Understanding Positive $Eu$ and $Sr$ Anomalies
Positive Europium ($Eu$) and Strontium ($Sr$) anomalies in primitive mantle-normalized multielement patterns signify enrichments of these elements relative to primitive mantle values and adjacent elements.
Explaining Positive $Eu$ Anomaly
A positive $Eu$ anomaly usually occurs when $Eu$ is retained in the melt or incorporated into accumulating minerals that are not plagioclase feldspar. This can happen if plagioclase crystallization is minimal or if specific minerals hosting $Eu$ accumulate.
Explaining Positive $Sr$ Anomaly
A positive $Sr$ anomaly points towards the accumulation of minerals that readily incorporate Strontium ($Sr$), often by substituting for Calcium ($Ca$). While plagioclase is a major host, other minerals can also contribute significantly.
Mineral Analysis for Anomalies
Assessing each mineral's potential to cause both positive $Eu$ and $Sr$ anomalies:
- Orthoclase: Primarily hosts Potassium ($K$) and Rubidium ($Rb$). It plays a minor role in generating substantial positive $Eu$ or $Sr$ anomalies.
- Apatite: This phosphate mineral is known to incorporate Rare Earth Elements (REEs), including $Eu$, and Strontium ($Sr$) via substitution for Calcium ($Ca$). The accumulation of apatite can thus increase the rock's $Eu$ and $Sr$ content.
- Orthopyroxene: As a silicate mineral, it is generally incompatible with large cations like $Sr$ and REEs. It is unlikely to be responsible for positive anomalies in both elements.
- Olivine: This magnesium-iron silicate is typically incompatible with trace elements such as $Eu$ and $Sr$. It does not usually cause positive anomalies for these elements.
Conclusion: Role of Apatite Accumulation
The accumulation of Apatite provides the most logical explanation for the presence of both positive $Eu$ and positive $Sr$ anomalies. Apatite's capacity to incorporate significant amounts of both $Eu$ and $Sr$ into its crystal lattice, relative to other minerals during igneous processes, results in their enrichment on normalized patterns.
Important Questions from Igneous Petrology (Trace Elements/Melting/Evolution)
- Choose the explanation(s) for negative Eu anomalies in upper crustal rocks like granite and granodiorite.
- A source rock undergoes melting. Assuming batch melting, $5\%$ partial melting and bulk distribution coefficient of $0.045$, the enrichment factor ($C_L/C_o$) of Rb in the melt will be ________. (Round off to $2$ decimal places)
- A basaltic magma has an initial nickel concentration of 300 ppm. Olivine crystallizes from this magma by equilibrium crystallization (Case I) or fractional crystallization (Case II). Then, the absolute value of the difference between the nickel concentrations of the liquids remaining after 25% crystallization in these two cases is ________.
(Use $K_{D,Ni}$ olivine/melt = 10). - The modal abundance in an ultramafic rock and the partition coefficients of lutetium (Lu) in clinopyroxene, orthopyroxene, olivine and plagioclase are tabulated below. The bulk distribution coefficient of lutetium ($D_{Lu}$) in the ultramafic rock is ________.
Mineral Modal abundance (%) Partition coefficient Clinopyroxene 45 0.506 Orthopyroxene 40 0.42 Olivine 10 0.045 Plagioclase 05 0.019 A hypothetical garnet peridotite composed of 60% olivine, 25% orthopyroxene, 10% clinopyroxene and 5% garnet undergoes 10% batch melting described by $\frac{C_L}{C_o} = \frac{1}{F+D-F*D}$ where F is degree of melting and D is bulk partition coefficient. The ratio of Ce in the melt to the original rock will be ___________ (round off to 2 decimal places).
(The $K_D$ values of Ce for olivine, orthopyroxene, clinopyroxene and garnet are 0.001, 0.003, 0.10 and 0.02, respectively)
