The date measured is almost certainly the formation date. My undertanding is that there are gaseous isotopes of daughter products which are used for ageing. When the rock/chondrule melts, it allows the gases to escape and this has the effect of resetting the clock. This is how we know that the rocks from the LPBE are 3.9bn ya. Chondrules, by the same method demonstrate an age in excess of 4.5bn ya.
The likelyhood of many atoms of similar type being in the same region of space is likely high. I've never studied the theories of what happens in a supernova but the energies which synthesise all the elements greater than 32(iron) would, I believe create large numbers of these elements in the same region which then gets dispersed into interstellar space, from which proto stars and planetessimals coalesce (bad spelling). It's fascinating stuff and I'm sure nobody understands it properly. Rob McC --- "E.P. Grondine" <[EMAIL PROTECTED]> wrote: > Hi Sterling, > > I did not post my reply to you to the list, so they > won't know what the extracts you cited came from - > if > you have a copy of that message please post it - > > The problem still remains what caused sufficient > number of atoms of the same type to be in the same > place at the same time to produce the crystals and > glasses observed. > > If you have the gravity of a source proto-planet > differentiating the components in an immiscible > melt, > then that problem is solved. I can't see any > differentiating mechanism for an "instellar" melt, > regardless of energy source. > > No doubt the dating techniques are accurate. And no > doubt the elements were frozen in time in the > chondrule glasses and crystals. But is what is being > dated, the elements' formation date, or the > chondrule's formation date? > > good hunting, > Ed > > --- "Sterling K. Webb" > <[EMAIL PROTECTED]> > wrote: > > > Hi, Ed, > > > > > ...but does this mean that the > > > formation of the condrules and > > > their matrices date to > > > that time? > > > > The "formation date" is when all the > > various materials can no longer be mixed > > with other material, be wetted, dried, migrate, > > be modified, interact chemically, be altered, > > or otherwise be messed with. The tiny packet > > of the chondrule is melted, fused, sealed -- > > ain't nothing going nowhere. From that point, > > the isotopes decay without any material being > > allowed to escape. The uranium turns slowly > > to a peculiar isotope of lead with a long halflife > > (billions of years). You count the uranium atoms; > > you count the odd lead atoms; calculate how > > long it took for some of the original uranium to > > that number of lead atoms. Since nothing can > > enter or leave the chondrule, it's pretty accurate > > (very accurate). > > > > > No doubt the constituent components of our solar > > > system date to that time, but does this mean > that > > the > > > formation of the condrules and their matrices > date > > to > > > that time? > > > > A solid rock, a melted lump (like a > chondrule), > > a piece of glass (like a tektite) are all good > > dating > > candidates because atoms can't go waltzing in > > and out like it was a border bordello... Once a > > rock or any lump shows signs of being altered > > by the environment, partial melting or heating, > > aqueous modification, alarm flags go up. > > Sometimes, it's a good thing: a tektite's K/Ar > > date turns out to be when it either impacted or > was > > impacted, but it's Rb/Sr shows (I think) its > > original > > formation date (curiously, about 480 mya). Many > > wouldn't agree with that, but they then have to > > explain > > why its "original" Rb/Sr ratio is radically > > different from > > ANY other rock, on Earth or off. (Mostly that > > detail's > > ignored.) At any rate, it's different from its > K/Ar > > date > > (each tektite type has its own K/Ar date). > > > > > If the dates are right, the problem becomes "how > > did > > > that many identical atoms get together in one > > place so > > > that the chondrules could form?" > > > > Not sure what you mean here. The chondrules > > have many elements in many compounds, just like > > the meteorites, many of the same ones. They were > > gas and dust before being flash melted, typical of > > > the inner solar nebula -- the usual crap. Lots of > > argument > > about what melted them, and the details, of > course, > > solar flare, electric currents in the disc, > magnetic > > effects, shock waves? > > > > Your theory of pressure release isn't > > necessarily > > dead. What if a sudden short heating event (solar > > flare for > > example) melts them radiatively and heats the gas > > around > > that region. After the chondrule is flash fried, > the > > hot gas > > (no longer being heated) expands rapidly and the > > heat and > > pressure around the chondrule drops as the gas > > expands and > > cools, letting them cool quickly by radiating > their > > heat > > away quickly (?). I should shut up; that's > > dangerously > > close to being chemistry... > > > > > > Sterling > > > --------------------------------------------------------------- > > My favorite two books on the formation of the > solar > > system are John S. Lewis "The Physics and > Chemistry > > of > > the Solar System." The 2 Ed. is $75, $35 used. (I > > was > > lucky; I caught it when it was remaindered out of > > print > > and bought it for $8. The other is Stuart Ross > > Taylor, > > "Solar System Evolution" (1992) also very > expensive. > > > > I bought a copy when 1st ed. was remaindered out > of > > print for $4. However, the 2nd Ed. (1999), much > > bigger, > > is available used for $20: > > > http://www.bookcloseouts.com/default.asp?R=0521641306B > > > --------------------------------------------------------------- > > ----- Original Message ----- > > From: "E.P. Grondine" <[EMAIL PROTECTED]> > > To: "Sterling K. Webb" > > <[EMAIL PROTECTED]> > > Sent: Monday, October 23, 2006 3:25 PM > > Subject: Re: [meteorite-list] Chondrule formation > > mechanism (Info Please) > > > > > > > Hi Sterling, > > > > > > If the dates are right, the problem becomes "how > > did > > > that many identical atoms get together in one > > place so > > > that the chondrules could form?" > > > > > > Since this question has no good answer, one is > > forced > > > to look at the dating and exactly what it is > that > > that > > > dating measured. > > > > > > No doubt the constituent components of our solar > > > system date to that time, but does this mean > that > > the > > > formation of the condrules and their matrices > date > > to > > > that time? > > > > > > I still want to look at that Krasnojarsk - the > > > mechanism for the olivine inclusions has to be > > > interesting, whatever it was - its the best > excuse > > I > > > can come up with - what do you think about that > > one? > > > > > > Thanks, > > > Ed > > > > === message truncated === __________________________________________________ Do You Yahoo!? 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