17 research outputs found

    Probing the lower continental crust with the petrology and geochemistry of Queensland xenoliths

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    This thesis investigated the poorly constrained composition and evolution of the lower continental crust in Queensland through geochemistry, geochronology, and petrological modelling. The analysis of deep crustal rocks brought to the surface by explosive volcanism shows that the lower crust in Queensland is nearly devoid of many geologically significant trace elements. The modelling demonstrates that this lower crust formed through solid-melt interactions at the base of the crust. The combined data show that this depleted, hybridised crust could be more common than previously recognised and that hybridisation is likely a major driver of continent formation

    Linking granulites, intraplate magmatism, and bi-mineralic eclogites with a thermodynamic-petrological model of melt-solid interaction at the base of anorogenic lower continental crust

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    This study reports results from thermodynamic phase equilibrium and trace element modelling of mafic magmatic underplating and solid-liquid interaction in the lower continental crust (LCC) in intraplate settings. The arrival of underplating basalt sills into thin (∼30 km at 8 kbar) and thicker (∼45 km at 12 kbar) andesitic and basaltic LCC precursors was simulated with heating and batch-melting to yield refractory residues. Continued magmatic invasion of the LCC was then modelled at the same pressures with hybridisation between the residual solids and basaltic and picritic liquids (at 80:20 and 50:50 proportions). The first finding is that hybridisation with basalt increases the stability fields of 2-pyroxene-plagioclase and garnet-clinopyroxene-plagioclase granulites, by far the dominant LCC xenoliths found in anorogenic settings. The second finding pertains to situations when the liquid proportion of the hybrid system is lower than the fraction of the incoming mafic liquid. The resulting liquids share many characteristics with silicic volcanic rocks in continental flood basalt (CFB) provinces, including low Al and high Ti-concentrations. Stoichiometric melting reactions from 80:20 residual andesite:basalt hybridisation reproduce trace element patterns of silicic eruptives with potential for decoupling of Sr-Nd-Pb isotope systematics, as found in CFB rhyolites and LCC xenoliths. Incongruent melting of hornblende and garnet (the A in AFC) from the residual solid and dominant peritectic fractional crystallisation of plagioclase (the FC in AFC) from the liquid occur. A notable feature of LCC-picrite hybridisation is that solid residues after felsic liquid extraction, when subjected to higher pressure, are buffered to olivine and quartz-free compositions. Consequently, if these high-pressure granulite residues delaminated, they would convert to the bi-mineralic high-Mg eclogites found as xenoliths in kimberlites. The modelled residues also have the Sr-enrichment and positive Eu-anomalies of eclogite xenoliths. Solid-liquid interaction in intraplate settings has been and remains a significant process in crustal growth and evolution.</p

    Evidence for highly refractory, heat producing element-depleted lower continental crust: Some implications for the formation and evolution of the continents

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    The composition of the lower continental crust is estimated via the analysis of granulite xenoliths, granulite terrains and geophysical properties. All three proxies generally agree on the lower crust's refractory nature, dominated by mafic granulites. Estimates weighted using seismic velocity reference models yield lower Th and U concentrations and higher K/U ratios than granulite xenolith averages, while terrain granulites are often much less refractory than xenoliths. Here we present new data for lower crustal xenoliths from central Queensland, an understudied part of the xenolith-bearing eastern Australian basalt provinces. The granulite chemistry was estimated using the reconstitution approach in which the modal mineralogy and in situ chemical analyses are combined. High-resolution energy-dispersive X-ray spectroscopy mapping revealed that K is enriched in anastomosing grain boundary networks and fractures in granulite and co-occurring mantle peridotite xenoliths. Laser-ablation inductively-coupled-plasma mass-spectrometry 2-D traverses show that the same networks are also significantly enriched in many highly incompatible elements. There are sharp concentration contrasts with neighbouring phases for elements with very different diffusivities (Li and U), suggesting that the networks formed during entrainment, decompression and heating within the host basalt. We propose that undetected inclusion of such late enrichment skews xenolith chemistry estimates to non-representative, overly fertile compositions. In the case of the studied xenoliths, the carrier basalts are not very strongly enriched in highly incompatible elements, and even when the K-rich networks are included in the reconstitution, the resulting granulite chemistry is very refractory with 0.52 wt% K2O, 0.07 ppm Th and 0.03 ppm U. The locally dominant lithology of lower crustal xenoliths is simple two-pyroxene, plagioclase, ilmenite granulite with few accessory phases. The granulite mineralogy and chemistry were compared with results from thermodynamic models of prograde anatexis of different metabasites, variably hydrated. The comparison shows that the granulites could be restitic calc-alkaline basalts or diorites that experienced episodic melt extraction accumulating up to 50–60% total melt loss at very high temperatures (950–1050 °C), implying that the temperatures recorded by two-pyroxene thermometry (750–830 °C) do not capture the thermal maximum. The corresponding upper crustal section of the northern New England Orogen exposes a range of Devonian to Cretaceous granitoids, some of which have complementary features to the granulites, including the low modal abundance of plagioclase; low relative abundance of Ti, very high Rb/Ba ratios, and high Th/U ratios. Together, the data suggest extensive and protracted melting of the original lower crust upon lithospheric thinning and concomitant magmatic underplating. The required high temperatures favour picritic over basaltic underplates. In such a setting, the gravity-driven delamination of more mafic garnet-rich restites and olivine-rich mafic-ultramafic underplating material is physically plausible. This delamination could help explain the long-established mass balance issue for the formation of continental crust in general.</p

    A reconstitution approach for whole rock major and trace element compositions of granulites from the kapuskasing structural zone

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    Current estimates for the composition of the lower continental crust show significant variation for the concentrations of the highly incompatible elements, including large uncertainties for the heat-producing elements. This has consequences for models of the formation of lower crust. For example, is lower continental crust inherently poor in incompatible elements or has it become so after extraction of partial melts caused by thermal incubation? Answering these questions will require better agreement between estimates for the chemistry of the lower crust. One issue is that granulite samples may have been altered during ascent. Xenoliths often experience contamination from the entraining alkaline magma, potentially resulting in elevated concentrations of incompatible trace elements when analysed by conventional bulk rock techniques. To avoid this, we assessed an in situ approach for reconstructing whole rock compositions with granulites from the Kapuskasing Structural Zone, Superior Province, Canada. As terrain samples, they have not been affected by host magma contamination, and as subrecent glacial exposures, they show minimal modern weathering. We used scanning electron microscope electron dispersive spectroscopy (SEM-EDS) phase mapping to establish the modal mineralogy. Major and trace element concentrations of mineral phases were determined by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), respectively. These concentrations were combined with the modal mineralogies to obtain reconstructed whole rock compositions, which were compared to conventional bulk rock analyses. The reconstructed data show good reproducibility relative to the conventional analyses for samples with massive textures. However, the conventional bulk rock chemistry systematically yields higher K concentrations, which are hosted in altered feldspars. Thus, even in terrain samples, minor alteration can lead to elevated incompatible element estimates that may not represent genuine lower continental crust.</p

    A new compositional estimate for refractory lower continental crust with implications for the first terrestrial Pb-isotope paradox

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    The lower continental crust, representing up to 50% of the continental mass, is largely inaccessible, making its composition difficult to constrain. Previous composite models based on geophysical evidence and geochemical data of granulite terrains and xenoliths have proposed varying results, from a mafic, relatively refractory lower crust to an intermediate-felsic, more enriched composition. Here, we investigated the mineralogy and geochemistry of predominantly mafic granulite xenoliths from eastern Australia and the Kola Peninsula, Russia, using an in situ analytical approach that minimises host magma contamination. The resulting xenolith compositions are variably and often strongly depleted in most highly incompatible trace elements, including the heat-producing elements. These xenoliths represent an extremely refractory component of the lower continental crust, likely formed after high degrees of partial melting or crystallisation from a depleted source. A lower crust composed solely of this refractory endmember would be too exhausted in heat-producing elements to satisfy heat-flow constraints. However, a volumetrically significant component of the lower crust is this mafic and refractory material, combined with undifferentiated material and a felsic or metapelitic portion. Using geophysical constraints on proportions of refractory (55%), undepleted (38%) and enriched (7%) components, a new estimate for average lower continental crust that satisfies heat flow limits was calculated, including for elements such as Be, B, Cs, W and Tl, where previous estimates relied on very few data. Finally, we show that because much of the lower continental crust is so refractory and depleted in incompatible elements, it is unlikely to be a reservoir that can balance radiogenic isotope (unradiogenic Pb) and trace element ratios (e.g. Rb/Cs, Nb/Ta) for which bulk silicate Earth departs from chondritic ratios.</p

    Evidence for evolved Hadean crust from Sr isotopes in apatite within Eoarchean zircon from the Acasta Gneiss Complex

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    Current models for the properties of Hadean-Eoarchean crust encompass a full range of possibilities, involving crust that is anywhere from thick and differentiated to thin and mafic. New data are needed to test and refine these models, and, ultimately, to determine how continents were first formed. The Rb-Sr system provides a potentially powerful proxy for crustal evolution and composition. However, this system has thus far been underutilized in studies on early crustal evolution due to its susceptibility to re-equilibration. Overcoming this issue requires new analytical approaches to micro-sample ancient Sr-rich mineral relics that may retain primary Rb-Sr systematics, allowing for the precise and accurate determination of initial Sr-87/Sr-86 values. In this study, we used a novel application of laser-ablation multi-collector inductively coupled plasma mass spectrometry to determine the Sr isotope composition of apatite inclusions in >3.6 Ga zircon grains from Eoarchean granodiorite gneisses of the Acasta Gneiss Complex, Slave Province, Canada. The Rb-87-corrected Sr-87/Sr-86 values of these inclusions are largely identical and are distinctly different from values obtained from altered matrix apatite. The inclusion data provide the first direct estimate of initial Sr-87/Sr-86 for these ancient rocks. Combining this result with information on the protolith and source-extraction age yields estimates for the range of Rb/Sr values, and by extension composition, that the source of these rocks may have had. The data indicate that continental crust containing over 60 wt% of SiO2 was present in the ca. 4.2 Ga source of the Acasta Gneiss Complex. Thus vestiges of evolved crust must have existed within the primitive proto-continents that were present on the Hadean Earth. (C) 2018 Elsevier Ltd. All rights reserved

    File 3: Robust laser ablation Lu–Hf dating of apatite: an empirical evaluation

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    Isochron and weighted mean plots for the Lu–Hf matrix-correction standard (OD-306) and secondary standards HR-1 and Bamble-1. MSWD, mean squared weighted deviation; n, number of analyses; P(χ2), Chi-squared probability for a single data population. Where required (i.e. insufficient spread along the isochron), the isochrons were anchored to an initial 177Hf/176Hf ratio of 3.55 ± 0.05 (see text)

    File 5: Robust laser ablation Lu–Hf dating of apatite: an empirical evaluation

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    Apatite, monazite and titanite U–Pb and trace element data for all analysed samples (tab names refer to the four study areas), including data for the analytical standards. Rho, error correlation. All trace element data are reported in ppm concentrations. Log(LREE), logarithm of the sum of the La, Ce, Pr and Nd concentrations, which was used to distinguish age populations (see text)
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