skin layer of a hot, radiation-dominated pair atmosphere, probably confined to the vicinity of the neutron star by a strong magnetic field. The width of this layer is only about 0.1 mm. In this layer, 10¹² generations of pairs are formed (by photon-photon collisions), cooled and annihilated during the 0.15 s duration of the impulsive phase. The very large burst energy implied by the distance of the LMC, and its very rapid release, are unsolved problems. We mention, nonetheless, the possibility of neutron star vibrations, which could transport the energy coherently to the surface, heat the atmosphere mechanically to a hot, pair-producing temperature, and have a characteristic damping time roughly equal to the duration of the impulsive phase.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts held at Toulouse, France, 26–29 November, 1979.     

OSO-8 observations of CaII H and K,MgII h and k,Lα and Lβ above a sunspot

Observations with the French (L.P.S.P.) experiment on board OSO-8 of a sunspot and nearby plage region are described. The behaviour of the emission cores of the Ca II H and K and Mg II h and k resonance lines is very similar and the correspondence in intensity between the four lines persists in all observed features. In contrast, the Lyman lines show little correlation with the other lines. Their emission regions appear broader in the spectroheliograms than the underlying sunspot structure and must not necessarily possess a counterpart in lower layers. From the central intensity of L above the umbra an electron density of 4.3 × 10¹⁰ cm^-3 n _e ^* 2.3 × 10¹¹ cm^-3 at 20 000 K is estimated.Mitteilungen aus dem Kiepenheuer-Institut Nr. 186.Stockholm Observatorium, S-13300 Saltsjöbaden, Sweden.Laboratoire de Physique Stellaire et Planétaire, CNRS, P.O. Box 10, F-91370 Verrières-le-Buisson, France.     

The eruption of active region filaments and its relation to the triggering of a solar flare

The formation and eruption of active region filaments is supposed to be caused by the increase of a concentrated current embedded in the active region background magnetic field of an active region according to the theory of Van Tend and Kuperus (1978).The onset of a filament eruption is due to either changes in the background magnetic field or the increase of the filament current intensity. Both processes can be caused by the emergence of new magnetic flux as well as by the motion of the photospheric footpoints of the magnetic field lines. It is shown that if the background field evolves from a potential field to a nearly force-free field the vertical equilibrium of the current filament is not affected, but large forces are generated along the filament axis. This is identified as the cause of filament activation and the increase in filament turbulence during the flare build-up phase. Depending on the evolution of the background field and the current filament, two different scenarios for flare build-up and filament eruption are distinguished.This work was done while one of the authors (M.K.) was participating in the CECAM workshop on Physics of Solar Flares held at Orsay, France, in June 1979.     

The development of X-ray flare onsets near active region filaments

Skylab X-ray images of the early phases of six active region transient brightenings were compared with simultaneous H images to study the spatial relationships between filaments and the X-ray brightenings. When the X-ray loops were roughly perpendicular to the axes of the H filaments, the filaments did not disappear. X-ray loops which appeared nearly parallel to the filaments were generally associated with the disappearances of those filaments. It is suggested that the perpendicular loops correspond to the class I X-ray flares of Pallavicini et al. (1977) while the parallel loops are the early phases of their class II flares characterized in the decay phases by arcades of large loops with low energy densities. Both kinds of X-ray flares can be associated with impulsive phases.     

Coronal holes,solar wind streams,and geomagnetic activity during the new sunspot cycle

We have extended our previous study of coronal holes, solar wind streams, and geomagnetic disturbances from the declining phase (1973–1975) of sunspot cycle 20 through sunspot minimum (1976) into the rising phase (1977) of cycle 21. Using daily He I 10830 Å spectroheliograms and photospheric magnetograms, we found the following results:

1. As the magnetic field patterns changed, the solar atmosphere evolved from a structure having a few, large, long-lived, low-latitude coronal holes to one having numerous small, short-lived, high-latitude holes (in addition to the polar holes which persisted throughout this 5-year interval).
2. The high-latitude holes recurred with a synodic rotation period of 28–29 days instead of the 27-day period already known to be characteristic of low-latitude holes.
3. During 1976–1977 many coronal holes were intrinsically ‘weak’ in the sense that their average intensities did not differ greatly from the intensity of their surroundings. Such low-contrast holes were rare during 1973–1975.

An updated Bartels display of the occurrence of holes, wind speed, and geomagnetic activity summarizes the evolution of their characteristics and interrelations as the sunspot cycle has progressed. Long-lived, low-latitude holes have become rare but remain terrestrially effective. The more common high-latitude holes are effective only when the Earth lies at a relatively high heliographic latitude in the same solar hemisphere.     

Lower Cambrian patch reefs and associated sediments: southern Labrador, Canada

The complex pattern of biological accretion, internal sedimentation, early lithification, and biological destruction, that characterizes modern reefs and many fossil reefs has been recognized in archaeocyathid-rich patch reefs of Lower Cambrian age in the Forteau Formation, southern Labrador. Patch reefs occur as isolated masses or complex associations of many discrete masses of archaeocyathid-rich limestone and skeletal lime sands, surrounded by well-bedded skeletal limestones and shales. Each reef is composed of many loafshaped mounds stacked on top of one another. The limestone of each mound comprises archaeocyathids and Renalcis or Renalcis-like structures in a matrix of argillaceous lime mud rich in sponge spicules, trilobite and salterellid skeletons. Numerous growth cavities roofed by pendant Renalcis-like organisms and Renalcis are partially to completely filled with geopetal sediment indicating that much of the matrix was deposited as internal sediment. Two stages of diagenetic alteration are recognized: (1) syn-depositional, which affected only the reefs, and (2) post-depositional, which affected both reefs and inter-reef sediments. On the sea floor reef sediments were pervasively cemented and fibrous carbonate was precipitated in intraskeletal and growth cavities. These limestones and cements as well as archaeocyathid skeletons, were subsequently bored by endolithic organisms. Later post-depositional subaerial diagenesis resulted first in dissolution of certain skeletons and precipitation of calcite cement above the water table, followed by extensive precipitation of pore-filling calcite below the water table. These carbonate reefs are similar in structure to the basal pioneer accumulations of much younger lower and middle Palaeozoic reefs. They did not develop into massive ‘ecologic’ reefs because archaeocyathids never developed the necessary large, massive, hemispherical skeletons. This occurrence indicates that reefs developed more or less coincident with, and not long after, the appearance of skeletal metazoans in the Lower Cambrian.     

Silicon monoxide and the 10 μm interstellar feature

Laboratory spectra of SiO particles of 1 m radius show a broad structureless extinction peak at 9.6m. The wavelength dependence of extinction from SiO, an amorphous silicon oxide, provides a good match to that of interstellar dust.     

Mathematical models for petroleum-forming processes: n-paraffins and isoprenoid hydrocarbons

Mathematical models have been developed which simulate both random and nonrandom thermal cracking of branched and straight-chain hydrocarbons. Application of these models to n-paraffins suggests that thermal cracking alone cannot be the dominant mechanism in formation of the n-paraffin distributions present in crude oils. Application to isoprenoid hydrocarbons indicates that nonrandom cracking could be important in producing the isoprenoid distributions found in oils.Results of the mathematical modeling show that methane formation should, as predicted from energy considerations, be kinetically disfavored. It therefore is likely that substantial quantities of methane are produced from saturated hydrocarbons only under thermal conditions more severe than those under which oil is produced.The mathematical models employed are adaptable for other geochemical applications, such as isotope fractionation.