TY - JOUR
T1 - Thulium anomalies and rare earth element patterns in meteorites and Earth
T2 - Nebular fractionation and the nugget effect
AU - Dauphas, Nicolas
AU - Pourmand, Ali
N1 - Funding Information:
We thank the Field Museum, the Muséum National d’Histoire Naturelle, American Museum of Natural History and Natural History Museum in Vienna for proving meteorite samples. We are also grateful to R.N. Clayton for a sample of Qingzhen (EH3) and Thomas J. Ireland, Arash Sharifi and Junjun Zheng for their assistance with purifying the commercial lithium metaborate flux and mass spectrometry. We are grateful to T.R. Ireland, M. Norman, and an anonymous reviewer for their comments that greatly improved the manuscript. Discussions with J.-A. Barrat, A.M. Davis, and L. Grossman, and were greatly appreciated. J.-A. Barrat indicated to us that Tm/Tm ∗ anomalies in terrestrial rocks are correlated with REE fractionation. F.L.H. Tissot provided mean REE concentrations of fine grain CAIs with group II REE patterns. This work was funded by grants from NASA , ( NNX12AH60G and NNX14AK09G ) and NSF ( EAR1144429 and EAR1444951 ) to N.D. and NSF ( EAR-1003639 ) to A.P.
PY - 2015/8/5
Y1 - 2015/8/5
N2 - This study reports the bulk rare earth element (REEs, La-Lu) compositions of 41 chondrites, including 32 falls and 9 finds from carbonaceous (CI, CM, CO and CV), enstatite (EH and EL) and ordinary (H, L and LL) groups, as well as 2 enstatite achondrites (aubrite). The measurements were done in dynamic mode using multi-collector inductively coupled plasma mass spectrometers (MC-ICPMS), allowing precise quantification of mono-isotopic REEs (Pr, Tb, Ho and Tm). The CI-chondrite-normalized REE patterns (La. N/Lu. N; a proxy for fractionation of light vs. heavy REEs) and Eu anomalies in ordinary and enstatite chondrites show more scatter in more metamorphosed (petrologic types 4-6) than in unequilibrated (types 1-3) chondrites. This is due to parent-body redistribution of the REEs in various carrier phases during metamorphism. A model is presented that predicts the dispersion of elemental and isotopic ratios due to the nugget effect when the analyzed sample mass is limited and elements are concentrated in minor grains. The dispersion in REE patterns of equilibrated ordinary chondrites is reproduced well by this model, considering that REEs are concentrated in 200. μm-size phosphates, which have high La. N/Lu. N ratios and negative Eu anomalies.Terrestrial rocks and samples from ordinary and enstatite chondrites display negative Tm anomalies of ~-4.5% relative to CI chondrites. In contrast, CM, CO and CV (except Allende) show no significant Tm anomalies. Allende CV chondrite shows large excess Tm (~+10%). These anomalies are similar to those found in group II refractory inclusions in meteorites but of much smaller magnitude. The presence of Tm anomalies in meteorites and terrestrial rocks suggests that either (i) the material in the inner part of the solar system was formed from a gas reservoir that had been depleted in refractory dust and carried positive Tm anomalies or (ii) CI chondrites are enriched in refractory dust and are not representative of solar composition for refractory elements. A new reference composition relevant to inner solar system bodies (CI*) is calculated by subtracting 0.15% of group II refractory inclusions to CI. The observed Tm anomalies in ordinary and enstatite chondrites and terrestrial rocks, relative to carbonaceous chondrites, indicate that material akin to carbonaceous chondrites must have represented a small fraction of the constituents of the Earth. Tm anomalies may be correlated with Ca isotopic fractionation in bulk planetary materials as they are both controlled by addition or removal of refractory material akin to fine-grained group II refractory inclusions.
AB - This study reports the bulk rare earth element (REEs, La-Lu) compositions of 41 chondrites, including 32 falls and 9 finds from carbonaceous (CI, CM, CO and CV), enstatite (EH and EL) and ordinary (H, L and LL) groups, as well as 2 enstatite achondrites (aubrite). The measurements were done in dynamic mode using multi-collector inductively coupled plasma mass spectrometers (MC-ICPMS), allowing precise quantification of mono-isotopic REEs (Pr, Tb, Ho and Tm). The CI-chondrite-normalized REE patterns (La. N/Lu. N; a proxy for fractionation of light vs. heavy REEs) and Eu anomalies in ordinary and enstatite chondrites show more scatter in more metamorphosed (petrologic types 4-6) than in unequilibrated (types 1-3) chondrites. This is due to parent-body redistribution of the REEs in various carrier phases during metamorphism. A model is presented that predicts the dispersion of elemental and isotopic ratios due to the nugget effect when the analyzed sample mass is limited and elements are concentrated in minor grains. The dispersion in REE patterns of equilibrated ordinary chondrites is reproduced well by this model, considering that REEs are concentrated in 200. μm-size phosphates, which have high La. N/Lu. N ratios and negative Eu anomalies.Terrestrial rocks and samples from ordinary and enstatite chondrites display negative Tm anomalies of ~-4.5% relative to CI chondrites. In contrast, CM, CO and CV (except Allende) show no significant Tm anomalies. Allende CV chondrite shows large excess Tm (~+10%). These anomalies are similar to those found in group II refractory inclusions in meteorites but of much smaller magnitude. The presence of Tm anomalies in meteorites and terrestrial rocks suggests that either (i) the material in the inner part of the solar system was formed from a gas reservoir that had been depleted in refractory dust and carried positive Tm anomalies or (ii) CI chondrites are enriched in refractory dust and are not representative of solar composition for refractory elements. A new reference composition relevant to inner solar system bodies (CI*) is calculated by subtracting 0.15% of group II refractory inclusions to CI. The observed Tm anomalies in ordinary and enstatite chondrites and terrestrial rocks, relative to carbonaceous chondrites, indicate that material akin to carbonaceous chondrites must have represented a small fraction of the constituents of the Earth. Tm anomalies may be correlated with Ca isotopic fractionation in bulk planetary materials as they are both controlled by addition or removal of refractory material akin to fine-grained group II refractory inclusions.
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U2 - 10.1016/j.gca.2015.03.037
DO - 10.1016/j.gca.2015.03.037
M3 - Article
AN - SCOPUS:84929417509
VL - 163
SP - 234
EP - 261
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
ER -