Kimberlite melts rich in alkali chlorides and carbonates: A potent metasomatic agent in the mantle

Kamenetsky, Maya B.; Sobolev, Alexander V.; Kamenetsky, Vadim S.; Maas, Roland; Danyushevsky, Leonid V.; Thomas, Rainer; Pokhilenko, N. P.; Sobolev, N. V.

doi:10.1130/g20821.1

Cited by 247 publications

(139 citation statements)

References 23 publications

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“…The analysis of melt and fluid inclusions in diamonds and other minerals found in kimberlites generated deep in the mantle (van Achterbergh et al 2003;Kamenetsky et al 2004;Korsakov and Hermann 2006;Tomlinson et al 2006;Klein-BenDavid et al 2007;Guzmics et al 2008) yielded compositions very similar to our experimental alkaline carbonatites. These inclusions are extremely rich in alkalis, with generally K 2 O [ Na 2 O, and also in volatiles (Cl, H 2 O and CO 2 ), and show a strong affinity to the cationic composition of our melts, especially the carbonate-rich end members (Fig.…”

Section: Evidence For Subducted Carbonates and K-rich Metasomatism Insupporting

confidence: 79%

Melting of carbonated pelites at 8–13 GPa: generating K-rich carbonatites for mantle metasomatism

Grassi

Schmidt

2010

Contrib Mineral Petrol

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The melting behaviour of three carbonated pelites containing 0-1 wt% water was studied at 8 and 13 GPa, 900-1,850°C to define conditions of melting, melt compositions and melting reactions. At 8 GPa, the fluidabsent and dry carbonated pelite solidi locate at 950 and 1,075°C, respectively; [100°C lower than in carbonated basalts and 150-300°C lower than the mantle adiabat. From 8 to 13 GPa, the fluid-present and dry solidi temperatures then increase to 1,150 and 1,325°C for the 1.1 wt% H 2 O and the dry composition, respectively. The melting behaviour in the 1.1 wt% H 2 O composition changes from fluid-absent at 8 GPa to fluid-present at 13 GPa with the pressure breakdown of phengite and the absence of other hydrous minerals. Melting reactions are controlled by carbonates, and the potassium and hydrous phases present in the subsolidus. The first melts, which composition has been determined by reverse sandwich experiments, are potassium-rich Ca-FeMg-carbonatites, with extreme K 2 O/Na 2 O wt ratios of up to 42 at 8 GPa. Na is compatible in clinopyroxene with D cpx=carbonatite Na ¼ 10À18 at the solidus at 8 GPa. The melt K 2 O/Na 2 O slightly decreases with increasing temperature and degree of melting but strongly decreases from 8 to 13 GPa when K-hollandite extends its stability field to 200°C above the solidus. The compositional array of the sediment-derived carbonatites is congruent with alkali-and CO 2 -rich melt or fluid inclusions found in diamonds. The fluid-absent melting of carbonated pelites at 8 GPa contrasts that at B5 GPa where silicate melts form at lower temperatures than carbonatites. Comparison of our melting temperatures with typical subduction and mantle geotherms shows that melting of carbonated pelites to 400-km depth is only feasible for extremely hot subduction. Nevertheless, melting may occur when subduction slows down or stops and thermal relaxation sets in. Our experiments show that CO 2 -metasomatism originating from subducted crust is intimately linked with K-metasomatism at depth of [200 km. As long as the mantle remains adiabatic, lowviscosity carbonatites will rise into the mantle and percolate upwards. In cold subcontinental lithospheric mantle keels, the potassic Ca-Fe-Mg-carbonatites may freeze when reacting with the surrounding mantle leading to potassium-, carbonate/diamond-and incompatible element enriched metasomatized zones, which are most likely at the origin of ultrapotassic magmas such as group II kimberlites.

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Section: Evidence For Subducted Carbonates and K-rich Metasomatism Insupporting

confidence: 79%

Melting of carbonated pelites at 8–13 GPa: generating K-rich carbonatites for mantle metasomatism

Grassi

Schmidt

2010

Contrib Mineral Petrol

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“…K-rich pyroxenes might form on the liquidus of a silicate magma mixed or coexisting with K-rich brine (42). Recent attention to chlorides is also due to new evidence of their participation in metasomatic processes in the mantle (43)(44)(45)(46) and deep zones of the crust (47). Speculation of the possible nature of Cl in the mantle has also been recently reviewed (48).…”

mentioning

confidence: 99%

The role of mantle ultrapotassic fluids in diamond formation

Palyanov

Shatsky

Sobolev

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

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Analysis of data on micro-and nano-inclusions in mantle-derived and metamorphic diamonds shows that, to a first approximation, diamond-forming medium can be considered as a specific ultrapotassic, carbonate/chloride/silicate/water fluid. In the present work, the processes and mechanisms of diamond crystallization were experimentally studied at 7.5 GPa, within the temperature range of 1,400 -1,800°C, with different compositions of melts and fluids in the KCl/K 2CO3/H2O/C system. It has been established that, at constant pressure, temperature, and run duration, the mechanisms of diamond nucleation, degree of graphite-to-diamond transformation, and formation of metastable graphite are governed chiefly by the composition of the fluids and melts. The experimental data suggest that the evolution of the composition of deep-seated ultrapotassic fluids/melts is a crucial factor of diamond formation in mantle and ultrahigh-pressure metamorphic processes. high-pressure experimentA n important stage in solving problems of diamond genesis is constructing a clear physical model of diamond formation (1). One of the key parameters to be modeled is the composition of the diamond crystallization medium, with a fluid being largely considered as a crucial agent in diamond formation. It is generally agreed that the mantle media of diamond crystallization were volatile-saturated melts or fluids (1-7). Carbonbearing fluids and carbonates could be sources of carbon, and the medium could have been saturated with it by redox reactions. Recent studies have been aimed at searching and examining fluid and fluid-containing inclusions in genetically different diamonds. Also, experiments have been performed to model the processes of diamond crystallization in the media compositionally corresponding to the inclusions. It is the combination of these two approaches that permits us to gain deeper insight into diamond genesis and to better understand the specific character of processes of deep-seated mineral formation.Studies of mineral inclusions in diamonds have determined the main types of diamondiferous parageneses: ultramaffic, eclogitic (4,8,9), and intermediate websterite (10) and calc-silicate types (11). Some mineral inclusions, e.g., intergrowths of phlogopite with pyroxene (8, 12, 13) and phlogopite with calcite (14), are unambiguously evidence that K and H 2 O were present in the diamond-forming medium. It is pertinent to note that, as early as 30 years ago, neutron activation analysis of diamonds without visible inclusions, taken from three South African deposits, revealed a mixture of mostly K and of other components, which was interpreted as evidence of the presence of entrapped melt (15). Principally, more recent information has accrued from micro-and nano-inclusions in mantle-derived and metamorphic diamonds. Analysis of the composition of the inclusions in fibrous or cloudy mantle diamonds shows that, at the time of entrapment, the inclusion matter was a specific fluid whose composition belongs to one of the three main types: (i) ...

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“…Melt inclusions in groundmass olivine contain alkali-Ca carbonates and CO 2 fluid similar to the mantle xenoliths and xenocryst inclusions in Vulture [44]. The alkali chlorides and alkali carbonates in the Udachnaya-East pipe are characterised by high 87 Sr/ 86 Sr ratios of 0.7069 and 0.7050 respectively [43], in the range of Vulture igneous rocks and carbonates. Alkali carbonate-chloride fluid compositions are also typical of micro-inclusions trapped in diamonds [64,65] at around 5-7 GPa.…”

Section: Discussionmentioning

confidence: 99%

“…Then nyerereite was found in chloride-carbonate nodules in the Udachnaya-East kimberlites [37,38]. Moreover, nyerereite, together with other alkaline carbonates, has been identified as a daughter phase of mineral-hosted melt inclusions in silicate rocks of carbonatite-related alkaline complexes and in kimberlites [39][40][41][42][43][44][45]. The chemical com- Table 1.…”

Section: Nyerereite Inclusionsmentioning

confidence: 99%

Nyerereite from carbonatite rocks at Vulture volcano: implications for mantle metasomatism and petrogenesis of alkali carbonate melts Research Article

2009

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Vulture volcano displays a wide range of mafic to alkaline, carbonate-, and/or CaO-rich volcanic rocks, with subvolcanic and plutonic rocks together with mantle xenoliths in pyroclastic ejecta. The roles of magmatic volatiles such as CO 2 , S, and Cl have been determined from compositions and trapping temperatures of inclusions in phenocrysts, which include the Na-K-Ca-carbonate nyerereite within melilite. We surmise that this alkali carbonate crystallised from an appropriate carbonatitic melt at relatively high temperature. Carbonatitic metasomatic features are traceable throughout many of the mantle xenoliths, and various carbonatitic components are found in the late stage extrusive suite. There is no evidence that alkali carbonatite developed as a separate magma, but it may have been an important evolutionary stage. We compare the rare occurrence of nyerereite at Vulture with other carbonatites and with an unaltered kimberlite from the Udachnaya pipe. We review the evidence at Vulture for associated carbonatitic metasomatism in the mantle, and we suggest that low viscosity alkali carbonatitic melts may have a primary and much deeper origin than previously considered.

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Kimberlite melts rich in alkali chlorides and carbonates: A potent metasomatic agent in the mantle

Cited by 247 publications

References 23 publications

Melting of carbonated pelites at 8–13 GPa: generating K-rich carbonatites for mantle metasomatism

Melting of carbonated pelites at 8–13 GPa: generating K-rich carbonatites for mantle metasomatism

The role of mantle ultrapotassic fluids in diamond formation

Nyerereite from carbonatite rocks at Vulture volcano: implications for mantle metasomatism and petrogenesis of alkali carbonate melts Research Article

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