# Fractional Crystallization Scenario for Magma Evolution on Mercury Inferred From Geochemical Variation Around the Caloris Basin

K Hirata, T Usui, E Caminiti, J Wright, S Besse

► To cite this version:

K Hirata, T Usui, E Caminiti, J Wright, S Besse. Fractional Crystallization Scenario for Magma Evolution on Mercury Inferred From Geochemical Variation Around the Caloris Basin. Journal of Geophysical Research. Planets, 2025, 130, 10.1029/2024je008788. hal-05443254

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# JGR Planets

## RESEARCH ARTICLE

10.1029/2024JE008788

**Key Points:**

* Five geochemical end-members were identified around Mercury's Caloris basin

* Two volcanic smooth plains show a compositional variation consistent with fractional crystallization of a common magma source

* Crustal diversity is possibly explained by small variations in compositions and/or melting conditions of the mantle

**Correspondence to:**

K. Hirata,
hirata-kaori444@g.ecc.u-tokyo.ac.jp

**Citation:**

Hirata, K., Usui, T., Caminiti, E., Wright, J., & Besse, S. (2025). Fractional crystallization scenario for magma evolution on Mercury inferred from geochemical variation around the Caloris basin. *Journal of Geophysical Research: Planets*, 130, e2024JE008788. https://doi.org/10.1029/2024JE008788

Received 16 OCT 2024

Accepted 16 MAR 2025

**Author Contributions:**

**Conceptualization:** K. Hirata

**Formal analysis:** K. Hirata

**Funding acquisition:** K. Hirata,

E. Caminiti, J. Wright

**Investigation:** K. Hirata, E. Caminiti,
J. Wright

**Methodology:** K. Hirata

**Software:** K. Hirata, E. Caminiti,
J. Wright

**Supervision:** T. Usui, S. Besse

**Validation:** K. Hirata, T. Usui,
E. Caminiti, J. Wright, S. Besse

**Visualization:** T. Usui

**Writing – original draft:** K. Hirata

**Writing – review & editing:** K. Hirata,
T. Usui, E. Caminiti, S. Besse

# Fractional Crystallization Scenario for Magma Evolution on Mercury Inferred From Geochemical Variation Around the Caloris Basin

<figure><img src="image_2.png" /></figure>

K. Hirata<fnref n="1" />,<fnref n="2" /> ID, T. Usui<fnref n="1" /> ID, E. Caminiti³,⁴ ID, J. Wright⁵ ID, and S. Besse⁶

**Abstract** The observed geochemical heterogeneity on the surface of Mercury is key to understanding the planet's volcanic activity and mantle conditions. The Caloris basin shows a diversity in elemental composition, spectral properties, and geomorphology, both within and around it. However, the relationship among these characteristics has not been well understood due to the mismatch in spatial resolutions of the available observation data. This study investigates the geochemical end-members around the Caloris basin, overcoming the limitation of the low spatial resolution of MESSENGER's X-Ray Spectrometer (XRS) data. End-member units are defined using spectral and geomorphological units from MESSENGER's VIS-NIR spectral data and high-resolution images, with the assumption of homogeneous elemental compositions within each unit. A mixing model is constructed to reproduce the XRS data by mixing the end-members, and we solve the inverse problem to calculate the respective end-member compositions. Five end-member compositions were determined, including those corresponding to the post-Caloris volcanic smooth plains interior and exterior to the basin and surrounding pre-Caloris crust. Two smooth plains units, which are geomorphologically indistinguishable but spectrally distinct, showed a compositional variation consistent with magma evolution through fractional crystallization. This suggests that they originated from parent magmas with a common composition. The pre-Caloris crust units showed a large compositional variation, ranging from low- to high-Mg content, implying the potential existence of high-Mg crusts comparable to the HMR. The observed crustal diversity could be explained by relatively minor heterogeneity in source mantle compositions and/or conditions of partial melting within the mantle.

**Plain Language Summary** The variations in Mercury's surface composition provide important clues about its volcanic history and interior conditions. The area around the Caloris basin, the planet's largest well-preserved impact feature, shows a range of elemental compositions and surface geology, characterized by distinct spectral and topographic features. However, the relationships between these features have not been well understood due to mismatched spatial resolutions of the observational data. This study combines low-resolution elemental composition data with high-resolution spectral data and images from NASA's MESSENGER mission to link surface compositions with geological information. We developed a model that reproduces compositional observations by mixing different types of surface compositions ("end-member compositions") corresponding to specific geological contexts. Five distinct types of surface compositions were identified, including those associated with two types of volcanic smooth plains inside and outside the basin, as well as the pre-basin crust. Two volcanic plains exhibit compositional variation consistent with magma evolution through fractional crystallization of a common parent magma. The pre-basin crust showed a large compositional variation, suggesting the presence of high-Mg crusts similar to the High-Mg Region elsewhere on Mercury. Small variations in the composition or melting conditions of Mercury's mantle could explain the observed crustal diversity.

## 1. Introduction

Observations by the Mercury Surface, Space Environment, Geochemistry, and Ranging mission (MESSENGER: Solomon et al., 2007) revealed the surface spectral and geomorphological diversity of Mercury,

<footnote n="1">¹Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan,</footnote>

<footnote n="2">²Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan, ³LIRA, Observatoire de Paris,</footnote>

<footnote>Université PSL, CNRS, Sorbonne Université, Université de Paris, Meudon, France, ⁴Université Grenoble Alpes, CNRS,</footnote>

<footnote>Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Saint-Martin-d'Hères, France, ⁵School of Physical</footnote>

<footnote>Sciences, The Open University Walton Hall, Milton Keynes, UK, ⁶European Space Astronomy Centre (ESAC), European Space Agency (ESA), Madrid, Spain</footnote>
using its payload instruments, the Mercury Dual Imaging System (MDIS: Hawkins et al., 2007) and the Mercury Atmospheric and Surface Composition Spectrometer (MASCS: McClintock & Lankton, 2007). For example, three main spectral units, often referred to as “color units” earlier in the literature, were found using MDIS spectral data: high-reflectance red plains (HRP), characterized by a higher spectral reflectance at 750 nm and spectral slope than Mercury's global average; low-reflectance blue plains (LBP), characterized by a lower spectral reflectance and slope than the average; and low-reflectance material (LRM), with lower spectral reflectance and slope than the LBP (Denevi et al., 2009; Ernst et al., 2010; Klima et al., 2018; Murchie et al., 2008, 2015; Robinson et al., 2008), as well as other minor units (Barraud et al., 2020; Blewett et al., 2011). These units were also observed using MASCS data (Helbert et al., 2013; Izenberg et al., 2014) and recently extended by Caminiti et al. (2023) through a study of the Caloris basin. Mercury's global geomorphological units have been studied using surface images from MDIS observations (Kinczyk et al., 2019), including: impact basins and craters, smooth plains characterized by lower densities of superposing impact craters and sharp contacts with surrounding units (Denevi et al., 2013; Trask & Guest, 1975), and intercrater plains that are spread among the other units and have relatively high densities of superposing impact craters and gently rolling topography (Trask & Guest, 1975; Whitten et al., 2014). The smooth plains exhibit evidence that they were largely emplaced by effusive volcanism (Denevi et al., 2013), and much of the intercrater plains appear to be merely older, more densely cratered volcanic plains (Whitten et al., 2014). While most smooth plains are associated with HRP spectra, intercrater plains have a spectral variation between HRP and LBP (Denevi et al., 2013). The LRM's strong tendency to be located within impact crater materials indicates that it is sourced from the bottom of Mercury's stratigraphy, which has led to it being interpreted as remnants of a carbon-rich primary crust (Klima et al., 2018; Peplowski et al., 2015; Vander Kaaden & McCubbin, 2015).

Since Mercury preserves crust formed in multi-stage volcanic activities, its surface materials are key to understanding the underlying interior conditions. Heterogeneity within Mercury's mantle has been invoked to account for the observed crustal heterogeneity. Petrological studies have suggested that the observed crustal compositions required the presence of different source mantle mineralogies, for example, lherzolitic and harzburgitic mantles (Beuthe et al., 2020; Charlier et al., 2013). Recent theoretical models have explained the required mantle sources by vertical mixing of stratified mantle caused by mantle overturn (Mouser & Dygert, 2023; Mouser et al., 2021). Correlation between the mineralogy estimated using the observed crustal compositions and the resurfacing ages has suggested a temporal evolution of partial melting conditions, from deep and hot to shallow and cool, in the mantle (Namur & Charlier, 2017; Namur, Collinet, et al., 2016; Padovan et al., 2017).

Understanding the mantle heterogeneity and thermal evolution remains incomplete and vague because of the discrepancies among geochemistry, mineralogy, and geomorphology. Spectral observations with MDIS and MASCS data have provided insights into the diversity of the surface materials, but the mineralogy was not specified directly from the observations. Therefore it is challenging to characterize the volcanism that formed the crust only using spectral and geomorphological data: The elemental composition data are needed as well.

The MESSENGER X-Ray Spectrometer (XRS: Schlemm et al., 2007) measured the surface elemental composition, revealing higher Mg and S and lower Al, Ca, and Fe abundances than those of other terrestrial bodies (Evans et al., 2012; Nittler et al., 2011). Along with the relatively large metallic iron core, the low Fe and high S contents in the crust indicate highly reducing conditions within Mercury (McCubbin et al., 2012; Namur, Charlier, et al., 2016; Zolotov et al., 2013). The geochemical heterogeneity of the crust of Mercury was divided into geochemical terranes throughout the northern hemisphere (Peplowski & Stockstill-Cahill, 2019; Vander Kaaden et al., 2017; Weider et al., 2015), although the southern hemisphere was not spatially well resolved in XRS observations because of the spacecraft's highly eccentric orbit.

The most geochemically distinct feature is the High-Mg Region (HMR) centered at ~30°N, 90°E. The HMR is characterized by Mercury's highest Mg/Si of ~0.7-0.8, as well as the planet's lowest Al/Si and highest S/Si and Ca/Si (Weider et al., 2015). Geomorphologically, the HMR is within intercrater plains. Its lower topography and thinner crust (James et al., 2015; Padovan et al., 2015) than the surrounding area suggested that the HMR coincides with ancient impact basins. This led to several hypotheses for the origin of the HMR, including: excavation of mantle materials by impacts (Weider et al., 2015), impact-induced volcanic origin (Mojzsis et al., 2018), or non-impact induced volcanic origin during early high-temperature volcanism (Frank et al., 2017). In contrast, the Northern Terrane exhibits lower Mg/Si and Al/Si than the surrounding area (Weider et al., 2012). The spatial extent of geochemically defined Northern Terrane correlates well with a geomorphological feature, Borealis
Planitia, which is Mercury's largest volcanic smooth plains located in the north polar region (Denevi et al., 2013; Head et al., 2011; Ostrach et al., 2015). However, the geochemical dichotomy within the Northern Terrane was found later (Weider et al., 2015) and is supported by neutron measurements (Lawrence et al., 2017).

Another important geochemical feature is the Caloris Interior Plain (CIP), characterized by low Mg/Si and high Al/Si (Weider et al., 2012, 2015). CIP corresponds to the floor of the Caloris basin, which is the largest well-preserved impact feature on Mercury. Based on its younger model age compared to the basin formation, estimated by crater measurements, and HRP spectral signature, it is also considered volcanic smooth plains formed by lava eruptions following the basin's formation (Denevi et al., 2009, 2013; Murchie et al., 2008).

The Caloris basin is surrounded by another smooth plain unit, the Caloris Exterior Plains (CEP), which are geomorphologically indistinguishable from CIP (Denevi et al., 2013; Rothery et al., 2017). Their indistinguishable geomorphology suggests that the two smooth plains formed by a sequence of magma eruptions that occurred after the basin's formation (Rothery et al., 2017). In contrast, CEP exhibits an LBP spectrum instead of an HRP spectrum characteristic of CIP and the majority of other volcanic smooth plains (Caminiti et al., 2023; Denevi et al., 2009; Murchie et al., 2008; Robinson et al., 2008). This spectral dichotomy as well as compositional dichotomy suggested by the previous elemental composition map (Nittler et al., 2020) implies a mineralogical dichotomy between the CIP and CEP (Caminiti et al., 2023).

Heterogeneity in the melting conditions and/or the compositions of the source mantle might reasonably explain the mineralogical and/or compositional diversity among distinct geochemical terranes, such as HMR, Northern Terrain, and CIP (Charlier et al., 2013; Namur, Collinet, et al., 2016; Vander Kaaden & McCubbin, 2016). However, considering that CIP and CEP are geomorphologically and geographically continuous, their compositional diversity could instead be caused by other processes operating in a shorter timescale than geological time. When magma temporarily stagnates in the crust, magma evolves and changes its composition over geological time. Fractional crystallization is a progressive removal of crystallized minerals from the residual melt. Alternatively, partially melted crustal materials would incorporate into the magma, which is called crustal assimilation. The comparison of the major element compositions between CIP and CEP will provide key information to distinguish the original process of crustal differentiation. However, the elemental composition of CEP has not been quantified from XRS data.

The spatial resolution of XRS (~100–1000 km) is much lower than that of MDIS (~100 m) or MASCS (~1 km). The elemental composition maps were produced from XRS measurements aiming to understand the global geochemical heterogeneity. The maps have effective spatial resolutions varying pixel by pixel depending on the number and size of original footprints used to calculate the pixel compositions, so they do not represent the variability in a spatial scale smaller than them, as noted by Nittler et al. (2020). This mismatch of the spatial resolutions makes the comparison among the data sets from different measurement techniques challenging.

This study investigates the relationship between the surface elemental compositions and geological contexts, that is, formation and evolution process of the crust and its mineralogy, indicated from both spectral and geomorphological properties. To overcome the limited spatial resolution of the XRS observation data, we used a mixing model that expresses the observed elemental compositions by mixing of multiple end-members related to geological contexts (Section 2). Using the geochemical, spectral, and geomorphological observation data acquired during the MESSENGER mission, we estimated the end-member compositions within and around the Caloris basin (Section 3). By comparing the derived end-member compositions, especially of those related to CIP and CEP, we discuss the geochemical diversity and the potential processes that led to this diversity (Section 4).

## 2. Methods

To investigate the variation of the elemental composition around the Caloris basin, which has major and minor axes of 1525 and 1315 km, respectively, and is centered at 32°N, 163°E (Fassett et al., 2009), the study area was defined as a quadrangle area between 90 and 240°E and -30–70°N (Figure 1).

This study analyzed XRS data, as well as MASCS and MDIS data, from the MESSENGER mission (Section 2.1). Defining the end-member units associated to geological contexts (Section 2.2) and considering the lower spatial resolution of compositional data compared with spectral and image data, we modeled the observed elemental composition data as a mixing of end-members (Section 2.3) and the end-member compositions were determined from unmixing analysis (Section 2.4).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** The study area with (a, b) elemental composition, (c, d) spectral, and (e, f) geomorphological data. (a) Mg/Si map (Nittler et al., 2020). The spatial resolution varies depending on the location. The average spatial resolution for each latitude is indicated with the corresponding length in the longitudinal direction (white-shaded area outlined in red). (b) XRS footprint distribution map showing the number of footprints that overlap the surface in color. (c) Individual MASCS footprints classified into three spectral units with criteria by Caminiti et al. (2023); high-reflectance red plains (HRP, red), low-reflectance blue plains (LBP, blue), and low-reflectance material (LRM, dark gray). (d) Spectral unit map showing the dominant spectral unit within each 1°-grid pixel by the same color as used in panel (c). (e) Original geomorphological map with scale of 1:300,000 showing the four geomorphological units, smooth plains (pink), intercrater plains (brown), the Caloris Group (light blue), and crater materials (yellow). (f) Resampled geomorphological map showing the dominant geomorphological unit within each 1°-grid pixel.</figcaption></figure>

## 2.1. Data Sets

### 2.1.1. Elemental Composition Data

By detecting X-ray photons emitted from Mercury's surface, XRS measured surface abundances of rock-forming elements, such as Mg, Al, Si, Ca, S, and Fe (Schlemm et al., 2007). Individual XRS measurements derived X-ray
energy spectra representing the average composition within footprint areas, that is, intersections of the in-
strument's field of view and the planetary surface (Nittler et al., 2011, 2020, 2023; Weider et al., 2012, 2015). For
individual footprint observations, elemental ratios derived from spectra were already compiled and published
along with the information about footprint geometries (Supplementary Information in Nittler et al. (2020)).
Among the provided elemental ratio data, this study analyzed Mg/Si, Al/Si, and Ca/Si ratios to characterize the
surface mineralogy. We considered the exact shapes of the hexagonal footprints, often stretched in the direction of
spacecraft's motion, using the data published on the Planetary Data System (Figure 1b).

Here we emphasize that this study reanalyzed the footprint data rather than used the elemental composition maps.
Elemental composition maps have already been produced (Figure 1a; Nittler et al., 2020; Weider et al., 2015).
These maps are useful for characterizing the global heterogeneity of surface elemental compositions, but do not
describe the compositional variations at local scales and their spatial resolutions vary from pixel to pixel (Nittler
et al., 2020). The previous map generation procedures include summing up the footprint data and assigning the
averaged compositions to an associated pixel, likely reducing the spatial variations (Nittler et al., 2020; Weider
et al., 2015). For our purpose of extracting resolved end-members, we analyzed the original footprints as primary
data.

We used 313 footprints within our study area (Figure 1b). The total area of individual footprints is 1.01 × 10⁸ km²,
~4.5 times larger than the study area, but covering only 68% of the study area. This is because some of the
observations associated with those footprints were performed consecutively and they overlap each other, showing
a tendency to gradually change the observed compositions.

**2.1.2. Spectral Units: VIS-NIR Spectral Data**

MASCS has two components, one of which is the Visible and InfraRed Spectrograph (VIRS) (McClintock & Lankton, 2007). Besse et al. (2015) provided a correction of VIRS data allowing for spectral analysis between 300 and 1450 nm wavelengths with a spectral resolution of 5 nm.

Caminiti et al. (2023) recently refined the definition of Mercury's major spectral units in the VIS-NIR wavelength range through a study of the Caloris basin. They sampled MASCS spectra from the HRP, LBP, and LRM units, extracted their spectral properties and highlighted the distribution of the spectral units around the Caloris basin. We followed their criteria to classify MASCS spectra within our study area into the three units, HRP, LBP, and LRM (Figure 1c).

MASCS has a 0.023°-circular field of view, which is much smaller than XRS's 6°-hexagonal field of view. Our study area is overlain by 227,343 MASCS footprints with a number density varying from ~0–10²·⁷/km². To eliminate the effect of their spatially heterogeneous distribution, we processed a resampled spectral map of 1°/pixel resolution by assigning to each pixel the dominant spectral unit (Figure 1d).

**2.1.3. Geomorphological Units: Surface Image Data**

MDIS is composed of two cameras, a monochrome narrow-angle camera (NAC), and a multispectral wide-angle
camera (WAC) (Hawkins et al., 2007). WAC had 11 filters that allowed observations of Mercury in the spectral
range of 395–1040 nm. MDIS observations revealed the geomorphic textures of the surface (Denevi et al., 2013;
Kinczyk et al., 2020; Whitten et al., 2014).

Using ArcGIS Pro 3.0, we mapped four geomorphological units at a constant digitization scale of 1:300,000 using
the 166 m/pixel MESSENGER MDIS Global Basemap BDR (Figure 1e). The four mapped geomorphological
units are: smooth plains, intercrater plains, the Caloris Group, and crater materials.

The smooth plains unit is generally level ground extending between impact craters with a low density of superposing craters (Trask & Guest, 1975), most of which are in relatively low states of degradation (Kinczyk et al., 2020). This was the first unit we mapped because it generally presents sharp contacts with surrounding units. The Caloris smooth plains are interpreted to have been formed by effusive volcanism (Denevi et al., 2013). We also mapped smooth plains of probable volcanic origin on the floors of impact craters that look too degraded to host smooth impact melt deposits.

The intercrater plains unit is generally level to gently rolling ground extending between impact craters with a high
density of superposing craters (Trask & Guest, 1975) in various states of degradation. This unit constitutes the
intervening space between the other units in our map, and thus this was the final unit we mapped. Most intercrater plains on Mercury are interpreted to have formed during a phase of effusive volcanism that predates smooth plains emplacement, although they probably also incorporate additional basin ejecta and impact melt (Whitten et al., 2014).

The Caloris Group encapsulates the different geomorphic textures attributed to the formation of the Caloris basin. These are: the Caloris Montes Formation, blocky massifs interpreted as the uplifted Caloris basin rim; the Odin Formation, knobby plains interpreted as Caloris impact ejecta; the Van Eyck Formation, plains characterized by troughs and ridges radiating from Caloris interpreted as ejecta sculpture, and; the Nervo Formation, hummocky material interpreted as fallback ejecta mantling the basin rim (Fassett et al., 2009; Mccauley et al., 1981). For our purposes it was not necessary to distinguish between these units and so we mapped them as a group united by their origin as material excavated by the Caloris impact, rather than volcanic plains.

Crater materials encompass all geomorphic textures, including rims, central uplift, floor and ejecta deposits of impact craters that superpose the other mapped units. Apparent impact melt materials were also mapped as crater materials. We chose to isolate these crater materials because we expected they would not form a single, coherent mineralogy because these craters are widely distributed over the study area and occur on different target units.

In order to match the spatial resolution with the spectral map (Section 2.1.2), the original geomorphological map was also resampled into a spatial resolution of 1°/pixel (Figure 1f). Every single pixel of the resampled map was assigned the unit that occupies the largest area within the associated 1°-grid area.

## 2.2. Definition of Spectromorphic Units

Integrating the current understanding of geological contexts according to both spectral (Section 2.1.2) and geomorphological units (Section 2.1.3), we defined seven spectromorphic units (Figure 2a): HRP-smooth plains (HRP-SP), HRP exterior to Caloris (HRP-ex), LBP-smooth plains (LBP-SP), LBP-intercrater plains (LBP-IP), Caloris Group (CG), Crater Materials (CM), and LRM units. Additionally, we treated pixels where spectral data are unavailable as unclassified (UN) and used for data selection (Section 2.4). For this we produced a spectromorphic map with a spatial resolution of 1°/pixel (Figure 2b).

Among the seven units, three end-member units, HRP-SP, LBP-SP, CG, and LRM units are related to the formation of the Caloris basin. The end-member units of the main interest are the two volcanic smooth plains, the CIP and CEP units located inside and outside the basin. Previous studies have found that these plains are indistinguishable based on geomorphology. The difficulty in estimating their relative ages from crater size-frequency distributions (Denevi et al., 2013; Fassett et al., 2009; Strom et al., 2008) or morphological observations of flow features (Rothery et al., 2017) suggests the deposition of both units spanning a geologically short period. On the other hand, their VIS-NIR spectra show a clear dichotomy, suggesting a difference in crustal materials between the CIP and CEP (Caminiti et al., 2023; Ernst et al., 2010, 2015). We defined the HRP-SP unit as smooth plains or crater materials with HRP within the basin (red in Figure 2b). The LBP-SP. Our LBP-SP unit was found on smooth plains with LBP spectra interior and exterior to the basin, as well as crater materials only within the basin (blue in Figure 2b).

Caminiti et al. (2023) examined the VIS-NIR spectral properties around several post-CIP craters, including Atget crater, and identified spectral signals associated with LRM and LBP. The outward transition of spectral properties, from LRM near the crater center to LBP and then to HRP at the outer rim of the ejecta materials, suggests a stratigraphy beneath the CIP, with HRP, LBP, and LRM arranged from shallow to deep.

The CG unit was determined from geomorphological Caloris Group unit (orange in Figure 2b). This unit includes all material excavated, transported, and deposited by the Caloris impact, and is considered to represent mixtures of the surface and subsurface materials at the time of basin formation.

LRM spectra were detected over Mercury's surface and considered to be indicative of the graphite primary crust (Klima et al., 2018; Murchie et al., 2015; Peplowski et al., 2016; Vander Kaaden & McCubbin, 2015). Ernst et al. (2015) examined the spectral properties of crater materials on CIP and suggested that the basin floor materials formed by impact melt have LRM spectra. Here, we defined the LRM unit according to spectral LRM unit (dark gray in Figure 2b).
<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) Definition of spectromorphic units. The seven units including HRP-smooth plains (HRP-SP), HRP-exterior to the Caloris (HRP-ex), LBP-smooth plains (LBP-SP), LBP-intercrater plains (LBP-IP), the Caloris Group (CG), Crater Materials (CM), and Low-Reflectance Material (LRM) units, as well as undefined area (UN), were defined based on spectral units (Section 2.1.2) and geomorphological units (Section 2.1.3). (b) Spectromorphic unit map with a spatial resolution of 1°/pixel showing the spatial distribution of the seven units using the same colors used in panel (a).</figcaption></figure>

The other four end-member units, CM, LBP-IP, and HRP-ex units are not related to the Caloris forming event. Since crater materials are surface and subsurface materials excavated and exposed during the cratering events, their compositions vary depending on the local stratigraphy and timing. Thus it is difficult to correlate the crater materials with a relevant original unit where the subsurface stratigraphy is not understood well. In this study area, subsurface stratigraphy was constrained only beneath the CIP, that is, within the basin rim, according to the spectral investigation of post-CIP craters (Caminiti et al., 2023). The area of geological crater material unit within the basin rim was classified into HRP-SP, LBP-SP, and LRM units based on the spectral unit, while those exterior to the basin rim were assigned as CM unit (light green in Figure 2b).

Pre-Caloris volcanic crust is geomorphologically classified as intercrater plains (Whitten et al., 2014). In this study area, intercrater plains show both HRP and LBP spectra, indicative of a difference in crustal materials. At first, the geomorphological intercrater plains unit was classified into LBP-IP (light blue in Figure 2b) and HRP-IP units based on spectral unit. However, the small spatial extent of HRP-IP unit made it difficult to perform the following compositional analysis.
HRP-ex unit was defined by integrating HRP-IP unit with HRP smooth plains exterior to the Caloris basin (pink in Figure 2b). This HRP-ex unit is largely distributed in the west part of the basin exteriors possibly being associated with the most distal HRP unit extending from Borealis Planitia (Caminiti et al., 2023; Ostrach et al., 2015).

## 2.3. Geometrical Mixing Model

We modeled the XRS observation data as representing spatial mixtures of the end-member compositions within the footprint areas. For our purpose of determining the compositions corresponding to mineralogical differences suggested by spectral and geomorphological observations, we used the spectromorphic units as end-member units and assumed homogenous compositions within each end-member unit, hereafter “end-member compositions.”

The mixing equation $Y = RX + E$ describes our model (Equation 1). For the target elemental species $e$ ($e = 1, ..., n_e$), the elemental ratio $y_{i,e}$ associated with the XRS footprint $i = 1, 2, ..., n_i$ was modeled as representing a mixture of end-member compositions $x_{u,e}$ ($u = 1, 2, ..., n_u$), as

$$ \begin{pmatrix} y_{1,1} & \cdots & y_{1,n_e} \\ \vdots & y_{i,e} & \vdots \\ y_{n_i,1} & \cdots & y_{n_i,n_e} \end{pmatrix} = \begin{pmatrix} r_{1,1} & \cdots & r_{1,n_u} \\ \vdots & r_{i,u} & \vdots \\ r_{n_i,1} & \cdots & r_{n_i,n_u} \end{pmatrix} \begin{pmatrix} x_{1,1} & \cdots & x_{1,n_e} \\ \vdots & x_{u,e} & \vdots \\ x_{n_u,1} & \cdots & x_{n_u,n_e} \end{pmatrix} + \begin{pmatrix} \epsilon_{1,1} & \cdots & \epsilon_{1,n_e} \\ \vdots & \epsilon_{i,e} & \vdots \\ \epsilon_{n_i,1} & \cdots & \epsilon_{n_i,n_e} \end{pmatrix}, \quad (1) $$

where the residual terms are represented as $\epsilon_{i,e}$. An associated mixing ratio $r_{i,u}$ was given as the surface area occupied by end-member unit $u$ relative to the surface area of footprint $i$. We calculated them by comparing the areal extent of the footprints (Figure 1b) and the end-member unit map (Figure 2b).

## 2.4. Unmixing Analysis

Before fitting, some footprints were filtered out according to the heterogeneity, sparseness, and uncertainty of the units. CM unit represents crater materials that superpose the other units. Since materials exposed on the surface by crater formation are those from the underlying unit, CM unit may exhibit geochemical diversity depending on local stratigraphy and excavation depth proportional to crater size (Holsapple, 1993; Susorney et al., 2016). Thus, assuming a homogeneous composition that represents the whole CM unit may not be reasonable. The LRM unit also has a limited spatial extent within the study area, making the calculation of the end-member composition by the unmixing analysis inaccurate. Therefore, footprints that have mixing ratio of CM and LRM units of more than 0.2 in total were filtered out from this analysis. The footprints with larger than 0.5 mixing ratios of UN were also filtered out. For the remaining 225 footprints, mixing ratios were standardized so that the sum of the mixing ratios of the five units were 1, that is, $r_{i,1} + r_{i,2} + r_{i,3} + r_{i,4} + r_{i,5} = 1$.

We calculated the end-member compositions that best reproduce the mixing equation, that is, that minimize the residual $E$ (Equation 1). Least-square fitting was performed to the 225 footprint data to derive the end-member compositions $x_{u,e}$, considering the fitting weight $w_{i,e}$ given as,

$$ w_{i,e} = \frac{1}{A_i \sigma_{i,e}^2}, \quad (2) $$

using the surface area $A_i$ and the observation error $\sigma_{i,e}$ of elemental composition $e$ of footprint $i$. The end-member compositions were derived as

$$ X = (R^T W R)^{-1} R^T W Y. \quad (3) $$

Note that for each elemental ratio $e$, the weight matrix $W$ was defined as a diagonal matrix with components $w_{i,e}$. The residual were calculated as

$$ \begin{aligned} \epsilon_{i,e} &= y_{i,e} - y_{\text{model},i,e} \\ &= y_{i,e} - \sum_{u} r_{i,u} x_{u,e}, \end{aligned} \quad (4) $$
<table><caption>**Table 1**
The Derived End-Member Compositions Compared With Previous Estimates</caption><thead><tr><th rowspan="2"></th><th colspan="5">This study</th><th colspan="2">Previous study</th></tr><tr><th>HRP-SP</th><th>HRP-ex</th><th>LBP-SP</th><th>LBP-IP</th><th>CG</th><th>LMg-HAl<sup>a</sup></th><th>HAl<sup>a</sup></th></tr></thead><tbody><tr><td>Mg/Si</td><td>0.23 ± 0.04</td><td>0.25 ± 0.09</td><td>0.40 ± 0.07</td><td>0.57 ± 0.11</td><td>0.37 ± 0.07</td><td>0.28 ± 0.03</td><td>0.38 ± 0.02</td></tr><tr><td>Al/Si</td><td>0.32 ± 0.03</td><td>0.27 ± 0.08</td><td>0.24 ± 0.05</td><td>0.18 ± 0.06</td><td>0.27 ± 0.04</td><td>0.32 ± 0.01</td><td>0.33 ± 0.01</td></tr><tr><td>Ca/Si</td><td>0.15 ± 0.01</td><td>0.12 ± 0.03</td><td>0.16 ± 0.03</td><td>0.18 ± 0.04</td><td>0.14 ± 0.03</td><td>0.15 ± 0.01</td><td>0.16 ± 0.01</td></tr></tbody></table>

where $y_{model,i,e}$ are calculated with the derived end-member compositions $x_{u,e}$ and mixing ratio $r_{i,e}$ (Equation 1). Fitting errors $\Delta x_{u,e}$ were calculated to evaluate to what extent the compositions converge to the estimated end-member composition $x_{u,e}$ as,

$$ \Delta x_{u,e} = \sqrt{\frac{\sum_i (r_{i,u}^2 e_{i,e}^2)}{\sum_i r_{i,u}^2}} \quad (5) $$

### 3. Results

The seven end-member units showed characteristic spatial distributions (Figure 2b). Inside the Caloris basin was mainly assigned to HRP-SP with small patches of LBP-SP while the annular region exterior to the basin showed a complicated distribution of LBP-SP, CG, and CM. Further beyond the basin rim, HRP-ex and LBP-IP composed the surface. Original MASCS footprints that represented LRM existed in the south and south-west of the basin (Figure 1c) although it was the least spatially extensive unit in the map (Figure 2d).

Our unmixing analysis derived the five end-member compositions (Table 1; Figure 3). HRP-SP and LBP-IP showed the smallest and largest Mg/Si ratios of 0.23 ± 0.04 and 0.57 ± 0.11, respectively; conversely, they showed the largest and smallest Al/Si ratios of 0.32 ± 0.03 and 0.18 ± 0.06, respectively. These two end-member compositions did not overlap within their error ranges. LBP-SP showed the intermediate composition of HRP-SP and LBP-IP. The compositions of HRP-ex and CG were close each other and similar to that of CIP. While the estimated Mg/Si and Al/Si values showed the variations among end-members, the estimated Ca/Si values, ranging from 0.12 to 0.18, overlapped each other with statistical errors of 0.01–0.04.

On previous elemental composition maps produced by Nittler et al. (2020), each pixel was assigned a weighted average of all footprints overlapping it (gray clouds in Figure 3). Our analysis derived the unmixed end components that constitute the variations shown by the previous elemental composition maps. Previously, Peplowski and Stockstill-Cahill (2019) performed Principal Component Analysis using the Mg/Si and Al/Si ratios (Nittler et al., 2020) along with the K abundances (Peplowski et al., 2012) to characterize the global geochemical diversity. Two geochemical terranes were related to the CIP and CEP: Low-Mg High-Al (LMg-HAl) and High-Al (HAl) units (Table 1). The majority of the basin interior and some of the basin exterior were the major locations of the LMg-HAl unit, showing Mg/Si of 0.28 ± 0.03. The estimated Mg/Si of 0.23 ± 0.04 of our HRP-SP unit was slightly lower than that of LMg-HAl unit, while Al/Si and Ca/Si were equivalent. The part of CEP located south of the Caloris basin was related to HAl unit, which showed similar Mg/Si, higher Al/Si and Ca/Si to that of our LBP-SP unit.

To evaluate the validity of our model, we compared the mixing ratios of specific end-member units and the observed compositions for individual XRS measurements (Figure 4). If an end-member unit represents a single homogeneous composition as assumed in our model, the observed compositions are expected to converge to a single value, that is, the derived end-member composition, when the mixing ratio of one end-member unit approaches 1. Our results showed that the observed and modeled Mg/Si compositions fit well, especially in the case of the HRP-SP, LBP-SP, and CG units (Figure 4, top row).

In contrast, if the observed composition does not converge to a specific value as the mixing ratio of a unit approaches 1, it suggests that there exists compositional heterogeneity within the end-member units that controls the

<footnote><sup>a</sup>Peplowski and Stockstill-Cahill (2019).</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** The derived end-member compositions showing the relationship between Mg/Si and (a) Al/Si and (b) Ca/Si ratios. Colored circles show the end-member compositions filled with the same colors used in the end-member unit map (Figure 2b), black bars representing their standard errors. Background gray squares indicate compositions of the pixels in our study area from the previous elemental composition maps (Nittler et al., 2020).</figcaption></figure>

observed compositional diversity rather than the compositional heterogeneity among different units. The derived end-member compositions of Ca/Si explained well only for HRP-SP unit (Figure 4, bottom row).

For Al/Si compositions, HRP-SP and CG appeared to converge to some extent, as seen in the Mg/Si case (Figure 4, middle row). On the other hand, the measurements with large mixing ratios of HRP-ex showed the minimum and maximum Al/Si of the whole data set, suggesting a large heterogeneity within this unit or the existence of sub-units with different compositions. Similarly, a significant downward offset from the model prediction was observed with the observations with $r_{LBP-IP} \sim 0.6-0.8$, likely indicating a compositional variation within the LBP-IP unit. The significant inconsistency between the observations and the model prediction was mainly related to high mixing ratios of HRP-ex and LBP-IP units.

# 4. Discussion

## 4.1. Geological Contexts of End-Member Units

Each of the spectromorphic units defined as end-member units in this study is considered to relate to different geological contexts. The estimated HRP-SP and LBP-SP compositions represent the magma compositions erupted after the Caloris-forming impact, spanning the same period of geological time. Caminiti et al. (2023) reported a spectral similarity between subsurface LBP materials beneath CIP and surface LBP materials on CEP. They hypothesized a scenario in which CIP was formed by sequential eruptions of LBP-SP magma corresponding to the CEP, followed by HRP-SP magma exposed at the CIP surface. These two end-member compositions may reflect magma evolution processes that occurred on timescales significantly shorter than geological timescales (Section 4.2).

It should be emphasized that this study also clarified the compositional variations exterior to the Caloris basin. While the CIP has been reported as a geochemical terrane (Peplowski & Stockstill-Cahill, 2019; Vander Kaaden et al., 2017; Weider et al., 2015), characterizing the composition of the CEP has been challenging due to the lack of spatial resolution in the elemental

composition data. Our unmixing approach enabled, for the first time, a comparison between the compositions of the Caloris interior and exterior plains, being represented by HRP-SP and LBP-SP units, respectively, which is crucial to discuss the volcanic activity that formed these smooth plains (Section 4.2).

The LBP-IP unit is associated with early volcanic activity, according to the older formation age of intercrater plains than smooth plains. Comparing the composition with those of HRP-SP and LBP-SP units is useful for understanding the temporal variations of interior conditions, such as magma compositions and source mantle compositions (Section 4.3).

The CG, which is defined geomorphologically as ejecta materials from the Caloris-forming impact, showed a similar composition to that of LBP-SP. The ejecta materials were considered to consist of pre-basin crustal materials and possibly upper mantle materials, since the Caloris-forming impact might have reached the depth of the mantle (Ernst et al., 2015; Potter & Head, 2017). This geological relation predicted the CG unit composition to have a Mg/Si ratio equivalent to or higher than that of the LBP-IP unit, which is considered to represent the pre-basin crustal materials. However, the derived Mg/Si ratio of the CG unit was as low as the smooth plains units (HRP-SP and LBP-SP units). This could be explained by the fact that we ignored CM, LRM, and UN for calculating mixing ratios (Section 4.5). Alternatively, possible explanations can be a later mixing of smooth plains materials by impact events at the top surface layer, the presence of low-Mg crusts before the basin formation, or a change in elemental composition due to physical processes during the basin-forming impact.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Relationships between the mixing ratios of end-member units and the elemental compositions for individual XRS measurements. Circles represent the 225 individual XRS measurements, showing the relationship between the mixing ratios of HRP-SP (red), HRP-ex (pink), LBP-SP (blue), LBP-IP (light blue), and CG (orange) units in horizontal axis and the observed Mg/Si (top row), Al/Si (middle row), and Ca/Si (bottom row) compositions with observational error (black line) in vertical axis. The transparency of the circles highlights the coefficient ($\frac{\sum u_i r_{iu}^2}{\sum u_i r_{iu}}$) used to calculate the fitting errors $\Delta x_{u,i}$ (Equation 5). With the derived end-member compositions (colored stars), our mixing model predicts the elemental composition ranges (shaded dark gray) with errors (shaded light gray).</figcaption></figure>

## 4.2. Fractional Crystallization Scenario

Fractional crystallization of magma leads to the evolution of the melt composition. Namur and Charlier (2017) experimentally showed that when magma with an elemental composition corresponding to that of Mercury's crust undergoes crystallization under low pressure (1 kbar) corresponding to shallow depths, crystallization of forsterite ($Mg_2SiO_4$) decreases Mg/Si and increases Al/Si of the residual melt (Figure 5). Other phases started crystallizing as temperature decreased (black crosses in Figure 5), such as diopside (CaMgSi$_2$O$_6$), enstatite (MgSiO$_3$), plagioclase (NaAlSi$_3$O$_8$ - CaAl$_2$Si$_2$O$_8$), or quartz (SiO$_2$) depending on the initial composition. The experiments represented equilibrium crystallization rather than fractional crystallization. However, the end-member composition estimated in this study corresponds mainly to a compositional range where only forsterite crystallizes. Given that the composition of the crystallized forsterite showed little variation with consistently high Mg/Fe ratios, the observed compositional evolution (colored lines in Figure 5) is similar to that of fractional crystallization (black arrows in Figure 5). Therefore we interpreted the results of Namur and Charlier (2017) as representing the compositional evolution during fractional crystallization and compared them with the estimated end-member compositions.

The compositions of the volcanic smooth plains represented by the HRP-SP and LBP-SP units lie on the same magma evolution trend (Figure 5). This indicates that both the HRP-SP and LBP-SP materials originated from parent magmas with common higher Mg compositions and that the compositional difference reflects different degrees of fractional crystallization: HRP-SP experienced a grater degree of fractional crystallization than LBP-SP. In the experiments by Namur and Charlier (2017), $\Delta \text{Mg/Si} \sim -0.1$ as when $\sim 10$ wt% forsterite crystallizes. Extrapolating this relationship, it was calculated that the crystallization of $\sim 17 \pm 8$ wt% forsterite
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Comparison between estimated end-member compositions and compositional evolution trend during fractional crystallization under low-pressure conditions (Namur & Charlier, 2017): The initial magma compositions (colored squares) were assumed based on observational crustal compositions within Smooth Plains (red), High-Mg and Low-Mg NVP (pink), IcP-HCT (light blue), and HMR (yellow). As the initial magma cooled, the compositions of the residual melt changed (colored lines) according to the crystallizing phases. From all five initial magmas, forsterite (Fo) was the first phase to crystallize: A black arrow shows the corresponding compositional evolution. As crystallization progressed, additional phases started to crystallize (black cross), such as diopside (Di), plagioclase (Pl), enstatite (En), and quartz (Qz).</figcaption></figure>

would account for the compositional change from LBP-SP to HRP-SP of $\Delta\text{Mg/Si} = -0.17 \pm 0.08$. For comparison, theoretical calculation also suggested that, assuming the initial magma composition of Si content of 25–30 wt%, which covers the variation of Si contents on Mercury's surface (McCoy et al., 2018), crystallization of ~19–23 wt% forsterite was required for $\Delta\text{Mg/Si} = -0.17 \pm 0.08$. This scenario implies that the deposition of the LBP-SP is relatively earlier than that of the HRP-SP. This is consistent with the previously suggested relative age relationship that CIP postdates CEP (Caminiti et al., 2023), spanning the same short period of geological time (Denevi et al., 2013; Fassett et al., 2009; Rothery et al., 2017; Strom et al., 2008). The differences in the degree of magma evolution of the finally erupted magma could be due to the difference in crustal thickness between the interior and exterior of the basin. Compared to the exterior, the basin interior likely had a thinner crust due to crustal removal by the basin-forming impact, and the prolonged eruption may have led to a higher degree of fractional crystallization.

Previously, the degree and/or depth of partial melting and the mantle source compositions were considered controlling factors for the elemental compositions of the volcanic surface (Beuthe et al., 2020; Caminiti et al., 2023; Charlier et al., 2013; Mouser & Dygert, 2023; Mouser et al., 2021; Namur, Collinet, et al., 2016; Vander Kaaden & McCubbin, 2016; Wang et al., 2022). This explains well the crustal diversity among temporally and spatially distinct geochemical terranes, such as CIP, HMR, and Northern Terrain. However, our HRP-SP and LBP-SP are continuous in both formation age and location, which makes it more difficult to imagine magma source diversity in this small area. The fractional crystallization scenario explains the observed variability between HRP-SP and LBP-SP more parsimoniously.

Another possible explanation for the compositions of HRP-SP and LBP-SP is crustal assimilation. The crustal compositions at the moment when the magma eruptions that formed the CIP and CEP occurred are expected to have the LBP-IP composition derived in this study. Since the LBP-SP unit has an intermediate composition between those of the HRP-SP and LBP-IP, compositional mixing between the crust with LBP-IP composition and the magmas with the HRP-SP composition can reproduce the LBP-SP composition. In this case, contrary to the fractional crystallization scenario, the CEP exhibits a more evolved magma composition. The thicker crust outside the basin would have delayed magma eruption, allowing the erupted magma to have undergone significant fractional crystallization within the crust. Even in

this scenario, a common magma source can account for the crustal diversity between the CIP and the CEP. However, the relative timing of the HRP-SP and LBP-SP depositions does not strongly support this scenario. When crustal assimilation occurs, magma compositions typically transit from source magma composition toward crustal composition, that is, from HRP-SP through LBP-SP toward LBP-IP, in this case. HRP-SP was predicted to predate LBP-SP, which is not consistent with the stratigraphic relationship of LBP-SP predating HRP-SP within the basin (Caminiti et al., 2023).

Note that the crustal compositions of smooth plains do not always represent the initial composition of their parent magma. Indeed, in the case of the fractional crystallization scenario, the crustal composition of the HRP-SP unit does not directly represent the initial composition of the parent magma. In the same way, the LBP-SP is potentially higher-Mg magma. Petrological experiments by Namur and Charlier (2017) indicated that magma with Mg/Si as high as 0.5, corresponding to that of geomorphologically old crust, that is, Intercrater Plains-Heavily Cratered Terrane (IcP-HCT), could also produce both LBP-SP and HRP-SP compositions through fractional crystallization (Figure 5). The parent magma for LBP-SP and HRP-SP could have had a composition either corresponding to or different from that of early volcanism which formed the LBP-IP unit. Thus, our results do not provide a constraint on the magma source diversity.
### 4.3. Implications for Early Volcanism

In this study, the compositions of the pre-Caloris crust were derived as the end-member compositions of the LBP-SP and HRP-ex. The two units showed contrasting compositions, with LBP-IP showing the highest Mg abundances and HRP-ex showing the second lowest Mg abundances after the HRP-SP, and large variabilities (Table 1). These results suggest that intercrater plains formed through early volcanism had a large compositional variation and that their spectral diversity is related to compositional diversity.

Compositional diversity within the intercrater plains was detected between the HMR and the remainder of the unit (Weider et al., 2015). However, the compositional similarity of the HMR and LBP-IP calculated in this study infers a scenario, where the existence of high-Mg crusts regions other than the HMR is possible: In early volcanism, magma with varying compositions from low-to high-Mg contents, reflecting the variations in degree and/or depth of partial melting or the composition of the source mantle (Frank et al., 2017; Mojzsis et al., 2018; Weider et al., 2015), erupted to cause global resurfacing. In late-stage volcanism, resurfacing by relatively low-Mg lavas equivalent to the HRP-SP or LBP-SP compositions occurred and partially obscured the high-Mg early crusts. The HMR shows high-Mg compositions at present because the effect of such resurfacing by low-Mg lava was small. Small patches of high-Mg crusts might exist but were not detected by the XRS observations due to the limitation of the spatial resolution.

The XRS observations with large footprints in the southern hemisphere might show the spatial average of the end-member compositions of the LBP-IP, HRP-ex, and potentially HRP-SP or LBP-SP with small fractions corresponding to the smooth plains. Future observations will enable a detection of the compositional diversity within the intercrater plains in the southern hemisphere and will improve the understanding of the early volcanism (Section 4.5).

### 4.4. Crustal and Magmatic Evolution

We propose a crustal formation scenario around the Caloris basin (Figure 6). As a primary crust, graphite crystallized and floated on top of the magma ocean (Peplowski et al., 2016; Vander Kaaden & McCubbin, 2015). Partial melting of the mantle generated magmas with various compositions first, which erupted to form the intercrater plains. Note that the pre-basin crusts were mostly related to LBP spectra around the Caloris basin but some of them showed HRP spectra. The Caloris-forming impact excavated and melted the crust, and possibly the mantle, as deep as the primarily formed graphite layer, resulting in the basin floor with LRM spectra (Ernst et al., 2015; Potter & Head, 2017). The basin floor and ejecta were formed as LRM and CG units, respectively. At the same time, possibly driven by the impact energy (Padovan et al., 2017), magma generation was promoted beneath the basin area, both interior and exterior (Figure 6a). Subsequently, volcanic eruptions occurred both inside and outside the basin, forming the LBP-SP unit. The magmas experienced fractional crystallization under the condition of low pressure, such as in magma chambers found on the Earth. Magma eruptions were prolonged only within the basin, where magma that experienced a larger degree of fractional crystallization formed the surface HRP-SP unit (Section 4.2; Figure 6b). Superposing craters younger than the other units excavated and exposed the subsurface materials, and formed the CM unit.

Around the basin, the radial spread of ejecta materials and the following deposition of the volcanic infill associated with the LBP-SP resulted in a complex distribution of the CG and LBP-SP units. Consequently, craters formed later in these regions are inferred to represent the LBP-SP, CG, LBP-IP, LRM units, or mixtures of them, depending on the craters' excavation depth and local stratigraphy.

The HRP-SP unit, indicative of post-fractional crystallization magma eruptions, can be observed within the basin, while it is not found in the surrounding area (Figure 2b). This discrepancy can be attributed to differences in crustal thickness. Thinner crustal areas tend to have higher magma eruption rates compared to thicker crustal regions, as suggested by comparison of volcanic activity between the near and far sides of the Moon (Morota & Furumoto, 2003; Taguchi et al., 2017) and discussed by comparing the magma eruption rates within the Caloris and Rembrandt interior plains (Hirata et al., 2022). The observed correlation between the locations of the impact basins and the volcanic smooth plains on Mercury also supports that the magma eruptions more likely occurred within the basin where the basin-forming impact made the crust thinner than the surroundings (Denevi et al., 2013). The difference in crustal thickness might have led to the more prolonged magma eruptions within CIP than within CEP, resulting in forming the HRP-SP unit with its more-evolved composition. Note that the
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Schematic cross section of the stratigraphy of the end-member units across the Caloris basin. (a) The Caloris-forming impact deposited LRM (dark gray) and the CG unit (orange) as the basin floor and ejecta, respectively, on top of the pre-Caloris crust (light gray). The impact energy might have promoted melt production beneath the basin. (b) After the formation of the basin, volcanic eruptions formed LBP-SP (blue) in the interior and exterior of the basin. HRP-SP (red) was formed within the basin interior, where volcanic activity was more active than the exterior.</figcaption></figure>

opposite relationship between the crustal thickness and degree of magma evolution is possible: Magma stays in the crust for a longer period before eruption and experiences a higher degree of magma evolution in the area with the thicker crust. This scenario is consistent with the compositional variation between the HRP-SP and LBP-SP when combined with the crustal assimilation scenario. However, it does not consistently explain the stratigraphic relation of the LBP-SP predating the HRP-SP within CIP (Section 4.2).

## 4.5. Limitations of the Model and Future Prospects

Here we discuss the limitations of our mixing model and analysis. Our model ignored several end-member units when calculating mixing ratios (Section 2.4). The CM unit was excluded due to the potential diversity of associated crustal materials and the LRM unit was excluded due to low mixing ratios resulting from its small spatial extent. Additionally, mixing ratios were normalized so that the mixing ratios of the calculated five units are 1 in total. Although measurements that met either of the conditions, $r_{SC} + r_{LRM} \ge 0.2$ or $r_{UN} \ge 0.5$, were filtered out from calculation, the derived end-member compositions might have influenced by these units.

The end-member units defined in this study could be divided into multiple subunits. Our HRP-ex unit was related to two geomorphological units, smooth plains and intercrater plains. Furthermore, some parts of the smooth plains area may be related to the southeastern extension of the Borealis Planitia, while other parts are related to crater floors. Subunits with these different geological contexts can exist inside our HRP-ex unit. Indeed, it is also suggested from a comparison between the measurement data and the mixing ratios (Figure 4). The measured Al/Si within footprints that meet $r_{HRP-ex} > 0.8$ ranged from 0.1 to 0.4, which covers the Al/Si range of the whole data set.

Differences in the geological contexts between the eastern and western regions outside the Caloris basin can also be considered. West of the basin is characterized by intercrater plains and smooth plains that possibly compose a part of Borealis Planitia, whereas the HMR lies east of the study area. Considering that the spatial extent of geochemical terranes could be larger than suggested by the previous elemental composition maps, our LBP-IP unit might partially overlap the HMR.
There is a potential application of this methodology to regions with distinct geomorphological and spectral characteristics to distinguish and understand the crustal compositions. Among the potential target regions are the Rembrandt basin in the southern hemisphere with interior volcanic smooth plains and the Rachmaninoff basin characterized by a concentrated distribution of LRM within and around it. However, it appears challenging to apply this methodology beyond the Caloris basin using XRS data because of the large size of footprints, particularly in the southern hemisphere, and the smaller size of the basins compared to the Caloris basin.

Upcoming measurements by BepiColombo, an ESA-JAXA collaboration mission for Mercury exploration, with its onboard Mercury Imaging X-ray Spectrometer (MIXS: Bunce et al., 2020) should help solve this issue. MIXS is composed of the two spectrometers, wide-field collimator (MIXS-C) and the narrow-field, high resolution, telescope (MIXS-T). MIXS-C is an imaging spectrometer and will achieve the global mapping of the surface elemental composition in spatial resolution of <50 – 100 km both on the northern and southern hemispheres. The higher spatial resolution of MIXS-C than that of XRS (~ 100 – 1000 km) will improve the understanding of the spatially heterogeneous surface elemental compositions of Mercury. Additionally, MIXS-T will probe the localized composition with a higher spatial resolution of <10 km, which will enable detailed compositional analysis of the small scale features, such as LRM and pyroclastic deposits.

## 5. Conclusions

The surface elemental compositions measured by MESSENGER/XRS provide key information to understand the interior conditions during the volcanic activities that formed the surface. We analyzed the surface elemental composition data measured around the Caloris basin using the mixing model to resolve the elemental compositions associated with different geological contexts.

1. Five end-member compositions within and around the Caloris basin were determined by assuming that the end-member units defined from spectral and geomorphological properties have homogeneous elemental compositions.

2. Compositional variations between the Caloris interior and exterior plains can be explained by a difference in degrees of fractional crystallization from parent magmas with a common composition. The spectral and compositional heterogeneity observed on Mercury's surface can be partially explained by magma evolution.

3. Two end-member units associated with the pre-Caloris crust showed contrasting compositions with large variability, indicating the compositional diversity within the intercrater plains. Small patches of intercrater plains with Mg contents comparable to that of the HMR may also exist. Future observations by BepiColombo MIXS have the potential to detect such compositional diversity at small spatial scales, particularly in the southern hemisphere.

## Data Availability Statement

The MESSENGER XRS data used in this study are available through NASA's Planetary Data System Geoscience node (https://pds-geosciences.wustl.edu/missions/messenger/xrs.htm, Nittler, 2018; Starr, 2018). Elemental composition data corresponding to MESSENGER XRS footprints are available through Nittler et al. (2020). The MESSENGER MDIS data used in this study are available through the USGS Astrogeology Science Center (https://astrogeology.usgs.gov/search/map/mercury_messenger_mdis_global_basemap_bdr_166m, PDS Cartography and Imaging Sciences Node, 2022). Spectral data used to generate the spectral unit map in this study are available through Caminiti et al. (2023).

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## Acknowledgments

KH acknowledges support from the Japan Society for the Promotion of Science (22K21344 and 23KJ0527). EC acknowledges the Domaine d'Intérêt Majeur en Astrophysique et Conditions d'Apparition de la Vie+ ile de France for their support. JW acknowledges support from the European Space Agency (ESA) as an ESA Research Fellow received during the write-up of this work. Kaori Hirata sincerely appreciates Noriyuki Namiki for providing helpful feedback. The authors acknowledge Larry Nittler and the anonymous reviewer for their constructive comments at the review stage.
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