Typhoon Lee

Toward an Astrophysical Theory of Chondrites

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.

Aluminum-26 in the early solar system - Fossil or fuel

The isotopic composition of Mg was measured in different phases of a Ca-Al rich inclusion in the Allende meteorite. Large excesses of /sup 26/Mg of up to 10% were found. These excesses correlate strictly with the /sup 27/Al//sup 24/Mg for four coexisting phases with distinctive chemical compositions. Models of in situ decay of /sup 26/Al within the solar system and of mixing of interstellar dust grains containing fossil /sup 26/Al with normal solar system material are presented. The observed correlation provides definitive evidence for the presence of /sup 26/Al in the early solar system. This requires either injection of freshly synthesized nucleosynthetic material into the solar system immediately before condensation and planet formation, or local production within the solar system by intense activity of the early Sun. Planets promptly produced from material with the inferred /sup 26/Al//sup 27/Al would melt within approx.3 x 10/sup 5/ yr.

The Origin of Chondrules and Refractory Inclusions in Chondritic Meteorites

Examples of calcium-aluminum-rich inclusions (CAIs) surrounded by thick chondrule mantles have been found in chondritic meteorites and cast doubt on the conventional belief that CAIs and chondrules possessed different spacetime origins in the primitive solar nebula. We study specific processes by which such objects, and the more common ordinary CAIs and chondrules, might have formed by flare heating of primitive rocks interior to the inner edge of a gaseous accretion disk that has been truncated by magnetized funnel flow onto the central proto-Sun. Motivated by the appearance of the chains of Herbig-Haro knots that define collimated optical jets from many young stellar objects (YSOs), we adopt the model of a fluctuating X-wind, where the inner edge of the solar nebula undergoes periodic radial excursions on a timescale of ~30 yr, perhaps in response to protosolar magnetic cycles. Flares induced by the stressing of magnetic fields threading both the star and the inner edge of the fluctuating disk melt or partially melt solids in the transition zone between the base of the funnel flow and the reconnection ring, and in the reconnection ring itself. The rock melts stick when they collide at low velocities. Surface tension pulls the melt aggregate into a quasi-spherical core/mantle structure, where the core consists mainly of refractories and the mantle mainly of moderate volatiles. Orbital drift of rocks past the inner edge of the disk or infall of large objects from the funnel flow replaces the steady loss of material by the plasma drag of the coronal gas that corotates with the stellar magnetosphere. In quasi-steady state, agglomeration of molten or heat-softened rocks leads to a differential size-distribution in radius R proportional to R-3e, where tL ~ 20 yr is the drift time of an object of fiducial radius L ≡ 1 cm and t is the time since the last inward excursion of the base of the funnel flow and X-wind. Thus, during the ~30 yr interval between successive flushing of the reconnection ring, flash-heated and irradiated rocks have a chance to grow to millimeter and centimeter sizes. The evaporation of the moderately volatile mantles above large refractory cores, or the dissolving of small refractory cores inside thick ferromagnesian mantles before launch, plus extended heating in the X-wind produce the CAIs or chondrules that end up at planetary distances in the parent bodies of chondritic meteorites.

The calibration of D[Sr/Ca]versus sea surface temperature relationship for Porites corals

A press for making parts from material comprising a die plate having first and second passages extending completely therethrough. One end of each of the passages defines a die. The die plate has first and second passage sections which intersect the first and second passages, respectively. The passage sections open at different locations on the periphery of the die plate. Appropriate tooling including the above-mentioned dies are provided for forming first and second parts from the material. The first and second parts are separately removed from the die plate by moving them through the first and second passage sections, respectively.

Protostellar Cosmic Rays and Extinct Radioactivities in Meteorites

Calcium-aluminum-rich inclusions (CAIs) and chondrules of chondritic meteorites may originate with the melting of dustballs launched by a magnetically driven bipolar outflow from the inner edge of the primitive solar nebula. Bombardment by protostellar cosmic rays may make the rock precursors of CAIs and chondrules radioactive, producing radionuclides found in meteorites that are difficult to obtain with other mechanisms. Reasonable scalings from the observed hard X-rays for the cosmic-ray protons released by flares in young stellar objects yield the correct amounts of 41Ca,53Mn, and 138La inferred for meteorites, but proton- and α-induced transformations underproduce 26Al by a factor of about 20. The missing 26Al may be synthesized by 3He nuclei accelerated in impulsive flares reacting primarily with 24Mg, an abundant isotope in the target precursor rocks. The mechanism allows a simple explanation for the very different ratios of 26Al/27Al inferred for normal CAIs, CAIs with fractionated and unidentified nuclear (FUN) anomalies, and chondrules. The overproduction of 41Ca by analogous 3He reactions and the case of 60Fe inferred for eucritic meteorites require special interpretations in this picture.

Extinct Radioactivities and Protosolar Cosmic Rays: Self-Shielding and Light Elements

We study the eUects 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 and The mantles have a R min R max . 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 —uence consistent with the product of their residence time in the reconnection ring and the —ux of solar cosmic rays obtained by a scaling of impulsive —ares from the hard X-rays observed from low-mass protostars. For in the 50 km regime and in the few centimeter regime, which corresponds to the R min R max range of sizes of observed CAIs in micrometeorites and chondrites, we recover approximately the canonical values quoted for the ratios 26Al/27Al, 53Mn/55Mn, and 41Ca/40Ca in CV3 meteorites. Moreover, the excess 138La (denoted as 138La*) produced by proton bombardment of 138Ba lies within the CAI range obtained in the experiments of Shen et al. When we include fragmentation reactions that produce 10Be from the impact of protons, alphas, and 3He on the 16O that is bound up in rocks, we further obtain a level of 10Be/9Be 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 10Be/9Be is particularly difficult to produce by any other astrophysical mechanism. Thus, the 10Be 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.

Dust: a diagnostic of the hydrologic cycle during the last glacial maximum

Dust concentrations in ice of the last glacial maximum (LGM) are high in ice cores from Greenland and Antarctica. The magnitude of the enhancements can be explained if the strength of the hydrologic cycle during the LGM was about half of that at present. This notion is consistent with a large decrease (5°C) in ocean temperature during the LGM, as recently deduced from measurements of strontium and calcium in corals.

A double island arc between Taiwan and Luzon: consequence of ridge subduction

Analysis of geomorphological, geochronological, geochemical and geophysical features in the segment of the Taiwan-Luzon Arc between Taiwan and Luzon (the Bashi Segment) allows the recognition of a double arc structure. The two volcanic chains are separated by 50 km just north of Luzon (18°N), and converge near 20°N. Islets in the western chain are older and largely composed of volcanic rocks of Miocene to Pliocene age. They all show low relief, lateritic platforms and wave-cut terraces, and are covered by massive recrystallized limestone. In contrast, all active volcanoes in this segment of the Taiwan-Luzon Arc belong to the eastern chain, where most islets are Quaternary in age. The volcanoes have well-developed cone shapes, and well-preserved deposits of near-vent facies. Magmas of the eastern chain have higher KSi, (La)n,(La/Yb)n, and lowereNd than their counterparts at the same latitude in the western chain. Therefore, the magmas erupted in the eastern chain were derived from more enriched mantle sources than the magmas erupted in the western chain. Moreover, the available seismological data seem to suggest an abrupt increase of the dip angle from 30° at 18°N to 80° at 20°N. Thus, the double arc structure is located in the region where the Benioff zone suddenly changes. In analogy with the Lesser Antilles Arc, we propose a geodynamic model in which the double arc in the Bashi Strait is the tectonic manifestation of the subduction of the aseismic Scarborough Seamount Chain, the extinct mid-ocean ridge of the South China Sea. Before that ridge reached the Manila Trench, the western chain was the volcanic front. When the ridge reached the subduction zone at 5–4 Ma, its buoyancy temporarily interrupted the subduction thus causing a time gap in magmatic activity. Furthermore, this ridge-arc collision was probably also responsible for regional uplift causing extensive sub-aerial weathering and erosion as well as massive reef formation in the western chain. When subduction started again, the dip angle became shallower in response to the extra buoyancy of the downgoing ridge. If the depth of magma generation remained constant, the shallower dip angle would have naturally led to an eastward shift of the volcanic front thus producing the younger eastern chain. Moreover, we speculate that the abrupt change of the dip angle may have torn the downgoing slab thus allowing more enriched continental lithospheric material to invade the mantle wedge from the northwest, thus imprinting a geochemically more enriched signature on the magmas of the eastern chain.

The partition of Be between soil and water

Abstract An extensive and systematic investigation of Be partition between solid and water was carried out using laboratory batch experiments with radioactive 7Be tracer. We found that: (1) Be is strongly held by the solid particles in natural environments under neutral conditions; (2) man-made δ-MnO2 and in situ weathering products of andesite have the highest Kd (> 106) while most mud, silt and clay minerals have Kd on the order of 105; (3) Kd is high under neutral or alkaline conditions but decreases rapidly by four orders of magnitude between pH 6 and 2; (4) Kd decreases with increasing concentration of suspended particles; (5) dissolved organic matter in Taiwan Strait surface water does not seem to greatly affect the value of adsorption Kd; (6) adsorption of Be onto soil is a two-stage process and the second stage is ∼ 1000 times slower than the first; (7) the reaction of surface ion exchange seems to proceed at a faster rate in river water than in seawater, probably because of the high ionic strength of the latter; (8) the activation barrier of sorption reaction for Be seems to be ∼ 10 kcal.mol−1 for river mud; and (9) the adsorption of Be onto soil seems to be a reversible process.

The Irradiation Origin of Beryllium Radioisotopes and Other Short-lived Radionuclides

Two explanations exist for the short-lived radionuclides (T1/2 ≤ 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, 7Be 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 7Be. Within model uncertainties associated mainly with nuclear cross sections, we obtain agreement with the experimental value. Moreover, if 7Be and 10Be 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 ~ 2 × 1010 cm-2 s-1. The X-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, 26Al, 36Cl, and 53Mn 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 3He abundance). The decoupling of the 26Al and 10Be observed in some rare CAIs receives a quantitative explanation when rare gradual events (shallow energy spectra and low 3He abundance) are considered. The yields of 41Ca are compatible with an initial solar system value inferred from the measured initial 41Ca/40Ca 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 60Fe in the solar accretion disk does not necessarily mean that other short-lived radionuclides have a stellar origin.