# Rubidium and potassium isotope compositions of enstatite meteorites: implications for the evolution of their parent body(ies)

Baoliang Wang, Frédéric Moynier, Yan Hu

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Baoliang Wang, Frédéric Moynier, Yan Hu. Rubidium and potassium isotope compositions of enstatite meteorites: implications for the evolution of their parent body(ies). Geochimica et Cosmochimica Acta, 2025, 397, pp.63-74. 10.1016/j.gca.2025.04.007. insu-05080894

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# Rubidium and potassium isotope compositions of enstatite meteorites: implications for the evolution of their parent body(ies)

Baoliang Wang<sup>a,*</sup> <sup>ID</sup>, Frédéric Moynier<sup>a</sup>, Yan Hu<sup>b ID</sup>

<figure><img src="image_3.png" /></figure>

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## ARTICLE INFO

Associate editor: Oliver Nebel

Keywords:

Rb isotopes

K isotopes

Enstatite meteorites

Moderately volatile elements

Thermal metamorphism

## ABSTRACT

Enstatite meteorites, including enstatite chondrites and enstatite achondrites (e.g., aubrites), formed under highly reducing conditions in the solar system. Enstatite chondrites underwent progressive thermal metamorphism from petrologic type 3 to type 6, potentially leading to vaporization and redistribution of volatile elements. Coupled Rb and K isotopic analyses of enstatite meteorites could provide complementary insights into the inherent isotopic variability and volatile depletion processes. In this study, we present Rb and K isotopic compositions for a suite of enstatite meteorites, including sixteen enstatite chondrites spanning metamorphic grades from 3 to 6, as well as four aubrites. Type 3 enstatite chondrites exhibit isotopic compositions similar to those of Earth for both Rb and K, which further underscores the isotopic resemblance between Earth and enstatite chondrites. From type 3–4 to type 5–6, the examined enstatite chondrites generally show a trend towards heavier Rb and K isotopic compositions, indicating volatilization and redistribution of Rb and K during open system thermal metamorphism of the parent body(ies). One EH5 (St. Marks) and two EL6 (Pillistfer and Atlanta) samples deviate from this trend with light K isotope compositions, which may result from an interplay of evaporation, vapor transport and recondensation. On the other hand, the Rb and K isotopic variations in aubrites—which originated from the melting and fractional crystallization of enstatite chondrite-like parent body(ies)—likely reflect more complex processes, possibly involving a combination of plagioclase-bearing melt extraction, magmatic differentiation, core segregation, and the back-condensation of volatiles after impact volatilization.

## 1. Introduction

Enstatite meteorites are comprised of undifferentiated enstatite chondrites and differentiated enstatite achondrites, such as aubrites. They are the most reduced group of meteorites known to have formed in the solar system (Larimer, 1968; Keil, 1989). Enstatite chondrites are primitive meteorites that share isotopic similarities with Earth for many elements, including non-mass dependent effects for elements such as O (Clayton et al., 1984), Ti (Trinquier et al., 2009) and Cr (Mougel et al. 2018), and mass-dependent effects for elements like Mg (Teng et al., 2010), Zn (Moynier et al., 2011), Fe (Wang et al., 2014), and K (Zhao et al., 2020). These isotopic similarities have been used as evidence for strong genetic connection between enstatite chondrites and Earth (e.g., Javoy, 1995; Javoy et al., 2010; Dauphas, 2017). However, the estimated ³⁰Si/²⁸Si ratio of the bulk silicate earth (BSE) is ~ 0.30 ‰ higher than that in enstatite chondrites (Georg et al., 2007; Fitoussi and Bourdon, 2012; Savage and Moynier, 2013). This discrepancy

necessitates either an unrealistically high Si abundance within the Earth's core, or much greater extent of metal/silicate Si isotopic fractionation, if the isotopic fractionation primarily arose from metal/-silicate differentiation (Fitoussi and Bourdon, 2012; Savage and Moynier, 2013; Hin et al., 2014; Moynier et al., 2020). Furthermore, a substantial discrepancy in the budget of elements such as Mg and Al exists between enstatite chondrites and the BSE (Larimer and Anders, 1970; Baedecker and Wasson, 1975), which is also difficult to reconcile with the idea that enstatite chondrite-like materials serve as the major building blocks of Earth.

Formed under extremely reducing conditions, enstatite chondrites feature a unique mineral assemblage predominantly composed of FeO-poor enstatite, Si-bearing metallic Fe-Ni and a variety of sulfides (Keil, 1968, 1989; Lin, 2022). Following the classification scheme for H and L groups of ordinary chondrites, enstatite chondrites have been divided into EH (high-Fe) and EL (low-Fe) subgroups (e.g., the CI-normalized Fe/Si ratio in the EH group is greater than that of the EL

<footnote><sup>a</sup> Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, 1 Rue Jussieu, 75005 Paris, France</footnote>

<footnote><sup>b</sup> Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154, USA</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: bwang@ipgp.fr (B. Wang).</footnote>
group by a factor of ~ 1.5; Sears et al. 1982). Weyrauch et al. (2018) further defined four distinct subtypes of enstatite chondrites (i.e., EH<sub>a</sub>, EH<sub>b</sub>, EL<sub>a</sub>, EL<sub>b</sub>), based on chemical variations in sulfides and metal, and suggested that enstatite chondrites are derived from at least four parent bodies, possibly up to eight considering some anomalous samples.

Aubrites are products of partial or total melting and igneous differ-
entiation of material similar to that of the enstatite chondrites parent
body(ies) under highly reducing conditions (e.g., Wolf et al., 1983; Keil,
2010; Barrat et al., 2016). They are predominantly pyroxenites con-
sisting mainly of FeO-free enstatite, with varying amounts of diopside,
plagioclase, and forsterite. Unlike enstatite chondrites, aubrites are
depleted in metallic Fe-Ni and sulfides (Keil, 2010). Most aubrites are
brecciated with several exceptions such as Shallowater, an anomalous
aubrite considered to have originated from a parent body different from
that of main-group aubrites, based on its complex cooling history (Keil,
1989), Hf-W systematics (Petitat et al., 2008), and Zn (Moynier et al.,
2011) and Cr (Zhu et al., 2021) isotopic compositions. The rare earth
element (REE) patterns of Mount Egerton and Larned suggest that these
two aubrites are also distinct from the main-group aubrites and likely
derived from a separate parent body (Barrat et al., 2016).

The heat generated by the decay of short-lived radionuclides and impacts caused thermal metamorphism within enstatite chondrites. Both EH and EL chondrites include various petrographic types that reflect varying degrees of thermal metamorphism, categorized as EH3–6 and EL3–6, with 3 indicating the lowest degree of thermal metamorphism (e.g., Huss et al., 2006). The texture and mineralogy of enstatite chondrites could be changed during progressive thermal metamorphism. In type 3 enstatite chondrites, chondrules remain largely intact. With increasing metamorphic grade, chondrules become less discernable, and even disappear in most type 6 chondrites. Monoclinic low-Ca pyroxene is prevalent in type 3 enstatite chondrites, while it is entirely replaced by orthopyroxene in types 5 and 6 enstatite chondrites. From type 3 to type 6, increasing amounts of plagioclase are recrystallized from the fine-grained matrix and become coarser-grained; conversely, the abundance of olivine gradually decreases, and is almost absent in type 5 and type 6 enstatite chondrites (Huss et al., 2006).

Accompanied by the recrystallization of minerals, the heating events have potentially led to volatilization, redistribution, and isotopic fractionation of volatile elements (Wang and Lipschutz, 2005; Huss et al., 2006; Wombacher et al., 2008; Moynier et al., 2011; Hellmann et al., 2021; Barrat et al., 2023). Moderately volatile elements (e.g., Se, Ag, In, Bi, Tl, Cd, Zn, Te) are depleted in types 5 and 6 relative to types 3 and 4 enstatite chondrites or CI chondrites (Wang and Lipschutz, 2005; Wombacher et al., 2008; Moynier et al., 2011; Hellmann et al., 2021). Correspondingly, enstatite chondrites with higher metamorphic grade (type 6) have been found to be highly enriched in the heavier isotopes of Zn and Cd (Wombacher et al., 2008, Moynier et al., 2011). A similar trend was recently observed for Te, with types 5 and 6 enstatite chondrites displaying heavier isotope compositions than those of types 3 and 4 (Hellmann et al., 2021). The systematically heavier isotopic compositions, coupled with larger elemental depletion in type 5–6 compared to type 3–4 enstatite chondrites (e.g., Cd, Zn, and Te), may be attributed to vaporization and volatile loss during the open-system thermal metamorphism of their parent body(ies) (Wombacher et al., 2008; Moynier et al., 2011; Hellmann et al., 2021).

Similar to Cd, Zn, and Te, both Rb and K are moderately volatile
elements, with 50 % condensation temperatures (T<sub>c</sub><sup>50</sup>) of 800 K and
1006 K, respectively (Lodders, 2003). However, no systematic variations
in K isotopic compositions have been observed for enstatite chondrites
across different chemical groups or petrological types (Zhao et al., 2020;
Hu et al., 2023; Nie et al., 2023). In contrast, a discernible trend exists
between Rb isotopic compositions and petrologic types (e.g., Pringle and
Moynier, 2017; Nie et al., 2023). Nevertheless, the robustness of this
trend remains uncertain given the limited number of samples analyzed.
In addition, an extremely heavy Rb isotopic composition was observed
in an EL6 sample (Atlanta), potentially due to intense Rb evaporation

during thermal events on the parent body, yet it was attributed to
terrestrial weathering (Nie et al., 2023).

Since Rb and K isotopes fractionate in opposite directions during terrestrial chemical weathering (Chen et al., 2020; Teng et al., 2020; Z.Y. Zhang et al., 2021) but behave coherently during volatility-controlled processes (e.g., Nie et al., 2021; Z.J. Zhang et al., 2021; Hu et al., 2022, 2023; Wang et al., 2024), coupled Rb and K isotopic analyses of a broader set of enstatite chondrites could reveal the inherent isotopic variability related to different volatile depletion processes. Here, we analyzed sixteen enstatite chondrites ranging in metamorphic grades from 3 to 6, along with four aubrites, to determine their Rb and K isotopic compositions. Combining Rb and K isotopes, we aim to offer further insights into the volatile inventories in enstatite meteorites and the thermal evolution of their parent body(ies).

**2. Samples and methods**

2.1. *Samples*

In this study, twenty enstatite meteorites were analyzed for Rb and K isotopic compositions. Sixteen enstatite chondrites include four EH3 (MIL07028, SAH97096, GRO95517, and Kota-Kota), two EH4 (EET96299 and Abee), two EH5 (EET96135 and St. Marks), one EL3 (MAC02837), one EL4 (MAC02747), and six EL6 (Ufana, North West Forrest, Atlanta, Eagle, LON 94100, and Pillistfer). Four aubrites were analyzed from the main group (Tiglit, Khor Temiki, Cumberland Falls, and Norton County). To evaluate the reproducibility of the measurements, a portion of enstatite chondrites were analyzed in replicates using the same homogenized powders. Additionally, two USGS reference materials, BHVO-2 and AGV-2 were analyzed following the same procedures, in order to verify the accuracy of our measurements.

## 2.2. Analytical methods

Chemical preparation of samples was carried out at the Institut de Physique du Globe de Paris (IPGP). To ensure sample homogeneity, over one gram of samples was ground into powder. Approximately 50 mg of powdered samples were weighed and dissolved in screw-top Teflon beakers with a mixture of 27 mol L⁻¹ HF and 16 mol L⁻¹ HNO₃ (3:1, v/v) at 140°C for 72 h. Afterwards, the sample solution was dried down and successively dissolved in aqua regia (a mixture of 10 mol L⁻¹ HCl and 16 mol L⁻¹ HNO₃ in a 3:1 ratio, v/v) and 6 mol L⁻¹ HCl to eliminate fluorides. After complete dissolution, dried samples were refluxed with 16 mol L⁻¹ HNO₃ and finally re-dissolved in 1 mL of 1 mol L⁻¹ HNO₃ and 0.5 mol L⁻¹ HNO₃ for Rb and K column chemistry, respectively.

Rubidium and K elemental concentrations of all studied meteorites as well as REE data for Atlanta were measured using an Agilent 7900 Quadrupole Inductively Coupled Plasma Mass Spectrometry (ICP-QMS) at IPGP. An aliquot of the digested solution was diluted by approximately a factor of 10,000 for elemental analysis. Sample solutions were introduced into a Scott spray chamber through a MicroMist nebulizer with an uptake rate of 0.2 mL/min. Count measurements were converted to solution concentrations by analyzing a mixture of certified standards with concentrations covering the sample range.

Rubidium purification was achieved using a four-step method as described in Wang et al. (2023), which involved one step of Eichrom DGA resin (50–100 µm, 1.8 mL), two steps of Bio-Rad AG50W-X12 resin (200–400 mesh, 20 mL and 10 mL, respectively), and one step of Eichrom Sr-spec resin (20–50 µm, 1 mL). The Rb recovery of this method was > 95 %, and the whole-procedure Rb blank was less than 0.03 ng, representing < 0.5 % of the total Rb (>6 ng) in the sample solution. Potassium was purified from matrices by cation-exchange chromatography (Moynier et al., 2021). A volume of 2 mL of Bio-Rad AG 50 W-X8 cation exchange resin (200–400 mesh) was packed into Bio-Rad PolyPrep columns. The resin was cleaned using 10 mL of 6 mol L⁻¹ HCl followed by 6 mol L⁻¹ HNO₃, with a rinse of 10 mL Milli-Q deionized water
after each acid wash. Subsequently, the resin was conditioned by 10 mL of 0.5 mol L⁻¹ HNO₃. Sample solutions were loaded onto the resin in 1 mL of 0.5 mol/L HNO₃, and 13 mL of 0.5 mol L⁻¹ HNO₃ was added to elute matrix elements. Potassium was collected in the subsequent 22 mL fraction using the same acid. This chromatography protocol provides a K yield > 99.3 %, and the total blank was less than 7 ng, which is insignificant (< 0.7 %) relative to the amount of K in sample aliquots (>1 µg).

Rubidium and K isotopic measurements were performed on the Nu Sapphire collision-cell multi-collector inductively-coupled plasma spectrometer (CC-MC-ICP-MS) at IPGP following previously established protocols (Moynier et al., 2021; Wang et al., 2023; Hu and Moynier, 2024). Rubidium isotopic ratios were measured using the conventional high-energy mode. A high Rb sensitivity of ~ 500 V per µg/g for ⁸⁵Rb was achieved in low-resolution mode. Purified sample solutions and standard solutions were diluted to 2 ng/g with 0.5 mol·L⁻¹ HNO₃, and introduced into the mass spectrometer via an Apex Omega desolvation nebulizer system with standard Ni cones (“dry plasma”). The isotopes ⁸⁵Rb and ⁸⁷Rb were collected in L3 and H1 Faraday cups with 10¹¹Ω

resistors. Simultaneously, ⁸⁸Sr was collected in the H3 Faraday cup to correct the interference of ⁸⁷Sr on ⁸⁷Rb, assuming an ⁸⁷Sr/⁸⁸Sr value of 0.085 (Nebel et al., 2005; Pringle and Moynier, 2017; Wang et al., 2023). Before each analysis, a 30-second blank measurement was performed to subtract the background signal. Each analysis comprised 40 cycles, with an integration time of 5 s per cycle. Common standard-sample bracketing approach was used to correct the instrumental mass bias. Rubidium isotopic compositions are reported relative to NIST SRM 984 in δ⁸⁷Rb notation: δ⁸⁷Rb (‰) = [(⁸⁷Rb/⁸⁵Rb)sample / (⁸⁷Rb/⁸⁵Rb)SRM 984 - 1] × 1000.

Potassium isotopic compositions were analyzed using the low-energy ion pathway (collision cell pathway). Standard and sample solutions were diluted to 100 ng/g with 0.5 mol·L⁻¹ HNO₃. Likewise, the Apex Omega desolvating system was employed for introducing the K solution into the mass spectrometer. Ion beams were firstly directed into a hexapole collision cell supplied with H₂ and He gases, facilitating the significant removal of Ar-related interfering species (e.g., ³⁸ArH⁺, ⁴⁰ArH⁺, and ⁴⁰Ar⁺) through reactions with H₂. Subsequently, the ⁴¹K⁺

<table><caption>**Table 1**

Rubidium and potassium isotope compositions of enstatite meteorites.</caption><thead><tr><th>Sample</th><th>Type</th><th>Fall/Find</th><th>location</th><th>Weathering grade</th><th>Shock level</th><th>Rb (µg/g)</th><th>δ<sup>87</sup>Rb (‰)</th><th>2SD</th><th>K (µg/g)</th><th>δ<sup>41</sup>K (‰)</th><th>2SD</th></tr></thead><tbody><tr><td>MIL07028</td><td>EH3</td><td>Find</td><td>Antarctica</td><td>B</td><td></td><td>3.0</td><td>-0.19</td><td>0.06</td><td>817.2</td><td>-0.64</td><td>0.06</td></tr><tr><td>MIL07028 (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.64</td><td>0.07</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.64</strong></td><td><strong>0.00</strong></td></tr><tr><td>SAH97096</td><td>EH3</td><td>Find</td><td>Sahara Desert</td><td></td><td>2</td><td>4.0</td><td>-0.09</td><td>0.06</td><td>848.6</td><td>-0.54</td><td>0.06</td></tr><tr><td>GRO95517</td><td>EH3</td><td>Find</td><td>Antarctica</td><td>C</td><td></td><td>3.2</td><td>-0.10</td><td>0.06</td><td>755.4</td><td>-0.40</td><td>0.04</td></tr><tr><td>GRO95517 (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.36</td><td>0.03</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.38</strong></td><td><strong>0.06</strong></td></tr><tr><td>Kota-Kota</td><td>EH3</td><td>Find</td><td>Africa</td><td></td><td>3</td><td>3.8</td><td>-0.11</td><td>0.03</td><td>824.8</td><td>-0.58</td><td>0.02</td></tr><tr><td>Kota-Kota (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.62</td><td>0.04</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.60</strong></td><td><strong>0.06</strong></td></tr><tr><td>EET96299</td><td>EH4</td><td>Find</td><td>Antarctica</td><td>B</td><td></td><td>3.4</td><td>-0.01</td><td>0.06</td><td>881.1</td><td>-0.24</td><td>0.06</td></tr><tr><td>EET96299 (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.18</td><td>0.06</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.21</strong></td><td><strong>0.08</strong></td></tr><tr><td>Abee</td><td>EH4</td><td>Fall</td><td>Canada</td><td></td><td>2-4</td><td>4.1</td><td>0.01</td><td>0.06</td><td>837.8</td><td>-0.34</td><td>0.05</td></tr><tr><td>EET96135</td><td>EH5</td><td>Find</td><td>Antarctica</td><td>B</td><td>3</td><td>3.2</td><td>0.07</td><td>0.05</td><td>713.1</td><td>-0.25</td><td>0.05</td></tr><tr><td>EET96135 (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.31</td><td>0.06</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.28</strong></td><td><strong>0.08</strong></td></tr><tr><td>St. Marks</td><td>EH5</td><td>Fall</td><td>South Africa</td><td></td><td>3</td><td>0.6</td><td>2.18</td><td>0.06</td><td>519.4</td><td>-0.36</td><td>0.06</td></tr><tr><td>St. Marks (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.38</td><td>0.05</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.37</strong></td><td><strong>0.03</strong></td></tr><tr><td>MAC02837</td><td>EL3</td><td>Find</td><td>Antarctica</td><td>C</td><td>3</td><td>2.7</td><td>-0.17</td><td>0.06</td><td>1003.3</td><td>-0.55</td><td>0.05</td></tr><tr><td>MAC02837 (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.54</td><td>0.04</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.55</strong></td><td><strong>0.01</strong></td></tr><tr><td>MAC02747</td><td>EL4</td><td>Find</td><td>Antarctica</td><td>B/C</td><td>4</td><td>3.6</td><td>-0.05</td><td>0.05</td><td>873.9</td><td>-0.55</td><td>0.05</td></tr><tr><td>MAC02747 (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.51</td><td>0.07</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.53</strong></td><td><strong>0.06</strong></td></tr><tr><td>UFANA</td><td>EL6</td><td>Fall</td><td>Tanzania</td><td></td><td>2</td><td>1.9</td><td>1.03</td><td>0.05</td><td>557.1</td><td>-0.07</td><td>0.06</td></tr><tr><td>UFANA (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.08</td><td>0.08</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.08</strong></td><td><strong>0.01</strong></td></tr><tr><td>North West Forrest (E6)</td><td>EL6</td><td>Find</td><td>Australia</td><td></td><td>2</td><td>2.7</td><td>0.45</td><td>0.07</td><td>804.7</td><td>-0.23</td><td>0.05</td></tr><tr><td>Atlanta</td><td>EL6</td><td>Find</td><td>US</td><td></td><td>2</td><td>0.8</td><td>2.50</td><td>0.05</td><td>518.6</td><td>-0.18</td><td>0.07</td></tr><tr><td>Atlanta (replicate 1)</td><td></td><td></td><td></td><td></td><td></td><td></td><td>2.51</td><td>0.06</td><td></td><td>-0.20</td><td>0.05</td></tr><tr><td>Atlanta (replicate 2)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.12</td><td>0.04</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.17</strong></td><td><strong>0.08</strong></td></tr><tr><td>Eagle</td><td>EL6</td><td>Fall</td><td>US</td><td></td><td>2</td><td>3.2</td><td>0.41</td><td>0.05</td><td>653.2</td><td>-0.02</td><td>0.01</td></tr><tr><td>Eagle (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.04</td><td>0.06</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>0.01</strong></td><td><strong>0.08</strong></td></tr><tr><td>LON94100</td><td>EL6</td><td>Find</td><td>Antarctica</td><td>A/Be</td><td>2</td><td>2.3</td><td>0.32</td><td>0.02</td><td>568.4</td><td>-0.27</td><td>0.02</td></tr><tr><td>LON94100 (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.25</td><td>0.03</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.26</strong></td><td><strong>0.03</strong></td></tr><tr><td>Pillistfer</td><td>EL6</td><td>Fall</td><td>Estonia</td><td></td><td>2</td><td>2.2</td><td>0.28</td><td>0.06</td><td>421.6</td><td>-0.55</td><td>0.07</td></tr><tr><td>Pillistfer (replicate)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.64</td><td>0.06</td></tr><tr><td>average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><strong>-0.60</strong></td><td><strong>0.13</strong></td></tr><tr><td>Tiglit</td><td>Aubrite</td><td>Fall</td><td>Morocco</td><td></td><td></td><td>0.2</td><td>-0.17</td><td>0.02</td><td>61.1</td><td>-0.89</td><td>0.07</td></tr><tr><td>Khor Temiki</td><td>Aubrite</td><td>Fall</td><td>Sudan</td><td></td><td></td><td>2.0</td><td>0.18</td><td>0.04</td><td>514.3</td><td>-0.27</td><td>0.05</td></tr><tr><td>Cumberland</td><td>Aubrite</td><td>Fall</td><td>US</td><td></td><td></td><td>0.6</td><td>0.66</td><td>0.04</td><td>82.8</td><td>-0.71</td><td>0.07</td></tr><tr><td>Norton county</td><td>Aubrite</td><td>Fall</td><td>US</td><td></td><td></td><td>0.3</td><td>-0.71</td><td>0.04</td><td>79.6</td><td>-0.38</td><td>0.05</td></tr><tr><td>Rock standards</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>BHVO-2</td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.11</td><td>0.05</td><td></td><td>-0.46</td><td>0.06</td></tr><tr><td>AGV-2</td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.15</td><td>0.06</td><td></td><td>-0.45</td><td>0.08</td></tr></tbody></table>

<footnote>ªThe weathering grade and shock level are from MetBase and previous studies (Zhang et al. 1995; Kong et al. 1997; Wang et al., 2014; Rubin 2015; Zhao et al., 2020). Weathering index A, B, or C represents “minor,” “moderate,” or “severe” rustiness, respectively. Letter “e” represents evaporite minerals visible.</footnote>
and ³⁹K⁺ ion beams were re-accelerated and directed through the magnet to the Faraday collectors. The isotopes ⁴¹K and ³⁹K were collected in L1 and L6 Faraday cups with 10¹¹Ω and 10¹⁰Ω resistors, respectively. Each analysis consisted of a 60-second blank measurement, and 50 cycles of sample measurement with an integration time of 5 s per cycle. The instrumental mass discrimination was corrected using the sample-standard bracketing technique. The K isotopic data are reported relative to the international standard NIST SRM 3141a in delta notation as δ⁴¹K (‰) = [(⁴¹K/³⁹K)sample / (⁴¹K/³⁹K)NIST SRM 3141a - 1] × 1000.

The long-term reproducibility of Rb and K isotopic analyses is 0.04 ‰ (2SD), based on repeated measurements of BHVO-2 (δ⁸⁷Rb = -0.12 ± 0.04 ‰, 2SD, n = 39, Wang et al., 2025; δ⁴¹K = -0.43 ± 0.04 ‰, 2SD, n = 11, Moynier et al., 2021).

### 3. Results

Rubidium and potassium abundance and isotopic compositions of twenty enstatite meteorites are presented in Table 1. Trace element concentrations in some of the enstatite chondrites have been previously analyzed (Barrat et al., 2014). The Rb concentrations of several of our samples (e.g., SAH 97096, Kota-Kota, Abee, and MAC 02837) display visible differences from their data, possibly due to analytical uncertainty and sample heterogeneity. Nevertheless, the whole-rock Rb concentrations in our samples generally follow a consistent trend with those reported by Barrat et al. (2014) for both whole-rock and residual silicates after leaching with 6 mol/L HCl (Fig. S1). Our analyses of EET 96135 (EH5), St. Marks (EH5), Atlanta (EL6), Eagle (EL6), LON 94100 (EL6), and Pillistfer (EL6) yield consistent δ⁸⁷Rb and δ⁴¹K values with those of previous studies (Zhao et al., 2020; Ku and Jacobsen, 2020; Nie et al., 2023). In comparison, the δ⁴¹K values for MIL 07028 (EH3; -0.64 ± 0.06 ‰) and MAC 02747 (EL4; -0.53 ± 0.06 ‰) differ from the values (-0.35 ± 0.02 ‰ for MIL 07028 and -0.76 ± 0.02 ‰ for MAC 02747) reported by Zhao et al. (2020). The δ⁸⁷Rb value (+0.01 ± 0.06 ‰) of Abee (EH4) is consistent with the value of -0.01 ± 0.08 ‰ in Pringle and Moynier (2017) but is higher than those (-0.19 ± 0.02 ‰ and -0.24 ± 0.02 ‰) reported by Nie et al. (2023). These differences may reflect an unrepresentative sampling of the bulk rock.

The enstatite chondrites exhibit significant Rb isotope variations, with δ⁸⁷Rb values varying from -0.19 ‰ to +2.51 ‰ (Table 1). The EH3 and EL3 chondrites have a restricted variation in δ⁸⁷Rb (-0.19 ‰ to -0.09 ‰), aligning closely with the estimated average of the BSE (δ⁸⁷Rb

= -0.13 ± 0.06 ‰, Wang et al., 2023). Compared to type 3, type 4 enstatite chondrites possess slightly higher δ⁸⁷Rb (-0.05 ‰ to +0.01 ‰) values, whereas type 5–6 enstatite chondrites display even heavier and more variable Rb isotope compositions (δ⁸⁷Rb: +0.07 ‰ to +2.51 ‰) (Fig. 1).

In contrast to the homogeneous Rb isotope compositions, the K isotope compositions in type 3 enstatite chondrites shows considerable variations. The variability is consistent with published K isotope data for type 3 enstatite chondrites (e.g., Zhao et al., 2020; Ku and Jacobsen, 2020; Hu et al., 2023). Nonetheless, both the EH and EL groups generally show a trend of increasingly heavier K isotopic compositions from type 3 (-0.64 ‰ to -0.41 ‰) to type 6 (-0.26 ‰ to +0.01 ‰), except one EH5 (St. Marks, δ⁴¹K = -0.37 ± 0.03 ‰) and one EL6 (Pillistfer, δ⁴¹K = -0.60 ± 0.13 ‰) which deviate from this trend with light K isotope compositions (Fig. 1).

Besides enstatite chondrites, the four analyzed aubrites also display a wide range of isotopic variation, with δ⁸⁷Rb values varying from -0.71 ‰ to +0.66 ‰, and δ⁴¹K values ranging from -0.89 ‰ to -0.27 ‰, respectively (Table 1 and Fig. 1).

### 4. Discussion

#### 4.1. Terrestrial alteration effects

In contrast to witnessed falls which typically have minimal terrestrial residence history, meteorite finds may experience alteration prior to their recoveries. The aubrites analyzed in this study are all witnessed falls with no significant terrestrial weathering. However, more than half of the enstatite chondrites examined in this study are “finds” from diverse locations (Table 1). Therefore, it is essential to assess the potential influence of terrestrial alteration on the studied enstatite chondrites.

The Rb isotopic compositions in the studied enstatite chondrites show a strong negative correlation (R² = 0.78) with their Rb concentrations (Fig. 2a). In comparison, there is a weaker negative correlation (R² = 0.43) between their K isotopic compositions and K concentrations (Fig. 2b). Additionally, except one EH5 (St. Marks) and two EL6 (Pillistfer and Atlanta), other enstatite chondrites exhibit a positive correlation (R² = 0.61) between their δ⁴¹K and δ⁸⁷Rb values (Fig. 2c). This coupling of Rb and K isotopes suggests a common origin for the isotopic variations of these two elements. The increase in δ⁸⁷Rb values with

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Rubidium and potassium isotopic compositions of enstatite chondrites and aubrites. From type 3 to type 6, the enstatite chondrites generally show a trend towards heavier Rb and K isotopic compositions. The Rb and K isotopic compositions of the BSE are from previous studies (Tuller-Ross et al. 2019; Hu et al., 2021; Wang et al., 2023).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>(caption on next column)</figcaption></figure>

**Fig. 2.** (a) Negative correlation between the δ<sup>87</sup>Rb values and Rb concentrations in enstatite chondrites. (b) A weaker negative correlation for the δ<sup>41</sup>K values and K concentrations. (c) Positive correlation between δ<sup>87</sup>Rb and δ<sup>41</sup>K values of enstatite chondrites. The circles represent the data reported by Nie et al. (2023), which are included because the Rb and K isotope compositions were also measured on aliquots from the same digestion, thus eliminating the potential artifact of sample heterogeneity. The open symbols correspond to St. Marks, Pillistfer, and Atlanta, which deviate from the “partial evaporation trend” characterized by other enstatite chondrites, likely reflecting an effect of incomplete K recondensation.

decreasing Rb concentrations (Fig. 2a) might reflect an effect of Rb removal during chemical weathering, as <sup>85</sup>Rb would be preferentially lost (Z.Y. Zhang et al., 2021). However, the positive correlation between Rb and K isotope compositions (Fig. 2c) precludes Rb and K loss induced by chemical weathering as a plausible explanation for the isotopic variations, since Rb and K isotopes exhibit distinct behaviors during this process. For example, chemical weathering would cause <sup>41</sup>K to be preferentially lost, which is the opposite of what occurs with Rb isotopes (Chen et al., 2020; Teng et al., 2020; Z.Y. Zhang et al., 2021).

In addition to weathering, terrestrial contamination may also affect the original volatile inventory of enstatite chondrites and their isotopic compositions. A previous study on eucrite meteorites has observed that hot desert finds typically exhibit lighter K isotopic compositions than eucrite falls and Antarctic finds, and this offset was attributed to the addition of terrestrial K (Tian et al., 2019). However, no discernible systematic deviation in Rb and K isotope compositions is observed among the enstatite chondrites analyzed in this study, whether comparing finds to falls, or examining meteorites from Antarctica, hot desert, and other locations (Fig. S2). The meteorite falls appear to record minimal terrestrial effects, yet their Rb and K isotopic compositions span nearly the entire range observed in the studied enstatite chondrites (Fig. 3), indicating that the Rb and K isotope variations are likely unrelated to terrestrial contamination.

The conclusion that the Rb and K isotope variations are unlikely to result from terrestrial alteration is consistent with the lack of correlation between Rb or K isotope compositions and chemical weathering degree (Fig. 3). Additional evidence for limited terrestrial chemical weathering includes the absence of Ce anomalies. In terrestrial environment, Ce (III) is partially oxidized to Ce (IV), which is less soluble than trivalent REEs (e.g., Floss and Crozaz, 1991; Mittlefehldt and Lindstrom, 1991). As a result, Ce anomalies—typically associated with aqueous alteration—have been commonly observed in weathered Antarctic meteorites (e.g., Floss and Crozaz, 1991; Mittlefehldt and Lindstrom, 1991; Harvey et al., 1993; Hsu and Crozaz, 1996, 1998). However, no noticeable Ce anomalies are detected in the studied enstatite chondrites (Fig. 4). Furthermore, among the type 3 (MIL 07028, SAH 97096, GRO 95517, Kota-Kota, and MAC 02837) and type 6 (Eagle, LON 94100, and Pillistfer) enstatite chondrites, the Ba and Sr abundances (Fig. S3), as well as REE patterns (Fig. 4) of finds, closely resemble those of witnessed falls (Barrat et al., 2014). This further supports that these samples have not undergone significant terrestrial alteration after falling to Earth.

It is worth noting that St. Marks, Pillistfer, and Atlanta deviate from the K-Rb isotope trend observed among other enstatite chondrites, displaying a positive correlation (R<sup>2</sup> = 0.89) between their δ<sup>41</sup>K and δ<sup>87</sup>Rb values (Fig. 2c). This different trend suggests the presence of an additional mechanism underlying the isotopic variations in these three samples. St. Marks and Pillistfer are both witnessed falls, thus experiencing minimal terrestrial alteration. The meteorite find Atlanta has the heaviest Rb isotope composition (+2.51 ‰) among the enstatite chondrites analyzed in this study. This extremely heavy Rb isotopic composition was previously attributed to Rb loss induced by terrestrial weathering (Nie et al., 2023). However, Atlanta follows the negative trend between Rb isotopic compositions and Rb concentrations defined by other enstatite chondrites (Fig. 2a). Additionally, it exhibits compatible REE pattern with other EL6 chondrites (including witnessed
<figure><img src="image_6.png" /><figcaption>Fig. 3. Rubidium and potassium isotopic compositions of enstatite chondrites as a function of chemical weathering degree.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. CI-normalized REE patterns of the studied enstatite chondrites. The REE data for Atlanta are analyzed in this study, and data for other samples are from Barrat et al. (2014). The CI values are from Barrat et al. (2012). Data can be found in supplementary Table S1.</figcaption></figure>

falls) (Fig. 4), indicating that its heavy Rb isotope composition may not have originated from terrestrial weathering as previously suggested.

Regardless of the heavy Rb isotopic compositions, St. Marks, Pil-
listfer, and Atlanta exhibit low δ⁴¹K values that diverge from the general
K-Rb isotope trend (Fig. 2b and c), indicating a distinct origin for their K
isotopic compositions. The positive correlation between δ⁴¹K and δ⁸⁷Rb
values in these three samples suggests that they may share a similar
history, contributing to their deviation from the K-Rb isotope trend
among other enstatite chondrites (Fig. 2c), which will be discussed in
section 4.2.1.

4.2. Origin of the Rb and K isotopic variations in enstatite meteorites

4.2.1. Enstatite chondrites

The EH and EL chondrites of petrologic types 3 examined in this
study exhibit indistinguishable isotope compositions from the average
values of the BSE (δ⁸⁷Rb = −0.13 ± 0.06 ‰, Wang et al., 2023; δ⁴¹K =
−0.43 ± 0.17 ‰, Tuller-Ross et al. 2019; δ⁴¹K = −0.42 ± 0.07 ‰, Hu

et al., 2021). From type 3 to type 6, both the EH and EL groups generally
reveal a trend towards heavier Rb and K isotopic compositions (Fig. 1).
This trend has been observed for Rb isotopes by Pringle and Moynier
(2017) and Nie et al. (2023); however, they considered that the sample
size is insufficient to determine its robustness.

Previous studies have suggested that higher metamorphic type
enstatite chondrites may have experienced melt extraction during
thermal metamorphism. For example, the extraction of plagioclase- and
oldhamite-bearing melt has been proposed to explain the calcium
depletion (Rubin et al., 2009) and the heavier δ⁴⁴/⁴⁰Ca values (Dai et al.,
2024) observed in EL6 chondrites. Plagioclase is the primary host
mineral for K and Rb in enstatite chondrites, and therefore,
plagioclase-bearing melt extraction may affect the K and Rb isotope
compositions in enstatite chondrites. However, the elevated δ⁴¹K and
δ⁸⁷Rb values in higher metamorphic grade enstatite chondrites are un-
likely to be caused by this process. This is because plagioclase is
significantly enriched in ⁴¹K (e.g., δ⁴¹K up to +1.91 ‰) relative to other
coexisting rock-forming minerals and typical igneous rocks (Kuhnel
et al., 2021; Huang et al., 2023), due to the decrease in K-O bond length (Li et al., 2019; Zeng et al., 2019). Consequently, the extraction of plagioclase-bearing melt would remove isotopically heavy K, leaving residual enstatite chondrites depleted in ⁴¹K, which is contrary to the K isotope trend observed among enstatite chondrites (Figs. 1 and 2). The enrichment of heavier isotopes in higher metamorphic grade enstatite chondrites has not been exclusively observed for Rb and K, but also for other volatile metals such as Cd (Wombacher et al., 2008), Zn (Moynier et al., 2011), and Te (Hellmann et al., 2021). Hence, the systematic Rb and K isotope variations in enstatite chondrites are likely associated with a common process that influenced the distribution of volatile elements on the parent body(ies).

Shock metamorphism can cause redistribution of volatile elements in chondrites and associated isotopic fractionations (e.g., Lipschutz and Woolum, 1988; Friedrich et al., 2004; Pringle and Moynier, 2017; Mahan et al., 2018; Hu et al., 2023). The Rb and K isotope variations observed among enstatite chondrites may be attributed to volatile loss by impact vaporization. In this case, enstatite chondrites of higher shock levels would experience a greater loss of Rb and K, thereby having a heavier Rb and K isotope composition. However, strongly shocked (shock stage S3 or S4) EL3-4 samples exhibit lighter Rb and K isotopic compositions compared to EL6 samples with lower shock stage S2. Additionally, no systematic difference in Rb and K isotope compositions

exists between highly shocked and less shocked EH samples (Fig. 5a and b). On the other hand, type 3 enstatite chondrites with different shock stages (e.g., SAH97096, shock stage S2; Kota-Kota and MAC02837, shock stage S3) display similar isotope compositions for both Rb and K. One EH3 chondrite (LAR 06252), with a shock stage of S4, has previously been analyzed for K isotope composition, and its δ⁴¹K value (−0.61 ± 0.02 ‰; Zhao et al., 2020) is similar to our data for type 3 enstatite chondrites. This indicates that heavily shock may not have caused significant Rb or K isotope variations. This inference is consistent with previous K isotope studies on tektites, which formed through high-energy meteoroid impacts yet show no notable K isotope fractionation (Jiang et al., 2019; Magna et al., 2021). Therefore, shock metamorphism may not be the main cause of the observed Rb and K isotope variations among enstatite chondrites.

Rubidium and K isotopic variations in non-carbonaceous chondrites have been suggested to be driven by diffusion (Nie et al. 2023). At elevated temperatures, alkali elements can diffuse from regions of higher concentration to those of lower concentration on a hand-specimen scale, leading to significant kinetic isotopic fractionation. Light isotopes migrate more rapidly than heavier ones during diffusion. In this case, K- and Rb-rich regions that lose these elements would develop higher δ⁸⁷Rb and δ⁴¹K values, while K- and Rb-poor regions that gain these elements exhibit lower δ⁸⁷Rb and δ⁴¹K values. However, this

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Rubidium and potassium isotopic compositions of enstatite chondrites plotted against the shock levels (a and b) and petrologic types (c and d). The white circles represent St. Marks (EH5), Pillistfer (EL6) and Atlanta (EL6), respectively, which have a distinct origin for their K isotopic compositions (see text for details).</figcaption></figure>
inference is not consistent with their Rb isotope data, as Rb-poor samples (with higher metamorphic grade) display heavier Rb isotope compositions, whereas Rb-rich samples (with lower metamorphic grade) have lower δ⁸⁷Rb values (Nie et al., 2023). Nevertheless, Nie et al. (2023) suggested that this discrepancy may be attributed to the small number of samples analyzed for Rb isotopes. With the expanded dataset from this study, enstatite chondrites with higher metamorphic grades generally exhibit lower elemental concentrations and heavier isotope compositions for both Rb and K. This observation, along with similar trends identified for other volatile metals such as Cd, Zn, and Te (Wombacher et al., 2008; Moynier et al., 2011; Hellmann et al., 2021), is difficult to reconcile with the diffusion model.

The positive trend between Rb-K isotope compositions and petrologic types (Fig. 5c and d) indicates that thermal metamorphism, driven by the heat generated by impacts and the decay of short-lived radio-nuclides (Wang and Lipschutz, 2005; Huss et al., 2006), is a more likely cause of Rb and K isotope variations in enstatite chondrites. This effect has previously been observed for K and Rb in heated CM chondrites (Pringle and Moynier, 2017; Amsellem et al., 2020; Hu et al., 2023). With increasing metamorphic grade, peak temperatures increase from below 500 °C in type 3 enstatite chondrites to above 800 °C in type 5–6 enstatite chondrites (e.g., Zhang et al., 1996; Huss et al., 2006). At such high temperatures, metal elements (e.g., Cd, Zn, and Te) with relatively high volatility could be partly vaporized and redistributed (Wombacher et al., 2008; Moynier et al., 2011; Hellmann et al., 2021), contributing to the trend of progressively more volatile-depleted and isotopically heavier compositions, from type 3–4 to type 5–6 enstatite chondrites.

However, a mathematical model has been developed to simulate the migration of Rb and K during the thermal metamorphism of chondrite parent bodies (Nie et al., 2023). The calculation suggests that thermal metamorphism-induced evaporation and gas transport may not adequately explain the Rb and K isotopic variation in non-carbonaceous chondrites. Notably, this model considers plagioclase as the host mineral for K and Rb, without accounting for sulfide minerals. While the amount of K in plagioclase is approximately equal to that in sulfide minerals of enstatite chondrites (Nie et al., 2023). Additionally, the calculation adopts an oxygen fugacity parameter of IW-1, which aligns more closely with that of ordinary chondrites, whereas enstatite chondrites exhibit a lower oxygen fugacity (approximately IW-3). From this perspective, Nie et al. (2023) suggest that their model is more applicable to ordinary chondrites than to enstatite chondrites.

The exceptions are observed in St. Marks, Pillistfer, and Atlanta, which deviate from the general K-Rb isotope trend, moving toward lighter K isotope compositions at given δ⁸⁷Rb values (Fig. 2c). These three samples are enriched in the heavier Rb isotope, suggesting the loss of volatile Rb under intense thermal metamorphic conditions. In contrast, the K isotope compositions of St. Marks and Pillistfer are indistinguishable from those of type 3 enstatite chondrites that experienced the lowest degree of thermal metamorphism. Enrichment of the Lighter K isotope has also been observed in ALHA 81021 (EL6) (e.g., -0.92 ‰, Zhao et al., 2020; -0.90 ‰, Hu et al., 2023). Plagioclase-bearing melt extraction, which previously suggested to have occurred in EL6 chondrites (e.g., Rubin et al., 2009), would preferentially remove ⁴¹K (e.g., Kuhnel et al., 2021; Huang et al., 2023). However, the deviation of St. Marks, Pillistfer, and Atlanta from other enstatite chondrites (Fig. 2c) is unlikely to be caused by plagioclase-bearing melt extraction, given the comparable REE patterns between St. Marks (EH5) and EH3 chondrites, as well as the similarity in REE patterns between Pillistfer, Atlanta and other EL6 chondrites (Fig. 4).

A previous study on Te isotopic systematics in enstatite chondrites suggested that it may involve a more complex history of evaporation and condensation than simple partial kinetic Rayleigh evaporation during thermal metamorphism, due to back-reaction and recondensation of volatiles as vapor transported through the parent body (Hellmann et al., 2021). The deviation of St. Marks, Pillistfer, and Atlanta with their lighter K isotopic compositions, may similarly be attributed to a complex

interplay of evaporation, vapor transport and recondensation. In contrast, their Rb isotopic compositions show no clear indications of recondensation (Fig. 2a). This may be attributed to the difference of T<sub>c</sub><sup>50</sup> between Rb and K, with the latter being less volatile. Further studies are necessary to ascertain the origin of the light K isotope compositions observed in high metamorphic grade enstatite chondrites.

### 4.2.2. Aubrites

Unlike enstatite chondrites, there is no resolvable trend between δ⁸⁷Rb values and Rb concentrations, nor between δ⁴¹K values and K concentrations among aubrites (Fig. 6a and b). Furthermore, aubrites do not exhibit a correlation between K and Rb isotopic compositions (Fig. 6c). These observations suggest more complex processes affected the distribution of Rb and K on the aubrites' parent body.

Aubrites are commonly considered to have originated from the melting and fractional crystallization of enstatite chondrite-like parent lithologies (e.g., Keil, 2010; Barrat et al., 2016). In this case, an enstatite-plagioclase basaltic composition would be expected to form, given that enstatite chondrites contain approximately 10 vol% plagioclase (e.g., Keil, 1968). However, there is a lack of enstatite-plagioclase basalts in the world's meteorite collections. Wilson and Keil (1991) attributed this absence to the loss of enstatite-plagioclase basaltic melt into space, driven by pyroclastic volcanism. The removal of basaltic melts is consistent with the generally lower contents of plagioclase in aubrites (<3 vol%) compared to enstatite chondrites (~10 vol%) (e.g., Keil, 2010). This model explains not only the lack of basalts, but also the deficiency of alkalis on the aubrite parent body (e.g., Keil, 2010). Because plagioclase is the main host mineral for K and Rb, and the studied aubrites have varying abundances (0.7 to 6.6 vol%) of plagioclase (e.g., Watters and Prinz, 1979), these differences may have contributed to K and Rb isotope variations among aubrites.

Magmatic differentiation and core segregation may have also played a role in shaping the distribution of these two alkali elements on aubrite parent body. Since K and Rb are both strongly incompatible during terrestrial magmatic evolution, Earth mantle partial melting or basaltic differentiation do not induce significant K and Rb isotope fractionation. This conclusion is supported by previous studies of terrestrial igneous suites (e.g., Tuller-Ross et al., 2019; Hu et al., 2021; Wang et al., 2023, 2025), and the indistinguishable δ⁴¹K values between mantle peridotites and oceanic basalts reported by Humayun and Clayton (1995) and Ionov and Wang (2021). However, aubrites were formed under highly reducing conditions (Keil, 1989, 2010), such that alkali metals, which are typically considered lithophile on Earth, become siderophile and/or chalcophile (Lodders and Fegley, 1993; Gannoun et al., 2011; Krot et al. 2014). Igneous processes on the aubrite parent body may have involved the crystallization of alkali-bearing metal and sulfide phases, causing associated Rb and K isotopic fractionations. For example, Zhao et al. (2020) attributed the exceptionally light K isotope composition (-2.34 ‰) in one aubrite (MIL13004) to the presence of abundant djerfisherite (K₆[Fe,Cu,Ni]₂₅S₂₆Cl), considering that this phase is predicted to be enriched in lighter K isotopes based on ab initio calculation. In addition, core formation may have sequestered a proportion of alkali metals. Lodders (1995) proposed that the deficiency of alkalis in aubrites can be explained by segregation of alkali sulfides into the aubrite parent body core. Correspondingly, this process may have influenced the isotopic compositions of Rb and K in the bulk silicate aubrite parent body. However, the present lack of experimental isotope partitioning data prevents a testing of this hypothesis.

Following its differentiation, the aubrite parent body was disrupted by an energetic collision, and the debris may have formed a “rubble pile” asteroid under gravitational contraction (Wolf et al., 1983). The light Rb and K isotope compositions observed in some aubrites cannot simply be attributed to impact volatilization, assuming the aubrites originally had Rb and K isotopic compositions similar to that of enstatite chondrites. Previous K isotope studies on tektites also show no evidence of shock-induced K isotope fractionation (e.g., Jiang et al., 2019; Magna et al.,
<figure><img src="image_9.png" /><figcaption>Fig. 6. (a) The δ$^{87}$Rb values plotted against Rb concentrations in aubrites. (b) The δ$^{41}$K values plotted against K concentrations in aubrites. (c) The correlation between δ$^{87}$Rb and δ$^{41}$K values of aubrites.</figcaption></figure>

2021). Nevertheless, impact events and related processes resulted in mechanical mixing of material from various regions of the original parent body, or the incorporation of chondritic clasts, which may have influenced volatile elements distribution in aubrites. The studied aubrites are all brecciated (Keil, 1989); therefore, the mechanical mixing of distinct fragments may complicate the interpretation of Rb and K isotopic variations in aubrites.

Additionally, the light Rb and K isotope compositions in some aubrites may partly be attributed to recondensation of volatiles after impact volatilization. The enrichment of lighter Zn isotopes in some aubrites (e.g., Pena blanca spring: δ$^{66}$Zn = -7.04 ‰; Aubres: δ$^{66}$Zn = -6.72 ‰; Mayo Belwa: δ$^{66}$Zn = -3.91 ‰) has previously been interpreted as the back-condensation of the vapor phase (Moynier et al., 2011). Similar effects of vapor recondensation have been proposed to explain the enrichments of isotopically light Zn and K detected in several lunar samples (Paniello et al., 2012; Day et al., 2017; Tian et al., 2020). While tektites generally show limited K isotope variations (e.g., Jiang et al., 2019; Magna et al., 2021), extremely light K isotope compositions have been observed in microtektites, which are attributed to the preferential recondensation of lighter K isotopes from vapor plume (e.g., Herzog et al., 2008; Koefoed et al., 2024). Therefore, the back-condensation mechanism originally proposed for light Zn isotope compositions in aubrites may possibly explain the isotopically light Rb and K in some of the studied aubrites.

Hence, diverse factors may affect the distribution of Rb and K in aubrites, leading to the wide dispersion of Rb and K isotope compositions. Due to the small number of aubrites analyzed in this study, the specific mechanism behind Rb and K isotope variations remains to be investigated.

### 4.3. Thermal evolution on enstatite chondrite parent body(ies)

The distribution and isotope variations of Rb and K among enstatite chondrites are most likely the result of open system thermal metamorphism that occurred on their parent body(ies). The heat driving these thermal metamorphism events was primarily provided by the decay of short-lived nuclides such as $^{26}$Al (Wang and Lipschutz, 2005; Huss et al., 2006). Radiogenic decay would result in varying degrees of heat distribution from the interior to the surface of planetesimals. An “onion shell” structure, proposed for ordinary chondrite parent asteroids to elucidate their early thermal evolution (McSween et al., 2002; Trieloff et al., 2003), is also applicable to enstatite chondrite parent body(ies). This is supported by previous studies on Cd and Te isotope compositions in enstatite chondrites (Wombacher et al., 2008; Hellmann et al., 2021), as well as the Rb and K isotope systematics in this study.

The “onion shell” model (Fig. 7) shows that the thermal structure of enstatite chondrite parent body(ies) is characterized by layers of increasing temperatures from the surface towards the center, with

<figure><img src="image_10.png" /><figcaption>Fig. 7. The thermal structure of enstatite chondrite parent body(ies) as illustrated by “onion shell” model. Internally heated by the decay of short-lived nuclides after accretion, the parent body reached higher maximum metamorphic temperatures at the center, resulting in higher petrologic types that cooled more slowly than the outer layers, where lower petrologic types formed and cooled more rapidly. The heating events have led to redistribution and volatilization of volatile elements, resulting in progressively more volatile-depleted and isotopically heavier compositions in the more intensely heated interior regions. While recondensation of isotopically light volatile elements may have contributed to the local heterogeneity in their distribution.</figcaption></figure>

successively increasing metamorphic grade. The parent body was heated to high temperatures by the decay of short-lived nuclides, and the center was cooled less effectively than the outer layers. Type 5–6 enstatite chondrites represent the inner shells, with peak temperatures exceeding 800 °C (e.g., Mason, 1968; Zhang et al., 1996; Huss et al., 2006). In contrast, type 3–4 enstatite chondrites were likely derived from the surface layer and experienced minimal metamorphic effects, with estimated maximum temperatures lower than 500 °C (Huss et al., 2006).

The decay of short-lived nuclides provided heat that led to the volatilization of volatile elements, whereby lighter isotopes preferentially entered the vapor phase. Vapor from the hotter inner regions migrated toward the cooler outer regions and was eventually lost to space. Consequently, these processes are expected to produce more volatile-depleted and isotopically heavy compositions in the more intensely heated interior regions of the parent body. This is consistent with the general trend of increasingly heavier Rb and K isotopic compositions from the shallower, less strongly heated shells (as sampled by type 3–4 enstatite chondrites) to the deeper, more strongly metamorphosed parts of the parent body (where type 5–6 enstatite chondrites
derived). It has been suggested that EH and EL chondrites are likely derived from separate parent bodies (e.g., Keil, 1989; Boyet et al., 2018; Weyrauch et al., 2018), which however, are not distinguishable based on Rb and K isotope systematics. The least metamorphosed EH3 and EL3 chondrites share similar BSE-like Rb and K isotope compositions.

On the other hand, thermal metamorphism of enstatite chondrites involves processes more complex than partial kinetic Rayleigh evaporation (Wombacher et al., 2008; Hellmann et al., 2021). As volatile elements migrate to the cooler, outer regions of the parent body, a fraction of them may recondense. Recondensation of isotopically light volatile elements could have contributed to the locally heterogeneous distribution of these elements within each layer of the parent body. This may explain the deviation of St. Marks, Pillistfer, and Atlanta from the “partial evaporation trend” with their lighter K isotopic compositions (Fig. 2c), and may also partly account for the relatively large range of K isotope variations observed in type 3 enstatite chondrites (this study; Zhao et al., 2020; Ku and Jacobsen, 2020; Hu et al., 2023), which are distinct from the homogeneous Rb isotope signature.

## 5. Conclusions

This study reports Rb and K isotope compositions of a large set of enstatite meteorites, including eight EH chondrites, eight EL chondrites, and four aubrites. The type 3 enstatite chondrites, which have experienced minimal thermal metamorphism, show Rb and K isotopic compositions that closely match the average values of the BSE, providing additional examples of the isotopic similarity between Earth and enstatite chondrites. As the thermal metamorphic grade increases from type 3 to type 6, both the EH and EL groups generally display a trend towards heavier Rb and K isotopic compositions, reflecting the volatilization and redistribution of volatile elements during the open system thermal metamorphism of the parent body(ies). In contrast, the Rb and K isotopic variations observed among aubrites possibly result from a complex interplay of melt extraction, magmatic evolution, core segregation, and the recondensation of volatiles following impact volatilization.

### CRediT authorship contribution statement

**Baoliang Wang:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. **Frédéric Moynier:** Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition. **Yan Hu:** Writing – review & editing.

### Data availability

Data are available through the Zenodo database at https://doi.org/10.5281/zenodo.15027613.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

We deeply thank three anonymous reviewers for their constructive comments that greatly improved the quality of this paper. We are grateful to Prof. Hailiang Dong and Prof. Oliver Nebel for efficient editorial handling of our manuscript. We thank Tu-Han Luu and Dimitri Rigoussen for their assistance with mass spectrometer maintenance. This work was partly supported by the IPGP analytical platform PARI, Region Ile-de-France SESAME Grants no. 12015908, DIM ACAV+, the ERC grant agreement No. 101001282 (METAL) (F.M.), and the UnivEarthS

Labex program (numbers: ANR-10-LABX-0023 and ANR-11-IDEX-0005-02) (F.M.). B.W. thanks the support of the CSC scholarship (No. 202106340025).

## Appendix A. Supplementary material

The supplementary figures (Fig. S1 to Fig. S3) and table (Table S1) are presented in the supplementary file. The Fig. S1 displays a consistent trend in whole-rock Rb concentrations of our samples compared to those reported by Barrat et al. (2014) for both whole-rock and residual silicates after leaching experiments. The Fig. S2 shows comparison of Rb and K isotopic compositions between enstatite chondrite falls and finds, and between different locations. The Fig. S3 exhibits trace element patterns of type 3 and type 6 enstatite chondrites. The REE data of enstatite chondrites are provided in Table S1. Supplementary material to this article can be found online at https://doi.org/10.1016/j.gca.2025.04.007.

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