The study of Mg isotopes has been carried out for about 40 years since 1970s. With analytical progress, the study is not only limited to the excess of 26Mg due to decay of short-lived 26Al in primitive meteorites, also extended to mass-dependent fractionation of Mg isotopes in meteorites and terrestrial rocks. This paper reviews recent development in Mg isotope researches

Daode, Wang

Guiqin, Wang

Yangting, Lin

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Chinese Journa l o f Polar Sc ience, Vo.l 20, No. 2, 209 214, Decem ber 2009The study ofM g isotope compositions of terrestrial rocks andm eteoritesW ang Gu iqin(王桂琴 ) 1, L in Y angting(林杨挺 ) 2 andW ang D aode(王道德 ) 11 K ey Laboratory of Isotope Geochronology and G eochem is try, Guangzhou Institute of Geochem istry, Chinese Academy of Sciences, Guangzhou 510640, Ch ina2 K ey Laboratory of the Ear th 'sD eep Inter ior, Institute of G eology and G eophysics, Chinese Academy of S ciences,Beijing 100029, ChinaRece ivedOctober 9, 2009Abstract The study o fM g isotopes has been carr ied out for about 40 years since1970 s. W ith ana ly tica l progress, the study is not on ly lmi ited to the excess o f26Mgdue to decay of short lived26A l in prmi itive m eteorites, a lso ex tended to m ass dependent fractionation ofM g isotopes inm eteo rites and terrestr ia l rocks. Th is paper rev iews recent deve lopm ent inMg iso tope researches.Keywords Mg iso topes, MC ICPMS, so lar nebular condensation, p lanet fo rm ation.1 IntroductionM g is a m ajor elem ent o f m ost terrestr ial m ateria ls and extraterrestria l rocks. It hasthree iso topes ( 24M g, 25Mg, and 26Mg), w ith re lative mass differences betw een them greater than 4% per atom icmass un it ( amu). M g iso top ic compositions can be changed by bothm ass dependent fractionation and decay of short lived rad ionuclide 26A .l H ence, M g isotopes can be used to trace geochem ical processes and as an isochronom eter.M g isotopes of extra terrestria lmateria ls have been extensively stud ied, especia lly thoseformed in the earlest so lar system. Because excess 26M g serves as a proxy for extinct 26A l( half l ife T1 /2= 0. 73M a), wh ich is one of the most im portant energy sou rces of meltingastero ids, A lM g system holds a prom ise of being a h igh resolu tion chronom eter for early solar system even ts, g iven 26A l was un iform ly d istribu ted throughout the inner so lar nebu la.Furthermore, mass dependen t fractionation ofM g iso topes can trace processes in the terrestr ial and the solar nebular evolu tion.A lm ost a ll Mg isotop ic compositions w ere usually m easured w ith therm al ion iza tionm ass spectrometry ( TIMS ) and, especially, secondary ion ization m ass spectrom etry( S IMS) [ 1 3] , 10 years ago. The analysis precisions of both techn iques are 1 2 [ 4 6] ,compatib le to the range ofM g isotop ic compositions due to mass dependent fractionation.M u ltip le collector inductive ly coupled plasm a m ass spectrom etry ( MC ICPMS ) providesw ith a possib ility of h igh precisionm easurem en t ofM g isotop ic ra tios. The reproductib ilityof 25M g/24M g and 26M g /24Mg measured by MC ICPMS is better than 30 to 60 ! 10- 6.H ence, mass dependen t fractionation ofM g isotopes can be used to trace cosmochem icaland geochem ical processes.2 Research progress inM g iso top ic compositions2. 1 Excess 26Mg and the refferecd 26A l /27A l ra tios in the so lar nebulaAssum ing a homogeneous distribution of 26A l in the ear ly so lar system, Mg iso topiccompositions of d iverse com ponents in prim itivemeteorites provide a high resolution ( on theorder of 70000 years[ 7 ] ) chronom eter for the earliest evolution of the solar system.The CaA l rich inclusions ( CA Is) and chondrules are m illimeter to cen tim eter sizedassemb lages in chondrites, probably formed at the earliest stage in the solar system hencepreserve h ints o f the solar nebu lar processes[ 1, 8, 9] . The accumu lated data of 26A l 26M g isotopes in prim itive solar system m aterial ( CA Is and chondru les) demonstra te that 26A l existed in the ear ly so lar system w ith a clear cutoff of in ferred 26A l/27A l ratios o f ~ 5 ! 10- 5 ( e.g. M acPherson et a l. [ 1] ) . Th is in ferred 26A l/27A l ratio suggests a h igh abundance of 26A lin the early so lar system, wh ich produced enough heat form elting paren ta l astero ids of basaltic achondrites. The systematic d ifference in the re ferred ( 26A l/27A l) 0 ratios of CA Is andchondrules im plies a form ing in terval of 2M a between them, w ith CA Is earlier than chondru les.The in itial 26A l/27A l ratios o fCA Isw ere usually determ ined in situ w ith SIMS. H owever, them ineral " isoch ron" cou ld be affected through re d istr ibu tion ofM g iso topes betweenphases du ing secondary events. In add ition, m ost o f chondru les have too lower A l/Mg forSIM S to detect excess of 26M g. In contras,t MC ICPM S provide us w ith a new techn ique toob ta in the bu lk A lM g iso topic compositions of CA Is, wh ichmay not be sign ificantly affected by the secondary events. In add ition, its h igh precision allow s us to determ ine small excess of 26Mg from samp les w ith low A l/M g ratios.B izzarro[ 10] m easured bu lk M g isotop ic compositions of severa l CA Is and chondru lesfrom A llende using MC ICPMS. Their Mg iso tope data indicate that A llende chondru lesformed as earlest the oldest CA Is, and continued for at least 2. 5 Myr. The " canon ical"( 26A l/27A l) 0 value of the solar system is ~ 5 ! 10- 5. R ecen tly, a few of researches suggested a " supra canon ical" value of ~ 5. 85 7 ! 10- 5 based on h igh precision m easurem ents o fM g isotop ic compositions of CA Iw ithMC SIMS andMC ICPM S[ 11] . In contrac,tthere are other analyese argueing against the " super canon ical" value[ 12 15] .2. 2 Mg iso topemass depend en t fractionation in T errestrial and ex terrestrial rocksVolatility ofmagnesium is betw een refractory elem ents ( e. g. , A l and Ca) andm oderately volatile e lemen ts ( e. g. , Fe) and so should prove usefu l as an ind icator of p lanetaryprovenance. S tud ies ofM g iso topic variations in terrestria l and extraterrestr ial rocks can beused to identify the processes of p lanetary accre tion and d ifferen tia tion. M g iso topes havebeen used to estim ate the rela tive contributions of chondru les and CA Is to the formation ofthe terrestria l p lanets and astero ids because of their d istinctive M g isotop ic com posi210 W ang Guiqin et al.tions[ 16] . H owever, Mg isotop ic composition of the Earth is still no twell constrained. Somestud ies on mantle peridotites indicate that the Earth has heavierM g iso top ic compositionscompared to chondrites[ 17] wh ile other studies, also based onm an tle per idotites asw ell as ocean ic basalts, reached opposite conclusion[ 18] . In add ition, it is still debated ifM g iso topic fractionation can happen among peridotite m inera ls. S tud ies on basalt differentiation reveal lim itedM g isotope fractionation dur ing basaltic d ifferentia tion[ 14] , wh ich ind icates thatM g iso tope fractionation shou ld be even smaller a t man tle temperatures consider ing that equ ilibr ium isotope fractionation decreases as the temperature increases. H owever, sign ificantM g iso topic fractionation among mantle m inera ls has been reported in recen t studies[ 16 ] . If the conclusions of these studies are indeed righ,t then it ism ore complica ted toconstrain theM g iso top ic composition of the earth since many previous stud ies only use oliv ine to represent the bulk Earth [ 16 ] . Late ly, H andler et a.l [ 19] investiga ted M g isotopiccompositions of o liv ine and a few coexisting pyroxenes and found lim ited in ter m ineralM g isotop ic fractionation. Based on their data, the earth is chondr itic or ig in.U p to now, pub lished data onm eteor ites and terrestria l rocks varied w idely[ 20, 21] .3 Published data ofMg isotopic compositions in ChinaThe research ofM g isotop ic compositions started late in Ch ina. W ith the in troductionof advanced analy tical equ ipm ent in recent years, more domestic researchers started to payatten tion to th is area of reseach and made important progress. The fo llow ing w ill introducesom e of their research work.3. 1 Analy tica lm ethod sA procedure has been developed for separating Mg from bu lk m eteorite and ultram aficrock sam ples byW ang et al. [ 22] . B io Rad AG50W X12, 200 400 m esh resin and 1MHNO3 are used to separa teM g from sample solutions. Pur if iedM g samp le solutionsw ere introduced to aMC ICPM S to analyzeMg isotope compositions. The new procedure allows fora long term external reproducib ility of 26M g and 25M g of 0. 10 and 0. 05 , and 0. 06 for 26M g* ( 2 , n= 211, over a period of 24 months), respective ly. R ecovery yie ld ofM g in th is separating procedure is > 99% . The total procedural b lank forMg is < 2. 0ng.3. 2 Mg A l iso topes inm eteoritesH su et a l. [ 23] m adeA lMg iso top icmeasuremen ts on severa lCA Is and chondru les fromthe N ingq iang by SIM S. Their in itia l ( 26A l/27A l)0ratios for the CA Is are genera lly consisten t w ith the value of 5 ! 10- 5. 2 chondrules have the ( 26 A l/27 A l) 0 values of0. 3 0. 6 ! 10- 5.A sm en tioned above, " supra canon ical" value of ( 26A l/27A l)0is in question. The orig in of CA Is is also a long stand ing issue. W ang et a l. [ 24] measured Mg isotop ic com positions of CA Is from both N ingqiang ( ungrouped[ 25] ) and A llende ( CV3 ) carbonaceous211The study o fM g isotope compositions o f terrestrial rocks and meteoriteschondrites w ithMC ICPM S, to con firm the canon ical value of ( 26A l/27A l) 0, and to clarifyM g m ass dependent fractionation of CA Is. In add ition, severa l amoeboid oliv ine aggregates( AOA s) from A llende were analyzed to illum inate the interval of so lar nebular condensation, because AOA s probably sam pled later condensates in compar ison w ith normalCA Is[ 26] .The resu lt show s that CA Is from A llende have a rela tively c losed A lM g system in theCA Is and no sign ificantM g isotope exchangew ith amb ientm ateria ls, they have an in ferredin itia l ( 26A l/27A l)0ra tio o f ( 4. 77 ∀ 0. 39) ! 10- 5 close to the canon ical value. The AOA scon ta in excess 26M g and p lot close to the CA I regression line, wh ich is suggestive of theircon temporary formation. None of the CA Is and AOA s stud ied in th iswork show s sign ifican tm ass fractionation w ith enrichm ent o f the heavierMg iso topes, argu ing against an evaporation orig in.3. 3 The comparisonMg Isotopic compositions betw een terrestria l and exterrestrial materialsIt is still few and inconsistent for the published Mg isotope data on terrestria l and extraterrestr ial sam ples. In add ition, whether terrestria lm ater ials ofM g isotop ic characteristics is sim ilar to chondrites or non chondrites has been dispu ted. W ang et al [ 27] analyzedsom eMg isotop ic data of the m an tle rocks, and four bu lk meteorites analyzed by MCICPMS. Their data ind icate that terrestria lmafic and ultram afic rocks that probab ly derivedfrom the m antle disp lay sign ificant variation of26Mg from - 0. 52 to 0. 19 , suggestiveof heterogeneous com positions ofMg isotopes of the terrestria lman tle. These var ia tion sm aynot be due to simp le partia lm elting and /or fractional crystall ization, because m inera l separates ( includ ing oliv ine, c linopyroxene and orthopyroxene) o f the peridotites show no fractionation ofM g iso topes. Mg isotop ic compositions of 3 chondr ites are ra ther hom ogeneous.H owever, to deduce whether the earth Mg isotop ic com positions orig in from chondrites orno ,t stil l need more data.Y ang et al. [ 28 ] have stud ied a set o fwell characterizedmantle peridotite xenoliths fromSanyitang and Beiyan, N orth Ch ina craton, wh ich have d iverse or ig ins w ith d ifferentm inera logy, chem ical composition and degree of partia lm elting and m etasom atism. The resu ltsd isp lay a sm all var iation in M g isotop ic composition. TheM g isotop ic com positions of thecoexisting oliv ine, orthopyroxene and clinopyroxene in all peridotites are iden tica l w ith intheirr external precision ( ~ ∀ 0. 1 , 2SD ), suggesting thatM g isotope fractionation betw een oliv ine and pyroxenes at the temperatu res of > 900# is insign ifican.t These resu ltsare thus consisten tw ith the absence ofM g isotope fractionation dur ing basalt differentiation.O veral,l the 26Mg of the mantle is estim ated to be - 0. 26 ∀ 0. 16 ( 2SD ). Th is value issim ilar to two chondrites ( A llende andM urch ison) ( ~ - 0. 3). Their data show the silicate earth has a chondriticM g isotop ic composition.L i et al. [ 29] found the 26M g relative to the orig inal solu tion range from - 2. 03 to- 0. 33 , posit ive ly correla ted w ith pH.4 Questions and directionsThere is a lot of con fusion on the research ofMg isotopes in early solar system and ter212 W ang Guiqin et al.restria l process. The questions focus on:( 1) W hether the upper lim it of in itia l ( 26A l/27A l)0ratio of 5 ! 10- 5 is true;( 2) W hether 26A l is hom ogenous in early solar nebular;( 3) W hether terrestr ialmater ia ls show M g isotop ic characteristics sim ilar to chondritesor non chondr ites;( 4) The orig in ofCA Is is also a long stand ing issue. Mg isotope compositions can provide better evidence.Fu rther research is benefic ia l to address these issues.Acknow ledgem ents GRV 99027 was supp lied by the Polar Research Institute o f Ch ina.This study was supported by the Pilo t Pro ject o f Know ledge Innovation Program of the Ch inese A cademy of Sciences ( kzcx2 yw Q08) and the N atural Science Foundation of Ch ina( G rants 40873054).References[ 1] M acPherson GJ, Davis AM, Z inner E ( 1995): The distribution of alum inum 26 in the early so lar system A reappra isa.l M eteo ritics, 30: 365 386.[ 2] GuanY, H uss GR, M acPherson GJ et al. 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Geochmi ica et Cosmochmi ica Acta Supp lemen,t 72: 545.214 W ang Guiqin et al.

The study of Mg isotope compositions of terrestrial rocks and meteorites

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The study of Mg isotope compositions of terrestrial rocks and meteorites

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