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Abstract The closure of the Mozambique Ocean defines the final assembly of the megacontinent Gondwana and is associated with a vast region of crustal growth in the Arabian-Nubian Shield. Despite this central paleogeographic position, there are few constraints on the position of terranes within and bounding the Mozambique Ocean. We report paleomagnetic data from ca. 726 Ma dikes exposed in southern Oman. Well-resolved magnetite magnetization is constrained to be primary by a conglomerate test on mafic clasts within overlying Cryogenian diamictite. The resulting paleomagnetic pole indicates that Oman was at a paleolatitude of 37 ± 2.5°N and was rotated ~80° counterclockwise from its present-day orientation. This position is consistent with Oman forming a contiguous plate with the India and South China cratons on the northern margin of the Mozambique Ocean in a distinct tectonic domain from Arabian-Nubian arcs to the south. This position reveals an ~5500-km-wide oceanic realm prior to subsequent closure that resulted in a major zone of Neoproterozoic crustal growth. 

Abstract. Chemical abrasion is a technique that combines thermal annealing and partialdissolution in hydrofluoric acid (HF) to selectively removeradiation-damaged portions of zircon crystals prior to U–Pb isotopicanalysis, and it is applied ubiquitously to zircon prior to U–Pb isotopedilution thermal ionization mass spectrometry (ID-TIMS). The mechanics ofzircon dissolution in HF and the impact of different leaching conditions onthe zircon structure, however, are poorly resolved. We present amicrostructural investigation that integrates microscale X-ray computedtomography (µCT), scanning electron microscopy, and Ramanspectroscopy to evaluate zircon dissolution in HF. We show that µCTis an effective tool for imaging metamictization and complex dissolutionnetworks in three dimensions. Acid frequently reaches crystal interiors viafractures spatially associated with radiation damage zoning and inclusionsto dissolve soluble high-U zones, some inclusions, and material aroundfractures, leaving behind a more crystalline zircon residue. Other acid pathsto crystal cores include the dissolution of surface-reaching inclusions andthe percolation of acid across zones with high defect densities. In highlycrystalline samples dissolution is crystallographically controlled withdissolution proceeding almost exclusively along the c axis. Increasing theleaching temperature from 180 to 210 ∘C results indeeper etching textures, wider acid paths, more complex internal dissolutionnetworks, and greater volume losses. How a grain dissolves strongly dependson its initial radiation damage content and defect distribution as well asthe size and position of inclusions. As such, the effectiveness of anychemical abrasion protocol for ID-TIMS U–Pb geochronology is likelysample-dependent. We also briefly discuss the implications of our findingsfor deep-time (U-Th)/He thermochronology.