147
Sm-
143Nd and
176Lu-
176Hf systematics of eucrite and angrite meteorites
Audrey BOUVIER
1*, Janne BLICHERT-TOFT
2, Maud BOYET
3, and Francis ALBAR
EDE
21
Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, London, ON N6A 3K7, Canada
2Laboratoire de Geologie de Lyon, CNRS UMR 5276, Ecole Normale Superieure de Lyon and Universite Claude Bernard Lyon
1, 46 Allee d’Italie, 69007 Lyon, France
3Laboratoire Magmas et Volcans, CNRS UMR 6524, Universite Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France *Corresponding author. E-mail: audrey.bouvier@uwo.ca
(Received 14 September 2014; revision accepted 15 August 2015)
Abstract–Comparative planetary geochemistry provides insight into the origin and
evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary
differentiation. We present 147Sm-143Nd and 176Lu-176Hf analyses of eight noncumulate
(basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of
ilmenite. We report a147Sm-143Nd mineral isochron age for the Moama cumulate eucrite of
4519 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal
layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and,
consequently, the erroneous 147Sm-143Nd and 176Lu-176Hf internal mineral isochron ages of
basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.
INTRODUCTION
The formation and internal evolution of planets and planetesimals in the inner solar system are recorded in the petrology, geochemistry, and isotope compositions of meteorites and samples brought back by space missions. Among these, angrites and eucrites are two groups of achondrite meteorites that show evidence of planetary differentiation such as core formation and
mantle–crust differentiation taking place within 5 Ma
after the formation of the solar system (e.g., Kleine et al. 2005a, 2012; Srinivasan et al. 2007). Similar timescales have been suggested for the accretion and differentiation of Mars (Dauphas and Pourmand 2011). Unmelted planetesimals such as the chondrite parent
bodies accreted within 2–10 Myr after
calcium-aluminum-rich inclusions (CAIs; e.g., Bouvier et al. 2007; Henke et al. 2013) when the main source of
radiogenic heat from the decay of short-lived
radionuclides such as 26Al (with a half-life of
~0.705 Ma) was already declining. Generally, igneous differentiation and formation of magma oceans on planets and planetesimals were driven by the release of
radioactive heat from the decay of 26Al and the
gravitational energy of early planetary accretion and differentiation (e.g., Ghosh et al., 2003). The exact timing and processes involved during planetary crust and mantle differentiation are, however, in need of better understanding. Radiogenic isotopes, such as the
146,147
Sm-142,143Nd and 176Lu-176Hf systems, are
1896
powerful isotopic tracers and chronometers of planetary
reservoirs and geological processes. Meteorite
collections host a variety of materials from unmelted and melted planetesimals that include a wide range of parent body compositions and differentiation histories. Early stages of planetary differentiation of many of
these meteorites have been identified by 182Hf-182W and
146
Sm-142Nd systematics (e.g., Kleine et al. 2005a; Boyet
et al. 2010; Touboul et al. 2015). We here present new
data pertaining to the 147Sm-143Nd and 176Lu-176Hf
systems for a suite of eucrite and angrite meteorites that shed new light on the crust formation of their respective parent bodies. The results also show evidence of isotopic disturbances at the mineral scale of the Sm-Nd and Lu-Hf chronometers in both basaltic eucrite and angrite meteorites, while the respective whole-rock
isochrons give ages consistent with solar system
formation. We therefore caution that determination of
the 176Lu decay constant by the internal isochron
method in eucrites and angrites, and perhaps in other achondrite groups as well, may be tenuous.
SAMPLES AND ANALYTICAL TECHNIQUES
We measured the 147Sm-143Nd and 176Lu-176Hf
compositions of eight basaltic eucrites, two cumulate eucrites, and three angrites. The Sahara (SAH) 99555 meteorite was obtained from Labenne Meteorites, while the rest of the meteorites were provided by a number of institutional collections (see Table 1). The complete sample list and collection numbers are given in Tables 1 and 2. All whole-rock samples were crushed at the Ecole Normale Superieure de Lyon (hereafter ENS Lyon) under clean laboratory conditions using a boron carbide mortar exclusively dedicated to meteorite work.
Mineral separates of enriched (>90%) fractions of
plagioclase and pyroxene from some of the eucrites were prepared at Washington State University following the protocol of Bouvier et al. (2005).
The present study reports whole-rock compositions of three basaltic eucrites, Haraiya, Nuevo Laredo, and
Emmaville, and one cumulate eucrite, Binda,
complementing a previous Sm-Nd and Lu-Hf isotopic study of whole-rock eucrite meteorites by Blichert-Toft et al. (2002). Moreover, individual pieces of Bouvante, Juvinas, Millbillillie, Stannern, and Moama, and several pieces of Bereba (all eucrite falls), weighing between 9 and 11 g, were processed for mineral separation and analyzed together with their corresponding whole-rocks. The Bereba fragments had patches of fusion crust as well as numerous glassy veins, all of which were removed by handpicking under the microscope before crushing. We also analyzed the Sm-Nd whole-rock isotopic compositions of three angrites, the quenched
angrites D’Orbigny and Sahara 99555, and the
clinopyroxene cumulate Angra dos Reis (Table 1), and
the Lu-Hf whole-rock isotopic composition of
D’Orbigny (Table 1). The attempts at measuring Hf isotope compositions of Sahara 99555 and Angra dos Reis failed because of the inadequate sensitivity of the first-generation multiple-collector inductively coupled
plasma–mass spectrometer (MC-ICP-MS) VG Plasma
54 used for the present measurements. We followed the
dissolution and chemistry procedures and mass
spectrometer protocols described in Blichert-Toft et al. (1997, 2002) and Bouvier et al. (2005). Additional analytical procedures pertaining to mineral separation and rare earth element (REE) column chemistry not outlined in these papers are described in the following.
Whole-rock powders were ground from inner chips of each meteorite sample. The remainder of each sample was crushed until monomineralic grains were obtained (ignoring microinclusions), typically below 145 and
350 lm for the fine-grained basaltic and coarser grained
cumulate eucrites, respectively. The powders were then sieved with acetone to achieve good separation of the different size fractions. For the basaltic eucrites, the fractions used for pyroxene and plagioclase separation
were 63–100 lm, 63–125 lm, or 100–145 lm depending
on the meteorite. Moama, the sole cumulate eucrite processed for mineral separation, had a coarser texture than the basaltic (noncumulate) eucrites and, hence, the 200–350 lm fraction was selected for mineral separation for this sample. Most magnetic grains (i.e., magnetite) were removed using a hand-magnet. Paramagnetic (i.e.,
pyroxene) and nonmagnetic (i.e., plagioclase and
phosphate) minerals were separated using a Frantz isodynamic magnetic separator. Mineral fractions were concentrated using methylene iodide and bromoform heavy liquids (Geoliquids, IL, USA). Mineral separates were finally handpicked using stainless steel tweezers. Owing to the small grain size of the basaltic eucrites, >98% pure mineral concentrates of pyroxene and plagioclase (some with abundant microinclusions, e.g., Millbillillie) were obtained. Mineral separates (pyroxene and plagioclase) were leached at room temperature for 10 min in 10% HF followed by 10 min in 6M HCl, while whole-rock powders were processed unleached such as not to alter their parent–daughter ratios to be determined by isotope dilution. All whole-rock powders
and leached mineral fractions were subsequently
dissolved using PTFE containers enclosed in high-pressure and -temperature steel-jacketed Parr bombs following the protocols described in Blichert-Toft et al. (2002) and Bouvier et al. (2005).
Rare earth element fractions were separated from
the sample matrices by standard cation-exchange
Table 1. Sm-Nd and Lu-Hf isotopic compositions of eucrites and angrites. Meteorite Split # & collection(#) a Sample Mass (g) [Lu] , ppm b [Hf] ppm b 176 Lu/ 177 Hf b,c 176 Hf/ 177 Hf d eHf i e [Sm] ppm f [Nd] ppm f 147 Sm/ 144 Nd f 143 Nd/ 144 Nd g eNd i e Hf/Sm Basaltic eucrites B er eba 1297 (1) Whole-rock c — 0.261 1.28 0.0289 0.282367 7 0.11 1.75 5.43 0.1952 0.512557 11 0.97 0.73 1297 & 3288 (1) Whole-rock 0.190 0.282452 6 1.98 6.11 0.1961 0.512616 6 0.34 Pyroxene 0.337 0.283583 6 0.354 0.859 0.2494 0.516291 5 + 40 Plagioclase 0.291 0.310 3.69 0.0119 0.280581 3 9.8 0.806 2.54 0.1919 0.512670 6 + 3.2 4.6 Bouvante 3223 (1) Whole-rock c — 0.434 2.53 0.0243 0.281908 3 1.9 3.40 10.8 0.1906 0.512442 7 0.48 0.74 3223 (1) Whole-rock 0.290 0.282014 2 3.12 9.85 0.1914 0.512486 9 0.09 Pyroxene 0.501 0.281491 4 0.628 1.24 0.3065 0.516634 5 + 13 Plagioclase 0.318 0.161 2.28 0.0100 0.280275 2 14 0.640 2.10 0.1845 0.512598 7 + 6.2 3.6 Emmaville DR6979 (8) Whole-rock 0.5 0.295 1.45 0.0289 0.282336 9 1.1 1.98 6.09 0.1966 0.512619 13 0.53 0.73 Haraiya 4062 (6) Whole-rock 0.565 0.262 1.32 0.0282 0.282343 10 + 1.3 1.65 5.03 0.1983 0.512686 13 + 0.24 0.80 Juvinas 40 (1) Whole-rock c — 0.245 1.14 0.0305 0.282471 5 1.5 1.58 4.84 0.1968 0.512647 9 0.39 0.72 40 (1) Whole-rock 0.432 0.282236 3 1.85 5.78 0.1942 0.512570 5 0.36 Pyroxene 0.325 0.285761 3 0.491 0.835 0.3557 0.517428 6 0.70 Plagioclase 0.202 0.039 1.35 0.0041 0.280045 3 4.2 0.234 0.870 0.1624 0.511785 17 + 3.4 5.7 Nuevo Laredo 1783 (7) WR 0.603 0.362 1.81 0.0285 0.282317 10 0.55 2.52 7.76 0.1963 0.512648 9 + 0.21 0.72 Millbillillie 3464 (1) Whole-rock c — 0.262 1.26 0.0296 0.282398 3 1.0 1.73 5.32 0.1963 0.512589 13 0.99 0.72 13188.4 (2) Whole-rock 0.416 0.282395 2 1.71 5.30 0.1953 0.512596 8 0.24 Pyroxene 0.290 0.282308 2 0.308 0.676 0.2760 0.515583 9 + 10 Plagioclase 0.189 0.173 1.84 0.0134 0.280957 4 1.2 0.377 0.847 0.2693 0.515134 9 + 5.6 4.9 Stannern gl. nr. 19 (3) Whole-rock c — 0.415 2.33 0.0253 0.282016 4 1.1 3.19 10.1 0.1910 0.512467 11 0.23 0.73 L6772 (4) Whole-rock 0.312 0.281998 3 2.91 9.20 0.1913 0.512482 6 0.11 Pyroxene 0.306 0.281680 3 0.443 0.867 0.3101 0.516395 8 + 6.15 Plagioclase 0.258 0.058 1.15 0.0071 0.280407 3 0.83 0.307 1.04 0.1790 0.512408 8 + 5.8 3.7 Cumulate eucrite Binda 1554 (1) Whole-rock 0.370 0.092 0.798 0.0164 0.281257 12 0.61 0.536 1.63 0.1987 0.512618 9 1.85 1.49 Moama E12415 (5) Whole-rock c — 0.059 0.076 0.1104 0.289702 14 + 3.3 0.167 0.392 0.2575 0.513817 9 13 0.46 E12415 (5) Whole-rock 0.811 0.289431 5 0.146 0.381 0.2317 0.513705 9 + 0.08 Pyroxene 0.788 0.292385 4 0.177 0.304 0.3523 0.517287 9 0.63 Plagioclase 0.444 — 0.071 0.372 0.1151 0.510184 17 0.38
Table 1. Continued. Sm-Nd and Lu-Hf isotopic compositions of eucrites and angrites. Meteorite Split # & collection(#) a Sample Mass (g) [Lu] , ppm b [Hf] ppm b 176 Lu/ 177 Hf b,c 176 Hf/ 177 Hf d eHf i e [Sm] ppm f [Nd] ppm f 147 Sm/ 144 Nd f 143 Nd/ 144 Nd g eNd i e Hf/Sm Angrites D’Orbigny (4) Whole-rock 0.8 0.262 1.44 0.0259 0.282077 11 1.0 1.74 5.40 0.1953 0.512587 13 0.45 0.83 Sahara 99555 (9) Whole-rock 1.4 —— 2.12 6.69 0.1916 0.512492 6 0.08 0.81 h Angra dos Reis (10) Whole-rock 0.628 —— 6.04 18.1 0.2023 0.512764 6 1.1 0.48 h Data fr om Bliche rt-Tof t et al. (2002 ). a Colle ctions (1) M u seum Nation al d’Hist oire Nature lle, Paris; (2) The W estern Aust ralian Muse um, Perth ; (3) Ge ologisk M useum, Cop enhagen ; (4) Natu rhistorisc hes Muse um, Vienna ; (5) M useum Victo ria, Melb ourne, (6) Americ an M useum of Natu ral Histo ry; (7) Smithsonian Nation al Museu m o f Natu ral History; (8 ) Aust ralian M useum, Sydn ey; (9) Labenn e M eteorites; (10) California Institu te of Tech nology. b Hf and Lu isoto pic compo sition s and co ncentrat ions measured by MC-ICP-M S (VG Plasm a 54). Hf and Lu co ncentrat ions determ ined by isotope dilut ion. [H f], [Lu], and 176 Lu/ 177 Hf < 0.5% (2 r errors). cWhole-ro ck data from Blichert-Toft et al. (2002 ). dUncertain ties repor ted on Hf me asured isotope ratios are 2r /√ n ana lytical errors in last de cimal place, w here n is the numbe r o f measu red isoto pic rat ios. 176 Hf/ 177 Hf norm alized for mass fr actiona tion to 179 Hf/ 177 Hf = 0.7325. 176 Hf/ 177 Hf of JMC-47 5 H f st andard = 0.282 160 10 (2 r ) (i.e., exte rnal rep roducib ility = 35 pp m). Hf standar d run altern ately w ith samples. eeHf i and eNd i are calc ulated for each sample at 4560 M a , and 4520 Ma for M oama, relativ e to the Lu-Hf and Sm-Nd CHUR compo sitions (Bo uvier et al. 2008) . fNd and Sm isoto pic comp ositions and concen trations me asured by MC-ICP-M S (VG Pl asma 54). [Nd ], [Sm], and 147 Sm / 144 Nd < 0.5% (2 r errors). Nd and Sm concen trations determined by isoto pe dilut ion. gUnce rtainties repor ted on Nd measured isoto pe ratios are 2r /√ n analyt ical errors in last decimal place, whe re n is the numbe r o f measu red isotopic ratio s. 143 Nd/ 144 Nd norm alized for mass fractio nation to 146 Nd/ 144 Nd = 0.721 9. 143 Nd/ 144 Nd of La Jolla Nd standar d = 0.511 858 18 (2 r ) (i.e., externa l repro ducibility = 35 ppm). Nd stand ard run altern ately with samples. hHf/Sm from Bizza rro et al. (2012 ) for SAH 9955 5, and fr om Mit tlefehldt and Lindst rom (1990 ) for Ang ra Dos Reis .
Table 2. Compilation of radiometric ages for eucrites and the D’Orbign y angrite. Meteorite 87 Rb-87 Sr 147 Sm-143 Nd Pb-Pb 53 Mn-53 Cr 244 Pu-Xe Ar-Ar Basaltic eucrites B er eba 4.17 0.26 Ga 4 ~ 4.52 Ga 13 4512 18 Ma 15 Bouvante 4.40 0.19 Ga 13 4510 4M a 13 4547 15 Ma 15 Cachari 3.99 0.21 Ga 13 ~ 4.46 Ga 13 3.48 0.04 Ga 10 Caldera 4537 12 Ma 12 Chervony Cut 4.30 0.46 Ga 6 4.70 0.33 Ga 6 4563.6 0.9 Ma 14 1.4 –4.3 Ga 6 Ibitira 4.52 0.25 Ga 4 4555 6M a 9 4557 þ 2 4 Ma 14 4.495 0.015 Ga 11 Juvinas 4.60 0.07 Ga 1 4.46 0.39 Ga (this study) 4539 4 M a (U-Pb) 8 4562.5 1M a 14 4548 23 Ma 15 4.0 –4.5 Ga 10 Millbillillie 4566 24 Ma 15 3.55 0.02 Ga 10 Nuevo Laredo 4514 15 Ma 13 Stannern 3.3 0.5 Ga 4 4.48 0.07 Ga 2 4128 16 Ma 13 4434 13 Ma 15 3.5 0.1 Ga 10 Cumulate eucrites Binda 3.45 0.34 Ga 4544 88 Ma 16 Moama 4.58 0.05 Ga 5 4460 30 Ma 7 4594 79Ma 16 4426 94 Ma 13 4519 34 Ma (this study) Moore County 4456 25 Ma 13 4542 85 Ma 16 4484 19 Ma 13 < 4549 Ma 14 Serra de Mag e 4410 20 Ma 3 4399 35 Ma 13 4553 þ 2 4 Ma 14 Angrites D’Orbigny 4507 89 Ma 18 4563.4 0.3 Ma 19 ~ 4G a 17 1All egre et al. (1975 ), 2Lugmair and Scheinin (1975 ), 3Lu gmair et al. (1977 ), 4Birc k and All egre (1978 ), 5Hamet et al. (1 978), 6Woo den et al. (1979 ), 7Jacobse n and Wasse rburg (1984 ), 8Man h es et al. (1984 ), 9Chen and Wasserb urg (1 985), 10 Ar-Ar age s co mpilated in Bogar d (1995 ), 11 Bogar d and Gar rison (1995 ), 12 Wadhwa and Lugm air (1 996), 13Tera et al. (1997 ), 14 Lugmair and Shuko lyukov (1998), 15Miura et al. (1998 ), 16 Boye t et al. (2010 ), 17 Gar rison and Bogar d (2003 ), 18 Sanb orn et al. (in press), 19 Bren necka and Wadhwa (2 012).
filled with in-house prepared HDEHP-coated Teflon powder to separate successively Nd, Sm, and Lu, the
latter with ~20% Yb. Regrettably, some samples from
the first two batches processed right after column
calibration had contaminated Lu-Yb fractions.
HDEHP’s partition coefficient for Lu is particularly high and a residual memory effect on Lu from the preceding calibration step was identified, but not until most of the samples had already been processed. As a consequence, the first two batches of eucrite whole-rocks and pyroxene separates and part of the third batch of plagioclase separates were compromised for Lu. We were able to check that this incidence had no effect on the Nd and Sm fractions owing to the fact that HDEHP’s partition coefficients for light to medium REEs in 6M HCl (used for cleaning the columns) are
very small. As reflected by the calculated eHfi values,
which are within 1 eHf unit of the chondritic value
(Bouvier et al. 2008) and corresponding whole-rock compositions (Blichert-Toft et al. 2002) at 4560 Ma, plagioclase from Millbillillie and Stannern were not affected by Lu memory from the calibration process. For the ensuing discussion, we will nevertheless be conservative and ignore all the mineral Lu data and therefore also the accompanying Hf isotope data which
are of no use without the corresponding Lu
concentrations determined on the same sample
dissolution. Contamination did not affect Hf, which was not separated by HDEHP, and we report the Hf
isotopic compositions for all samples in Table 1.
Lutetium from the Haraiya, Nuevo Laredo, Emmaville, and Binda whole-rock samples had been separated at an earlier stage on a different set of HDEHP columns, and hence were not affected by this mishap either. For the Lu-contaminated whole-rock samples, we use the Lu-Hf isotope data measured by Blichert-Toft et al. (2002),
which is permissible because the eucrite samples
analyzed by Blichert-Toft et al. (2002) were from the
same rock fragments as those used for mineral
separation (Table 1). The correspondence between the Sm-Nd whole-rock isotope analyses of this study and those of Blichert-Toft et al. (2002) is good, except for Moama, which is a coarse-grained cumulate (Table 1). We will discuss the Sm-Nd eucrite compositions from this study and Blichert-Toft et al. (2002) together.
Spiked Hf, Lu+Yb, Sm, and Nd of eucrites and angrites were analyzed by MC-ICP-MS on the VG Plasma 54 instrument at ENS Lyon using an Aridus desolvating system for Hf and the standard glass expansion nebulizer and spray chamber for Sm, Nd,
and Lu+Yb. Data acquisition and mass bias corrections
are described in Blichert-Toft et al. (2002) and Bouvier et al. (2005). The JMC-475 Hf and La Jolla Nd standards were analyzed in alternation with the samples
and gave, respectively, 0.282160 0.000010 (2r) and
0.511858 0.000018 (2r) during the analytical sessions
of this study. As these mean values are identical within the quoted uncertainties to the accepted values of
0.282163 0.000009 (Blichert-Toft et al. 1997) for
JMC-475 and 0.511858 (Lugmair et al. 1983) for La Jolla, no further corrections were applied to the data. Total procedural blanks for Sm, Nd, Hf, and Lu, with the exception of the samples discussed above, were all less than 20 pg and thus negligible.
RESULTS
The Lu-Hf and Sm-Nd isotope compositions of mineral separates and whole-rocks of the eucrites and angrites of this study are listed in Table 1. For ready comparison, we also listed the data from Blichert-Toft
et al. (2002). The Lu-Hf and Sm-Nd ages were
calculated using the Excel macro Isoplot/Ex version
4.14 from Ludwig (1991) with k176Lu = 1.867
9 1011 yr1(Scherer et al. 2001; S€oderlund et al. 2004)
and k147Sm= 6.54 9 1012 yr1 (Lugmair and Marti
1978). The epsilon notation values were calculated using the Sm-Nd and Lu-Hf CHUR values of Bouvier et al. (2008).
Eucrites
The Sm/Nd and Lu/Hf ratios and Nd and Hf isotope compositions of the newly analyzed basaltic eucrites Emmaville, Haraiya, and Nuevo Laredo fall within the range previously reported by Blichert-Toft et al. (2002), Patchett and Tatsumoto (1980), and Bizzarro et al. (2003) for basaltic eucrites. Their Sm/Nd ratios are near-chondritic. Assuming an age of 4.56 Ga for basaltic
eucrites, the eNdi values of Emmaville and Haraiya are
slightly negative (0.53 and 0.24, respectively), while
slightly positive for Nuevo Laredo (+0.21). These samples
further have slightly subchondritic Lu/Hf with negative
eHfifor Emmaville and Nuevo Laredo (1.1 and 0.55,
respectively) and positive eHfi for Haraiya (+1.3).
The whole-rock sample of the cumulate eucrite Binda
is nearly chondritic in the Sm-Nd system
(147Sm/144Nd= 0.1987 and 143Nd/144Nd= 0.512618,
eNdi = 1.9 assuming an age of 4.52 Ga), which differs
from basaltic eucrites. In contrast, its Lu/Hf is
subchondritic (176Lu/177Hf = 0.0164), which contrasts
with the superchondritic Lu/Hf compositions of other cumulate eucrites previously analyzed by Blichert-Toft et al. (2002) (Fig. 1a). The Sm-Nd isotope analysis of the new fragment of the cumulate eucrite Moama has the highest Sm/Nd of all the analyzed whole-rock eucrites and falls on the trend of the other basaltic and cumulate eucrites (Fig. 1b) from this study and those of
Blichert-Toft et al. (2002). Our new data on Moama are consistent with previous Sm-Nd analyses of Moama by Hamet et al. (1978), Jacobsen and Wasserburg (1984), and Boyet et al. (2010) (Fig. 2). With the exception of the Moama fragment analyzed by Blichert-Toft et al. (2002),
all the eNdi values obtained for whole-rock samples
(from the same collection of chips for all the basaltic eucrites) from this study and Blichert-Toft et al. (2002)
are consistent within 0.7 epsilon units (which we will
use as the external precision on estimates of initial isotopic compositions) calculated using an assumed crystallization age of 4.56 Ga for all basaltic eucrites (Table 1).
The Lu-Hf eucrite whole-rock isochron age using the four new whole-rock samples (Binda, Emmaville, Haraiya, and Nuevo Laredo) together with modern literature data obtained by MC-ICP-MS, including data for 15 basaltic eucrites and five cumulate eucrites from Blichert-Toft et al. (2002) without Palo Blanco and four basaltic eucrites
from Bizzarro et al. (2003), is 4615 39 Ma with an
initial 176Hf/177Hf = 0.279751 30 (MSWD= 8.4)
without Binda, and 4611 38 Ma with an initial
176
Hf/177Hf= 0.279755 29 (MSWD = 8.7) with Binda
(Fig. 1a). For the Sm-Nd system, regressing the 23 basaltic and cumulate eucrites together (9 from this study and 14 from Blichert-Toft et al. 2002) (avoiding duplicates of the same eucrites, and Caldera and Pasamonte which were
excluded in Blichert-Toft et al. 2002) yields an isochron
age of 4532 170 Ma (initial 143Nd/144Nd = 0.50671
0.00023, MSWD= 2.1) (Fig. 1b).
Most of the individual Sm-Nd mineral isochrons (whole-rock plus pyroxene and plagioclase separates) obtained on eucrites (this study and the literature) do not show statistically significant alignments (all have MSWD values higher than 20) due to limited spread of the parent/daughter ratios (see Table 1) and possibly high degrees of isotopic disturbance of pyroxene. Evidence for late perturbation of the Sm-Nd system in pyroxene is provided by the model ages calculated with the initial
solar system 143Nd/144Nd of Bouvier et al. (2008):
although all the measured minerals but Bereba’s
pyroxene approximately fall on an errorchron of 4.57 Ga (Fig. 3), the differences between pyroxene and plagioclase model ages are large and nonsystematic (0.18 to 1 Ga). Pyroxenes have higher Sm/Nd ratios than their respective whole-rocks (Table 1; Figs. 1b and 2). The opposite is found for plagioclase with low, very homogeneous Sm/ Nd (except for Millbillillie, which can be explained by the presence of microinclusions of opaque or pyroxene minerals as recognized by Yamaguchi et al. [1994] and also observed for the present sample during mineral separation). The internal Sm-Nd isochron ages of basaltic
eucrites vary between~4459 Ma for Juvinas and aberrant
“ages” (older than the age of the solar system) without 0.278 0.280 0.282 0.284 0.286 0.288 0.290 0.292 0 0.02 0.04 0.06 0.08 0.10 176 Hf/ 177 Hf 176Lu/177Hf D'O Binda CHUR 143 Nd/ 144 Nd 147Sm/144Nd plagioclase pyroxene whole-rock Eucrites
whole-rock, cumulate eucrite whole-rock, basaltic eucrite
Angrites a) 0.5120 0.5124 0.5128 0.5132 0.5136 0.5140 0.18 0.20 0.22 0.24 Binda D'O SAH ADR CHUR b)
Fig. 1. a) Lu-Hf systematics of the whole-rock angrite D’Orbigny and whole-rock eucrites (Emmaville, Harayia, Nuevo Laredo, and Binda) from this study (see text for further details) compared with data for whole-rock basaltic and cumulate eucrites from the literature (Blichert-Toft et al. 2002; Bizzarro et al. 2003). All whole-rock basaltic eucrites and Binda together give an age of 4587 150 Ma (MSWD = 8) and an initial176Hf/177Hf= 0.279773 0.00008. All basaltic and cumulate eucrites together give
an age of 4615 45 Ma (MSWD = 11) and an initial 176Hf/177Hf= 0.279758 0.000038. b) Sm-Nd systematics of quenched (SAH 99555, D’Orbigny) and plutonic (Angra Dos Reis) whole-rock angrites from this study and basaltic (Bereba, Bouvante, Emmaville, Harayia, Juvinas, Millbillillie, Nuevo Laredo, and Stannern) and cumulate (Binda and Moama) whole-rock eucrites from this study and Blichert-Toft et al. (2002) (excluding duplicated eucrite samples, Caldera, and Pasamonte). Regressing the 23 basaltic and cumulate eucrites together (this study and Blichert-Toft et al. 2002) yields an isochron age of 4532 170 Ma (initial 143Nd/144Nd= 0.50671 0.00023, MSWD = 2.1). For each panel, the CHUR composition and the isochron regression line through the whole-rock eucrite compositions are shown. ADR= Angra Dos Reis, D’O = D’Orbigny, and SAH= Sahara 99555.
geological significance for Bereba, Bouvante, and Millbillillie, all with large MSWD values and all controlled mostly by the pyroxene compositions (Fig. 3). The Sm-Nd isotope data for pyroxene separates from basaltic eucrites show stronger isotopic disturbance than the corresponding plagioclase and whole-rocks when compared to a 4560 Ma solar system reference evolution line (Fig. 3). The six plagioclase fractions from Bereba, Bouvante, Juvinas, Millbillillie, Stannern, and Moama
give an errorchron with an age of 4.81 0.35 Ga and an
initial143Nd/144Nd of 0.50660 0.00044 (MSWD = 30),
i.e., within the error bars of the parameters of the whole-rock isochron, while pyroxene scatter greatly in the Sm-Nd isochron diagram (Fig. 3). Only the cumulate eucrite Moama gives a meaningful and statistically
significant Sm-Nd internal isochron age of 4521 34 Ma
and an initial 143Nd/144Nd of 0.506736 000045
(MSWD = 1.4) (Fig. 2), which is a younger but more
precise age than those previously reported by Hamet et al. (1978), Jacobsen and Wasserburg (1984), and Boyet et al. (2010). When all the Sm-Nd data reported for Moama are regressed together, the Sm-Nd mineral
isochron age of Moama becomes 4512 90 Ma with
143
Nd/144Ndi= 0.50681 0.00015, and a higher MSWD
of 16 (Fig. 2). There was not enough Hf in the plagioclase fraction of Moama to obtain a three-point Lu-Hf mineral isochron.
Angrites
The 147Sm/143Nd ratios of the three whole-rock
samples of Angra Dos Reis, D’Orbigny, and SAH 99555 fall between 0.1916 and 0.2023. We were only able to measure the Lu-Hf isotopic composition of the
D’Orbigny whole-rock and found a distinctly
subchondritic 176Lu/177Hf of 0.0259 (Fig. 1a). The
initial eNdi (0.08 to 1.1) and eHfi (1.0)
compositions calculated at 4563 and 4557 Ma, which are the corrected Pb-Pb crystallization ages obtained for quenched and Angra Dos Reis angrites, respectively (Brennecka and Wadhwa 2012), are chondritic for D’Orbigny and SAH 99555 to slightly subchondritic for Angra Dos Reis.
DISCUSSION
Disturbance of Mineral Isotopic Compositions
When calculating a radiometric mineral age of an igneous event, it is assumed that the closure of the relevant chronometric system for all coexisting minerals takes place simultaneously and that the system remains closed thereafter. Meteorites that were ejected from part of a larger disrupted parent body were probably subjected to several impact events subsequent to their
crystallization, such that ejected meteoroids may
contain a history of those events. Different
chronometric systems react differently to chemical 0.508 0.510 0.512 0.514 0.516 0.518 0.05 0.15 0.25 0.35 0.45 143
Nd/
144Nd
147Sm/
144Nd
Moama (this study) Sm-Nd age = 4519 ± 34 Ma 143Nd/144Nd i= 0.506736 ± 0.000045 MSWD = 1.3 plag WR px
Fig. 2. Sm-Nd internal isochron age of 4519 34 Ma (MSWD= 1.3) obtained from pyroxene and plagioclase separates and whole-rock analyses of this study (red symbols) of the cumulate eucrite Moama. For comparison, analyses of plagioclase, pyroxene, and whole-rocks of different fragments of Moama by Hamet et al. (1978), Jacobsen and Wasserburg (1984), and Boyet et al. (2010) are also shown (gray symbols). Together, the Sm-Nd errorchron age of Moama is 4512 90 (MSWD= 16), which is consistent with but less precise than the internal isochron age reported here.
Béréba Bouvante Juvinas Millbillillie Stannern Moama Béréba Bouvante Stannern Juvinas Millbillillie Moama 0.510 0.512 0.514 0.516 0.518 0.10 0.20 0.30 0.40 143
Nd/
144Nd
147Sm/
144Nd
4568 M a CHURFig. 3. Sm-Nd isotopic compositions of eucrite pyroxene (diamonds) and plagioclase fractions (squares) analyzed in this study indicate disturbance of the Sm-Nd isotope system in basaltic eucrites at the mineral scale. The solid line is the chondritic evolution at 4568 Ma. The intercept of the gray dashed lines corresponds to the CHUR composition (Bouvier et al. 2008).
diffusion and resetting because of different elemental diffusion rates and depending on whether these element pair systems are based on parent/daughter (e.g., Rb-Sr) or daughter-only (Pb-Pb) isotopic compositions. A
compilation of the ages given by the long-lived
87
Rb-87Sr, 147Sm-143Nd, and 207Pb-206Pb and extinct
53
Mn-53Cr and 244Pu-Xe chronometers of eucrites from
the literature (Table 2) shows a broad range, well outside the analytical uncertainties and even the cooling interval of a molten asteroid. As seen from Table 2, most of the long-lived chronometers used to date the formation of eucrites and, to a lesser extent, angrites show large variations and inconsistencies. The U-Pb and Pb-Pb ages typically are between 4.51 and 4.54 Ga for the basaltic eucrites (Table 2) (Manhes et al. 1984; Tera et al. 1997; Wadhwa and Lugmair 1996); Cachari and Stannern have younger and imprecise ages at 4.46 and 4.12 Ga (Tera et al. 1997), respectively. Zircon U-Pb ages (Misawa et al. 2005) indicate that basaltic eucrites formed both early in solar system history and
over a relatively short period of ~15 Ma. Even U-Pb
and Hf-W isotope systematics of zircons, considered the most resistant to disturbance (if not metamict; Cherniak 1993), show evidence that some zircons equilibrated or re-equilibrated at a rather late stage (Misawa et al. 2005; Srinivasan et al. 2007; Hopkins et al. 2015). The Rb-Sr, Sm-Nd, and Ar-Ar dates are the most disturbed ranging between 1.4 and 9.2 Ga for eucrites (Table 2). Impact events at 3.54 and 3.64 Ga were recently identified by Ar-Ar chronometry in the brecciated
anomalous basaltic achondrite Bunburra Rockhole
(Jourdan et al. 2014). High-pressure polymorphs of mineral phases (Miyahara et al. 2014), glass veins, and brecciated textures observed in the howardite, eucrite, and diogenite (HED) suite physically bear witness to impacts disrupting the crust and forming a regolith at the surface of 4 Vesta (Metzler et al. 1995), the asteroid held to be the HED parent body. Impact craters and basins as well as ejecta blankets were observed by spacecraft during the Dawn mission when it visited 4 Vesta (Jaumann et al. 2012).
Cumulate eucrites have variable Pb-Pb ages that are 70–150 Ma younger than basaltic eucrites with
4399 35 Ma for Serra de Mage, 4426 94 Ma for
Moama, and 4484 19 Ma for Moore County (Tera
et al. 1997), but older apparent Sm-Nd ages of
4544 88 for Binda and 4542 85 Ma for Moore
County (Boyet et al. 2010). Variable Sm-Nd ages have
been obtained for Moama, 4466 42 Ma (Jacobsen
and Wasserburg 1984), 4520 50 Ma (Hamet et al.
1978), 4594 79 Ma (Boyet et al. 2010), and
4519 34 Ma (this study) indicating possible sample
heterogeneity though three of four studies suggest that Moama could have crystallized later than basaltic
eucrites. This would be consistent with protracted cooling of deep-seated cumulate eucrites allowing for a longer period of exchange between mineral phases (Metzler et al. 1995). Alternatively, cumulates may have been excavated by a late impact, in which case the Sm-Nd chronometer could have been reset instantaneously. This, however, is at odds with the cumulate eucrites, Moama in particular (Lovering 1975), which lack any evidence of shock effects (e.g., brecciation, mineral transformation).
The Sm-Nd and Lu-Hf whole-rock isochrons of
cumulate and basaltic eucrites indicate ages of ~4.6 Ga
(Figs. 1a and 1b), but the Sm-Nd data for the six pyroxene–plagioclase pairs do not consistently plot on their respective whole-rock isochrons for basaltic eucrites (Figs. 1 and 2). For angrites, Sahara 99555 indicates an
anomalous~4.8 Ga Lu-Hf age interpreted to reflect early
irradiation processes in the protoplanetary disk (Bizzarro et al. 2012). This result has not been confirmed by other Lu-Hf internal isochrons of angrites (Sanborn et al. 2012). The most straightforward explanation for the inconsistent results of the mineral and whole-rock isochron ages for both the Sm-Nd and Lu-Hf isotope systems, and for other chronometers as well (Figs. 2 and 4 and Table 2), is open-system behavior at the mineral scale in eucrites and angrites.
Inconsistent ages are found between different
studies and different chronometers, and there is
disagreement on the time interval between the
formation of basaltic and cumulate eucrites. A period of ~100 Ma was initially deduced from the Sm-Nd whole-rock (basaltic and cumulate) eucrite isochron giving an
age of 4464 75 Ma controlled by the fractionated
Sm/Nd of the cumulate eucrites (Blichert-Toft et al. 2002), and also from Pb-Pb dates (Tera et al. 1997). When including in the regression the new eucrite data of this study, we now find a Sm-Nd whole-rock
(basaltic and cumulate) isochron age of 4673 130 Ma,
which no longer distinguishes the cumulate eucrite group from the basaltic eucrites. The Lu-Hf ages for both basaltic and cumulate eucrites from Blichert-Toft et al. (2002), which were calculated using the “old”
decay constant for Lu (k176
Lu = 1.93 9 1011yr1)
become ~150 Ma older when using the current accepted
value of 1.8679 1011yr1 (Scherer et al. 2001;
S€oderlund et al. 2004). In addition, anomalous 176
Hf excesses leading to apparent ages older than the age of the solar system and low initial values were found in whole-rock chondrites and eucrites (Blichert-Toft et al. 2002; Bizzarro et al. 2003) as well as in the SAH 99555 angrite (Bizzarro et al. 2012). Again, with our new
(basaltic and cumulate) eucrite whole-rock Lu-Hf
isotope data, and using k176Lu= 1.867 9 1011yr1,
uncertainties of the corresponding Sm-Nd whole-rock eucrite age and the age of formation of the solar system
(Figs. 1 and 4). Moreover, we find an initial 176Hf/177Hf
of 0.279755 0.000029, which is consistent with the
value of 0.279793 0.000019 obtained for the
whole-rock chondrite Lu-Hf isochron of Bouvier et al. (2008)
and the value of 0.279718 0.000029 obtained for
chondrites by Blichert-Toft and Albarede (1997, 1998). Sanborn et al. (in press) reporting on Lu-Hf internal isochrons of angrites confirm this initial Hf isotope ratio for the solar system based on data for the
quenched angrite D’Orbigny (4510 97 Ma and initial
176
Hf/177Hf = 0.279790 0.000043) and the plutonic
angrite NWA 4801 (4563 50 Ma and initial
176
Hf/177Hf = 0.279759 0.000046), although another
angrite, NWA 4590, indicates mineral isochron
disturbances and a higher initial ratio (4523 24 Ma
and initial 176Hf/177Hf= 0.279908 0.000036) (the
corresponding 147Sm-143Nd ages are listed in Table 2).
We therefore do not find evidence of an anomalous Lu-Hf isochron slope among whole-rock eucrites.
Our more precise Sm-Nd internal isochron for the
cumulate eucrite Moama with an age of 4519 34 Ma
(MSWD = 1.3) constrains the time interval between
basaltic and cumulate eucrite crystallization to
40 40 Ma (Fig. 2). This Sm-Nd age could represent
the end of equilibration of this rock buried deep in 4 Vesta’s crust or a late period of magmatic activity on the eucrite parent body. The age gap is consistent with Pb-Pb and Hf-W data on basaltic and cumulate eucrites, which indicate a time interval of at least 50 Ma between the crystallization of the two groups (Tera et al. 1997; Touboul et al. 2015). The apparent Sm-Nd and Pb-Pb age differences (Table 2) found between basaltic and cumulate eucrites may be the consequence of late igneous activity or heterogeneous thermal disturbances by impacts on the parent body. The eucrite parent body subsequently experienced several late events of thermal and shock metamorphism (Metzler et al. 1995) as testified to by the howardite group, also surmised to originate from 4 Vesta, which are all polymict breccias with mixtures of eucrite and diogenite lithologies (of presumed 4 Vesta origin as well). Howardites have Ar-Ar ages indicating a major period of disturbance between 3.3 and 3.8 Ga (Cohen 2013). Eucrites (and diogenites) can also be polymict or monomict breccias of the individual and corresponding lithologies and have a broader range of Ar-Ar ages
(Table 2). The <4.5 Ga ages found in some eucrites
(whether basaltic or cumulate) likely reveal late
disturbance by shock metamorphism due to numerous and strong impacts visible on the surface of 4 Vesta, which left vast impact craters, particularly one 400 km in diameter, that could have excavated deeper seated
rocks from the planetesimal. It is important to
emphasize that while impact is a process that may not have affected the whole series of eucrite and diogenite meteorites, late magmatism and thermal metamorphism from cooling of a magma ocean would be recorded on a global scale for such planetesimals. The observations of shock-melt veins and high-pressure polymorph phases (coesite, stishovite, and maskelynite) (Miyahara et al. 2014), brecciation (Metzler et al. 1995), and foreign chondritic clasts (e.g., Zolensky et al. 1992) found within the HED suite also indicate that dynamic events
have affected many of these rocks since their
crystallization. The claims from experiments that shock effects cannot affect radiometric ages (e.g., Gaffney et al. 2011; Bloch and Ganguly 2014) do not take into account that phase transitions may be solid–liquid–solid transitions (e.g., formation of glass or maskelynite with schlieren indicating flow and not a solid–solid phase transformation). Nor do they consider the duration of peak shock pressure or the formation of high-pressure polymorphs such as those found in natural samples. The fact that some achondrites, including eucrites
(Table 2), have Rb-Sr, Sm-Nd, and U-Pb ages <4.5 Ga
demonstrates that these radiometric systems can indeed be affected by late thermal processes unrelated to the cooling of their respective parent bodies.
0.279 0.281 0.283 0.285 0.287 0.289 0.291 0.00 0.02 0.04 0.06 0.08 0.10 0.12 176
Hf/
177Hf
176Lu/
177Hf
all eucrite WR Lu-Hf age = 4611±38 Ma 176Hf/177Hf 0 = 0.279755 ± 0.000029 MSWD = 8.7 whole -rock eucrit es WR1&2 Ol Ol Px1&2 CHUR slope = 0.08990±0.00076 slope = 0.09516±0.00069 SAH 99555
Fig. 4. Lu-Hf whole-rock isotope systematics of the cumulate (red circles) and basaltic (black circles) eucrites from this study and the literature (Blichert-Toft et al. 2002; Bizzarro et al. 2003), and mineral separates (pyroxene and olivine fractions) and whole-rock data for the SAH 99555 angrite (blue squares) (Bizzarro et al. 2012). While Pb-Pb ages of basaltic eucrites and quenched angrites are similar at 4560–4563 Ma, the slopes of the Lu-Hf isochrons differ by a time span equivalent to ~300 Ma, which suggests isotopic disturbances of mineral fractions as observed in basaltic eucrites for both the Lu-Hf and Sm-Nd isotope systems (see Figs. 1a and 1b and Fig. 3).
A more rapid magmatic evolution history is inferred for angrites. The fine-grained quenched angrites (e.g.,
D’Orbigny) have been dated precisely at ~4563 Ma,
while the coarse-grained plutonic angrites (e.g., LEW
88516) have been dated at ~4557–4558 Ma using U-Pb
systematics (Amelin 2008; Brennecka and Wadhwa 2012). An anomalous group of metal-rich angrites includes NWA 2999 and NWA 6291 with reported Pb-Pb “intermediate” ages between those of the quenched
and plutonic angrites at ~4560 Ma (Amelin and Irving,
2007; Bouvier et al. 2011; Brennecka and Wadhwa 2012). Only these anomalous plutonic angrites (NWA 2999 and NWA 6291) are brecciated and appear to contain a significant chondritic component (Gellissen et al. 2007) on the basis of their bulk chemistry and Hf-W isotope systematics (Markowski et al. 2007; Kleine et al. 2009). Nevertheless, the Ar-Ar systematics of
D’Orbigny indicate an age of~4 Ga and significant loss
of 40Ar (Garrison and Bogard 2003). The Sm-Nd
and Lu-Hf ages for D’Orbigny are 4507 89 Ma
(MSWD = 16) and 4510 97 Ma (MSWD = 1.9, with
an initial 176Hf/177Hf of 0.279790 0.000043)
respectively (Sanborn et al. in press). These results
contrast with the apparent ~4.87 Ga Lu-Hf age for
SAH 99555 reported by Bizzarro et al. (2012). The Lu-Hf isotope systematics of SAH 99555 are compared with the Lu-Hf whole-rock eucrite isochron in Fig. 4. The difference in slope corresponds to an apparent ~300 Ma discrepancy between SAH 99555 and eucrites. This apparent age difference is inconsistent with the ~4560 Ma Pb-Pb ages of zircons from basaltic eucrites
(Misawa et al. 2005) and the precise 4563.37 0.25 Ma
Pb-Pb age (adjusted for its U isotopic composition) for the D’Orbigny angrite (Amelin 2008; Brennecka and Wadhwa 2012). It has been suggested that anomalous Lu-Hf ages found in meteorites may be the consequence of irradiation processes in the early solar system (Albarede et al. 2006; Bizzarro et al. 2012). There is, however, no evidence of such processes affecting the whole-rock isochron of eucrites presented here (Figs. 1a and 4). The quenched angrite D’Orbigny provides evidence of mineral disturbance recorded by the Ar-Ar, Sm-Nd, and Lu-Hf isotope systems on the angrite parent body (Garrison and Bogard 2003; Sanborn et al. in press), but these redistributions were not recorded in the Pb-Pb, Hf-W, Al-Mg, or Mn-Cr isotope systematics of either quenched or plutonic angrites (Amelin 2008; Spivak-Birndorf et al. 2009; Kleine et al. 2012). The most plausible explanation of the inconsistent results of internal U-Pb, Sm-Nd, and Lu-Hf isochrons is open-system behavior of some mineral phases in the eucrites and angrites. Late thermal events associated with impacts also are recorded by the Ar-Ar and Sm-Nd chronometers in shocked chondrites (Bogard 1995;
Bogomolov et al. 2013; Swindle et al. 2014), and by the Ar-Ar, Al-Mg, and U-Pb isotope systems in the anomalous achondrite Bunburra Rockhole (Jourdan et al. 2014). The various isotope systems thus react
differently to late disturbances caused by impact
processes depending on their mineral carriers and ambient physical conditions (pressure, temperature, peak shock duration, and recrystallization via a solid or melt phase).
By documenting the chronology of eucrites and angrites from this study and the literature, we have shown how secondary processes such as impacts can affect various chronometers in planetary objects. As a consequence, any attempt at dating rocks for which petrological observations indicate shock effects becomes
uncertain. Likewise, inferring the initial 176Hf/177Hf of
the solar system from eucrite and angrite mineral isochrons is not a robust approach.
Differentiation of Crustal Reservoirs on the Eucrite and Angrite Parent Bodies
The accretion and differentiation of some
planetesimals took place very early in solar system history, as early as less than 1 Ma after the formation of CAIs, for the parent bodies of magmatic iron
meteorites as deduced from 182Hf-182W systematics
(Kleine et al. 2005b; Kruijer et al. 2014) and Pb
isotopes (Blichert-Toft et al. 2010). Excesses in 26Mg
produced by the short-lived radionuclide 26Al have been
found in bulk samples of basaltic and cumulate eucrites, mesosiderites (e.g., Bizzarro et al. 2005; Schiller et al. 2010), and mineral isochrons of angrites (Baker et al. 2005; Spivak-Birndorf et al. 2009), and are in agreement
with 53Mn-53Cr chronology (Lugmair and Shukolyukov
1998), indicating that the magma sources of these rocks formed within 3 Ma of the beginning of the solar system defined here as the age of CAI formation at ~4568 Ma (Bouvier et al. 2007; Bouvier and Wadhwa
2010). This was possible because the internal
temperature of the planetary bodies of eucrites and angrites increased from the radiogenic heat produced by
the decay of 26Al until reaching metamorphic and
melting temperatures in the internal parts of
the planetesimals (e.g., Bizzarro et al. 2005).
Hafnium-tungsten isotope systematics further indicate
that metal–silicate fractionation took place within
~2 Ma after CAIs for the angrite parent body and
3.8 1.3 Ma after CAI formation for basaltic eucrites
(Kleine et al. 2004, 2009). Mantle–crust differentiation
may have started as early as 1 Ma after core formation
(Quitte and Birck 2004). The 182
Hf-182W isotope
systematics in eucrite zircons indicate a time of
differentiation and a subsequent ~9 Ma period of thermal metamorphism on the eucrite parent body (Srinivasan et al. 2007). Metamorphic processes could have lasted as long as 30 Ma in the shallow layers of 4 Vesta as attested to by U-Pb and Pb-Pb ages of zircon domains in basaltic eucrites (Misawa et al. 2005; Zhou et al. 2013; Hopkins et al. 2015). This period may correspond to the end of thermal metamorphism associated with cooling of the magma ocean and its overlying crust, but the finding of zircons with different subdomain ages within the same eucrites suggest that such late diffusion and corresponding age records were probably caused by the physical effects (temperature and pressure) of impacts (Hopkins et al. 2015). This mechanism was also inferred from Hf-W isotope systematics of basaltic eucrite metals (Kleine et al. 2005a; Touboul et al. 2015) and disturbed zircons
(Srinivasan et al. 2007). The absence of 182W variations
among cumulate eucrites suggests a much later
formation, at least 50 Ma after the basaltic eucrites, and thus a protracted magmatic history of the eucrite parent body (Touboul et al. 2015). Our internal isochron age of Moama (Fig. 3) supports a late crystallization period for cumulate eucrites, which formed within deeper layers than basaltic eucrites on their common parent body. Cumulate eucrites are rarely brecciated and hence are less likely than basaltic eucrites to have been affected by early impact events.
If formed 50 Ma apart, the genetic relationships between basaltic and cumulate eucrite magmas remain uncertain (e.g., Treiman 1997; Barrat et al. 2000). Although cumulate eucrites share a common mineralogy with basaltic eucrites (clinopyroxene and orthopyroxene; plagioclase; and minor amounts of metal, troilite, chromite, phosphate, ilmenite, and zircon), the former is the sole known group with an adcumulate gabbroic texture (Treiman 1997). Differences in major element
compositions, based on TiO2and FeO*/MgO, and trace
element abundances, separate cumulate and noncumulate eucrites and also divide the noncumulate eucrites into two distinct chemical series, the Nuevo Laredo and the Stannern trends, respectively (e.g., Barrat et al. 2000, 2007). The Sm-Nd and Lu-Hf isotope systematics
indicate additional compositional differences. The
cumulate eucrites have high Lu/Hf and Sm/Nd (Blichert-Toft et al. 2002), with the exception of Binda (this study), compared to basaltic eucrites, which are near to slightly subchondritic within one epsilon unit (Figs. 1a and 1b).
In Table 1, we report the calculated Hf/Sm ratios of eucrites and angrites of this study and Blichert-Toft et al. (2002). In Fig. 5, we compare these data with Hf/ Sm ratios for quenched (SAH 99555, LEW 87051) and plutonic (Angra Dos Reis, LEW 86010, NWA 4590, NWA 4801) angrites from Mittlefehldt and Lindstrom
(1990), Bizzarro et al. (2012), and Sanborn et al. (in press). The geochemical compositions of whole-rock basaltic eucrites and the quenched angrites (D’Orbigny, SAH 99555, and LEW 87051) and one plutonic angrite (LEW 86010) indicate that their Hf/Sm ratios are similar to the mean value of unequilibrated whole-rock
chondrites of 0.74 0.03 (2SE, n = 30) (Bouvier et al.
2008). The cumulate eucrites, except Binda, and the plutonic angrite Angra Dos Reis have subchondritic Hf/
Sm (Moama Hf/Sm= 0.45 and ADOR Hf/Sm = 0.48).
In contrast, Binda and the two plutonic angrites NWA 4590 and 4801 have high, superchondritic Hf/Sm. Binda with Hf/Sm of 1.5 attests to petrogenetic processes different from those of the other cumulate eucrites (Table 1). Likewise, NWA 4590 and NWA 4801 have higher Hf/Sm than other plutonic angrites with values of, respectively, 1.25 and 1.12. Because Sm and Hf are
characterized by similar incompatibilities during
melting, the Hf/Sm ratio is relatively insensitive to
magmatic differentiation until ilmenite begins to
crystallize, presumably at a rather late stage. In lunar magmas, ilmenite starts crystallizing from the late-stage liquid when 89–95% of the magma ocean has solidified (Hess and Parmentier 1995; Van Orman and Grove 2000). Remelting of ilmenite-rich cumulates also may play a role in the variability of Ti contents and Hf/Zr ratios in basalts. The high-field strength elements (Zr, Hf, Nb, and Ta) and some transition metals (V, Cr, Co,
Sn) are moderately compatible in iron–titanium oxides
such as ilmenite, whereas other elements, including REEs and W, are strongly incompatible (Klemme et al. 2006). The observed differences in Hf/Sm ratios between cumulate and noncumulate eucrites therefore have been
0 2 4 6 8 10 12 14 16 18 chondrites Hf/Sm N basaltic eucrites cumulate eucrites plutonic angrites quenched angrites ADOR LEW86
NWA 4801 NWA 4590 Binda
Fig. 5. Histogram showing the distribution of 38 Hf/Sm ratios for whole-rock basaltic and cumulate eucrites (from Table 1) and quenched and plutonic angrites (D’Orbigny from Table 1; SAH 99555 from Bizzarro et al. 2012; LEW 87051, Angra Dos Reis, and LEW 86010 from Mittlefehldt and Lindstrom 1990; NWA 4590 and NWA 4801 from Sanborn et al. in press). The range of compositions of Hf/Sm = 0.74 0.03 (2SE, n= 30) of bulk unequilibrated chondrites is from Bouvier et al. (2008) and shown with a gray bar for reference.
attributed to the role of ilmenite in fractionating Hf from Sm during the formation of cumulate eucrites (Blichert-Toft et al. 2002). Based on trace element characteristics (e.g., Barrat et al. 2000), basaltic eucrites
appear to represent large (>10%) degrees of melting of
a chondritic source involving pyroxene, plagioclase, and olivine (Blichert-Toft et al. 2002). The mineralogy of angrites is ultramafic with clinopyroxene, olivine, and minor amounts of plagioclase compared to eucrites but, nevertheless, quenched and plutonic angrites seem to follow petrogenetic paths similar to those of basaltic
and cumulate eucrites, respectively. The replicate
analyses of the same whole-rock eucrites (Table 1)
indicate that the eNdi and eHfi values may vary within
0.7 e units, also dependent on the age of last isotopic equilibration, and thus the eucrites and angrites seem to derive from reservoirs that had a globally chondritic evolution in terms of Sm-Nd and Lu-Hf isotope
systematics. The near-chondritic Hf/Sm ratios of
basaltic eucrites and angrites indicate that their parent melts derived from a source that had not undergone fractionation of ilmenite or other phases potentially able to fractionate Hf from Sm (e.g., phosphate, zircon). The small range of Sm/Nd and Lu/Hf and identical Hf/Sm of basaltic eucrites and quenched angrites (Fig. 5) indicate that they formed by large degrees of melting of a near-chondritic source and subsequently underwent fractional crystallization without ilmenite fractionation such as to conserve unfractionated Hf/Sm ratios. It has been suggested that the Stannern and Nuevo Laredo
eucrite trends, which show different degrees of
enrichment in incompatible elements and have different
TiO2concentrations, could be explained by assimilation
of small degrees of partial melts derived from a typical main-group eucrite (e.g., Juvinas) source (Barrat et al. 2000). We observe that the eucrites from these two trends have identical, nearly chondritic Hf/Sm ratios (Barrat et al. 2007), which means that the observed
excesses in TiO2are not due to the presence of ilmenite.
The composition of the cumulate eucrites and plutonic angrites are more variable and more complex to interpret. Binda, NWA 4590, and NWA 4801 have superchondritic Hf/Sm, while Angra Dos Reis has subchondritic Hf/Sm (Fig. 5) suggesting that these two groups derived from either ilmenite-poor or -rich sources or that ilmenite was fractionated in or out during crystallization. Cumulate eucrites have been suggested to be the product of fractional crystallization of the main group and Nuevo Laredo trend of basaltic eucrites (Treiman 1997). Most of the cumulate eucrites (besides Binda) have low Hf/Sm suggesting that ilmenite
crystallized out and was extracted by fractional
crystallization during their formation, while Binda records enrichment in ilmenite. The plutonic LEW
86010 is anomalous in that it is the only plutonic angrite with Hf/Sm (0.82) similar to all the quenched angrites and basaltic eucrites. These observations point to possible layering and ilmenite segregation during magmatic differentiation of both the eucrite and angrite parent bodies. The segregation of ilmenite took place from previous parental melts. Despite the low-gravity fields of these relatively small parent bodies, ilmenite somehow accumulated and produced these chemically distinct elemental signatures. There are thus at least three viable crustal reservoirs for the eucrite groups and we propose a similar petrogenetic model for angrites.
The difference in source composition and the ~50 Ma age gap between basaltic and cumulate eucrites based on Sm-Nd (this study) and Hf-W (Touboul et al. 2015) isotope systematics suggest that the two groups did not originate by simple fractional crystallization but rather underwent a protracted history of igneous differentiation. This long duration of magmatism is difficult to reconcile with the rapid timescales involved in the crystallization of a magma ocean on the eucrite parent body if analogous to 4 Vesta (Schiller et al. 2010; Mandler and Elkins-Tanton 2013). Moreover, the magma ocean cooling model does not constrain the petrogenesis of diogenites (Barrat and Yamaguchi 2014). A similar history but more rapid evolution is inferred for quenched and plutonic angrites, which
formed over 6 Ma (from 4563.4 0.3 Ma for
D’Orbigny to 4557.0 0.3 Ma for NWA 4801;
Brennecka and Wadhwa 2012) after the accretion of a possibly smaller parent body than that proposed for eucrites. For both meteorite groups, early magmatic processes therefore included fractional crystallization and subsequent formation and isolation of mantle and
crustal reservoirs over ~6 Ma for the angrite parent
body, and over 40 Ma for the eucrite parent body. CONCLUSIONS
We analyzed eucrite and angrite whole-rocks and separated minerals from eucrites for Sm-Nd and Lu-Hf
isotope compositions. We found that whole-rock
Sm-Nd and Lu-Hf isochrons define reasonable ages at ~4.6 Ga in contrast to the highly disturbed mineral ages for basaltic eucrites. We also found a statistically significant Sm-Nd mineral isochron with an age of
4519 34 Ma for the Moama cumulate eucrite,
suggesting a gap of about 40 40 Ma between the
formation of basaltic and cumulate eucrites. The difference in Hf/Sm among basaltic eucrites, cumulate eucrites, and Binda, and the age gap, further suggest that cumulate eucrites may have formed by late remelting of eucrite sources and fractional crystallization processes. We propose a similar petrogenetic model for angrites.
Quenched angrites display sub- to chondritic Sm-Nd and Lu-Hf evolution as also recorded by basaltic eucrites, while plutonic angrites share similarities with cumulate eucrites. The Hf/Sm ratios of angrites indicate that three potential crustal reservoirs were produced by chemical differentiation. These are embodied by the compositions of the clinopyroxene cumulate Angra Dos Reis (similar to cumulate eucrites), quenched angrites (similar to basaltic eucrites), and plutonic angrites (NWA 4590 and NWA 4801; similar to the Binda cumulate eucrite). The internal Lu-Hf isochron of SAH 99555 (Bizzarro et al. 2012) is inconsistent with the whole-rock Lu-Hf isochron of eucrites and angrites presented together here. The disturbed mineral ages of eucrites and angrites suggest
that minerals were affected by thermal diffusion
subsequent to impact events on their respective parent bodies. This precludes the use of these meteorite groups to constrain precisely the Lu-Hf system parameters of the solar system and the bulk silicate earth.
Acknowledgments—We are grateful to the curators of the
different meteorite collections who provided the
meteorites for this study (Museum d’Histoire Naturelle
de Paris, Naturhistorisches Museum in Vienna,
Smithsonian National Museum of Natural History,
American Museum of Natural History, Western
Australian Museum in Perth, Museum Victoria in Melbourne, Australian Museum in Sydney, Geologisk
Museum in Copenhagen, California Institute of
Technology, and Labenne Meteorites). We further thank Philippe Telouk for assistance with the VG Plasma 54, Chantal Douchet for maintenance of the clean lab, and Jeff Vervoort and Bill McClelland for providing access to
mineral separation facilities at Washington State
University and University of Idaho. J. Blichert-Toft and F. Albarede acknowledge financial support from the French Institut National des Sciences de l’Univers and the Programme National de Planetologie. We also thank AE G. Srinivasian and EC T. Jull for handling this manuscript, and E. Scherer, A. Yamaguchi, and an anonymous reviewer for constructive suggestions.
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