Kimberlites reveal 2.5-billion-year evolution of a deep, isolated mantle reservoir

The widely accepted paradigm of Earth's geochemical evolution states that the successive extraction of melts from the mantle over the past 4.5 billion years formed the continental crust, and produced at least one complementary melt-depleted reservoir that is now recognized as the upper-mantle source of mid-ocean-ridge basalts1. However, geochemical modelling and the occurrence of high 3He/4He (that is, primordial) signatures in some volcanic rocks suggest that volumes of relatively undifferentiated mantle may reside in deeper, isolated regions2. Some basalts from large igneous provinces may provide temporally restricted glimpses of the most primitive parts of the mantle3,4, but key questions regarding the longevity of such sources on planetary timescales—and whether any survive today—remain unresolved. Kimberlites, small-volume volcanic rocks that are the source of most diamonds, offer rare insights into aspects of the composition of the Earth’s deep mantle. The radiogenic isotope ratios of kimberlites of different ages enable us to map the evolution of this domain through time. Here we show that globally distributed kimberlites originate from a single homogeneous reservoir with an isotopic composition that is indicative of a uniform and pristine mantle source, which evolved in isolation over at least 2.5 billion years of Earth history—to our knowledge, the only such reservoir that has been identified to date. Around 200 million years ago, extensive volumes of the same source were perturbed, probably as a result of contamination by exogenic material. The distribution of affected kimberlites suggests that this event may be related to subduction along the margin of the Pangaea supercontinent. These results reveal a long-lived and globally extensive mantle reservoir that underwent subsequent disruption, possibly heralding a marked change to large-scale mantle-mixing regimes. These processes may explain why uncontaminated primordial mantle is so difficult to identify in recent mantle-derived melts

Liquid immiscibility and the origin of alkali-poor carbonatites

The work on liquid immiscibility in carbonate-silicate systems of Freestone and Hamilton (1980) has been extended to include alkali-poor and alkali-free compositions. Immiscibility isshown to occur on the j oins albite-calcite and anorthite-calcite at 5 kbar. These results make it possible to interpret ocellar structure between calcite-rich spheroids in lamproite or kimberlite host rock as products of liquid immiscibility. The common sequence of rock types found in carbonatite complexes of melilitite-ijolite-urtite-phonolite is interpreted as being the result of both fractional crystallization and liquid fractionation, the corresponding carbonatite composition changing from nearly pure CaCO 3 ( + MgCO3) progressively tonatrocarbonate. A carbonate melt cooling in isolation will suffer crystal fractionation, the residual liquid producing the rarer ferrocarbonatites, tc., whilst the crystal accumulate of calcite (dolomite) plus other phases uch as magnetite, apatite, baryte, pyrochlore, etc., are the raw material for the coarse-grained intrusive carbonatites commonly found in ring complexes

A Re-Os isotope and PGE study of kimberlite-derived peridotite xenoliths from Somerset Island and a comparison to the Slave and Kaapvaal cratons

En el excelente libro de T. Nagell citado en la bibliografía aparece el siguiente resultado

The geological record of base metal sulfides in the cratonic mantle: A microscale Os-187/Os-188 study of peridotite xenoliths from Somerset Island, Rae Craton (Canada)

We report detailed petrographic investigations along with Os-187/Os-188 data in Base Metal Sulfide (BMS) on four cratonic mantle xenoliths from Somerset Island (Rae Craton, Canada). The results shed light on the processes affecting the Re-Os systematics and provide time constraints on the formation and evolution of the cratonic lithospheric mantle beneath the Rae craton. When devoid of alteration, BMS grains mainly consist of pentlandite + pyrrhotite +/- chalcopyrite. The relatively high BMS modal abundance of the four investigated xenoliths cannot be reconciled with the residual nature of these peridotites, but requires addition of metasomatic BMS. This is especially evident in the two peridotites with the highest bulk Pd/Ir and Pd/Pt. Metasomatic BMS likely formed during melt/fluid percolation in the Sub Continental Lithospheric Mantle (SCLM) as well as during infiltration of the host kimberlite magma, when djerfisherite crystallized around older Fe-Ni-sulfides. On the whole-rock scale, kimberlite metasomatism is visible in a subset of bulk xenoliths, which defines a Re-Os errorchron that dates the host magma emplacement. The Os-187/Os-188 measured in the twenty analysed BMS grains vary from 0.1084 to > 0.17 and it shows no systematic variation depending on the sulfide mineralogical assemblage. The largest range in Os-187/Os-188 is observed in BMS grains from the two xenoliths with the highest Pd/Ir, Pd/Pt, and sulfide modal abundance. The whole-rock T-RD ages of these two samples underestimate the melting age obtained from BMS, demonstrating that bulk Re-Os model ages from peridotites with clear evidence of metasomatism should be treated with caution. The T-RD ages determined in BMS grains are clustered around 2.8-2.7, similar to 2.2 and similar to 1.9 Ga. The 2.8-2.7 Ga T-RD ages document the main SCLM building event in the Rae craton, which is likely related to the formation of the local greenstone belts in a continental rift setting. The Paleoproterozoic T ages can be explained by addition of metasomatic BMS during (i) major lithospheric rifting at similar to 2.2 Ga and (ii) the Taltson-Thelon orogeny at similar to 1.9 Ga. The data suggest that even metasomatic BMS can inherit Os-187/Os-188 from their original mantle source. The lack of isotopic equilibration, even at the micro-scale, allowed the preservation of different populations of BMS grains with distinct Os-187/Os-188, providing age information on multiple magmatic events that affected the SCLM. (C) 2017 Elsevier Ltd. All rights reserved

Craton reactivation on the Labrador Sea margins: 40Ar/39Ar age and Sr-Nd-Hf-Pb isotope constraints from alkaline and carbonatite intrusives

The once-contiguous North Atlantic craton (NAC) is crosscut by the Labrador Sea that opened during the Early Cenozoic after extensive Mesozoic continental rifting and removal of cratonic mantle. This large-scale structural change within the cratonic lithosphere was followed at about 150 Ma by the cessation of ultrapotassic and potassic-to-carbonatitic magma production, which had prevailed throughout much of the NAC history. At Aillik Bay, a sequence of olivine lamproites (1374.2 ± 4.2 Ma, 2σ), aillikites/carbonatites (590–555 Ma), and nephelinites (141.6 ± 1.0 Ma, 2σ) erupted through the southern NAC edge on the present-day Labrador Sea margin. Links between these alkaline magma types with diverse petrogeneses as a consequence of large-scale processes in the lithospheric mantle over a period of 1200 Myr are demonstrated utilizing their Sr–Nd–Hf–Pb isotope compositions. The Mesoproterozoic olivine lamproites are characterized by unradiogenic Nd (εNd(i) = − 8.4 to − 5.4), Hf (εHf(i) = − 11 to − 7.8), and Pb (206Pb/204Pb(i) = 14.2–14.8) but moderately radiogenic Sr isotope compositions (87Sr/86Sr(i) = 0.7047–0.7062) fingerprinting long-term enriched cratonic mantle, which must have reached to depths of more than 150 km at this time. In contrast, Neoproterozoic carbonate-rich aillikites and carbonatites have fairly radiogenic Nd (εNd(i) = 0.1–1.8), Hf (εHf(i) = − 0.9 to + 2.6), and Pb (206Pb/204Pb(i) = 17.5–18.8) but unradiogenic Sr isotope compositions (87Sr/86Sr(i) = 0.7033–0.7046) that point to the involvement of convective upper mantle material during melting. Simple binary mixing calculations coupled with the observation that carbonate-rich magmatism prevailed for over 30 Myr in the area imply a complex pattern of lithosphere–asthenosphere interaction at depths between ∼180 and 140 km. The Cretaceous nephelinites have slightly unradiogenic Nd (εNd(i) = − 4 to − 1.4), moderately radiogenic initial 87Sr/86Sr (0.7044–0.7062), but initial εHf (− 3.3 to + 1.4) similar to the aillikites and highly radiogenic Pb (206Pb/204Pb(i) = 19.1–20.2) isotope compositions. Their sodic mafic alkaline nature reflects partial melting at a higher level of the cratonic mantle tapping metasomatic components that had been introduced during the > 30 Myr of Neoproterozoic aillikite/carbonatite magmatism. The new 40Ar/39Ar age and Sr–Nd–Hf–Pb isotope data, along with petrological arguments, suggest that at least 30 km of the cratonic mantle beneath the southern NAC edge had been replaced by the hotter upwelling asthenosphere between ca. 550 Ma, when a thick diamond-bearing lithosphere was present, and 150 Ma. This lithospheric thinning presumably occurred shortly prior to Cretaceous continental rifting in response to enhanced plate-tectonic stresses focused at this zone of persistent lithospheric weakness. It appears, however, that the recurrent volatile-rich alkaline magmatism and associated mantle metasomatism played an important role in destroying the structural integrity of the cratonic mantle thereby aiding the subsequent lithosphere thinning