Timing of the magmatic activity and upper crustal cooling of differentiated asteroid 4 Vesta

Journal Pre-proofs Timing of the magmatic activity and upper crustal cooling of differentiated asteroid 4 Vesta F. Jourdan, T. Kennedy, G.K. Benedix, E. Eroglu, C. Mayer PII: DOI: Reference:

S0016-7037(20)30059-4 https://doi.org/10.1016/j.gca.2020.01.036 GCA 11616

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Geochimica et Cosmochimica Acta

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Please cite this article as: Jourdan, F., Kennedy, T., Benedix, G.K., Eroglu, E., Mayer, C., Timing of the magmatic activity and upper crustal cooling of differentiated asteroid 4 Vesta, Geochimica et Cosmochimica Acta (2020), doi: https://doi.org/10.1016/j.gca.2020.01.036

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Timing of the magmatic activity and upper crustal cooling of differentiated asteroid 4 Vesta

F. Jourdan1,2, T. Kennedy1,2, G.K. Benedix2, E. Eroglu3, C. Mayer1

1Western

Australian Argon Isotope Facility, John de Laeter Centre, TIGeR, Curtin University,

Australia 2Space

Science and Technology Centre, School of Earth and Planetary Sciences, Curtin

University, Australia Discipline of Chemical Engineering, WA School of Mines: Minerals, Energy and Chemical

3

Engineering, Curtin University, Perth, WA 6845, Australia Abstract Eucrites are extraterrestrial basalts and cumulate gabbros formed, and subsequently more or less metamorphosed, at the crustal level of the HED (Howardite-Eucrite-Diogenite) parent body, thought to be the asteroid 4 Vesta. Unbrecciated eucrites offer the best way to understand the igneous, metamorphic and cooling processes occurring in the crust of Vesta following accretion since they were not substantially affected/altered by secondary impact processes. The 40Ar/39Ar

system of unbrecciated eucrites should be in a relatively pristine state, and thus can

inform us on the early volcanic and thermal history of the HED parent body, and, in particular, the cooling history of various crustal parts below the ~300 °C isotherm, which represents the average closure temperature of the Ar diffusion in plagioclase. We analysed plagioclase and pyroxene (± groundmass) separates of two cumulate (Moore County and Moama), and five (Caldera, BTN 00300, EET 90020, GRA 98098, QUE 97053) equilibrated basaltic eucrites with the 40Ar/39Ar technique using a Thermo© ARGUS VI multicollection mass spectrometer. The two cumulate unequilibrated gabbros also gave cooling ages of 4531 ± 11 Ma and 4533 ± 12 Ma and combined with a fast cooling rate estimated from lamella thicknesses, suggest that magmatic activity persisted up to 4533 ± 11 and 4535 ± 12 Ma and that the plutons were intruded in a relatively shallow part of the crust, above the metamorphosed regions. Four equilibrated eucrites yielded a well-defined cluster of ages

between 4523 ± 8 Ma to 4514 ± 6 Ma. Those ages indicate when the part of the upper crust, where those eucrites probably resided (~10-15 km deep), cooled below ~300°C at a rate of 17.3 ± 3.6°C/Ma (2σ). Such a slow cooling rate combined with available global thermal models, supports the hypothesis of a global crustal metamorphism by burial and reheating of lava flows. Finally, an age of 4531 ± 5 Ma was obtained for metamorphosed eucrite EET 90020 and, combined with petrographic observations, indicates the age of a major crustal excavation event by impact. 40Ar diffusion models suggest that it is possible to differentiate impact vs crustal cooling provided that a sufficient quantity of pyroxene is measured by 40Ar/39Ar.

1. Introduction Asteroid 4 Vesta is the second biggest asteroid in the asteroid belt and is the only largely preserved proto-planet left in the solar system which was differentiated into a core, mantle and crust (Zuber et al., 2011). As such and despite is small size (i.e. ~525 km in diameter; Russell et al., 2012), Vesta mirrored the behaviour of major planets shortly after their formation without subsequent tectonic or volcanic modification. Therefore, understanding the duration of the magmatic activity of Vesta, as well as the thermal evolution and structure of its crust can give powerful constraints on the evolution of differentiated bodies. Current models favour the formation of a short-lived global to partial magma ocean below a thin pre-volcanism chondritic crustal lid (Mandler and Elkins‐Tanton, 2013; Neumann et al., 2014) followed by abundant surface eruptions of basaltic lavas and thermal metamorphism of the most deeply buried volcanic layers (Yamaguchi et al., 2001). Several models of the crustal structure of asteroid 4 Vesta have been proposed ranging from a 30-40 km thick onion-shell like crust with a basaltic upper crust, a middle crust made of gabbroic cumulates and an orthopyroxene rich lower crust estimated from HED meteorites (e.g., Mandler and Elkins‐Tanton, 2013) to a thicker (~80 km) crust with more diffuse structure peppered with orthopyroxene-rich diogenite and gabbroic intrusions estimated from impact modelling and data from the DAWN mission (Clenet et al., 2014). A series of models have attempted to simulate the early evolution of 4 Vesta (Ghosh and McSween Jr, 1998; Gupta and Sahijpal, 2010; Neumann et al., 2014; Zhou et al., 2013) but are scenario-specific and heavily parameters-dependent so need anchoring by in situ analysis of 4 Vesta’s rocks. For instance, the models from Ghosh and McSween Jr (1998; model A), Zhou et al. (2013) and Neumann et

al. (2014) predict contrasting crustal temperatures of ca. 500°C, 250°C and 200°C, respectively at a depth of 15 km, 50 Ma after formation. Geochronology and thermochronology applied to specific geologic formations of Vesta can bring some important constraints on all those models, the crustal structure and the history of Vesta in general. Eucrites are extraterrestrial pigeonite-plagioclase basalts and cumulate gabbros formed, and subsequently more or less metamorphosed, at the crustal level of their parent body. The consensus is that the eucrites have been excavated from 4-Vesta during the Veneneia and/or Rheasilvia south pole impact basins (Clenet et al., 2014; Mandler and Elkins‐Tanton, 2013). Therefore, eucrites represent a unique and more direct opportunity to understand the igneous, cooling and metamorphic history of Vesta. Dating early geological processes on Vesta can thus be achieved by using thermal resistant isotopic systems that are difficult to reset by impact events such as short-lived chronometers (e.g., 53Mn-53Cr, 26Al-26Mg) and in particular chronometers associated with zircon (182Hf–182W, U-Pb and Pb-Pb) (Hopkins et al., 2015; Iizuka et al., 2015; Roszjar et al., 2016; Zhou et al., 2013). A systematic use of zircons as geo- or thermo-chronometer is a difficult task due to their general absence in eucritic material (Zhou et al., 2013), and when present, their average grain size being usually around 10 µm or less (Misawa et al., 2005) requiring the use of ion probe techniques associated with relatively low analytical precision (Hopkins et al., 2015). The 40Ar/39Ar geo- and thermo-chronometer is a very powerful tool to investigate igneous and metamorphic history of terrestrial rocks (McDougall and Harrison, 1999) but this system is plagued by its low resistance to impact-induced high temperatures (Bogard, 2011; Jourdan, 2012), due to the relatively low closure temperature of plagioclase (ca. 300 ± 50 °C; Cassata and Renne, 2013). Conditions met during large impact tend to erase, or at least significantly blur any information recorded by the

40Ar/39Ar

system about the rock early formation

processes. Unbrecciated eucrites represent only about 5% of all the eucrites meteorite (Paniello et al., 2012) and include weakly- to non-metamorphosed basaltic eucrites and thus provide an opportunity to access a range of conditions in the HED parent’s crust by the 40Ar/39Ar system (Bogard, 2011). In this study, we analysed 2 cumulate (Moore County and Moama) and 5 granoblastic (BTN 00300, EET 90020, GRA 98098, QUE 97053 and Caldera) basaltic eucrites with the 40Ar/39Ar technique using a Thermo© ARGUS VI multi-collection mass spectrometer (Jourdan et al., 2017; Kennedy et al., 2019) to better constrain the igneous and metamorphic history of the Vestan crust.

2. Previous geochronology of the first 70 Ma of Vesta. Short lived chronometers such as 53Mn-53Cr, 26Al-26Mg and 182Hf–182W suggest that the coremantle differentiation and first magmatic activity at the surface of Vesta started less than 3 Ma after CAI formation with an overwhelming majority of these data yielding ages around 4564 Ma (Bizzarro et al., 2005; Hublet et al., 2017; Lugmair and Shukolyukov, 1998; Trinquier et al., 2008; Yin et al., 2002). Recently, Zhou et al. (2013) proposed an age-filtered compilation of the first 50 Ma of the history of Vesta, using published zircon Lu-Hf and ion probe U-Pb (and Pb-Pb) ages, as well as their own ion probe U-Pb data. These authors retained all ages with an uncertainty of ± 50 Ma or less (2σ). While data with a precision of ± 30-40 Ma (e.g., 4519 ± 34 Ma; Bouvier et al., 2015) or statistical scatter (e.g., 4545 ± 6 Ma; MSWD = 6 / P = 4 x10-4; Touboul et al., 2015) still might provide some semi-quantitative information about the thermal history of a rock, they are either not precise and/or not accurate enough to study the first 70 Ma of the history of Vesta in detail. In this study, we will use this age compilation as a starting point, but will somewhat tighten the filtering criteria and only use ages determined on individual rocks and individual crystal phase (since mixing results from different rocks with likely different thermal histories or crystals with different closure temperature is not correct from a thermochronological perspective), with an uncertainty of ± 20 Ma or less at 2σ and based only on statistically concordant data assessed using χ² statistical test (P ≥ 0.05; i.e. statically acceptable MSWD; e.g., Jourdan, 2012) to avoid age data that have been perturbed by secondary processes. Filter ages from the data compilation of Zhou et al. (2013) and new zircon ages from Roszjar et al. (2016; Hf-W) and Hopkins et al. (2015; U-Pb) reveal that robust ages range from 4564 ± 1 to 4531 ± 10 Ma, with a dominant concentration of ages near ~4553 Ma and three young ages of ~4531 Ma. The three youngest ages of 4532 +6/-11 Ma (Hf-W), 4531 ± 10 Ma (Pb-Pb) and 4531 ± 20 Ma (Pb-Pb) were obtained on three distinct single zircon crystals from NWA 5073, Camel Donga and the rim of a zircon crystal from Millbillillie, respectively. These ages have been interpreted to represent the age of zircon crystallizing directly from intrusive igneous bodies (Zhou et al., 2013) whereas the three ages around 4531 Ma have been attributed to either igneous (Roszjar et al., 2016; Zhou et al., 2013) or impact melt-sheet (Hopkins et al., 2015; Roszjar et al., 2016) processes whereas a more recent study on mesosiderites suggests they are part of a large scale impact event at ca. 4525 Ma (Haba et al., 2019). The size of metamorphic

zircons relate with their metamorphic grade and large (ca. 80 µm in length) zircon crystals from the Agoult meteorite, interpreted to be of metamorphic origin, were dated by high-precision TIMS U-Pb at 4555 ± 2 Ma, suggesting that this rock was buried deep enough in the crust at this time to undergo thermal metamorphism by burial (Iizuka et al., 2015). We note that this age result is roughly in agreement with error-ages obtained using the Hf-W technique used to date the metamorphic activity on Vesta (Kleine et al., 2005). Based on the discrepancy between Mn-Cr and Pb-Pb ages of zircons from Juvinas, Iizuka et al. (2015) proposed that the young ~4531 Ma zircons from Camel Donga and analysed by Zhou et al. (2013) could have crystallized during thermal metamorphism as well as thus casting some doubt on the true duration of the magmatic activity on Vesta. More recently, Iizuka et al. (2019) also obtained two concordant plagioclase Pb-Pb isochron and plagioclase 40Ar/39Ar plateau ages of 4532 ± 1 Ma and 4494 ± 9 Ma, respectively, from the Agoult meteorite. When combined with the zircon U-Pb metamorphic age of 4555 ± 2 Ma, these results suggest a relatively slow linear cooling rate of 10.8 ± 1.9 °C/Ma (2σ) for this rock (Iizuka et al., 2019). Note, and this is important for this study, that following our filtering criteria, there is no statistically valid (i.e. low MSWD and P > 0.05) and/or precise enough individual age data for the cumulate eucrites Moama and Moore County (Boyet et al., 2010). Bogard and Garrison (2003) analysed 10 unbrecciated eucrites by the 40Ar/39Ar technique and derived apparent ages around 4.5 Ga for most of them, but all of these apparent ages were based on significantly perturbed age spectra (error ages), preventing establishment of firm ages for those meteorites. Such a scatter of the data is due to the difference diffusion characteristics plagioclase, glass (groundmass) and pyroxene and thus, to the variable response of these phases to any subsequent impacts, even minor ones (Kennedy et al., 2013). Consequently, 40Ar/39Ar data measured on whole rocks will represent a mixture of apparent ages between different phases and only when no post emplacement shock occurred, will a whole rock sample yield plateau ages. Kennedy et al. (2013) proposed a quality filtering of the new and published 40Ar/39Ar

ages of unbrecciated eucrites and were left with only two out of eleven ages having

yielded robust plateau ages. EET 90020 which yielded two matrix plateau ages combined in a mean age of 4511 ± >20 Ma (Yamaguchi et al., 2001; J-value uncertainties not included; age recalculated using the 40K decay constants of Renne et al., 2011) and Lake Carnegie which yielded two plagioclase plateau ages with a weighted mean age of 4507 ± 20 Ma (Kennedy et al., 2013) although the significance (i.e., impact-related vs. crustal cooling) of these ages was not clear.

3. Sample descriptions Hereafter, we provide brief petrographic descriptions of the seven samples investigated in this study. Descriptions are based on available published literature and the Meteoritical Bulletin database (https://www.lpi.usra.edu/meteor/metbull.php). Photos of the samples before or after light crushing are provided in Fig. 1 and show the significant abundance of pristine plagioclase crystals for all eucrites. Bates Nunataks 00300, 39 (BTN 00300) This meteorite is one that was described thoroughly by Mayne et al. (2009). We summarise here the main mineral and petrological properties. BTN 00300 is a medium-grained granulite with some trace subophitic texture and pyroxene exsolution lamellae ranging from 1 to 5 μm in width. It contains plagioclase (48 vol%), pyroxene (46 vol%), silica (5 vol%) and opaque minerals (1 vol%) (Mayne et al., 2009). Opaque phases include metal, sulphide and oxide minerals. Low-Ca pyroxenes are magnesian (#Mg ~47, Fs58.1Wo6.8), high-Ca pyroxene is Fs32.5Wo38.1 on average and the plagioclase is An81. The oxides are Ti-poor chromite, Ti-rich spinel, and ilmenite. BTN 00300 has an unfractionated incompatible trace element pattern (Mayne et al., 2009; Mittlefehldt and Galindo Jr, 2002). The degree of metamorphic equilibration suggests an exsolution temperature of 1024 ± 74oC (Mayne et al., 2009). Caldera, NM6394 (Caldera) Caldera consists dominantly of coarse-grained pyroxene (39 vol%), plagioclase (An91; 47 vol%) and silica (12 vol%), in a hypidiomorphic granular texture, and chemical composition similar to the main-group eucrites (Boctor et al., 1994). Accessory minerals are chromite, ilmenite, troilite and rare metal (Boctor et al., 1994). The main pyroxene is pigeonite (Wo6En37Fs57) showing up to ~100 µm coarse exsolutions of augite (Wo42En29Fs29), both of which show very fine-grained subsequent exsolution lamellae of augite and low-Ca pyroxene respectively; suggesting prolonged slow annealing at subsolidus temperature (Boctor et al., 1994). The presence of tridymite suggests that equilibrium was not maintained below 850oC at which temperature it should have inverted to quartz (Boctor et al., 1994). The degree of metamorphic equilibration suggests an exsolution temperature of 1034 ± 88oC (Mayne et al., 2009). Elephant Moraine 90200, 66 (EET 90020)

This basaltic eucrite contains both coarse- and fine-grained lithologies, and shows equigranular granular granoblastic textures (Mayne et al., 2009; Mittlefehldt and Galindo Jr, 2002; Yamaguchi et al., 2001). Ferroan low-Ca pyroxenes (Wo5En34Fs61; 49% average modal abundance) has #Mg ~37; plagioclase (An88; 50% average modal abundance) is moderately zoned with cores of An91. The sample contains <1% average modal abundance of chromite/ulvöspinel, ilmenite, sulphide and metal, and <1% silica (Mayne et al., 2009). The rare earth elements in this sample resemble those of cumulate eucrites; however, the Hf and Ta values are 11-13 x CI similar to those of basaltic eucrites (Mittlefehldt and Galindo Jr, 2002). EET 90020 is thought to have undergone partial melting which was extracted, as it lacks mesostasis as observed in less metamorphosed, basaltic eucrites (Barrat et al., 2007). The degree of metamorphic equilibration suggests an exsolution temperature of 1095 ± 75oC (Mayne et al., 2009). Graves Nunataks 98098, 62 (GRA 98098) This unbrecciated, inequigranular, granoblastic eucrite contains coarse- and fine-grained granulitic regions with low-Ca pyroxene (Wo7En36Fs57; 50% average modal abundance) that has a #Mg ~41 (Mittlefehldt and Galindo, 2002; Mayne et al. 2009). Strongly zoned plagioclase (41% average modal abundance) has cores of An85, and the opaque minerals (<1% average modal abundance) are Ti-rich spinel and ilmenite (Mittlefehldt and Galindo Jr, 2002). Also observed are cross-cutting veins composed of elongated tridymite grains (8% average modal abundance of silica) enclosing grains of plagioclase and pyroxene. The REE pattern shows LREE-enrichment and Hf and Ta are 20-21 x CI (Mittlefehldt and Galindo Jr, 2002). The lack of mesostasis in the granoblastic unbrecciated eucrites (BTN 00300, EET 90020, GRA 98098), indicates that the formation of the silica grains is related to the high degree of metamorphism that they experienced (Mayne et al., 2009). The degree of metamorphic equilibration suggests an exsolution temperature of 985 ± 78oC (Mayne et al., 2009). Moore County, USNM929 (Moore County) A full description of Moore County, and accompanying publications, is available on the web at: https://curator.jsc.nasa.gov/antmet/hed/pdf/moore%20county-final.pdf. It is an unbrecciated, coarse-grained equigranular cumulate eucrite with pyroxene (Wo6En47Fs47) and plagioclase (An91) average modal abundances of 52% and 44%, although these values depend on the section analysed. Opaque minerals of chromite/ulvöspinel, ilmenite, sulphides and metal (Hsu and Crozaz, 1997; Mayne et al., 2009) occur in minor amounts. This eucrite also contains

3% silica (Mayne et al., 2009). The degree of metamorphic equilibration suggests an exsolution temperature of 934 ± 54oC (Mayne et al., 2009). Moama, E12415 (Moama) Moama is also an unbrecciated equigranular cumulate eucrite (Hsu and Crozaz, 1997; Mayne et al., 2009) listed as coarse-grained by Hsu and Crozaz (1997); however Mayne et al. (2009) list this eucrite as fine-grained, so possibly this meteorite contains both coarse- and fine-grained components as does GRA 98098. Pyroxene (Wo3En57Fs40) and plagioclase (An94) have average modal abundances of 44% and 55% respectively. It also contains minor components (<1% average modal abundances) chromite, troilite and metal and <1% silica (Hsu and Crozaz, 1997; Mayne et al., 2009); although sulphides were not evident in the sample examined by Mayne et al. (2009). The degree of metamorphic equilibration suggests an exsolution temperature of 906 ± 33oC (Mayne et al., 2009). Queen Alexandra Range 97053, 29 (QUE 97053) Mayne et al. (2009) examined QUE 97053,6 and found that it exhibited a sub-ophitic textured basaltic eucrite, with pyroxene that are almost entirely equilibrated. The section is dominated by plagioclase (51vol%) with somewhat less pyroxene (Wo2En35Fs63; 48 vol%). The rock contains <1% silica, <1% chromite/ulvöspinel, ilmenite and no sulphides or metal (Mayne et al., 2009). Shock features such as mosaicized pyroxene, and undulose plagioclase are evident (Mayne et al., 2009), indicating that, while the meteorite is unbrecciated, it still experienced some level of shock due to nearby impact. The degree of metamorphic equilibration suggests an exsolution temperature of 801 ± 24oC (Mayne et al., 2009). 4. Analytical technique 4.1. Sample preparation and irradiation Small pieces (~200 mg or less) of each of the meteorites, excluding fusion crust, were crushed with care, washed with distilled water and dried. Material was hand-picked for analysis using a binocular microscope. Plagioclase, pyroxene and/or matrix grains were selected from each meteorite depending on the individual petrology of the samples. For Caldera, silica crystals with conchoidal fractures (tridymite) were also selected in an attempt to see if this mineral could yield meaningful results (cf. description and Energy Dispersive X-ray (EDX) analyses in Annex 1). For each crystal type, grains of as similar a size as possible (two populations of

212-355 µm, and >355-500 µm grain size in diameter) were selected to optimise consistent 39Ar

release.

The samples were loaded into two 1.9 cm-diameter and 0.3 cm-depth Al discs that contain multiple smaller sample wells; all sample wells containing the meteorite grains were surrounded by sample wells that carried the WA1ms neutron fluence monitor, a well characterized and inter-calibrated neutron fluence monitor (2613 [±0.09%] Ma; Jourdan et al., 2014b), and the Fish Canyon sanidine (28.294 [±0.13%] Ma; Jourdan and Renne, 2007; Renne et al., 2011). The sample disks were Cd-shielded (to minimize undesirable nuclear interference reactions) and irradiated for 40 h in the TRIGA reactor (Oregon State University, USA), in a central position. The J-values calculated from the WA1ms standard grains in the surrounding pits are 0.0108400 (± 0.19 %) and 0.0108103 (± 0.09 %) for disc #1 and disc #2, respectively. The correction factors for interfering isotopes were (39Ar/37Ar)Ca = 6.95 ∙ 10-4 (± 1.3%), (36Ar/37Ar)Ca = 2.65 ∙ 10-4 (± 0.83%) measured on CaF2 and (40Ar/39Ar)K = 7.02 ∙ 10-4 (± 12%) determined on K-Fe glass (Renne et al., 2013). Ar isotopic data are corrected for blank, mass discrimination, and radioactive decay. Individual uncertainties are reported in Annex 3-9 at the 1σ level unless otherwise indicated. 40Ar/39Ar

analyses were undertaken carried out in the Western Australian Argon Isotope

Facility, at Curtin University, Perth on both a MAP 215-50 and ARGUS VI mass spectrometers. All age spectra are reported in Annex 2. All raw data, K/Ca plots and Inverse Isochrons are given in Annex 3-9. 4.2 Argon isotope measurements: MAP 215-50 Multi-grain aliquots were wrapped in low-blank niobium foil and step-heated using a 110 W Spectron Laser Systems, with a continuous Nd-YAG (IR; 1064 nm) laser rastered over the sample for 1 minute to ensure a homogenously distributed temperature. The gas was purified in a stainless-steel extraction line using two SAES AP10 and one GP50 getters. Ar isotopes were measured in static mode using a MAP 215-50 mass spectrometer (resolution of ~500; sensitivity of 4x10-14 mol/V) with a Balzers SEV 217 electron multiplier using 9 to 10 cycles of peak-hopping. Data acquisition was performed with the Argus program written by M.O. McWilliams and running within a Labview environment. Blanks were monitored every 3 to 4 steps and typical 40Ar blanks range from 1 x 10-16 to 2 x 10-16 mol. Mass discrimination was monitored using an automated air pipette and provided a range in mean values of 1.003286 (±

0.05%) to 1.003319 (± 0.04%) per dalton (atomic mass unit) relative to an air ratio of 298.56 ± 0.31 (Lee et al., 2006). 4.3 Argon isotope measurements: ARGUS VI multicollector. For each sample, either a single grain or an aliquot of five grains, each with the minimum possible size to ensure maximum sample homogeneity, were step-heated using a continuous 100 W PhotonMachine© CO2 (IR, 10.4 µm) laser fired on the aliquot material for 60 seconds. All standard crystals were measured on the ARGUS VI and fused in a single step. The gas was purified in an extra low-volume stainless steel extraction line of 240 cm3, set up to run with a single SAES AP10 getter. Ar isotopes were measured in static mode using a low-volume (600 cm3) ARGUS VI mass spectrometer from Thermo Fisher© (Kennedy et al., 2019) set with a permanent resolution of ~200. Measurements were carried out in multi-collection mode using three Faraday cups equipped with three 1012 ohm (masses 40; 38; and 37) and one 1013 ohm (mass 39) resistor amplifiers and a low background compact discrete dynode (CDD) ion counter to measure mass 36. We measured the relative abundance of each mass simultaneously during 10 cycles of peak-hopping and 16 seconds of integration time for each mass. Detectors were calibrated to each other through air shot beam signals. We used three different approaches to measure the isotopic abundance of mass 39: (1) we measured the argon isotopes in static multi-collection mode using faraday cups coupled with 1012 ohm resistors on mass 40 to 37 and a low noise CDD (conventional differential detector) ion counter on mass 36, (2) we use a procedure similar as (1) except we used a 1013 ohm resistor on mass 39 and (3) we used a procedure similar to (1) except mass 39 was measured a second time on the CDD which was used to calculate the final ratios. Annex 10 and Table 1 illustrates the difference in age precision obtained using these various configurations and also compared the MAP 215-50 and ARGUS VI results on four aliquots of EET 90020 with a similar sample weight. This suggests that the most optimal precision is obtained on the ARGUS VI using either a faraday cup with 1013 ohm resistor to measure mass 39 or a peak jumping approach using the CDD to measure mass 39. Blanks were analyzed for every three to four incremental heating steps and typical 40Ar blanks range from 1  10–16 to 2  10–16 mol. Mass discrimination was monitored using an automatic air pipette, and yielded mean values of 0.992610 (± 0.02 %) to 0.997440 (± 0.02 %; 1σ) per Dalton (atomic mass unit). 4.4 Data processing

The raw data (Annex – Table 1) were processed using the ArArCALC software (Koppers, 2002), and the ages have been calculated using the decay constants recommended by Renne et al. (2011). All analytical parameters and relative abundance values are provided in TableAnnex 1 and have been corrected for blanks, mass discrimination and radioactive decay. Individual errors in Table-Annex 1 are given at the 1σ level. Importantly, no inverse isochron age could be calculated as all samples yield data that cluster on the radiogenic axis within error. This means that the trap component is extremely small and thus negligible. The consequence of this is that the age is not sensitive to the choice of the trapped ratio. We adopt a trapped ratio of 1 ± 1 (Korochantseva et al., 2007) in this study, but again noting that the choice has no consequence on the plateau age calculation. The only exception is for plagioclase sample EET 90020-3 (Annex 5) which yielded an inverse isochron with a spreading factor of 11% (Jourdan et al., 2009) and a trapped

40Ar/36Ar

ratio of 149 ± 20 suggesting fractionated atmosphere

contamination. For this sample, we use the trapped 40Ar/36Ar ratio to calculate a plateau age. After correction, the age obtained for EET 90020-3 is indistinguishable from the three other aliquots (Table 1). Cosmic ray 38Arc exposure ages have been calculated using the cosmochron approach following the procedure detailed by Kennedy et al. (2013) and Kennedy et al. (2019) and are given in Table 1. Criteria for the determination of a plateau are as follows: (1) plateaus must include at least 70% of 39Ar; (2) the plateau should be distributed over a minimum of 3 consecutive steps agreeing at 95% confidence level and satisfying a probability of fit (P) of at least 0.05. The use of these criteria means that perturbed spectra are not used for age determination. Plateau ages are given at the 2𝜎 level and are calculated using the mean of all the plateau steps, each weighted by the inverse variance of their individual analytical error. Mini-plateaus are defined similarly except that they include between 50% and 70% of 39Ar, but these ages are only accepted where there is a corroborating plateau age (>70% cumulative 39Ar release) from the same sample. Since we compare predominantly 40Ar/39Ar ages throughout the text, uncertainty on the decay constant and standard age are not systematically indicated but are provided in Table 1 for each of the final age and add between ± 6 to ± 9 Ma to the internal uncertainty. 5. Results All results are given in Table 1, while Fig. 2 shows a selection of successful age spectra obtained for each of the meteorite on plagioclase, pyroxene and groundmass. All age spectra are provided in Annex 2 and all data, and K/Ca and Inverse isochron plots are given in Annexes

3 to 9. For each meteorite, Fig. 3 provides statistically concordant weighted mean ages calculated using a selection of plateau and mini-plateau ages obtained on plagioclase and groundmass from each meteorite. Plateau ages obtained on pyroxene and tridymite (cf. Annex 1 for a discussion about Tridymite) and with age >4.4 Ga are also indicated on Fig. 3. Fig. 4 shows all the 40Ar/39Ar weighted mean ages obtained in this study coupled with the filtered age set of the first 50 million years of the history of Vesta (Annex 11). Since the large majority of the 40Ar/39Ar data is very well-behaved and give plateau ages (Fig. 2; Annex 2), only a succinct description of the 40Ar/39Ar results is given below. Bates Nunataks 00300, 39 (granulite) Four anorthitic plagioclase aliquots were analysed as single-grain or multi-grain populations and all yielded plateau ages ranging from 4458 ± 51 Ma to 4563 ± 136 Ma, including between 76 and 100% of the total 39Ar released, albeit with a variable precision (Table 1). A combined weighted mean yielded an age of 4515 ± 9 Ma (P=0.09; Fig. 3). Average K/Ca ratios range from 0.0030 – 0.0033 (Annex 3; corresponding to Ca/K of ~ 300 – 320). Two multi-grain low-Ca pyroxene aliquots yielded two 78% and 84% (i.e., including 78% and 84% of the total 39Ar released) plateau ages of 3920 ± 142 Ma to 4075 ± 133 Ma respectively, and combined in a weighted mean age of 4003 ± 97 Ma (P=0.11). The K/Ca spectra are characteristic of the Ar degassing behaviour of pyroxene (Ware and Jourdan, 2018) with a virtual absence of Ca in the first part of the spectrum marked by very large K/Ca error bars and likely associated with Ar released from orthopyroxene, and ultra-low K/Ca ratio near the end of the spectrum probably associated with high-Ca pyroxene lamellae exsolutions. The average K/Ca value is 0.0001. A pyroxene age of 4003 ± 97 Ma is probably related to a small impact event which opened the K/Ar system in pyroxene but not in plagioclase due to the diffusion kinetic crossover between the two phases for ultra-brief heating events (cf. discussion below; Cassata et al., 2010; Kennedy et al., 2019). We obtained a single CRE age of 21 ± 6 Ma (2σ; P=0.96) on an aliquot of plagioclase (BTN 00300-3; Table 1). Caldera, NM6394 (granulite) Four single- and multi-grain plagioclase aliquots yielded two 100% plateau ages of 4541 ± 35 Ma and 4522 ± 8 Ma from which a weighted mean age of 4523 ± 8 Ma (P=0.29; Fig. 3) was

calculated. The third aliquot yielded a 54% mini-plateau age 4475 ± 10 Ma whereas the last aliquot failed to return an age, and with both spectra showing a strong diffusion-like (i.e. 40Ar loss) profile. Plagioclase average K/Ca varies between 0.0031 and 0.0035 (Ca/K ~ 285 – 320; Annex 4). Two tridymite aliquots (cf. Annex 1) yielded two 100% plateau ages of 4551 ± 34 Ma (Caldera1) and 4486 ± 16 Ma (Caldera-2), with associated K/Ca of 0.016 (Ca/K ~ 62) and 0 due to the absence of Ca (Annex 4). Caldera-1 has an age indistinguishable from the age obtained on plagioclase and a K/Ca value suggesting a mixture of plagioclase and Tridymite. Caldera-2 has no Ca and hence is more likely to represent a pure Tridymite endmember. This aliquot yielded an age distinctively younger than the age obtained on plagioclase, perhaps related to a subsequent impact event, although it is hard to speculate since little is known about Ar behavior in tridymite. A third aliquot returned a strongly perturbed age spectrum and failed to yield an age as a result (Annex 2). Two aliquots of pyroxene yielded 57% and 63% mini-plateau ages of 3989 ± 12 Ma and 4115 ± 37 Ma and showing strong evidence of perturbation. Their K/Ca spectra is as expected for a mixture of orthopyroxene with exolution of high-Ca pyroxene (Annex 4). As mentioned for BTN 00300, the K/Ar system in pyroxene can be easily reset by very small impacts whereas plagioclase remains unaffected (Kennedy et al., 2019). We obtained a single CRE age of 9 ± 3 Ma an aliquot of plagioclase (Caldera-6). Elephant Moraine 90020, 66 (granulite) Four plagioclase aliquots yielded 90-100% plateau ages ranging from 4536 ± 8 Ma to 4525±16 Ma. Note that the plagioclase aliquot EET 90020-3 was the only sample that yield a true inverse isochron with a spreading factor of 11% departing from the radiogenic axis and as such, the age calculation is dependent on the value of the 40Ar/36Ar trapped ratio. For this age we use a trapped 40Ar/36Ar ratio of 149 ± 20 measured by the inverse isochron (Annex 5) to calculate a plateau age of 4536 ± 8 Ma. All four aliquots yielded a weighted mean age of 4531 ± 5 Ma (P=0.39). The average K/Ca ratios range from 0.0035 to 0.0042 (Ca/K ~ 240 – 285; Annex 5). A single pyroxene aliquot yielded an 80% plateau age of 4530 ± 21 Ma (Fig. 3 and Fig. 4), indistinguishable from the plagioclase age with a K/Ca spectrum typical of pigeonite (K/Ca ~ 0.004 with very large uncertainties) with Ca-rich pyroxene exolutions (K/Ca ~ 0.0002) similar with K/Ca spectra observed on terrestrial pyroxene from basalts (Ware and Jourdan, 2018).

We obtained a single CRE age of 12 ± 2 Ma on an aliquot of plagioclase (EET 90020-3).

Graves Nunataks 98098, 62 (granulite) We analysed eight single- and multi-grain aliquots of plagioclase and obtained three plateau ages ranging from 4524 ± 56 Ma to 4501 ± 25 Ma and including more than 79 % of total 39Ar released. We obtained three 65% - 51% mini-plateau ages ranging from 4549 ± 66 Ma to 4500 ± 12 Ma, although the youngest one shows clear sign of 40Ar loss by diffusion. Since all the plateau and the two oldest mini-plateau ages are concordant within error, we combined them in a weighted mean age of 4514 ± 6 Ma (P=0.45; Fig. 4). Adding the youngest mini-plateau age affected by diffusion loss would result in a weighted mean age of 4511 ± 5 Ma (P=0.12). K/Ca ratios vary significantly in between each aliquot with values ranging from 0.0031 to 0.0183 (Ca/K 54 ~ 320; Annex 6). A single aliquot of pyroxene yielded a 50% mini-plateau age of 4504 ± 27 Ma based on the high temperature steps of a diffusion-shaped age spectrum. As for EET 90020, the K/Ca spectrum suggest a mixture between Ar degassing from pigeonite (Ca/K ~ 0.004 with large error bars) and exsolution lamellae of high-Ca pyroxene (Ca/K ~ 0.0001; Annex 6). None of the sample returned a valid CRE age (Table 1). Moore County, USNM929 (cumulate) We analysed four plagioclase aliquots that returned two plateau ages of 4533 ± 13 Ma (100%) and 4524 ± 21 Ma (88%), respectively with a weighted mean age of 4531 ± 11 Ma (P=0.47; Fig. 3). Those two plagioclase aliquots yielded average K/Ca of 0.0024 and 0.0025 (Ca/K ~ 400; Annex 7). A third imprecise 100% “plateau” age of 4406 ± 104 Ma is based on only two steps, hence not robust, whereas the fourth multi-grain aliquots yielded a strongly perturbed age spectrum. Two pyroxene aliquots yielded imprecise plateau (77%) and mini-plateau (67%) age of 4535 ± 128 Ma and 3537 ± 265 Ma, respectively. Their K/Ca spectra is compatible with Ar degassing from orthopyroxene with high-Ca pyroxene exsolutions (Annex 7). None of the sample returned a valid CRE age (Table 1). Moama, E12415 (cumulate)

Four single- and multi-grain plagioclase aliquots yielded 100% to 76% plateau ages ranging from 4421 ± 58 Ma to 4552 ± 153 Ma. The weighted mean age from the three older concordant ages is 4533 ± 12 Ma (P=0.48). Note that the younger plateau age of 4421 ± 58 Ma was excluded from the weighted mean age calculation as (1) it was not concordant with the rest of the age population within uncertainties, (2) is based on a borderline concordant plateau (P=0.10) and (3) has been obtained using the MAP 250-50 instrument which yielded a poorly defined age spectrum that could in turn mask some complexity (c.f. Kennedy et al., 2019). As this is the case with most other unbrecciated (Table 1) and monomict brecciated meteorites (Kennedy et al., 2019), some crystals will be affected by small impacts more than other depending their type (e.g. plagioclase vs. pyroxene), chemical compositions and even location within a given meteorites (e.g., one plagioclase crystal can be affected, whereas another one will remain untouched). Ar loss (and the associated decrease in apparent ages) caused by such impacts can easily be identified for well-defined age spectra, but not for low precision one such as the age spectrum associated with the 4421 ± 58 Ma age (Annex 2 and 8). The average K/Ca ratio for all four aliquots varies between 0.0011 and 0.0012 (Ca/K ~ 830 – 910; Annex 8). A pyroxene aliquot failed to yield a plateau age and displayed a wavy pattern. None of the sample returned a valid CRE age (Table 1).

Queen Alexandra Range 97053, 29 (granulite) Four single- and multi-grain plagioclase aliquots yielded three 88% to 100% plateau ages ranging from 4526 ± 8 Ma to 4502 ± 50 Ma and a 68% mini-plateau age of 4512 ± 9 Ma. The four ages are concordant and yielded a weighted mean age of 4520 ± 5 Ma (P=0.10; Fig. 3). Plagioclase average K/Ca values are relatively homogenous for each aliquot and range from 0.005 to 0.008 (Ca/K =125 – 200; Annex 9). A single aliquot of multi-grain groundmass yielded a 100% age of 4522 ± 12 Ma indistinguishable from the plagioclase age and with an average K/Ca of 0.003 (Ca/K ~ 330). A second aliquot failed to give a plateau age. A pyroxene multi-grain aliquot yielded a 90% plateau age of 4545 ± 53 Ma and showed a typical K/Ca ratio spectrum of pigeonite (K/Ca ~ 0.002 with large uncertainties) with high-Ca pyroxene exolutions (K/Ca ~ 0.0001). A single plagioclase aliquot yielded a CRE age of 24 ± 4 Ma (P=1.0).

6. Discussion We obtained two plagioclase ages of 4531± 11 Ma and 4533 ± 12 Ma on cumulate eucrites Moore County and Moama, respectively and five plagioclase ages ranging from 4531 ± 5 Ma to 4514 ± 6 Ma supported by a groundmass and pyroxene ages of 4522 ± 12 Ma and 4521 ± 21 Ma on five granulitic unbrecciated eucrites. We also obtained five significantly younger plateau and mini-plateau ages on pyroxene from meteorite BTN 00300, Caldera and Moore County ranging from 3537 ± 265 Ma to to 4115 ± 37 Ma suggesting that those three meteorites have experienced brief high-temperature shock-heating events that only have affected the K/Ar chronometer in pyroxene and not plagioclase (i.e., Boehnke et al., 2016; Cassata et al., 2011; Kennedy et al., 2013). Such an effect has been modelled and explained by Cassata et al. (2011) and measured by Kennedy et al. (2013) and is due to the diffusion kinetic crossover of Ar between plagioclase and pyroxene at high temperature. In short, Ar will diffuse extremely fast out of pyroxene at a temperature of few thousand of degrees without any significant Ar diffusion loss in plagioclase provided that the heat is only maintained for a very short amount of time (i.e., micro- to milliseconds), as expected during impact events (Boehnke et al., 2016; Cassata et al., 2011). We obtained a plateau age of 4486 ± 16 Ma on tridymite from Caldera possibly reflecting the effect of a substantial impact event shortly after plagioclase cooling, rather than tridymite cooling. We obtained four

38Ar

CRE ages ranging from 24 ± 4 Ma to 9 ± 3 Ma from four

equilibrated eucrites.

6.1 Moore County and Moama - Plutonic intrusions Moore County and Moama are medium to coarse-grained unequilibrated eucrites that yielded similar 40Ar/39Ar plagioclase ages of 4531 ± 11 Ma and 4533 ± 12 Ma, respectively. Unlike zircon, plagioclase does not record the age of the melt phase, but rather the cooling of the solidified igneous rock below a temperature of ca. 300°C (Cassata and Renne, 2013). According to the thickness of their pyroxene exsolutions Moore County and Moama experienced a relatively slow cooling history and are thus like cumulates found in terrestrial layered intrusions (Miyamoto and Takeda, 1994). It has been proposed that cumulate gabbros crystallized at the very base of the crust along with diogenite (Mandler and Elkins‐Tanton,

2013) or represent the end of the magmatic process and the liquid that did not erupted and crystallized in magma chambers (Patzer and McSween, 2012). 6.1.1 Cooling history Moore County exhibits up to ~100 µm thick augite exsolution lamellae within orthopyroxene (inverted to pigeonite) and shows no sign of metamorphic re-equilibration (Miyamoto and Takeda, 1994). Based on the thickness of the augite lamellae, Miyamoto and Takeda (1994) calculated a cooling rate of ~160°C/Ma for such exsolutions and calculated a depth of ca. 8 km. Assuming a standard solidus temperature of 1060°C for a basaltic rock (Stolper, 1977) and a steady cooling rate, we can calculate that Moore County cooled down below 300 °C, the average closure temperature of plagioclase, in ~4.7 Ma. However, because of the presence of a second set of much finer (~100 nm) augite lamellae, Miyamoto and Takeda (1994) proposed that Moore County was re-heated from ~730 °C to ~940 °C by an impact, about 1 Ma after crystallization and then rapidly cooled/quenched (cooling rate of 0.35°C/a) over a few ka due to the excavation of Moore County near the surface. More recent diffusion experiments by Stimpfl et al. (2003) suggested that the diffusion coefficients of Ca in pyroxene might have been previously underestimated and, as a result, suggested that the cooling rate recorded by the thick augite lamellae of Moore County might have been three times faster than calculated by Miyamoto and Takeda (1994). Furthermore, Stimpfl et al. (2003) also suggested that their calculation alleviates the need for a reheating event to explain the finer augite lamellae, thus suggesting a cooling duration of ~1.5 Ma after crystallization, down to 300°C. Interestingly, both approaches suggest that the host pluton of Moore County cooled down below the plagioclase closure temperature in ≤ 1.5 Ma. Moama also exhibits augitic exsolution lamellae within orthopyroxene, with a maximum thickness of ~41 µm (Miyamoto and Takeda, 1977), hence finer than observed for Moore County. However, since the cooling rate is dependent on the Ca content and the Ca content of Moama is lower than for Moore County, Miyamoto and Takeda (1977) calculated a cooling rate of 96°C/Ma, lower than calculated for Moore County. Taking into consideration updated Ca coefficient diffusion proposed by Stimpfl et al. (2003), this means that Moama would have cool down below the closure temperature of plagioclase ca. 2.4 Ma after reaching solidus. Therefore, these results have two major implications that we will detail below: 6.1.2 Magmatic activity

Since Moama and Moore County are unequilibrated cumulate eucrites and are therefore unambiguously related to magmatic intrusions, this implies that magma chambers were still active around 4533 ± 11 and 4535 ± 12 Ma (40Ar/39Ar ages + 2 Ma to account for cooling time post-solidus) in the upper crust (ca. 8 km or shallower; Miyamoto and Takeda (1994); Stimpfl et al. (2003)) of asteroid 4 Vesta. These ages thus support a possible igneous origin for three zircon crystals from NWA 5073, Camel Donga and Millbillillie (rim) that yielded ages of 4531 ± 10 Ma, 4531 ± 20 Ma and 4532 +6/-11 Ma (Fig. 4; Hopkins et al., 2015; Roszjar et al., 2016; Zhou et al., 2013), but whose origins were not fully resolved. When looking in more detail though, Camel Donga is a metamorphosed and equilibrated eucrite, and yielded a plagioclase (oxides) Pb-Pb age of 4515.4 ± 0.4 Ma (Fig. 4) suggesting that the Pb-Pb system recorded the crystallization of metamorphic zircons and subsequent cooling history of Camel Donga (cf. discussion below and by (Iizuka et al., 2019) and cannot be used to decipher the magmatic history of Vesta. The external rim of a zircon grain from Millbillillie recorded an Pb-Pb age of 4531 ± 20 Ma, whereas the interior of the crystal gave a statistically indistinguishable age of 4555 ± 17 Ma (Hopkins et al., 2015). Those authors preferred explanation was that the rim recorded an impact event (either from diffusion Pb-loss or zircon overgrowths recrystallization in the impact melt sheet) although they could not rule out rim overgrowth crystallization from a magmatic melt (Hopkins et al., 2015). These two zircons (Camel Donga and Millbillillie (rim)) have been recently used to support a mega-impact scenario on the northern pole of Vesta at ~ 4525 Ma and recorded by zircon crystals from several mesosiderites (Haba et al., 2019). A more convincing case as a record of magmatic activity is given by a zircon crystal from NWA 5073 as this eucrite is unshocked, unbrecciated and unequilibrated, hence escaped any kind of impact and/or burial metamorphism. NWA 5073 yielded three relatively precise Hf-W ages with a youngest age of 4532 +6/-11 Ma (Fig. 4, which Roszjar et al. (2016) interpreted to indicate a crystallization age, either from Vesta’s own igneous activity or from an impact melt sheet. Their preferred interpretation, based on crystal shapes, was that the zircon had grown due to prolonged magmatism on Vesta but without being able to firmly rule out crystallization from an impact melt sheet. Therefore, since our

40Ar/39Ar

ages on Moama and Moore County unambiguously

demonstrated that magma pocket/intrusions where still solidifying at this time, this lends some support to the suggestion that the ~ 4532 Ma zircon crystal from NWA 5073 (Roszjar et al., 2016) indeed, crystallized as a result of igneous processes and altogether, this adds to the body of evidence suggesting that magmatic activity on Vesta lasted for at least 30 to 35 million years

post Vesta formation. Such a relatively long duration is in contrast with the assumption that no magmatic activity should occur that late in the history of Vesta since all the 26Al was extinct ~5 Ma after formation. However, recent numerical thermal model such as the one from Zhou et al. (2013) suggest that, whereas >50% of the mantle was molten ~5 Ma after accretion, partial melt persisted for several tens of Ma in the mantle, (or even up to 150 million years near the core-mantle boundary; Neumann et al., 2014), and basaltic melts probably migrated within the crust and formed magma chambers as a consequence. We note that their temperature derived from their model for Vesta’s crust is well in agreement with the cooling rates of 17 ± 4 °C/Ma (this study) and 11 ± 2 °C/Ma (Agoult; Iizuka et al., 2019) determined on slowly cooled metamorphosed equilibrated eucrites using geo/thermochronometry (cf. discussion below). The lack of geochemical differentiation of Moama and Moore County eucrites (e.g., high Mg#) makes remelting of existing crust as the result of intense thermal metamorphism (Yamaguchi et al., 2009) an unlikely scenario for the genesis of these two cumulates. 6.1.3. Shallow emplacement The cooling rates recorded by Moore County and Moama are significantly faster than the average cooling rate of the upper crust ranging from ca. 10 to 30°C / Ma determined by various thermal models (e.g, Neumann et al., 2014; Zhou et al., 2013) and the cooling rate of ~17 ± 4 °C/Ma obtained in this study on metamorphosed eucrites (cf. discussion below) thus suggesting that both crystal mushes intruded a part of the crust that was already cold at the time of intrusion. If the intruded crust was still equilibrating, or was >> 300 °C, the cooling rate recorded by pyroxene crystal lamellae would been much slower, which is not compatible with the petrographic observations (Miyamoto and Takeda, 1977). Therefore, the fact that cumulate eucrites have an age older than the metamorphosed eucrites implies that the cumulate cooled down below 300 °C before the equilibrated eucrites (4517 ± 5 Ma; cf. below). Although the cooling rate of Moore County could have been drastically accelerated by an excavation event (cf. discussion above), such an excavation must have been relatively shallow. Indeed, these two cumulate eucrites could not have been located at the base of the crust as previously postulated (Mandler and Elkins‐Tanton, 2013), since this would necessitate a basin forming event that would excavate cumulate eucrites near the time of their emplacement (to prevent equilibration) but would also have excavated and quenched all the equilibrated eucrites allegedly located above the cumulate layer. Such a senario would be in flagrant contradiction with the younger 40Ar/39Ar ages measured on the equilibrated eucrites (this study and Iizuka et al., 2019). Thus, a fast cooling rate, absence of metamorphic equilibration and age older than

the equilibrated eucrites clearly argue for an emplacement of the plutons above the region of the crust that was being metamorphosed, possibly around 8 km depth (Miyamoto and Takeda, 1994) or shallower (Stimpfl et al., 2003). Such a shallow emplacement level is in fact in excellent agreement with crustal density maps showing quasi-circular areas best interpreted as shallow pluton intrusions and the higher proportion of cumulate eucrite and diogenite intrusive material observed in howardites suggesting a high proportion of intrusive material in the upper crust of Vesta (McSween et al., 2019; Raymond et al., 2017). A shallow emplacement is also compatible with rapid quenching by excavation from a shallow impact (Moore County? Fig. 7), without affecting the equilibrated eucrites which are still undergoing cooling, deeper in the crust. This does not exclude that some intrusion occurred within deeper crustal levels (Fig. 7) where burial metamorphism occurred of course (e.g., Iizuka et al., 2019), but our results imply that part of the shallower most crust was still being intruded by active magma chambers around 4535 Ma.

6.2 BTN 00300, Caldera, GRA 98098 and QUE 97053 – Cooling of the crust We obtained plagioclase ages for four equilibrated eucrites (BTN 00300, Caldera, GRA 98098 and QUE 97053), ranging from 4523 ± 8 Ma to 4514 ± 6 Ma and supported by a plagioclasebearing matrix age of 4522 ± 12 Ma for QUE 97053 (Fig. 2 and 3; Table 1) which, altogether, can be compared to the less precise age of 4507 ± 20 Ma obtained by Kennedy et al. (2013) on the Lake Carnegie unbrecciated equilibrated eucrite (Fig. 4). 6.2.1 Cooling rate of the four equilibrated eucrites Those four 40Ar/39Ar ages are remarkably similar (P = 0.23) and record when the parts of the crust hosting these eucrites cooled below ~300°C after peak metamorphism temperatures of 1000-900 °C (Yamaguchi et al., 1996). In other words, plagioclase records the time where the 300 °C isotherm moving from the uppermost crust to the lower crust reached the location of the equilibrated eucrites analysed in this study. We calculated a cooling rate of 17.3 ± 3.6 °C / Ma (2σ) by carrying out Monte Carlo simulations to fully propagate all uncertainties and realistic ranges of values in the calculations (e.g., Scibiorski et al., 2015). All input parameters were modeled using Gaussian distributions and we used a peak metamorphism age of 4555 ± 5 Ma (Iizuka et al., 2015; Kleine et al., 2005), cooling ages of 4517 ± 5 Ma (averaged from the four equilibrated eucrite plagioclase 40Ar/39Ar ages), metamorphic peak temperature of 950 ±

50 °C (Yamaguchi et al., 1996) and Ar closure temperatures of 300 ± 25 °C (Cassata and Renne, 2013). Note that a peak metamorphism age set at 4555 ± 5 Ma assumes that all the volume of Vesta’s upper crust was already emplaced and thus that bulk of the crust between 10-20 km reached its peak metamorphic temperature, roughly at the same time, between 4560 and 4550 Ma. A cooling rate of 17.3 ± 3.6 °C/Ma indicates a relatively rapid cooling of the crustal level of Vesta. Of course, this rate is an average over ~ 40 million years because cooling rates tend to follow logarithmic decay functions with a faster cooling rate at the beginning of the cooling process (e.g., Ghosh and McSween Jr, 1998)). Such a cooling rate provides an elegant explanation for the discrepancy between Mn-Cr ages and Pb-Pb error-ages in pyroxene (Iizuka et al., 2019) as it has been proposed that cooling rate between 1 to 30°C / Ma can create such an age difference (Ganguly et al., 2007). 6.2.2 Comparison with Agoult, Camel Donga and Caldera Agoult - These results can be compared to age data obtained on Agoult, an equilibrated eucrite with a granulitic texture and large metamorphic zircon crystals. Agoult yielded a recrystallization age of 4555 ± 2 Ma (Iizuka et al. (2015); U-Pb on zircon), and cooling ages of 4520 ± 11 Ma (Pb-Pb on apatite) obtained by Koike et al. (2018), and of 4532 ± 1 Ma (Pb-Pb on leaching oxides-rich residues of plagioclase-rich fractions) and 4494 ± 9 Ma (40Ar/39Ar on plagioclase), complemented by a pyroxene error-age with individual date ranging from 4523 to 4530 Ma obtained by (Iizuka et al., 2019). For comparison, the closure temperatures for Pb diffusion in crystals with a diameter < 100 μm and for a cooling rate of 10°C/Ma are 953 ± 65 °C for zircon, 711 ± 40 ˚C for pyroxene, and 539 ± 24 Ma for apatite whereas no diffusion data are available on oxides (the U-carrier phases in the plagiocalse crystals) although a closure temperatre of ~700 °C was estimated (Iizuka et al., 2019). Iizuka et al. (2019) calculated a linear cooling rate of 10.8 ± 1.9 °C/Ma with an excellent fit to all the data. Such a cooling rate is significantly lower than calculated for the four equilibrated eucrites analysed in this study and in agreement with the 40Ar/39Ar age of Agoult being statistically younger than the eucrites analysed in this study (note that all the 40Ar/39Ar ages from both studies were obtained at Curtin University). Considering that Agoult presents a higher degree of metamorphism as suggested by the presence of uniquely large zircon crystals (which size correlates to the degree of metamorphism), this implies that Agoult was located deeper in the crust compared to BTN 00300, Caldera, GRA 98098 and QUE 97053. How much deeper is hard to accurately determinate and any calculation is most certainly model dependent but, based on a higher

degree of metamorphism and significant

40Ar/39Ar

age differences, an estimate of few

kilometers deeper than these four eucrites is not unreasonable. Camel Donga - Our results can also be compared to U-Pb and Pb-Pb results obtained on Camel Donga. Zircon U-Pb and oxides-bearing plagioclase Pb-Pb ages of 4531 ± 10 Ma (Zhou et al., 2013) and 4515.4 ± 0.4 Ma (Iizuka et al., 2019) where obtained for the equilibrated (and brecciated) eucrite Camel Donga. An age difference of 16 ± 10 Ma between the zircon U-Pb and oxides-bearing plagioclase Pb-Pb ages of Camel Donga is similar within uncertainty to the age difference of 23 ± 3 Ma (i.e., 4555 ± 2 vs. 4532 ± 1 Ma) observed for the same phases of Agoult (Iizuka et al., 2019). This suggests that Camel Donga too recorded the cooling of metamorphic crust post burial-metamorphism, yet it seems that this eucrite started its metamorphism and cooling process 24 ± 10 Ma later than Agoult. The reason for the slightly different cooling history between those two equilibrated eucrites is not clear. One can speculate that Camel Donga could have been located even deeper in the crust than Agoult. Alternatively, Camel Donga’s different metamorphic history could be related to late and deep intrusions instead of burial metamorphism (Iizuka et al., 2019). Another possibility. is that, since Camel Donga was brecciated during an impact event at ~3.75 Ga as recorded by its

40Ar/39Ar

age

(Iizuka et al., 2019; Kennedy et al., 2013), the Pb-Pb system could have been partially open during such an impact event. Finally, Camel Donga thermal history could be more similar to the series of mesosiderites formed at ~ 4525 Ma during a giant impact on Vesta (Haba et al., 2019) since it has been shown to contain trace amount of Fe-Ni metal from an impactor (Warren et al., 2017). Caldera - A multi-mineral

147Sm/143Nd

(coupled with short-lived

146Sm/142Nd

and 53Mn/53Cr

data) age of 4537 ± 12 Ma was obtained on Caldera by Wadhwa and Lugmair (1996) interpreted as dating its crystallization age. However, considering that the data have been obtained on a plagioclase, pyroxene and whole rock (x2) four-point isochron, all phases being less retentive for Nd than for Pb in zircon (Prinzhofer et al., 1992), and considering that Caldera is an equilibrated eucrite, this suggests that an age of 4537 ± 12 Ma represents a mixture of cooling ages from different phases, hence was not retained in our filtered compilation of accurate ages. Nevertheless, assuming a Nd average (yet rough) closure temperature for a mixture between pyroxene, plagioclase and whole rock roughly half way between zircon (UPb; ~950 °C) and plagioclase (40Ar/39Ar; 300 °C) or even if the average closure temperature of such a mixture has a minimum value of ~850 °C as estimated by Kagami et al. (2019) using

indirect approaches, this age agrees within uncertainty with a relatively slow cooling of 17 °C/Ma determined in this study for Caldera and the other equilibrated eucrites.

Global metamorphism of Vesta’s upper crust The most successful model to explain a relatively slow cooling rate for metamorphosed and equilibrated, yet unshocked eucrites is where equilibration occurred following burial metamorphism and then the crust proceeds to cool down slowly and steadily from peak metamorphism conditions (Iizuka et al., 2019; Yamaguchi et al., 1996, 1997). In short, Yamaguchi et al. (1996) proposed that the basalts that erupted first were progressively buried by younger flows deeper in the crust and, because of a relatively high crustal thermal gradient (i.e. closer to the magma ocean / partially molten mantle), were reheated above 800 °C, and equilibrated as a result. Refinement of the model by Yamaguchi et al. (2009) involves the role of magmatic intrusions and eruption of superjacent lava flows (Warren, 1997) and impact events superimposed on crustal metamorphism to explain brief reheating events recorded by some eucrites (cf. discussion below about

40Ar/39Ar

results obtained on EET 90020;

Yamaguchi et al. (2001)). According to the thermal models of Yamaguchi et al. (1996) and Zhou et al. (2013), the upper crust should reach a temperature of 300°C at 4520 Ma at a depth of ca. 17 km (cooling at 10°C/Ma) and 15 km (cooling at 13°C/Ma), respectively. However, a depth of 15 km in the model of Zhou et al. (2013) corresponds to 1000-900°C peak metamorphic temperatures reached at ~4560 Ma, ~5 Ma older than measured by Iizuka et al. (2015) on Agoult, at the oldest age range used in our Monte Carlo simulations (4555 ± 5 Ma). Thus, using such a model, our cooling rate implies that the metamorphosed crust cooled down slightly more rapidly than what has been calculated for a depth of 15 km and by inference that the four equilibrated eucrites analyzed in this study were probably located slightly above 15 km below surface. On the other hand, Agoult was probably located slightly deeper than 15 km below surface as suggested by its slower cooling history. It should be kept in mind that, like any numerical models, there are large uncertainties associated with some of the parameters and that the solution given by the model is an approximation of nature. Nevertheless, those depth estimates firmly locate the equilibrated eucrites in the uppermost part of the crust assuming either a crustal thickness of 40 km (Mandler and Elkins‐Tanton, 2013) or 80 km (Clenet et al., 2014). Note that the uncertainties pertinent to any thermal model calculations (generally not calculated/provided) and the uncertainty on the cooling rates calculated in this study (17 ± 4

°C/Ma) and for Agoult (11 ± 2 °C/Ma) make the agreement between the thermal model of Zhou et al. (2013) for the first 10-20 km of the crust and the 40Ar/39Ar and Pb-Pb thermochronological data, quite an excellent match.

6.3 EET 90020 – age of a deep crustal excavation Our new plagioclase age of 4531 ± 5 Ma is in agreement with, albeit much more precise than the groundmass 40Ar/39Ar age of 4511 ± >20 Ma (recalc. from Yamaguchi et al., 2001; error on the J-value were not included). The plagioclase age is supported by a less precise orthopyroxene age of 4531 ± 21 Ma. Although statistically indistinguishable, the groundmass 40Ar/39Ar

results obtained by Yamaguchi et al. (2001) showed signs of diffusive 40Ar* loss

probably due to re-heating by a small-scale impact, thus it is not surprising that our plagioclase age is at the older end of the age range (i.e., age + uncertainties) obtained by Yamaguchi et al. (2001) on groundmass. 6.3.1. Age of crustal-scale impact event Small plagioclase grain sizes and the presence of vesicles in the groundmass suggest that EET 90020 was initially emplaced close to the surface either as impact melt rock or a thin volatilerich lava flow/dyke (McCoy et al., 2006). Two-pyroxene exsolutions attest to a postemplacement burial depth of several kilometers that caused re-equilibration temperatures of ~1000°C followed by slow cooling to a temperature lower than 870 °C (Yamaguchi et al., 2001). If the K/Ar system would record a monotonous cooling of the crust, then this would imply that EET 90020 was located much closer to the surface compared to BTN 00300, Caldera, GRA 98098 and QUE 97053 and cooled down at a much faster rate of ca. 27 °C/Ma (cf. cooling calculation described above). However, chemical disequilibrium between opaque minerals (mostly ilmenite and spinel) prompted Yamaguchi et al. (2001) to suggest that EET 90020 was then rapidly reheated and cooled at a rate of several °C/day, most likely associated with an impact event that caused the excavation and quenching of EET 90020. Such an excavation event could even be associated with the very large impact recorded by the U-Pb system in zircon crystals from a series of mesosiderites at 4525 ± 1 Ma (Haba et al., 2019) as the ages overlap within error when the decay constants uncertainties are fully propagated in the plagioclase 40Ar/39Ar weighted mean age (4531 ± [14] Ma; Table 1). Abrupt cooling due to excavation has a very dramatic consequence on the interpretation of the plagioclase 40Ar/39Ar

age of EET 90020 as the latter provides only a maximum cooling age for the crust at this depth (i.e., natural crustal cooling below 300 °C would occur ˂< 4531 Ma), yet a precise age for the impact event itself. This would suggest that part of the crust located several kilometers below the surface was still several hundred degrees hot at this time and was probably excavated to the surface during this early period of intense bombardment (Hopkins et al., 2015). Such an impact age is supported by a maximum Hf-W age of <4533 Ma, based on the low abundance of 182W in zircon (Srinivasan et al., 2007). Therefore, the

40Ar/39Ar

plagioclase age of 4531 ± 5 Ma

reflects the timing of the excavation of EET 90020 while this part of the crust was still above the closure temperature of plagioclase and thus provide (1) a maximum age of < 4531 Ma for the cooling of the crust where this equilibrated eucrite resided, thus indicating a cooling rate << 27°C/Ma and (2), more importantly, the age of a relatively large impact that excavated few kilometers of the crust. 6.3.2. Testing the impact scenario through Ar diffusion modelling Since we have relatively precise

40Ar/39Ar

age data on both plagioclase (4531 ± 5 Ma) and

pyroxene (4530 ± 21 Ma) for this eucrite, two minerals with significantly different diffusion characteristics (cf. below), we can use these results to test if the impact scenario is a better match for the 40Ar/39Ar data. We used the ArArDIFF algorithm (Jourdan and Eroglu, 2017; Jourdan et al., 2017) to model the diffusion response recorded by

40Ar/39Ar

age spectra of

plagioclase and orthopyroxene spheres with a 200 µm radius exposed to (1) slow cooling using a cooling rate of 18°C (in order to generate a plagioclase age with a center of mass of 4531 Ma and in agreement within error with a cooling rate of 17 ± 4 °C/Ma (Fig. 5a and 5b)) or (2) an impact in hot (700°C; i.e. the temperature expected at 4531 Ma) target rock with a post-shock temperature of 1000°C followed by a complete cooling in a year (Fig. 5c and 5d). Note that even at temperature as high as 1000°C, plagioclase is expected to remain largely solid (cf. numerical model and discussion by Jourdan et al., 2014a). We use the diffusion characteristics derived from Cassata et al. (2011) and Cassata and Renne (2013) for orthopyroxene (D0 = 6x102 cm²/s; Ea = 371 kJ/mol) and mid-retentive plagioclase (D0 = 6x101 cm²/s and Ea = 205 kJ/mol), respectively. We assume a peak metamorphic age of 4555 Ma (Iizuka et al., 2015). We also simulate the diffusion of W in zircon as a virtual step-heating Ar-like spectra for those two scenarios using the diffusion parameters of Roszjar et al. (2016) (D0 = 1.13x106 cm²/s; Ea =756 kJ/mol) to illustrate that neither of the two scenarios would affect the Hf/W chronometer in zircon as no W loss occurs. The simulations show that there is very little difference in the plagioclase age spectra for both scenarios, but the orthopyroxene model spectra show a slightly

older plateau age of ~ 4544 Ma in the slow cooling scenario compared to the impact scenario which shows full reset at 4531 Ma (Fig. 5a and 5c). Although the center of mass of the plagioclase and pyroxene 40Ar/39Ar ages obtained in this study would suggest that the impact scenario is correct and, despite our new age of 4530 ± 21 Ma being (to the best of our knowledge) the most precise pyroxene 40Ar/39Ar age on extraterrestrial meteorites obtained so far, our results are compatible with both the impact and cooling models within uncertainties, preventing us from forming a conclusion from the

40Ar/39Ar

data alone. Was our diffusion

modeling a pointless exercise then? Definitely not because we showed that if one manages to obtain a pyroxene age with sufficient precision (i.e. by running six aliquots of EET 90020, the uncertainty would decrease to ca. ± 8 Ma), one can then compare precise pyroxene and plagioclase ages to test an impact vs. crustal cooling scenarios. In our case, this would have simply necessitated the analysis of a few more pyroxene aliquots but such a scenario was not foreseen at the time of the sample preparation. Again, we reiterate that regardless of the model inconclusive results, an impact scenario seems a better fit to explain the petrographic observations (Yamaguchi et al., 2001), similarity with the zircon Hf-W minimum age of <4533 Ma (Srinivasan et al., 2007), and a seemingly overly rapid cooling rate for an equilibrated eucrite. Lastly, the fact that the K/Ar clock in pyroxene suffered no impact shock-related perturbations despite pyroxene being very sensitive to brief heating events (Cassata et al., 2010; Kennedy et al., 2013; Kennedy et al., 2019), suggests that EET 90020 might have been ejected directly upon impact and protected from any impact perturbation since then (cf. discussion below).

6.4 Implication for the magmatic and crustal evolution of 4 Vesta Our new 40Ar/39Ar data combined with statistically filtered literature data now provides a solid set of constraints on the crustal evolution of asteroid 4 Vesta. Zircon U-Pb and extinct chronometer ages show that the peak magmatic activity of Vesta occurred between ca. 4565 and 4555 Ma (Roszjar et al., 2016; Zhou et al., 2013). A global (Mandler and Elkins‐Tanton (2013) or partial shallow (Neumann et al., 2014) magma ocean on 4 Vesta would have been very short-lived during this period and must have been progressively confined to increasing depth in Vesta’s mantle, until only a small fraction of the mantle remained molten for several tens or even up to 150 million years after accretion near the core-mantle boundary, as suggested by the thermal models of and Zhou et al. (2013) and Neumann et al. (2014). During 4565-4555

Ma, such an intense rate of eruption resulted in basaltic flows rapidly being buried quickly after their emplacement, causing thermal metamorphism, causing in turn chemical re-equilibration in various minerals (Yamaguchi et al., 1996).

Peak metamorphic temperatures formed

metamorphic zircons at 4555 ± 2 Ma, as recorded in the Agoult eucrite (Iizuka et al., 2015) but were still resetting the Hf-W system at 4547 ± 2 Ma (Kleine et al., 2005). Asteroid 4 Vesta was still magmatically active until at least ~ 4533 Ma with evidence for active magma chambers (Moama and Moore County) at 4535 ± 12 and 4533 ± 11 Ma. This is in agreement with an igneous zircon age of 4532 +6/-11 Ma from NWA 5073 (Roszjar et al., 2016) suggesting that the magmatic activity lasted at least 33 ± 5 Ma after the formation of Vesta. We note that such a long-lasting magmatic activity is in agreement with thermal models predicting that melt pockets were present in the mantle for several tens of Ma after accretion (Neumann et al., 2014; Zhou et al., 2013) and that the HEDs meteorites came from a large asteroid like 4 Vesta, since much shorter duration estimates would imply an origin from a much smaller asteroid (Schiller et al., 2011). The rapid cooling rate of at least 160 °C/Myr (Miyamoto and Takeda, 1994; Stimpfl et al., 2003) recorded in these cumulates and the lack of subsequent equilibration, showed that they were emplaced in a relatively cold crust, probably located a few kilometres below the surface (McSween et al., 2011; McSween et al., 2019) hence at a shallower level than the metamorphosed eucritic layers that were well above 300°C at this time. The lack of subsequent burial metamorphism of the two cumulate eucrites demonstrates that the crust was almost, if not entirely formed by ~4535 Ma, and burial of the uppermost crust after this time became very limited. An excavation age of 4531 ± 5 Ma (EET 90020) shows that large impacts were excavating portions of relatively hot (~870°C) metamorphosed crust located well below the depth of active magma chambers. Between 4523 ± 8 Ma to 4514 ± 6 Ma, the buried parts of the crust (~ 12-15 km deep?) which contained BTN 00300, Caldera, GRA 98098 and QUE 97053 cooled down below ~300°C at an average rate of 17.3 ± 3.6 °C/Ma assuming a peak metamorphism age of ~ 4555 Ma. The 300°C isotherm kept propagating deeper in the crust and by 4494 ± 9 Ma reached the part of crust that contained Agoult (~ 15-20 km deep). When compared with thermal models of the cooling of the crust of 4 Vesta such as the recent model proposed by Zhou et al. (2013), this suggest that the suite of equilibrated eucrites analysed in this study combined with Agoult (Iizuka et al., 2019) are all located in the middle to lowest part of the upper crust depending upon Vesta’s full crustal thickness of either 40 km (Mandler and Elkins‐Tanton, 2013) or 80 km (Clenet et al., 2014).

6.5 Ejection and preservation of the unbrecciated eucrites. One or several subsequent impact events including at least one impact large enough to excavate at least ca. 15-20 kilometres of crust is needed to eject fragments of the crust of Vesta containing the unbrecciated eucrites. Since the unbrecciated eucrites show little sign of shock and the

40Ar/39Ar

system in plagioclase show little sign of thermal perturbation since their

cooling below closure temperature, the parental rocks containing the suite of unbrecciated eucrites with 40Ar/39Ar age >4.5 Ga must have been ejected with a sufficient quantity of rocks that it would be protected from subsequent impacts event by overlying rocks in a small enough asteroid to avoid frequent and large collisions. Unless such an ejection event occurred very recently, small km-size asteroid fragments have been shown to be quickly destroyed within several million years by impact in the asteroid belt (Bottke et al., 2005). However, orbital constraints on the Vestoids suggest that they have been ejected from Vesta at least ~ 1 Ga ago (Marchi et al., 2012) whereas crater counting model curves estimate that Rheasilvia, the youngest and biggest of the two south pole basins was formed at ~ 3.5 Ga (Schmedemann et al., 2014), in agreement with plagioclase 40Ar/39Ar plateau ages obtained on brecciated eucrites (Kennedy et al., 2019). Both estimates are in stark contrast with an ejection age from Vesta being a few million years old. One possibility is that since the crust has been massively excavated during the Veneneia and Rheasilvia basin forming events, any subsequent impacts would eject material located at any position along the crater bowl and central peak. However, since this particular suite of eucrites is unbrecciated, it is not clear how the material located on the crater wall and floor would have avoided being shocked for several billions of years since basin formation. Rather, it is perhaps more intuitive to assume that the parental rocks of the unbrecciated eucrites were ejected during one of the basin forming events, yet this latter scenario implies that the ejected material must have been protected from further impacts for several billions of years following their ejections. Following Kennedy et al. (2019), we propose that the fragments recombined in rubble pile asteroids. Rubble pile asteroids, such as Itokawa, Bennu or Ryugu from which we have close-up views, have been shown to be quite abundant in the solar system and the results of relatively large impacts on parent asteroids (e.g., Walsh, 2018). In addition, some Vestoids thought to be collisional fragments from Vesta, have been shown to possess a low density, likely associated with a high porosity (Carry, 2012). Small rubble pile asteroids are ideal candidates for preserving rocks from shock since their porous nature will confine the impact heat to the shallower most levels of the asteroid (Davison et al., 2010). It was recently shown that the presently ~500 m wide rubble pile asteroid Itokawa was

formed more than 2 billion years ago (Jourdan et al., 2017) from the breakup of a >20 km large parent body (Nakamura et al., 2011) thus demonstrating that rubble pile asteroids can survive for an extended period of time in the asteroid belt (Jourdan et al., 2017). It is important to note that the unbrecciated eucrites were not completely immune to impactrelated ultra-transient heating events since four pyroxene samples from BTN 00300 and Caldera yielded plateau and mini-plateau ages around 4 Ga (Table 1; Annex 2) indicating K/Ar reset in pyroxene (but not plagioclase) at this time. However, pyroxene tends to be easily perturbed by ultra-brief heating events associated with small impacts (Cassata et al., 2010; Kennedy et al., 2013; Kennedy et al., 2019). Several plagioclase aliquots (e.g., Caldera-5; GRA 98098-7; Moore County-3) show significant sign of perturbation as well despite many aliquots from the same meteorites yielding plateau ages (Table 1 and Annex 2). Since the eucrites investigated in this study are not brecciated, this suggests that ultra-transient heat waves caused by small impact events can cause variable diffusion responses within plagioclase from the same rock. As discussed by Kennedy et al. (2019), this is likely due to a combination between slightly different diffusion characteristics between plagioclase crystals (e.g., linked to the K/Ca value) and a heterogenous temperature distribution (focused heating) during such kind of impact within the target rock, especially if the latter is porous. Whereas storage deep from within one or several rubble pile asteroids presents an elegant mechanism to preserve eucrites from further major heating/shock events (Kennedy et al., 2019), it is not clear when the deeper crustal levels (~ 15-20 km) containing the equilibrated eucrites were ejected. Recent 40Ar/39Ar dating carried out on a series of plagioclase crystals from several unrelated polymict brecciated eucrites show a cluster of ages ranging from 4534 ± 56 Ma to 4491 ± 16 Ma, with a combined weighted mean age of 4500 ± 4 Ma (P=0.16), best interpreted as the age of a single large impact event that ejected a large range of crustal material (Kennedy et al., 2019). Such an age is compatible within error with the older ages obtained for the equilibrated eucrites, and overlaps with the plagioclase

40Ar/39Ar

age of 4494 ± 9 Ma

obtained for Agoult (Iizuka et al., 2019). Furthermore, we obtained an 40Ar/39Ar age of 4486 ± 16 Ma measured on a tridymite crystal from Caldera (Table 1) possibly related to an impact that was large enough to have caused reset of tridymite (but not plagioclase) either as deep as 10-15 km in the crust during excavation, or by pore collapse (Davison et al., 2010; Jourdan et al., 2017) when Caldera was already ejected in a rubble pile parent body. However, there is no obligation for the unbrecciated eucrites to be ejected at the same time since the metamorphosed eucrite EET 90020 was excavated and quenched at 4531 ± 5 Ma while other eucrites where

still slowly cooling at a similar depth in the crust. Therefore, there are no time constraints required for the ejection of deeper crustal material such as cumulate or metamorphosed eucrites as they could be well protected within Vesta itself before ejection. Lastly, the lack of fully reset 40Ar/39Ar plagioclase ages after ~3.47 Ga prompted Kennedy et al. (2019) to suggest that the complete brecciated eucrite suites have all been ejected at this time and clustered upon ejection into a rubble pile asteroid. Such a large impact could possibly be associated with the formation of Rheasilvia, the younger of the two South Pole impact basins.

Conclusion: New 40Ar/39Ar crystallization and cooling ages on plagioclase, pyroxene and groundmass allow us to reach the following conclusions: 1. Vesta was still magmatically active around 4535 Ma with evidence for active magma chambers (Moama and Moore County) at 4532 ± 11 and 4535 ± 12 Ma and located no more than a few kilometres below the surface. 2.

Between 4523 ± 8 Ma to 4514 ± 6 Ma, the part of the crust which hosted a series of equilibrated eucrites and initially reached peak metamorphism conditions around 4555 Ma, cooled down below ~300°C at an average rate of 17.3 ± 3.6 °C/Ma. This locates the equilibrated eucrites in the mid to lowest part of the upper crust depending upon Vesta’s full crustal thickness.

3. The unshocked nature of the unbrecciated eucrite suite and the lack of perturbation of the K/Ar system suggest that unbrecciated eucrites have been ejected during one or several crustal scale impact event(s), and recombined in rubble pile asteroids where they were protected from further significant shock heating events.

Acknowledgment: We thank Zdenka Martelli and Adam Frew for technical help with the sample preparation and analysis. Dermot Henry is thanked for one meteorite sample from the collection of Museum Victoria, Dr Kevin Righter, Lunar and Planetary Institute, is thanked for four samples from the NASA Antarctic Meteorite Collection, and Dr Linda Welzenbach, Smithsonian Institute,

National Museum of Natural History, for two meteorite samples. US Antarctic meteorite samples are recovered by the Antarctic Search for Meteorites (ANSMET) program which has been funded by NSF and NASA, and characterized and curated by the Department of Mineral Sciences of the Smithsonian Institution and Astromaterials Acquisition and Curation Office at NASA Johnson Space Center. Two anonymous reviewers and K. Min are thanked for their review of this manuscript.

Fig. captions:

Fig. 1. Low magnification stereomicroscope microphotographs of the seven unbrecciated eucrites analysed in this study. Meteorites are shown either as the original piece received (BTN 00300, 39 / GRA 98098, 62 / Moama, E12415/ QUE 97053, 29) or right after gentle crushing (Caldera, USNM6394/ EET 90020, 66 / Moore County, USNM929). All eucrites have visible crystals of fresh plagioclase and pyroxene. Fig. 2. Selection of the most successful

40Ar/39Ar

age spectra obtained for each of the

unbrecciated eucrites. Age spectra in red color: plagioclase; green color: pyroxene and blue color: groundmass. Each aliquot was step-heated with a CO2 laser and analysed with an ARGUS VI mass spectrometer using. Mass 39Ar was measured either utilising a faraday cup coupled with a 1013 ohm resistor or by the CDD ion counter. Ages have been calculated using the 40K decay constants of Renne et al. (2011) and R-values of Jourdan and Renne (2007; FCs) and Jourdan et al. (2014b; WA1ms). MSWD (Mean weighted square deviation) and P (Probability) values are indicated and show full concordancy between steps for each age spectrum. All spectra are provided in Annex 2. Fig. 3. Plateau (solid fill) and mini-plateau (semi-transparent fill) ages for the seven unbrecciated eucrites analysed in this study. Red box (plagioclase), green box (pyroxene), blue box (groundmass), grey box (tridymite; Annex 1). For each eucrite, a weighted mean age has been calculated using either plagioclase or plagioclase and groundmass. Fig. 4. Age compilation of the first 70 Ma history of the HED parent body based on radioisotopic ages obtained on eucrites and modified from Zhou et al. (2013). This compilation

only includes fully published isotopic ages (1) determined on single minerals from individual rocks, (2) with an uncertainty of ± 20 Ma or less, (3) based only on statistically concordant data assessed using χ² statistical test (P ≥ 0.05; i.e. statically acceptable MSWD; e.g., Jourdan, 2012) to avoid age data that have been perturbed by secondary processes. All data and their sources are provided in Annex 11. References: Short lived chronometers: Bizzarro et al. (2005), Hublet et al. (2017); Lugmair and Shukolyukov (1998); Trinquier et al. (2008); Yin et al. (2002); UPb, Pb-Pb, Lu-Hf and Sm-Nd: Zhou et al. (2013), Hopkins et al. (2015), Roszjar et al. (2016), Iizuka et al. (2015), Iizuka et al. (2019); Plagioclase

40Ar/39Ar:

*Iizuka et al. (2019) and

**Kennedy et al. (2013). The pre-filtered list of all reported age data until 2013 is given by Zhou et al. (2013) and is given in Annex 11. Fig. 5. ArArDiff synthetic age spectra calculated for 200 µm radius spheres using the diffusion characteristics derived from Cassata et al. (2011) and Cassata and Renne (2013) for orthopyroxene (D0 = 6x102 cm²/s; Ea =371 kJ/mol) and mid-retentive plagioclase (D0 = 6x101 cm²/s and Ea = 205 kJ/mol), respectively. We assume a peak metamorphic age of 4555 Ma (Iizuka et al., 2015). Model 1 (a, b) slow crustal cooling using a monotonous cooling rate of 18°C (Fig. 5a and 5b). Model 2 (c, d) impact in a hot (700°C) target rock generating a postshock temperature of 1000°C followed by a complete cooling in a year (Fig. 5c and 5d). Although Hf/W isotope analyses of zircon are not generated by incremental extractions, we applied our model to W using diffusion parameters from Roszjar et al. (2016) for zircon (D0 = 1.13x106 cm²/s; Ea =756 kJ/mol) to illustrate that neither scenario trigger any W loss and thus, the Hf/W chronometer should record the formation age of zircon. All parameters and thermal histories are given in Annex 12. Fig. 6. Histogram and probability distribution curve used to calculate the cooling rate of the upper crust using Monte Carlo simulations. This allows propagating all sources of uncertainties. The uncertainty on the calculated cooling rate is given at 95% confidence level. Parameters used in the simulations (all given hereafter at 2σ). Metamorphic peak temperature: 950 ± 50 °C; Closure temperature of plagioclase to Ar diffusion: 300 ± 26 °C (Cassata and Renne, 2013); Age of peak metamorphism: 4555 ± 5 Ma (Iizuka et al., 2015); cooling age range for the equilibrated eucrites analysed in this study: 4517 ± 5 Ma. Fig. 7. Simplified cross section of the 20 first kilometres of Vesta’s upper crust modified from Iizuka et al. (2019). Approximate locations of unbrecciated eucrites with a robust 40Ar/39Ar age analysed in this study and from Iizuka et al. (2019; Agoult) and Kennedy et al. (2013; Lake

Carnegie). The depths are approximate but in respect to the relative position of the meteorites relative to each other based on the cooling ages of each of the eucrites (cf. discussion). BTN: BTN 00300 / GRA98: GRA 98098 / Moa.: Moama / QUE97: QUE 97053 / Cald.: Caldera / EET90: EET 90020 / Mo. Cou.: Moore County.

Table captions: Table 1. Summary table of 40Ar/39Ar ages for eight unbrecciated eucrites analysed in this study. Abbreviations: plg = plagioclase (red font), pyx = pyroxene (green font). gm = groundmass (blue font), tri = tridymite (gray font; Annex 1). Machines: MAP = MAP 215-50; AVI = ARGUS VI. Setup refers to the detector used to measure mass 39. CDD = Compact discrete dynode. MG = multi-grain (between 3-5 grains for plag, 10mg for pyroxene), SG = single grain. Age in bold type = robust plateau (> 70 % total 39Ar released) age. Age in non-bold type = mini-plateau (50-70 % total 39Ar released) ages which are considered less reliable than their plateau counterpart. MSWD = mean square weighted deviation and “P” = probability. Note that the effect of the choice of the 40Ar/36Ar trap value is negligible within error as the data cluster on the radiogenic axis thus preventing calculation of any inverse isochrones (Annexes 3-9). **Age calculated using the 40Ar/36Ar trapped ratio measured using the inverse isochron (cf. text; Annex 5). Errors in bracket for the final ages include all sources of uncertainties.

Annex captions: Annex 1: Methodology, results and discussion of Electron dispersive X-ray analyses of tridymite crystals. Annex 2. All 40Ar/39Ar age spectra for each of the unbrecciated eucrites. Age spectra in red color: plagioclase; green color: pyroxene, blue color: groundmass, grey color: tridimite. Annex 3-9:

40Ar/39Ar

data and K/Ca and inverse isochron plots for each aliquots of each

meteorite. Annex 3: BTN 00300; Annex 4: Caldera; Annex 5: EET 90020; Annex 6: GRA 98098; Annex 7: Moore County; Annex 8: Moama; Annex 9: QUE 97053. Each aliquot consists of three tabs of the same colour for an easy reading. Tab 1:

40Ar/39Ar

relative

abundances; J-values and constants along with age calculations, associated statistics and K/Ca

computed using ArArCALC (Koppers, 2002). Tab 2: K/Ca plot. Tab 3: inverse isochron plot showing for all but one sample that the data cluster near/on the radiogenic axis. Gray line: inverse isochron forced to a cosmogenic value of 1 (Korochantseva et al., 2007) Pink line: inverse isochron without any anchor point. Annex 10. Graph showing the various precision obtained on plagioclase aliquots with similar weight from EET 900200 using different mass spectrometers (MAP 215-50 and ARGUS VI) and, in the case of the ARGUS VI multicollector, we experimented with different detector configurations. See text for details Annex 11. Summary table including all the published data used in this study and accompanying references. Note that this table only includes statistically valid mineral ages with an uncertainty equal or lower than ± 20 Ma (cf. discussion in the main text). Annex 12. Tables showing the Temperature, time and diffusion parameters used in the generation of the ArArDIFF diffusion models of Fig. 6.

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Table 1

General characteristics

39

Final Ar/ Ar age Weighted mean age of all plateaus (Ma; 2σ). [including all uncertainties]

CRE Age (Ma; 2σ)

0.09

2 3 18 24

not enough signal not enough signal 21 ± 6 not enough spread

4003 ± 97 [97]

0.11

11 18

not enough spread not enough spread

4523 ± 8

0.29

2 2 23 26

not enough signal not enough signal 9 ± 3 not enough spread

11 21

not enough signal not enough spread

4 7 34 34

not enough signal not enough signal 12 ± 2 not enough spread

102

not enough spread

14 8 11 9

not enough signa not enough signal not enough signal not enough signal

15 20 44 54

not enough spread not enough spread not enough spread not enough spread

433

not enough spread

<1 4 7 14

not enough signal not enough signal not enough signal not enough spread

10 5

not enough signal not enough signal

9 6 10 75

not enough signal not enough signal not enough signal not enough spread

159

not enough spread

42 14

not enough spread not enough signal

9 2 34 35

not enough signal not enough signal 24 ± 4 not enough spread

123

not enough spread

Machine (setup)

single or multi grain

BTN 00300-1 BTN 00300-2 BTN 00300-3 BTN 00300-4

plg plg plg plg

MAP AVI (10-12 ohm) AVI (10-13 ohm) AVI (CDD)

SG SG MG MG

100 100 76 86

4458 4563 4524 4514

51 136 19 11

1.0 0.2 1.5 0.8

0.53 1.0 0.12 0.66

* * * *

4 (plg)

4515 ± 9

[16]

BTN 00300-5 BTN 00300-6

pyx pyx

AVI (CDD) AVI (CDD)

MG MG

78 84

3920 ± 142 4075 ± 133

0.7 0.9

0.68 0.53

* *

2 (pyx)

4551 ± 34

2(plag)

BTN 00300, 39

Caldera, USNM6394

P

± ± ± ±

Tri

MAP

SG

100

Tri Tri

AVI (10-12 ohm) AVI (10-13 ohm)

SG MG

100 -

Caldera-4 Caldera-5 Caldera-6 Caldera-7

plg plg plg plg

MAP AVI (10-12 ohm) AVI (10-13 ohm) AVI (CDD)

SG SG MG MG

Caldera-8 Caldera-9

pyx pyx

AVI (CDD) AVI (CDD)

MG MG

57 63

3989 ± 12 4115 ± 37

0.4 1.7

0.88 0.08

plg plg plg plg

MAP AVI (10-12 ohm) AVI (10-13 ohm) AVI (CDD)

SG SG MG MG

100 90 100 94

4526 4525 4536 4527

33 16 8** 9

0.6 0.5 0.9 0.4

0.64 0.96 0.56 0.96

pyx

AVI (CDD)

MG

80

4530 ± 21

1.3

0.23

plg plg plg plg

MAP AVI (10-12 ohm) AVI (10-13 ohm) AVI (CDD)

SG SG MG MG

60 100 65 79

4549 4524 4521 4511

2.7 0.8 1.9 0.9

0.07 0.68 0.07 0.5

* * * *

plg plg plg plg

MAP AVI (10-12 ohm) AVI (10-13 ohm) AVI (CDD)

SG SG MG MG

84 51 -

4501 ± 25 0.7 0.56 >4500 ± 12 0.5 0.76 diffusion profile (>4.5 Ga) Strong perturbation

*

pyx

AVI (CDD)

MG

50

plg plg plg plg

MAP AVI (10-12 ohm) AVI (10-13 ohm) AVI (CDD)

SG SG MG MG

pyx pyx

AVI (CDD) AVI (CDD)

MG MG

67 77

3537 ± 265 4535 ± 128

0.5 0.3

0.93 0.94

plg plg plg plg

MAP AVI (10-12 ohm) AVI (10-13 ohm) AVI (CDD)

SG SG MG MG

100 100 76 99

4421 4552 4526 4541

2.0 0.3 1 0.7

0.10 1.0 0.49 0.73

pyx

AVI (CDD)

MG

-

slight perturbation

gm gm (F)

AVI (10-13 ohm) AVI (10-13 ohm)

MG MG

100 -

4522 ± 12 slight perturbation

QUE 97053-3 QUE 97053-4 QUE 97053-5 QUE 97053-6

plg plg plg plg

MAP AVI (10-12 ohm) AVI (10-13 ohm) AVI (CDD)

SG SG MG MG

99 88 68 100

4510 4502 4512 4526

QUE 97053-7

pyx

AVI (CDD)

MG

90

GRA 98098-9

0.96

1.1

0.37

4486 ± 16 0.9 Strong perturbation

0.54

100 (2steps) 4541 ± 35 0.2 Strong perturbation 82 4522 8 0.8 54 4475 ± 10 0.8

0.64

*

0.66 0.48

*

[15]

0.3

0.99

± ± ± ±

* * * *

4 (plg)

4531 ± 5

[14]

0.39

0.3

0.99

± ± ± ±

66 56 11 7

>4504 ± 27

1.8

0.06

4406 ± 104 2.0 100 (2 steps) 88 4524 ± 21 0.4 Strong perturbation 100 4533 ± 13 0.8

0.16 0.91

*

0.61

*

5 (plg)

4514 ± 6

[15]

0.45

0.1

1

no

Moore County-5 Moore County-6

2 (plg)

4531 ± 11 [17]

0.47

no

Moama-5

QUE 97053, 29 QUE 97053-1 QUE 97053-2

0.4

yes

GRA 98098-5 GRA 98098-6 GRA 98098-7 GRA 98098-8

Moama, E12415 Moama-1 Moama-2 Moama-3 Moama-4

P

yes

EET 90020-5

Moore County, USNM929 Moore County-1 Moore County-2 Moore County-3 Moore County-4

MS WD

yes

Caldera-2 Caldera-3

GRA 98098, 62 GRA 98098-1 GRA 98098-2 GRA 98098-3 GRA 98098-4

P

yes

Caldera-1

EET 90020, 66 EET 90020-1 EET 90020-2 EET 90020-3 EET 90020-4

Cosmic ray exposure age 38 Ar signal (fA)

Included number of in final aliquots age (*)

Material analysed

Sample name

Equilibrated

40

Plateau characteristics 39 Total Ar Plateau or miniMS released plateau Age (Ma; WD 2σ) (%)

± ± ± ±

58 153 17 19

* * *

3 (plg)

4533 ± 12 [18]

0.48

yes 1.5

0.13

*

33 50 9 8

1.5 0.2 1.6 0.6

0.16 1 0.1 0.87

* * * *

4545 ± 53

1.1

0.34

Table 1: Jourdan et al.

± ± ± ±

5 (4 plg + 1 gm)

4520 ± 5

[14]

0.17

Figure 1

BTN 00300, 39

3.0 mm

Caldera, USNM6394

2.5 mm

GRA 98098, 62

EET 90020, 66 3.0 mm 3.0 mm

Moore County, USNM929 

5.0 mm

5.0 mm 3.0 mm

QUE 97053,29

Jourdan et al.: Fig. 1

Moama, E12415 

Figure 2

6000

5400

6000

Plagioclase

5700

BTN 00300-4

Plagioclase

5700

5400

5400

5100

5100

Caldera-6

Plagioclase

5300

EET 90020-3

5200 5100

4500 4200 3900

Age [ Ma ]     

Age [ Ma ]     

Age [ Ma ]     

5000

4800

4800 4500 4200 3900

4514 ± 11 Ma

3600

30

40

50

60

4500 4400 4300 4200 4000 3900

3000 20

4600

4100

3000 10

MSWD=0.9; P=0.56

4700

MSWD=0.8; P=0.66

3300

0

4536 ± 8 Ma

4800

4522 ± 8 Ma

3600

MSWD=0.8; P=0.66

3300

4900

70

80

90

100

0

10

20

30

40

50

60

70

80

90

0

100

10

20

30

7000

Pyroxene

40

50

60

70

80

90

100

Cumulative 39Ar Released [ % ]

Cumulative 39Ar Released [ % ]

Cumulative 39Ar Released [ % ]

6000

EET 90020-5

6500

5300

6000

5000

Plagioclase

Plagioclase

GRA 98098-4

Moore County-4

5000 4500

Age [ Ma ]     

5500

Age [ Ma ]     

Age [ Ma ]     

5500

4700

4400

4000

MSWD=0.9; P=0.50

4530 ± 21 Ma MSWD=1.3; P=0.23

3500 3000 0

10

20

30

40

50

60

4500

4000

4511 ± 7 Ma

4100

5000

70

80

90

3800

3500

3500

3000

100

0

10

20

30

40

50

60

70

80

90

4533 ± 13 Ma MSWD=0.8; P=0.61

0

100

10

20

30

Cumulative 39Ar Released [ % ]

Groundmass

QUE 97053-1

5700

80

90

100

4541 ± 19 Ma MSWD=0.7; P=0.73

5100

5100

4522 ± 12 Ma

4900

MSWD=1.5; P=0.13

Age [ Ma ]     

4500

4700 4500 4300

3000

QUE 97053-6

5400

5300

Age [ Ma ]     

Age [ Ma ]     

70

Plagioclase

5700

5500 5000

3500

60

6000

5900

Moama-4

Plagioclase

4000

50

Cumulative 39Ar Released [ % ]

6000 5500

40

4800 4500 4200

4526 ± 8 Ma

3900

MSWD=0.6, P=0.87

4100 3600

3900

2500

3300

3700 3500

2000 0

10

20

30

40

50

60

Cumulative 39Ar Released [ % ]

70

80

90

100

3000 0

10

20

30

40

50

60

70

80

Cumulative 39Ar Released [ % ]

Figure 2: Jourdan et al.

90

100

0

10

20

30

40

50

60

Cumulative 39Ar Released [ % ]

70

80

90

100

Figure 3

Caldera, USNM6394

BTN 00300, 39

4720

4590 Plagioclase (n=4)

4680

4515 ± 9 Ma

4640

4570

MSWD = 2.1, P = 0.093

4550

Age (Ma)

Age (Ma)

4600 4560 4520 4480 Groundmass/glass Tridymite

4360

Plagioclase (n=2)

4523 ± 8 Ma

4470

Pyroxene

4400

4510 4490

Plagioclase

4440

4530

MSWD = 1.1, P = 0.29

4450

Analysis #

Analysis # 4570

EET 90020, 66

4630

4560

4610

4550

4590

4540

4570

GRA 98098, 62

Age (Ma)

Age (Ma)

Plagioclase (n=5)

4530 4520 4510 4500

MSWD = 0.9, P = 0.45

4550 4530 4510 4490

Plagioclase (n=4)

4531 ± 5 Ma

4490

4514 ± 6 Ma

4470

MSWD = 1.0, P = 0.39

4480

4450

Analysis #

Analysis # 4700 4660 4620

Moama, E12415

Moore County, USNM929

4533 ± 12 Ma

4531 ± 11 Ma

MSWD = 0.72, P = 0.48

MSWD = 0.53, P = 0.47

4600

Age (Ma)

4580

Age (Ma)

Plagioclase (n=3)

4700

Plagioclase (n=2)

4540 4500 4460

4500

4400

4420 4300

4380

Not included in the age calculation

Analysis #

Analysis # QUE 97053, 29

4610 4590 4570

Groundmass & plagioclase (n=5)

4520 ± 5 Ma MSWD = 1.6, P = 0.17

Age (Ma)

4550 4530 4510 4490 4470 4450 4430

Analysis #

Fig. 3: Jourdan et al.

Figure 4

4620

4600

Magmatic (± Metamorphic?) zircon ages

EET 90020

4580

(rap. cooling to <300°C) post-

Al-Mg

Zircon U-Pb Plagioclase Pb-Pb Apatite Pb-Pb Zircon Hf-W Sm/Nd (multi min.) Plag. Ar/Ar (Equilibrated) OPX Ar/Ar (Equilibrated) Plag. Ar/Ar (Cumulate)

impact?

Age (Ma)

4560

4540

4520

NWA 5073 magma. or impact melt crystal. ages

4500

Camel Donga metam. Cooling

*

(<300°C)

cumulate intrusions

Agoult - metam. Cooling ages

4480

4460 0

5

10

15

20

**

4532 Ma Cooling

25

# Analysis Fig. 4: Jourdan et al.

30

4525-4510 Ma Cooling (<300°C) Equilibrated eucrites 35

40

(a) Slow crustal cooling 

(b) Slow crustal cooling (time‐temperature history)

Zircon (Hf/W) Pyroxene (40Ar/39Ar) Plagioclase (40Ar/39Ar)

1000 800 600 400 200

0

20

40

60

80

100

% 39Ar released

Age (Ma)

1200

Temperature (C)

4600 4590 4580 4570 4560 4550 4540 4530 4520 4510 4500

4600 4590 4580 4570 4560 4550 4540 4530 4520 4510 4500

4560

4550

4540

4530

4520

4510

0 4500

Age (Ma)

(C) Impact excavation at 4531 Ma 

(d) Impact excavation at 4531 Ma (time‐temp. history)

1200 1000 800 600 400 200

0

20

40

60

80

100

4560

4550

% 39Ar released

4540

4530

Age (Ma)

Fig. 5: Jourdan et al.

4520

4510

0 4500

Temperature (°C)

Age (Ma)

Figure 5

Number of simulations (/10,000)

Figure 6

17.3 ± 3.6°C/Ma

500 400 300 200 100 0 10.4

12.4

14.4

16.4

18.4

20.4

22.4

Cooling rate (°C/Ma)

Fig. 6: Jourdan et al.

24.4

26.4

28.4

Figure 7

Incresing degree of metamorphism (approximate depth)

Polymict breccia

0 km -

Unequilibrated Equilibrated

Monomict breccia

Gabbroic intrusion Mo.Cou. Moa.

(4531 Ma)

(4533 Ma)

10 km Cald., QUE97, BTN, GRA98, L.Carn. Impact melt sheet

(4523-4510 Ma) EET90

20 km -

Agoult

(4531 Ma)

(4494 Ma)

Fig. 7: Jourdan et al.

Declaration of interests

☒ 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.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: