Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt

<p>The distribution of U and Th between crystals and quenched silicate melt has been measured in experimentally crystallized natural and synthetic starting compositions. Clinopyroxene was crystallized from two natural basalts and a synthetic composition on the diopside-anorthite join, olivine...

Full description

Bibliographic Details
Main Author: LaTourrette, Thomas
Format: Others
Language:en
Published: 1993
Online Access:https://thesis.library.caltech.edu/7374/1/LaTourrett_t_1993.pdf
LaTourrette, Thomas (1993) Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4ww8-x445. https://resolver.caltech.edu/CaltechTHESIS:01072013-160430261 <https://resolver.caltech.edu/CaltechTHESIS:01072013-160430261>
id ndltd-CALTECH-oai-thesis.library.caltech.edu-7374
record_format oai_dc
collection NDLTD
language en
format Others
sources NDLTD
description <p>The distribution of U and Th between crystals and quenched silicate melt has been measured in experimentally crystallized natural and synthetic starting compositions. Clinopyroxene was crystallized from two natural basalts and a synthetic composition on the diopside-anorthite join, olivine from one of the natural basalts, and garnet from a synthetic andesite. Clinopyroxene and olivine were crystallized at atmospheric pressure under controlled oxygen fugacity and U and Th distributions were determined by particle track radiography. Garnet crystals were grown at 27 Kbars and actinide distributions were determined by secondary ion mass spectrometry. Crystals were grown by slow cooling in an effort to maintain chemical equilibrium at the crystal-melt interface.</p> <p>Clinopyroxene- and olivine-melt partition coefficients (D<sub>i</sub><sup>xtl/liq</sup> = C<sub>i</sub><sup>xtl</sup>/C<sub>i</sub><sup>liq</sup>) were calculated from track distributions by correcting for chemical zoning in the crystals (a result of fractional crystallization) and particle range differences between the crystals and glass. Over the range of ƒO<sub>2</sub>s and compositions studied, D<sub>U</sub><sup>cpx/liq</sup> = 0.0021 - 0.0093, = D<sub>Th</sub><sup>cpx/liq</sup> = 0.0047- 0.021, and D<sub>U;Th</sub><sup>ol/liq</sup> &#60; 0.00005. Garnet-melt partition coefficients are D<sub>U</sub><sup>gt/liq</sup> = 0.0013 and D<sub>Th</sub><sup>gt/liq</sup> = 0.0122. In order to study the effect of the U valence state distribution on U-Th fractionation, clinopyroxene crystallization experiments were run at oxygen fugacities corresponding to the Ni-NiO (NNO), Fa-Mt-Qz (FMQ) and 1 log unit more oxidizing than Fe-FeO (IW+ 1) oxygen buffers. All compositions show an increase in D<sub>U</sub><sup>cpx/liq</sup> with decreasing ƒO<sub>2</sub>, presumably from the increasing proportion of U<sup>4+</sup> in the melt. D<sub>Th</sub><sup>cpx/liq</sup> displays little variation with ƒO<sub>2</sub>, consistent with the fact that Th is solely tetravalent D<sub>Th</sub><sup>cpx/liq</sup> does show an apparent small decrease with decreasing ƒO<sub>2</sub> in one composition, and this is interpreted to be due to compositional changes stemming from variable amounts Na-loss with ƒO<sub>2</sub>. A limited compositional analysis indicates that actinides may be incorporated into clinopyroxene by means of a (U,Th)<sup>4+</sup> + 2Na<sup>+</sup>,⇔ 3Ca<sup>2+</sup> coupled substitution in the M2 site. This mechanism predicts a positive dependence of the actinide partition coefficients on (D<sub>Ca</sub><sup>cpx/liq</sup>)<sup>3</sup> / (D<sub>Na</sub><sup>cpx/liq</sup>)<sup>2</sup>.</p> <p>While there is evidence for deviations from interfacial equilibrium during crystal growth, D<sub>Th</sub><sup>xtl/liq</sup> and D<sub>U</sub><sup>xtl/liq</sup> are affected similarly. This results in D<sub>Th</sub><sup>xtl/liq</sup>/D<sub>U</sub><sup>xtl/liq</sup>, which gives a measure of the degree of U-Th fractionation possible by crystal-melt partitioning, being insensitive to these deviations. D<sub>Th</sub><sup>cpx/liq</sup>/D<sub>U</sub><sup>cpx/liq</sup> is well behaved, approximately independent of composition, and decreases by a factor of 2 - 3 as the ƒO<sub>2</sub> decreases from NNO to IW+1. This indicates that U-Th fractionation by crystal-melt partitioning is ƒO<sub>2</sub> dependent.</p> <p>These results indicate that U-Th fractionation by clinopyroxene-melt partitioning during partial melting will result in a melt with Th/U less than the clinopyroxene, U-Th fractionation by olivine will be insignificant for physically realistic melt fractions, and U-Th fractionation by garnet will result in a melt with ThiU greater than the garnet. The observed pattern of <sup>238</sup>U-<sup>230</sup>Th disequilibrium in oceanic basalts (MORB and OIB) requires a net UTh fractionation that is large, ubiquitous, and results in (<sup>238</sup>U-<sup>230</sup>Th) &#60; 1. With these constraints, fractionation by partial melting of spinel lherzolite is ruled out. The sense of U-Th fractionation by garnet is consistent with oceanic basalts, but the magnitude of fractionation is much smaller than the entire range in observed fractionations. Since partial melts of neither spinel nor garnet lherzolite can match the sense and magnitude of <sup>238</sup>U-<sup>230</sup>Th disequilibrium in MORB and OIB, partial melting is considered unimportant in generating <sup>238</sup>U-<sup>230</sup>Th disequilibrium in oceanic basalts. <sup>238</sup>U-<sup>230</sup>Th disequilibrium in MORB and OIB must therefore result from some process other than partial melting, and possible alternative mechanisms are presented.</p>
author LaTourrette, Thomas
spellingShingle LaTourrette, Thomas
Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt
author_facet LaTourrette, Thomas
author_sort LaTourrette, Thomas
title Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt
title_short Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt
title_full Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt
title_fullStr Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt
title_full_unstemmed Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt
title_sort experimental determination of u and th partitioning between clinopyroxene, garnet, olivine, and natural and synthetic silicate melt
publishDate 1993
url https://thesis.library.caltech.edu/7374/1/LaTourrett_t_1993.pdf
LaTourrette, Thomas (1993) Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4ww8-x445. https://resolver.caltech.edu/CaltechTHESIS:01072013-160430261 <https://resolver.caltech.edu/CaltechTHESIS:01072013-160430261>
work_keys_str_mv AT latourrettethomas experimentaldeterminationofuandthpartitioningbetweenclinopyroxenegarnetolivineandnaturalandsyntheticsilicatemelt
_version_ 1719397003460345856
spelling ndltd-CALTECH-oai-thesis.library.caltech.edu-73742021-04-17T05:02:01Z https://thesis.library.caltech.edu/7374/ Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt LaTourrette, Thomas <p>The distribution of U and Th between crystals and quenched silicate melt has been measured in experimentally crystallized natural and synthetic starting compositions. Clinopyroxene was crystallized from two natural basalts and a synthetic composition on the diopside-anorthite join, olivine from one of the natural basalts, and garnet from a synthetic andesite. Clinopyroxene and olivine were crystallized at atmospheric pressure under controlled oxygen fugacity and U and Th distributions were determined by particle track radiography. Garnet crystals were grown at 27 Kbars and actinide distributions were determined by secondary ion mass spectrometry. Crystals were grown by slow cooling in an effort to maintain chemical equilibrium at the crystal-melt interface.</p> <p>Clinopyroxene- and olivine-melt partition coefficients (D<sub>i</sub><sup>xtl/liq</sup> = C<sub>i</sub><sup>xtl</sup>/C<sub>i</sub><sup>liq</sup>) were calculated from track distributions by correcting for chemical zoning in the crystals (a result of fractional crystallization) and particle range differences between the crystals and glass. Over the range of ƒO<sub>2</sub>s and compositions studied, D<sub>U</sub><sup>cpx/liq</sup> = 0.0021 - 0.0093, = D<sub>Th</sub><sup>cpx/liq</sup> = 0.0047- 0.021, and D<sub>U;Th</sub><sup>ol/liq</sup> &#60; 0.00005. Garnet-melt partition coefficients are D<sub>U</sub><sup>gt/liq</sup> = 0.0013 and D<sub>Th</sub><sup>gt/liq</sup> = 0.0122. In order to study the effect of the U valence state distribution on U-Th fractionation, clinopyroxene crystallization experiments were run at oxygen fugacities corresponding to the Ni-NiO (NNO), Fa-Mt-Qz (FMQ) and 1 log unit more oxidizing than Fe-FeO (IW+ 1) oxygen buffers. All compositions show an increase in D<sub>U</sub><sup>cpx/liq</sup> with decreasing ƒO<sub>2</sub>, presumably from the increasing proportion of U<sup>4+</sup> in the melt. D<sub>Th</sub><sup>cpx/liq</sup> displays little variation with ƒO<sub>2</sub>, consistent with the fact that Th is solely tetravalent D<sub>Th</sub><sup>cpx/liq</sup> does show an apparent small decrease with decreasing ƒO<sub>2</sub> in one composition, and this is interpreted to be due to compositional changes stemming from variable amounts Na-loss with ƒO<sub>2</sub>. A limited compositional analysis indicates that actinides may be incorporated into clinopyroxene by means of a (U,Th)<sup>4+</sup> + 2Na<sup>+</sup>,⇔ 3Ca<sup>2+</sup> coupled substitution in the M2 site. This mechanism predicts a positive dependence of the actinide partition coefficients on (D<sub>Ca</sub><sup>cpx/liq</sup>)<sup>3</sup> / (D<sub>Na</sub><sup>cpx/liq</sup>)<sup>2</sup>.</p> <p>While there is evidence for deviations from interfacial equilibrium during crystal growth, D<sub>Th</sub><sup>xtl/liq</sup> and D<sub>U</sub><sup>xtl/liq</sup> are affected similarly. This results in D<sub>Th</sub><sup>xtl/liq</sup>/D<sub>U</sub><sup>xtl/liq</sup>, which gives a measure of the degree of U-Th fractionation possible by crystal-melt partitioning, being insensitive to these deviations. D<sub>Th</sub><sup>cpx/liq</sup>/D<sub>U</sub><sup>cpx/liq</sup> is well behaved, approximately independent of composition, and decreases by a factor of 2 - 3 as the ƒO<sub>2</sub> decreases from NNO to IW+1. This indicates that U-Th fractionation by crystal-melt partitioning is ƒO<sub>2</sub> dependent.</p> <p>These results indicate that U-Th fractionation by clinopyroxene-melt partitioning during partial melting will result in a melt with Th/U less than the clinopyroxene, U-Th fractionation by olivine will be insignificant for physically realistic melt fractions, and U-Th fractionation by garnet will result in a melt with ThiU greater than the garnet. The observed pattern of <sup>238</sup>U-<sup>230</sup>Th disequilibrium in oceanic basalts (MORB and OIB) requires a net UTh fractionation that is large, ubiquitous, and results in (<sup>238</sup>U-<sup>230</sup>Th) &#60; 1. With these constraints, fractionation by partial melting of spinel lherzolite is ruled out. The sense of U-Th fractionation by garnet is consistent with oceanic basalts, but the magnitude of fractionation is much smaller than the entire range in observed fractionations. Since partial melts of neither spinel nor garnet lherzolite can match the sense and magnitude of <sup>238</sup>U-<sup>230</sup>Th disequilibrium in MORB and OIB, partial melting is considered unimportant in generating <sup>238</sup>U-<sup>230</sup>Th disequilibrium in oceanic basalts. <sup>238</sup>U-<sup>230</sup>Th disequilibrium in MORB and OIB must therefore result from some process other than partial melting, and possible alternative mechanisms are presented.</p> 1993 Thesis NonPeerReviewed application/pdf en other https://thesis.library.caltech.edu/7374/1/LaTourrett_t_1993.pdf LaTourrette, Thomas (1993) Experimental Determination of U and Th Partitioning Between Clinopyroxene, Garnet, Olivine, and Natural and Synthetic Silicate Melt. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4ww8-x445. https://resolver.caltech.edu/CaltechTHESIS:01072013-160430261 <https://resolver.caltech.edu/CaltechTHESIS:01072013-160430261> https://resolver.caltech.edu/CaltechTHESIS:01072013-160430261 CaltechTHESIS:01072013-160430261 10.7907/4ww8-x445