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Ferroan anorthosite suite (FAS) rocks are widely interpreted to represent primordial lunar crust. Despite their importance in pinpointing the timing of lunar crust formation, robust chronological investigations for this rock type are scarce. Here, we report the Ar-Ar, Rb-Sr, and Sm-Nd isotopic systematics for the FAS troctolitic anorthosite 62237. The Ar-Ar isotopic system has been reset by a thermal event at 3710 ± 48 Ma, and the Rb-Sr isotopic systematics has been disturbed such that a Rb-Sr isochron age cannot be determined. However, an internal isochron for the Sm-Nd isotopic system has yielded an age of 4350 ± 73 Mamore » (MSWD = 2.0) with an initial ε¹⁴³Nd_CHUR of -0.53 ± 0.26. The mineral and whole-rock fractions of 62237 plot on the same internal isochron as FAS sample 60025. The combined datasets define an age of 4372 ± 35 Ma (MSWD = 4.0) with an initial ε¹⁴³Nd_CHUR of -0.17 ± 0.22. Literature Sm-Nd data for FAS and Mg-suite whole-rocks also plot on the 60025-62237 isochron. The coherence of data from both FAS and Mg-suite rocks examined thus far suggests that both rock suites formed contemporaneously from identical, or nearly identical, sources. In addition, the concordance of FAS and Mg-suite ages suggests that primordial crust solidification either involved both magmatic suites, or that Mg-suite magmatism was contemporaneous with FAS magmatism within resolution of the Sm-Nd chronometer. The ages for FAS and Mg-suite also coincide with the formation ages of the mare basalt source regions and urKREEP. Ferroan anorthosite suite rocks and urKREEP are thought to represent primordial LMO solidification products, whereas Mg-suite and the mare basalt source regions are argued to represent mixtures of various LMO crystallization products that were formed during density-driven overturn of the LMO. The concordance of ages implies that the 4372 ± 35 Ma Sm-Nd isochron records the age of mantle overturn, and that overturn occurred during, or shortly after, solidification of the LMO.« less

The mantle of Mars probably differentiated through the crystallization of a magma ocean during the first tens of million years (Ma) of Solar System evolution. However, the exact timescale of large-scale silicate differentiation of the martian mantle is debated, and in particular, it remains unclear when differentiation commenced. In this paper, we applied the short-lived ⁵³Mn-⁵³Cr system to martian meteorites in order to date the onset of large-scale mantle differentiation on Mars. The new Cr isotope data demonstrate that martian meteorites exhibit no resolvable radiogenic ⁵³Cr variations, and instead have a uniform +20.2±1.2 (95% conf.) parts-per-million excess in ⁵³Cr/⁵²Cr relativemore » to the terrestrial mantle. The investigated groups of martian meteorites are lithologically varied and derive from diverse mantle sources that probably had variable Mn/Cr. Hence, the lack of ⁵³Cr variability among martian meteorites demonstrates that silicate differentiation on Mars occurred after the extinction of ⁵³Mn. Provided that the sources of the martian meteorites have Mn/Cr variations that are typical of the terrestrial planets, this result implies that the onset of large-scale silicate differentiation must have occurred later than 20±5 Ma after Solar System formation. The onset of silicate differentiation on Mars inferred here is significantly later than time estimates for segregation of the martian core which conservatively occurred within <10 Ma after Solar System formation. Thus, the new Mn-Cr data imply that there was a small, but resolvable, time gap of at least 5 Ma between core formation and magma ocean solidification on Mars. If the age of core segregation is taken at face value, our results imply that the martian magma ocean remained mostly molten over several Ma. This inferred longevity of the magma ocean is inconsistent with thermal models predicting rapid (<1 Ma) solidification of the martian magma ocean. Although there is currently no unique solution to this conundrum, our results can potentially be explained by a protracted history of impact bombardment that delayed differentiation in a shallow magma ocean on Mars, or perhaps more readily, by the presence of an early and dense atmosphere that acted as an insulator and prevented the magma ocean from cooling quickly.« less

Herein, mare basalt sources and ferroan anorthosite suite cumulates define a linear array on a ¹⁴⁶Sm/¹⁴⁴Nd versus ¹⁴²Nd/¹⁴⁴Nd isochron plot demonstrating these materials were derived from a common reservoir at 4336⁺³¹/_–32 Ma. The minimum proportion of the Moon that was in isotopic equilibrium at this time is estimated to be 1-3% of its entire volume based on the geographic extent from which the analyzed samples were collected and the calculated depths from which the samples were derived. Scenarios in which large portions of the Moon were molten to depths of many hundreds of kilometers are required to produce the observedmore » Sm-Nd isotopic equilibrium between the mantle and crustal rocks at 4.34 Ga. This is a consequence of the fact that limited heating of a solid Moon above the blocking temperature of the Sm-Nd isotopic system is insufficient to diffusively homogenize radiogenic Nd throughout the mantle and crust. There are three scenarios that might account for global-scale isotopic equilibrium on the Moon relatively late in Solar System history including: (1) Sm-Nd re-equilibration of a solid Moon resulting from widespread melting in response to mantle overturn or a very large impact, (2) early accretion of the Moon followed by delayed cooling due to the presence of an additional heat source that kept a large portion of the Moon molten until 4.34 Ga, or (3) late accretion of the Moon followed by rapid cooling of the magma ocean late in Solar System history. Neither density-driven overturn of the mantle, nor a large impact, are likely to homogenize the mantle and crust to the extent required by the Sm-Nd isochron. Likewise, secondary heating mechanisms, such as tidal heating or radioactive decay, are not efficient enough to keep the Moon molten to the depth of the mare basalt source regions for many tens to hundreds of millions of years. Instead, the age of equilibrium between such a compositionally diverse set of rocks, produced on a global scale, likely records the time of primordial solidification of the Moon from a magma ocean. This scenario accounts for both the petrogenetic characteristics of lunar rock suites, as well as their Sm-Nd isotopic systematics. It is supported by the preponderance of ~4.35 Ga ages obtained for other hypothetical magma ocean crystallization products, such as ferroan anorthosite suite rocks and K, REE, and P enriched cumulates that are thought to represent flotation cumulates of the magma ocean and the last vestiges of magma ocean solidification, respectively.« less

We report that zinc isotopes fractionate during evaporation, and thus can potentially be used to calculate the proportion of volatile elemental loss from objects such as tektites, nuclear fallout melt glasses formed from silicate soils, and rocks from the Moon. The utility of the Zn isotope system in constraining the magnitude of volatile loss depends on accurate knowledge of its fractionation behavior during evaporation, i.e., the Zn isotope fractionation factor. In this work, we present new results of Zn isotope analyses of experimentally heated soil samples, together with analyses of environmentally derived fallout melt glasses, and australite tektites, to bettermore » constrain how and to what extent Zn isotopes fractionate during thermally-driven evaporation.« less

High precision Sm-Nd isotopic analyses have been completed on a suite of 11 martian basaltic meteorites in order to better constrain the age of silicate differentiation on Mars associated with the formation of their mantle sources. Our data is used to evaluate the merits and disadvantages of various mathematical approaches that have been employed in previous work on this topic. Ages determined from the Sm-Nd isotopic systematics of individual samples are strongly dependent on the assumed Nd isotopic composition of the bulk planet. This assumption is problematic given differences observed between the Nd isotopic composition of Earth and chondritic meteoritesmore » and the fact that these materials are both commonly used to represent bulk planetary Nd isotopic compositions. Ages determined from the slope of Sm-146-Nd-142 whole rock isochrons are not dependent on the assumed Nd-142/Nd-144 ratio of the planet, but require the sample suite to be derived from complementary, contemporaneously-formed reservoirs. In this work, we present a mathematical expression that defines the age of formation of the source regions of such a suite of samples that is based solely on the slope of a Nd-143-Nd-142 whole rock isochron and is also independent of any a priori assumptions regarding the bulk isotopic composition of the planet. This expression is also applicable to mineral isochrons and has been used to successfully calculate Nd-143-Nd-142 model crystallization ages of early refractory solids as well as lunar samples. This permits ages to be obtained using only Nd isotopic measurements without the need for Sm-147/Nd-144 isotope dilution determinations. When used in conjunction with high-precision Nd isotopic measurements completed on martian meteorites this expression yields an age of formation of the martian basaltic meteorite source regions of 4504 +/- 6 Ma. Because the Sm-Nd model ages for the formation of martian source regions are commonly interpreted to record the age at which large scale mantle reservoirs formed during planetary differentiation associated with magma ocean solidification, the age determined here implies that magma ocean solidification occurred several tens of millions of years after the beginning of the Solar System. Recent thermal models, however, suggest that Mars-sized bodies cool rapidly in less than similar to 5 Ma after accretion ceases, even in the presence of a thick atmosphere. In assuming these models are correct, an extended period of accretion is necessary to provide a mechanism to keep portions of the martian mantle partially molten until 4504 Ma. Late accretional heating of Mars could either be associated with protracted accretion occurring at a quasi-steady state or alternatively be associated with a late giant impact. If this scenario is correct, then accretion of Mars-sized bodies takes up to 60 Ma and is likely to be contemporaneous with the core formation and possibly the onset of silicate differentiation. This further challenges the concept that isotopic equilibrium is attained during primordial evolution of planets, and may help to account for geochemical evidence implying addition of material into planetary interiors after core formation was completed.« less