Over about the last 5 to 10 years evidence from the study of rocks has indicated that Ca-rich, alkali-poor carbonatite magmas were much more common than the alkali-rich Oldoinyo Lengai type and therefore should receive much more scientific attention. The results of an experimental study on CaCO 3 -rich rock compositions confirm that a melt with a CaCO 3 -content of at least 90% can be formed by liquid immiscibility from a peralkaline silicate parent magma. Thus it is possible that any of the common carbonitite rocks could owe their origin to a liquid immiscibility process
Abstract Thirty new high‐precision U‐Pb perovskite and zircon ages from kimberlites in central North America delineate a corridor of mid‐Cretaceous (115–92 Ma) magmatism that extends ∼4000 km from Somerset Island in Arctic Canada through central Saskatchewan to Kansas, USA. The least contaminated whole rock Sr, Nd, and Hf isotopic data, coupled with Sr isotopic data from groundmass perovskite indicates an exceptionally limited range in Sr‐Nd‐Hf isotopic compositions, clustering at the low ɛ Nd end of the OIB array. These isotopic compositions are distinct from other studied North American kimberlites and point to a sublithospheric source region. This mid‐Cretaceous kimberlite magmatism cannot be related to mantle plumes associated with the African or Pacific large low‐shear wave velocity province (LLSVP). All three kimberlite fields are adjacent to strongly attenuated lithosphere at the edge of the North American craton. This facilitated edge‐driven convection, a top‐down driven processes that caused decompression melting of the transition zone or overlying asthenosphere. The inversion of ringwoodite and/or wadsleyite and release of H 2 O, with subsequent metasomatism and synchronous wet partial melting generates a hot CO 2 and H 2 O‐rich protokimberlite melt. Emplacement in the crust is controlled by local lithospheric factors; all three kimberlite fields have mid‐Cretaceous age, reactivated major deep‐seated structures that facilitated kimberlite melt transit through the lithosphere.
The volume, nature and origin of continental crust that existed on Earth before 3 Ga is poorly understood.One of the greatest obstacles in addressing these topics is the apparent paucity of preserved ancient (>3 Ga) crust.Despite being one of the largest Archean nuclei on Earth, the record of ancient crust in the Rae craton is virtually unexplored.Previous work has hinted at the presence of pre-3 Ga crust at the southern [1] and western [2] margins of the craton, but the true aerial extent, age, nature and origin of that ancient crust is unknown.In this study, we present whole-rock Sm-Nd isotope and elemental data, and zircon U-Pb, Hf and O isotope data, for granitoids from the western margin of the Rae craton.Samples with crystallization ages >3.0 Ga and/ or Sm-Nd depleted mantle model ages >3.2 Ga define an ~1000 x 100 km belt that stretches from central Canada to the Arctic coast, which we call the Kugyoak terrane.Pre-3.0Ga granitoids from this terrane are broadly similar in composition to typical Archean tonalitetrondhjemite-granodiorite [3].These granitoids yield crystallization ages between 3.25 and 3.07 Ga, initial zircon εHf values between +3 and -2, mantle-like zircon O isotope compositions and rare ca.3.3 Ga inherited zircon.Collectively, these data show that one of the largest Paleo to Mesoarchean terranes on Earth was previously unrecognized, and that this terrane represents a volumetrically significant addition of juvenile ca.3.3-3.1 Ga continental crust.Granitoids from this giant terrane yield little-to-no evidence for interaction with the large volumes of pre-3.5 Ga continental crust (up to 45% of present-day continental volume) that are inferred from proxy records, such as detrital zircon U-Pb-Hf isotope data, to have been present at the time of its formation [4].
Experimental data are presented for an evaluation of the solubility of tantalum in the ternary system CaCO 3 – Ca(OH) 2 – NaTaO 3 (or calcite – portlandite – sodium tantalate) over the temperature range 550 to 900°C at 0.1 GPa pressure. Near-liquidus phase relationships are given for the pseudobinary join ([CaCO 3 ] 45 [Ca(OH) 2 ] 55 ) 100– x – (NaTaO 3 ) x , 30 x 3 . The primary crystallization fields of calcite and microlite are separated by an exceptionally steep thermal valley located at about 55 wt.% NaTaO 3 . Quenched liquids contain calcite, portlandite, Na–Ca carbonates and calcium–tantalum oxide. The maximum solubility of tantalum in this system is estimated to be on the order of ~48 wt.% Ta 2 O 5 . At higher NaTaO 3 contents, microlite (isostructural with pyrochlore) crystallizes. In contrast, our previous work has demonstrated that lueshite, a perovskite-structured compound, crystallizes in the system CaCO 3 – Ca(OH) 2 – NaNbO 3 at high NaNbO 3 contents. The addition of 1.75 wt.% F to theTa system studied depresses the liquidus by >150°C, and tantalum solubility exceeds 61 wt.% Ta 2 O 5 . In experiments in the system CaCO 3 – Ca(OH) 2 – NaTaO 3 – NaNbO 3 , we found primary niobian microlite to crystallize at high temperature, and tantalian pyrochlore at lower temperature.
Abstract This paper provides a summary of selected diamond exploration techniques used in the glaciated terrain of Canada, focusing on indicator mineral methods and till geochemistry but also including geochemistry of lake sediments and vegetation. Diamond exploration in Canada focuses on kimberlite, the primary host rock for diamonds in this country. Kimberlite is a mineralogically and chemically distinct point source which may yield discrete dispersal trains in glacial sediments. Understanding the ice flow history and depositional history of glacial sediments and identifying multiple till sheets in areas covered by thick glacial sediments are essential for successful sampling, interpretation and follow-up of indicator mineral and geochemical anomalies related to these rocks. Orientation studies over known kimberlites provide important information on the mineralogical and geochemical signatures of kimberlite, and the size fractions of glacial sediments that are best suited to indicator mineral and geochemical analysis. Kimberlite indicator minerals survive glacial transport over long distances and the relative abundance of each mineral in till is a function of the primary mineralogy of individual kimberlites. Indicator mineral distributions observed at a regional scale define the net effect of glacial dispersal, often along different ice flow directions. Local scale distributions define individual dispersal trains. The finer (0.25 to 0.5 mm) fraction of heavy mineral concentrates prepared from till samples is best suited for indicator mineral surveys. Till geochemistry is gaining popularity in diamond exploration because it is significantly cheaper than indicator mineral analysis and it can be performed quickly. Important kimberlite pathfinder elements that provide good contrast in till geochemical surveys include Ni, Cr, Ba, Co, Sr, Rb, Nb, Mg, Ta, Ca, Fe, K, Ti and REE, the relative importance of which will depend on kimberlite composition as well as that of the surrounding bedrock. Biogeochemical studies over kimberlites in Canada reveal geochemical signatures in vegetation despite the glacially transported substrate.
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