Geochemical Characterization of Zircon in Fyfe Hills of the Napier Complex, East Antarctica

Ultra-high temperature (UHT) metamorphism plays an essential role in the development and stabilization of continents through accretionary and collisional orogenesis. The Napier Complex, East Antarctica, preserves UHT metamorphism, and the timing is still debated. U–Pb zircon geochronology integrated with rare earth element (REE) and oxygen isotope was applied to a garnet-bearing quartzo-feldspathic gneiss to confirm the timing of UHT metamorphism in Fyfe Hills in the western part of the Napier Complex. The zircons are analyzed using a sensitive high-resolution ion microprobe (SHRIMP). The cathodoluminescence observation and U–Pb ages allowed us to classify the analytical domains into three types: inherited domains (Group I), metamorphic domains (Group II), and U–Pb system disturbed domains (Group III). The REE patterns of Group II are characterized by a weak fractionation between the middle REE and heavy REE, which reinforces the above classification. The 207Pb*/206Pb* ages of Group II have an age peak at 2501 Ma, therefore, the gneiss experienced high temperature metamorphism at 2501 Ma. δ18O of zircons are homogeneous among the three groups (5.53 ± 0.11‰, 5.51 ± 0.14‰, and 5.53 ± 0.23‰), which suggests re-equilibration of oxygen isotopes after metamorphism at ca. 2501Ma under dry UHT conditions.

The Hadean origin of the Archean Napier Complex (East Antarctica)

<p>Details regarding the early evolution of the mantle-crust system are still poorly constrained due to the great scarcity of >3.7 Ga rocks in the geological record. The Napier complex (East Antarctica) is an Eoarchean craton that contains some of Earth’s oldest rocks. This complex recorded Meso- and Neoarchean metamorphism that reached extreme conditions corresponding to granulite facies at 2.5 Ga (1050-1120°C and 7-11 kbar). As a consequence, most samples exhibit disturbed radiogenic isotope systematics (e.g., Rb-Sr, Sm-Nd) and zircon crystals found in such samples are very complex rendering isotopic systematics interpretations challenging. The analytical methods employed in previous studies do not allow these complexities to be understood, which motivated the present contribution.</p><p>Here we studied two granulitic orthogneisses labelled 78285007 (Mount Sones) and 78285013 (Gage Ridge) that correspond to the oldest available rocks from the Napier Complex. Mount Sones displays typical characteristics of Archean tonalite-trondhjemite-granodiorite (TTG) suites (e.g., high Na<sub>2</sub>O/K<sub>2</sub>O, high Sr/Y, fractionated REE patterns with low heavy REE concentrations) with a normative composition intermediate between tonalite and trondhjemite whereas Gage Ridge has a composition closer to that of granite despite a strongly fractionated REE pattern and a pronounced positive Eu anomaly. We have conducted zircon texture assessment using cathodoluminescence and back-scattered electron images in annealed and not annealed crystals. We have subsequently combined U-Pb age profiling by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) and Lu-Hf isotope systematics measurement by LA-MC-ICP-MS in these zircon crystals. Finally, we analysed <sup>146,147</sup>Sm-<sup>143,142</sup>Nd isotopesystematics in corresponding whole-rock samples to better constrain the early history of their source.</p><p>Our results reveal that Mount Sones and Gage Ridgeorthogneisses formed at 3794 ± 40 and 3857 ± 39 Ma, respectively, with initial ɛ<sub>Hf </sub>of -2.6 ± 1.5 and -3.6 ± 2.5, respectively. Sm-Nd isotope measurements indicate a μ<sup>142</sup>Nd of -8.7 ± 3.9 and a ɛ<sub>Nd </sub>of -2.0 ± 0.3 at 3794 Ma for Mount Sones, whereas Gage Ridge exhibits a μ<sup>142</sup>Nd of -12.1 ± 6.2 and a disturbed <sup>147</sup>Sm-<sup>143</sup>Nd systematics. Taken altogether our results indicate that the oldest granitoids of the Napier Complex formed by reworking of 4456-4356 Ma mafic protocrust(s). Our inferred petrogenesis is similar to what has been proposed for other Eoarchean terranes worldwide (e.g., Itsaq Gneiss Complex, the Acasta Gneiss Complex, the Nuvvuagittuq Supracrustal Belt, and the North China craton). We propose that Hadean protocrusts were massively reworked in the Eoarchean to form cratons which, in turn, would account for both the absence of Hadean crust in the geological record and its little influence throughout the Archean despite crustal growth models proposing that ≤ 25% of present-day volume of continental crust was formed by the end of the Hadean.</p>

Petrographically-controlled elemental mobility in monazite and rutile in ultrahigh temperature granulites

<p>In situ age and trace element determinations of monazite and rutile grains from an ultrahigh temperature (UHT) metapelite hosted leucosome from the Napier Complex using laser split-stream analysis reveals highly variable behavior in both the U–Pb, REE and trace element systematics that are directly linked to the petrographic setting of individual grains.</p><p>Monazite grains armored by garnet and quartz retain a concordant 2.48 Ga age that is the same as the age for peak UHT metamorphism in the Napier Complex. Yttrium in the armored grains are unzoned with contents around 700 ppm in the garnet-hosted monazite and range between 400-1600 ppm in the monazite enclosed within quartz. A monazite grain hosted within a mesoperthite grain records a spread of concordant ages from 2.42 to 2.20 Ga and Y contents ranging between 400 to 1700 ppm. This grain exhibits core to rim zoning in both Y and age with the cores enriched in Y relative to the rim and younger ages in the core relative to the rim. A monazite grain that sits on a grain boundary between mesoperthite and garnet records the largest spread in ages­– from 2.42 to 2.05 Ga. The youngest ages in this grain are within a linear feature that reaches the core and is connected to the grain boundary between the garnet and mesoperthite, the oldest ages are observed where monazite is in contact with garnet. Yttrium in the grain is enriched in the core and depleted at the rim with the strongest depletions where monazite in adjacent to grain boundaries between the silicate minerals or in contact with garnet.</p><p>By contrast, rutile which is petrologically part of the peak-UHT assemblage and therefore inferred to have grown at c. 2.48 Ga records a complex discordant array of ages with the oldest concordant ages at 1.90 Ga with a spread down concordia to 1.70 Ga and a lower, imprecisely defined intercept at 0.55 Ga. The most discordant rutile grains sit within the residual garnet-sillimanite-spinel domains and record Zr-in rutile temperature of <800 °C. The least discordant and oldest grains sit within the leucosome and record Zr-in-rutile temperatures of >1000 °C. There is no correlation between grain size and age/degree of discordance.</p><p>The age and chemical relationships outlined above illustrate decoupling between the geochemical and geochronological systems in monazite and rutile. Individual grains are suggestive of a range of processes that modify these systems, including volume diffusion, flux-limited diffusion and recrystallisation, all operating at the scale of a single thin section and primarily controlled by the host minerals and their microstructural setting. These relationships, while complicated, can be interpreted in terms of the thermal history of this rock allowing the potential identification of a previously cryptic thermal event. This would not be possible without the petrographic information for the location of individual grains enabled through analysis of the different accessory minerals in thin section.</p>

Pb nanospheres in ancient zircon yield model ages for zircon formation and Pb mobilization

Nanospheres of lead (Pb) have recently been identified in zircon (ZrSiO4) with the potential to compromise the veracity of U-Pb age determinations. The key assumption that the determined age is robust against the effects of Pb mobility, as long as Pb is not lost from the zircon during subsequent geological events, is now in question. To determine the effect of nanosphere formation on age determination, and whether analysis of nanospheres can yield additional information about the timing of both zircon growth and nanosphere formation, zircons from the Napier Complex in Enderby Land, East Antarctica, were investigated by high-spatial resolution NanoSIMS (Secondary Ion Mass Spectrometry) mapping. Conventional SIMS analyses with >µm resolution potentially mixes Pb from multiple nanospheres with the zircon host, yielding variable average values and therefore unreliable ages. NanoSIMS analyses were obtained of 207Pb/206Pb in nanospheres a few nanometres in diameter that were resolved from 207Pb/206Pb measurements in the zircon host. We demonstrate that analysis for 207Pb/206Pb in multiple individual Pb nanospheres, along with separate analysis of 207Pb/206Pb in the zircon host, can not only accurately yield the age of zircon crystallization, but also the time of nanosphere formation resulting from Pb mobilization during metamorphism. Model ages for both events can be derived that are correlated due to the limited range of possible solutions that can be satisfied by the measured 207Pb/206Pb ratios of nanospheres and zircon host. For the Napier Complex zircons, this yields a model age of ca 3110 Ma for zircon formation and a late Archean model age of 2610 Ma for the metamorphism that produced the nanospheres. The Nanosphere Model Age (NMA) method constrains both the crystallization age and age of the metamorphism to ~±135 Ma, a significant improvement on errors derived from counting statistics.