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Uniformitarians believe that the elements and their respective isotopes originated from stellar nucleosynthetic processes.
These nuclear processes are usually preferred over isotopic fractionation to explain isotopic anomalies, whenever isotopes of an element show a pattern that is inconsistent with mass-dependent isotope fractionation.
each element’s isotopes being fractionated during separate episodes).
Furthermore, magnesium-isotope anomalies within inclusions of the Allende meteorite show a linear relationship between Al is invoked.
Strontium isotope measurements are therefore routinely corrected by measuring the (fractionated) Sr) using the appropriate mass dependence of fractionation.
This process is inherently incapable of distinguishing between natural and mass spectrometer generated fractionation.
However, the trends in the calcium and magnesium anomalies are the opposite of what would result from mass dependent isotope fractionation.
This prompts the rather cumbersome suggestion that a complex process of vaporization and condensation had been responsible (i.e.
Nd is concentrated in melt, while Sm remains in solid. High Sm/Nd rocks contain more 143Nd Low Sm/Nd rocks contain less 143Nd NOTE: Sm parent will be enriched in “depleted” sources (i.e.
When mixed with variable amounts of terrestrial nonradiogenic strontium, these remnants generate mixing lines indistinguishable from age-indicative Rb-Sr isochrons.
Such mixtures also produce elevated Sr as the y-axis of the ‘isochron’.
Because igneous rocks are so heterogeneous, different mineral phases will have different Rb/Sr ratios, even though they have the same crystallization age and the same 87Sr/86Sr initial.
MANTLE 87Sr/86Sr = 0.702 87Rb-87Sr isochrons A schematic Rb-Sr isochron measured measured when you crystallize a rock, you will always have some Sr present Sample with lower [Rb] Sample with higher [Rb] Bushveld granite Rb-Sr isochron Ifx=(87Rb/86Sr)m Andy=(87Sr/86Sr)m We have y=b mx Where interceptb=(87Sr/86Sr)i And slope m=(eλt-1) More than just an age tool - tracking (87Sr/86Sr)i through time BABI - Basaltic Achondrite Best Initial = Bulk Earth, undifferentiated Rb-Sr isochron From meteorites 87Sr/86Sr ratios of igneous rocks: MORB0.7025 Continents0.7119 Ocean Islands More than just an age tool - tracking (87Sr/86Sr)i through time Average continental crust continuing continental growth enriched early continental differentiation BABI Ocean islands MORB depleted continuing upper mantle depletion A rock’s (87Sr/86Sr)i value call tell you how enriched or depleted its mantle source was. (87Sr/86Sr)i = 0.7020 at 2Ga means a depleted source How would you explain a (87Sr/86Sr)i value of 0.728 at 1.4Ga? Shasta lavas span a wide range of Sr isotopic chemistries As crystals form, Rb enriched in melt, eventually can get ultra-enriched (87Sr/86Sr) Crystals form in magma chamber, Rb stays in melt Or magma melts host rock, which has high 87Sr/86Sr Or magma chambers with different histories mix prior to eruption Seawater (87Sr/86Sr) through time Controls on Seawater Sr Isotopic composition Questions: Why is the river Sr isotope value the highest?