The subsurface biosphere in the basaltic ocean crust is potentially of major importance in affecting chemical exchange between the ocean and lithosphere. Alteration of the oceanic crust commonly ...yields secondary pyrite that is depleted in 34S relative to igneous sulfides. Although these 34S depleted sulfur isotope ratios may point to signatures of biological fractionation, previous interpretations of sulfur isotope fractionation in altered volcanic rocks have relied on abiotic fractionation processes between intermediate sulfur species formed during basalt alteration. Here, we report results for multiple S-isotope (32S, 33S, 34S) compositions of altered basalts at ODP Site 801 in the western Pacific and provide evidence for microbial sulfate reduction within the volcanic oceanic crust. In-situ ion-microprobe analyses of secondary pyrite in basement rocks show a large range of δ34S values, between −45‰ and 1‰, whereas bulk rock δ34S analyses yield a more restricted range of −15.8 to 0.9‰. These low and variable δ34S values, together with bulk rock S concentrations ranging from 0.02% up to 1.28% are consistent with loss of magmatic primary mono-sulfide and addition of secondary sulfide via microbial sulfate reduction. High-precision multiple sulfur-isotope (32S/33S/34S) analyses suggest that secondary sulfides exhibit mass-dependent equilibrium fractionation relative to seawater sulfate in both δ33S and δ34S values. These relationships are explained by bacterial sulfate reduction proceeding at very low metabolic rates. The determination of the S-isotope composition of bulk altered oceanic crust demonstrates that S-based metabolic activity of subsurface life in oceanic basalt is widespread, and can affect the global S budget at the crust–seawater interface.
In the absence of a firm link between individual meteorites and their asteroidal parent bodies, asteroids are typically characterized only by their light reflection properties, and grouped ...accordingly into classes. On 6 October 2008, a small asteroid was discovered with a flat reflectance spectrum in the 554-995nm wavelength range, and designated 2008 TC3 (refs 4-6). It subsequently hit the Earth. Because it exploded at 37km altitude, no macroscopic fragments were expected to survive. Here we report that a dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95kg. Analysis of one of these meteorites shows it to be an achondrite, a polymict ureilite, anomalous in its class: ultra-fine-grained and porous, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as F class, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.
The Middle Jurassic Barcroft mafic granodiorite and Late Cretaceous, ternary-minimum McAfee Creek Granite are important components of the igneous arc sited along the SW North American margin. ...Bulk-rock analyses of 11 samples of the metaluminous, I-type Barcroft comagmatic suite have an average i18O value of 7.4-0.6% (all values-1+). Four Barcroft specimens average kNd=m3.6-1.8, 87Sr/86Sr=0.707-0.001. The pluton consists of petrochemically gradational, Ca-amphibole-rich gabbro/diorite, granodiorite, metadiorite, and rare alaskite-aplite; for most of the pluton, oxygen isotope exchange of quartz, feldspar(s), biotite, and Ca-amphibole accompanied local deuteric alteration. Eight specimens of slightly peraluminous granitic rocks of the muscovite-bearing McAfee Creek series have an average i18O of 8.6-0.5%. Four McAfee-type samples average kNd=m7.8-1.7, 87Sr/86Sr=0.711-0.004. For both plutons, bulk-rock evidence of exchange with near-surface water is lacking, suggesting ~5-10 km cooling depths. Barcroft minerals exhibit regular oxygen isotopic partitioning from high to low i18O in the sequence quartz>plagioclase>K-feldspar>>amphiboleSbiotite. Along the SE margin of the pluton, quartz and biotite in Lower Cambrian quartzites are higher in i18O, and show slightly larger fractionations than igneous analogues. Exchange with fluids derived from these heated, contact-metamorphosed country rocks increased bulk 18O/16O ratios of Barcroft border rocks (and constituent plagioclase+subsolidus tremolite-actinolite), especially of granitic dikes transecting the wall rocks. Oxygen isotope thermometry for seven Barcroft pluton quartz-amphibole and six quartz-biotite pairs indicate apparent subsolidus temperatures averaging 519-49 degrees C. Quartz-plagioclase pairs from two Barcroft granodiorites yield values of 519 and 515 degrees C. A quartz-biotite pair from a quartzite adjacent to the Barcroft pluton yields an apparent temperature of 511 degrees C, in agreement with estimates based on contact metamorphic parageneses. Except for its SE margin, Barcroft pluton silicates evidently exchanged oxygen isotopes under local deuteric conditions. Compatible with Ca-amphibole thermobarometric analyses, areal distributions for quartz-plagioclase, quartz-amphibole, and quartz-biotite pairs reveal that putative annealing temperatures are lowest in NE-trending axial portions of the Barcroft body, so it simply cooled inwards. Intrusion ~70 million years later by the McAfee Creek Granite had no discernable effect on i18O values of Barcroft minerals and bulk rocks. PUBLICATION ABSTRACT
In the absence of a firm link between individual meteorites and their asteroidal parent bodies, asteroids are typically characterized only by their light reflection properties, and grouped ...accordingly into classes. On 6 October 2008, a small asteroid was discovered with a flat reflectance spectrum in the 554-995 nm wavelength range, and designated 2008 TC(3) (refs 4-6). It subsequently hit the Earth. Because it exploded at 37 km altitude, no macroscopic fragments were expected to survive. Here we report that a dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg. Analysis of one of these meteorites shows it to be an achondrite, a polymict ureilite, anomalous in its class: ultra-fine-grained and porous, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as F class, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.
Ferry and Rumble discuss the formation and destruction of periclase formed in steeply dipping marbles from the Beinn an Dubhaich aureole in Scotland and the Silver Star aureole in Montana.
New and previously published stable isotope (oxygen and hydrogen) and
40
Ar/
39
Ar data are compared from selected outcrops of the Sulu UHP terrain near Qinglongshan (Jiangsu Province, China). These ...rocks exhibit unusually low, heterogeneous δ
18
O and δD values acquired in a Neoproterozoic geothermal system despite undergoing Triassic (220-240 Ma) UHP metamorphism. Incremental heating
40
Ar/
39
Ar analyses of muscovite, biotite, and K-feldspar from metagranite, quartzite, and gneiss (all metamorphosed to the coesite-eclogite facies) yield cooling ages between 190 and 204 Ma. In contrast, phengite
40
Ar/
39
Ar data from eclogite, quartzite, and gneiss contain variable amounts of extraneous argon, consistent with inheritance from a Neoproterozoic protolith. Plots comparing phengite δ
18
O, δD, and
40
Ar/
39
Ar total gas ages from different lithologies within individual Qinglongshan outcrops only meters apart highlight significant inter-outcrop isotopic heterogeneities (ΔD
phengite
= 26‰; Δ
18
O
phengite
= 8.8‰; Δ
40
Ar/
39
Ar
phengite
= 664 Ma); however maximum intra-outcrop isotopic variations between lithologies are limited (ΔD
phengite
= 5‰; Δ
18
O
phengite
= 1.8‰). The oxygen and hydrogen isotopic variations are interpreted to reflect primary isotopic heterogeneities acquired during Neoproterozoic hydrothermal fluid circulation with cold-climate meteoric waters. The limited intra-outcrop isotopic variations suggest that extensive isotopic exchange occurred during UHP metamorphism within discrete outcrops, irrespective of lithology. Likewise, extraneous argon within the phengites reflects inheritance from a Neoproterozoic protolith, and the age variations between outcrops is probably due to differential argon loss during thermal and baric equilibration accompanying differential exhumation following UHP metamorphism. The retention of extraneous argon in phengite is partially controlled by the host rock lithology, inasmuch as as the armoring effects of basaltic eclogite are greater than quartzite or gneiss. Collectively, these data indicate that the length-scale of isotopic exchange is defined by contiguous blocks of coherent rock. Isotopic exchange during UHP metamorphism occurred between different lithologies restricted to discrete blocks within an accretionary prism, but did not communicate with other blocks. This study underscores the closed system (outcrop scale) behavior of isotopic exchange that can occur during continental subduction, collision, and uplift under UHP conditions.
Thermal emission from cold dust grains in giant molecular clouds can be used to probe the physical properties, such as density, temperature and emissivity in star-forming regions. We present the ...Submillimetre Common-User Bolometer Array (SCUBA-2) shared-risk observations at 450 and 850 μm of the Orion A molecular cloud complex taken at the James Clerk Maxwell Telescope (JCMT). Previous studies showed that molecular emission lines can contribute significantly to the measured fluxes in those continuum bands. We use the Heterodyne Array Receiver Programme 12CO J = 3−2 integrated intensity map for Orion A in order to evaluate the molecular line contamination and its effects on the SCUBA-2 maps. With the corrected fluxes, we have obtained a new spectral index α map for the thermal emission of dust in the well-known integral-shaped filament. Furthermore, we compare a sample of 33 sources, selected over the Orion A molecular cloud complex for their high 12CO J = 3−2 line contamination, to 27 previously identified clumps in OMC 4. This allows us to quantify the effect of line contamination on the ratio of 850–450 μm flux densities and how it modifies the deduced spectral index of emissivity β for the dust grains. We also show that at least one Spitzer-identified protostellar core in OMC 5 has a 12CO J = 3−2 contamination level of 16 per cent. Furthermore, we find the strongest contamination level (44 per cent) towards a young star with disc near OMC 2. This work is part of the JCMT Gould Belt Legacy Survey.
Carbon K-edge soft X-ray spectromicroscopy was used to obtain information on the orientation, purity and chemical composition of graphite within natural samples. Highly crystalline natural graphite ...had spectra indistinguishable from highly ordered pyrolytic graphite, but contained a wide variety of internal structures in the 100–500
nm size range. These structures were both physical (domains with differing sheet orientations) and chemical in nature. Graphite sheet orientations could vary from normal to the X-ray beam to nearly perpendicular within a few hundred nanometers. Chemical compositions ranged from pure crystalline material to nearly amorphous carbon. Little evidence for significant addition of O- or N-bearing groups was observed in these samples, although this may have been a result of the extraction techniques used. The range of carbon types found in the environment may provide clues to the types of thermal alteration and source mechanisms behind graphitic kerogen fractions.
The discovery of mass-independent isotope effects observed in Archean rocks, certain classes of meteorites, and atmospheric aerosols has had profound implications to our understanding of ancient and ...present atmospheric sulfur chemistry. We present a new technique that takes advantage of continuous He flow
isotope-ratio-monitoring gas chromatography–mass spectrometry to achieve precise analysis of all four stable sulfur isotopes (
32S,
33S,
34S, and
36S) at nanomole level samples. The technique involves fluorination of sulfide (silver sulfide or pyrite), and separation of product gas by gas chromatography and the removal of mass-131 interference by a liquid-nitrogen ethanol slush at −
110 °C. This technique works with an optimum sample size of 100 to 200 nmol with precision for Δ
33S and Δ
36S at 0.1 and 0.5‰ (2
σ). Samples, as small as tens of nanomole, can be analyzed using this new method. One of the major sources of error in
irm-GCMS is found to be tailing of the major ion beam (
32SF
5
+) onto minor beams (
33SF
5
+ and
36SF
5
+), which results in contraction of the measured
δ
33S and
δ
36S scales. This effect is corrected by measuring a series of reference sulfide samples with mass-dependent sulfur isotope compositions. This methodology increases the spatial resolution of the laser ablation in situ analysis and considerably reduces the analysis time as compared with conventional dual inlet methods.
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