Natural garnets are robust silicate minerals stable over large ranges of temperature and pressure that can provide useful geochemical constraints on petrogenetic conditions. The purpose of this study is to determine how the size of cation sites controls rare earth elements (REE) incorporation in mantle eclogitic garnets. Major and lanthanide element data in a suite of mantle-derived eclogite garnets were combined with new single-crystal structure refinements (SREFs) to examine the effects of major-element chemistry and site dimension on the incorporation of REE into the garnet structure. Several distinct trends are apparent. Pyrope-rich samples similar to mantle lherzolitic garnets are enriched in the smaller heavy rare earth elements (HREE). Almandine-rich garnets are also HREE-enriched, but low rare earth elements (LREE) values are lower than in the pyrope-rich garnets. Intermediate garnet compositions are more depleted in HREE and enriched in LREE (Ce, Nd, and Sm). Finally, Ca-rich garnets (50% grossular component) are depleted in LREE and HREE, but are enriched in MREE. Hence, the X site dimension does exert a crucial role in REE incorporation into the garnet structure. Crystal structure refinements provide further evidence of this influence.
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Research Article|
May 01, 2019
X-site control on rare earth elements in eclogitic garnets – an XRD study
Flore A. Caporuscio;
1
Earth Observations
, EES-14, Los Alamos National Laboratory
, Los Alamos, NM87545, USA
Corresponding author, e-mail: floriec@lanl.gov
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Roberta Oberti;
Roberta Oberti
2
CNR-Istituto di Geoscienze e Georisorse, sede secondaria di Pavia
, Via Ferrata 1, 27100Pavia, Italy
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Joseph R. Smyth
Joseph R. Smyth
3
Department of Geological Sciences, University of Colorado
, Boulder, CO80309, USA
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European Journal of Mineralogy (2019) 31 (3): 453–463.
Article history
received:
20 Oct 2017
rev-recd:
12 Dec 2018
accepted:
17 Dec 2018
first online:
09 Jan 2019
Citation
Flore A. Caporuscio, Roberta Oberti, Joseph R. Smyth; X-site control on rare earth elements in eclogitic garnets – an XRD study. European Journal of Mineralogy 2019;; 31 (3): 453–463. doi: https://doi.org/10.1127/ejm/2019/0031-2828
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Index Terms/Descriptors
- alkaline earth metals
- almandine
- bonding
- calcium
- cations
- cell dimensions
- crystal chemistry
- crystal structure
- eclogite
- electron probe data
- garnet group
- grossular
- igneous rocks
- lherzolite
- major elements
- mantle
- metals
- metamorphic rocks
- nesosilicates
- neutron activation analysis data
- orthosilicates
- P-T conditions
- peridotites
- plutonic rocks
- pyrope
- rare earths
- refinement
- silicates
- trace elements
- ultramafics
- unit cell
- X-ray diffraction data