I did confuse ratio and slope in my previous statement. The ratio between the isotopes is assumed to be the same (homogenous) throughout the mix and when plotted makes a slope of 0. In the case of Rb/Sr, the claimed homogenous isotopes would be the 87Sr/86Sr ratio.
From the source mentioned below:
"Since D and Di are isotopes of the same element, they have identical chemical properties*. Minerals may include varying quantities of that element, but all will inherit the same D/Di ratio as the source material. This results in an identical Y-value for the data points representing each mineral (matching the Y-value of the source material)."
This is saying that the ratio of D/Di is homogenous throughout the magma.
The idea of isotopic homogeneity is mentioned later in the article.
"Consider some molten rock in which isotopes and elements are distributed in a reasonably homogeneous manner." and "They did not separate at about the same time from an isotopically homogeneous pool of matter. "
No where in this article is it established that magma is isotopically homogenous. No references are offered that show that this true. The closest we get is "Consider some molten rock in which isotopes ... are distributed in a ... homogenous manner." We are to suppose that magma is homogenous. Where are the facts that show this is true?
It still stands that no one was there to measure the original ratio of isotopes, it must be assumed that they were homogenous.
The rest of the article is of little value, unless it can be shown that magma is isotopically homogenous. You would think that this vital point would deserve at least a quote or reference. As I said before, there is more and more evidence that magma is not homogenous. And I talking of more variation than isotope fractionation can claim.
Considering the source of this article. I have learned when dealing with Talk.Origins you need to adopt an attitude a lot like the lying salesman joke. You know, "How do you know when a salesman is lying?" Answer: "His lips are moving." How do you know when you cannot trust Talk.Origins? When you read it in their web pages.
Allen
----- Original Message -----
From: Hofmann, Jim <jhofmann@Exchange.FULLERTON.EDU>
To: 'Allen & Diane Roy ' <Dianeroy@peoplepc.com>; <evolution@calvin.edu>
Sent: Wednesday, May 03, 2000 10:29 PM
Subject: RE: How is this for an Anti-Evolutionist's use of quotes?
> Calm down and try again. Do you mean slope or ratio? The old "no one was
> there" stuff is not very telling against the arguments in support of
> isochrone methods used in the article you dismiss.
>
> JH
>
>
> -----Original Message-----
> From: Allen & Diane Roy
> To: evolution@calvin.edu
> Sent: 5/3/00 9:49 PM
> Subject: Re: How is this for an Anti-Evolutionist's use of quotes?
>
> The Isochron methods makes the assumption that at time zero, the ratio
> between the measured isotopes is 0 (or, in other word, they are
> homogeneously mixed throughout the magma). This ratio makes a
> horizontal
> line across the chart and this is the reference point from which one
> compares the slope of an isochron line. The problem is, no one was
> there to
> see if the ratio between the isotopes was really zero (i.e. if they were
> homogenous). Thus, this method is no better than any other method. One
> has
> to assume a zero slope at time zero. But, no one knows for sure and
> they
> cannot know for sure. The more I read of the mix of magma, the more I
> read
> of it being non-homogenous in any way.
>
> The original mix of measured isotopes may have been at slope 1 and the
> current reading may be at slope 1.005. The actual time lapse would be
> the
> difference between slope 1 and slope 1.005 (= 0.005) not between slope
> 0
> and 1.005.
>
> All radiometric methods depend upon assumptions which are falsified or
> unknowable.
>
> Allen
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