Bernt, Irene

Breuer, Doris

Collinet, Max

Plesa, Ana-Catalina

Schwinger, Sabrina

Institute of Transport Research:Publications

EFFECTS OF MAGMA OCEAN CRYSTALLIZATION ON THE LONG-TERM THERMOCHEMICALEVOLUTION OF THE MOON I. Bernt1, A.-C. Plesa1, S. Schwinger1, M. Collinet1, D. Breuer1, 1GermanAerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin (irene.bernt@dlr.de) .Introduction: The early stages of lunar evolutionhave been characterized by the existence of a magmaocean. Fractional crystallization of the latter resulted inan inhomogeneous mantle with cumulate layers of in-creasing density towards the surface, due to iron enrich-ment in the residual magma ocean [1]. Such an unsta-ble density distribution is prone to overturn and sets thestage for the subsequent thermochemical history of themantle. In particular, this initial stage can significantlyaffect mixing of mantle material and the partial melt pro-duction, as well as the melt composition during the laterstages of lunar evolution. The melt composition is di-rectly linked to the composition of lunar basalts, whosevariability at the lunar surface indicates complex meltingprocesses and the presence of various mantle reservoirs.In this study we model the solid-state convection,mixing, and partial melt production in the lunar mantle.We investigate the effects of an initially layered mantleas a consequence of the fractional crystallization ofthe lunar magma ocean and compare it to an initiallyhomogeneous mantle, as it was used in many previousstudies. While previous mantle convection studiesinvestigated lunar mantle melting in a purely homo-geneous mantle [2] or with a localized KREEP layer[3], here we combine petrological and geodynamicalmodels to investigate the degree of heterogeneity thatcan be produced by melting and mixing of an initiallyheterogeneous lunar mantle. In addition, we constrainpossible model parameters for both homogeneous andheterogeneous mantle scenarii using the estimatedsecondary crustal thickness of the Moon.Petrological Model: Heterogeneous lunar mantlecompostion case: The petrological model computesthe crystallization sequence of the lunar magma ocean(LMO) in the heterogeneous case. For the calculationswe follow the approach described by [4], which is ingood agreement with experimental data [5, 6] for thewhole solidification process. We assume fractional crys-tallization of a spherical shell with an initial LMO thick-ness of 1350 km and a bulk lunar mantle compositionfrom [7].The resulting mantle structure is characterized by 5compositional layers, where the predominant mineralsare olivine, orthopyroxene, clinopyroxene, clinopyrox-ene and ilmenite (ilmenite bearing cumulates = IBC) andplagioclase (crust), respectively. For each of these layersan average density is calculated at the end of the crystal-lization sequence, as well as the change in density as afunction of mantle depletion due to melting. The solidusand liquidus temperature profiles are calculated for theaverage compositions in each layer using alphaMELTS.The results from the petrological model are shown inFig. 1. The densities of the mantle layers increase withradius due to the enrichment of iron in the remaining meltduring crystallization. The crust has a comparative lowdensity (Fig. 1a). The temperature profile follows thecrystallization temperatures of the cumulates. In Fig. 1c,the temperature profile is shown together with the indi-vidual layers and their corresponding solidi.Homogeneous lunar mantle composition case:For the homogeneous case we use a KLB-1 peridotite[8] mantle composition, for which the solidus andliquidus, as well as the density-update due to mantledepletion were calculated using alphaMELTS. Theinitial temperature profile follows the approach used in[3], and is shown in Fig. 1b).Geodynamical Model: We use the mantle convec-tion code GAIA [9] in a 2D quarter cylinder geometry tomodel the thermochemical evolution of the composition-ally homogeneous or heterogeneous lunar mantle. Weemploy an Arrhenius law to calculate the temperature-and depth-dependent viscosity. To track the compositionof the mantle we use a particle-in-cell (PIC) method [10],where tracer particles carry information about materialproperties such as density, melting temperature, degreeof depletion, amount of heat producing elements etc. Inour simulations we account for core cooling and radioac-tive decay, as well as mechanical mixing. In addition,we consider the effects of melting by accounting for thelatent heat during mantle melting, as well as solidus in-crease and density decrease due to mantle depletion [11].Results: We use estimates of the average thicknessof the basaltic crust that was formed by convection andmelting processes during the lunar evolution after thecrystallization of the LMO. These estimates show largeuncertainties and vary from roughly 2 to 5 km [12].Here we use a conservative range of 2 to 10 km, sincebasaltic melt may remain trapped in the lithospherein form of intrusive melt. Homogeneous case:The initial temperature profile has a significant impacton the amount of melt produced in the homogeneouscase. Previous studies found that even intermediatetemperature profiles produce rather large amounts ofsecondary crust [3]. Our models indicate that to pro-duce a secondary basaltic crust of thickness consistentwith current estimates (max. 10 km) a relatively cold1471.pdf53rd Lunar and Planetary Science Conference (2022)Figure 1: Initial input for the geodynamical model, generated by the petrological model. a) The density profiles forboth scenarii (initial homogeneous and initial heterogeneous mantle) together with the initial layers in the heteroge-neous case. b) Initial temperature profile and solidus of the peridotite mantle in the homogeneous case. c) Initialtemperature profile and solidi of the heterogeneous mantle composition case. (rc = rich cumulates, het = heteroge-neous case, hom = homogeneous case, Sol = solidus, ρ= density).initial potential mantle temperature of 1506–1555 K isrequired. This temperature profile (Fig. 1b) would implyan abnormally small initial magma ocean depth of only106–145 km. In addition, such low initial temperatureslead to a delayed secondary crust production (Fig. 2).Heterogeneous case: In contrast to the homo-geneous case, the initial temperature profile of theheterogeneous case is given by the solidification temper-ature (Fig. 1c). One important aspect in this case is thestability of the IBC layer. Sinking of the IBC cumulatesinfluences the interior dynamics of the lunar mantle andthe amount of melt produced. If the IBC layer remainstrapped in the stagnant lid, the models show an averagethickness of 1.3 km of basaltic crust produced during theevolution. On the other hand, if about 35% of the IBCsinks into the mantle it leads to an average thickness ofthe basaltic crust of 2 km. The higher amount of meltproduced in this case results from the fact that sinkingFigure 2: Comparison of the crustal thicknesses for a het-erogeneous case with a homogeneous case. The initialtemperature profile for the homogeneous case has a po-tential temperature of 1555 K, corresponding to an ini-tial magma ocean depth of 145 km. In the heterogeneouscase about 35% of the IBC layer sinks into the mantle.The acceptable crustal thickness range is shown by thegrey vertical line..of the IBC cumulates lead to a stronger return flow thatpromotes mantle melting and due to melting of IBCcumulates themselves, which have the lowest meltingtemperature of all mantle components.Conclusion: With our coupled petrological-geodynamical model we are able to constrain thermalevolution parameters of an initially homogeneous andan initially heterogeneous lunar mantle. Using thesecondary crustal thickness as a constraint, we findthat a heterogeneous mantle composition yields morecomparable results to observational estimates than ahomogeneous mantle composition. In the latter caseonly a cold initial temperature profile would be com-patible with a basaltic crustal thickness as suggestedby observations. In an initially heterogeneous mantle,at least parts of the IBC layer need to be recycled intothe mantle to obtain a secondary crustal thickness in therange of the inferred basaltic crust estimates. In futurework we will compare the melt composition obtainedin our calculations to the observed composition of marebasalts to include additional constraints for our models.Furthermore, we plan to investigate convection andmixing of the cumulates during the LMO crystallization.Acknowledgements: We thank Brian Doherty for hiscontribution to the numerical implementation. I.B. and S.S.were supported by the German Research Foundation (DeutscheForschungsgemeinschaft) SFB-TRR170, (subprojects C4 andA5).References: [1] Elkins-Tanton L.T. (2012) Annu. Rev.Earth Planet. Scie. [2] Ziethe R. et al. (2009) P&SS; [3] La-neuville M. et al. (2013) JGR: Planets; [4] Schwinger S. andBreuer D. (2021) PEPI; [5] Rapp J.F. and Draper D.S. (2018)MAPS; [6] Charlier B. et al. (2018) Geochim. Cos- mochim.Acta; [7] O’Neill H.St.C. (1991) Geochim. Cosmochim. Acta;[8] Zhang J. and Herzberg C. (1994) JGR: Solid Earth; [9]Hüttig C. et al. (2013) PEPI; [10] Plesa A.C. et al. (2013)IGI Global; [11] Breuer D. et al. (2018) AGU Fall Meeting;[12] Shengxia G. et al. (2016) JGR: Planets.1471.pdf53rd Lunar and Planetary Science Conference (2022)

Effects of magma ocean crystallization on the long-term thermochemical evolution of the Moon

Abstract

Similar works

Full text

Available Versions

Institute of Transport Research:Publications