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/lp/de-gruyter/isotope-geochronology-models-versus-reality-yCGwf85dIY
- Publisher
- de Gruyter
- Copyright
- Copyright © 2015 by the
- ISSN
- 2300-7559
- eISSN
- 2300-7559
- DOI
- 10.1515/physica-2015-0007
- Publisher site
- See Article on Publisher Site
Abstract
Ma orit o the a ers on isoto ic atin o minerals an rocks have een evote to some new eochronolo ical ata im ortant or eolo or to evelo ments o a aratus an im rovements o la orator techni ues an roce ures owever, there are some asic ro lems concernin cre i ilit o the ata u lishe inclu in the error rackets , an their eolo ical meanin , which rarel are touche on The issues to e raise ma e rou e into two cate ories 1 istortion in the course o re arator o erations an inal measurements, an 2 some ou ts concernin eolo ical inter retation o the ata an the mo els use irst o all it shoul e reali e that what we reall anal se in a s ectrometer is not an e istin rock or mineral ut a ow er ro uce man ste s o consecutive roce ures, each o them ca a le o irreversi l istortin the ori inal com osition Keywords: isoto e eochronolo , mo els, atin , mass s ectrometr , uncertaint The routine se uence o o erations e ins with crushin an rin in , when a selective ulverisation attacks el s ars much more than micas not to mention uart The resultin mo i ication o the ori inal com osition ma e ver 116 J. BURC ART _ su stantial or whole-rock samples. Special care an microscopic control are necessary when a e homo eneity may e suspecte . This is the case o mi matites an other non-homo eneous rocks e. . paleovolcanites consistin o relic pyro enes surroun e y secon ary chlorites, or rocks with well- evelope reaction ones . The pro lem o homo eneity applies also to separate minerals, which o ten contain tiny inclusion o minerals i erin in isotopic composition an /or a e. Simple sievin may result in complete loss o tiny ircon rains in avour o lar e ones, the process stron ly istortin the composition o the entire ircon population which o ten is a mi ture, not only o si es ut also a es. hile the a oresai e ects seem to e important only in some special cases, the aw-crusher memory constitutes u i uitous an er o serious, thou h un ervalue , contamination o the whole-rock samples. Even the most care ul cleanin o any aw-crusher cannot uarantee that the machine ecomes a solutely evoi o any traces o a previously treate rock. There are too many recesses, nooks, scratches an some sur ace imper ections. A si ni icance o such an e ect was teste y a simple e periment 1 : the 87Sr/86Sr ratio was measure in a recam rian neiss an a Jurassic limestone in two se uences neiss limestone neiss, an limestone neiss limestone . Contamination o the limestone ranulate a ter the neiss was insi ni icant, while the e ect was very serious or the neiss ranulate a ter the limestone, the i erence etween the two rocks in the Sr a un ances ein the likely e planation. It appears that the ma nitu e o the contamination epen s on three parameters: the ratio o the mass o contaminatin matter irt or some remnants o the previous rock to the mass o the sample; the ratio o the element concentrations; an the ratio o the isotope ratios to e measure . It turne out that when the consecutive rocks are very i erent in the isotope ratios the e ect may a ect the r ecimal place even at very care ul cleanin . There ore, not only thorou h cleanin ut also a rational se uence o rocks is stron ly recommen e . An isotopic analysis o a Tertiary asalt crushe a ter a recam rian neiss may e completely worthless even at the most sterile chemical operations an a very hi h precision o the spectrometric measurements. It shoul e stresse that contamination intro uce in the course o preliminary operations is irreversi le an it ecisively in luences the inal spectrometric values. Besi es, it is i icult to etect, assess uantitatively, an make proper corrections. Contamination may occur also in the course o consecutive chemical operations an it can a ect cleanness o water, rea ents, vessels, air, relia ility o the spike solutions an their sta ility, e iciency o chromato raphic separation ut that is relatively easy to etect, assess an re uce. u lications concernin the ma nitu e o errors pre era ly uncertainties are a un ant, ut almost completely restricte to spectrometric analyses. Actually the apparently precisely etermine error rackets e presse y relate to an a e comprise the spectrometric errors only. It is true that only such uncertain- ISOTO E EOC RONOLO : MODELS VERSUS REALIT ties can e precisely etermine . owever, we are elu e y apparently hi h precision o an analysis, while the spectrometric error shoul e rossly enlar e y the errors o i erent parenta e, i they are to e un erstoo as uncertainty o a e. Various mo els so ar intro uce usually are e presse in a orm o isochrone ia rams. The initial ratio is either taken rom minerals evoi o the ra ioactive nucli e, or o taine y the least s uares metho . owever, as pointe y hen 2 to accept such a calculate value, an to take it or rante or a series o whole rock samples sometimes may e an oversimpli ication in view o the processes o ractionate crystallisation, partial meltin , mi in , or metamorphism. An assumption o a per ect homo eneity o the isotopic ratios in rocks at the initial moment is clearly relate to the scale o the o ect. It may e accepta le or a ran e o meters, ut seems impro a le or a lar e o y o anatectic ori in. Conse uently it is relate to the samplin pattern an its conscious choice. The analytical ata representin isotopic ratios are converte to an a e value e presse in years y means o a tacitly assume mo el. There ore, in or er to o tain a relia le a e value it is not enou h to carry out the analytical operations with the reatest possi le care ut also to e con i ent o the proper mo el applie , a mo el which e presses the eochemical evolution o the rock e amine . It touches upon the pro lem o what we ate. or a eolo ist an a e o a rock relates to the rock s position in the architecture o the comple . It is an a e o the orm, not su stance. Se uence o rock layers, intrusive contacts, crosscuttin o veins or microtectonic elements all the classical tools o eolo y re ers to the orms. On the other han , the isotope eochemistry stu ies the matter only, isre ar in the orms o occurrence. Thus there is a su stantial i erence as re ar s the o ect o an analysis, an conse uently it may e the source o misun erstan in . Analo ous misun erstan in appears when a coolin a e is tacitly re ar e as an a e o crystallisation. Another remark is relate to the mo els use to escri e the evolution o N -Sm system. The asis o the mo el are the CHUR or DM values taken as the initial ratios. Conse uently the very i ea o the a e o a rock ecomes lurre . The a e value o taine is relate to the isotopic evolution o the matter which later ecame soli i ie as the rocks. By no means it may e treate as the a e o a rock. Sometimes it is la elle as a crust-formation age thou h some authors raise ou ts which resulte in the two-sta e mo els. A process o eolo ical interpretation o the isotopic ata is ase on comparison o the ata with the mo el e pectation which is only a simpli ication o the processes which actually occurre in nature. Such an approach stems rom the principle o uni ormity. owever, the pro lem ecomes isputa le when we procee to very istant times an epth ones. Certainly our knowle e o the 118 J. BURC ART _ processes which operate urin recam rian times is e initely insu icient. It especially applies to the processes actin within the mantle, an the e chan e reactions etween the mantle an the crust. ave such processes een constant or the last 4.6 illion years ow ar ack in time are we entitle to apply the eolo y- ase mo els to pre- eolo ical times These are somewhat perple in uestions to which it is i icult to ive a well- roun e answer. ithout it, any petro enetic interpretations o the Sm-N or Lu- systems remain nothin more than workin hypotheses, purely theoretical mo els. Certainly the mo els we use as a asis or atin are ross simpli ication as compare to the comple an poorly reco nise processes which un erwent in nature. This lecture was ase on some e cerpts rom a mono raph y J. Burchart an J. ral Isotope record of the Earth's past in olish Izotopowy ) prepare or pu lication. RE ERENCES 1. Burchart J., Bachli ski R. 2000) i mielenia. V O lnopolska Ses a Naukowa Datowanie Minera w i Ska , rak w 10-12.02.2000, 2-4. 2. hen h . . 1989) Influences of the nature of the initial Rb-Sr system on isochron validity. Chem. eol. Isot. eosci. Section) 80, 1-16.
Journal
Annales UMCS, Physica – de Gruyter
Published: Mar 1, 2015