The recent fall of the relatively unaltered CM chondrite Maribo provides a unique opportunity to study the early stages of aqueous alteration on the CM chondrite parent body. We show using ...transmission electron microscopy of a matrix FIB-section from Maribo that this meteorite mainly appears to consist of tochilinite-cronstedtite intergrowths (TCIs), but also contains regions of amorphous or nanocrystalline silicates, anhydrous silicates and FeNi metal aggregates with thin iron oxide rims, suggesting that it experienced aqueous alteration to a relatively small degree. A comparison of Maribo with increasingly altered CM chondrites such as Jbilet Winselwan and Bells shows that during progressive aqueous alteration (1) the TCIs are replaced by coarser sulfides and increasingly Mg-rich serpentine, and (2) the abundance of 15N-rich hotspots increases, whereas the magnitude of their 15N enrichment decreases. We observe that the overall N isotope variability related to aqueous alteration is an order of magnitude lower than the variability observed between different chondrite groups. We suggest these high order differences are the result of heterogeneous accretion of insoluble or soluble organic carriers of 15N to the different chondrite parent bodies. D/H ratios of matrices from Maribo, Jbilet Winselwan and Bells increase with progressive aqueous alteration, a trend that is opposite to expectations of mixing between D-poor water and D-rich organic matter. We argue that this behaviour cannot be related to Fe oxidation or serpentinization reactions and subsequent loss of D-poor H2 gas. We offer an alternative hypothesis and suggest that CM chondrites experienced two-stage aqueous alteration. During the first stage occurring at relatively low temperature, mixing of increasing amounts of D-poor water with D-rich organic matter results in a decrease of D/H ratio with increasing degree of alteration. During the second stage of alteration occurring at relatively high temperature (T < 300 °C), decomposition of TCIs in CMs of petrologic type <2.7 releases gaseous D-poor water that results in increase of the D/H ratio of the CM matrices. Finally, we report on changes in the organic structure of Maribo, Jbilet Winselwan and Bells using Carbon-K and Nitrogen-K edge electron energy loss spectroscopy. The organic matter initially has higher aromatic/aliphatic ratios (e.g., Maribo) and lower abundances of ketone and carboxyl functional groups, which we suggest are the result of chemical degradation of double bonded carbon from oxidation during hydrothermal alteration. Consequently, we propose that the organic matter of the CM chondrite Paris, for which lower aromatic/aliphatic ratios have been observed, may have been different from Maribo, perhaps reflecting the early accretion of Paris relative to Maribo.
Some very large (>0.1μm) presolar grains are sampled in meteorites. We reconsider the lifetime of very large grains (VLGs) in the interstellar medium focusing on interstellar shattering caused by ...turbulence-induced large velocity dispersions. This path has never been noted as a dominant mechanism of destruction. We show that, if interstellar shattering is the main mechanism of destruction of VLGs, their lifetime is estimated to be ≳108yr; in particular, very large SiC grains can survive for ~1Gyr. The lifetimes obtained for VLGs are comparable to the longest residence time derived for some presolar grains based on the cosmic-ray exposure time. However, most presolar SiC grains show residence times significantly shorter than 1Gyr, which may indicate that there is a more efficient mechanism than shattering in destroying VLGs, or that VLGs have larger velocity dispersions than 10kms−1. We also argue that the enhanced lifetime of SiC relative to graphite can be the reason why we find SiC among μm-sized presolar grains, while the abundance of SiC in the normal interstellar grains is much lower than graphite.
•A new estimate of the lifetime of very large grains (>0.1μm) in the interstellar medium is given.•The estimated lifetimes (~1Gyr) for SiC very large grains is compared with the residence times derived for presolar grains.•Reasons why most presolar SiC grains have shorter residence times than estimated above are discussed.
Two explanations exist for the short-lived radionuclides (T sub(1/2) , 5 Myr) present in the solar system when the calcium-aluminum-rich inclusions (CAIs) first formed. They originated either from ...the ejecta of a supernova or by the in situ irradiation of nebular dust by energetic particles. With a half-life of only 53 days, 7 super(B)e is then the key discriminant, since it can be made only by irradiation. Using the same irradiation model developed earlier by our group, we calculate the yield of super(7)Be. Within model uncertainties associated mainly with nuclear cross sections, we obtain agreement with the experimental value. Moreover, if super(7)Be and super(10)Be have the same origin, the irradiation time must be short (a few to tens of years), and the proton flux must be of order F 6 2 x 10 super(10) cm super(-2) s super(-1). The X-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, super(26)Al, super(36)Cl, and super(53)Mn are also generated at the measured levels within model uncertainties, provided that irradiation occurs under conditions reminiscent of solar impulsive events (steep energy spectra and high 3 super(H)e abundance). The decoupling of the super(26)Al and super(10)Be observed in some rare CAIs receives a quantitative explanation when rare gradual events (shallow energy spectra and low 3 super(H)e abundance) are considered. The yields of super(41)Ca are compatible with an initial solar system value inferred from the measured initial super(41)Ca/ super(40)Ca ratio and an estimate of the thermal metamorphism time (from Young et al.), alleviating the need for two-layer proto-CAIs. Finally, we show that the presence of supernova-produced super(60)Fe in the solar accretion disk does not necessarily mean that other short-lived radionuclides have a stellar origin.
Toward an Astrophysical Theory of Chondrites Shu, Frank H.; Shang, Hsien; Lee, Typhoon
Science (American Association for the Advancement of Science),
03/1996, Letnik:
271, Številka:
5255
Journal Article
Recenzirano
The chondrules, calcium-aluminum-rich inclusions (CAIs), and rims in chondritic meteorites could be formed when solid bodies are lifted by the aerodynamic drag of a magnetocentrifugally driven wind ...out of the relative cool of a shaded disk close to the star into the heat of direct sunlight. For reasonable self-consistent parameters of the bipolar outflow, the base and peak temperatures reached by solid bodies resemble those needed to melt CAIs and chondrules. The process also yields a natural sorting mechanism that explains the size distribution of CAIs and chondrules, as well as their fine-grained and coarse-grained rims. After reentry at great distances from the original launch radius, the CAIs, chondrules, and their rims would be compacted with the ambient nebular dust comprising the matrices, forming the observed chondritic bodies.
We study the effects of self-shielding in the X-wind model of protosolar cosmic-ray irradiation of early solar-system rocks. We adopt a two-component picture of protoCAIs consisting of cores with the ...elemental abundances of type B1 CAIs (calcium-aluminum-rich inclusions) and mantles of less refractory material. The cores have a power-law distribution of sizes between R{sub min} and R{sub max}. The mantles have a uniform thickness, whose value is chosen to bring the total inventory of elements at least as refractory as sulfur to cosmic abundances for the entire population of protoCAIs. Each object is irradiated with a fluence consistent with the product of their residence time in the reconnection ring and the flux of solar cosmic rays obtained by a scaling of impulsive flares from the hard X-rays observed from low-mass protostars. For R{sub min} in the 50 {mu}m regime and R{sub max} in the few centimeter regime, which corresponds to the range of sizes of observed CAIs in micrometeorites and chondrites, we recover approximately the canonical values quoted for the ratios {sup 26}Al/{sup 27}Al, {sup 53}Mn/{sup 55}Mn, and {sup 41}Ca/{sup 40}Ca in CV3 meteorites. Moreover, the excess {sup 138}La (denoted as {sup 138}La*) produced by proton bombardment of {sup 138}Ba lies within the CAI range obtained in the experiments of Shen et al. When we include fragmentation reactions that produce {sup 10}Be from the impact of protons, alphas, and {sup 3}He on the {sup 16}O that is bound up in rocks, we further obtain a level of {sup 10}Be/{sup 9}Be that agrees approximately with the report of McKeegan et al. for a CAI from the Allende meteorite. Similar calculations for the expected anomalies in the stable isotopes of lithium show rough consistency with the measured values and further support our interpretation. The value for {sup 10}Be/{sup 9}Be is particularly difficult to produce by any other astrophysical mechanism. Thus, the {sup 10}Be discovery greatly strengthens the case for an origin in early solar-system irradiation, rather than external stellar seeding, for the shortest-lived radionuclides inferred from CAIs in chondritic meteorites.
X-rays and Fluctuating X-Winds from Protostars Shu, Frank H.; Shang, Hsien; Glassgold, Alfred E. ...
Science (American Association for the Advancement of Science),
09/1997, Letnik:
277, Številka:
5331
Journal Article
Recenzirano
Protostars emit more x-rays, hard and soft, than young sunlike stars in more advanced stages of formation. The x-ray emission becomes harder and stronger during flares. The excess x-rays may arise as ...a result of the time-dependent interaction of an accretion disk with the magnetosphere of the central star. Flares produced by such fluctuations have important implications for the x-wind model of protostellar jets, for the flash-heating of the chondrules found in chondritic meteorites, and for the production of short-lived radioactivities through the bombardment of primitive rocks by solar cosmic rays.
Taiwan is located at the collision boundary between the Philippine Sea Plate and the Asian Continental Plate and is one of the most active orogenic belts in the world. Fluids sampled from 9 ...sub-aerial mud volcanoes distributed along two major geological structures in southwestern Taiwan, the Chishan fault and the Gutingkeng anticline, were analyzed to evaluate possible sources of water and the degree of fluid-sediment interaction at depth in an accretionary prism. Overall, the Taiwanese mud volcano fluids are characterized by high Cl contents, up to 347 mM, suggesting a marine origin from actively de-watering sedimentary pore waters along major structures on land. The fluids obtained from the Gutingkeng anticline, as well as from the Coastal Plain area, show high Cl, Na, K, Ca, Mg and NH
4, but low SO
4 and B concentrations. In contrast, the Chishan fault fluids are much less saline (1/4 seawater value), but show much heavier O isotope compositions (δ
18O=5.1–6.5 ‰). A simplified scenario of mixing between sedimentary pore fluids and waters affected by clay dehydration released at depth can explain several crucial observations including heavy O isotopes, radiogenic Sr contents (
87Sr/
86Sr=0.71136–0.71283), and relatively low salinities in the Chishan fluids. Gases isolated from the mud volcanoes are predominantly CH
4 and CO
2, where the CH
4–C isotopic compositions show a thermogenic component of δ
13C=−38 ‰. These results demonstrate that active mud volcano de-watering in Taiwan is a direct product of intense sediment accretion and plate collision in the region.
Taking advantage of the availability of a continuous sea surface temperature (SST) record at Kenting, southern Taiwan, we have carried out a calibration of
DSr/Ca-SST (
D: distribution coefficient) ...relationships for
Porites lobata and
P. lutea. Between 22°C ∼ 28°C, the best fitting linear relationships for the two species agree within their respective errors with a maximum deviation less than 0.3°C. Our calibration overlaps with the only published calibration for
Porites (
deVilliers et al., 1994) but is somewhat less temperature sensitive. The maximum temperature differences for the two calibrations between 21°C and 27°C are ± 1.2°C. The
D-SST relationship calculated using the data of
Beck et al. (1992, 1994) and their unpublished seawater data lies between these two calibrations. Our calibration removes some discrepancies previously described in the studies of
McCulloch et al. (1994a) on the Great Barrier Reef and of
Min et al. (1995) on New Caledonia. We found little growth rate effect on the calibration for
P. lutea between 18 mm/yr and 23 mm/yr. We have also monitored the seawater Sr/Ca for the entire 1993. The observed variation of 0.033 mmol/mol can cause a temperature artifact of 0.7°C and is thus a dominant error in using this new paleo-thermometer. To carry out this experiment, we have developed a microsurgical technique that can be used to sample corals at better than 0.15 mm resolution. The analytical precision should ultimately allow SST reconstruction of better than 0.2°C. Furthermore, the insensitivity of the calibration against growth rate variation and species differences suggests a promising future for this new thermometer, in contrast to the pessimism of
deVilliers et al. (1995). However, inter-laboratory differences and the question of seawater Sr/Ca variation need to be addressed first. We recommend a single SST calibration for the Sr/Ca thermometer on coral
Porites: Sr/Ca
N = 10.286–0.0514 × SST (N: normalized to Hawaii seawater Sr/Ca = 8.510 mmol/mol) or
DSr/Ca = 1.2077 – 0.006011 × SST.
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![The calibration of D[Sr/Ca]...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0016-7037_1872-9533/small "The calibration of D[Sr/Ca]versus sea surface temperature relationship for Porites corals")