Troctolite 76535

Troctolite 76535
Name	Troctolite 76535
Type	Igneous rock (troctolite, coarse-grained olivine-plagioclase)
Location	Hadley–Apennine
Collected by	Apollo 17
Date collected	1972
Specimen number	76535

Troctolite 76535 is a coarse-grained lunar troctolite returned by Apollo 17 from the Taurus–Littrow valley during the Apollo program. The sample has been central to debates involving the Lunar Magma Ocean hypothesis, the timing of lunar mantle differentiation, and the dynamics of the lunar crust. Multiple institutions including the Smithsonian Institution, NASA, and university laboratories have performed studies linking results to broader questions addressed by missions such as Lunar Reconnaissance Orbiter, GRAIL, and proposals like Artemis program.

Discovery and Context

Troctolite 76535 was collected during the final Apollo surface expedition led by commander Eugene Cernan with crewmates Harrison Schmitt and Ronald Evans at station near the base of the South Massif in the Taurus–Littrow valley. The sample context connects to regional geology mapped by Gene Shoemaker-influenced field strategies and remote sensing from Surveyor program and Lunar Orbiter imagery. Field notes, photographed by Jack Schmitt, place the specimen amid breccias and mare basalts sampled in proximity to materials interpreted in comparisons with returned samples from Apollo 15 and Apollo 16. The provenance links into stratigraphic interpretations made by teams including researchers from US Geological Survey and Brown University.

Petrology and Mineralogy

Petrographic analyses at facilities such as California Institute of Technology, University of Arizona, and Massachusetts Institute of Technology characterize 76535 as an olivine-plagioclase cumulate dominated by forsterite-rich olivine and calcic plagioclase (bytownite). Thin section studies using polarizing microscopes and electron microprobes at Carnegie Institution for Science and Lunar and Planetary Institute identify accessory phases including orthopyroxene, chromite, ilmenite, and trace troilite. Textural relations exhibit cumulate layering and subsolidus annealing similar to textures reported in troctolites from Sierra de las Minas (Earth analog studies) and in lunar samples like those from Apollo 11 and Apollo 12 suites. Scanning electron microscopy work at Jet Propulsion Laboratory and NASA Johnson reveals exsolution features and intergrowths analogous to those observed in pallasitic meteorites curated at the Smithsonian Institution.

Geochemistry and Isotopic Studies

Major- and trace-element geochemistry measured by inductively coupled plasma mass spectrometry at University of California, Berkeley, University of New Mexico, and Rutgers University indicate low incompatible element concentrations with enrichment patterns consistent with olivine-plagioclase fractionation models advocated by researchers at University of Tokyo and Open University. Isotopic systems including Sm–Nd, Rb–Sr, Pb–Pb, and Oxygen isotope measurements performed by teams at California Institute of Technology and University of Oxford constrain source reservoirs and evaluate mixing with KREEP-like components associated with interpretations by J. T. Wasson and G. J. Taylor. High-precision IMG-MS and TIMS datasets generated at Scripps Institution of Oceanography and University of Chicago illustrate Nd model ages that have been compared with ages for ferroan anorthosite suite samples analyzed at NASA Goddard.

Age and Formation History

Radiometric ages from Sm–Nd and Rb–Sr systems, reported by groups at Wiscosin–Madison and University of Bern, place crystallization of 76535 within a timeframe debated between ~4.3 and ~4.5 billion years ago, similar to ages proposed for early lunar mantle processes by scholars at Brown University and California Institute of Technology. These ages are integrated with impact-reset signatures dated using Ar–Ar methods conducted at Lawrence Livermore National Laboratory and University of Colorado Boulder, providing constraints on subsequent thermal events linked to basin-forming impacts like Imbrium basin and Nectaris basin. Thermodynamic modeling carried out by groups at Carnegie Institution for Science and MIT supports formation through cumulate crystallization within a cooling lunar magma reservoir, with later metamorphism and brecciation related to regional impact history reconstructed using crater counts from Lunar Reconnaissance Orbiter datasets.

Significance for Lunar Science

Troctolite 76535 has played a pivotal role in testing models of early lunar differentiation central to theories advanced by researchers such as Alfred Nier-era isotope pioneers and modern proponents at Brown University and Imperial College London. The sample informs debates about the longevity of the lunar magnetic field inferred from paleomagnetic work at University of California, San Diego and Imperial College London, and contributes to understanding mantle heterogeneity invoked in studies by NASA Ames Research Center and Goddard Space Flight Center. Comparative studies with lunar meteorites analyzed at Natural History Museum, London and Antarctic collections at Japanese Antarctic Research Expedition laboratories have broadened perspectives on sampling bias and global lunar geology raised during meetings of the Lunar and Planetary Science Conference.

Curation and Analyses Performed

Curation of 76535 has been managed by the Johnson Space Center curation facility with aliquots distributed under protocols involving NASA, Smithsonian Institution, and international partners including Russian Academy of Sciences and CNES. Analytical campaigns have included petrography, electron microprobe analysis at Caltech, isotopic dating at University of New Mexico, noble gas studies at ETH Zurich, paleomagnetic experiments at Imperial College London, and synchrotron X-ray mapping at Argonne National Laboratory. Ongoing proposals for additional work have been discussed at panels convened by National Academies of Sciences, Engineering, and Medicine, linking legacy sample science to future missions like Lunar Reconnaissance Orbiter follow-ons and the Artemis program sample return objectives.

Category:Lunar samples