¹⁸²Hf–¹⁸²W age dating of a ²⁶Al-poor inclusion and implications for the origin of short-lived radioisotopes in the early Solar System

Abstract

Refractory inclusions [calcium–aluminum-rich inclusions, (CAIs)] represent the oldest Solar System solids and provide information regarding the formation of the Sun and its protoplanetary disk. CAIs contain evidence of now extinct short-lived radioisotopes (e.g., ²⁶Al, ⁴¹Ca, and ¹⁸²Hf) synthesized in one or multiple stars and added to the protosolar molecular cloud before or during its collapse. Understanding how and when short-lived radioisotopes were added to the Solar System is necessary to assess their validity as chronometers and constrain the birthplace of the Sun. Whereas most CAIs formed with the canonical abundance of ²⁶Al corresponding to ²⁶Al/²⁷Al of ∼5 × 10⁻⁵, rare CAIs with fractionation and unidentified nuclear isotope effects (FUN CAIs) record nucleosynthetic isotopic heterogeneity and ²⁶Al/²⁷Al of <5 × 10⁻⁶, possibly reflecting their formation before canonical CAIs. Thus, FUN CAIs may provide a unique window into the earliest Solar System, including the origin of short-lived radioisotopes. However, their chronology is unknown. Using the ¹⁸²Hf–¹⁸²W chronometer, we show that a FUN CAI recording a condensation origin from a solar gas formed coevally with canonical CAIs, but with ²⁶Al/²⁷Al of ∼3 × 10⁻⁶. The decoupling between ¹⁸²Hf and ²⁶Al requires distinct stellar origins: steady-state galactic stellar nucleosynthesis for ¹⁸²Hf and late-stage contamination of the protosolar molecular cloud by a massive star(s) for ²⁶Al. Admixing of stellar-derived ²⁶Al to the protoplanetary disk occurred during the epoch of CAI formation and, therefore, the ²⁶Al–²⁶Mg systematics of CAIs cannot be used to define their formation interval. In contrast, our results support ¹⁸²Hf homogeneity and chronological significance of the ¹⁸²Hf–¹⁸²W clock.

Meteorites and their components contain evidence for the presence of now extinct short-lived (t_1/2 < 10 Ma) radionuclides (e.g., ⁴¹Ca, ²⁶Al, ⁶⁰Fe, ⁵³Mn, and ¹⁸²Hf) during the earliest stages of the Solar System’s evolution. These radioisotopes are believed to have an external, stellar origin, and were either inherited from the ambient interstellar medium or injected into the protosolar molecular cloud before or contemporaneously with its collapse (1). Understanding how and when these radioisotopes were added to the nascent Solar System can constrain the astrophysical environment where our Sun formed and, therefore, test models of Solar System formation. The oldest Solar System solids preserved in chondritic meteorites are calcium–aluminum-rich inclusions (CAIs), which define an absolute age of 4,567.30 ± 0.16 Ma (2). These millimeter-to-centimeter objects are believed to have formed as fine-grained condensates from a ¹⁶O-rich gas of approximately solar composition in a region with high ambient temperature (>1,300 K) and low total pressures (∼10^–4 bar). This environment existed in the innermost part of the protoplanetary disk during the early stage of its evolution characterized by high mass accretion rates (∼10⁻⁵ M_☉ y^–1) to the proto-Sun (3). Formation of CAIs near the proto-Sun is also indicated by the presence in these objects of the short-lived radioisotope ¹⁰Be formed by solar energetic particle irradiation (4). Some of the CAIs subsequently experienced melting and evaporation to form distinct coarser igneous inclusions, such as the compact Type A and Type B CAIs commonly observed in CV meteorites (carbonaceous chondrite of the Vigarano type) (5).

The majority of CAIs in unmetamorphosed chondrites contain high abundance of radiogenic ²⁶Mg (²⁶Mg*), the decay product of ²⁶Al (t_1/2 ∼ 0.7 Ma), corresponding to an inferred initial ²⁶Al/²⁷Al ratio of ∼(4.5–5.5) × 10^–5 (6). Recent high-precision ²⁶Al–²⁶Mg systematics of bulk CV CAIs define the so-called canonical ²⁶Al/²⁷Al ratio of (5.252 ± 0.019) × 10^–5 (7). The uncertainty of the canonical ²⁶Al/²⁷Al ratio corresponds to ∼4,000 y, implying a brief episode of condensation and melt evaporation that resulted in Al/Mg fractionation event(s). However, it is uncertain whether the canonical ²⁶Al/²⁷Al ratio reflects that of the bulk Solar System or, alternatively, only a local snapshot of the evolving inhomogeneous protoplanetary disk.

A rare subset of refractory grains [platy hibonite crystals (PLACs) and blue aggregates (BAGs), ref. 8] and inclusions (6, 9) have low initial ²⁶Al/²⁷Al ratios (<5 × 10^–6). Of particular interest are the coarse-grained igneous inclusions with fractionation and unidentified nuclear effects (FUN CAIs, ref. 10), which, in addition to their low initial abundance of ²⁶Al, are characterized by large mass-dependent fractionation effects and nucleosynthetic anomalies in several elements. These observations are interpreted to reflect formation of FUN CAIs by thermal processing of presolar dust aggregates before the injection of ²⁶Al and its homogenization in the protoplanetary disk (11). If this interpretation is correct, FUN CAIs can provide insights into the timing of admixing of ²⁶Al to the forming protoplanetary disk and, in turn, the origin of short-lived radioisotopes in the early Solar System. However, a late formation of ²⁶Al-poor CAIs after decay of ²⁶Al cannot be excluded. Indeed, CAIs are known to have experienced multistage thermal processing in the protoplanetary disk and/or on their chondrite parent bodies (5) that could have erased their radiogenic ²⁶Mg. Thus, obtaining a robust age estimate for a FUN inclusion is a critical step toward a better understanding of the significance of ²⁶Al-poor inclusions.

Results and Discussion

Most known FUN CAIs were discovered >30 y ago and were largely consumed during destructive isotopic measurements. To identify additional FUN CAIs suitable for age dating, we conducted a systematic search of coarse-grained refractory inclusions in the Allende CV carbonaceous chondrite, given that CV chondrites contain the highest proportion of igneous CAIs among the distinct chondrite groups. Of ∼220 inclusions investigated, only one FUN CAI was identified on the basis of its bulk magnesium–isotope composition. This FUN inclusion, named STP-1, is a coarse-grained igneous Type B2 CAI composed of melilite, spinel, Al,Ti-diopside, and anorthite. STP-1 contains only minor amounts of secondary minerals (nepheline, sodalite, grossular, and monticellite), indicating that it largely avoided secondary alteration processes. Similar to most previously identified FUN CAIs (10, 12), STP-1 shows mass-dependent enrichment in the heaviest isotopes of magnesium, as well as deficits in the mass-independent components of ²⁶Mg (²⁶Mg*) and ⁵⁴Cr of ∼300 and ∼3,500 ppm, respectively (Table 1). Trace element analysis demonstrates that STP-1 is characterized by a Group II rare-earth element (REE) pattern (Fig. 1), indicative of condensation from a gas depleted in the most refractory REEs (13). On a three-isotope oxygen diagram, compositions of spinel, anorthite, hibonite, and most Al,Ti-diopside grains plot along a mass-dependent fractionation line with a slope of 0.52 and Δ¹⁷O value of –24 ± 1\x{2030}. Oxygen–isotope compositions of melilite and some Al,Ti-diopside grains deviate from this line and show Δ¹⁷O values ranging from −17 to –4\x{2030} and from –24 to –17\x{2030}, respectively (Fig. 2). The igneous texture and the fractionated magnesium and oxygen–isotope composition favoring the heaviest isotopes imply that STP-1 experienced melt evaporation at low total pressure (14) following condensation of its precursor material. To define the initial abundance of ²⁶Al at the time of crystallization of STP-1, we have investigated the ²⁶Al–²⁶Mg systematics of its primary minerals by secondary ionization mass spectrometry. Multiple analyses of spinel, Al,Ti-diopside, melilite, hibonite, and anorthite crystals define an internal isochron corresponding to an initial ²⁶Al/²⁷Al ratio of (2.94 ± 0.21) × 10^–6 (Fig. 3A), that is, much lower than the canonical value of ∼5 × 10^–5.

Table 1.

¹⁸²Hf–¹⁸²W systematics of mineral fractions and bulk Mg and Cr isotope compositions of the STP-1 FUN CAI

Sample	27Al/24Mg	μ25Mg	μ26Mg*	μ53Cr	μ54Cr	180Hf/183W	μ182W (6/3)	μ184W (6/3)	μ182W (6/4)	μ183W (6/4)
Bulk	3.18 ± 0.06	9,331 ± 20	−303 ± 10	260 ± 9	−3,579 ± 15
Melilite						1.978 ± 0.119	−18 ± 27	−28 ± 15	69 ± 22	41 ± 23
Anorthite						2.675 ± 0.161	35 ± 27	−27 ± 16	115 ± 35	43 ± 24
Diopside						14.22 ± 0.85	622 ± 52	−31 ± 24	676 ± 58	46 ± 37

Isotope ratios expressed in the μ-notation, which reflect 10⁶ (ppm) deviations from the terrestrial reference standard. Uncertainties represent the external reproducibility or internal precision, whichever is larger. Mg and Cr isotope data were acquired following techniques outlined in Bizzarro et al. (33) and Trinquier et al. (34), respectively. (6/3), internally normalized to ¹⁸⁶W/¹⁸³W; (6/4), internally normalized to ¹⁸⁶W/¹⁸⁴W.

Fig. 1.

Chondrite-normalized bulk REE abundances in the Allende STP-1 FUN CAI. This CAI is characterized by a Group II REE pattern and a small negative Ce anomaly.

Fig. 2.

Three-isotope oxygen diagram of oxygen–isotope compositions of individual minerals in the Allende STP-1 FUN CAI. Similarly to the majority of FUN CAIs (31), the oxygen–isotope compositions of anorthite, spinel, hibonite, and most Al,Ti-diopside grains in STP-1 plot along a mass-dependent fractionation line defining an initial Δ¹⁷O value of ∼ −24\x{2030}, that is, similar to the oxygen–isotope composition of canonical CAIs and that of the Sun (31, 32). Oxygen–isotope compositions of melilite and some of the Al,Ti-diopside grains plot along a line with a slope of ∼1, suggesting subsequent isotope exchange with a ¹⁶O-depleted gaseous reservoir. In contrast with most FUN CAIs from CV chondrites characterized by ¹⁶O-poor compositions of melilite and anorthite, melilite in STP-1 shows a range of Δ¹⁷O values, whereas anorthite is uniformly ¹⁶O-rich. These observations indicate that STP-1 is more pristine than all previously known FUN CAIs.

Fig. 3.

Internal mineral ²⁶Al–²⁶Mg (A) and ¹⁸²Hf–¹⁸²W (B) isochron diagrams for the Allende STP-1 FUN CAI. The well-defined ²⁶Al–²⁶Mg isochron based on spinel, Al,Ti-diopside, melilite, anorthite, and hibonite shows no evidence for late-stage disturbance, consistent with its pristine mineralogy and the ¹⁶O-rich composition of most primary phases. Thus, we infer that the ²⁶Al–²⁶Mg isochron defines the initial ²⁶Al/²⁷Al ratio in STP-1 at the time of its crystallization. M.S.W.D., mean square of weighted deviations. Using the ¹⁸⁶W/¹⁸⁴W instead of the ¹⁸⁶W/¹⁸³W ratio for internal normalization returns an initial ¹⁸²Hf/¹⁸⁰Hf value of (9.22 ± 1.1) × 10⁻⁵ and an intercept of –26 ± 26 ppm (M.S.W.D. = 0.31). This ¹⁸²Hf/¹⁸⁰Hf value is identical within analytical uncertainty to that obtained using the ¹⁸⁶W/¹⁸³W for internal normalization, although marginally lower. However, repeated analysis of a number of distinct aliquots of column-processed BCR-2 (Basalt, Columbia River) rock standard and the Allende carbonaceous chondrite using a quantity of W comparable to that present in the mineral fractions of the STP-1 FUN CAI indicates a superior external reproducibility when the ¹⁸⁶W/¹⁸³W is used for internal normalization. Therefore, our preferred approach is to use the ¹⁸⁶W/¹⁸³W for internal normalization, in agreement with earlier studies (17, 18, 35, 36).

Absolute age dating of CAIs by the Pb–Pb method requires knowledge of the uranium–isotope composition of individual inclusions (2). However, the uranium concentration in STP-1 is depleted by a factor of ∼100 compared with canonical CAIs, possibly due to loss during melt evaporation under oxidizing conditions. As such, STP-1 in particular, and FUN inclusions in general, may not be suited for uranium-corrected absolute Pb–Pb dating using current state-of-the-art mass spectrometry techniques. To define the formation age of STP-1, we have instead investigated its ¹⁸²Hf–¹⁸²W systematics by the internal isochron approach. With a half-life of ∼9 Ma, the ¹⁸²Hf-to-¹⁸²W decay scheme is one of the most widely used chronometers to understand the timing of solid formation in the early Solar System (15). In addition, currently available data support the proposal that the ¹⁸²Hf nuclide was uniformly distributed in the early Solar System with an initial ¹⁸²Hf/¹⁸⁰Hf ratio of (9.85 ± 0.40) × 10⁻⁵ (16–18). First, the ¹⁸²Hf–¹⁸²W isochron of canonical CAIs intersects the carbonaceous chondritic composition, suggesting that the precursor material of primitive asteroids that presumably accreted in the outer Solar System had similar initial ¹⁸²Hf content as CAIs (16). Second, there is excellent agreement between uranium-corrected Pb–Pb and Hf–W ages of rapidly cooled magmatic meteorites (19). Lastly, CAIs with variable tungsten nucleosynthetic anomalies define the same initial ¹⁸²Hf abundance (17), indicating that the source of ¹⁸²Hf is decoupled from that responsible for nucleosynthetic heterogeneity in refractory elements such as tungsten. The Al,Ti-diopside, anorthite and melilite fractions separated from STP-1 define a statistically significant ¹⁸²Hf–¹⁸²W isochron corresponding to an initial ¹⁸²Hf/¹⁸⁰Hf ratio of (9.60 ± 1.10) × 10^–5 and intercept of –113 ± 27 ppm, when the ¹⁸⁶W/¹⁸³W is used to correct for instrumental mass fractionation (Fig. 3B). Using a different isotope pair for internal normalization yields an identical initial ¹⁸²Hf/¹⁸⁰Hf within uncertainty but a different intercept of –26 ± 26 ppm (see the legend of Fig. 3). These two intercept values are distinct from the inferred Solar System initial μ¹⁸²W value of – 351 ± 10 ppm (18), indicating the presence of tungsten nucleosynthetic heterogeneity in STP-1. In contrast, the internal ¹⁸²Hf–¹⁸²W isochron of STP-1 corresponds to a ¹⁸²Hf/¹⁸⁰Hf ratio that is identical within analytical uncertainty to the Solar System initial ¹⁸²Hf/¹⁸⁰Hf ratio of (9.85 ± 0.40) × 10⁻⁵ inferred from canonical CAIs (18). Accepting the inferred initial ¹⁸²Hf/¹⁸⁰Hf ratio of STP-1 and associated uncertainty at face value, we calculate an age difference of Ma between formation of STP-1 and canonical CAIs, which is not consistent with the time interval of Ma inferred from the ²⁶Al–²⁶Mg system. Thus, we conclude that the formation age of STP-1 and, by extension, that of ²⁶Al-poor FUN CAIs, is coeval with canonical CAIs within the uncertainty of our measurements. These results are consistent with the proposal that ¹⁸²Hf was homogeneously distributed in the solar protoplanetary disk at the time of formation of the Solar System’s first solids (16–18).

The contrasting initial abundances of ²⁶Al recorded by CAIs with identical initial ¹⁸²Hf/¹⁸⁰Hf ratios indicate that ²⁶Al was heterogeneously distributed in the protoplanetary disk during the epoch of CAI formation. Therefore, the ²⁶Al–²⁶Mg systematics of CAIs cannot be used to define the duration of the CAI-forming event(s). Likewise, the apparently restricted range of inferred initial ²⁶Al/²⁷Al ratios defined by bulk analyses of canonical CAIs from CV carbonaceous chondrites does not necessarily imply a homogeneous distribution of ²⁶Al throughout the solar protoplanetary disk during and after the epoch of CAI formation. Assessing the degree of ²⁶Al homogeneity in the disk requires careful comparison between the ²⁶Al–²⁶Mg and uranium-corrected Pb–Pb ages of objects with simple thermal histories. We note that the age difference between the formation of canonical CAIs and rapidly cooled angrite meteorites inferred from the assumption-free uranium-corrected Pb–Pb dating method is not consistent with that suggested by the ²⁶Al–²⁶Mg system, supporting widespread ²⁶Al heterogeneity in the solar protoplanetary disk (2, 7).

The decoupling between the initial abundances of ²⁶Al and ¹⁸²Hf in early-formed refractory inclusions requires distinct stellar sources to account for the presence of these short-lived nuclides in the early Solar System. In addition, the Hf–W data reported here for the STP-1 FUN CAI are most easily understood in the context of a homogeneous distribution of ¹⁸²Hf at the birth of the Solar System. If correct, this implies that the carrier of this short-lived radioisotope was well mixed within the Solar System’s parental molecular cloud. Heavy r-process isotopes such as ¹⁸²Hf are thought to be synthesized during the explosions of core-collapse supernovae of less than 11 M_☉ (20). Because the lifetime of these “lower mass” massive stars is significantly longer than the typical lifetime of giant molecular clouds (21), they are not expected to contribute appreciable amounts of freshly synthesized radionuclides into star-forming regions. Therefore, in agreement with models of the chemical evolution of the galaxy (20, 22), we infer that the initial ¹⁸²Hf/¹⁸⁰Hf ratio of ∼1 × 10^–4 recorded by FUN and canonical CAIs reflects long-term, steady-state galactic stellar nucleosynthesis before the formation of the protosolar molecular cloud. A galactic origin for ¹⁸²Hf in the early Solar System is consistent with the view that this radionuclide was homogeneously distributed in the protoplanetary disk at the time of formation of the Solar System's first solids as inferred from earlier work and the ¹⁸²Hf–¹⁸²W data presented here for the STP-1 FUN inclusion.

In contrast with ¹⁸²Hf, the Solar System’s initial inventory of ²⁶Al is approximately 10 times higher than the background levels of the galaxy inferred from γ-ray astronomy (23) and/or models of the galactic chemical evolution (20, 22), requiring late-stage addition of stellar debris to the Solar System’s parental molecular cloud. A possibility is that the observed variable ²⁶Al abundances during the epoch of CAI formation reflect heterogeneity in the CAI precursor material (24). Such heterogeneity could result from selective thermal processing of presolar carriers, including the carrier(s) of ²⁶Al, thereby generating reservoirs enriched or depleted in presolar components (25). However, ²⁶Al-poor objects such as FUN CAIs, PLACs, and BAGs show large-scale nucleosynthetic heterogeneity in the stable ⁴⁸Ca and ⁵⁰Ti nuclides, including both enrichments and depletions (8, 10, 12, 26, 27), implying that the heterogeneity preserved in these objects is unrelated to ²⁶Al. Moreover, the mineralogy of STP-1, coupled with its group II REE pattern and ¹⁶O-rich composition, suggest that the precursor material of this inclusion formed by condensation from a gas of solar composition depleted in the most refractory REEs, similar to the majority of fine-grained CAIs. Thus, although it is possible that the variable nucleosynthetic anomalies present in ²⁶Al-poor objects reflect selective thermal processing of their precursors, we conclude that the initial ²⁶Al/²⁷Al of ∼3 × 10^–6 recorded by STP-1 represents the ²⁶Al abundance in the CAI-forming region when this inclusion crystallized.

Solids ultimately thermally processed in the CAI-forming region are believed to represent molecular cloud material accreting to the proto-Sun from the infalling envelope via the protoplanetary disk (28). In contrast with ¹⁸²Hf, our results suggest that the carrier of ²⁶Al was heterogeneously distributed in the protosolar molecular cloud and, by extension, during infall of envelope material to the protoplanetary disk. However, at the time of formation of the earliest Solar System solids, the innermost protoplanetary disk is thought to have been physically well mixed (29), implying that the observed ²⁶Al heterogeneity during formation of FUN and canonical CAIs may be temporal rather than spatial. Thus, the different levels of ²⁶Al in these inclusions could reflect admixing of stellar derived carrier(s) of ²⁶Al to the protoplanetary disk during the epoch of CAI formation. Progressive admixing of ²⁶Al to the disk can be understood in the framework of the inside-out collapse model of prestellar cores, where the innermost portion of the core collapses first, followed by the successive outer layers (30). This interpretation requires the innermost part of the protostellar molecular cloud to have been depleted in ²⁶Al compared with the remaining cloud, and that the formation of FUN CAIs predates canonical CAIs, the latter being allowed by the ¹⁸²Hf–¹⁸²W age uncertainty of the STP-1 FUN CAI.

A galactic origin for the initial abundance of ¹⁸²Hf recorded by the FUN and canonical CAIs requires ∼18 Ma of free decay between last nucleosynthetic event that produced the heavy r-process nuclides and formation of the Solar System (22). This time interval, however, is not compatible with the initial ²⁶Al/²⁷Al ratio of ∼3 × 10⁻⁶ defined by the STP-1 FUN CAI, accepting that this value represents that of the ²⁶Al-poor region of the protosolar molecular cloud before the last addition of freshly synthesized ²⁶Al present in canonical CAIs. If correct, this interpretation suggests that the protosolar molecular cloud was part of a giant molecular cloud complex (GMC) chemically enriched in freshly synthesized matter by earlier generation(s) of massive stars to account for the level of ²⁶Al present in FUN CAIs. Whether our Sun formed during the early or late evolutionary stages of the GMC requires knowledge of the ⁶⁰Fe/²⁶Al value of canonical and FUN CAIs, given that most of the ²⁶Al in the early life of the GMC will be synthesized and ejected by the stellar winds of massive stars where ⁶⁰Fe is not produced.

Materials and Methods

We conducted a systematic search for new FUN CAIs by investigating the Mg–isotope composition of numerous igneous CAI-like objects in cut sections of an ∼3 kg fragment of the Allende CV carbonaceous chondrite. All igneous CAI-like inclusions of appropriate size were sampled with a computer-assisted microdrilling device fitted with 300-μm–diameter diamond-coated microdrills. The sampled material was digested using hydrofluoric (HF)–HNO₃ acid mixtures and, after complete dissolution, a 5% aliquot of the sample was taken for Al/Mg ratio determination to 5% accuracy using a ThermoFisher X-Series II inductively coupled plasma source mass spectrometer (ICPMS) at the Centre for Star and Planet Formation in Copenhagen. The magnesium from samples with Al/Mg ratios typical of CAIs was purified by ion-exchange chromatography and its isotopic composition analyzed using a ThermoFisher Neptune multiple collector inductively coupled plasma source mass spectrometer (MC-ICPMS) at the Centre for Star and Planet Formation in Copenhagen, following protocols outlined in Bizzarro et al. (33). One inclusion was typified by a resolvable deficit in ²⁶Mg* of ∼300 ppm as well as a stable Mg–isotope composition enriched in the heavy isotopes by ∼1%/amu. This inclusion, named STP-1, was classified as a FUN CAI and selected for further analysis. Present on the surfaces of two 3-mm–thick sections, the STP-1 FUN CAI is a spherical inclusion of ∼10 mm in diameter. Once the inclusion was liberated from the Allende meteorite, polished sections were made from the extracted material for petrographic characterization, mineral chemistry and in situ ²⁶Al–²⁶Mg and O–isotope work.

Elemental maps of sections and electron microprobe analyses of individual minerals were performed with the University of Hawaii (UH) field-emission electron JEOL JXA-8500F operated at 15-kV accelerating voltage, 15-nA beam current, and fully focused beam using five wavelength spectrometers. The STP-1 inclusion is a coarse-grained igneous CAI composed of pure anorthite, gehlenitic melilite (Åk₆₋₂₈), and igneously zoned Al,Ti-diopside (Al₂O₃ = 17.7−28.5 wt %, TiO₂ = 0.03−8.7 wt %), all poikilitically enclosing euhedral compositionally pure spinel grains. Lath-shaped hibonite grains and spinel–hibonite intergrowths occur in the outermost portion of the inclusion. The hibonite grains have low contents of MgO (0.2−1.7 wt %) and TiO₂ (0.09−3.2 wt %). No multilayered Wark–Lovering rim sequence is observed around STP-1. The oxygen isotope composition and Al–Mg systematics of primary minerals in STP-1 was investigated using the UH Cameca ims-1280 ion microprobe based on techniques described in SI Materials and Methods.

Following removal from the Allende slab and cleaning, bulk fragments of STP-1 were preserved for bulk isotope and elemental analyses. The remaining material was gently crushed in an agate mortar under distilled ethanol and minerals were handpicked under binocular microscopes in both plain and back lighting. REE abundances were determined on the X-Series II ICPMS from a bulk aliquot of STP-1, using sample-standard bracketing techniques. The chromium isotope composition of a separate bulk aliquot was determined based on previously published techniques (34) using a ThermoFisher Triton thermal ionization mass spectrometer at the Centre for Star and Planet Formation. Following handpicking, the anorthite, melilite, and Al,Ti-diospide fractions were rinsed in distilled ethanol followed by 0.02 M HNO₃ in an ultrasonic bath for 15 min. After complete dissolution in mixtures of concentrated HF:HNO₃:H₂O₂, a 15% aliquot of each sample was extracted and spiked with a mixed ¹⁸⁰Hf/¹⁸⁶W tracer for elemental abundance determinations. Tungsten was purified by ion-exchange chromatography in a two-step procedure inspired from Fritz et al. (37) and Strelow et al. (38). Samples were dissolved in 0.25 M HNO₃ + 0.1 M HF + 0.1% H₂O₂ and loaded on a column containing 2–4 mL of AG50W-X8 resin, and W was eluted along with high-field strength elements with 1–2 column volumes (c.v.) of 0.25 M HNO₃ + 0.1 M HF + 0.1% H₂O₂ followed by 2.5 c.v. of 0.1 M HF. After dry down, samples were dissolved in 1 M HF and loaded on a column containing 1 mL AG1-X4 resin. Residual elements were eluted sequentially with 5 c.v. of 1 M HF, 5 c.v. of 2 M HCl + 0.1% H₂O₂ and 2 c.v. of 6 M HCl + 0.01 M HF, whereas the W was recovered with 4 c.v. of 6 M HCl + 1 M HF.

Following tungsten purification, isotope data were acquired in static mode using the ThermoFisher Neptune MC-ICPMS at the Centre for Star and Planet Formation. Samples were converted to nitrate form, dissolved in a 2% HNO₃ solution containing traces of HF, and introduced into the plasma source by means of an Aridus II desolvating nebulizer. Mass fractionation was corrected with the exponential law using the ¹⁸⁶W/¹⁸³W = 1.98594 (39). Samples were analyzed only once, and the ratios are reported as relative deviations from the mass-bias corrected NIST 3163 tungsten reference material in the μ-notation (10⁶ deviations). A blank correction was applied to all samples, and the uncertainty of this correction is propagated in the final uncertainties of the isotope measurements reported in Table 1. Full analytical details of procedures used for Hf/W and W isotope measurements, as well as all other data reported in this paper, are presented in SI Materials and Methods.