Isotope 4 Flashcards
Radiogenic isotopes as tracers
• Isotope tracer studies use only initial daughter isotope ratios (D0)
• Radiogenic isotopes rarely used in isolation;
□ But have distinct advantages over element concentration data:
1. Daughter isotope ratios are NOT fractionated during melting.
2. Daughter isotope ratios are NOT fractionated during closed-system fractional crystallisation (FC)
3. Parent/daughter ratios ARE fractionated during melting and FC
Initial isotope ratios statements
Initial isotope ratios allow certain key statements to be made about igneous rocks:
- If a suite of igneous rocks has no variation in D0 composition this indicates they came from a homogenous source and +/- closed system fractional crystallisation.
- If D0 is variable, then mixing of components must have occurred (either a heterogeneous source and/or open system FC).
Inner Earth
• Rock samples are rare
• Only come up in kimberlites– bias
o Smaller amounts from deeper parts
• Earth is isotopically heterogenous – how do we find D-0
o Look at meteors
o Look in nebulas – proplids – each will have its own isotopic composition – to find Earth’s look at meteorites
o 4567 billion years old – oldest material
o Rhb is much more volatile than strontium – ratio was fractionated during early days of solar system – blown from solar system
The Nd-Sr Isotope Diagram
Enriched = in incompatible elements Depleted = in incompatible elements
o Mantle
o High Sm/Nd
o Low Rb/Sr
Crust
o Low Sm/Nd
o High Rb/Sr
Nd Isotopic evolution and fractionation of Earth
Nd143/144
Mantle = depleted residue compared to CHUR
Crust = enriched compared to CHUR
o The higher the Sm/Nd ratio the steeper the slope
o In terrestrial rocks the departure of 143Nd/144Nd away from the CHUR evolution line is quite small, because there is limited fractionation of Sm from Nd during melting.
o Therefore we describe the deviation away from CHUR in parts per 10000, termed epsilon units (eNd).
Sr Isotopic evolution and fractionation of Earth
o BABI – what we think Earth started as
o Partial melting event – leads to melt and residue
o Partial Melting processes – led to fractionation of isotopes in Earth
Mantle = depleted Rb/Sr - below CHUR Crust = enriched Rb/Sr - above CHUR
Isotopic variation and mixing processes:
o Isotope ratios unaffected by melting processes (and/or closed system fractional crystallisation)
o Thus, initial isotope variations (D0) must reflect mixing processes….
o …Including…simple magma-mixing (mixing of two melts), assimilation (solid-melt mixing), melting of a heterogeneous source (solid-solid mixing) or post emplacement alteration/interaction with water.
o Any type of mixing between two components (A+B) can be described by a simple mass balance equation (as for atomic mass calculation!):
o Used for any isotope ratio, but ensure that you use the concentration of the daughter element where required
(ie. Sr conc with 87Sr/86Sr and Nd conc with 143Nd/144Nd).
Mixing: Radiogenic Isotope mixtures
Mixing of materials with different isotopic ratios and elemental concentrations produces predictable arrays.
• These general types of mixing patterns hold for both element concentrations and isotope and trace element ratios.
• Binary mixing will always produce straight lines on element vs element plots and isochron plots
o The two axes have the same denominator so that it is essentially a parent vs daughter element-element plot
• Isotope-isotope plots a curved mixing line is usually produced as the axis are different (different isotopes)
Isochrons and mixing lines or errorchrons
o Differentiate the two – measure different isotope systems – if valid age they will be in error of each other – if not they will be just mixing age
o If it is mixing it has to be mixing on every plot – crystallization the trace elements will not
o If the ages of isotopes are not similar then it’s not an isochron but erorrchrons,
o It represents a mixing line
