Trace Element Geochemistry Questions and Answers

Q: A chondrite normalised REE pattern of quartzo-feldspathic gneiss shows a sharp positive anomaly of Eu, which indicate presence of :

A sharp positive Eu anomaly indicates plagioclase in the sample.Under reducing magmatic conditions europium occurs partly as Eu2+, which is about the same size as Ca2+ and substitutes for it in plagioclase feldspar. Plagioclase therefore takes in much more Eu than the neighbouring rare earths, so a rock rich in plagioclase (or formed from plagioclase accumulation) shows a positive Eu anomaly on a chondrite-normalised plot.Quartz and sillimanite take in almost no rare earth elements, and clinopyroxene does not single out europium in this way.

Q: Which one of the following lunar materials shows positive europium anomaly ?

The lunar material with a positive europium anomaly is anorthosite.Under the reducing conditions of the Moon most europium is divalent, Eu2+, which substitutes readily for calcium in plagioclase while the other rare earths, all trivalent, do not. The lunar highland anorthosites, which floated as plagioclase on the early magma ocean, are therefore enriched in Eu relative to its neighbours.The complementary melts left behind are depleted in Eu. KREEP, the residual liquid rich in potassium, rare earths and phosphorus, and the mare basalts, including the Apollo 11 high-K basalts and quartz tholeiites, all show negative europium anomalies.

Q: This trace element is mostly concentrated in the crust and substitute for a major element in feldspar, muscovite, biotite and sodic plagioclase.

The trace element is Rubidium.Rubidium is a large-ion lithophile element with an ionic radius close to that of potassium, for which it substitutes in potassium-bearing minerals: K-feldspar, muscovite and biotite. Because it is incompatible in mantle minerals it is strongly enriched in the continental crust, which is why Rb-Sr dating works so well on granites and micas.Nickel is concentrated in the mantle and in olivine, neodymium is a rare earth element that does not substitute for a major element in micas and feldspar, and calcium is itself a major element, not a trace element.

Q: Consider the following statements :Statement-1 : The ratio of Ni : Mg is highest in early formed crystals like olivine and shows a steady decline in late formed minerals.Statement-2 : The nickel ion has essentially the same radius as magnesium.Which of the following is correct with respect to the above statements ?

The answer is Both statements are true and statement-1 is not the correct explanation of statement-2.Statement-1 is true: nickel enters olivine readily, so the Ni : Mg ratio is highest in the earliest crystals and falls steadily in later minerals and residual liquids. Statement-2 is also true: the Ni2+ ion (about 0.69 Å) is very close in size to Mg2+ (about 0.72 Å), so it substitutes for magnesium.The causal link runs the other way: the similar radius (statement-2), together with nickel's stronger preference for the olivine site, explains the early concentration of nickel (statement-1). Statement-1 does not explain statement-2, so option A, which claims it does, is wrong, and the options calling either statement false are wrong too.

Q: What is a concentration of Ni in an olivine crystal, in equilibrium with a melt having 150 ppm Ni and \({}^{Ni}D_{01-liq.} = 12\)

The olivine holds about 0.18 wt% Ni.The partition coefficient is the ratio of the concentration in the crystal to that in the melt, so:\[ \begin{aligned} C_{ol} &= D \times C_{liq} = 12 \times 150 \\ &= 1800\ \text{ppm} \\ &= 1800 / 10000 = 0.18\ \text{wt\%} \end{aligned} \]One weight per cent is 10,000 ppm. 18 wt% comes from dividing by 100 instead of 10,000, and 12.5 and 1.25 come from dividing 150 by 12 instead of multiplying, which would put nickel in the melt rather than in the olivine that strongly prefers it.

Q: Which of the following is True for the compatable elements ?

For compatible elements, the partition coefficient D > 1 and they are preferentially retained in the solid.The partition coefficient is the ratio of an element's concentration in a mineral to that in the coexisting melt, \(D = C_{\text{solid}}/C_{\text{liquid}}\). A compatible element, such as Ni and Cr in olivine and pyroxene, enters the crystal structure readily, so D exceeds 1 and the element stays in the residual solid during melting or is removed early during crystallisation.D < 1 with enrichment in the liquid describes incompatible elements, such as K, Rb and the light rare earths. The mixed options are contradictory: D > 1 cannot go with preference for the liquid, nor D < 1 with preference for the solid.

A chondrite normalised REE pattern of quartzo-feldspathic gneiss shows a sharp positive anomaly of Eu, which indicate presence of :

A.Plagioclase in the sample
B.Quartz in the sample
C.Clinopyroxene in the sample
D.Sillimanite in the sample

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

A sharp positive Eu anomaly indicates plagioclase in the sample.

Under reducing magmatic conditions europium occurs partly as Eu²⁺, which is about the same size as Ca²⁺ and substitutes for it in plagioclase feldspar. Plagioclase therefore takes in much more Eu than the neighbouring rare earths, so a rock rich in plagioclase (or formed from plagioclase accumulation) shows a positive Eu anomaly on a chondrite-normalised plot.

Quartz and sillimanite take in almost no rare earth elements, and clinopyroxene does not single out europium in this way.

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Which one of the following lunar materials shows positive europium anomaly ?

A.KREEP
B.Anorthosite
C.Apollo 11 Hi-K Basalt
D.Apollo 11 Quartz Tholeiite

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

The lunar material with a positive europium anomaly is anorthosite.

Under the reducing conditions of the Moon most europium is divalent, Eu²⁺, which substitutes readily for calcium in plagioclase while the other rare earths, all trivalent, do not. The lunar highland anorthosites, which floated as plagioclase on the early magma ocean, are therefore enriched in Eu relative to its neighbours.

The complementary melts left behind are depleted in Eu. KREEP, the residual liquid rich in potassium, rare earths and phosphorus, and the mare basalts, including the Apollo 11 high-K basalts and quartz tholeiites, all show negative europium anomalies.

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This trace element is mostly concentrated in the crust and substitute for a major element in feldspar, muscovite, biotite and sodic plagioclase.

A.Nickel
B.Neodymium
C.Rubidium
D.Calcium

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: C

The trace element is Rubidium.

Rubidium is a large-ion lithophile element with an ionic radius close to that of potassium, for which it substitutes in potassium-bearing minerals: K-feldspar, muscovite and biotite. Because it is incompatible in mantle minerals it is strongly enriched in the continental crust, which is why Rb-Sr dating works so well on granites and micas.

Nickel is concentrated in the mantle and in olivine, neodymium is a rare earth element that does not substitute for a major element in micas and feldspar, and calcium is itself a major element, not a trace element.

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Consider the following statements :

Statement-1 : The ratio of Ni : Mg is highest in early formed crystals like olivine and shows a steady decline in late formed minerals.
Statement-2 : The nickel ion has essentially the same radius as magnesium.

Which of the following is correct with respect to the above statements ?

A.Both statement-1 and statement-2 are true and statement-1 is the correct explanation of statement-2
B.Both statement-1 and statement-2 are true and statement-1 is not the correct explaination of statement-2
C.Statement-1 is true but statement-2 is false
D.Statement-1 is false but statement-2 is true

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: B

The answer is Both statements are true and statement-1 is not the correct explanation of statement-2.

Statement-1 is true: nickel enters olivine readily, so the Ni : Mg ratio is highest in the earliest crystals and falls steadily in later minerals and residual liquids. Statement-2 is also true: the Ni²⁺ ion (about 0.69 Å) is very close in size to Mg²⁺ (about 0.72 Å), so it substitutes for magnesium.

The causal link runs the other way: the similar radius (statement-2), together with nickel's stronger preference for the olivine site, explains the early concentration of nickel (statement-1). Statement-1 does not explain statement-2, so option A, which claims it does, is wrong, and the options calling either statement false are wrong too.

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What is a concentration of Ni in an olivine crystal, in equilibrium with a melt having 150 ppm Ni and \({}^{Ni}D_{01-liq.} = 12\)

A.0.18 wt%
B.12.5 wt%
C.18 wt%
D.1.25 wt%

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

The olivine holds about 0.18 wt% Ni.

The partition coefficient is the ratio of the concentration in the crystal to that in the melt, so:

\[ \begin{aligned} C_{ol} &= D \times C_{liq} = 12 \times 150 \\ &= 1800\ \text{ppm} \\ &= 1800 / 10000 = 0.18\ \text{wt\%} \end{aligned} \]

One weight per cent is 10,000 ppm. 18 wt% comes from dividing by 100 instead of 10,000, and 12.5 and 1.25 come from dividing 150 by 12 instead of multiplying, which would put nickel in the melt rather than in the olivine that strongly prefers it.

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Which of the following is True for the compatable elements ?

A.The value of partition coefficient, D > 1 and they are preferentially retained in liquid
B.The value of partition coefficient D > 1 and they are preferentially retained in solid
C.The value of partition coefficient D < 1 and they are preferentially retained in solid
D.The value of partition coefficient D < 1 and they are preferentially retained in liquid.

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: B

For compatible elements, the partition coefficient D > 1 and they are preferentially retained in the solid.

The partition coefficient is the ratio of an element's concentration in a mineral to that in the coexisting melt, \(D = C_{\text{solid}}/C_{\text{liquid}}\). A compatible element, such as Ni and Cr in olivine and pyroxene, enters the crystal structure readily, so D exceeds 1 and the element stays in the residual solid during melting or is removed early during crystallisation.

D < 1 with enrichment in the liquid describes incompatible elements, such as K, Rb and the light rare earths. The mixed options are contradictory: D > 1 cannot go with preference for the liquid, nor D < 1 with preference for the solid.

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