Magnesium Stable Isotopes Support the Lunar Magma Ocean Cumulate Remelting Model for Mare Basalts

Magnesium stable isotopes support the lunar magma ocean cumulate remelting model for mare basalts

Fatemeh Sedaghatpoura,1 and Stein B. Jacobsena

aDepartment of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138

Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved November 13, 2018 (received for review July 3, 2018) We report high-precision Mg isotopic analyses of different types physical separation of CAIs and chondrules in the protoplanetary of lunar samples including two pristine Mg-suite rocks (72415 disk (37, 43). However, the extent and mechanisms of Mg isotopic and 76535), basalts, anorthosites, breccias, mineral separates, and fractionation during the magmatic evolution of planetary bodies lunar meteorites. The Mg isotopic composition of the dunite 72415 remain unexplored. More recent studies of Mg isotopes in the (δ25Mg = −0.140 ± 0.010‰, δ26Mg = −0.291 ± 0.018‰), the most Mg- Moon, asteroid Vesta, ureilites, and Martian meteorites report rich and possibly the oldest lunar sample, may provide the best esti- significant Mg isotope variations perhaps related to their igneous mate of the Mg isotopic composition of the bulk silicate Moon (BSM). differentiation history (22, 23, 31, 35, 44). Most Mg isotopic studies This δ26Mg value of the Moon is similar to those of the Earth and of lunar samples show a significant dichotomy between low- and chondrites and reflects both the relative homogeneity of Mg isotopes high-Ti basalts (23, 37, 39). The isotope dichotomy in lunar basalts in the solar system and the lack of Mg isotope fractionation by the is also seen for other elements and is suggested to be the result of Moon-forming giant impact. In contrast to the behavior of Mg iso- heterogeneities produced by the lunar magmatic differentiation (18, topes in terrestrial basalts and mantle rocks, Mg isotopic data on 20, 27, 45); however, this assumption for Mg has not yet been well lunar samples show isotopic variations among the basalts and pristine studied. Moreover, the isotopic composition of the lunar basalts is anorthositic rocks reflecting isotopic fractionation during the early commonly used to estimate the isotopic composition of the BSM, lunar magma ocean (LMO) differentiation. Calculated evolutions of but it remains unclear whether the basalts are the most represen- 26 δ Mg values during the LMO differentiation are consistent with the tative of the BSM (27, 32). To evaluate such possibilities and con- 26 observed δ Mg variations in lunar samples, implying that Mg isotope strain the behavior of Mg isotopes during the lunar magmatic

variations in lunar basalts are consistent with their origin by remelting differentiation, we analyzed a suite of representative samples in- EARTH, ATMOSPHERIC, of distinct LMO cumulates. cluding pristine anorthositic and Mg-suite rocks, lunar basalts, and AND PLANETARY SCIENCES mineral separates (Materials and Methods). magnesium isotopes | the Moon | magmatic differentiation | isotope fractionation | lunar basalts Discussion Previous Mg isotopic studies have found no measurable Mg he early work on lunar anorthosites separated from the isotope fractionations in terrestrial whole rocks formed by partial TApollo 11 lunar regolith led to the development of the lunar melting and magmatic differentiation (41, 42, 46). However, magma ocean (LMO) hypothesis in the early 1970s (1, 2). This significant intermineral Mg isotope fractionations have been hypothesis has later been elaborated by lunar meteorites and found among terrestrial minerals due to both kinetic and equi- remote sensing (3), chronology of pristine lunar rocks (4, 5), and librium effects (43, 47–51). In contrast to terrestrial rocks, the complementary Eu anomalies in anorthosites and mare basalts lunar samples studied here (Fig. 1 and SI Appendix, Tables S1 (6). Fractional crystallization of the LMO resulted in the for- and S2) show Mg isotope variations similar to what has been mation of a mafic mantle and a feldspathic crust, with late-stage reported for other differentiated bodies (22, 23, 31, 35). This Mg ilmenite-rich cumulates and the materials enriched in potassium (K), rare earth elements (REE), and phosphorus (P) (KREEP) Significance crystallizing beneath the lunar anorthositic crust (7, 8). Many experimental and theoretical models along with elemental Soon after the return of the first lunar samples by the Apollo and isotopic data of lunar samples, including both stable and ra- missions, it became clear that the Moon is a highly differenti- ’ diogenic isotopes, were used to determine the Moon soriginand ated object, with a plagioclase-rich crust formed by mineral magmatic evolution as well as the origin of lunar basalts, which are flotation in a very early magma ocean. The younger lunar mare presumably partial melts of the cumulates produced during the basalts were interpreted to result from remelting of these lu- – LMO crystallization (9 19). Over the last 2 decades, the non- nar magma ocean (LMO) cumulate layers. Here we report sig- traditional stable isotopes have also provided new insights into the nificant Mg isotope variations produced by the Moon’s early accretion and magmatic evolution of planetary bodies including the magmatic differentiation. These results support models in – Moon (20 35). Most of the data (except for volatile elements such which lunar basalts are formed by partial melting of distinct as K and Zn) show that among bodies in the solar system the bulk cumulate sources produced during the LMO crystallization and silicate Earth (BSE) and bulk silicate Moon (BSM) are uniquely imply that the bulk lunar Mg isotope composition is similar to similar, despite some variations among the lunar rocks. Magnesium, that of the inner solar system. a major element with three stable isotopes, is potentially an important tool to study the Moon’s early magmatic differentiation Author contributions: F.S. designed research; F.S. performed research; S.B.J. contributed because its isotopic fractionation is only influenced by mineral new reagents/analytic tools; F.S. and S.B.J. analyzed data; and F.S. and S.B.J. wrote crystallization and is not affected by core formation processes (23, the paper. 31, 35–39). Most studies indicate that Mg isotopic compositions in The authors declare no conflict of interest. the inner solar system are homogeneous and vary perhaps as a This article is a PNAS Direct Submission. result of igneous differentiation processes (22, 23, 36, 38–42). This Published under the PNAS license. homogeneity of Mg isotope compositions among planetary bodies 1To whom correspondence should be addressed. Email: [email protected]. has been questioned by a recent high-precision Mg isotope study This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. (29) based on the idea that some variations should be expected due 1073/pnas.1811377115/-/DCSupplemental. to vapor loss from growing planetesimals (29) or sorting and Published online December 17, 2018.

www.pnas.org/cgi/doi/10.1073/pnas.1811377115 PNAS | January 2, 2019 | vol. 116 | no. 1 | 73–78 Downloaded by guest on October 1, 2021 is suggested that the Fe isotope fractionation during the early crystallization produced isotopically light olivines of the deeper mantle, which balance the heavy Fe isotope composition of lunar basalts (27). This, in turn, leads to similar Fe isotopic compositions of the BSE and BSM. Also, Sossi and Moynier (32) analyzed more Mg-suite rocks and found that the compositions of the BSE and BSM are broadly indistinguishable, but the Fe isotopic composition of the dunite 72415 was still lighter (∼0.40‰ in δ56Fe) than that of the BSM. They interpreted the Fe isotopic composition of this sample as an anomaly resulting from Fe–Mg diffusion in olivine rather than equilibrium fractionation during olivine crystallization. The lack of Fe isotope fractionation during olivine crystallization from a basaltic melt experiment (57) may also rule out the equilibrium Fe isotope fractionation via olivine crystallization in the early LMO. However, the δ26Mg value of the dunite 72415 measured in this study (−0.291 ± 0.018‰) agrees well with the previous estimate of Mg isotopic composition of the Moon (−0.26 ± 0.16‰) (23). Therefore, the suggested diffusive fractionation origin for the light Fe isotopic composition of the dunite 72415, which is not recorded in the Mg isotopic composition of this rock, still remains enigmatic. The δ26Mg values of this dunite and the pristine troctolite 76535 (−0.336 ± 0.031‰) overlap with the estimated values of the Moon (−0.26 ± 0.16‰) (23), the Earth (−0.25 ± 0.07‰), chondrites (−0.28 ± 0.06‰) (41), and Mars (−0.271 ± 0.040‰) (35). These two Mg-suite samples of deep origin (72415 and 76535) have high modal abundance of olivine; hence, the similarity between their Mg isotopic compositions and that of the BSM (23) hints at no Mg isotopic fractionation during crystallization of the most magnesian olivine from the LMO (Fig. 1). The lack of Mg isotope fractionation δ26 Fig. 1. The δ26Mg values of terrestrial and lunar samples analyzed in this study during olivine crystallization is also confirmed by the Mg value of (SI Appendix,TablesS1andS2). The solid and dotted lines are Mg isotopic the olivine separate (SI Appendix,TableS2) and will be evaluated in composition of the BSM and two SEs, respectively (δ26Mg = −0.291 ± 0.018‰). our isotopic model in the next section. These results agree with the The literature data are from refs. 23 (Moon), 35 (Mars), and 41 (Earth and Fe isotopic study of Mg-suite samples (32) suggesting that the Mg- chondrites). The shaded blue and yellow boxes are Mg isotopic compositions suite rocks may be the most representative of the BSM for both Mg measured for low-Ti and high-Ti basalts, respectively (this study). and Fe isotopic compositions. Our results contradict a recent study of Mg isotope compositions of planetary bodies (29) suggesting that all differentiated bodies are isotopically heavier (∼0.02 ± 0.010‰, isotopic variation could be a tracer of planetary differentiation. 2σm) than chondrites. The latter was explained by equilibrium iso- In the standard LMO model, fractional crystallization resulted in tope fractionation between silicate liquid and vapor lost during the flotation of low-density minerals like feldspar forming the an- Moon’s accretion (29). Nevertheless, significant Mg isotope varia- orthositic crust, whereas sinking of the denser ferromagnesian tions among the rocks from different planetary bodies such as Mars, minerals produced olivine and pyroxene layers (7, 8, 52). Here the asteroid Vesta, and the Moon do exist implying that igneous we first discuss the Mg isotope compositions of the Mg-suite (the differentiation fractionates stable isotopes. Therefore, a model that most Mg-rich samples) and FAN samples that are thought to be can link measured isotopic compositions of individual rocks to the the direct products of LMO crystallization (53). bulk compositions of their source planets/planetesimals is a pre- The highland Mg-suite samples are plutonic rocks with distinctive requisite for evaluating potential differences in stable Mg isotope – = characteristics of a high Mg # (60 95, where Mg # molar [MgO/ compositions among planetary bodies in the solar system. + × MgO FeO] 100); an enrichment in KREEP material; and a Ferroan-anorthosites (FAN) are assumed to be flotation cumu- depletion in Cr, Ni, and Co (5, 14). They are likely formed by partial lates of a global LMO. The three FAN samples studied here are melting of a hybrid parent magma produced in the early lunar pristine highland rocks with very low siderophile and incompatible magmatic evolution (5, 14). The model that fits best with the pe- element abundances (58). Two anorthosites, 60015 and 60025, are 26 trology, geochemistry, and chronology of Mg-suite rocks (14) pre- enriched in heavy Mg isotopes (Δ Mgsample-BSM ∼ 0.3‰). How- dicts that their parental magmas were formed from less dense early ever, anorthosite 62236 has a δ26Mg value of −0.249‰ similar to cumulates at high temperature (∼1,400–1,800 °C). Then, these ris- that of the BSM (Fig. 1). Considering the fractionation factor for 26 ing hot and low-density magmas were mixed with KREEP and plagioclase-melt (Δ Mgplagioclase-melt = 0.869‰)(SI Appendix, plagioclase at the base of the crust. We studied two Mg-suite rocks, Table S7), the δ26Mg values of liquids in equilibrium with samples 72415 and 76535. The lunar dunite 72415, one of the oldest lunar 60015 and 60025 could be −0.849 and −0.787‰, respectively. samples of the Mg-suite, contains chromite symplectites indicative These values can reflect a late stage of LMO evolution as is shown of crystallization at 40–50 km rather than at a shallow depth of in Isotopic Fractionation Model.However,theLMOoriginof ∼1 km (5, 54, 55). The deep cumulate origin of this sample has been younger FAN samples is more controversial (59–61). The third questionedbyRyder(55)basedonitsslightlyzonedolivines(Fo# FAN sample 62236 is a noritic anorthosite with 83% plagioclase 86–89) with relatively high CaO (∼0.1%) compared with plutonic (Pl), 7% orthopyroxene (Opx), 5% clinopyroxene (Cpx), and 5% olivines. However, based on the existence of slightly zoned olivines olivine (Ol) (58). The MgO contents (wt %) of the pyroxenes and (Fo # 84–88) in pallasites (56) and high CaO content of San Carlos plagioclase of this sample (Opx, 22.1; Cpx, 14–21; and Pl, 0.00; ref. olivine, Wang et al. (27) argued that none of these observations 62) indicate that Opx, Cpx, and Ol are the main hosts of Mg, provides strong evidence against deep origin of the dunite 72415. resulting in a Mg isotopic composition similar to that of the BSM. The Fe isotopic composition of this dunite is distinctly lighter The young age of this sample (63) also implies its formation by (∼0.35–0.45‰ in δ56Fe)thanthatoftheBSEandBSM(27,32).It processes other than a simple flotation from the LMO. The lack of

74 | www.pnas.org/cgi/doi/10.1073/pnas.1811377115 Sedaghatpour and Jacobsen Downloaded by guest on October 1, 2021 significant correlation between δ26Mg and MgO in the low-Ti ba- Three of the lunar meteorites analyzed here have δ26Mg values 26 salts, dunite, and troctolite implies no major Mg isotope fraction- that are ∼0.10‰ differentfromthatoftheBSM(δ MgNWA7007 = 26 ation by crystallization of olivine cumulates, which account for most −0.193 ± 0.016‰, δ MgDhofar1625 = −0.382 ± 0.032‰,and δ26 26 of the Mg in the BSM. However, the significant variation in Mg of δ MgNWA6570 = −0.369 ± 0.027‰)(Fig.1).Becauseourme- anorthosites and high-Ti lunar basalts with lower MgO content teorite samples were small (2–8 mg) and probably not represen- suggests possible Mg isotope fractionation during crystallization of tative of the bulk compositions, it is likely that measured isotopic clinopyroxene and plagioclase from the LMO (SI Appendix,Fig.S5). variations are due to different proportions of major minerals within 26 Three low-Ti lunar basalts have average δ Mg of −0.285 ± these samples. 0.109‰ similar to those of the BSM (this study and ref. 23), Mars To constrain whether the measured Mg isotope compositions (35), Earth, and chondrites (41). In contrast, high-Ti basalts have were formed during the LMO crystallization, we have analyzed 26 substantially lower δ Mg values (−0.694 to −0.312‰)withtheav- several lunar mineral separates (SI Appendix, Table S2) and have 26 erage δ Mg of −0.462 ± 0.084‰ (Fig. 1 and SI Appendix,TableS1), modeled the Mg isotope fractionation during the LMO crystal- which is similar to earlier reports (23, 37, 39). Stable isotope studies of lization using the estimated mineral-melt fractionation factors other elements such as O, Ti, Li, and Fe have also shown a dichotomy based on these lunar mineral measurements and terrestrial between low- and high-Ti basalts with low-Ti basalts being similar to mineral data in the literature. the BSE and high-Ti basalts departing toward heavier/lighter isotope compositions (17, 18, 20, 34, 45, 64, 65). Sedaghatpour et al. (23) Isotope Fractionation Model predicted the isotopically light ilmenite produced at the late stage of We model Mg isotope fractionation during the LMO differen- LMO solidification to be the main source of the isotopically light Mg tiation to test if the isotopic dichotomy seen in low- and high-Ti observed in high-Ti basalts. However, our analyses of lunar ilmenites basalts is consistent with the cumulate remelting model (9, 12, show insignificant Mg isotope fractionation by ilmenite crystallization 18). In this study, we used the Snyder et al. (11) model for the SI Appendix at the late stage of LMO solidification ( ). LMO differentiation. The combination of both equilibrium and δ26 − ± The Mg values of two splits of sample 15555, 0.723 fractional crystallization (suggested in ref. 11) was not used in ‰ − ± ‰ 0.037 (15555, 19) and 0.778 0.017 (15555, 999), are the Fe isotope model of ref. 27; hence, to be consistent with this significantly lighter than other low-Ti basalts. However, another model and evaluate our model with different major elements, we split of this sample has an unusually heavy δ26Mg value of −0.02 ± ‰ present results for both calculated Mg and Fe isotope ratios. 0.03 (23), which was explained by the heterogeneous mineral In ref. 11, Mg and Fe concentration evolutions are given as

distribution in this sample. Oxygen isotopic and chemical variations = EARTH, ATMOSPHERIC,

a function of percent solidification (PCS) defined by PCS AND PLANETARY SCIENCES are also observed in different chips of this sample, which consists of 100(1 − Fm), where Fm is the mass fraction of residual melt. This olivine, pyroxene, and plagioclase (17, 66). Because chemical vari- model has six stages with the first two being equilibrium crys- ations among Apollo 15 olivine-normative mare basalts are mostly tallization of Ol (up to PCS = 40) followed by Opx until PCS = controlled by olivine, it was suggested that representative samples of 76. To use their result (figure 3 in ref. 11), each stage is ap- Apollo 15 olivine-normative rocks should be >1 g and ideally >5g proximated with a constant bulk solid–melt partition coefficient (66). Moreover, a recent Mg isotopic study of a zoned olivine grain i (D s/m value) for element i. The concentration evolution of i in from sample 15555 has shown a significant effect of the Mg–Fe i the magma (C m) for equilibrium crystallization stages is given by intermineral diffusion (67). A similar kinetic Mg–Fe isotope variation has also been observed in olivine megacrysts from a Martian Ci 1 meteorite (68). During the Mg–Fe diffusion, the mineral becomes m = [1] Ci F + Di ð − F Þ, isotopically lighter than coexisting mineral or melt if Mg diffuses T m s=m 1 m into it and becomes isotopically heavy if Mg diffuses out, due to the i faster diffusion of light isotopes. Therefore, different mineral where C T is the concentration of i in the total (T) or bulk system. abundances in small subsamples could affect the isotopic compo- Then, the rest of the concentration evolution is controlled by sitions of different splits of the basalt 15555. We have dissolved fractional crystallization forming Opx (PCS = 76–78%), Ol + ∼0.053 g of 15555, 19 and ∼1.004 g of 15555, 999, much larger Pl + pigeonite (Pig) (PCS = 78–86%), Cpx + Pl + Pig (PCS = amounts than the ∼0.010 g subsample dissolved in ref. 23. The 86–95%), and Cpx + Pl + Pig + ilmenite (Ilm) (PCS = 95– relatively consistent δ26Mg values in our two larger subsamples 99.5%). We use constant D values for each stage in the Rayleigh suggest that the δ26Mg value of −0.778‰ measured in the largest fractional crystallization law: subsample may be representative of the bulk 15555. However, this Ci Di − isotopic composition is still anomalous among the low-Ti basalts. m = F s=m 1 [2] i m . Among the breccias analyzed in this study, only sample 14321, CT 1803 has a significantly lighter Mg isotopic composition than the Δ26 = − ‰ estimated BSM ( Mg14321-BSM 0.869 ) (Fig. 1). Chem- This allows us to reproduce the Snyder et al. (11) model to calculate ical and isotopic compositions of this sample indicate that it is the Mg and Fe concentration of the melt at different stages of LMO derived from KREEP and high-Al basalt-rich impactites formed crystallization. The parameters used for this calculation are given in in pre-Imbrian craters (69, 70). The light Mg isotopic composi- SI Appendix, Tables S5 and S6.TheresultsinFig.2A show Mg and tion of the sample 14321 is not likely a result of evaporation/ Fe concentration evolutions of the LMO that closely reproduce the deposition during the impact events, because Mg is a moderately curves in ref. 11. For an isotope ratio (i/j), where i and j are different refractory element (71), and its isotope composition is unlikely to isotopes of an element E, we need to define the mass fraction of the be fractionated by the impact events. In addition, none of the reference isotope j in the melt: other breccias and impact melts (this study and ref. 23) shows a Cj F Cj ½ − ðPCS= Þ significant Mg isotopic fractionation relative to the BSM. Based f j = m m = m 1 100 [3] m j j . on the isotope fractionation model in the following section, CT CT KREEP components are likely to have very low δ26Mg. Thus, the KREEP-rich nature of this sample may explain its extreme Mg The following equations are used to calculate the isotopic isotope composition. However, analysis of the KREEP basalt compositions of the melt and cumulate as a function of PCS at 15386 yielded δ26Mg of −0.349 ± 0.038‰, which is not as low as each stage that is approximated with a constant fractionation i=j could be expected if it was only related to KREEP components. factor ðαs=mÞ for different isotope ratios between solid (s) and

Sedaghatpour and Jacobsen PNAS | January 2, 2019 | vol. 116 | no. 1 | 75 Downloaded by guest on October 1, 2021 A

B Fig. 2. Calculated evolution of δ26Mg and δ56Fe values vs. PCS during the LMO differentiation. The model is based on the magma ocean crystallization model of ref. 11 with equilibrium crystallization up to 76% PCS of the LMO followed by fractional crystallization (Cpx, clinopyroxene; Ilm, ilmenite; Ol, olivine; Pig, pigeonite; Pl, plagioclase). (A) The Fe and Mg concentration evolutions during LMO crystallization from ref. 11 are reproduced to have the Mg and Fe contents of the melt and cumulate at different stages to calculate the Mg and Fe isotopic evolutions. (B) The solid and dashed lines show evolution of Fe and Mg isotope compositions of residual melts and instantaneous cumulates, respectively. The shaded blue and yellow boxes are Mg and Fe isotopic compositions measured for low- Ti and high-Ti basalts, respectively (this study and refs. 35 and 39). For low-Ti basalts, δ26Mg is in the range of −0.394 to −0.220‰, and δ56Fe is in the range of 0.038–0.110‰. For high-Ti basalts, δ26Mg is in the range of −0.312 to −0.694‰, and δ56Fe is in the range of 0.130–0.212‰. The results show that Fe and Mg isotope variations in lunar basalts are consistent with the LMO cumulate remelting model for their origin (9, 10, 12).

melt (m). The evolution of the isotope ratios in the melt for Fe concentrations for crystallizing minerals, and the resulting equilibrium crystallization in δ notation is bulk isotope fractionation factors, respectively. The calculated h i curves for the evolution of 26Mg/24Mg and 56Fe/54Fe of the i=j 1,000 + δ LMO are shown in δ values in Fig. 2B. The solid and dashed i=j T δ Em = h i − [4] j j i=j 1,000. curves represent melt and cumulate compositions, respectively, fm + 1 − fm αs=m that evolve during the LMO solidification. Lunar basalts have been produced by partial melting of the LMO cumulates (9, 12, The evolution of the isotope ratios in the melt for the stages with 18, 72). Hence, melting of LMO cumulates following the dashed Rayleigh fractional crystallization is curves in Fig. 2B tends to produce melts that are similar to the " # LMO melt shown by solid curves. This is because the isotope h i i=j α − fractionation during melting to form lunar basalts involves the i=j i=j j s=m 1 δ Em = 1,000 + δT fm − 1,000 . [5] same minerals (the source cumulate minerals), but the process is a melt-mineral fractionation instead of a mineral-melt fractionation. Our calculations show that primarily crystallization of Opx In both equilibrium and fractional crystallization, the cumulate and Cpx with slightly heavier Mg isotopic compositions com- isotope ratio evolutions are given by pared with that of the BSM can produce a light residual melt up to −1.716‰ by the end of LMO crystallization (PCS = δi=jE − δi=jE = αi=j [6] cum m 1,000 ln s=m . 99.5%) (Fig. 2B). The blue and yellow rectangles on the Mg isotope data in Fig. 2B show the measured ranges of δ26Mg SI Appendix, Tables S7–S9 list the isotope fractionation factors values of low- and high-Ti lunar basalts, respectively. The mea- between minerals and melt used in our calculation, the Mg and sured values of these basalts overlap the Mg isotope evolution

76 | www.pnas.org/cgi/doi/10.1073/pnas.1811377115 Sedaghatpour and Jacobsen Downloaded by guest on October 1, 2021 curves of the melt and cumulates in our model, which shows that sufficiently similar, the differences between the two models are the large isotopic variation of lunar basalts can be the result of insignificant. We also tested our model considering only frac- the magmatic processes producing these basalts. This implies tional crystallization through the LMO crystallization suggested that a simple average of the isotopic compositions of the basalts recently (19), which gives similar results for low- and high-Ti is not likely to be representative of the BSM. A similar argument basalts except for the less depleted residual melt (−0.76‰)by can be made for any other differentiated body, which is ignored the end of LMO crystallization (PCS = 99.5%). by Hin et al. (29). The isotopic compositions of the lunar basalts are, however, consistent with the cumulate remelting model. In Conclusions particular, the positions of low- and high-Ti basalt fields in Fig. Our model shows that both Mg and Fe isotope variations in 2B are consistent with the type of source mineral assemblages lunar basalts can be explained by the LMO cumulate remelting inferred from the Lu–Hf and Sm–Nd isotope systematics of these model. The Mg isotope measurements of lunar samples com- basalts (12, 15) as well as some other stable isotopic studies of bined with this model support the notion that (i) the BSM and these samples (17, 18, 20, 27, 45). Based on these studies (12, BSE have similar Mg isotopic compositions, (ii) Mg isotopes 15), low-Ti basalts can be produced from an assemblage of oliv- preserve the signature of lunar magmatic differentiation, and ine and orthopyroxene with trace amount of clinopyroxene that (iii) the isotopic composition of the lunar basalts may not be the crystallized early in the history of the LMO, whereas high-Ti best representative of the BSM. Magnesium isotopic composi- basalts can be produced from a variety of late ilmenite-bearing tions of the low-Ti basalt 15555 and the KREEP basalt 15386 do mineral assemblages. The calculated enrichment of the heavy Fe not fit our calculated model. Furthermore, although Mg isotopic isotopes at late stages in the LMO is also shown in Fig. 2B. This composition of lunar breccia 14321 fits the model, its chemical enrichment is caused primarily by crystallization of olivine and composition with high Mg content does not match the late cu- pyroxenes that produced the cumulate sources of low-Ti basalts. mulate of LMO crystallization with low Mg content. These Crystallization of ilmenite with high Fe content at the end of anomalies might be related to different and more complex ori- LMO solidification produced the cumulate sources of high-Ti gins for these samples. basalts. The blue and yellow rectangles on the Fe isotope data in Fig. 2B show the measured ranges of δ56Fe values for low-Ti Materials and Methods and high-Ti lunar basalts, respectively, which also overlap with Samples. Different types of lunar samples including pristine anorthositic and the Fe isotope fractionation curves for the LMO crystallization. Mg-suite rocks, lunar basalts, few mineral separates, and several Mg stan- This model shows that the crystallization of pyroxenes signifi- dards are studied here. Petrology, mineralogy, and chemical compositions EARTH, ATMOSPHERIC,

cantly affects both Mg and Fe isotopic compositions of the of these samples are available in the lunar sample compendium (76) and the AND PLANETARY SCIENCES source of low-Ti basalts. On the other hand, clinopyroxene crys- R. Korotev list of lunar meteorites (meteorites.wustl.edu/lunar). A brief de- tallization could be the most important factor for explaining the scription of these samples is given in SI Appendix. light Mg isotopic composition of high-Ti basalts, whereas ilmen- Analytical Methods. ite crystallization has a larger effect on the Fe isotopic compo- The sample dissolution process was done in a mixture of sition of these basalts. Lunar basalts show an Fe isotopic HF-HCl-HNO3 using a CEM MARS 6 microwave digestion system through a three-step procedure. Ion-exchange chromatography procedure established composition that is generally heavier than that of the BSE, with in our group (38) was used for Mg purification. Magnesium isotope ratios the Fe isotopic composition of low-Ti basalts being closer to the were measured with a Nu Plasma II MC-ICPMS in low-resolution mode and ∼ ‰ δ56 BSE and lighter ( 0.1 in Fe) than high-Ti basalts (27, 32, wet plasma analysis using the sample-standard bracketing method. The re- 45). It has been suggested that the heavy Fe isotope composition sults are reported in δ notation relative to the DSM3 standard in per mil (‰) of the Moon estimated based on the basalts is due to either (SI Appendix, Tables S1 and S2). The uncertainties of the measurements are

evaporation of Fe by the giant impact or isotopic fractionation reported as two SEMs (2σm). The long-term reproducibility, which is the 2SD during the lunar magmatic differentiation (45, 64, 73–75). The of multiple isotopic measurements of the standards over 18 mo, is better isotopic fractionation during the lunar magmatic differentiation than ±0.08‰ (SI Appendix, Fig. S3 and Table S4). The Mg isotopic compo- is favored in our model because the more recent Fe isotope data sitions of our in-house pure standards (38), Cambridge1 (77), and the US of Mg-suite rocks as better analogs of the BSM suggest similar Geological Survey standards (78) are within error of the recommended val- Fe isotopic compositions for the BSE and BSM (27, 32). Mag- ues (SI Appendix, Table S4). The Mg isotopic compositions of all standards and samples measured here lie on a single mass-dependent fractionation nesium loss during the Moon formation has also been suggested curve following the exponential law (SI Appendix, Fig. S4), which is in as a cause of small variation seen between the BSM and chon- agreement with the previous studies (47). More details of the analytical drites (29). However, our study demonstrates that accurate ac- method are given in SI Appendix. counting for the Mg and Fe isotopic fractionation during the LMO differentiation is needed before any inferences about evap- ACKNOWLEDGMENTS. We thank the reviewers for their constructive com- orative loss of these elements from the Moon can be made. ments, Misha Petaev and Chris Parendo for discussion, Dimitri Papanastassiou Although a new model by Lin et al. (13) makes improvements for providing lunar mineral separates, Randy Korotev for providing lunar meteorites, and Kun Wang for meteorites’ dissolution process. This work upon (11), our results are not particularly sensitive to the dif- was partly funded by NASA Grants NNX12AH65G and NNX15AH66G. NASA ferences between these two models as can be seen from Eq. 3. Johnson Space Center and Curation and Analysis Planning Team for Extrater- Because the changes in fractionating mineral assemblages are restrial Materials kindly provided the Apollo samples for this study.

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