High pressure trace element partitioning between clinopyroxene and alkali basaltic melts

•Trace elements partition coefficients (D) between clinopyroxene and primitive K-basalts.•Influence of temperature, chemical composition and growth rate on D.•Less efficient rejection of incompatible elements during rapid crystal growth.•Apparent increase in DREE values with time imputable to melt p...

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Published in:Geochimica et cosmochimica acta Vol. 305; pp. 282 - 305
Main Authors: Bonechi, Barbara, Perinelli, Cristina, Gaeta, Mario, Fabbrizio, Alessandro, Petrelli, Maurizio, Strnad, Ladislav
Format: Journal Article
Language:English
Published: Elsevier Ltd 15-07-2021
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Summary:•Trace elements partition coefficients (D) between clinopyroxene and primitive K-basalts.•Influence of temperature, chemical composition and growth rate on D.•Less efficient rejection of incompatible elements during rapid crystal growth.•Apparent increase in DREE values with time imputable to melt polymerization.•Deep differentiation of Campi Flegrei Volcanic District parental magmas linked to fractionation of 20–30% of a clinopyroxenitic mineral assemblage. We present new experimental data on major and trace element partition coefficients (D) between clinopyroxene and a K-basaltic melt from Procida Island (Campi Flegrei Volcanic District, south Italy). Time-series experiments were conducted at 0.8 GPa and 1080–1250 °C aiming to investigate the role of the crystallization kinetics on trace elements partitioning behaviour at a pressure relevant for deep magmatic reservoirs. Results indicate that large ion lithophile elements (LILE) are incompatible (e.g., DSr ≤ 0.15), light rare elements (LREE; e.g., DLa ≤ 0.20) are always more incompatible than heavy rare elements (HREE), which in some cases result to be compatible with clinopyroxene (e.g., DDy = 1.40); high field strength elements (HFSE) are generally incompatible (DHFSE ≤ 0.8), while transition elements (TE) range from slightly incompatible (e.g., DV = 0.6) to highly compatible (e.g., DCr = 63). The calculated D values for LILEs, REEs, HFSEs, and TEs tend to decrease with the increase of temperature and to increase with increasing tetrahedrally-coordinated aluminium content, in agreement with the previous studies. Moreover, we observed the influence of the growth rate on the partition coefficients, with the highest DREE values calculated in the runs with the highest growth rate (~10−7 cm s−1), due to the less efficient rejection of incompatible elements during rapid crystal growth, that in this study is not linked to disequilibrium conditions, but to the presence of pre-existing nuclei. Additionally, the apparent increase in DREE values with time observed in some runs is not referable to a change in time but rather to the different degrees of polymerization, expressed as the ratios NBO/T of these melts, strictly related to a loss of Fe occurred during the experiments, and thus to a different melt viscosity. Finally, the application of the experimental clinopyroxene-melt partition coefficients highlights that the deepest step of the magmatic differentiation in the Campi Flegrei Volcanic District is represented by the fractionation of about 20–30% of a clinopyroxenitic mineral assemblage from a basaltic parental magma.
ISSN:0016-7037
1872-9533
DOI:10.1016/j.gca.2021.04.023