Stead, and three co-authors, reported in 2017 that the systematics of the rare earth element (REE) abundances in olivine are poorly understood and this undermines their usefulness in deciphering petrogenetic processes. Here we report a novel REE dataset for olivine spanning a wide range in Mg number (Mg# 1–59) from the Skaergaard Layered Intrusion. Olivine's La and Lu concentrations vary from 0.68 to 56 ppb and 66 to 571 ppb, respectively. Apparent olivine/ melt partition coefficients for the heavy to middle REE define near-symmetrical parabolas, consistent with lattice strain theory, indicating that these elements are structurally bound within the olivine lattice. The parabolas define peaks with variable optimal radii (ro). This suggests that the Fe-content of olivine exerts a strong influence on REE partitioning, as fayalitic olivines display more open parabolas (higher apparent Young's modulus, E) than forsteritic olivines. The greater site elasticity is manifest as higher REE concentrations in the fayalitic samples.
The observation that E and ro are not constant across a range of Mg# has implications for predictive models for olivine REE. The CI chondrite-normalized olivine REE patterns show smooth trends from the heavy to the middle REE, with greater variation among the light REE (LREE). Variation and enrichment of the LREE are ubiquitous within published data for natural olivine, yet are not predicted by experimental results. This study also presents two-dimensional REE maps illustrating the behavior of the REE in olivine in unprecedented detail. Mapping of magnesian olivine shows core to rim increase in Al, LREE, and Eu and the development of a positive Eu anomaly in olivine, resulting from solid-state, likely diffusive re-equilibration with adjacent plagioclase.
The authors proposed that secondary redistribution is the principal cause of elevated LREE patterns in natural olivine. The results from the present investigation show that from a petrological point of view, MREE to HREE systematics of olivine hold promise for study of olivine petrogenesis and that the LREE in olivine may be further
developed to understand diffusion time scales and cooling rates in magmatic systems.
The authors concluded that rare earth element concentrations were determined for olivine and clinopyroxene from seven samples from the Skaergaard Layered Intrusion, over an olivine Mg# range of 1 to 59. Olivine La and Lu concentrations vary from 0.68 to 56 ppb and 66 to 571 ppb, respectively, with highest REE concentrations observed in the most fayalitic olivines. There are systematic variations between olivine Mg#, apparent partition coefficients and crystal lattice strain model parameters.
Olivine/melt partitioning parabolas define peaks with ro from 0.70 to 0.82 Å. Significantly, there is systematic variation in the tightness of the parabolas, whereby the rigidity of the lattice site (E) is apparently dominantly influenced by the Mg# of the olivine. The fayalitic olivine display more relaxed parabolas and therefore greater accommodation of the MREE and LREE than forsteritic olivine.
To a first order, the authors confirmed that the REE in olivine behave according to the crystal lattice strain theory and that experimental Dol/melt published by Sun and Liang (2013, 2014), Beattie (1994), and Evans et al. (2008) are appropriate for modelling REE behavior during mantle melting. However, in detail, widely used predictive models for olivine REE Dol/melt (e.g. Sun and Liang, 2013) with constant crystal lattice parameters are likely in need of refinement, even over narrower ranges of Mg# and temperatures. This conclusion arises from the insight gained from the studied olivines, which are much more Fe-rich than mantle olivine or olivine crystallizing from typical mantle melts. They show systematic variation of E with Mg#, which is significant to interpretations of REE patterns for this phase in general.
An important discovery of this study is that REE concentrations in olivine were affected by subsolidus, solid-state re-equilibration with an adjacent more enriched phase, in this case plagioclase. This was documented with 2-D elemental mapping and has caused core to rim increases in Al, LREE, and Eu in magnesian olivine. The discovery that LREE enrichment is accompanied by a positive Eu anomaly points to uptake of REE from neighboring plagioclase. The distribution of LREE concentration profiles and the coupling with Al, suggest that uptake may have occurred via diffusion. In the case of La, the original concentration contrast between plagioclase and olivine was in excess of 7,000 ppm. The extent of concentration contrast apparently influenced the extent to which original REE systematics in olivine were subject to redistribution.
In natural olivine, solid-state redistribution of the REE, and especially the LREE, should be considered a common process (see also Sun and Liang, 2014).The authors proposed that secondary redistribution is a major reason for the elevated LREE Dol/melt values reported throughout the literature for natural olivines. The nature of the process likely involves volume diffusion but may be aided by other parameters such as the micro-structure of adjacent crystals. Regardless of process, the results of this study argue against the analysis of olivine crystals that neighbor higher REE phases (such as clinopyroxene) in holocrystalline rocks and favor targeting grain interiors of optically pure crystals that are free of fractures and inclusions for pheno- or antecrysts in volcanic samples.
Although REE analysis in olivine remains technically challenging, the possibility of systematic diffusive redistribution of the LREE into olivine has potential for future use of olivine to illuminate interesting geological processes, including particularly study of cooling rates in magmatic systems.
Supplementary data to this article can be found online at: https://www.sciencedirect.com/science/article/pii/S0009254117305843?via%3Dihub
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