To determine the thickness of sedimentary cover overlying Proterozoic basement, an ambient-noise surface-wave tomo-graphic (ANSWT) survey was conducted in the Boulia region of the Mt Isa Province in Queensland, Australia. An array of 100 three-component seismic nodes was deployed along a 30 km section and ambient seismic noise was recorded for 19 days. In the resultant seismic model, the top-basement contact is resolved as a sharp, subhorizontal interface at a depth of about 700 m, where seismic velocity (Vs) increases downward from about 2500 to 3500 m/s. The accuracy of the retrieved top-basement contact was confirmed by comparison with drill hole intersections and with results from active seismic and magnetotelluric surveys. This study demonstrates that the ANSWT method can accurately and inexpensively map the thickness of sedimentary cover that obscures potentially mineralised regions globally.
Abstract A new thermodynamic model for silicate melt in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–Fe2O3–Cr2O3 model system is presented, building on the tholeiitic through to granitic melt model of Holland et al. (2018) [Journal of Petrology, 59, 881–900] but extending for the first time into anhydrous alkaline systems. The new melt model is accompanied by new thermodynamic models for nepheline, kalsilite, leucite, melilite and ilmenite. Collectively these models enable pseudosection modelling of alkaline-silicate magmatic systems, providing a new tool for investigating this geologically- and economically-important compositional space. The models are calibrated with respect to experimental data on phase relations among minerals and melt, and the fit is benchmarked here via detailed comparison with seven experimental datasets, which encompass a range of pressure (0–22 kbar), temperature (680–1350°C), oxygen fugacity (log fO2 ΔFMQ-3 to +1), total alkali (3–16 wt%) and silica (37–70 wt%) conditions. The calculated pseudosections successfully reproduce experimental crystallisation sequences and phase compositions, indicating that the thermodynamic models are well calibrated across this spectrum of conditions. Redox buffered experimental conditions are simulated using oxygen buffered pseudosections. Contouring of oxygen buffered pseudosections with XFe3+ (mol. Fe3+ /Fetotal), or pseudosections of varying XFe3+ with ΔFMQ, reveals (i) often complex and non-intuitive relationships between these two representations of oxidation state, and (ii) substantial variation in ferric iron over narrow temperature intervals in some oxygen buffered sets of experiments. An implication is that simulating oxygen buffering is vital when benchmarking thermodynamic models using experimental results. Furthermore, because natural igneous systems likely feature a near-constant XFe3+, it is important to assess experimental results in this framework when making inferences about natural systems, recognising that oxygen fugacity is a consequence not a control of phase equilibria in nature. Overall, our new models provide a novel tool to explore the role of variables such as pressure, fractional crystallisation and crustal assimilation in the petrogenesis of alkaline-silicate magmatic systems and their associated mineralisation.
Abstract Clinopyroxene is a key fractionating phase in alkaline magmatic systems, but its impact on metal enrichment processes, and the formation of REE + HFSE mineralisation in particular, is not well understood. To constrain the control of clinopyroxene on REE + HFSE behaviour in sodic (per)alkaline magmas, a series of internally heated pressure vessel experiments was performed to determine clinopyroxene–melt element partitioning systematics. Synthetic tephriphonolite to phonolite compositions were run H2O-saturated at 200 MPa, 650–825°C with oxygen fugacity buffered to log f O2 ≈ ΔFMQ + 1 or log f O2 ≈ ΔFMQ +5. Clinopyroxene–glass pairs from basanitic to phonolitic fall deposits from Tenerife, Canary Islands, were also measured to complement our experimentally-derived data set. The REE partition coefficients are 0·3–53, typically 2–6, with minima for high-aegirine clinopyroxene. Diopside-rich clinopyroxene (Aeg5–25) prefer the MREE and have high REE partition coefficients (DEu up to 53, DSm up to 47). As clinopyroxene becomes more Na- and less Ca-rich (Aeg25–50), REE incorporation becomes less favourable, and both the VIM1 and VIIIM2 sites expand (to 0·79 Å and 1·12 Å), increasing DLREE/DMREE. Above Aeg50 both M sites shrink slightly and HREE (VIri ≤ 0·9 Å ≈ Y) partition strongly onto the VIM1 site, consistent with a reduced charge penalty for REE3+ ↔ Fe3+ substitution. Our data, complemented with an extensive literature database, constrain an empirical model that predicts trace element partition coefficients between clinopyroxene and silicate melt using only mineral major element compositions, temperature and pressure as input. The model is calibrated for use over a wide compositional range and can be used to interrogate clinopyroxene from a variety of natural systems to determine the trace element concentrations in their source melts, or to forward model the trace element evolution of tholeiitic mafic to evolved peralkaline magmatic systems.
Abstract Enrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitized phonolite breccia, focusing on small satellite occurrences 1–2 km from the Songwe Hill carbonatite, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile and minor huttonite/thorite, as well as fluorite and apatite. A genetic link between HREE mineralization and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitization. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focusing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid–rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite. At a maximum Y content of 3100 µg g −1 , HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of carbonatite-related HREE enrichment suggests that all form via a common mechanism linked to fenitization. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focusing conduit for fenitizing fluids, reducing fluid – country-rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralization within the source carbonatite body at depth.
The Natkusiak continental flood basalts and Franklin sills of Victoria Island preserve an exceptional record of the ca. 716–723 Ma Franklin large igneous province and are synchronous with major climatic variations and breakup of the supercontinent Rodinia. The Natkusiak Formation basalts record an early phase of discontinuous rubbly flows (<100 m, low-Ti Type 1 magmas) overlain by a thicker series of extensive tholeiitic sheet flows (∼1100 m, high-Ti Type 2 magmas). Coeval intrusions hosted by underlying Shaler Supergroup sedimentary rocks are differentiated low-Ti Type 1 Franklin sills and doleritic high-Ti Type 2 sills, both of which show correlations in isotope plots with the northernmost basalts on Victoria Island. Whole-rock Pb-Sr-Nd-Hf isotopic compositions from 66 samples indicate that the earliest magmas (Type 1) had similar primary melt compositions (Fo90 olivine) to oceanic island basalts and incorporated up to 10% granitoid basement (initial εNd = –0·8 to –7, Nb/La = 0·42 to 0·67), a relatively weak continental signature compared to many other continental flood basalt provinces. Type 2 doleritic sills and the northern sheet flow basalts incorporated up to 5% granitoid (initial εNd = +0·9 to +5·5), consistent with a waning continental influence during maturation of the magmatic system. Radiogenic isotope ratios are not correlated with indices of fractional crystallisation, which indicates that the continental material was either dispersed within the melt source, or that the magmas were heterogeneously contaminated prior to differentiation. In the southwestern part of Victoria Island, Type 1 basalts show negligible continental influence (Nb/La = 0·81 to 0·94) and have unusually high initial εNd ratios (+4·4 to +11·8) that are decoupled from initial εHf (+0·8 to +11·1). These radiogenic εNd compositions persist throughout the southern volcanic stratigraphy and indicate involvement of a component with high time-integrated Sm/Nd that lacked correspondingly high Lu/Hf. We propose that the source region for the southwestern Natkusiak basalts and related sills included isotopically matured oceanic crust, which was recycled through the asthenospheric mantle into a laterally heterogeneous plume. The distinct trace element signatures of the southern and northern sources became attenuated with the onset of voluminous melting (corresponding to emplacement of the Type 2 doleritic sills and sheet flow basalts) and may reflect contributions from hydrous eclogitic material emplaced into the lithospheric mantle during the ca. 1·9 Ga Wopmay Orogeny. As both the northern and southern volcanic rocks exhibit contrasting isotopic signatures throughout the preserved stratigraphy, the magma plumbing system must have experienced limited lateral mixing and homogenisation, which allowed for the expression of distinct mantle source signatures in the high-level sills and basaltic lavas.
Alkaline-silicate complexes host some of the world's largest resources of rare-earth elements and high-field-strength elements (REE & HFSE) and represent the most fractionated magmatic systems on our planet. Geochemical evidence indicates that they are mantle melts, but while various studies highlight a role for lithospheric mantle, we do not know the precise origin of their contained REE and HFSE, and whether enrichment of the mantle source for these magmas can be attributed to specific geodynamic processes or events. We present new Nd-Hf isotope measurements (/ & / ) made by LA-MC-ICP-MS, as well as a compilation of existing isotopic data for a suite of alkaline igneous rocks from the Gardar Province, a Mesoproterozoic continental rift in southern Greenland. Neodymium and hafnium isotopes are unaffected by crystal fractionation and can directly fingerprint the source of REE and HFSE. The dataset covers both phases of Gardar magmatism (1325–1261 and 1184–1140 Ma) and incorporates mafic dyke swarms and km-scale intrusive complexes, including Ilimmaasaq (Ilímaussaq) and Motzfeldt, which host some of the world's largest REE and HFSE deposits. The majority of Gardar complexes have a narrow range of positive median initial εNd (0 to +3.3) and εHf values (+0.2 to +6.0). Only two granite intrusions and the Eriksfjord basaltic lavas have crustally contaminated Nd-Hf isotope compositions, with the vast majority of Gardar igneous rocks preserving the isotope signature of their mantle source. Considering the diversity of rock types in the Gardar Province, initial εNd -εHf compositions are remarkably homogeneous, indicating a derivation of the Gardar's REE and HFSE from a laterally-extensive mantle melt source. Several Gardar systems have low initial εHf for a given εNd (εHf to -9.7), a distinctive signature as few geological processes decouple the Nd and Hf isotope systems. The decoupled Nd-Hf isotope signatures are consistent with contributions from isotopically-matured phlogopite-bearing metasomatic veins (commonly known as PIC: phlogopite-ilmenite-clinopyroxene) in the lithospheric mantle. The metasomatising fluids that formed these source rocks were introduced via Palaeoproterozoic subduction, but the Gardar isotopic signatures indicate that REE and HFSE enrichment of these metasomes was not derived from subducted sediment; instead it is likely that metals were scavenged from the mantle wedge overlying the ancient subduction zone. The Gardar Nd-Hf isotope evolution trends overlap with a global compilation of kimberlites through time and allow us to tie the origin of the PIC metasomes to the regional geodynamic history of South Greenland. We identify PIC metasomes as a key metal source for the Gardar and by extension perhaps other REE-mineralised igneous provinces globally.
The Neoproterozoic (~723–716 Ma) Franklin Large Igneous Province exposed on Victoria Island in the Canadian Arctic is comprised of a sill-dominated magma plumbing system overlain by the coeval Natkusiak flood basalts. We have investigated three sections, separated by a total of >50 km of distance, of a sill (the Fort Collinson Sill Complex) emplaced just above a prominent sedimentary marker unit. The sill is characterized by a basal olivine-enriched layer (OZ: up to 55 % olivine) and an upper gabbroic unit. The observed diversity of olivine compositions in the OZ implies that bulk-rock MgO versus FeO arrays reflect accumulation of a heterogeneous olivine crystal cargo. We suggest that the OZ was formed as a late olivine slurry replenishment in a partially crystallized gabbroic sill, propagating for over 50 km along strike. This interpretation is consistent with Pb-isotope data, which show that at least three geochemically distinct magmas were emplaced into the Fort Collinson Sill Complex. The OZs exhibit a gradual westward evolution toward more Fe-rich bulk compositions. This is best explained by progressive mixing of the replenishing olivine slurry with a resident gabbroic mush during westward flow. Pb-isotopic signatures suggest that magmas near the inferred conduit feeder assimilated small amounts (<10 %) of dolostone country rock, which may have locally buffered olivine compositions to high-Fo contents.
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