182W anomalies in mantle:a brief review

Yining Zhang · Yun Liu

Abstract With significant advances in mass spectrometry for isotope analysis in the last decade,e.g.,negative thermal ionization mass spectrometry and multi-collector inductively coupled plasma mass spectrometry,high-precision (ppm-level) measurements of tungsten (W) isotopes have been widely used for early earth differentiation processes,such as metal-silicate segregation,melting and crystallization processes during the magma ocean,and putative core-mantle exchange and dynamics.Here,we give a brief review of works on 182W anomalies in terrestrial samples,including methods,results,explanations,implications,and prospects.The review will be presented by including the following parts:the introduction of W isotopes and the short-lived radioactive 182Hf-182W system;data notations and W isotope measurement methods;182W anomalies observed in terrestrial samples;a summary of models developed for interpreting origins of positive and negative 182W anomalies;future prospects.

Keywords 182W anomalies · Mantle samples · MC-ICPMS · N-TIMS · Normalization

1 W isotopes and the 182Hf-182W system

1.1 W isotopes

There are five stable tungsten isotopes,i.e.,180W,182W,183W,184W,and186W,with averaged present-day abundances of 0.12(1),26.50(16),14.31(4),30.64(2),and 28.43(19) in percentage,respectively.“The values in paratheses following the isotopic abundances indicate the range of probable isotope-abundance variations among different materials as well as measurement uncertainties”wrote by Meija et al.(2016).The nucleosynthesis theory shows that those five isotopes have different nucleosynthetic origins.Except for the180W that is believed to be ap-process dominated nuclide,182W,183W,184W,and186W nuclides are all products of boths-andr-processes with different proportions (e.g.,Arlandini et al.1999;Bisterzo et al.2011).The magnitudes of nucleosynthetic W isotope anomalies can be obtained by measuring W isotope compositions (183W/184W) because different proportions ofsandr-process will lead to correlated W isotope anomalies(182W/184W vs.183W/184W) (Burkhardt and Schönbächler 2015;Kleine and Walker 2017).Except for those primary neutron/proton capture processes,the secondary neutron capture related to galactic cosmic rays (GCR) cannot be ignored neither,especially for182W.Detailed information on these nucleosynthetic and GCR-related secondary neutron capture effects is out of the scope of this study,and the review of Kleine and Walker (2017) is recommended.

1.2 Basic principles of the 182Hf-182W chronometer

182Hf is a now-extinct short-lived radioactive nuclide that decays into182W through two β–processes(182Hf →182W)with a half-lifet1/2of(8.90 ± 0.09)×106yr(Vockenhuber et al.2004).Considering the geochemical properties for elements Hf and W,which Hf is lithophile and W is siderophile,indicating significant Hf/W differentiation during metal-silicate segregation processes,such as core formation where W will be partitioned into the metallic core and Hf will be retained in silicate mantles.Besides,both Hf and W are refractory with similar 50% condensation temperatures(50%TC)of～1700 K(Lodders 2003),indicating little Hf/W differentiation during condensation of the nebula that can be safely ignored.Furthermore,experimental results showed that Hf is relatively more compatible than W during crystallization and melting processes involving silicates,where significant Hf/W differentiation will be produced by mineral phases such as ilmenite,high-Ca clinopyroxene,and garnet(Shearer and Righter 2003).These basic principles for the182Hf-182W chronometer are summarized in Table 1.

Table 1 Summarized basic principles for 182Hf-182W chronometer

2 Data notations and W isotope measurements

2.1 Data notations

Generally,two different notations are being widely used to denote the relative deviations of182W/183W and182W/184W to reference materials:ε and μ values can be expressed as(Here we take182W/184W as an example)

Here the ε values are deviations of part per ten thousand and μ values are those of part per million and μ182W=100×ε182W.

2.2 W isotope measurements and correction methods

There are two different methods for high-precision W isotope analysis,N-TIMS,and MC-ICP-MS.A brief introduction of these two methods will be presented below including several correction methods developed in recent years.

2.2.1 Mass bias correction or normalization

To correct all possible natural and instrumental mass-dependent fractionations,mass bias correction,also known as normalization must be applied by fixing one of the isotope ratios,such as186W/184W,186W/183W,and184W/183W(hereafter will be denoted as 6/4,6/3 and 4/3) for W isotopes and correcting other ratios with a theoretical mass fractionation law (linear,equilibrium,kinetic,power and exponential).Theoretically,such normalization will guarantee that all statistically resolvable non-zero anomalies must be caused by these possible mechanisms,such as incomplete mass bias corrections,mass-independent fractionations,radioactive decay,and nucleosynthesis.For W isotopes,both 6/3 and 6/4 normalizations with an exponential fractionation law have been widely used and can be briefly expressed with Eq.(3)and(4)below(taking 6/4 as an example).

Here (186W/184W)measuredis the measured raw ratio by mass spectrometer for186W/184W andm186/m184is the atomic mass ratio between186W and184W.Normalization is thus performed by firstly obtaining the β value using Eq.(3)and then calculating other corrected ratios with this β value using Eq.(4).

2.2.2 N-TIMS analysis

Because of the high first ionization potential (FIP) of W atom(FIP ≈8.0 eV=772 kJ/mol),a[WO3]-anion rather than [W]+ion is produced through negative thermal ionization with electron emitter (generally La and Gd solutions) and transmitted into the mass spectrometer for isotope analysis.However,there are 3 different stable isotopes for O,i.e.,16O,17O,and18O,leading to oxide interferences among different isotopologues of [WO3]-,such as [182W16O217O]-vs.[183W16O3]-.Thus,the effect of such interferences and potential variations of O isotope composition must be carefully handled and corrected.Furthermore,as the prepared and purified sample solutions are generally loaded on a Re filament for thermal ionization during N-TIMS analysis and part of the Re filament will also be ionized into [ReO3]-,along with the fact of two stable isotopes for Re,185Re,and187Re,leading to another mass interference caused by [ReO3]-,such as[185Re16O3]-versus[184W16O217O]-and[183W16O218O]-.Therefore,Re interferences correction also needs to be included.In a word,to obtain182W anomalies with high precision using N-TIMS,all measured [WO3]-and[ReO3]-data must be reasonably corrected for oxygen and Re interferences (also known as oxide interferences) and then correcting all mass biases with normalization.

Correction methods for oxide interferences were initially developed by Heumann et al.(1989) and Harper and Jacobsen (1996).Detailed introductions about these methods are out of our scope here.In 2012,Touboul and Walker (2012) modified the method of Harper and Jacobsen (1996) and improved the precision for182W/184W into ±4.5 ppm by adopting a double normalization correction method.This method can be briefly summarized into 3 steps:(1)Initial oxide interferences correction using predefined O isotope compositions;(2) 6/4 normalization;(3)Double normalization by fixing183W/184W with a linear fractionation law.The reason why this method used a double normalization was based on the observation that there are theoretically unexpected correlations between182W/184W and183W/184W ratios after 6/4 normalization.In addition,Touboul and Walker (2012) found that precision for182W/184W ratio without double normalization will be larger than ±65 ppm,which is meaningless for the high-precision requirement on the W isotope anomaly study today.

However,Trinquier et al.(2016) pointed out that the double normalization method cannot give reasonable results unless183W/184W ratios are constant,which is probably invalid.Therefore,they developed a new analytical protocol that can simultaneously monitor the18O/16O variations when measuring the W isotope ratios with long-term 2 standard deviations(2 SD)of 10–11 ppm and 17–18 ppm for182W/184W and183W/184W.Unfortunately,similar correlations between normalized W isotope ratios were still observed and show a time dependence.

Based on the work of Trinquier et al.(2016),Archer et al.(2017)proposed a new correction technique to further improve the precision for measuring W isotope ratios along with simultaneous monitoring of O isotope compositions.Their methods can be divided into 5 steps:(1)Initial oxide interferences correction using a newly in-lab determined O isotope composition;(2) Recalculate per-integration18O/16O for [WO3]-and [ReO3]-anions;(3) Calculate per-integration17O/16O for [WO3]-and [ReO3]-anions using18O/16O obtained in step 2 with the in-lab determined correlation line between17O/16O and18O/16O ratios;(4)Second oxide interferences using calculated O isotope composition in step 2 and 3;(5) Normalization.This method achieved long-term external precisions of 5.7 ppm(2 SD) and 6.6 ppm (2 SD) for182W/184W and183W/184W ratios.However,correlations between182W/184W and183W/184W ratios were again observed.Archer et al.(2017)inferred that such correlations were potentially caused by Faraday cup degradation and were eliminated after cup maintenance.

Finally,the main differences between two oxide correction methods developed by Touboul and Walker(2012)and Archer et al.(2017) can be summarized as:One uses fixed oxygen isotope composition for oxide correction and uses183W/184W for double normalization.One uses simultaneously monitored oxygen isotope composition,which avoids using183W/184W and potential non-massdependent fractionation effects on183W.

2.2.3 MC-ICP-MS analysis

Different from N-TIMS,W isotopes are directly analyzed through the form of [W]+in MC-ICP-MS.Therefore,normalization can be directly performed for measured raw data.Interestingly,correlations between two W isotope ratios involving182W,183W,and184W after normalization were also observed in MC-ICP-MS analysis just like those using N-TIMS (Willbold et al.2011;Shirai and Humayun 2011;Kruijer et al.2012;Cook and Schönbächler 2016;Kruijer and Kleine 2018;Tusch et al.2019).Detailed introductions of such correlations are beyond the scope of this brief review.These works cited above are recommended.

2.2.4 Correcting coupled W isotope ratios

To correct the effect of correlated W isotope ratios on182W anomalies,several empirical correcting methods have been proposed.For MC-ICP-MS analysis,Kruijer et al.(2012)proposed the following empirical correction equations:ε182/183W (6/3)corr.=ε182/183W (6/3)meas.-(-2)×ε183/184W (6/3)meas.;ε182/184W (6/3)corr.=ε182/184W (6/3)meas.--(-1) ε183/184W (6/3)meas..After using this correction method,identical ε182/183W (6/3) and ε182/184W (6/4) were obtained.They inferred that the possible origin for such correlations was a mass-independent isotope fractionation between odd and even W isotopes related to W-loss during the sample preparation and purification process.Similarly,by analyzing W isotope compositions of the standard material NIST SRM 129c,Cook and Schönbächler (2016)found that these observed correlations between ε182/183W(6/3) and ε184/183W (6/3) can be well explained by the nuclear field shift effect (NFSE) and thus proposed another correction method as ε182/183W(6/3)corr.=ε182/183W(6/3)meas.-(1.962×ε184/183W (6/3)meas.).Their work showed that the previously inferred mass-independent fractionation process might be NFSE-induced fractionation due to the production of polytungstates and precipitate during chemical preparations.As for N-TIMS analysis,Trinquier et al.(2016) proposed that observed correlations can be mitigated by averaging data over shorter sessions.Kruijer and Kleine (2018) proposed that double normalization developed by Touboul and Walker(2012)shall not be used because they found that non-mass-independent fractionations of183W caused by NFSE or mass interferences by organic matter can significantly affect183W/184W ratios and182W/184W ratios when using the double normalization method.In other words,effects of incomplete recovery of W,O isotope variations,and NFSE-caused mass-independent fractionations must be all carefully handled during high-precision W isotope measurement.

3 182W anomalies in terrestrial samples

3.1 Basic observations and assumptions

To determine182W anomalies in terrestrial samples,three basic assumptions must be introduced here:

1.The bulk silicate Earth (BSE) is chosen to be the reference for W isotope anomalies,indicating ε180-W=ε182W=ε183W=ε184W=0 for BSE.

2.The bulk Earth is assumed to have a chondritic Hf/W ratio and W isotope composition,ε182WBulk-Earth-=ε182Wchondirtes=-1.9 ± 0.1 (Kleine et al.2002;Yin et al.2002) compared to the BSE.Traditionally,such difference between the BSE and chondrites indicates an early core formation event for the Earth within the first 60 Ma of the solar system.

3.Using mass balance calculation,182W anomaly for Earth’s core,i.e.,ε182Wcore,is assumed to be～ -2.2 ε (-220 ppm).

3.2 Positive 182W anomalies

Using high-precision W isotope analysis with MC-ICPMS,Willbold et al.(2011) firstly observed an averaged positive182W anomaly ε182/184W (6/4) of+0.13 ± 0.04(after correcting minor variations of183W) in～3.8 Ga Isua Archean samples,which consist of orthogneisses,metabasalts,amphibolite enclaves,and metasediments,but no resolvable anomalies (-0.01 ± 0.02) in post-Archean samples including Iceland dactite,La Palma basalt,Sao Miguel basalt and Guernsey gneiss.They speculated that such positive anomalies may be a residue of the Earth mantle before the late veneer because the addition of 0.3–0.8 wt% chondritic Earth-mass during late accretion with ε182W of～ -1.9 can lower the182W/184W ratio of the mantle by 10–30 ppm on average.The observed decrease on positive182W anomalies since Archean may result from convective mixing of the mantle.

In 2012,Touboul et al.(2012) observed positive182W anomalies in 2.8 Ga Kostomuksha komatiites using N-TIMS.Averaged μ182/184W(6/4)=+15.0 ± 4.8 ppm,along with estimated～80% highly siderophile elements(HSE)abundances of the modern primitive mantle(Puchtel and Humayun 2005)and coupled enrichments in187Os and186Os (Puchtel et al.2005).These anomalies were explained by the long-term preservation of early mantle differentiation products formed within the first 30 million years of the solar system history through metal-silicate equilibration or large-scale magmatic differentiation of the mantle.In other words,Touboul et al.(2012) inferred that observed positive182W anomalies were caused by radiogenic growth with the decay of182Hf before 4.53 Ga.

In 2014,Touboul et al.(2014) observed positive182W anomalies μ182/184W (6/4) ranging from+6 to+17 ppm in ≥3.66 Ga ultramafic,mafic,and felsic supracrustal rocks from the Nuvvuagittuq Greenstone Belt using N-TIMS.Based on measured HSE concentration,Os isotope,and major elements data,they inferred that those positive anomalies are caused by metasomatism from a182W-rich crustal component been recycled into the mantle via subduction or delamination.However,the source of such crustal components carrying182W excess remains unclear.

In 2015,Willbold et al.(2015) observed positive182W anomalies ε182/184W (6/4) in～3.96 Ga mafic and felsic samples from Acasta Gneiss Complex (AGC) ranging from+0.06 to+0.15 using MC-ICP-MS.However,no resolvable anomalies were observed in～3.6 Ga felsic samples from the same terrane.Similar to those of Touboul et al.(2012),they attributed those anomalies and decreasing trends to incomplete homogenization of the mantle where geochemical signals of these residues of pre-late veneer mantle,like positive182W anomalies were gradually erased by mantle convection.In addition,similar anomalies in AGC samples were observed by later works of Reimink et al.(2018,2020).

In 2016,Rizo et al.(2016a) reported stable W isotopic compositions,182W/184W ratios,HSE abundances,and187Re–187Os systematics in 3.8–3.3 Ga mafic and ultramafic rocks from Isua supracrustal belt.They observed resolvable182W anomalies up to～+21 ppm by MCICP-MS analysis.Unlike those from Willbold et al.(2011),they inferred that positive182W anomalies were not caused by the isolation of a pre-late veneer mantle,rather than a result of Hf/W fractionation within the first 50 Ma of solar system history,and the potential process might be early magma ocean fractionation.Meanwhile,Rizo et al.(2016b)reported first observations of positive182W anomalies[μ182/184W (6/4)] in modern flood basalts from Baffin Bay(up to+48.4 ± 4.6 ppm) and Ontong Java Plateau(sample ID:192-1187A-009R-04R;+23.9 ± 5.3 ppm)using N-TIMS.However,later work of Kruijer and Kleine(2018) reported no resolvable positive anomalies in the same Ontong Java Plateau sample (192-1187A-006R-06W).They proposed that those anomalies might be analytical artifacts due to mass-independent biases of183W and double normalization and cast some doubt on Rizo et al.(2016b)’s data on Baffin Bay samples.By measuring some Eoarchean (＞3.8 Ga) ultramafic rocks in Saglek Block using N-TIMS,Liu et al.(2016) also observed uniform positive anomalies of～+11 ppm.With other analyzed geochemical data like trace lithophile elements,they speculated that such anomalies were derived from the recycling of182W-rich crustal rocks into the mantle due to the fluid mobility of tungsten.

Dale et al.(2017) analyzed HSE abundances and182W compositions in the similar 3.8 Ga Isua mafic and ultramafic samples from Isua Greenstone Belt like those previous works.A+13 ± 4 ppm182W excess[μ182/184W(6/4) with MC-ICP-MS] and 50–60% of HSE abundances of the modern primitive mantle were observed and they attributed these data to incomplete mixing between those pre-late veneer mantle and late veneer materials after the putative Moon-forming giant impact.

Puchtel et al.(2018) observed positive182W anomalies[μ182/184W (6/4) with N-TIMS] of+11.7 ± 4.5 ppm,chondritic initial187Os composition,and low estimated source HSE abundances (35 ± 5% of that modern primitive mantle) in Boston Creek komatiitic basalt lava flow(BCF) in 2.7 Ga Abitibi Greenstone Belt.They inferred that derivation of the parental BCF magma from a mantle domain characterized by HSE-deficient,differentiated accreted materials can produce observed182W anomalies and HSE deficiency.

Tusch et al.(2019) reported comprehensive data for different rock types from Itsaq Gneiss Complex (IGC,3.2–3.8 Ga) including high-precision W isotope with W-Th-U-Ta abundance data.A uniform positive182W anomalies [μ182/184W (6/4) with MC-ICP-MS] of+12.8± 1 ppm was obtained.Along with decoupled182W anomalies versus HSE abundances and Platinum Group Element (PGE) patterns in the Isua region,they proposed that observed182W excesses are a residue of early silicate differentiation processes before 4.50 Ga,which might be an early magma ocean differentiation event.

Mei et al.(2020) observed182W excesses(+14.5 ± 4.0 ppm)in 4.0–3.8 Ga diorite and trondhjemite samples in North China Craton (NCC) but normal182W compositions in 3.36–2.95 Ga TTG rocks in NCC.They inferred that those positive anomalies may be caused by either early mantle differentiation that occurred within the lifetime of182Hf or a partial lack of late accreted material.Similarly,with the help of W/Th ratios who can evaluate the secondary alternation of samples due to fluid mobility of W,Tusch et al.(2020)also observed a uniform pristine (samples with canonical W/Th ratios)182W excess of+12.6 ± 1.4 ppm in rock suite from Pilbara Craton spanning an age range from 3.58 to 2.76 Ga.Along with the previously reported HSE data,they proposed that the missing late veneer hypothesis can serve as a plausible explanation.Furthermore,using W/Th ratios and the fact of missing142Nd anomalies in these samples,they also claimed that the early silicate crystal-liquid differentiation model can be reasonably ruled out.

In summary,almost all182W excesses were observed in Archean samples from Isua Greenstone Belt (～3.8 Ga),Kostomuksha komatiites (2.8 Ga),Nuvvuagittuq Greenstone Belt (≥3.66 Ga),Acasta Gneiss Complex(～3.96 Ga),Saglek Block (＞3.8 Ga),Abitibi Greenstone Belt (～2.7 Ga),Itsaq Gneiss Complex (3.2 to 3.8 Ga),North China Craton (4.0 to 3.8 Ga) and Pilbara Craton (3.58 to 2.76 Ga) except for those data for modern flood basalts in the Baffin Bay and Ontong Java Plateau by Rizo et al.(2016b).However,because results obtained by Rizo et al.(2016b)were still debatable(Kruijer and Kleine 2018),thus we conservatively conclude here that resolvable182W excesses were only observed in Archean samples so far.A summary of these observations is listed in Table 2.

3.3 Negative 182W anomalies

In 2002,Schoenberg et al.(2002) reported significant182W/183W depletions (ε182/183W (4/3) down to-1.23 ± 0.38) in 3.7–3.8 Ga metasediments in Isua Greenstone Belt relative to in-lab reference material ACQUIRE-W using MC-ICP-MS.However,considering later measurements for similar samples,Willbold et al.(2011) stated that those negative182W anomalies were likely to be analytical artifacts.

Since then,the “rediscovery” of negative182W anomalies was performed by Puchtel et al.(2016) in 3.55 Ga Schapenburg komatiites using N-TIMS.An averaged μ182/184W (6/4)=-8.4 ± 4.5 ppm,along with coupled μ142Nd=-4.9 ± 2.8 ppm and estimated 29 ± 5% HSE abundances in source relative to modern primitive mantle were observed and treated to be derived from a mantle domain that formed within the first～30 Ma of solar system history who enriched in incompatible elements (relatively low Hf/W and Sm/Nd ratios).

In 2017,Mundl et al.(2017) reported negative182W anomalies (down to -18 ppm) in modern ocean island basalts (OIB) from Hawaii,Samoa,and Iceland relative to terrestrial W isotope standard material Alfa Aesar using N-TIMS with correction protocols of Archer et al.(2017).In addition,they also observed that those182W anomalies were negatively correlated with3He/4He ratios,suggesting that the OIB sources accessed signals formed within the first 60 million years of the solar system history and the most likely candidate was mega-ultralow-velocity zones(ULVZ).

Using MC-ICP-MS,Mei et al.(2018)observed negative182W anomalies (ε182W=-0.09 ± 0.05,2 SD,n=12)in BHVO-2 standard material compared with commonly used Alfa Aesar W reference.They inferred that such anomalies might be a result of organic interferences on mass 183 or mass-independent fractionation of183W.However,the detailed physiochemical mechanisms remain unclear.

Mundl et al.(2018) analyzed some fine-grained glacial diamictites deposited between～3.0 and～0.3 Ga using N-TIMS and found an averaged μ182/184W (6/4) value of-12.5 ± 5.0 ppm(2 SD).Based on accompanied high Ni and HSE abundances,they inferred that such negative182W anomalies reflect contributions from deep mantle upwellings that produced some of the komatiites.

Also using N-TIMS and newly developed correction methods by Archer et al.(2017),Mundl-Petermeier et al.(2019) analyzed basaltic samples erupted in Greenland-Iceland plume and found two groups of samples with μ182W ranging from+1.7 to-9.1 ± 4.5 ppm and -0.6 to -11.7 ± 4.5 ppm.Combined with Pb isotope and3He/4He data,they proposed a mixing model between a minimum of three mantle source domains,where one of the domains may be characterized by negative182W anomalies that likely formed within the lifetime of182Hf before 4.5 Ga.

Using N-TIMS,Rizo et al.(2019) analyzed182W and183W compositions for several plume-related volcanic rocks from the Réunion Island and the Kerguelen Archipelago hotspots and also the BHVO-2 standard.All plume samples were found yielding negative μ182W values ranging from -5.2 ± 3.7 to -20.2 ± 5.1 ppm.In addition,they also found a minor but resolvable negative μ182W value of -6.6 ± 1.9 ppm in BHVO-2 like those of Mei et al.(2018).Along with other geochemical data,Rizo et al.(2019) revealed a temporal shift on182W isotope composition since Archean (positive μ182W) to modern(negative μ182W) might be best explained by core-mantle exchange and decreasing of positive μ182W at～2.7 Ga could be related to the onset of inner core crystallization and post-Archean deep slab subduction.

In 2020,Mundl-Petermeier et al.(2020) reported182W compositions and3He/4He data for rocks from 15 different hotspots with N-TIMS analysis.They found these rocks yielding μ182W values from～0 to -23 ± 4.5 ppm.By using3He/4He data and a mixing model,they speculated a mantle source domain characterized by μ182-W ≤ -23 ppm may be formed in the lowermost mantle as a result of core-mantle isotopic equilibration and the ultra-low-velocity-zones (ULVZ) likely contributed some of the observed negative μ182W values in some OIB samples.

Later on,Jackson et al.(2020) performed systematic analysis for those OIB samples from Iceland with the highest3He/4He ratios,including182W composition (NTIMS analysis)and Sr-Nd-Hf-Pb isotopes.They found that OIB samples hold the highest3He/4He and largest-magnitude182W anomalies can only be found in geochemically depleted mantle domains with high143Nd/144Nd and low206Pb/204Pb without signatures of recycled materials.Based on this inference,they speculated that one possible origin of these negative182W anomalies may be core-mantle interactions.

Recently,Nakanishi et al.(2021) reported homogenous μ182W values of -5.9 ± 3.6 ppm (2 SD,n=13) in“primitive” kimberlites from 10 localities worldwide ranging in age from 1153 to 89 Ma with N-TIMS.Also using the W/Th ratio to evaluate the effect of second fluid alternation for W and other geochemical data,they concluded that these negative anomalies are inherited from their mantle source,which might be created by contamination of core materials,a result of early silicate fractionation (with low Hf/W ratio),an overabundance of late veneer (elevated HSE contents),or a combination of all these effects.

Except for those results from Puchtel et al.(2016),Mundl et al.(2018) and Nakanishi et al.(2021),all reported samples with negative μ182W values are plumerelated basalts.In addition,except for the results of Mei et al.(2018),all the other negative182W anomalies were obtained using N-TIMS analysis.A summary is also listed in Table 3.

4 A summary of origin models for 182W anomalies in the mantle

4.1 Positive 182W anomalies:Hf/W differentiation and decay of 182Hf within the first 50-60 Ma of the solar system history

As we mentioned in Sect.1.2,processes that can induce Hf/W differentiations that occur within the lifetime of182Hf (i.e.,5×to 7×oft1/2for182Hf:44.5–62.3 million years) will cause enrichment or depletion in182W,such as metal-silicate segregation (negative μ182W in metal and positive μ182W in silicates),partial melting and crystallization of silicates (negative μ182W in melt and positive μ182W in solids) and so on.Because of the non-zero value of the solar system’s initial182Hf/180Hf ratio(Kruijer et al.2014),several different processes were proposed for explaining positive182W anomalies,such as metal-silicate segregation during core formation events (Touboul et al.2012) and early magma ocean differentiation (Touboul et al.2012,2014;Rizo et al.2016a;Tusch et al.2019;Mei et al.2020).In addition,these processes were also used to account for some negative182W anomalies (Puchtel et al.2016;Mundl et al.2017,2018)as residues of early magma ocean differentiation enriched in incomplete elements(low Hf/W ratio).

However,several important issues must be pointed out here:

1.These mechanisms for positive182W anomalies require long-term preservation (isolation) of source domains,which are absent from mantle convection;

2.Possible collateral geochemical signals are pressingly needed to put more constraints on both mechanisms,such as HSE-and MSE-abundances after the metalsilicate segregation,the differentiation of Re-Os(186Os and187Os),Sm-Nd (147Sm-143Nd and146Sm-142Nd),and Lu-Hf (176Hf) for magma ocean period;

3.Partitioning behaviors of Hf and W among different phases and controlling factors (oxygen fugacity,temperature,pressure,bulk compositions,diffusion coefficients,and so on) need to be carefully evaluated to constrain the degree of Hf/W differentiations;

4.Contaminations from crustal materials and fluid mobility of W must be carefully checked (e.g.,Liu et al.2016;Tusch et al.2020;Nakanishi et al.2021).A good summary of some of those issues can be found in Tusch et al.(2019).

4.2 Positive 182W anomalies:the role of the late veneer

The late veneer hypothesis assumes adding 0.3 to 0.8 wt%of the Earth’s mass composed of chondritic materials on average (μ182W=～ -190 ppm) into the mantle after core-formation ceased and all182Hf are extinct.Therefore,the late veneer hypothesis was believed to be capable of lowering the μ182W values of pre-late veneer mantle by～15 ppm (Willbold et al.2015).In addition,as the late veneer hypothesis is also suggested as the source of HSE in the mantle,thus the covariations between positive μ182W anomalies and abundances of HSE in the source region(182W anomalies are negatively correlated with HSE abundances) were believed to be a result of incomplete mixing of late veneer materials into the mantle (Willbold et al.2011,2015;Dale et al.2017;Puchtel et al.2018).However,some studies have also shown that decoupling between μ182W and HSE abundances was possible such as no lower than the modern level of HSE abundances were observed in samples with positive182W anomalies (Touboul et al.2012;Rizo et al.2016a).Therefore,under such circumstances,the hypothesis of late veneer cannot be used anymore.

4.3 Negative 182W anomalies in modern basalts:core-mantle interactions

Most of the negative182W anomalies observed in modern basalts were regarded as a result of core-mantle exchangerelated processes (Mundl et al.2017;Mundl-Petermeier et al.2019,2020;Rizo et al.2019;Yoshino et al.2020).This model was initially proposed by Mundl et al.(2017)based on the correlated3He/4He ratios and negative182W anomalies.However,later works argued that different core material contamination was probably infeasible because significant collateral geochemical effects would be expected even when a tiny amount (＜1 wt%) of the core material was added,such as elevated HSE concentrations(Mundl-Petermeier et al.2019) that has not been observed yet.

Additionally,the core-mantle interactions through silicate-metal equilibration in a basal magma ocean,diffusional exchange of W between core and lower mantle,or Si-Mg-Fe exsolutions from the core were proposed to eliminate inconsistency caused by those collateral geochemical effects(Mundl-Petermeier et al.2019,2020;Rizo et al.2019;Yoshino et al.2020).

4.4 Possible analytical artifacts

Another possible origin for these observed182W anomalies was believed to be analytical artifacts (N-TIMS with a double normalization correction method) by Kruijer and Kleine (2018).With careful reanalysis of nearly the same sample as those of Rizo et al.(2016b) using MC-ICP-MS,they speculated those unusually high positive182W anomalies reported in Ontang Java Plateau basalts with N-TIMS might result from several reasons:(1) Relatively low W concentrations of ＜23 ppb;(2) Relatively low W yields of～50%;(3) Effects of analytical non-mass-dependent183W fractionations during sample purifications on182W by using the double normalization correction method.Therefore,Kruijer and Kleine (2018) stated that the +23.9 ± 5.3 ppm positive anomaly observed by Rizo et al.(2016b) with N-TIMS was probably an analytical artifact and they also cast some doubt on the much larger anomaly observed in Baffin Bay basalt of+48.4 ± 4.6 ppm.

Apart from N-TIMS analysis,Cook and Schönbächler(2016) observed significant mass-independent biases on182W and183W using MC-ICP-MS and attributed these biases to the result of NFSE.Mei et al.(2018) also used MC-ICP-MS observed negative182W anomalies in BHVO-2 standard material and found instrumental fractionations involving183W couldn’t be explained by any types of commonly used fractionation laws (power,exponential,kinetic,and equilibrium),so they inferred that such bias might result from organic interferences on mass-independent fractionation of183W.

Recently,some works have proposed possible solutions to avoid these analytical artifacts.For MC-ICP-MS analysis,Breton and Quitté (2014) showed that improved separation procedure based on anion-exchange chromatography that capable of achieving quantitative W yield of 99.8 ± 1.2 % can inhibit the non-mass-dependent effects of183W.A similar conclusion was arrived at by Tusch et al.(2019) through improved dry-down protocols with concentrated HNO3and H2O2.Additionally,Breton and Quitté (2014) proposed that non-mass-dependent effects of183W during MC-ICP-MS analysis might be caused by the potential production of WH+ions.Thus,their results indicate that any improvements on sample preparation,purification,and mass spectrometer configurations that can inhibit the production of WH+ions may also help avoid analytical artifacts.As for N-TIMS analysis,almost all recent works adopted the analytical protocols developed by Archer et al.(2017).Therefore,except for improving chemical protocols to achieve W yield as high as possible,regular maintenance of the Faraday cup may be a good choice.

5 Future prospects

5.1 W isotope measurements

Although high-precision W isotope analysis using MCICP-MS and N-TIMS has improved precision of μ182W values to～5 ppm.However,there are still several problems that need urgently to be evaluated:

1.Mass independent biases of183W even after normalization in both MC-ICP-MS (Shirai and Humayun 2011;Willbold et al.2011;Kruijer et al.2012;Cook and Schönbächler 2016;Kruijer and Kleine 2018;Tusch et al.2019;Takamasa et al.2020) and N-TIMS analyses (Touboul and Walker 2012;Trinquier et al.2016;Archer et al.2017).

2.Physical and chemical mechanisms behind those previously proposed correction methods by Kruijer et al.(2012) and Cook and Schönbächler (2016) are still unclear and remain to be revealed.What are their physical meanings?

3.Based on our summarized information in Tables 2 and 3,we find that for those positive182W anomalies,both MC-ICP-MS and N-TIMS with several different standard materials reported resolvable anomalies.However,when it comes to negative182W anomalies,except for the study of Mei et al.(2018)with MC-ICPMS,all the other studies chose the N-TIMS with Alfa Aesar reference and reported resolvable negative anomalies (e.g.,Puchtel et al.2016;Mundl et al.2017;Mundl-Petermeier et al.2020).The physiochemical origin of this difference remains unknown.

5.2 Stable W isotope fractionations

All high-precision W isotope analyses are based on the assumption that the isotope ratio (186W/184W,186W/183W,184W/183W) is constant in nature.Whereas,recent works have shown that there are resolvable stable W isotope fractionations in terrestrial samples (Kurzweil et al.2018,2019,2020).In addition,considering possible NFSEinduced mass-independent fractionations (e.g.,Fujii et al.2006;Schauble 2007;Cook and Schönbächler 2016;Zhang and Liu 2020),possible mass-dependent and independent(NFSE) fractionations of W isotope in metal-silicate segregation and early magma ocean differentiation processes must be carefully evaluated to estimate their possible effects on those observed182W anomalies.

AcknowledgementsThis review is supported by the Strategic Priority Research Program (B) of CAS (XDB41000000),Pre-research Project on Civil Aerospace Technologies No.D020202 is funded by the Chinese National Space Administration(CNSA)and Chinese NSF projects (42130114).We thank Dr.Tong Fang and Prof.Liping Qin for their very helpful comments and suggestions that improved our manuscript.

Declarations

Conflict of interestOn behalf of all authors,the corresponding author states that there is no conflict of interest.