Gas hydrates (clathrates) causing pore-water freshening and oxygen isotope fractionation in deep-water sedimentary sections of terrigenous continental margins

The occurrence of gas hydrates in deep-water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep-sea drilling results. On the Pacific continental slope off Guatemala gas hydrates were brought up for the first time from two holes (497, 498A) drilled during Leg 67 of the DSDP in water depths of 2360 and 5500 m, respectively. The hydrates occur in organic matter-rich Pleistocene to Miocene terrigenous sediments. In the hydrate-bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10–20 m subbottom depth. Pore waters at the bottom of the holes (near 400 m subbottom) have as little as half the chlorinity of seawater (i.e. 9‰). Similar, but less pronounced, trends were observed during previous legs of the DSDP in other hydrate-prone segments of the continental margins where recharge of fresh water from the continent can be excluded (e.g. Leg 11). The crystallization of hydrates, like ice, excludes salt ions from the crystal structure. During burial the dissolved salts are separated from the solids. Subsidence results in a downward motion of the solids (including hydrates) relative to the pore fluids. Thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation. The volume of the latter decreases downhole thus causing downward decreasing salinity (chlorinity). Hydrate formation is responsible for oxygen isotope fractionation with¹⁸O-enrichment in the hydrate explaining increasingly more positive δ¹⁸O values in the pore fluids recovered (after hydrate dissociation) with depth.     

Investigations of an intrusive contact,northwest Nelson,New Zealand—I. Thermal,chronological and isotopic constraints

In the Nelson area of New Zealand, intrusion of the Separation Point Batholith 114 Ma ago caused thermal metamorphism in the adjacent Devonian Rameka Gabbro that allowed radiogenic ⁴⁰Ar to diffuse partially out of hornblendes in the gabbro.Comparison of mineral ages in the batholith (obtained by the K-Ar, ${}^{40}\text{Ar}{}^{39}\text{Ar}$, Rb-Sr, U-Pb and fission track methods) with closure temperature estimates for the various isotopic systems has allowed thermal histories of several samples to be resolved. These curves indicate that, following emplacement, the batholith cooled at an exponentially decreasing rate reaching 100°C by about 75 Ma ago.Oxygen isotope measurements on minerals from the gabbro and granodiorite give calculated equilibrium water compositions of +7 and +9 of δ¹⁸O_SMOW respectively. Measured porosities from samples of the gabbro yield a model permeability of 3 × 10^?17 m¹². These results confirm that heat transfer from the batholith to the gabbro was predominantly by conduction. A numerical heat flow model has been developed for the region, and shows that samples in the gabbro near the contact reached ~600°C. whereas 5 km away from the contact, the samples were only raised ~ 50°C above the ambient temperature.Excess ⁴⁰Ar was identified as causing a significant scatter in the K-Ar ages of hornblendes from the gabbro. Though obscured by this effect, these data suggest a crystallization age of the Rameka Gabbro of 367 Ma, which is consistent with geological evidence.     

Paleomagnetism of samples from the axial zone of the Afar depression

We have studied SmNd systematics in pyroxene and phosphate mineral separates of Angra dos Reis. A pyroxene-phosphate internal isochron age ofT₂ = 4.55 ± 0.04AE is obtained, in excellent agreement with reported Pb-Pb ages.¹⁴²Nd/¹⁴⁴Nd ratios in pyroxene samples are systematically larger than those in phosphates by 6 parts in 10⁵. This variation is tentatively assigned to a radiogenic contribution from extinct¹⁴⁶Sm. Fission xenon components in pyroxene and phosphate separates are characterized by discrete ratios of fission/spallation and evidence is presented for a third ratio in celsian. It is shown that this characteristic is due to a close association of²⁴⁴Pu with the light REE. Computed ratios²⁴⁴Pu/Nd are the same in pyroxene and phosphate separates, but²⁴⁴Pu/²³⁸U and²⁴⁴Pu/²³²Th ratios are not. Taking the fission xenon retention age to be 4.55 AE, we obtain an abundance ratio²⁴⁴Pu/Nd= 1.5 × 10^?4 (or an atomic ratio²⁴⁴Pu/¹⁵⁰Nd= 1.6 × 10^?3) at this time and in the region of the solar system where the Angra dos Reis parent body formed. The exposure age of Angra dos Reis, as obtained by the⁸¹Kr-⁸³Kr method is55.5 ± 1.2m.y. Neutron capture during the 55.5-m.y. exposure to cosmic rays increased the ratio¹⁵⁰Sm/¹⁴⁹Sm in Angra dos Reis by 6 parts in 10⁴.     

40Ar/39Ar age and thermal history of the Kirin chondrite

The Kirin meteorite, a large (2800kg) H5 chondrite, fell in Kirin Province, China in 1976. A sample from each of the two largest fragments (K-1, K-2) yield⁴⁰Ar/³⁹Ar total fusion ages of 3.63 ± 0.02b.y. and 2.78 ± 0.02b.y. respectively.⁴⁰Ar/³⁹Ar age spectra show typical diffusional argon loss profiles. Maximum apparent ages of 4.36 b.y. (K-1) and ～4.0 b.y. (K-2) are interpreted as possible minimum estimates for the age of crystallization of the parent body.The⁴⁰Ar/³⁹Ar ages found for gas released at low temperature are about 2.2 b.y. for K-1 and about 0.5 b.y. for K-2, suggesting that this meteorite may have suffered two discrete collisional events that caused degassing of radiogenic argon. Modelling of possible thermal events in the parent body indicates that samples K-1 and K-2 were at a depth of less than 3 m from the base of an impact melt of a thickness less than 7 m and separated by no more than ～2 m from one another at the time of the heating event about 0.5 b.y. ago. Further, the duration of heating was probably less than a few years.Calculations from³⁸Ar data yield exposure ages for samples K-1 and K-2 of about 5 m.y., similar to that found for many other H chondrites.     

Critical sections in the Western Andes of Central Peru

Summary Seven valley-sections across the Pacific slopes of the Andes provide evidence interpreted as follows. Cretaceous and post-Senonian beds were folded, thrust and eroded before an immense pile of lava and ash about 60 miles across was erupted. A granitic batholith about 30 miles wide cut through them and the edge of the folded sediments. Erosion unroofed this batholith and then more lava and ash erupted whilst normal faulting proceeded with down throw towards the Pacific. Uplift of about 12 000 feet in rather recent time is called for to account for high Andean plateau. In brief folding due to compression ended. Vulcanicity, magmatic emplacement, faulting and vertical movement succeeded.

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Zusammenfassung Die Profile in sieben die Westhänge der Anden querenden Tälern lassen folgenden Sachverhalt erkennen. Ablagerungen der Kreidezeit und der Zeit nach dem Senon wurden gefaltet, überschoben und erodiert, ehe die Eruption einer ungeheuren, über 100 km breiten Masse von Lava und Aschen erfolgte. Ein etwa 50 km breiter granitischer Batholith durchdrang die Vulkanite und den Rand der gefalteten Sedimentgesteine. Die Erosion entblößte diesen Batholithen, und dann wurden wiederum Lava und Aschen gefördert, während Abschiebungen, mit Absenkung des pazifischen Flügels, in Tätigkeit traten. Man muß mit Hebungen von etwa 4 km in ziemlich junger Zeit rechnen, um das Andenhochplateau erklären zu können. Kurz gesagt, die Faltung durch Einengung endete. Sie wurde abgelöst von Vulkanismus, Platznahme plutonischer Massen, Bruchtektonik und Vertikalbewegung.

     

Sterane distribution of solid bitumen pyrolyzates. Changes with biodegradation of crude oil in the Ouachita Mountains,Oklahoma

Solid bitumens (grahamite and impsonite) of southeastern Oklahoma have been shown to originate from near-surface alteration of crude oil (Curiale, 1981; Curiale and Harrison, 1981). Pyrolysis of these solids has been employed to compare the sterane distribution of geographically proximate oils to that of the bitumens. The ratio of rearranged to regular steranes is higher in the pyrolyzates than in the oils, a finding consistent with a bitumen origin due to biodegradation of oil. The remaining presence of steranes, particularly regular steranes, in the bitumens suggests that sterane occlusion may have occurred prior to or during the alteration process, thus removing tetracyclic compounds from the influence of microbial attack. These data suggest that pyrolysis-$\text{GC}\text{MS}$ offers a viable approach to correlation problems involving solid bitumens.     

Dynamics of Blinkers

The relative Doppler and non-thermal velocities of quiet-Sun and active-region blinkers identified in Ov with CDS are calculated. Relative velocities for the corresponding chromospheric plasma below are also determined using the Hei line. Ov blinkers and the chromosphere directly below, have a preference to be more red-shifted than the normal transition region and chromospheric plasma. The ranges of these enhanced velocities, however, are no larger than the typical spread of Doppler velocities in these regions. The anticipated ranges of Doppler velocities of blinkers are 10–15 km s^–1 in the quiet Sun (10–20 km s^–1 in active regions) for Hei and 25–30 km s^–1 in the quiet Sun (20–40 km s^–1 in active regions) for Ov. Blinkers and the chromosphere below also have preferentially larger non-thermal velocities than the typical background chromosphere and transition region. Again the increase in magnitude of these non-thermal velocities is no greater than the typical ranges of non-thermal velocities. The ranges of non-thermal velocities of blinkers in both the quiet Sun and active regions are estimated to be 15–25 km s^–1 in Hei and 30–45 km s^–1 in Ov. There are more blinkers with larger Doppler and non-thermal velocities than would be expected in the whole of the chromosphere and transition region. The recently suggested mechanisms for blinkers are revisited and discussed further in light of the new results.     

Stratigraphic control of hot fluids on anthracitization, Lackawanna synclinorium, Pennsylvania

The Lackawanna synclinorium of Pennsylvania contains abundant coal that was altered to anthracite rank during the Late Paleozoic Alleghanian orogeny. Why did the coal in the synclinorium undergo anthracitization? Two alternatives have been suggested. (1) The region was buried deeply during and/or after the Alleghanian orogeny and thus became hot in response to Earth's geothermal gradient; and (2) hot fluids migrating from the orogen toward the foreland carried heat into the coal and caused anthracitization. If the second hypothesis is correct, the region should contain evidence that hot fluids passed through the coal. Field observations, illite-crystallinity studies and fluid-inclusion analyses indicate that the coal-bearing Pottsville and Llewellyn Formations, and an underlying detachment fault, called the Pottchunk fault (at or slightly below the base of the Pottsville Formation), acted as a regional aquifer for the migration of hot fluids during the Alleghanian orogeny. The presence of quartz veins and the hydrothermal minerals tosudite and pyrophyllite in strata above the Pottchunk fault, and the apparent absence of an illite-crystallinity burial gradient in the underlying strata, suggest that fluids migrated through large pores in coarse-grained sandstones, through abundant fractures that developed in response to Alleghanian deformation, and along the Pottchunk fault. The quartz veins, tosudite and pyrophyllite mineralization do not occur below the Pottchunk fault (except at one locality), suggesting that fluid flow was greater above the fault, perhaps because fracturing accompanying deformation increased permeability of the fault's hanging wall. Fluid inclusions in the Pottsville and Llewellyn Formations indicate fluids achieved a minimum temperature of 270 °C at a depth of 3.1–8.5 km. Heat-flow calculations constrained by fluid-inclusion data show the heat carried by the migrating fluids through the coal-bearing strata was sufficient to elevate the rock temperature to anthracite-grade conditions (250 °C), assuming that the fluid event lasted about 1 year. Thus, deep burial (6–9 km) of the coal-bearing strata in the Lackawanna synclinorium was not required for anthracitization. Anthracitization was likely the result of stratigraphically controlled hot fluid migration through the coal-bearing strata at shallow depths (≤5 km).     

Aurora diaries

Giles Harrison uses the diary of an 18th-century apothecary from Stroud to document appearances of the aurora borealis. From this record and sightings from Europe over the same period, he deduces that aurorae were visible further south some 200 years ago.