Isotopes

Major Element Behavior

• Major elements controls the mineral phases that form

• The tendency for a major element to go into a phase depends on the overall chemistry of the system and what phases are competing for that element (P, T, phase diagrams)

• The concentration of a major element in a phase is usually buffered by the system, so that it varies little in a phase as the system composition changes

Trace Element Behavior

• Trace element concentrations in the Henry;s law region of concentration, so their activity varies in the direct relation to their concentration in the system

Major Element Behavior

• Major elements controls the mineral phases that form

• The tendency for a major element to go into a phase depends on the overall chemistry of the system and what phases are competing for that element (P, T, phase diagrams)

• The concentration of a major element in a phase is usually buffered by the system, so that it varies little in a phase as the system composition changes

Trace Element Behavior

• Trace element concentrations in the Henry;s law region of concentration, so their activity varies in the direct relation to their concentration in the system

Trace elements during mantle melting

Compositional differences can reflect:

• Heterogenous source rocks

  • Chemically and/ or mineralogically

• Different degrees of partial melting

• Varying conditions under which the melting takes place (depth= P, H2O)

Stable and Radioactive Isotopes

• Stable isotopes are those that remain indefinitely

• Radioactive isotopes decay to other nuclides

• The rate of decay is constant, and not affected by P,T,X…

• Parent nuclide= radiative nuclide that decays

• Daughter nuclide(s) are the radiogenic atomic products

  • they may be stable but also can be radioactive and decay again at different rate (U-series)

What creates the observed differences in isotopic ratios in rock and minerals?

• mass fraction (stable isotopes)- paleoclimate

• compatibility to different reservoirs- magmatic processes; source signature (Sr-Rb, Sm-Nd)

• Time-dating (Sr-Rb, Sm-Nd, U-Th-Pb)

Mass Fractionation (as for stable isotopes)

• Generally most effective for light element isotopes where the differences in mass between isotopes is greatest

  E.g H, He, C, O, S

• But also new techniques measure Mg,Fe,Si,V

• If any mass fractionation does take place, the light isotope always fractionates, preferably into the phase with weaker bonding, and is generally favored in the vapor over the liquid and in the liquid over the solid

δ 18O/16O (or written as just δ 18O) application on Paleoclimate

Evaporation preferably takes lighter oxygen isotope (16O); condensation preferably takes heavier oxygen isotope

Daughters produced in varying proportions resulting from previous event of chemical
fractionation

40K → 40Ar by radioactive decay
Basalt → rhyolite by FX (a chemical fractionation process)
Rhyolite has more K than basalt
40K → more 40Ar over time in rhyolite than in basalt
40Ar/39Ar ratio will be different in each

Time

The longer 40K → 40Ar decay takes place, the greater the difference between the basalt and rhyolite will be

Radioactive Decay

→ age of a sample (t) if we know:

D the amount of the daughter nuclide produced
N0 the amount of the original parent nuclide
N the amount of remaining nuclide
y the decay constant for the system in question

Isotopic Systematics

• Heavy Isotopic ratio values are determined by the parent/daughter ratios in the original solid source

• Magmatic processes (FC, Melting) DO NOT change ratios

• The time over which this “closed system” evolves

• Mixing of sources or melts will alter the ratios as will assimilation

Sr-Rb System

• Rb⁸⁷→Sr⁸⁷+ a beta particle (y = 1.42 x 10-11 a-1)

• Half life is 48.8 Ga

• Rb behaves like K → micas and alkali feldspar

  • commonly highly incompatible

• Sr behaves like Ca → plagioclase and apatite (but not clinopyroxene)

  • Commonly moderately incompatible

• Sr⁸⁸:Sr⁸⁷:Sr⁸⁶:Sr⁸⁴ ave. sample= 10:0.7:1:0.07

• Sr⁸⁶ is a stable isotope, and not created by breakdown of any other parent

Isochron Technique

Requires 3 or more cogenetic samples with a range of Rb/Sr

Could be:
• 3 cogenetic rocks derived from a single source by partial melting, FX, etc• 3 coexisting minerals with different K/Ca (~ Rb/Sr) ratios in a single rock

Isochron technique produces 2 valuable things:

1. The age of the rocks (from the slope = yt)
2. (⁸⁷Sr/⁸⁶Sr)o = the initial value of ⁸⁷Sr/⁸⁶Sr

Isotopic Systematics

• Heavy Isotopic ratio values are determined by the parent/daughter ratios in the original solid source

• AND

• The time over which this “closed system” evolves

• Mixing of sources or melts will alter the ratios as will assimilation

• Magmatic processes (FC, Melting) DO NOT change ratios

Using magmas as “windows” to the mantle

Spider diagram for oceanic basalts

The Sm-Nd System

l Both Sm and Nd are LREE
F Incompatible elements fractionate → melts
F Nd has lower Z → larger → liquids > does
Sm

Geochemical Reservoirs

• Conceptual sources of melts/magmas that have distinct, trace element and isotopic compositions

• They appear to be “end-member” types of compositions that = developed as the mantle evolved throughout Earth history

• Incompatible trace elements became enriched or depleted from various melting and/or recycling events THUS the isotopic compositions evolved through time

Geochemical Reservoirs Examples

The U-Pb-Th System

• Very complex system.

  • 3 radioactive isotopes of U: ²³⁴U, ²³⁵U, ²³⁸U

  • 3 radiogenic isotopes of Pb: ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb
    - Only 204Pb is strictly non-radiogenic

• U, Th, and Pb are incompatible elements, & concentrate in early melts

• Isotopic composition of Pb in rocks =…
  

The U-Pb-Th System