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9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

The timing of lunar solidification and mantle overturn recorded in ferroan anorthosite 62237

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 Ma (MSWD = 2.0) with an initial ε 143 Nd 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 ε 143 Nd 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.

58 GEOSCIENCES↗
DOE OSTI2023

Chronology of alkali anorthosite 14304 clast “b” records basin forming impact at ~3.95 Ga

Alkali-suite rocks constitute one of three major suites of lunar crustal rocks. As such, constraining their formation timescales and petrogenesis is important for understanding the earliest magmatic history of the Moon. However, the magmatic history of alkali-suite rocks is partly obscured by superimposed effects of major basin-forming impact events on the lunar nearside. Consequently, unambiguous crystallization ages of samples from this suite of rocks have not been determined. Here, the aim of this study is to better understand the petrogenetic history of the alkali-suite and the potential superimposed effects of impact metamorphism by determining Sm-Nd, Rb-Sr, and 40 Ar/ 39 Ar ages for an alkali anorthosite clast from Apollo 14 lithic breccia 14304 termed clast “b”. The new chronologic measurements of clast “b” yield concordant Sm-Nd, Rb-Sr, and 40 Ar/ 39 Ar ages of 3947 ± 13 Ma, 3975 ± 34 Ma, and 3937 ± 37 Ma respectively, resulting in a weighted mean age of 3949 ± 11 Ma. This age is not interpreted to date an igneous event related to production of the lunar highlands crust and instead the chronology and petrography of clast “b” are most readily explained by an impact event at ~3.95 Ga that caused near-complete isotopic re-equilibration of the Sm-Nd, Rb-Sr, and 40 Ar/ 39 Ar chronometers. The weighted mean age of 3949 ± 11 Ma of clast “b” is several hundred Ma younger than 4.3–4.4 Ga ages typically determined for samples of other crustal rock suites but in very good agreement with independent estimates for the formation of Imbrium basin ejecta and other marginally older impact events which are thought to have been sampled at the Apollo 14 landing site. Thus, although petrologic and geochemical examination suggest that clast “b” is a pristine igneous clast, its age likely records an impact event at the Apollo 14 landing site. Whereas the various determined ages of clast “b” do not reflect the timescales of alkali-suite magmatism, the relatively low initial Sr and Nd isotopic compositions of clast “b” indicate that its protolith evolved with very low 147 Sm/ 144 Nd and 87 Rb/ 86 Sr that are distinct from estimates for urKREEP but similar to that of lunar plagioclase. This implies that the igneous protolith of clast “b” derived from a plagioclase-dominated KREEP-rich source that must have formed after the formation of the urKREEP source at ~4.35 Ga but well before the impact event recorded by clast “b” at ~3.95 Ga.

58 GEOSCIENCES↗
DOE OSTI2023

Gallium isotopic constraints for the origin of the Earth-Moon system

Comparatively heavy isotopic compositions of moderately volatile elements (MVE) in lunar rocks have been advocated to reflect the loss of light isotopes during devolatilization processes from the Moon. In this study we present new gallium (Ga) isotope data for lunar highland rocks, with a focus on the Ferroan Anorthosite Suite (FAS). These are commonly thought to be direct crystallization products from the late lunar magma ocean (LMO) and should contain the majority of the Ga inventory of the Moon. As such, FAS rocks are crucial for identifying the processes that drove Ga isotope fractionation as well as for inferring the Ga isotopic composition of the bulk Moon. Our data reveal that FAS samples have a range in δ 71 Ga from -0.27 to 0.22‰ and are generally isotopically light in Ga compared to other lunar lithologies, but straddle values typical of terrestrial rocks. Although Ga is defined as an MVE, these Ga isotope variations do not correspond with concentrations of more volatile elements, indicating that Ga isotope variations in the FAS are not primarily controlled by devolatilization processes. Instead, the Ga isotopic compositions of bulk FAS rocks broadly correlate with the composition of plagioclase, with the calcium content of plagioclase decreasing as Ga becomes isotopically heavier. This suggests that fractionation of Ga isotopes in FAS rocks was caused by the preferential incorporation of isotopically light Ga into plagioclase during the later solidification stages of the LMO. The progressive crystallization and extraction of plagioclase forces the residual melt towards increasingly heavier Ga isotope ratios, corroborating similar conclusions derived from correlations between δ 71 and Eu* in the mare basalt suite. Using Ga isotope partitioning calculations, we demonstrate that an isotope fractionation coefficient between plagioclase and coexisting melt of -0.3 to -0.4‰ could explain the observed range of δ 71 values in FAS, mare basalt suite rocks, and KREEP. These calculations allow for a first order estimate of the Ga isotopic composition of the bulk silicate Moon prior to plagioclase fractionation and suggest it was close to the composition of the bulk silicate Earth. This would imply that the Moon did not lose a substantial fraction of its Ga inventory during accretion, consistent with new constraints from Rb-Sr isotope systematics that indicate the Moon's volatile deficit was primarily inherited from Theia. In conjunction with the overlap in non-mass-dependent isotope ratios, these collective observations could be reconciled if Theia and the proto-Earth formed in similar regions of the inner Solar System that were already volatile-depleted.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

The formation and evolution of the Moon’s crust inferred from the Sm-Nd isotopic systematics of highlands rocks

Ages determined for magnesian and ferroan anorthosite crustal rock suites overlap, suggesting they formed contemporaneously about 4.3–4.5 Ga. A notable exception is the Sm-Nd age previously determined on Mg-suite gabbronorite 67667 which is at least 100 Ma younger than the youngest ferroan anorthosite. New chronologic measurements of 67667 presented here yield concordant Sm-Nd and Rb-Sr mineral isochron ages of 4349 ± 31 Ma and 4368 ± 67 Ma, suggesting the sample is older than previous estimates. Furthermore, a whole rock Sm-Nd isochron of Mg-suite rocks from the Apollo 14, 15, 16, and 17 landing sites yields an age of 4348 ± 25 Ma, indicating that Mg-suite magmatism was widespread and roughly contemporaneous on the lunar nearside. Here, analysis of Sm-Nd internal isochron ages confirms that Mg-suite magmatism was restricted to a period between about 4.33 and 4.35 Ga at the Apollo 14, 15, 16, and 17 landing sites and was synchronous with magmatism at the Apollo 16 site associated with the ferroan anorthosite suite between 4.35 and 4.37 Ga. Magnesian- and ferroan anorthosite suite rocks with ages younger than ~4.33 Ga appear to have experienced slow cooling in the deep lunar interior, so that the ages record when the samples cooled below the closure temperature of the Sm-Nd isotopic system and not the time they crystallized.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide insights into deep (~20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist-to-eclogite facies oceanic- and continental-affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P-T-D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn-subduction return flow along similar P-T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top-to-the-S-SW shear (prograde-to-peak subduction), top-to-the-NE shear (blueschist facies exhumation), and then E-W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi-mineral Rb-Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ~50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ~50–40 and ~43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene-Oligocene subduction channel proceeded at rates of ~1.5–5 mm/yr and accommodated ~80% of the total exhumation of this HP/LT complex.

58 GEOSCIENCES↗
DOE OSTI2020

Materials Data on Rb3Sr by Materials Project

Rb3Sr is Uranium Silicide structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Rb is bonded to eight equivalent Rb and four equivalent Sr atoms to form RbRb8Sr4 cuboctahedra that share corners with twelve equivalent RbRb8Sr4 cuboctahedra, edges with eight equivalent SrRb12 cuboctahedra, edges with sixteen equivalent RbRb8Sr4 cuboctahedra, faces with four equivalent SrRb12 cuboctahedra, and faces with fourteen equivalent RbRb8Sr4 cuboctahedra. All Rb–Rb bond lengths are 4.83 Å. All Rb–Sr bond lengths are 4.83 Å. Sr is bonded to twelve equivalent Rb atoms to form SrRb12 cuboctahedra that share corners with twelve equivalent SrRb12 cuboctahedra, edges with twenty-four equivalent RbRb8Sr4 cuboctahedra, faces with six equivalent SrRb12 cuboctahedra, and faces with twelve equivalent RbRb8Sr4 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on RbSr3 by Materials Project

RbSr3 is Uranium Silicide structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Rb is bonded to twelve equivalent Sr atoms to form RbSr12 cuboctahedra that share corners with twelve equivalent RbSr12 cuboctahedra, edges with twenty-four equivalent SrRb4Sr8 cuboctahedra, faces with six equivalent RbSr12 cuboctahedra, and faces with twelve equivalent SrRb4Sr8 cuboctahedra. All Rb–Sr bond lengths are 4.42 Å. Sr is bonded to four equivalent Rb and eight equivalent Sr atoms to form SrRb4Sr8 cuboctahedra that share corners with twelve equivalent SrRb4Sr8 cuboctahedra, edges with eight equivalent RbSr12 cuboctahedra, edges with sixteen equivalent SrRb4Sr8 cuboctahedra, faces with four equivalent RbSr12 cuboctahedra, and faces with fourteen equivalent SrRb4Sr8 cuboctahedra. All Sr–Sr bond lengths are 4.42 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Rb3Sr by Materials Project

Rb3Sr is alpha bismuth trifluoride structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. there are two inequivalent Rb sites. In the first Rb site, Rb is bonded in a body-centered cubic geometry to four equivalent Rb and four equivalent Sr atoms. All Rb–Rb bond lengths are 4.71 Å. All Rb–Sr bond lengths are 4.71 Å. In the second Rb site, Rb is bonded in a body-centered cubic geometry to eight equivalent Rb atoms. Sr is bonded in a body-centered cubic geometry to eight equivalent Rb atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Rb3Sr by Materials Project

Rb3Sr crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. there are two inequivalent Rb sites. In the first Rb site, Rb is bonded to eight Rb and four equivalent Sr atoms to form RbRb8Sr4 cuboctahedra that share corners with twelve equivalent RbRb8Sr4 cuboctahedra, edges with eight equivalent SrRb12 cuboctahedra, edges with sixteen RbRb8Sr4 cuboctahedra, faces with four equivalent SrRb12 cuboctahedra, and faces with fourteen RbRb8Sr4 cuboctahedra. There are four shorter (4.82 Å) and four longer (4.86 Å) Rb–Rb bond lengths. All Rb–Sr bond lengths are 4.86 Å. In the second Rb site, Rb is bonded to eight equivalent Rb and four equivalent Sr atoms to form RbRb8Sr4 cuboctahedra that share corners with four equivalent RbRb8Sr4 cuboctahedra, corners with eight equivalent SrRb12 cuboctahedra, edges with twenty-four RbRb8Sr4 cuboctahedra, faces with six equivalent SrRb12 cuboctahedra, and faces with twelve RbRb8Sr4 cuboctahedra. All Rb–Sr bond lengths are 4.82 Å. Sr is bonded to twelve Rb atoms to form SrRb12 cuboctahedra that share corners with four equivalent SrRb12 cuboctahedra, corners with eight equivalent RbRb8Sr4 cuboctahedra, edges with eight equivalent SrRb12 cuboctahedra, edges with sixteen equivalent RbRb8Sr4 cuboctahedra, faces with four equivalent SrRb12 cuboctahedra, and faces with fourteen RbRb8Sr4 cuboctahedra.

36 MATERIALS SCIENCE↗