Abstract
Mass-dependent stable isotopic variations recorded in lunar samples provide novel resolution to the formation and differentiation history of the Moon. In this study, we report new ...high-precision Ca-isotope measurements for lunar rocks and minerals. Ca-isotope data and modeling of the lunar magma ocean together demonstrate indistinguishable mass-dependent Ca isotopic compositions of the bulk silicate Earth and Moon. This implied Earth-Moon isotope equilibration is consistent with the Moon’s high-energy giant-impact (Synestia) origin and not readily compatible with the traditional giant-impact models. Moreover, a cross-comparison between Ca and Mg isotopic data for an important anorthosite sample (60025) consistently clarifies its formation near the completion of the lunar magma ocean crystallization. Therefore, the various existing radiometric dating for 60025 sets the lunar magma ocean to have fully solidified by either 4.51 or 4.38 billion years ago, constraining the two respective lunar differentiation timescales to <30 (short-lived) or ~130–150 (long-lived) million years.
Magnesium isotopic compositions are reported for twenty‐four international geological reference materials including igneous, metamorphic and sedimentary rocks, as well as phlogopite and serpentine ...minerals. The long‐term reproducibility of Mg isotopic determination, based on 4‐year analyses of olivine and seawater samples, was ≤ 0.07‰ (2s) for δ26Mg and ≤ 0.05‰ (2s) for δ25Mg. Accuracy was tested by analysis of synthetic reference materials down to the quoted long‐term reproducibility. This comprehensive dataset, plus seawater data produced in the same laboratory, serves as a reference for quality assurance and inter‐laboratory comparison of high‐precision Mg isotopic data.
Les compositions isotopiques du magnésium sont fournies pour vingt‐quatre matériaux géologiques de référence internationaux, comprenant des roches ignées, métamorphiques et sédimentaires, ainsi qu'une phlogopite et des serpentines. La reproductibilité à long terme de la détermination isotopique du Mg, basée des analyses sur quatre ans d’échantillons d'olivine et d'eau de mer, était ≤ 0.07% (2s) pour δ26Mg et ≤ 0.05% (2s) pour δ25Mg. La précision a été testée par l'analyse de matériaux de référence synthétiques jusqu’à la reproductibilité à long terme indiquée. Cette base de données complète, ainsi que des données d'eau de mer produites dans le même laboratoire, servent de référence pour l'assurance qualité et la comparaison inter‐laboratoires de haute précision des données isotopiques du Mg.
A new type of potentiometric sensor based on a recently constructed carbon ionic liquid electrode (CILE) is described. Two kinds of ionic liquids, i.e., N‐octylpyridinium hexafluorophosphate (OPFP) ...and 1‐butyl‐3‐methylimidazoluim hexafluorophosphate (BMFP) were tested as binder for construction of the carbon composite electrode. The characteristics of these electrodes as potentiometric sensors were evaluated and compared with those of the traditional carbon paste electrode (CPE). The results indicate that potentiometric sensors constructed with ionic liquid show an increase in performance in terms of Nernstian slope, selectivity, response time, and response stability compared to CPE.
To evaluate the interlaboratory mass bias for high‐precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were ...analyzed in five laboratories by using three types of multicollector inductively coupled plasma mass spectrometer (MC‐ICPMS). Magnesium isotopic compositions from all labs are in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3‰ in 26Mg/24Mg, >4 times of uncertainties) different for some mafic samples. The interlaboratory mass bias does not correlate with matrix element/Mg ratios, and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well‐calibrated standards with matrix elements matching samples should be used to reduce the interlaboratory mass bias.
Key Points:
First interlaboratory comparison of Mg isotopic data of geostandards
Analysis of standards with composition ranging from ultramafic to felsic
We report high-precision Mg isotopic analyses of different types of lunar samples including two pristine Mg-suite rocks (72415 and 76535), basalts, anorthosites, breccias, mineral separates, and ...lunar meteorites. The Mg isotopic composition of the dunite 72415 (δ25Mg = −0.140 ± 0.010‰, δ26Mg = −0.291 ± 0.018‰), the most Mg-rich and possibly the oldest lunar sample, may provide the best estimate of the Mg isotopic composition of the bulk silicate Moon (BSM). This δ26Mg value of the Moon is similar to those of the Earth and chondrites and reflects both the relative homogeneity of Mg isotopes in the solar system and the lack of Mg isotope fractionation by the Moon-forming giant impact. In contrast to the behavior of Mg isotopes in terrestrial basalts and mantle rocks, Mg isotopic data on lunar samples show isotopic variations among the basalts and pristine anorthositic rocks reflecting isotopic fractionation during the early lunar magma ocean (LMO) differentiation. Calculated evolutions of δ26Mg values during the LMO differentiation are consistent with the observed δ26Mg variations in lunar samples, implying that Mg isotope variations in lunar basalts are consistent with their origin by remelting of distinct LMO cumulates.
Magnesium isotopic compositions of 22 well-characterized differentiated meteorites including 7 types of achondrites and pallasite meteorites were measured to estimate the average Mg isotopic ...composition of their parent bodies and evaluate Mg isotopic heterogeneity of the solar system. The δ26Mg values are −0.236‰ and −0.190‰ for acapulcoite–lodranite and angrite meteorites, respectively and vary from −0.267‰ to −0.222‰ in the winonaite–IAB-iron silicate group, −0.369‰ to −0.292‰ in aubrites, −0.269‰ to −0.158‰ in HEDs, −0.299‰ to −0.209‰ in ureilites, −0.307‰ to −0.237‰ in mesosiderites, and −0.303‰ to −0.238‰ in pallasites. Magnesium isotopic compositions of most achondrites and pallasite meteorites analyzed here are similar and reveal no significant isotopic fractionation. However, Mg isotopic compositions of D′Orbigny (angrite) and some HEDs are slightly heavier than chondrites and the other achondrites studied here. The slightly heavier Mg isotopic compositions of angrites and some HEDs most likely resulted from either impact-induced evaporation or higher abundance of clinopyroxene with the Mg isotopic composition slightly heavier than olivine and orthopyroxene. The average Mg isotopic composition of achondrites (δ26Mg=−0.246±0.082‰, 2SD, n=22) estimated here is indistinguishable from those of the Earth (δ26Mg=−0.25±0.07‰; 2SD, n=139), chondrites (δ26Mg=−0.28±0.06‰; 2SD, n=38), and the Moon (δ26Mg=−0.26±0.16‰; 2SD, n=47) reported from the same laboratory. The chondritic Mg isotopic composition of achondrites, the Moon, and the Earth further reflects homogeneity of Mg isotopes in the solar system and the lack of Mg isotope fractionation during the planetary accretion process and impact events.
High alkali (Na2O + K2O) low SiO2 lavas generally have highly fractionated trace element compositions and their petrogenesis is debated. Both pyroxene-rich (pyroxenite) and olivine-rich (peridotite) ...mantle sources have been proposed for such lavas. HIMU (high 238U/204Pb) lavas usually have such compositions and are often interpreted as being derived from garnet-pyroxenite (eclogite) residue of subducted recycled oceanic crust, possibly including a carbonate component. In some cases, an origin from a garnet peridotite source has also been considered.
We present new Mg, Pb, Nd, and Sr isotopic data and elemental data for alkalic ultrabasic lavas (melilitites, nephelinites) from the Oslo Rift, Norway. The magmatic province of the Permo-Carboniferous Oslo Rift is interpreted as part of the much larger Skagerrak-Centered Large Igneous Province (SCLIP) which may be related to a mantle plume source originating from the African Large Low Shear Velocity Province (LLSVP) at the core-mantle boundary. The range of δ26Mg is narrow (−0.32 to −0.28) and is similar to typical mantle values, and therefore is inconsistent with a carbonated source as has been suggested for other similar lavas. The 206Pb/204Pb values of all the lavas are high, which identifies them as HIMU lavas. The εNd of +1 to +2 are substantially lower than for typical HIMU lavas, and in the εNd-εSr diagram they plot on the LoNd array between HIMU and EMI. The lavas are more like archetypal kimberlites (formerly, Group I kimberlites), as these also have εNd slightly higher than the chondritic value and Pb isotopes consistent with a HIMU source. The REE patterns of the lavas are also almost as fractionated as in Group I kimberlites.
We used major and trace elements to infer the primary magma composition of these lavas. This primary magma composition is compared with experimental melts of both carbonated peridotite and carbonated eclogite source compositions and shows that their major element compositions are only consistent with either a very high degree of melting of eclogite or a very low degree melting of garnet peridotite. The strongly fractionated REE patterns of the lavas can only be made by low-degree melting of either eclogite or garnet peridotite, which therefore effectively rules out eclogite as a major component of the mantle source of these lavas. Further trace element modeling points to a mantle source composition with >97% garnet peridotite that was lightly carbonated (<0.5 wt% CO2) and melted to a very low degree (∼0.4%) to produce these lavas. The modeled source trace element pattern was constrained by the Nd, Sr, and Pb isotopic compositions of the lavas. The measured high Ti contents of these lavas (3.8–6 wt%) are quantitatively shown to be the result of enrichment by both partial melting and fractional crystallization, using experimentally determined DTi for melting of this type of source. We also show that a low degree of melting of a slightly carbonated source is sufficient to result in the likely very high CO2 content (10–15 wt%) of the primary magma.
The rather uniform Pb isotope composition of HIMU magmas suggests that the HIMU source is rather uniform in composition, but the Pb isotope composition requires this source to form late in Earth's history. The similarity of these lavas and kimberlites suggests a very deep source, possibly in the LLSVP that gave origin to the SCLIP. This means that LLSVPs do not represent some primordial layer around the core that escaped homogenization of the mantle by the Moon-forming giant impact. Instead, this layer must have formed late in Earth's history. The constraint that this source contains <3% of garnet-pyroxenite (eclogite) residue of recycled basaltic oceanic crust shows that this layer is primarily not a subducted slab graveyard. The process by which the LLSVPs formed, however, remains obscure.
•Highly alkalic Oslo Rift lavas exhibit radiogenic, HIMU Pb isotope signatures.•The HIMU-like Olso Rift lavas are derived from a carbonated garnet peridotite source.•206Pb/204Pb isotopes are not a tracer of a pyroxenite-source lithology.•HIMU-like Pb isotope signatures may have origins in the LLSVP.
The main objectives of this thesis are to estimate Mg isotopic compositions of the Moon and achondrites, to understand the behavior of Mg isotopes during magmatic differentiation processes in ...different planetary bodies, and to evaluate the extent of Mg isotopic heterogeneity in the solar system. In order to achieve these goals, Mg isotopes have been measured for 47 well-characterized lunar samples and 22 differentiated meteorites by MC-ICPMS. The limited Mg isotopic variations among mare and highland regolith, mare breccias, and highland impact-melt rocks reflect negligible Mg isotope fractionation during lunar surface processes (e.g. solar wind, cosmic rays, micrometeorite bombardments, meteorite impacts, etc.). However, the significant Mg isotopic variation (~0.628 ‰ for δ 26Mg) between high- and low-Ti basalts suggests the source heterogeneity produced during the lunar magmatic differentiation. High abundance of ilmenite with lighter Mg isotopic composition than coexisting olivine and pyroxene may cause lighter Mg isotopic composition of high-Ti basalts. The δ 26Mg values of differentiated meteorites, including 7 types of achondrites and pallasites, range from -0.318 ‰ to -0.183 ‰. The significant variation of Mg isotopic compositions of these meteorites versus their major chemical compositions suggests that the isotopic variation in achondrites is caused by different mineralogical sources produced during the magmatic differentiation of their parent bodies. Overall, the average Mg isotopic compositions of the Moon (δ26Mg = 0.259 ± 0.162 ‰) and achondrites (δ26Mg = -0.260 ± 0.046 ‰) estimated in this study are identical to those of the Earth (δ 26Mg = -0.25 ± 0.07 ‰) and chondrites (δ 26Mg = -0.28 ± 0.06 ‰), indicating a homogeneous Mg isotopic distribution in the solar system. Magnesium is a moderately refractory element, and unlike Fe and Si cannot be fractionated during the planetary core formation. Therefore, homogeneous distribution of its stables isotopes implies the lack of the separation and sorting of chondrules objects in protoplanetary disk during the solar system formation. It also suggests negligible Mg isotopic fractionation by volatilization during the Moon-forming giant impact.
Recent high-precision isotopic measurements show that the isotopic similarity of Earth and Moon is unique among all known planetary bodies in our Solar System. These observations provide fundamental ...constraints on the origin of Earth–Moon system, likely a catastrophic Giant Impact event. However, in contrast to the isotopic composition of many elements (e.g., O, Mg, Si, K, Ti, Cr, and W), the Fe isotopic compositions of all lunar samples are significantly different from those of the bulk silicate Earth. Such a global Fe isotopic difference between the Moon and Earth provides an important constraint on the lunar formation – such as the amount of Fe evaporation as a result of a Giant Impact origin of the Moon. Here, we show through high-precision Fe isotopic measurements of one of the oldest lunar rocks (4.51±0.10 Gyr dunite 72 415), compared with Fe isotope results of other lunar samples from the Apollo program, and lunar meteorites, that the lunar dunite is enriched in light Fe isotopes, complementing the heavy Fe isotope enrichment in other lunar samples. Thus, the earliest olivine accumulation in the Lunar Magma Ocean may have been enriched in light Fe isotopes. This new observation allows the Fe isotopic composition of the bulk silicate Moon to be identical to that of the bulk silicate Earth, by balancing light Fe in the deep Moon with heavy Fe in the shallow Moon rather than the Moon having a heavier Fe isotope composition than Earth as a result of Giant Impact vaporization.
•We report Fe isotope compositions of lunar meteorites and Apollo samples.•Lunar dunite 72 415 is enriched in light Fe isotopes, unlike any other lunar samples.•The heavy Fe isotope in the shallow Moon can be balanced with light Fe in the deep.•The Lunar Magma Ocean differentiation has fractionated the Fe isotopes of the Moon.•The Fe isotope composition of the Moon is most likely identical to that of the Earth.
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