Trivalent titanium in high-titanium lunar ilmenite

Domain: biology
Published: 2026-03-27
Editorial outcome: minor_revision

Abstract
Lunar mare basalts are often rich in titanium, hosted predominantly within the mineral ilmenite (Fe2+Ti4+O3). Here, we examine ilmenite in a ~3.8 billion-year-old mare basalt (Apollo rock 75035) using high-resolution electron microscopy and electron energy loss spectroscopy. A key finding is that 75035 ilmenite is itself enriched in Ti, beyond the end member of the conventional solid solution series. Using energy loss near-edge spectroscopy, we determine that the excess Ti is trivalent, with Ti3+ accounting for 13% of the total Ti content. This discovery confirms the presence of trivalent Ti in lunar ilmenite, long hypothesized based on the Moon’s reducing environment. Accounting for the change in implied stoichiometry, a review of literature data suggests that Ti3+ may be present in ilmenite across a wide range of lunar samples. We extrapolate known relationships from the literature to connect Ti3+ to redox conditions, estimating the oxygen fugacity during crystallization of 75035 to be below the iron-wüstite buffer, ΔIW ≤ −1.6. Further quantifying the relationship between Ti valence state and oxygen fugacity would allow Ti3+-bearing ilmenite to serve as an oxybarometer able to access the reducing conditions found on many planetary bodies.

Introduction
Lunar basalts preserve the geological history of the Moon and offer insights into the evolution of the Earth-Moon system. The compositional diversity of lunar basalts (e.g., in Ti content) reflects variations in mantle source regions and degrees of partial melting across both time and space, offering a window into the composition and structure of the lunar mantle. Petrological studies of mare basalts have been essential to establishing the environmental conditions, especially oxygen fugacity (fO2) and temperature (T), during volcanic activity on the ancient Moon (e.g., ref. 1). Apollo 17 returned more than one hundred high-Ti basalts from the Taurus-Littrow Valley 2,3. Of these, two samples from the rim of Camelot crater (75035 & 75055) have low Mg content and contain no olivine (Type II in ref. 3). These date to ~3.8 Ga, coinciding with the initial peak of lunar volcanic activity 4–7, and likely represent the deepest lava flows sampled by the Apollo 17 crew 6,8. The distinctive composition of these Type II basalts makes them compelling for investigating the magmatic environment during their formation.

Ilmenite (modal abundance of 14–17% in 75035 2,9) is a major titanium reservoir in high-Ti basalts, such as 75035. Petrological studies of 75035 generally suggest that ilmenite is a phenocryst phase, possibly the liquidus phase, and that its crystallization interval overlaps with plagioclase and perhaps even later-stage minerals 9,10. Due to its prolonged and early crystallization in 75035, ilmenite could preserve evidence of the oxygen fugacity and temperature of its host magma. Deviations from the relatively simple FeTiO3 stoichiometry can, in principle, be used to trace the thermal and oxidation-reduction (redox) history of ilmenite basalts.

In this Article, we use scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) to study a ~7 mg chip from 75035-232. Our high-resolution microscopy and spectroscopy results show that the 75035 high-Ti basalt is not simply enriched in stoichiometric ilmenite, but that the ilmenite itself contains a significant excess of Ti. We find that the excess Ti exists as Ti3+, occupying both Fe2+ and Ti4+ sites and confirming prior speculation 11. From energy-loss near-edge structure (ELNES) of the Ti L2,3 core-level spectra, we directly measure the oxidation states of Ti, showing that the amount of Ti3+ matches that of the excess Ti found via EELS. This implies an underlying stoichiometry of Fe2+1−xTi4+1−xTi3+2xO3. Our EELS measurements determine xEELS = 0.079 ± 0.040 by assigning the excess Ti to Ti3+, and we find xELNES = 0.067 ± 0.016. This level of excess Ti places 75035 ilmenite well beyond the stoichiometric end-point of the conventional solid-solution series and suggests a highly reducing formation environment. Employing existing petrology experiments for Ti-enriched ilmenite 11–15, we use the Ti3+ content to estimate an upper bound on the oxygen fugacity, relative to the iron-wüstite buffer (IW), of ΔIW ≤ −1.6 during ilmenite crystallization. More generally, our research suggests that Ti3+ in ilmenite may be common on the Moon, as we show by a re-examination of ilmenite analyses within the AstroMat database.

Results
It is important to establish at the outset that 75035-232 shows no signatures of space weathering (Fig. S1): we find no Si-bearing vapor deposits or glasses, nanophase iron, vesicles, or microcraters 16–20. This indicates that 75035-232 was not sun-facing, consistent with our analysis of its orientation from the lunar compendium 21 and the reconstructed orientation reported in ref. 22. Therefore, we attribute the trivalent excess titanium in a lunar ilmenite exclusively to a magmatic origin.

Mineralogy and tomography of 75035
Mineral identification on the exterior surface of the chip from 75035 (Fig. 1a) reveals (by area) 45% pyroxene, 43% plagioclase, 11% ilmenite, and <1% SiO2-phase, with the SiO2-phase likely to be underestimated due to the limited spatial resolution in X-ray fluorescence (XRF) measurements (see Supplementary Fig. S2 and Methods). These results are consistent with prior characterizations of 75035 2,9, confirming that our chip is representative of the larger rock. A density map from X-ray tomography highlights a distinct ilmenite domain running through the chip (white region in Fig. 1b). The ilmenite domain is also highlighted in the energy-dispersive spectroscopy (EDS) map of Fig. 1g (Ti distribution in light blue). Slices from the tomography (Fig. 1c–f), interpreted as virtual thin sections, show that the ilmenite region appears as large laths penetrating the surrounding plagioclase (darker gray) and pyroxene (lighter gray), suggesting that ilmenite crystallized early in the sequence. The plagioclase appears well-formed and is encased by pyroxene (Fig. 1f and Supplementary Movie 2), consistent with plagioclase largely crystallizing before pyroxene 9,10.

The crystallization sequence of high-Ti basalts is inherently complex, influenced by factors such as the cooling rate, Fe2+/(Fe2++Mg) ratio, and oxygen fugacity at the time of formation 23. Fe-Ti oxides in general can record the thermal and redox histories of magmas, thanks to well-characterized exchange and redox reactions involving di- and trivalent Fe, along with tetravalent Ti 24,25. The trigonal R3 crystal structure of ilmenite (FeTiO3) consists of alternating layers of two octahedrally coordinated M-sites—one occupied by Ti4+ and the other typically by Fe2+, with possible cation substitutions such as Mg2+ 26. Compositional and structural variations in ilmenite can serve as indicators of the magmatic conditions present during crystallization 1,25,27,28, which is especially useful for 75035, where ilmenite is the dominant and earliest Fe-Ti oxide phase to crystallize 9,10.

Nanoscale characterization of 75035 ilmenite
We employed high-resolution STEM and EELS and used a focused-ion beam (FIB) to lift out thin lamellae from the ilmenite regions of 10–50 nm thickness and 5–8 μm lateral extent (see Methods and Supplementary Fig. S3 for the FIB procedure; lift-out regions are shown in Fig. 1g). Lift-out samples extracted from locations I through VIII were selected in different orientations to access different crystallographic zones in STEM. Laue X-ray diffraction (XRD), performed on sample V, confirms that the mineral extracted is ilmenite (Supplementary Fig. S4), showing the expected unit cell with lattice constants of a = 5.0971 ± 0.034 Å and c = 14.1084 ± 0.093 Å. Point mapping from XRD over a 30 × 30 μm2 lift-out (Fig. S4a) shows little to no variation in the diffraction pattern, indicating large single-crystal domains, consistent with observations from STEM. Four-dimensional STEM (4D-STEM), performed on the same lift-outs used for EELS analyses, confirms the crystal orientation from XRD (Supplementary Fig. S4) and high-resolution STEM imaging (Supplementary Fig. S5).

EDS, EELS, and XRF all measure excess Ti in 75035 ilmenite (i.e., Ti/Fe ratio greater than one in Fig. 2a). The quantification results from EELS provide accurate measurements of elemental ratios based on fitting the Hartree-Slater cross-sections (Fig. 2b) to determine the height of the ionization edges for each element (Methods). A quadrant section of the Fe–Ti–O ternary diagram is shown in Fig. 2c, where the ilmenite and spinel solid solution series are indicated by gray shaded regions (adapted from ref. 11). Our EELS quantification results are plotted in green in Fig. 2c, with the terrestrial ilmenite sample falling within the ilmenite solid solution series, while the 75035 ilmenite extends beyond it.

However, this raises a question: where does the excess Ti go? To retain its Ti4+ oxidation state would require defects in the ilmenite, which are not evidenced by our STEM and XRD results. It was proposed by Simons & Woermann (1978) that substituting Ti for Fe in ilmenite could result in compositions extending toward Ti2O3 with trivalent titanium 15. Subsequently, Lattard (1995) synthesized Ti-enriched ilmenite and spinel (red shaded regions, Fig. 2c), inferring a Ti3+ abundance in ilmenite of up to 0.026 apfu 11. Here, we find that 75035 ilmenite exhibits an even higher Ti content (green dots, Fig. 2c).

By assigning the excess Ti content to Ti3+, based on the elemental ratios of the ionization edges obtained from EELS, we can determine the amount of Ti3+ in ilmenite using the following substitution mechanism 11:

Fe2+ + Ti4+ ↔ 2Ti3+, (1)

where 2Ti3+ are substituted into both M-sites in ilmenite. This substitution results in a stoichiometric formula of

Fe2+1−x Ti4+1−x Ti3+2x O3, (2)

where x represents the deviation from the ideal stoichiometric ilmenite 11. From the excess Ti content in Fig. 2a, we extract xEELS = 0.079 ± 0.040, with the average of values from model fits weighted by their inverse variance (Methods). The complete elemental analysis is provided in Table 1. The table shows that Ti is still in excess after accounting for the substitution of minor elements (Methods). Although xEELS is large, it remains within the expected range for Ti3+ incorporation under the reducing conditions of the lunar environment, given the bulk basalt composition (cf. ref. 29 and Discussion below).

ELNES of the Ti L2,3-edge provides direct evidence of Ti3+ in 75035 ilmenite. ELNES is sensitive to sample thickness (t) due to multiple scattering effects from bulk plasmons, making it essential to acquire spectra from regions of the FIB lamella thinner than ~0.3λ, where λ is the inelastic mean-free path (83 nm to 105 nm for our system and samples). Figure 3a shows a STEM high-angle annular dark-field (HAADF) image of sample II (see Supplementary Fig. S6a for the full region of ELNES data acquisition), with the corresponding thickness profile of the FIB lamella in Fig. 3b. The difference in ELNES intensity between 75035 and terrestrial ilmenite (I75035 − Iilm) is shown in Fig. 3c, normalized by t/λ, to account for the thickness-dependent change in total scattering probability. The 75035 ilmenite spectra exhibit higher intensity in the valley regions of the L3 and L2 edges (Fig. 3d), a characteristic spectral signature of Ti3+ 30 that manifests as bright regions between the two vertical dotted lines in Fig. 3c. In the thin region (t < 15 nm), the normalized difference spectrum, (I75035 − Iilm)/(t/λ) (black, Fig. 3e), exhibits a line shape consistent with the Ti2O3 ELNES reference (green, Fig. 3e). As thickness increases, the evidence of Ti3+ becomes less clear due to multiple scattering obscuring the ELNES details (Fig. 3f). Complementary O K-edge EELS measurements (Supplementary Fig. S7) show an intensification of the second peak, presumably due to hybridization of the Ti 3d xy and O 2p states 31,32, which provides additional evidence for Ti3+.

The percentage of Ti3+ can be quantified from the Ti L2,3-edge through linear least-squares fitting, using a weighted sum of two reference spectra (Methods): Fe2+Ti4+O3 (terrestrial ilmenite) and Ti3+O2 (from ref. 30). Supplementary Fig. S8 illustrates the fits to the Ti L2,3-edge of sample II. The fitting directly provides the modal abundance of Ti2O3, yielding a weighted average of xELNES = 0.067 ± 0.016 within the 10 nm to 15 nm thickness range, where ELNES quantification is most reliable. The quantification of excess Ti content, described in the previous section, yields a value of xEELS = 0.081 ± 0.016 (sample II, Table 1), which is within 1σ uncertainty of xELNES. We note that the presence of Ti3+ in lunar ilmenite was previously suggested for an Al-rich mare basalt (Apollo rock 14053) 33,34, but no quantitative estimate was obtained due to instrumental limitations 34. Our work provides a quantitative measurement of Ti3+ in lunar ilmenite.

Distribution of Ti3+ in the lunar collection
Along with two other mare basalts (Table 1), 75035 has been recognized as unique for its lack of olivine and low Mg content 2,3. Therefore, it is reasonable to ask whether Ti3+ enrichment is limited to this class of mare basalt. A definitive answer would require new analyses of many samples, but the literature can provide some guidance. Within the broader collection of lunar samples, we identified 530 lunar ilmenite analyses in the AstroMat data repository 35 that potentially contain Ti3+, based on calculated x values (Methods, Eq. (3)). Among the 530 samples (gray dots, Fig. 4a), assuming all excess Ti to be trivalent, the highest values are nearly half of our measured Ti3+ content (x = 0.069 ± 0.015, weighted average of xEELS and xELNES). Samples with the highest apparent Ti3+ content (x ~ 0.030) are observed across a range of sample types, including Chang’e-5 young basalts 36 and an Apollo 12 basalt 12003 37 from Oceanus Procellarum, an Apollo 17 basaltic fragment 79513 38, and an Apollo 14 crystalline matrix breccia 14321 39. Thus, we postulate that the conditions necessary to stabilize Ti3+ may have been widespread on the Moon.

Our measured minor element fractions (Table 1) are consistent with prior electron microprobe analyses of 75035 ilmenite 10 (brown circle, Fig. 4a) and with the two similar mare basalts (75055 40, 14053 33). The Ti/Fe content of 75035 ilmenite in ref. 10 falls at the lower edge of our measurements (Ti/Fe ~ 1.047, Table 1), implying only a small enrichment of Ti3+. However, comparisons with the literature are limited by differences in analytical methods, which may presume oxidation states and stoichiometries. Our ELNES results for 75035 ilmenite require the presence of Ti3+ and imply the stoichiometry of Eq. (2). As a corollary, our measurements also suggest that re-analysis of other lunar ilmenite samples could reveal Ti3+ levels beyond those we estimated from the AstroMat database.

Estimate of oxygen fugacity at formation
Our discovery of Ti3+-bearing lunar ilmenite provides important context for the oxygen-poor conditions recorded on the Moon. Here, we use our measurements to estimate redox conditions during the crystallization of 75035 ilmenite, based on the few experimental results in the literature. Conventional oxybarometers typically require equilibrium between two or more specific phases, restricting their utility to systems containing these phases and to samples sufficiently large to demonstrate their coexistence. For example, the two-oxide thermo-oxybarometer relies on Fe exchange and oxidation in coexisting titanomagnetite and hemoilmenite solid solution series (cf. refs. 14,24,25). We connect thermo-oxybarometers of titanomagnetite-ilmenite assemblages with our quantification of Ti3+ to estimate possible fO2-T conditions of 75035 ilmenite, which has been found to coexist with a spinel phase 10. Data from refs. 11,13,14 are fit to a line to determine an fO2-Ti3+ relationship at 1300°C (Methods). Based on our analysis of 75035 ilmenite, the fitted line (black dashed line, Fig. 4b) establishes an upper bound of ΔIW = −1.6. Accounting for our uncertainty in Ti3+ (green region, Fig. 4b), in addition to the inherent error of the linear extrapolation (gray region, Fig. 4b), the upper limit could reach IW − 0.8 and the lower limit of fO2 could, in principle, extend to values as low as IW − 10. Given that prior estimates of lunar fO2 range from IW − 0.5 to IW − 2 41–43, and that the fO2 of our Solar System’s most reduced body, Mercury, is roughly IW − 3 to IW − 7 44, a lower bound of IW − 10 is unreasonable. Nonetheless, our ΔIW = −1.6 in Fig. 4b approximately overlaps the equilibration region of Ti3+-rich armalcolite (down to IW − 1.5 45,46) and falls within the range of fO2 estimates for soils newly returned from the lunar farside by Chang’e-6 (South Pole-Aitken basin; IW − 1.9 43). These farside samples have an fO2 more than a full log unit lower than the average fO2 of nearside basalts (IW − 0.8 41), underscoring the significance and timeliness of our findings.

The crystallization temperature of experiments in Fig. 4b (1300°C) exceeds the expected range for most mare basalts, prompting us to use experimental constraints from ref. 12 to estimate a more realistic upper bound on the temperature. A bulk composition for low-Al2O3, high-Ti mare basalts was used in ref. 12, similar to that of 75035. Initial temperatures around 1200°C and slow cooling rates result in subhedral, tabular ilmenite textures, similar to those observed in 75035. The ilmenite-in curve for the slowest cooling rate (~2°C h−1) is extrapolated to lower fO2 values to provide a plausible upper bound on the formation temperature, yielding a value of T ~ 1150°C (Fig. 4c, blue line). Isopleths of constant ilmenite mole fraction (Xilm) within titanomagnetite-hemoilmenite assemblages are used to contextualize the fO2-T dependence of specific ilmenite compositions. In this case, however, providing a quantitative isopleth is challenging because the excess Ti content in 75035 ilmenite places its composition beyond the conventional solid solution series (Fig. 2c). Instead, we use the isopleths (Fig. 4c, red lines) at Xilm = 0.992 (calculated from microprobe analyses of ilmenite and ulvöspinel in 75035 10, see Methods) and Xilm = 0.999 (in the limit where there is no Fe3+ but ilmenite is still in equilibrium with the magnetite-ulvöspinel phase) to estimate the fO2-T dependence. The isopleth at Xilm = 0.992 shows little temperature dependence below its intersection with the ilmenite-in curve from ref. 12, suggesting that even though the upper bound of fO2 is based on data at 1300°C (Fig. 4b) it still provides a reasonable approximation without significantly overestimating the fO2 at lower temperatures. The isopleth at Xilm = 0.999 provides additional context for the fO2-T dependence, for ilmenite containing essentially no Fe3+. Although the temperature dependence is more pronounced than for Xilm = 0.992, refs. 13,14 discuss the lack of significant ilmenite compositional change with temperature and the general uncertainty of dependencies at high Xilm, especially at low oxygen fugacities. In any case, Ti3+-bearing ilmenite would be constrained to lie below this Xilm = 0.999 curve, with the caveat that the isopleth comes from a system that also contains a spinel phase.

Petrological experiments have established direct relationships between Ti3+ and fO2 in armalcolite and pyroxene (e.g., refs. 34,45,47,48). Ti valence in pyroxene has even been used to estimate the fO2 of mare basalt 14053, which is similar in composition to 75035 and has been hypothesized to contain Ti3+ in ilmenite 34. Indeed, pyroxene Ti3+/Ti4+ (as well as Cr2+/Cr3+ and V2+/V3+) suggests an fO2 close to IW − 2 48, consistent with our extrapolated fO2 range for 75035. Moreover, recent studies use X-ray absorption near edge structure (XANES) spectroscopy to examine the Ti K-edge in Fe-Ti oxides from synthetic basalts in a modified CaO–MgO–Al2O3–SiO2 system (1100°C, IW − 1.8) 49. These authors find that the average Ti valence state in Fe–Ti oxides (armalcolite and ilmenite) is 3.64 ± 0.08 (i.e., ~40% of Ti atoms are Ti3+) 49. Another XANES study of Apollo 17 basalts finds that an average of 15%−20% of Ti atoms in pyroxene can be in the trivalent state 29. Consequently, our measurements of trivalent titanium in 75035 ilmenite (~13% of the total Ti) and our fO2 estimate (ΔIW ≤ −1.6) are both consistent with these recent measurements 29,49.

Beyond 75035
Lunar basalts preserve the Moon’s evolutionary history. Our work advances lunar petrology by integrating modern microscopy techniques with existing petrological experiments. And we confirm a longstanding hypothesis: the presence of Ti3+ in lunar ilmenite.

The geographic distribution of Ti-enriched ilmenite on the Moon could provide insights into the evolution of local inhomogeneities in the lunar mantle, using Ti3+ content as a tracer of magmatic redox conditions. As discussed above, Chang’e-6 basalts from the lunar farside record an oxygen fugacity much lower than the mean fO2 estimated from nearside minerals 43. Any viable model of lunar evolution should explain such large fluctuations. Our work shows that modern characterization methods applied to the existing lunar collection could expose similar variability of redox environments, even within the set of nearside samples.

As a ubiquitous lunar mineral, ilmenite also has the potential to serve as a single-phase oxybarometer based on its Ti3+ content, extending methodologies previously applied to armalcolite. Since it is more abundant than armalcolite in lunar rocks, ilmenite provides a larger and more readily accessible phase for oxybarometer analyses. This potential single-phase oxybarometer should be explored with petrological experiments conducted on a variety of bulk compositions and cooling rates at known fO2-T conditions. Experiments covering the relevant range of stoichiometry and temperature would allow us to place more accurate bounds on the fO2-T conditions recorded by Ti3+-bearing ilmenite, strengthening and refining the application of our work. With such experiments, our small-volume analytical techniques could provide crucial constraints on the redox environment of magmas relevant to the Moon and other low-fO2 planetary systems.

Methods
X-ray fluorescence spectroscopy
Before XRF measurements, an optical image of the chip from 75035-232 was constructed from a z-stack using a Keyence VHX-7000 microscope (Fig. 1a). XRF was then acquired on the 2 mm chip before coating the surface with Au-Pd for SEM imaging. The Bruker Tornado system, available through the Georgia Tech Materials Characterization Facility (GT-MCF), uses polychromatic X-rays with a Rh source (spot size of 20 μm) and operates under low vacuum. The system was operated in point mode, acquiring a single spectrum with a pixel size of 20 × 100 μm2 across the exterior surface of the 2 mm chip. Mineral identification was performed using the M4 TORNADO AMICS software package, which contains pre-loaded standards for oxide quantification. The identification result is shown in Supplementary Fig. S2a, and excess Ti, calculated from Ti/Fe ratio, is shown in Fig. 2a.

X-ray tomography
Tomography data were acquired on the 2 mm chip from 75035-232, mounted on a SEM stub. Synchrotron X-ray microtomography was performed at Beamline 8.3.2 of the Advanced Light Source (ALS). The sample was scanned using monochromatic X-rays at 32 keV and an optical chain consisting of a 500 mm Ce-doped LuAG scintillator (Crytur), 2× Mitutoyo objective lens with long working distance (0.055 numerical aperture), and a pco.edge sCMOS detector (2560 × 2160 pixels), resulting in a pixel size of 3.5 μm and a lateral field of view of 5 mm. Two tiles were collected with a 30-pixel overlap and stitched together during post-processing. Raw data were reconstructed using the TomoPy software package 50. Data visualization and analysis were performed using ORS Dragonfly (results shown in Fig. 1b–f and Supplementary Movies).

Scanning electron microscopy
Prior to SEM imaging, the chip was coated with ~50 nm of Au-Pd to protect the surface. SEM was performed using a Thermo Scientific Helios 5CX at GT-MCF. EDS was used to acquire elemental maps at 15 keV for preliminary mineral identification on the 2 mm chip from 75035-232 (Supplementary Fig. S2b–g).

Focused ion beam lift-out procedure
The Thermo Scientific Helios 5CX is a dual-beam system capable of producing an electron beam with energies ranging from 0.5 keV to 30 keV (maximum beam current of 176 nA) and a Ga+ ion beam with energies also from 0.5 keV to 30 keV (beam current between 1 pA and 100 nA). Each critical step of the lift-out procedure is illustrated in Supplementary Fig. S3, which expands upon the standard FIB methodology 51 to accommodate the complex surface topography of our chip.

Before fabricating a lamella, we determined the appropriate angle for extraction using the micromanipulator needle, as it could influence the milling parameters discussed below. Supplementary Fig. S3a shows the deposition of a carbon protective layer (~2 μm thick). The front and back trenches (Supplementary Fig. S3b) were prepared using the standard lift-out procedure 51. When working with materials that have a complex surface topography, it is not always possible to view the lamella at the standard angles typically used for bottom cuts (between 0° and −10° for Helios 5CX). Therefore, extra care must be taken to make these cuts at alternative angles. The bottom and side cuts of the lamella were made in parallel to minimize issues from sputtered redeposition (yellow boxes, Supplementary Fig. S3c).

The extraction angle (between 0° and 52°) was carefully chosen to ensure that the gas injection system (GIS) and micromanipulator needle did not collide with the chip when inserted. Supplementary Fig. S3d shows the SEM view of the inserted GIS at the selected extraction angle, followed by the FIB view of the needle at the lift-out location in Supplementary Fig. S3e. It is sometimes useful to mill a trench into a corner (or side) of the lamella before attaching the needle, which helps to guide the needle into place and provides additional protection during extraction. The needle was then welded to the lamellae using a concurrent electron beam to enhance gas-assisted deposition (Supplementary Fig. S3f).

After extraction (Supplementary Fig. S3g), the lamella was attached to a FIB grid using the standard procedure 51. To ensure robust attachment, a Y-probe was used, and a trench was milled into the probe to allow diffusion of sputtered material (Supplementary Fig. S3h). For polishing the lamella, we began at 30 keV with a beam current of 0.77 nA, then reduced the current to 83 pA, followed by polishing at 5 keV and 43 pA, and finally at 2 keV and 10 pA. The final 2 keV polishing step was to flush the surface and reduce Ga implantation (SEM image of the polished lift-out in Supplementary Fig. S3i). The thinnest regions of the lift-out samples were ~10 nm, determined from the EELS zero-loss peak (e.g., Supplementary Fig. S8a), with a more typical thickness range between 20 nm and 50 nm in the bulk.

Scanning transmission electron microscopy
High-resolution STEM was performed using a Nion UltraSTEM-200X, equipped with a Gatan Enfinium ER spectrometer for EELS attached to a Merlin EELS direct electron detector and a Bruker silicon drift EDS detector, located at the Naval Research Laboratory. The STEM was operated at 200 keV and 40 pA with a 0.1 nm probe and a 27 mRad convergence angle. The annular bright field (ABF, ~9 mRad to 22 mRad) and HAADF (> 80 mRad) were collected for high-resolution STEM (Supplementary Fig. S6a). EDS quantification was performed using Bruker Esprit software (samples IA, IB, and IV in Table 1).

Electron energy loss spectroscopy
EELS core-loss and low-loss spectra were acquired sequentially using two dispersion settings: (1) 0.4 eV/ch for EELS quantification (acquiring two core-loss spectra at 390 eV and 650 eV to improve channel overlap) and (2) 0.1 eV/ch for each element (Ti at 435 eV; O at 510 eV; Fe at 687 eV) to capture the ELNES. The thickness of lift-out samples was measured from the ratio of the zero-loss peak to the total transmitted intensity, then converted from mean free path to absolute thickness 52.

Core-loss and low-loss spectra were used to determine the stoichiometry of the sample using a standard fitting procedure implemented in the exspy extension of Hyperspy 53,54. The core-loss spectra were iteratively fit to a power-law background and ionization edges, determined from the Hartree-Slater cross-sections, which were forward-convolved with the low-loss spectra to include the effects of plural scattering 55–57. The Ti L2,3- and O K-edges were fit simultaneously due to their spectral overlap, whereas the Fe L2,3-edge was fit separately because O K reaches the continuum before the onset of Fe L2,3. Elemental quantification of terrestrial ilmenite was performed to calibrate our fitting procedure. Figure 2b shows an example of the quantification fits of sample IV. The fine structure width, a region excluded from the fitting to remove the effects of the near-edge structure, was adjusted for each ionization edge to achieve the best fitting result. The quality of the fit was assessed both visually to determine whether the ionization edges reached the continuum and numerically to ensure the desired oxygen-conserving ratio of O/(Fe+Ti+O) = 0.60. The ratios of the ionization edges provide the stoichiometric ratios.

Our work shows a significant concentration of Ti3+, which is proposed to replace Fe2+ and convert the neighboring Ti4+ to Ti3+ so that charge balance is maintained (Eq. (1)). A complete model must consider the substitution of minor elements; here, we use y to label the minor-element occupation of the Fe-M site and y0 for the minor-element occupation of the Ti-M site. By accounting for minor elements, Eq. (2) is modified as,

Fe2+(1−y)−x Ti4+(1−y0)−x Ti3+2x O3, (3)

The values for y and y0 are determined from EDS measurements of elemental concentrations of minor elements: Mg, Mn, Cr, and Al. Both Mg2+ and Mn2+ are assumed to occupy the Fe-M site, as expected from the known ilmenite–geikielite and ilmenite–pyrophanite solutions series. Cr3+ and Al3+ can occupy both M sites equally, similar to Ti3+. With element symbols denoting respective site occupancies, the resulting values of y and y0 are:

y = Mg + Mn + 1/2 (Cr + Al),
y0 = 1/2 (Cr + Al): (4)

Our elemental quantification of Fe and Ti by EELS (Table 1) uses Eq. (3) with y and y0 determined by their average values over EDS measurements from the ilmenite samples (y = 0.053, y0 = 0.010) to obtain the corrected apfu values.

Linear least-squares minimization of oxidation state standards was used to fit the ELNES and determine the abundance of oxidation states for a given element. Specifically, a linear combination of terrestrial ilmenite (containing only Ti4+) and Ti2O3 (containing only Ti3+) was used to fit the Ti L2,3-edge of 75035 ilmenite:

(1−xELNES) [Ti L2,3]TiFeO3 + xELNES [Ti L2,3]Ti2O3, (5)

to determine the coefficient xELNES. Here, Ti2O3 serves as a suitable proxy for Ti3+ in 75035 ilmenite because it has the same octahedral coordination as ilmenite. Fitting results are summarized in Supplementary Fig. S8. It is important to note that this approach is only valid for very thin regions (< 15 nm) of the lift-out, where single scattering dominates, and subtle changes in the fine structure are not masked by bulk plasmons.

Transmission electron microscopy
The FEI Tecnai F30, accessible through GT-MCF, is a super-twin field-emission-gun transmission electron microscope (TEM) operated at 300 kV. The TEM was operated in STEM mode to acquire 4D-STEM diffraction patterns using a dwell time of 500 ns, a convergence angle of 0.40 mRad, and a pixel size of 4 × 4 nm2 with a Gatan OneView camera. The pyxem extension in HyperSpy 58 was used to correct for elliptical lens distortions and to calibrate the diffraction patterns using the Au-Pd protective coating on the lift-out samples.

The JEOL F200, located at the Naval Research Laboratory, is equipped with a high-throughput, high-accuracy EDS analysis system featuring two large-area silicon drift detectors. This setup enables high-sensitivity EDS measurements while minimizing sample damage. The EDS on the JEOL was used to provide an additional measurement of the minor elements in the 75035 ilmenite from sample VIII in Table 1. EDS quantification was performed using the vendor-supplied software.

Laue X-ray diffraction
XRD measurements were conducted at ALS Beamline 12.3.2 using a polychromatic beam with a spot size of 1 μm and an energy range of 5 keV to 24 keV. The beamline was configured in diffraction geometry and calibrated using a YAG (Y3Al5O12) crystal mounted on a glass substrate. A 45° angle of incidence was used. After performing a 2D scan to orient the beamline control software, a central location within the target area was selected, and a Laue diffraction pattern was collected using a DECTRIS Pilatus 1M detector placed directly above the sample, with an exposure time of 1 s per frame. XRD measurements result in an ilmenite unit cell with lattice constants of a = 5.0971 ± 0.034 Å and c = 14.1084 ± 0.093 Å, where the relatively large error bars are due to the use of a polychromatic source.

Data from AstroMat repository
Data from AstroMat Synthesis 35 is shown in Fig. 4a, where the presence of Ti3+ was calculated by attributing the excess Ti in literature analyses of lunar ilmenite to Ti3+. Data were downloaded in May 2025 using the following parameters: Collection = Lunar Samples; Analyzed Materials = Mineral: (Ilmenite, Opaque Mineral, Oxide). For all 2216 analyses, the excess Ti content (x in Eq. (3)) was calculated by expressing the ratio of Ti/Fe in terms of the substitution of the minor elements (i.e., y and y0 in Eq. (4)). The data were parsed to exclude entries that did not meet the following stoichiometric criteria: (1) Δcat < 0.02, where Δcat is the absolute difference between the actual cation sum and 2, the expected cation number for the ilmenite solid solution series; (2) Fe [apfu] ≥ 0.80, to ensure a sufficiently high Fe content for ilmenite; and (3) excess Ti content of x > 0 to select ilmenite analyses that are most likely to contain Ti3+. The literature containing space-weathered samples was excluded, further reducing the number of points to 530 (gray dots in Fig. 4a, from refs. 36–39, 60–87). An Excel file with the processed AstroMat dataset is provided in Supplementary Data 1. Confirmation of the Ti valence of these identified ilmenite analyses from the AstroMat database would require analyses similar to those presented in this work.

Petrology experiments from literature
Lattard (1995) synthesized spinel-ilmenite assemblages at 1300°C and oxygen fugacities between −0.2 < ΔIW < +2.2, using powders free of Mg 11; this study focuses solely on the formation of ilmenite-spinel intergrowths at 1300°C within the Fe–Ti–Cr–O system. Figure 4a, b shows stoichiometric values from ref. 11 (purple triangles), restricted to data with nonzero Ti3+ content. Lattard et al. (2005) used coexisting ilmenite and pseudobrookite in the Fe–Ti–O system as an oxybarometer for a temperature range of 1000°C to 1300°C 13. Data with Ti [apfu] ≥ 1.02 are chosen to ensure a reliably high Ti3+ concentration and are shown in Fig. 4a, b (orange triangles) for ilmenites synthesized at 1300°C.

Sauerzapf et al. (2008) extended the existing thermo-oxybarometer by including Mg and Al-bearing systems 14. The authors synthesized titanomagnetite-ilmenite assemblages over a temperature range of 1100°C to 1300°C and oxygen fugacities between −0.5 < ΔIW < +6.5 14; they inferred the presence of Ti3+ in ilmenite under certain conditions based on microprobe analyses, assigning Ti above 1 apfu to Ti3+. Data of ilmenite assemblages from tables provided in ref. 14 are used in Fig. 4a, b (red triangles). We specifically selected experimental configurations that satisfy the following criteria: (1) Ti3+ content in synthesized ilmenite of x > 0.02 to ensure a reliably high trivalent content; and (2) low-Mg content, Mg [apfu] < 0.01, and a formation temperature of 1300°C, for optimal comparison with refs. 11,13. Using the relative amounts of ilmenite and ulvöspinel from microprobe measurements in ref. 10, we calculate an ilmenite mole fraction of Xilm = 0.992 14. The isopleths (red lines, Fig. 4c) are then calculated for a given ilmenite mole fraction using the Excel file in Supplementary Data of ref. 14.

Simons and Woermann (1978) also proposed the presence of Ti3+ in ilmenite based on electron microprobe measurements and reported a minimum temperature of 1200°C required for its preservation 15. In addition, Sauerzapf et al. (2008) observed excess Ti content at 1100°C and IW − 0.2, but in a Fe–Ti–Al–Mg–O system rather than a pure Fe–Ti–O system 14. Nevertheless, we use the experimental results of ref. 12 since the minimum temperature required for the onset of Ti3+ remains unclear. Usselman et al. (1975) synthesized high-Ti basalts under various cooling rates and used the resulting textures to infer a cooling rate of < 1°C h−1 for 75035 12. Using the ilmenite phase boundary (solid blue line, Fig. 4c), derived from experiments at a cooling rate of ~2°C h−1, as an upper bound for temperature, we assume that the trend of the ilmenite phase can be extrapolated to lower fO2 (dashed blue line, Fig. 4c) and remains invariant under slower cooling rates.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript is well-written and focuses on the novel discovery of Ti3+ in ilmenite from Apollo lunar samples. In most cases, Titanium is considered to be stable with fixed valence states (4+), but the occurrence of Ti3+ (ilmenite and pyroxene) should be a valuable indicator for redox conditions in some extreme circumstance (planet surface, early Solar system, impact processes...). This study using high-resolution and cutting-edge method (STEM and related analyzing tools) confirmed the occurrence of Ti3+ in lunar ilmenite, which is quite different from the terrestrial ilmenite. This is intriguing and welcome for lunar and planetary scientists, and I believe that this study will be highly indexed for those researchers on lunar samples. However, I have some concerns about the distribution forms of the phases with Ti3+, and effectiveness on its applications in lunar samples. After well answering this, minor revision is needed for the acceptance in Nature Communications.\nThe main questions are as follows,\n(1) The distribution and form of Fe2+1-xTi4+1-xTi3+2xO3 phase in extracted FIB foils maybe identified based on the contrast shown HAADF images due to the density difference between Fe2+1-xTi4+1-xTi3+2x O3 and FeTiO3. Is it true? How about the distribution of this kind of phase bearing Ti3+. Heterogeneous or homogeneous? Was this basaltic clast altered by other exogenic geological processes, such as space weathering or shock processes, please exclude these possibilities.\n(2) Is it possible that the excess of Ti indicated by Ti/Fe>1 is caused by other substitution processes, (such as Mg2+→Fe2+, or other elements, Mn2+, Al3+, Cr3+, V4+ in ilmenite). Could you provide all element concentrations rather than just Ti/Fe ratio in ilmenite that was detected by EDS.\n(3) It seems that the Ti3+ in ilmenite mainly occur in high-Ti lunar basalt, so is it in the same case in some Low-Ti lunar basalt.\n(4) The redox conditions indicated by the occurrence of Ti3+ pyroxene in lunar basalts ranges from IW-2 to IW-1 (Simon and Sutton., 2018), is there any advantage when using the Ti3+ in ilmenite as an .\nOther comments:\n1.The percentage numbers mentioned in this manuscript should be clarified as wt%, atom% or other.\n2.We can observe the standard spectra of ilmenite in 75035 (Fig. 3d), does it mean that some area in extracted foil are still FeTiO3. In figure e and f, the mean spectra does not well resemble with the Ti2O3 ELNSE spectra, does it mean that the analyzed regions contain Ti3+ and Ti4+, or other phases. Is it right?", "reviewer_2": "This manuscript presents a compelling and technically sophisticated study on the detection and quantification of Ti3+ in ilmenite from the Apollo 75035 basalt. The use of high-resolution STEM/EELS and ELNES provides convincing evidence for non-stoichiometric ilmenite enriched in Ti3+, which has been hypothesized but never quantitatively confirmed. The implications for lunar magma redox conditions and the potential development of a single-phase oxybarometer are significant. The data in this manuscript is of high quality. However, several major issues in the manuscript must be addressed before considering for publication.\n(1) The potential role of space weathering must be addressed\nThe authors attribute the excess Ti in ilmenite solely to primary magmatic processes (Ti3+). However, space weathering is also known to cause surface enrichment of Ti in lunar soil ilmenites (e.g., Guo, Z., Zhang, M., Jia, B., Li, Y., Fa, W., Pang, Y., ... & Ouyang, Z. (2024). Space-weathered rims on lunar ilmenite as an indicator for relative exposure ages of regolith. Communications Earth & Environment, 5(1), 426.). This represents a competing hypothesis that must be rigorously evaluated and ruled out.\n1. The analytical techniques used (XRF, EDS, EELS) are sensitive to the near-surface region (tens to hundreds of nanometers), which is precisely the domain affected by space weathering. The current data alone cannot distinguish between a magmatic and a space-weathered origin for the Ti enrichment.\n2. The manuscript provides no characterization of space weathering features (e.g., using TEM to identify amorphous rims, nanophase Fe or Ti particles, or vapor-deposited layers) on the studied ilmenite grains. Without this, it is impossible to estimate the contribution of space weathering to the observed excess Ti signal.\n3. Consequently, the Discussion must be revised to explicitly consider space weathering as a potential mechanism for Ti enrichment. The authors should provide a balanced argument, based on their microstructural observations or the lack thereof, to justify why a primary magmatic origin is preferred.\n(2) About the representativeness of Apollo 75035 basalt\nThe discovery of Ti3+ in the Apollo 75035 basalt is robust. However, its broader significance remains unclear. Is this phenomenon unique to this specific basalt type or common across the Moon? How does the redox condition infer from 75035 compare to that of other mare basalts? The discussion should be strengthened to answer these questions.\n(3) For the Ilmenite as an oxybarometer\nThe proposal that Ti3+-bearing ilmenite could serve as a single-phase oxybarometer is exciting but speculative. The authors should discuss the potential limitations, such as the influence of bulk composition (e.g., Mg, Al content) on Ti3+ incorporation, or the effect of cooling rate on Ti3+ preservation. Have the authors considered performing similar analyses on other lunar or terrestrial reduced ilmenites to test the robustness of this method?\n(4) Comparison with existing Ti3+ studies\nThe authors mention previous studies that report Ti3+ in pyroxene and armalcolite. A more direct comparison between the Ti3+ contents in coexisting phases (if available) would help contextualize the uniqueness of the ilmenite results.\n(5) Expand the discussion on the implications for lunar evolution\nThe manuscript provides a thorough and compelling technical account of the detection and quantification of Ti3+ in ilmenite. However, the broader discussion of the geological significance of this discovery for understanding lunar evolution is currently underdeveloped. I recommend the authors add several paragraphs (or subsection) in the Discussion to explicitly address how this finding informs our models of the Moon's magmatic and thermal history. For instance, does this result support a specific model of the lunar mantle's redox heterogeneity? How does it relate to the temporal evolution of volcanic activity on the Moon? Placing the specific results from 75035 into this wider context will significantly elevate the impact of the study."}, "author_response": "Response to reviewer letter, 7 November 2025\nTrivalent Titanium in High-Titanium Lunar Ilmenite, Vira, A. D. et al.,\nManuscript #NCOMMS-25-64328-T\nThe authors thank the reviewers for their feedback on the manuscript. The responses to the reviewers’\ncomments are interleaved below. The reviewers’ comments are shown in black, followed by the authors’\nresponses in blue.\nReviewer 1\nThis manuscript is well-written and focuses on the novel discovery of Ti3+ in ilmenite from Apollo\nlunar samples. In most cases, Titanium is considered to be stable with fixed valence states (4+), but\nthe occurrence of Ti3+ (ilmenite and pyroxene) should be a valuable indicator for redox conditions in\nsome extreme circumstances (planet surface, early Solar system, impact processes...). This study, using\nhigh-resolution and cutting-edge methods (STEM and related analysis tools), confirmed the occurrence\nof Ti3+ in lunar ilmenite, which is quite different from the terrestrial ilmenite. This is intriguing and\nwelcome for lunar and planetary scientists, and I believe that this study will be highly indexed for those\nresearchers on lunar samples. However, I have some concerns about the distribution forms of the phases\nwithTi3+,andeffectivenessonitsapplicationsinlunarsamples.Afterwellansweringthis,minorrevision\nis needed for the acceptance in Nature Communications.\nThe main questions are as follows,\n(1.1) The distribution and form of Fe2+ Ti4+ Ti3+O phase in extracted FIB foils maybe identified\n1−x 1−x 2x 3\nbased on the contrast shown HAADF images due to the density difference between Fe2+ Ti4+ Ti3+O\n1−x 1−x 2x 3\nand FeTiO . Is it true? How about the distribution of this kind of phase bearing Ti3+? Heterogeneous\n3\nor homogeneous? Was this basaltic clast altered by other exogenic geological processes, such as space\nweathering or shock processes? Please exclude these possibilities.\nWe thank the reviewer for these insightful comments, which we address in reverse order. The chip from\n75035-232doesnotshowfeaturesconsistentwithspaceweathering,leadingustoconcludethattheobserved\nenrichment in Ti is of magmatic origin. To make this clear from the beginning, we have added the following\nsentences in the introduction:\n•\nIn this Article, we use scanning transmission electron microscopy (STEM) coupled with electron\nenergylossspectroscopy(EELS)tostudya∼7mgchipfrom75035(75035-232).Importantly,this\nchip shows no signatures of space weathering (Fig. S1): we find no nanophase iron, vesicles, Si-\nbearingvapor depositsorglasses, ormicrocraters[11–15]. Thesefindings indicatethat75035-232\nwasnotsun-facing,consistentwithouranalysisofitsorientationbasedoninformationinthelunar\ncompendium[16]andwiththereconstructedorientationreportedinRef.[17].\n•\nBased on the lack of evidence for space weathering, we attribute the excess Ti in 75035 ilmenite\nexclusivelytoamagmaticorigin.\nAs for the compositional distribution, we do see changes, as quantified in our revised Table 1, which\nwe have moved into the main text. However, distribution implies a size scale. Our measurements show the\ncompositiontobeveryuniformonthescaleofalift-outsample(microns),butvariessomewhatfromsample\nto sample (tens to hundreds of microns). In Table 1, the consistency of composition is now shown for two\npositions (ia and ib) on a single lift-out. For different samples, compositional variations can be judged\nfrom Table 1, given the lift-out locations shown in Fig. 1. We attribute compositional fluctuations to both\nmeasurement uncertainties and natural fluctuations. While major compositional variations in lunar magmas\noccur at significantly larger scales, the reviewer brings up an excellent point that may become relevant for\nexperimental studies at high-Ti content. For example, under some conditions, there could be exsolution\nlamellae consisting of a high-Ti phase and ilmenite, which could influence the distribution on the order of\ntens of nm to a few microns in length. However, we have not observed such structures in the present sample,\n75035.\n3\nthe contrast in the HAADF images due to the relatively small difference in Fe and Ti atomic numbers and\nthe even more subtle influence of the Ti oxidation state. This is why EELS and ELNES are important new\ntools for petrological analysis.\n(1.2) IsitpossiblethattheexcessofTiindicatedbyTi/Fe>1iscausedbyothersubstitutionprocesses,\nsuch as Mg2+→Fe2+, or other elements, Mn2+, Al3+, Cr3+, V4+ in ilmenite cause the excess of Ti\nindicated by Ti/Fe>1). Could you provide all element concentrations rather than just Ti/Fe ratio in\nilmenite that was detected by EDS?\nWe agree with the reviewer that it is important to consider the minor elements, as this will affect the\nquantification of the excess Ti. In our initial submission, we had a table with the full elemental analysis from\nEDS in the Supplemental Information. However, to address this and other concerns raised by reviewer 1, we\nhave moved this to the main text as Table 1 and inserted details underlying our analysis of the stoichiometry\nin Methods (refer to pages 14-15 of the revised manuscript). The revised table more clearly displays our\nresults (page 7 of the revised manuscript) and shows that the conclusion of excess Ti in 75035 ilmenite is\nnot a consequence of other substitution processes. In addition, we adjusted Fig. 2a to show the Ti/Fe ratio\ncalculated from the stoichiometric values in Table 1, which accounts for the minor elements. We thank the\nreviewer for pointing out this issue and believe that the revised manuscript is strengthened substantially by\naddressing it in the main text.\n(1.3) It seems that the Ti3+ in ilmenite mainly occurs in high-Ti lunar basalt, so is it in the same case\nin some Low-Ti lunar basalt?\nThank you for bringing up this intriguing point. We agree that it would be interesting to determine if Ti3+\nis also found in low-Ti lunar basalts in addition to other lunar rocks. It is difficult for us to speculate one\nwayoranotherduetothelimitedcontextofourhigh-resolutionmicroscopyandspectroscopymeasurements.\nHowever, to illustrate the point that Ti3+ in ilmenite could be more widespread than previously recognized,\nwe have added the following subsection to the Discussion:\nDistribution of Ti3+ in the Lunar Collection\nAlongwithtwoothermarebasalts(Table1),75035hasbeenrecognizedasuniqueforitslackofolivine\nand low Mg content [2, 3]. Therefore, it is reasonable to ask whether Ti3+ enrichment is limited to this\nclassofmarebasalt.Adefinitiveanswerwouldrequirenewanalysesofmanysamples,buttheliterature\nprovidessomeguidance.Withinthebroadercollectionoflunarsamples,weidentified530lunarilmenite\nanalysesintheAstroMatdatarepository[38]thatpotentiallycontainTi3+,basedoncalculatedxvalues\n(Methods, Eq. 2a). Among the 530 samples (gray dots, Fig. 4a), assuming all excess Ti to be trivalent,\nthe highest values are nearly half of our measured Ti3+ content (x = 0.069±0.015, weighted average\nofx andx ).ThosesampleswithhighestapparentTi3+ content(x ∼ 0.030)areobservedina\nEELS ELNES\nrangeofsampletypes,e.g.:Chang’e-5youngbasalts[39]andApollo12basalt12003[40]fromOceanus\nProcellarum,anApollo17basalticfragment79513[41],andApollo14crystallinematrixbreccia14321\n[42]. Thus, we postulate that the conditions necessary to stabilize Ti3+ may have been widespread on\ntheMoon.Ourmeasuredminorelementfractions(Table1)areconsistentwithpriorelectronmicroprobe\nanalysesof75035ilmenite[10](browncircle,Fig.4a)andwiththetwosimilarmarebasalts(75055[27],\n14053 [33]). The Ti/Fe content of 75035 ilmenite in Ref. [10] falls at the lower edge of our measure-\nments(Ti/Fe ∼ 1.047,Table1),implyingonlyasmallenrichmentofTi3+.However,comparisonswith\nthe literature are limited by differences in analytical methods, which may presume oxidation states and\nstoichiometries.OurELNESresultsfor75035ilmeniterequirethepresenceofTi3+andtheimpliedstoi-\nchiometryofEq.2.Asacorollary,ourmeasurementsalsosuggestthatre-analysisofotherlunarilmenite\nsamplescouldrevealTi3+levelsbeyondthoseweestimatedfromtheAstroMatdatabase.\n2\n(1.4) The redox conditions indicated by the occurrence of Ti3+ pyroxene in lunar basalts range from\nIW-2toIW-1(SimonandSutton.,2018),isthereanyadvantagewhenusingtheTi3+ inilmeniteasan.\nThetransmittedcopyunfortunatelyomitstheendofthefinalsentence,butwepresumethattherevieweris\nasking whether there is any advantage to using the Ti3+ content in ilmenite as an oxybarometer. Ilmenite is\nmore abundant and can be easier to locate (in comparison to armalcolite), and its simpler structure makes it\nan attractive phase to consider. Additional analytical tools are valuable, as not every rock contains sufficient\narmalcolite or pyroxene for conventional oxybarometry. We cannot speculate further until this approach is\nfullydevelopedintoareliableoxybarometer,whichwouldrequireasequenceofcarefulexperiments.However,\nwe adjusted the implications section to highlight the potential advantages of developing Ti3+ in ilmenite as\nan oxybarometer:\nIlmenite as an Oxybarometer:\nIlmenite,aubiquitouslunarmineral,alsohasthepotentialtoserveasasingle-phaseoxybarometerbased\nonitsTi3+content,extendingmethodologiespreviouslyappliedtoarmalcolite.Sinceitismoreabundant\nthanarmalcoliteinlunarrocks,ilmeniteprovidesalargerandmorereadilyaccessiblephaseforpotential\noxybarometer analyses. This potential single-phase oxybarometer should be explored with petrological\nexperimentsconductedonavarietyofbulkcompositionsandcoolingratesatknownfO −T conditions.\n2\nbyTi3+-bearingilmenite,strengtheningandrefiningtheapplicationofourwork.Withsuchexperiments,\nour small-volume analytical techniques could provide crucial constraints on the redox environment of\nmagmasrelevanttotheMoonandotherlow-fO planetarysystems.\n2\n(1.5) Other comments:\n1. The percentage numbers mentioned in this manuscript should be clarified as wt%, atom% or other.\n2. We can observe the standard spectra of ilmenite in 75035 (Fig. 3d). Does it mean that some areas\nin the extracted foil are still FeTiO ? In figures e and f, the mean spectra does not well resemble\n3\nwith the Ti O ELNSE spectra, does it mean that the analyzed regions contain Ti3+ and Ti4+, or\n2 3\nother phases. Is it right?\nThank you for these detailed comments.\nWe have clarified the percentage values by making the following adjustments to the main text:\n• Clarified that the XRF percentage values are the area percentage, as opposed to wt% or atom%:\nMineralidentificationontheexteriorsurfaceofthechipfrom75035-232(Fig.1a)reveals(byarea)\n45%pyroxene,43%plagioclase,11%ilmenite,and<1%SiO -phase,...\n2\n• Clarifiedthepercentagevaluesinthediscussionbyspecifyingthatthepercentageisofthetotalnumber\nof Ti atoms:\nThese authors find that the average Ti valence state in Fe-Ti oxides (armalcolite and ilmenite) is\n3.64±0.08 (i.e., ∼ 40% of Ti atoms are Ti3+) [53].Another XANES study of Apollo 17 basalts\nfindsthatanaverageof15%−20%ofTiatomsinpyroxene...\nRegarding the second question: Yes, the mean spectrum in Fig. 3e contains a mixture of Ti3+ and Ti4+. In\naddition, it is possible that some regions at the same depth remain as FeTiO . However, we do not observe\n3\nany inhomogeneity or domain formation. The imperfect fit in Fig. 3e and the deviation in Fig. 3f are most\nlikely related to multiple scattering effects due to the increase in the foil thickness.\nelectrons travel. If the sample is too thick, multiple scattering broadens spectral features and obscures the\ncontributionsofminorcomponents(e.g.,Ti O ).InFig.3e,weshowaqualitativecomparisonofthedifference\n2 3\nspectra to Ti O , to motivate that the differences in the peaks could be explained by a contribution to this\n2 3\nlunar ilmenite from Ti3+. Figure 3f shows that the measurement cannot be reliably done for depths that are\ntoo large. (Spectra are averaged over 3-nm depth ranges for mean depths of 11.75nm in (e) and 20.5nm in\n(f); we have included a new note at the end of the caption to better identify the averaged regions.)\nReviewer 2\nThis manuscript presents a compelling and technically sophisticated study on the detection and quan-\ntification of Ti3+ in ilmenite from the Apollo 75035 basalt. The use of high-resolution STEM/EELS and\nELNES provides convincing evidence for non-stoichiometric ilmenite enriched in Ti3+, which has been\nhypothesizedbutneverquantitativelyconfirmed.Theimplicationsforlunarmagmaredoxconditionsand\nthe potential development of a single-phase oxybarometer are significant. The data in this manuscript is\nof high quality. However, several major issues in the manuscript must be addressed before considering\nfor publication.\n(2.1) The potential role of space weathering must be addressed: The authors attribute the excess Ti in\nilmenitesolelytoprimarymagmaticprocesses(Ti3+).However,spaceweatheringisalsoknowntocause\nsurface enrichment of Ti in lunar soil ilmenites (e.g., Guo, Z., Zhang, M., Jia, B., Li, Y., Fa, W., Pang,\nY.,...&Ouyang,Z.(2024).Space-weatheredrimsonlunarilmeniteasanindicatorforrelativeexposure\nages of regolith. Communications Earth & Environment, 5(1), 426.).\nThis represents a competing hypothesis that must be rigorously evaluated and ruled out.\n1. The analytical techniques used (XRF, EDS, EELS) are sensitive to the near-surface region (tens\nto hundreds of nanometers), which is precisely the domain affected by space weathering. The cur-\nrent data alone cannot distinguish between a magmatic and a space-weathered origin for the Ti\nenrichment.\n2. The manuscript provides no characterization of space weathering features (e.g., using TEM to\nidentify amorphous rims, nanophase Fe or Ti particles, or vapor-deposited layers) on the studied\nilmenite grains. Without this, it is impossible to estimate the contribution of space weathering to\nthe observed excess Ti signal.\n3. Consequently, the Discussion must be revised to explicitly consider space weathering as a poten-\ntial mechanism for Ti enrichment. The authors should provide a balanced argument, based on\ntheir microstructural observations or the lack thereof, to justify why a primary magmatic origin is\npreferred.\nWe thank reviewer 2 for perceptive comments. From comparison of the shape of 75035-232 with pho-\ntographs in the lunar compendium, we concluded that 75035-232 was not sun-facing, which is in agreement\nwiththedetailedorientationanalysisprovidedintheSupplementalInformationinRef.[17].Therefore,75035-\n232didnotexperiencesignificantspaceweatheringmodifications.Furthermore,HAADFimagesdonotshow\nany exogenic alterations. We have added a figure to the Supplemental Information (Figure S1) that shows\nan example of the unaltered ilmenite, providing a comparable region to the results in Guo et al. 2024. The\nlack of space weathering features leads us to attribute the observed excess Ti to magmatic origin.\nWe agree with the reviewer that this point needs to be addressed in the main text. We have made the\nfollowing modifications to the introduction to establish this from the beginning:\n4\n•\nIn this Article, we use scanning transmission electron microscopy (STEM) coupled with electron\nenergylossspectroscopy(EELS)tostudya∼7mgchipfrom75035(75035-232).Importantly,this\nchip shows no signatures of space weathering (Fig. S1): we find no nanophase iron, vesicles, Si-\nbearingvapor depositsorglasses, ormicrocraters[11–15]. Thesefindings indicatethat75035-232\nwasnotsun-facing,consistentwithouranalysisofitsorientationbasedoninformationinthelunar\ncompendium[16]andwiththereconstructedorientationreportedinRef.[17].\n•\nBased on the lack of evidence for space weathering, we attribute the excess Ti in 75035 ilmenite\nexclusivelytoamagmaticorigin.\n(2.2) About the representativeness of Apollo 75035 basalt: The discovery of Ti3+ in the Apollo 75035\nbasalt is robust. However, its broader significance remains unclear. Is this phenomenon unique to this\nspecificbasalttypeorcommonacrosstheMoon?Howdoestheredoxconditioninferfrom75035compare\nto that of other mare basalts? The discussion should be strengthened to answer these questions.\nWe agree with the reviewer that the discussion should be strengthened to include a more comprehensive\ncomparison with other mare basalts. We have substantially expanded our comparison with the AstroMat\ndatabase and added the following subsection to the discussion:\nDistribution of Ti3+ in the Lunar Collection\nAlongwithtwoothermarebasalts(Table1),75035hasbeenrecognizedasuniqueforitslackofolivine\nand low Mg content [2, 3]. Therefore, it is reasonable to ask whether Ti3+ enrichment is limited to this\nclassofmarebasalt.Adefinitiveanswerwouldrequirenewanalysesofmanysamples,buttheliterature\nprovidessomeguidance.Withinthebroadercollectionoflunarsamples,weidentified530lunarilmenite\nanalysesintheAstroMatdatarepository[38]thatpotentiallycontainTi3+,basedoncalculatedxvalues\n(Methods, Eq. 2a). Among the 530 samples (gray dots, Fig. 4a), assuming all excess Ti to be trivalent,\nthe highest values are nearly half of our measured Ti3+ content (x = 0.069±0.015, weighted average\nofx andx ).ThosesampleswithhighestapparentTi3+ content(x ∼ 0.030)areobservedina\nEELS ELNES\nrangeofsampletypes,e.g.:Chang’e-5youngbasalts[39]andApollo12basalt12003[40]fromOceanus\nProcellarum,anApollo17basalticfragment79513[41],andApollo14crystallinematrixbreccia14321\n[42]. Thus, we postulate that the conditions necessary to stabilize Ti3+ may have been widespread on\ntheMoon.Ourmeasuredminorelementfractions(Table1)areconsistentwithpriorelectronmicroprobe\nanalysesof75035ilmenite[10](browncircle,Fig.4a)andwiththetwosimilarmarebasalts(75055[27],\n14053 [33]). The Ti/Fe content of 75035 ilmenite in Ref. [10] falls at the lower edge of our measure-\nments(Ti/Fe ∼ 1.047,Table1),implyingonlyasmallenrichmentofTi3+.However,comparisonswith\nthe literature are limited by differences in analytical methods, which may presume oxidation states and\nstoichiometries.OurELNESresultsfor75035ilmeniterequirethepresenceofTi3+andtheimpliedstoi-\nchiometryofEq.2.Asacorollary,ourmeasurementsalsosuggestthatre-analysisofotherlunarilmenite\nsamplescouldrevealTi3+levelsbeyondthoseweestimatedfromtheAstroMatdatabase.\n(2.3) For the Ilmenite as an oxybarometer: The proposal that Ti3+-bearing ilmenite could serve\nas a single-phase oxybarometer is exciting but speculative. The authors should discuss the potential\nlimitations, such as the influence of bulk composition (e.g., Mg, Al content) on Ti3+ incorporation, or\nthe effect of cooling rate on Ti3+ preservation. Have the authors considered performing similar analyses\non other lunar or terrestrial reduced ilmenites to test the robustness of this method?\nWe thank the reviewer for appreciating the implications of an ilmenite-based single-phase oxybarometer for\nreducing conditions. And the reviewer’s opinion is exactly the same as ours: it is exciting but speculative\n(which is why we thought it worth proposing!). As opposed to the suggested studies of natural ilmenites, the\npath that we believe would be necessary to bring a quantitative oxybarometer to fruition would be a series of\npetrological experiments under controlled, highly reducing conditions to investigate the potential effects that\nthebulkcompositionandcoolingratescouldhaveontheincorporationofTi3+.Wehavenotundertakenthose\nexperiments. Our present manuscript focuses solely on our discovery of trivalent titanium in lunar ilmenite\n5\nusing advanced microscopy and spectroscopy techniques to quantify Ti3+ content. We view the possibility of\na new single-phase oxybarometer for highly reducing conditions as an important implication of our discovery.\nBut, we feel that the discovery itself is worth reporting rapidly, without specialized experiments covering the\nsolution series, since our new observations and methods could have many different impacts.\nThatsaid,tomoreclearlyoutlinethepotentialofTi3+-bearingilmeniteasanoxybarometer,andwhatwould\nbe required to achieve this goal, we added the following subsection to the end of the manuscript:\nIlmenite as an Oxybarometer:\nIlmenite,aubiquitouslunarmineral,alsohasthepotentialtoserveasasingle-phaseoxybarometerbased\nonitsTi3+content,extendingmethodologiespreviouslyappliedtoarmalcolite.Sinceitismoreabundant\nthanarmalcoliteinlunarrocks,ilmeniteprovidesalargerandmorereadilyaccessiblephaseforpotential\noxybarometer analyses. This potential single-phase oxybarometer should be explored with petrological\nexperimentsconductedonavarietyofbulkcompositionsandcoolingratesatknownfO −T conditions.\n2\nbyTi3+-bearingilmenite,strengtheningandrefiningtheapplicationofourwork.Withsuchexperiments,\nour small-volume analytical techniques could provide crucial constraints on the redox environment of\nmagmasrelevanttotheMoonandotherlow-fO planetarysystems.\n2\n(2.4) Comparison with existing Ti3+ studies: The authors mention previous studies that report Ti3+\nin pyroxene and armalcolite. A more direct comparison between the Ti3+ contents in coexisting phases\n(if available) would help contextualize the uniqueness of the ilmenite results.\nUpon delving deeper into the literature on Ti3+ in pyroxene and armalcolite, we were able to find oxygen\nfugacity measurements that compared well with our estimate. We have modified the first paragraph in the\ndiscussion subsection “Estimate of fO at Formation” to include:\nTi3+ toestimatepossiblefO −T conditionsof75035ilmenite,whichhasbeenfoundtocoexistwitha\n2\nspinelphase[10].DatafromRefs.[18,35,43]arefittoalinetodetermineanfO −Ti3+relationshipat\n2\n1300◦C(Methods).Basedonouranalysisof75035ilmenite,thefittedline(blackdashedline,Fig.4b)\nestablishesanupperboundof∆IW = −1.6.AccountingforouruncertaintyinTi3+ (greenregion,\nupper\nFig.4b),inadditiontotheinherenterrorofthelinearextrapolation(grayregion,Fig.4b),theupperlimit\ncouldreachIW−0.8andthelowerlimitoffO could,inprinciple,extendtovaluesaslowasIW−10.\n2 2\nSolarSystem’smostreducedbody,Mercury,isroughlyIW−3toIW−7[49],alowerboundofIW−10\nisunreasonable.Nonetheless,our∆IW = −1.6inFig.4bapproximatelyoverlapstheequilibration\nupper\nregionofTi3+-richarmalcolite(downtoIW−1.5[37,50])andfallswithintherangeoffO estimates\n2\nforsoilsnewlyreturnedfromthelunarfarsidebyChang’e-6(SouthPole–Aitkenbasin;IW−1.9[48]).\nThese farside samples have an fO more than a full log unit lower than the average fO of nearside\n2 2\nbasalts(IW−0.8[46]),underscoringthesignificanceandtimelinessofourfindings.\nIn addition, we have added the following subsection to the discussion to provide a more direct comparison\nbetween Ti3+ contents in pyroxene and armalcolite with our quantified Ti3+ contents in 75035 ilmenite:\nPetrologicalexperimentshaveestablisheddirectrelationshipsbetweenTi3+ andfO inarmalcoliteand\n2\npyroxene(e.g.,Refs.[34,37,51,52]).TivalenceinpyroxenehasevenbeenusedtoestimatethefO of\n2\nmarebasalt14053,whichissimilarincompositionto75035andhasbeenhypothesizedtocontainTi3+\nin ilmenite [34]. Indeed, pyroxene Ti3+/Ti4+ (as well as Cr2+/Cr3+ and V2+/V3+) suggests an fO\n2\nclose to IW−2 [52], consistent with our extrapolated 75035 fO range. Moreover, recent studies use\nfrom synthetic basalts in a modified CaO–MgO–Al O –SiO system (1100◦C, IW−1.8) [53]. These\n2 3 2\nauthors find that the average Ti valence state in Fe-Ti oxides (armalcolite and ilmenite) is 3.64±0.08\n(i.e.,∼40%ofTiatomsareTi3+)[53].AnotherXANESstudyofApollo17basaltsfindsthatanaverage\nof15%−20%ofTiatomsinpyroxenecanbeinthetrivalentstate[26].Consequently,ourmeasurements\noftrivalenttitaniumin75035ilmenite(∼ 15%ofthetotalTi)andourfO estimate(∆IW ≤ −1.6)are\n2\nbothconsistentwiththeserecentmeasurements[26,53].\n(2.5) Expand the discussion on the implications for lunar evolution: The manuscript provides a thor-\nough and compelling technical account of the detection and quantification of Ti3+ in ilmenite. However,\nthebroaderdiscussionofthegeologicalsignificanceofthisdiscoveryforunderstandinglunarevolutionis\ncurrently underdeveloped. I recommend the authors add several paragraphs (or subsection) in the Dis-\ncussion to explicitly address how this finding informs our models of the Moon’s magmatic and thermal\nhistory. For instance, does this result support a specific model of the lunar mantle’s redox heterogene-\nity? How does it relate to the temporal evolution of volcanic activity on the Moon? Placing the specific\nresults from 75035 into this wider context will significantly elevate the impact of the study.\nWeagreewiththereviewerthatitcouldbestrengthenedbyabroaderdiscussionofthegeologicalsignificance\nof the new measurement of Ti3+. While a lengthy discussion of specific scenarios for lunar evolution would\nbe highly speculative, given our limited context from 75035 ilmenite, we have added the following subsection\nto the implications section of the manuscript to help readers place our specific Ti3+ quantification into the\nwider geological context on the Moon:\nImplications for Lunar Evolution:\nThegeographicdistributionofTi-enrichedilmeniteontheMooncouldprovideinsightintolocalinhomo-\ngeneitiesinthelunarmantle,usingTi3+ contentasatracerofmagmaticredoxconditions.Asdiscussed\nabove,Chang’e-6basaltsfromthelunarfarsiderecordanoxygenfugacitymuchlowerthanthemeanfO\n2\nestimatedfromnearsideminerals[48].Anyviablemodeloflunarevolutionshouldexplainsuchlargefluc-\ntuations. Our work shows that modern characterization methods applied to the existing lunar collection\ncouldexposesimilarvariabilityofredoxenvironments,evenwithinthesetofnearsidesamples.\n7"}, {"round": 2, "reviewer_comments": {"reviewer_1": "After the first run of revision, authors they have carefully excluded the other possible formation mechanisms such as space weathering on the possible origin for the occurrence of Ti3+., and further confirm the magmatic origin. Also, authors they provided the compelling data for the occurrence of Ti3+. After the previous revisions, the manuscript looks much more attractive. I would like to recommend it to be published in Nature Communications.\nIn Abstract, authors they stress the unique character of olivine-free basalt that they analyzed, which can constrain models of lunar evolution. For me, it is still not very clear, what is the significance of olivine-free basalt for indicating the lunar magmatic evolution. So please add one or two sentences to stress the broad meaning of your studied sample.\nPlease directly add the higher TiO2 content for ilmenite in your Result (and show what is the normal TiO2 content in lunar ilmenite), not just cite the reference [18]. I guess this could be important criteria for identifying the Ti3+ in lunar ilmenite.\nIn introduction, “Lunar basalts preserve the geological history of the Moon and offer indispensable insights into the early evolution of the Earth–Moon system, recording processes no longer observable on Earth.” Here I think it is not accurate, why lunar basalts can offer the early evolution of the Earth-Moon system. I guess you refer to those old basalts, not some young basalts. Please rephrase it.\nIn table 1, I find that in the region of VIII, the EDS data show that there is no excess of Ti in ilmenite, which means that there may be no clear Ti3+ in this ilmenite region. Therefore, authors may give some explanations. In this study, authors mainly focus on the phenocryst of ilmenite in 75035, how to explain the heterogeneity of Ti3+ occurrence. In VIII region, the ilmenite looks small in Fig. S10, is it phenocryst or matrix ilmenite.", "reviewer_2": "I thank the authors for their careful consideration of my comments and for the revisions made to the manuscript. My previous concerns have been adequately addressed in the current version. The manuscript has been improved, and I have no further substantive issues."}, "author_response": "Response to reviewer letter, 4 February 2026\nTrivalent Titanium in High-Titanium Lunar Ilmenite, Vira, A. D. et al.,\nManuscript #NCOMMS-25-64328B\nThe authors thank the reviewers for their feedback on the manuscript. The responses to the reviewers’\ncomments are interleaved below. The reviewers’ comments are shown in black, followed by the authors’\nresponses in blue.\nReviewer 1\nAfter the first run of revision, authors they have carefully excluded the other possible formation mech-\nanisms such as space weathering on the possible origin for the occurrence of Ti3+, and further confirm\nthe magmatic origin. Also, authors they provided the compelling data for the occurrence of Ti3+. After\nthe previous revisions, the manuscript looks much more attractive. I would like to recommend it to be\npublished in Nature Communications.\n(1.1) In Abstract, authors they stress the unique character of olivine-free basalt that they analyzed,\nwhich can constrain models of lunar evolution. For me, it is still not very clear, what is the significance\nof olivine-free basalt for indicating the lunar magmatic evolution. So please add one or two sentences to\nstress the broad meaning of your studied sample.\nThank you for pointing out the inconsistency in the abstract. We have signifcantly modified the abstract to\nreflect the changes made to theDiscussion section (duringthe previous revision). Theabstract now readsas:\nLunar mare basalts are often rich in titanium, hosted predominantly within the mineral ilmenite\n(Fe2+Ti4+O ).Here,weexamineilmeniteina∼3.78billion-year-oldmarebasalt(Apollorock75035)\n3\nusing high-resolution electron microscopy and electron energy loss spectroscopy. A key finding is that\n75035ilmeniteisitselfenrichedinTi,beyondtheendmemberoftheconventionalsolidsolutionseries.\nUsing energy loss near-edge spectroscopy, we determine that the excess Ti is trivalent, with Ti3+ mole\nfraction of 0.069±0.015. This discovery confirms the presence of trivalent Ti in lunar ilmenite, long\nhypothesized based on the Moon’s reducing environment. Accounting for the change in implied stoi-\nchiometry,areviewofliteraturedatasuggeststhatTi3+ maybepresentinilmeniteacrossawiderange\noflunarsamples.WeextrapolateknownrelationshipsfromtheliteraturetoconnectTi3+ toredoxcondi-\ntions,estimatingtheoxygenfugacityduringcrystallizationof75035tobebelowtheiron-wu¨stitebuffer,\n∆IW ≤ −1.6.FurtherquantifyingtherelationshipbetweenTivalencestateandoxygenfugacitywould\nallowTi3+-bearingilmenitetoserveasanoxybarometerabletoaccessthereducingconditionsfoundon\nmanyplanetarybodies.\n(1.2) Please directly add the higher TiO content for ilmenite in your Result (and show what is the\n2\nnormal TiO content in lunar ilmenite), not just cite the reference [18]. I guess this could be important\n2\ncriteria for identifying the Ti3+ in lunar ilmenite.\nWehavemodifiedthesentenceintheResultssectiontoincludethetrivalentTimolefractionfromRef.[18]\n(relabelled to Ref. [11]):\nSubsequently, Lattard (1995) synthesized Ti-enriched ilmenite and spinel (red shaded regions, Fig. 2c),\ninferring a Ti3+ mole fraction in ilmenite of up to 0.026apfu [11]. Here, we find that 75035 ilmenite\nexhibitsanevenhigherTicontent(greendots,Fig.2c).\n1\n(1.3) Inintroduction,“LunarbasaltspreservethegeologicalhistoryoftheMoonandofferindispensable\ninsights into the early evolution of the Earth–Moon system, recording processes no longer observable on\nEarth.”HereIthinkitisnotaccurate,whylunarbasaltscanoffertheearlyevolutionoftheEarth-Moon\nsystem. I guess you refer to those old basalts, not some young basalts. Please rephrase it.\nWe agree with the reviewer that the phrasing of this sentence is misleading as we are specifically referring\nto old basalts. We have made the following modification:\nLunar basalts preserve the geological history of the Moon and offer insights into the evolution of the\nEarth–Moonsystem.\n(1.4) In table 1, I find that in the region of viii, the EDS data show that there is no excess of Ti in\nilmenite, which means that there may be no clear Ti3+ in this ilmenite region. Therefore, authors may\ngive some explanations. In this study, authors mainly focus on the phenocryst of ilmenite in 75035,\nhow to explain the heterogeneity of Ti3+ occurrence. In viii region, the ilmenite looks small in Fig. S10,\nis it phenocryst or matrix ilmenite.\nTherevieweriscorrectthatthereisnotrivalentTiinsampleviiibasedontheEDSquantificationpresented\nin Table 1. The EDS quantification on sample viii was acquired nearby the grain boundary of ilmenite and a\nplagioclasegrain(Fig. S10). Ratherthanspeculatingon thepotentialheterogenityoftheTi3+ occurrence,it\nis more likely that the differences in composition could be due to the close proximity of the phase boundary.\nTo address this comment, we have added clarification as a footnote in Table 1 (page 23 of the revised\nmanuscript). In addition, we added the following sentence to Supplementary Discussion (b):\nTheEDSquantificationoversampleVIIIyieldednoexcessTiinilmenite(Table1),likelyduetotheclose\nproximityoftheneighboringphase(acquiredwithin<1µmofplagioclasegrainboundary,Fig.S10c).\nWith the exception of sample viii, all other quantifications featured in the manuscript were performed on\nFIB sections of large ilmenite grains, far from any grain boundaries (e.g., Fig. S1, S4, S6).\nReviewer 2\nI thank the authors for their careful consideration of my comments and for the revisions made to the\nmanuscript.Mypreviousconcernshavebeenadequatelyaddressedinthecurrentversion.Themanuscript\nhas been improved, and I have no further substantive issues.\nThank you for reviewing our manuscript.\n2"}]

Decision letter