Joined: Sep. 2006
OK AFDave, Here comes the summary....
I'm going summarize "How crystallised olivine, originating from a homogeneous source, that contains Rb and Sr constituents can be tested using the Rb/Sr whole rock Isochron method and result in a data set forming a linear relation."
First let's find out what Olivine really is. I think the 72nd Edition of my Handbook of Chemistry and Physics has some mineral tables. I won that book in freshman chemistry with the highest grade in the class. Chapter 4 - Page 4-150 - Physical Constants of Minerals.
Olivine has a chemical formula of (Mg,Fe)SiO4. It must be a polymetalic mineral with an SiO4 crystal backbone.
The X-Ray Crystallographic Data of Minerals on pg. 4-157 lists all the different chemically pure crystaline minerals, but the geologic mineral olivine is made up of numerous different crystaline minerals. Starting on pg 4-167 we have the following crystals that ALL are classified as Olivine since they ALL have an (xx)SiO4 chemical make-up.
Lime Olivine Ca2SiO4
Nickel Olivine Ni2SiO4
Cobalt Olivine Co2SiO4
A chemically mixed homogenous melt that contains, say, Mg Fe and Ni (and Rb and Sr of course) will solidify with a crystal structure that is not only uneven in crystal size but also crystal distribution. I won't go into the mechanics of this right now. It is easier to show you a pertinent example of this. Review this advertisement for an elemental analysis machine. The pictures on page 2 clearly indicate
Since the crystal distribution is somewhat randomized, and the Rb and Sr atoms are substitued in different quantities depending on if the crystal is Forsterite, Fayelite, or Nickel Olivine then we can clearly see how even a whole rock sample can give various Rb/Sr ratios if tested. There is no part of that 4x3cm sample that has the same crystal distribution as any other part (unless you gerrymander the sample like congressional districts, which geologists don't do). Also, I feel confident in stating that another 4x3cm sample will have a different crystal distribution and compisition that would give a different Rb/Sr ratio if tested.
|XGT-5000 analysis of a 4x3 cm² section of kimberlite quickly allowed the rock’s mineral distribution to be visualised. The rock contains abundant crystals of olivine (Mg,Fe,Ni)2SiO4 and one zoned, partially altered crystal of garnet. The garnet crystal is immediately identified by its alteration rim comprised of potassium rich mica. High potassium content also shows the locations of mica crystals within the matrix.|
The olivine crystals are black in the potassium and calcium images but have various shades in the iron and nickel images. These variations indicate the remarkable extent to which the compositions of these elements vary from crystal to crystal. In the Fe image, the olivine grains are seen to have thin Fe-rich rims. Notice also the additional information on physical structure provided by the transmission x-ray imaging.
There's my summary. Pick it apart if you can OR drop your statements about mixing. I think the above summary is enough to counteract Arndts and Overns argument that
I don't think they, or you, were looking critically on how crystal formation is actually done.
|What is needed but missing in the whole rock isochron is a mechanism to establish initial homogeneity, and then to extract heterogeneous samples. The mineral crystals do the job in an elegant way. Each type accepts a different level of contamination of the parent isotope, chemically determined. One cannot rationally extend this process back to the whole rock. It has been tried, but there is a fallacy .|
We could actually use the analysis machine to identify and extract the seperate minerals and accomplish a mineral Isochron analysis too. Neat!!!
p.s. AFDave, Just say you were mistaken about the whole rock thingy and start arguing about radioactive decay.