A granite shows a strongly negative Eu anomaly (Eu/Eu* = 0.3) on its chondrite-normalized REE pattern. What process is most likely responsible?
ACrystallization of garnet from the granitic magma
BFractionation of plagioclase, which selectively incorporates Eu2+ (substituting for Ca2+ in the crystal structure) while excluding trivalent REE, depleting Eu in the residual melt relative to its neighboring elements Sm and Gd
CWeathering of the granite at Earth's surface
DContamination by seawater
Europium is unique among the REE because it can exist as Eu2+ under reducing conditions typical of silicate melts. Eu2+ has a similar ionic radius to Ca2+ and substitutes readily into the plagioclase structure. When plagioclase crystallizes and is removed (or was fractionated from a parent magma), Eu is selectively depleted from the melt while the other REE follow normal trivalent partitioning. The resulting negative Eu anomaly is the hallmark of intracrustal differentiation involving plagioclase.
Question 2 True / False
MORB (mid-ocean ridge basalt) characteristically shows a flat to slightly LREE-depleted chondrite-normalized REE pattern, while ocean island basalt (OIB) shows strong LREE enrichment.
TTrue
FFalse
Answer: True
MORB forms by moderate-degree (~10-20%) melting of depleted upper mantle (spinel peridotite) at shallow depth. Without residual garnet, HREE are not preferentially retained, producing a flat pattern. The depleted source contributes to slight LREE depletion. OIB forms by smaller-degree melting of enriched mantle sources, often with residual garnet at depth (retaining HREE), producing steep LREE-enriched patterns. The contrast in REE slope between MORB and OIB is a first-order diagnostic of source composition and melting conditions.
Question 3 Short Answer
Explain what a positive Ce anomaly in marine chemical sediments indicates about the redox conditions during deposition.
Think about your answer, then reveal below.
Model answer: Cerium can be oxidized from Ce3+ to Ce4+ in oxygenated seawater. Ce4+ is highly insoluble and is scavenged onto manganese oxide surfaces and ferromanganese nodules, depleting dissolved Ce in seawater (producing the characteristic negative Ce anomaly in seawater). When these Mn-Fe oxides are incorporated into sediments, they carry excess Ce, producing a positive Ce anomaly. Alternatively, positive Ce anomalies in sediments can indicate incorporation of terrestrial material (which has no Ce anomaly relative to seawater patterns). The Ce anomaly in marine sediments and waters is therefore a proxy for ocean oxygenation: stronger negative anomalies in seawater indicate more oxidizing conditions driving Ce4+ scavenging.
The Ce anomaly is the only REE anomaly controlled by redox chemistry in the marine environment, making it a unique tracer of ocean oxygenation through Earth history.