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Lunar & Planetary Science Conference / Chondrule Formation
Peter Abrahams wrote:
A model for formation of chondrules; as planetesimals move through the
nebula, they form a 'bow shock' that could increase pressure and heat,
to facilitate accretion of chondrules. This shock layer would be near
the planetesimal's surface, and consist of both vaporized incoming
material and evaporated planetesimal material, all confined by the gas
pressure from 'above'. If accurate, the sizes and quantity of chondrules
should decrease with distance from the sun. The percent volume of
chondrules found in volatile rich CI chondrites (probably formed far
from sun), compared to percent volume in volatile poor ordinary
chondrites (possibly formed closer to sun) might reinforce this idea.
(Hood)
Hello, List Members!
There is an article by L. HOOD in Meteoritics 33-1, 1998, pp. 97-107:
Thermal processing of chondrule precursors in planetesimal bow shocks
Here’s an excerpt from the introduction:
As reviewed previously, earlier work strongly suggests that chondrules
formed in the protoplanetary nebula via RAPID MELTING AND
RESOLIDIFICATION OF CONDENSED AGGREGATES (Taylor et al., 1983; Grossman
et al., 1988; Hewins et al., 1996). A similar statement may be made
about Type B and C Ca-Al-rich inclusions (CAls) during an earlier
high-temperature nebula phase (MacPherson et al., 1988). In the case of
chondrules, the PREEXISTING NEBULA was apparently RELATIVELY COOL (<650
K) and the HEATING OF PRECURSORS WAS RAPID enough (minutes or less) to
explain the retention of volatiles and FeS as chondrule constituents.
The PREEXISTING NEBULA was apparently DUSTY enough (>1-10 mm-sized
particles per m^3) to account for the number of collisions between
plastic chondrules (Gooding and Keil, 1981) and the high inferred O
fugacity of the ambient nebula (Wood, 1985; Kring, 1988). Smaller-scale
dust is also indicated by the PRESENCE OF DUSTY RIMS ON MANY CHONDRULES
(e.g., Metzler et al., 1992).
The HEATING EVENTS that formed chondrules were apparently LOCALIZED
rather than nebula-wide judging by the relatively rapid chondrule
cooling rates (about 10^3 K/h) inferred from textural and compositional
data (Hewins and Radomsky, 1990; Alexander and Wang, 1997). On the other
hand, cooling rates of about 10^3 K/h are still much slower than that
due to free radiation to space (about 10^6 K/h), which suggests the
persistence of hot surroundings. Based on petrologic evidence for the
accretion of hot material onto cold chondrules, the spatial scale of at
least some heating events may have been <10 km (Kring, 1991). Finally,
many chondrules have been thermally processed repeatedly and contain
recycled fragments of previous generations of chondrules (Alexander,
1994; Kring, 1988, 1991). MULTIPLE HEATING EVENTS are therefore
indicated. POSSIBLE HEAT SOURCES FOR CHONDRULE FORMATION INCLUDE
ELECTROMAGNETIC RADIATION, GAS-DUST ENERGY TRANSFER, AND ENERGETIC
ELECTRON AND ION BOMBARDMENT. It has been found experimentally that the
textures of certain opaque inclusions in chondrules are consistent with
heating by visible and infrared radiation (Eisenhour et al., 1994).
However, it is possible that a combination of heating from other sources
(e.g., gas-dust energy transfer) and radiation from surrounding heated
gas and dust could also produce these textures.
There is some evidence that the EFFICIENCY OF CHONDRULE FORMATION MAY
HAVE DECREASED WITH INCREASING RADIAL DISTANCE FROM THE SUN. As seen in
Table 1, the volume percentage of chondrules in major chondrite groups
ranges from zero for the volatile-rich Cl chondrites to 60-80% for the
volatile-poor ordinary chondrites. Current experimental and chemical
evidence indicates that most elemental fractionations in chondrites
predated the chondrule formation process; specifically, CHONDRULE
PRECURSORS APPARENTLY EXPERIENCED A PERIOD OF THERMAL PROCESSING AND
CHEMICAL FRACTIONATION DURING AN EARLIER HIGH-TEMPERATURE NEBULA PHASE
(Grossman, 1996) ...
Regards, Bernd