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41.
Sample 73235 is one of several aphanitic impact melt breccias collected by the Apollo 17 mission at stations 2 and 3 on the slopes of the South Massif. This study presents a detailed investigation of internal structures and U-Pb ages of large zircon grains from this breccia sample. New data combined with the results of previous studies of zircon grains from the same location indicate that most zircon clasts in breccias from stations 2 and 3 formed during multiple magmatic events between 4.37 and 4.31 Ga, although the oldest zircon crystallized at about 4.42 Ga and the youngest at 4.21 Ga. In addition, zircons from the aphanitic breccias record several impact events prior to the ∼3.9 Ga Late Heavy Bombardment. The results indicate that the zircons probably crystallized at different locations within the Procellarum KREEEP Terrane and were later excavated and modified by several impacts and delivered to the same locality within separate ejecta blankets. This locality became a source of material that formed the aphanitic impact melt breccias of the South Massif during a ∼3.9 Ga impact. However, the zircons, showing old impact features, are not modified by this ∼3.9 Ga impact event suggesting that (i) this common source area was located at the periphery of excavation cavity, and (ii) the > 3.9 Ga ages recorded by the zircon grains could date large (basin-forming) events as significant as major later (∼3.9 Ga) collisions such as Imbrium and Serenitatis. 相似文献
42.
The sensitive high-resolution ion microprobe (SHRIMP) developed at the Australian National University (ANU) was the first of the high-resolution ion microprobes. The impact of this instrument on geochronological research over the last twenty years has been immense. This is particularly so for lunar geochronology where it has opened up avenues of research that were not possible using conventional TIMS techniques. The great advantage of SHRIMP is that it provides a means for determining precise U–Pb isotopic ratios on selected micron-size areas on polished grains of zircon and other U-bearing minerals. One of the first projects undertaken on the newly invented SHRIMP I was an investigation of U–Pb ages of lunar zircon. Using SHRIMP, multiple analyses could be made on areas of individual zircons to test the stability of U–Pb systems in shocked grains. Also, by analysing grains “in situ”, textural relationships between the analysed zircon and the components of the sample breccia could be used in the interpretation of the SHRIMP data. As a result of this research it was realised that most lunar zircons have ages up to 500 Ma older than the Imbrium and Serenitatis impacts at ca. 3.9 Ga, demonstrating that the zircons have not been affected by the these impact events although heating and shock effects have profoundly disturbed other dating systems. This has opened the way for research into the early lunar magmatic and bombardment record. For example, recent SHRIMP results have revealed profound differences in the ages of zircons from breccias from the Apollo 14 and Apollo 17 sample sites, raising new questions about the evolution of lunar magmatism. Also, multiple SHRIMP analyses on complex lunar zircons have shown that these grains can record U–Pb disturbance by later impact events. SHRIMP U–Pb age determinations on phosphates in lunar meteorites has identified lunar events not recognised in samples from the Apollo program. SHRIMP-based research on lunar materials is ongoing and, in combination with other chemical and structural evidence, continues to stimulate new ideas on the early evolution of the Moon. 相似文献
43.
Martina?MennekenEmail authorView authors OrcID profile Thorsten?Geisler Alexander?A.?Nemchin Martin?J.?Whitehouse Simon?A.?Wilde Biliana?Gasharova Robert?T.?Pidgeon 《Contributions to Mineralogy and Petrology》2017,172(8):66
The discovery of Hadean to Paleoarchean zircons in a metaconglomerate from Jack Hills, Western Australia, has catalyzed intensive study of these zircons and their mineral inclusions, as they represent unique geochemical archives that can be used to unravel the geological evolution of early Earth. Here, we report the occurrence and physical properties of previously undetected CO2 inclusions that were identified in 3.36–3.47 Ga and 3.80–4.13 Ga zircon grains by confocal micro-Raman spectroscopy. Minimum P–T conditions of zircon formation were determined from the highest density of the inclusions, determined from the density-dependence of the Fermi diad splitting in the Raman spectrum and Ti-in-zircon thermometry. For both age periods, the CO2 densities and Ti-in-zircon temperatures correspond to high-grade metamorphic conditions (≥5 to ≥7 kbar/~670 to 770 °C) that are typical of mid-crustal regional metamorphism throughout Earth’s history. In addition, fully enclosed, highly disordered graphitic carbon inclusions were identified in two zircon grains from the older population that also contained CO2 inclusions. Transmission electron microscopy on one of these inclusions revealed that carbon forms a thin amorphous film on the inclusion wall, whereas the rest of the volume was probably occupied by CO2 prior to analysis. This indicates a close relationship between CO2 and the reduced carbon inclusions and, in particular that the carbon precipitated from a CO2-rich fluid, which is inconsistent with the recently proposed biogenic origin of carbon inclusions found in Hadean zircons from Jack Hills. 相似文献
44.
R. T. Pidgeon A. A. Nemchin G. J. Hitchen 《Contributions to Mineralogy and Petrology》1998,132(3):288-299
Internal structures in zircons from granitoids from the late Archaean Darling Range Batholith show secondary features revealed
by HF etching, which record reconstitution of the zircons and modification of the distribution of trace elements during post
crystallisation cooling of the granitoid. Zircons from the granites commonly contain unzoned to weakly zoned cores surrounded
by rims showing oscillatory zoning which has been modified by recrystallisation. The most striking feature is the development
of high trace element concentration areas found in zircons from a number of granites. These structures range from enhanced
trace element concentrations in primary zones to a single accumulation of most trace elements in one band, about half way
between the outer edge and the centre of the zircon. In any zircon the extent of the concentration of trace elements towards
the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory
zones in the outer rim. This suggests that the trace element bands formed by migration of trace elements from the outer primary
zones to new concentration sites on an inner set of primary zones. This explanation is supported by the formation of multiple
curved trace element bands that transgress primary zoning and the determination of younger SHRIMP ages on depleted zircon
outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres. Also observed in some granite
zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs
throughout the zircon. This structure is explained in terms of secondary migration and reconcentration of trace elements in
curved bands. All structures can be transgressed by generally rounded lobes and patches of low U, weakly nebulously zoned
zircon. This is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation
of the granitoid, resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements
from the zircon.
Received: 6 January 1998 / Accepted: 8 May 1998 相似文献