Lunar basalt chronology, mantle differentiation and implications for determining the age of the Moon

Despite more than 40 years of studying Apollo samples, the age and early evolution of the Moon remain contentious. Following the formation of the Moon in the aftermath of a giant impact, the resulting Lunar Magma Ocean (LMO) is predicted to have generated major geochemically distinct silicate reservoirs, including the sources of lunar basalts. Samples of these basalts, therefore, provide a unique opportunity to characterize these reservoirs. However, the precise timing and extent of geochemical fractionation is poorly constrained, not least due to the difficulty in determining accurate ages and initial Pb isotopic compositions of lunar basalts. Application of an in situ ion microprobe approach to Pb isotope analysis has allowed us to obtain precise crystallization ages from six lunar basalts, typically with an uncertainty of about ±10Ma, as well as constrain their initial Pb-isotopic compositions. This has enabled construction of a two-stage model for the Pb-isotopic evolution of lunar silicate reservoirs, which necessitates the prolonged existence of high-μ reservoirs in order to explain the very radiogenic compositions of the samples. Further, once firm constraints on U and Pb partitioning behaviour are established, this model has the potential to help distinguish between conflicting estimates for the age of the Moon. Nonetheless, we are able to constrain the timing of a lunar mantle reservoir differentiation event at 4376±18Ma, which is consistent with that derived from the Sm–Nd and Lu–Hf isotopic systems, and is interpreted as an average estimate of the time at which the high-μ urKREEP reservoir was established and the Ferroan Anorthosite (FAN) suite was formed

Ancient volcanism on the Moon: Insights from Pb isotopes in the MIL 13317 and Kalahari 009 lunar meteorites

Lunar meteorites provide a potential opportunity to expand the study of ancient (>4000 Ma) basaltic volcanism on the Moon, of which there are only a few examples in the Apollo sample collection. Secondary Ion Mass Spectrometry (SIMS) was used to determine the Pb isotopic compositions of multiple mineral phases (Ca-phosphates, baddeleyite K-feldspar, K-rich glass and plagioclase) in two lunar meteorites, Miller Range (MIL) 13317 and Kalahari (Kal) 009. These data were used to calculate crystallisation ages of 4332 ±2Ma (95% confidence level) for basaltic clasts in MIL 13317, and 4369 ±7Ma (95% confidence level) for the monomict basaltic breccia Kal 009. From the analyses of the MIL 13317 basaltic clasts, it was possible to determine an initial Pb isotopic composition of the protolith from which the clasts originated, and infer a 238U/204Pb ratio (μ-value) of 850 ±130(2σ uncertainty) for the magmatic source of this basalt. This is lower than μ-values determined previously for KREEP-rich (an acronym for K, Rare Earth Elements and P) basalts, although analyses of other lithological components in the meteorite suggest the presence of a KREEP component in the regolith from which the breccia was formed and, therefore, a more probable origin for the meteorite on the lunar nearside. It was not possible to determine a similar initial Pb isotopic composition from the Kal 009 data, but previous studies of the meteorite have highlighted the very low concentrations of incompatible trace elements and proposed an origin on the farside of the Moon. Taken together, the data from these two meteorites provide more compelling evidence for widespread ancient volcanism on the Moon. Furthermore, the compositional differences between the basaltic materials in the meteorites provide evidence that this volcanism was not an isolated or localised occurrence, but happened in multiple locations on the Moon and at distinct times. In light of previous studies into early lunar magmatic evolution, these data also imply that basaltic volcanism commenced almost immediately after Lunar Magma Ocean (LMO) crystallisation, as defined by Nd, Hf and Pb model ages at about 4370Ma

The timing of basaltic volcanism at the Apollo landing sites

Precise crystallisation ages have been determined for a range of Apollo basalts from Pb-Pb isochrons generated using Secondary Ion Mass Spectrometry (SIMS) analyses of multiple accessory phases including K-feldspar, K-rich glass and phosphates. The samples analysed in this study include five Apollo 11 high-Ti basalts, one Apollo 14 high-Al basalt, seven Apollo 15 low-Ti basalts, and five Apollo 17 high-Ti basalts. Together with the samples analysed in two previous similar studies, Pb-Pb isochron ages have been determined for all of the major basaltic suites sampled during the Apollo missions. The accuracy of these ages has been assessed as part of a thorough review of existing age determinations for Apollo basalts, which reveals a good agreement with previous studies of the same samples, as well as with average ages that have been calculated for the emplacement of the different basaltic suites at the Apollo landing sites. Furthermore, the precision of the new age determinations helps to resolve distinctions between the ages of different basaltic suites in more detail than was previously possible. The proposed ages for the basaltic surface flows at the Apollo landing sites have been reviewed in light of these new sample ages. Finally, the data presented here have also been used to constrain the initial Pb isotopic compositions of the mare basalts, which indicate a significant degree of heterogeneity in the lunar mantle source regions, even among the basalts collected at individual landing sites

Provenance record of Laurentian passive-margin strata in the northern Caledonides: Implications for paleodrainage and paleogeography

Siliciclastic sequences accumulated along the eastern margin of Laurentia during the late Neoproterozoic to early Paleozoic as a result of the breakup of Rodinia and formation of the Iapetus Ocean. Detrital zircon data show considerable variability in provenance of time-equivalent units that has implications for paleodrainage and paleogeography. Samples from northwest Scotland and northeast Greenland show detrital zircon U-Pb age groupings dominated by Archean and Paleoproterozoic populations consistent with derivation largely from the West Greenland segment of the North Atlantic craton. In Scotland, timeequivalent outboard sedimentary sequences show contrasting Archean and Proterozoic populations, including a substantial ca. 1.1-1.0 Ga component, indicative of derivation from the Grenville orogen to the southwest. These contrasting paleodrainage patterns, consistent with paleocurrent data, must have developed during the early phase of passive-margin thermal subsidence and may have been accentuated by remnant rift shoulders. In Newfoundland and the U.S. Appalachians, time-equivalent sedimentary sequences are located within the hinterland of the Grenville orogen and are dominated by ca. 1.1-1.0 detritus with very few preMesoproterozoic grains. The Grenville deformation front in these areas may have constituted a drainage divide that limited sediment input from the cratonic interior.</p

Early palaeozoic orogenesis along the Indian margin of Gondwana: Tectonic response to Gondwana assembly

SHRIMP U-Pb dating of zircons from a peralkaline S-type Lesser Himalayan granite from the Kathmandu region, Nepal indicate an age of emplacement of 475 Ma. The granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from Archaean to early Palaeozoic with prominent late Mesoproterozoic (Grenvillian) and Neoproterozoic (Pan-African) age peaks and have a maximum age of 500 Ma based on the youngest detrital grains. Deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a Cambro-Ordovician orogenic event, herein termed the Bhimphedian Orogeny that can be traced across the Himalaya from Pakistan to the eastern Himalaya and possibly extends west into Afghanistan. We interpret the orogeny as being related to Andean-type orogenic activity on the northern margin of the Indian continent, following Gondwana assembly. The magmatic arc was associated with andesitic and basaltic volcanism and was active from c. 530 to 490 Ma. The arc activity overlaps with, and is succeeded by, regional deformation, crustal melting and S-type granite emplacement that extended to 470 Ma. Orogenic activity was driven by coupling across the plate margin either during on-going subduction or through accretion of microcontinental ribbons, possibly represented by the Lhasa and Qiangtang blocks. It represents the termination of an orogenic cycle, termed the North Indian Orogen that commenced with Neoproterozoic rifting and passive margin development and terminated with the Bhimphedian Orogeny. This was succeeded by a return to passive margin setting along the northern Indian margin of Gondwana which continued until the Cenozoic Himalayan Orogeny. (c) 2007 Elsevier B.V. All rights reserved.</p

High-spatial-resolution geochronology

High-spatial-resolution isotope analyses have revolutionised U–(Th–)Pb geochronology. These analyses can be done at scales of a few tens of microns or less using secondary ion mass spectrometry or laser ablation inductively coupled plasma mass spectrometry. They allow determination of the internal age variation of uranium- and thorium-bearing minerals and as a consequence much greater understanding of Earth system processes. The determination of variation on the micron scale necessitates the sampling of small volumes, which restricts the achievable precision but allows discrimination of discrete change, linkage to textural information, and determination of multiple isotopic and elemental data sets on effectively the same material. High-spatial-resolution analysis is being used in an increasing number of applications. Some of these applications have become fundamental to their scientific fields, while others have opened new opportunities for research

Rapidity of orogenesis in the Paleoproterozoic Halls Creek Orogen, northern Australia; evidence from SHRIMP zircon data, CL zircon images, and mixture modeling studies

Combining U-Pb SHRIMP zircon geochronology with cathodoluminescence imaging enables the resolution of temporally closely-spaced geological events important for understanding tectonothermal processes in the Paleoproterozoic Halls Creek Orogen of northern Australia. The youngest detrital zircon grains from a low-grade quartz-muscovite psammite of the Tickalara Metamorphics have a 207 Pb/ 206 Pb SHRIMP age of 1864+ or -4 Ma, defining a maximum depositional age for the unit. Zircon crystals from a high-grade garnet-biotite metapelite ( approximately 5-10 volume percent leucosome) from the same sequence are considerably more complex, and SHRIMP analyses form a single, large concordant group in the range approximately 1885 to 1830 Ma. The zircon crystals contain three distinct CL zoning patterns, and individual SHRIMP spots show a corresponding variation in 207 Pb/ 206 Pb ages. Concentric oscillatory-zoned zircon dominates pre-1850 Ma ages and is interpreted as detritus from igneous source rocks. Narrow structureless zircon rims infrequently overgrow and truncate the oscillatory zoning. These rims comprise most of the post-1850 Ma analyses and are inferred to be the product of uppermost amphibolite facies metamorphism, reflecting the interaction of a robust pre-existing detrital zircon suite with a very limited melt volume. In addition, some zircon cores of uncertain geological affinity contain large areas devoid of oscillatory zoning with individual analyses clustering around 1850 Ma. Dividing the data into "detrital" and "metamorphic" suites solely on the basis of CL imaging yields an older group of 18 analyses ( 207 Pb/ 206 Pb age = 1867+ or -4 Ma) and a younger group of 10 analyses ( 207 Pb/ 206 Pb age = 1843+ or -4 Ma), not including five SHRIMP spots within areas of unzoned zircon. Mixture modeling of all 33 analyses in the post-1900 Ma data set resulted in a best-fit solution composed of two distinct components: (1) an older group of 19 analyses with an age of 1867+ or -4 Ma, and (2) a younger group of 14 analyses with an age of 1845+ or -4 Ma. These results suggest that the unzoned patches of zircon might be related to metamorphism rather than being detrital cores. Importantly, the ages and proportions of populations predicted by mixture modeling are otherwise very similar to those derived from analysis of CL zoning patterns. These data imply that high-temperature metamorphism occurred in the metasedimentary rocks less than 25 my after the crystallization of the igneous detrital source. Such rapid rates of erosion, deposition, and burial have rarely been proposed for Proterozoic rocks, despite evidence for analogous orogenic processes in the Mesozoic and Cainozoic on a comparable timescale. Careful evaluation of geological and geochronological data in Proterozoic provinces elsewhere may reveal similar patterns, with potential implications for the possible rates of Proterozoic orogenesis and crustal evolution

Early palaeozoic orogenesis along the Indian margin of Gondwana: Tectonic response to Gondwana assembly

SHRIMP U-Pb dating of zircons from a peralkaline S-type Lesser Himalayan granite from the Kathmandu region, Nepal indicate an age of emplacement of 475 Ma. The granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from Archaean to early Palaeozoic with prominent late Mesoproterozoic (Grenvillian) and Neoproterozoic (Pan-African) age peaks and have a maximum age of 500 Ma based on the youngest detrital grains. Deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a Cambro-Ordovician orogenic event, herein termed the Bhimphedian Orogeny that can be traced across the Himalaya from Pakistan to the eastern Himalaya and possibly extends west into Afghanistan. We interpret the orogeny as being related to Andean-type orogenic activity on the northern margin of the Indian continent, following Gondwana assembly. The magmatic arc was associated with andesitic and basaltic volcanism and was active from c. 530 to 490 Ma. The arc activity overlaps with, and is succeeded by, regional deformation, crustal melting and S-type granite emplacement that extended to 470 Ma. Orogenic activity was driven by coupling across the plate margin either during on-going subduction or through accretion of microcontinental ribbons, possibly represented by the Lhasa and Qiangtang blocks. It represents the termination of an orogenic cycle, termed the North Indian Orogen that commenced with Neoproterozoic rifting and passive margin development and terminated with the Bhimphedian Orogeny. This was succeeded by a return to passive margin setting along the northern Indian margin of Gondwana which continued until the Cenozoic Himalayan Orogeny. (c) 2007 Elsevier B.V. All rights reserved.</p