Garnet stability in the deep lunar mantle: Constraints on the physics and chemistry of the interior of the Moon

Jie- Jun Jing; Yanhao Lin; Jurrien S. Knibbe; Wim van Westrenen

doi:10.1016/j.epsl.2022.117491

Garnet stability in the deep lunar mantle: Constraints on the physics and chemistry of the interior of the Moon

Jie- Jun Jing^*, Yanhao Lin, Jurrien S. Knibbe^*, Wim van Westrenen

^*Corresponding author for this work

Research output: Contribution to Journal › Article › Academic › peer-review

Abstract

High-pressure, high-temperature experiments have been conducted at deep lunar mantle conditions to constrain the garnet stability field. Using the Taylor Whole Moon composition, garnet is found to be stable at pressures above 3 GPa and temperatures below 1700 °C, yielding a smaller stability field than previously suggested on the basis of thermodynamic calculations. Experimental data are used to model equilibrium crystallization in a ‘two-stage’ model of lunar magma ocean (LMO) crystallization starting from a fully molten Moon. In the first stage, isothermal (1600 °C) equilibrium crystallization of the LMO would produce garnet-bearing lherzolite cumulates (containing up to ∼20 wt.% garnet) in the lowermost lunar mantle. Garnet in the deep lunar mantle would significantly decrease the Al₂O₃ content of the residual LMO and impact HREE/MREE fractionation. Numerical modeling of the second stage (residual LMO fractional crystallization) shows a delay in plagioclase saturation compared to models of single-stage fractional crystallization of a whole-Moon LMO of the Taylor Whole Moon composition, thinning the anorthositic crust from 95 km to 75 km. To reach the upper limit of current estimates of the average lunar crustal thickness (∼45 km), the two-stage scenario needs to be accompanied by a total of 10% liquid trapped in cumulates and 70% efficiency of plagioclase flotation. We also conduct trace element evolution modeling and reproduce a REE pattern identical to high K, REE, and P (KREEP) compositions after 99.8% solidification, when starting with a CI chondritic REE abundance. The density of a garnet-bearing deep lunar mantle is significantly higher than the density of olivine/orthopyroxene mixtures without garnet. The present-day lowest mantle in the Moon could therefore be characterized by chemical interactions between the earliest (garnet-bearing) and latest (ilmenite-bearing) products of LMO crystallization.

Original language	English
Article number	117491
Pages (from-to)	1-13
Number of pages	13
Journal	Earth and Planetary Science Letters
Volume	584
Early online date	21 Mar 2022
DOIs	https://doi.org/10.1016/j.epsl.2022.117491
Publication status	Published - 15 Apr 2022

Bibliographical note

Funding Information:
This work was supported financially by a Vici grant ( 865.13.006 ) from the Dutch Research Council to WvW, a China Scholarship Council fellowship ( 201904910721 ) to JJ and a Marie Sklodowska-Curie grant MERCURYREFINEMENT (No 845354 ) from the European Commission to JSK. We thank two anonymous reviewers for their thorough and constructive comments and F. Moynier for editorial work. We also thank E. Hellebrand and T. Bouten for technical assistance during microprobe analyses. JJ thanks Wulinwaizhuan for help during writing the manuscript.

Publisher Copyright:
© 2022 The Author(s)

Keywords

density
KREEP
lunar magma ocean
mantle cumulates
rare earth elements

UN SDGs

This output contributes to the following UN Sustainable Development Goals (SDGs)

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10.1016/j.epsl.2022.117491

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title = "Garnet stability in the deep lunar mantle: Constraints on the physics and chemistry of the interior of the Moon",

abstract = "High-pressure, high-temperature experiments have been conducted at deep lunar mantle conditions to constrain the garnet stability field. Using the Taylor Whole Moon composition, garnet is found to be stable at pressures above 3 GPa and temperatures below 1700 °C, yielding a smaller stability field than previously suggested on the basis of thermodynamic calculations. Experimental data are used to model equilibrium crystallization in a {\textquoteleft}two-stage{\textquoteright} model of lunar magma ocean (LMO) crystallization starting from a fully molten Moon. In the first stage, isothermal (1600 °C) equilibrium crystallization of the LMO would produce garnet-bearing lherzolite cumulates (containing up to ∼20 wt.% garnet) in the lowermost lunar mantle. Garnet in the deep lunar mantle would significantly decrease the Al2O3 content of the residual LMO and impact HREE/MREE fractionation. Numerical modeling of the second stage (residual LMO fractional crystallization) shows a delay in plagioclase saturation compared to models of single-stage fractional crystallization of a whole-Moon LMO of the Taylor Whole Moon composition, thinning the anorthositic crust from 95 km to 75 km. To reach the upper limit of current estimates of the average lunar crustal thickness (∼45 km), the two-stage scenario needs to be accompanied by a total of 10% liquid trapped in cumulates and 70% efficiency of plagioclase flotation. We also conduct trace element evolution modeling and reproduce a REE pattern identical to high K, REE, and P (KREEP) compositions after 99.8% solidification, when starting with a CI chondritic REE abundance. The density of a garnet-bearing deep lunar mantle is significantly higher than the density of olivine/orthopyroxene mixtures without garnet. The present-day lowest mantle in the Moon could therefore be characterized by chemical interactions between the earliest (garnet-bearing) and latest (ilmenite-bearing) products of LMO crystallization.",

keywords = "density, KREEP, lunar magma ocean, mantle cumulates, rare earth elements",

author = "Jing, {Jie- Jun} and Yanhao Lin and Knibbe, {Jurrien S.} and {van Westrenen}, Wim",

note = "Funding Information: This work was supported financially by a Vici grant ( 865.13.006 ) from the Dutch Research Council to WvW, a China Scholarship Council fellowship ( 201904910721 ) to JJ and a Marie Sklodowska-Curie grant MERCURYREFINEMENT (No 845354 ) from the European Commission to JSK. We thank two anonymous reviewers for their thorough and constructive comments and F. Moynier for editorial work. We also thank E. Hellebrand and T. Bouten for technical assistance during microprobe analyses. JJ thanks Wulinwaizhuan for help during writing the manuscript. Publisher Copyright: {\textcopyright} 2022 The Author(s)",

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doi = "10.1016/j.epsl.2022.117491",

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AU - Jing, Jie- Jun

AU - Lin, Yanhao

AU - Knibbe, Jurrien S.

AU - van Westrenen, Wim

N1 - Funding Information: This work was supported financially by a Vici grant ( 865.13.006 ) from the Dutch Research Council to WvW, a China Scholarship Council fellowship ( 201904910721 ) to JJ and a Marie Sklodowska-Curie grant MERCURYREFINEMENT (No 845354 ) from the European Commission to JSK. We thank two anonymous reviewers for their thorough and constructive comments and F. Moynier for editorial work. We also thank E. Hellebrand and T. Bouten for technical assistance during microprobe analyses. JJ thanks Wulinwaizhuan for help during writing the manuscript. Publisher Copyright: © 2022 The Author(s)

PY - 2022/4/15

Y1 - 2022/4/15

N2 - High-pressure, high-temperature experiments have been conducted at deep lunar mantle conditions to constrain the garnet stability field. Using the Taylor Whole Moon composition, garnet is found to be stable at pressures above 3 GPa and temperatures below 1700 °C, yielding a smaller stability field than previously suggested on the basis of thermodynamic calculations. Experimental data are used to model equilibrium crystallization in a ‘two-stage’ model of lunar magma ocean (LMO) crystallization starting from a fully molten Moon. In the first stage, isothermal (1600 °C) equilibrium crystallization of the LMO would produce garnet-bearing lherzolite cumulates (containing up to ∼20 wt.% garnet) in the lowermost lunar mantle. Garnet in the deep lunar mantle would significantly decrease the Al2O3 content of the residual LMO and impact HREE/MREE fractionation. Numerical modeling of the second stage (residual LMO fractional crystallization) shows a delay in plagioclase saturation compared to models of single-stage fractional crystallization of a whole-Moon LMO of the Taylor Whole Moon composition, thinning the anorthositic crust from 95 km to 75 km. To reach the upper limit of current estimates of the average lunar crustal thickness (∼45 km), the two-stage scenario needs to be accompanied by a total of 10% liquid trapped in cumulates and 70% efficiency of plagioclase flotation. We also conduct trace element evolution modeling and reproduce a REE pattern identical to high K, REE, and P (KREEP) compositions after 99.8% solidification, when starting with a CI chondritic REE abundance. The density of a garnet-bearing deep lunar mantle is significantly higher than the density of olivine/orthopyroxene mixtures without garnet. The present-day lowest mantle in the Moon could therefore be characterized by chemical interactions between the earliest (garnet-bearing) and latest (ilmenite-bearing) products of LMO crystallization.

AB - High-pressure, high-temperature experiments have been conducted at deep lunar mantle conditions to constrain the garnet stability field. Using the Taylor Whole Moon composition, garnet is found to be stable at pressures above 3 GPa and temperatures below 1700 °C, yielding a smaller stability field than previously suggested on the basis of thermodynamic calculations. Experimental data are used to model equilibrium crystallization in a ‘two-stage’ model of lunar magma ocean (LMO) crystallization starting from a fully molten Moon. In the first stage, isothermal (1600 °C) equilibrium crystallization of the LMO would produce garnet-bearing lherzolite cumulates (containing up to ∼20 wt.% garnet) in the lowermost lunar mantle. Garnet in the deep lunar mantle would significantly decrease the Al2O3 content of the residual LMO and impact HREE/MREE fractionation. Numerical modeling of the second stage (residual LMO fractional crystallization) shows a delay in plagioclase saturation compared to models of single-stage fractional crystallization of a whole-Moon LMO of the Taylor Whole Moon composition, thinning the anorthositic crust from 95 km to 75 km. To reach the upper limit of current estimates of the average lunar crustal thickness (∼45 km), the two-stage scenario needs to be accompanied by a total of 10% liquid trapped in cumulates and 70% efficiency of plagioclase flotation. We also conduct trace element evolution modeling and reproduce a REE pattern identical to high K, REE, and P (KREEP) compositions after 99.8% solidification, when starting with a CI chondritic REE abundance. The density of a garnet-bearing deep lunar mantle is significantly higher than the density of olivine/orthopyroxene mixtures without garnet. The present-day lowest mantle in the Moon could therefore be characterized by chemical interactions between the earliest (garnet-bearing) and latest (ilmenite-bearing) products of LMO crystallization.

KW - density

KW - KREEP

KW - lunar magma ocean

KW - mantle cumulates

KW - rare earth elements

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JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

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ER -

Garnet stability in the deep lunar mantle: Constraints on the physics and chemistry of the interior of the Moon

Abstract

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UN SDGs

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