Геохимия тантала и ниобия Aleksandr Stepanov CODES,UTAS March,

Геохимия тантала и ниобия Aleksandr Stepanov CODES,UTAS March, 2013

University of Tasmania One of the oldest universities in Australia School of Earth Sciences has main specializations in geology of ore deposits, volcanology and geochemistry.

LA-ICP-MS imaging of trace elements

Nb and Ta Nb and Ta are very similar elements with same charge and radius “Some think that the abundance of Nb on Earth should be much greater, but that the "missing" niobium may be located in the Earth’s core due to the metal's high density.” Wikipedia

Birth of “missing Nb paradox” McDonough, (1991) proposed that eclogites can be important host for Nb and Ta in the mantle. Green, (1995) demonstrated that Nb/Ta ratio is variable in terrestrial rocks. Nb/Ta of mantle eclogites counterbalance continental crust and mantle and thus it allows to calculate amount of eclogites stored in mantle: ~10 %. Much more than estimates from xenoliths counting. Rudnick et al., 2000, Science In eclogites Nb and Ta are stored in rutile, thus it should be effect of rutile which produced Nb/Ta fractionation. from Rudnick et al., 2000 chondrite crust GLOSS DM

Rutile effect Xiong, 2011 Schmidt, 2004 Numerous studies demonstrated that residual rutile can only increase Nb/Ta of the melt and decrease Nb/Ta ratio of the residue, or make insignificant effect. Schmidt, (2004), Xiong et al., (2011) Ilmenite and titanite do the same thing as rutile, even more efficiently (Prowatke&Klemme, 2005, John, et al. 2011). from Rudnick et al., 2000

Radical solutions Missing Nb is in the core (Wade and Wood, 2001, Nature) Earth has non chondrtic Nb/Ta ratio (Campbell and O’Naill, 2012, Nature) Hadean crust hypothesis (Nebel et al, 2010) Eclogites with high Nb/Ta ratio are not representative, and insignificant From Wade and Wood, 2001 from Rudnick et al., 2000 “missing Nb paradox”: where is Nb, which is necessary to balance crust and modern mantle? Complementary question is: how have formed residual rocks with high Nb/Ta ratios. Nb-Ta anomaly is unrelated question!

The solution and plan LA-ICP-MS data (15 spots) on partitioning of Nb and Ta by biotite and phengite. These minerals are able to fractionate Nb/Ta ratio efficiently. Nb/Ta ratio is increased during partial melting if residue contains biotite or phengite. Feasibility of this mechanism will be demonstrated by several lines of evidence: Nb and Ta distribution between minerals and melts in enclaves from El Hayazo, Spain. Nb and Ta fractionation in restites after lower crustal anatexis in Ivrea Verbano zone, Italy. Bulk rock geochemistry of the Kokchetav UHP gneisses Origin of Ta deposits Summary on Ta and Nb deposits. They are extremely different! Model for Ta rich melts Further directions Conclusions

Nb incompatible Nb/Ta in melt high Nb compatible Nb/Ta in melt low Partitioning coefficients summary Rt Amph Nb compatible Nb/Ta in melt high Nb incompatible Nb/Ta in melt low Nbmineral Nbmelt Tamineral Tamelt D(Ta)= D(Nb)= Different combinations form 4 quadrants. Rock forming minerals have low D(Nb) Ti minerals have D(Nb)>1, but D(Nb)/D(Ta)<1 Amphibole D(Nb)<1, and sometimes D(Nb)/D(Ta)>1

Partitioning coefficients for biotite and phengite Rt Amph Biotite and phengite have high D(Nb) Biotite and phengite have high D(Nb)/D(Ta) Range of D(Nb) and D(Ta) is large. D(Nb) is proportional to Ti content of mica. 4 high pressure experiments with large flakes of biotite or phengite Melt and biotite/phengite were analyzed by LA-ICP-MS. Also D(Nb) and D(Ta) calculated for studies with published compositions of melts and micas.

Case example 1: enclaves from El Hoyazo, Spain Antonio Acosta-Vigil et al. 2010 present perfect data on composition of melt (inclusions) and minerals in migmatite enclaves from rhyolites. Trace element compositions were measured by LA-ICP-MS. Biotite has high Nb content and controlled Nb/Ta ratio of melt. Ilmenite has very high Nb content but it appears that its effect on Nb-Ta in melt was insignificant.

Case example 2: Ivrea Verbano zone, Italy From Bea and Montero, 1999 Restites have high Nb concentrations and high Nb/Ta ratios Partial melts have low Nb/Ta ratios and concentrations. chondrite From Quick et al., 2009

Two stage melting for mica free restites In order to increase Nb/Ta ratio in mica-free restite required two stage melting: Stage 1: melting at the presence of residual mica. Rutile/ilmenite/titanite either absent of make little effect on the Nb/Ta ratio. Stage 2: incongruent melting of mica, produces residual rutile. Nb and Ta are retained and Nb-Ta fractionation is small. from Xiong, 2011

Case example 3: UHP gneisses from Kokchetav From K2O decrease and FeO+MgO increase in restite calculated amount of extracted melt. From LREE content estimated efficiency of melt extraction UHP gneisses from the Kokchetav complex represent metasediments, which experienced melting in subduction zone and then returned to the surface. 52 elements were analyzed by XRF and LA-ICP-MS in UHP gneisses and protolith rocks. Nb/Ta ratios are highly variable

Theoretical basis of partial melting From K2O decrease and FeO+MgO increase in restite calculated amount of extracted melt. From LREE content estimated fraction of melt extracted Amount of residual melt Amount of minerals (Grt, Coe, Phe, Cpx) in residue Presence/absence of rutile

Host phases for Ti in UHP gneisses There are three types of samples: Samples with residual phengite and without rutile have high Nb/Ta ratios. Samples with rutile and without phengite have Nb/Ta ratios slightly low than protolith. Samples with both rutile and phengite have elevated Nb/Ta ratios. Effect of phnegite is large than of rutile.

Origin of high Nb/Ta subduction related magmas High-K basalts with high Nb/Ta ratios are reported from several active plate margins. Features of these basalts (high K, high Nb/Ta) are similar to the features of the Kokchetav restitic gneisses with fertile composition: high abundance of K together with high Nb/Ta ratios Therefore these magmas demonstrate very remarkable thing: that restites very similar to the Kokchetav gneiss exist in subduction zone and that they experience further melting there! Stolz, et al. (1996) Kokchetav gneisses

Economic geology of Nb vs. Ta Nb ≈50 USD/kg 4,000 tons per year Used in Fe alloys Ta 300-500 USD/kg 500 tons per year Used in electronics Both Nb and Ta are elements of considerable economic interest and despite their general similarity they are extracted from completely different types of deposits.

Ta deposits Ta deposits are rare-metal pegmatites and apogranites. Important examples: Wodgina, Greenbushes, WA. Nb/Ta=0.1 Tanco pegmatite, Canada Volta Grande (MIBRA) Brasil. Nb/Ta=0.5 Kvarcevoe deposit, Kazakhstan (the only one which I have seen) Abu Dabbab, Naweibi, Gippsland, Egypt Nb/Ta=0.5 Volta Grande, Brasil Ta pegmatites are formed from highly evolved granitic melts (Pohl, 2011). Their features: High Ta, Rb, Cs, Sn contents, Low Nb/Ta ratios Low Ti, Zr and LREE content.

Ta minerals [(Mn2+, Fe2+) (Ta, Nb)2O6] columbite–tantalite series with intermediate solid solutions commonly referred as coltan, wodginite Mn2+(Sn,Ta)Ta2O8, microlite (Na,Ca)2Ta2O6(O,OH,F). Ta present in cassiterite Important source of Ta Columbite

Origin of Ta deposits I propose that formation of Ta pegmatites is result of fractional crystallization of biotite from granitic melts. Mica fractionation results decrease of Ti and Nb content and increase of Ta. More importantly biotite fractionation results in decrease of Ti content, and thus rutile and/or titanite are never saturated. Nece Nb abd Ta remain in the melt. Modeling of biotite crystallization reproduces trends.

Ta/Nb vs Ta

Was columbite saturated during evolution of Ta rich melts? Many papers conclude that solubility of columbite-tantalita in melts is the main factor in charge of Ta deposition. However Ta solubility in experiments are much higher than in deposits. Nb increases with fractionation, hence columbite fractionation is unlikely. Muscovite in granites and pegmatites shows high Nb contents (10-100 ppm Nb) and high Nb/Ta. Muscovite fractionation is important. wt.% in melt

Nb deposits Nb is mined from carbonatite and alkaline rocks with high Nb/Ta ratios. Main Nb is in pyrochlore NaNbO3. Araxa, Brazil. 80 % of world Nb supply. Weathered crbonatite. Catalao, Brasil. 12 % of world Nb supply. Weathered crbonatite. Niobec, Canada. 7% of world Nb supply. Fresh carbonatite. Mount Weld, WA. Weathered crbonatite. Nb deposits are likely formed by magmatic fractionation of Alcaline magmas. Nb-Ta in carbonatites from Chakhmouradian 2006 carbonatites

Conclusions: missing Nb Phengite and biotite have D(Nb)

Conclusions: Geochemistry of Nb and Ta deposits Nb and Ta deposits are formed by magmatic differentiation. Hydrothermal processes are insignificant, weathering can upgrade ore. Nb and Ta deposits occur in evolved melts, though of completely different composition: Ta in granites, Nb in carbonatites. Ta-rich granitic melts are formed by fractional crystallization of biotite from granitic melts. Biotite crystallization is very important because it prevents crystallization of rutile/titanite Muscovite fractionation can be important for final enrichment of Ta to ore garde

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