Indlela ye-Geochronology ekunqumeni ubudala bomhlaba
I-Geochronology igatsha lesayensi yomhlaba elifunda isikhathi sezinto ze-geological kanye nokulandelana kwezenzakalo ezabumba uMhlaba. Nge-geochronology, ososayensi bangaphendula imibuzo eyisisekelo: ukuthi uMhlaba uneminyaka emingaki, ukuthi amatshe athile akheka nini, nokuthi umlando weplanethi uqhubeka kanjani ngokuhamba kwesikhathi. Ukunquma ubudala boMhlaba akwenziwa ngokumane "ubale ubuyele emuva" kusukela emcimbini owodwa, kodwa ngokuhlanganiswa kwezindlela eziningi eziqinisanayo—ikakhulukazi ukuqopha kwe-radiometric, i-stratigraphy, kanye nokuhlangana kwezinsalela. Njengamanje, ubudala boMhlaba buyamukelwa kabanzi ukuthi bungaba yiminyaka eyizigidigidi ezingu-4,54, umphumela wokuvumelana kobufakazi be-geochemical kanye ne-astronomical.
Izisekelo ze-Geochronology
Ngokuvamile, izindlela ze-geochronological zihlukaniswe ngamaqembu amabili amakhulu: ukuqomisana okuhlobene kanye nokuqomisana okuphelele. Ukuqomisana okuhlobene akunikezi ubudala ngeminyaka, kodwa kunalokho kunquma ukulandelana kwemicimbi—okudala nokuthi obani abancane. Izibonelo zifaka phakathi isimiso sokubeka izinto phezulu (izingqimba eziphansi ze-sedimentary zivame ukuba zindala), ubudlelwano bokunqunywa (ukungena kwe-magma okunqamula izingqimba ezindala kuncane), kanye nokungavumelani, okubonisa "isikhathi esilahlekile" ngenxa yokuguguleka noma ukuntuleka kokubekwa.
Ngokuphambene nalokho, ukuqomisana okuphelele (ngokuqondile ukuqomisana kwezinombolo) kunikeza isilinganiso seminyaka ngezinombolo, isibonelo "leli dwala lineminyaka eyizigidi ezingu-250 ubudala." Indlela yezinombolo ethembeke kakhulu yeminyaka yobudala kakhulu ukuqomisana kwe-radiometric, okusekelwe ekuboleni kwezinto ezinemisebe zibe yizinto eziyindodakazi ngesivinini esingabikezelwa.
Izimiso Zokuqomisana Nge-Radiometric
Ukuqopha nge-radiometric kusebenza ngoba ama-athomu athile awazinzile futhi abola ngokwemvelo abe amanye ama-athomu. Izinga lalokhu kubola livezwa ngokwengxenye yokuphila, okuyisikhathi esidingekayo ukuze ingxenye yama-athomu omzali abe ama-athomu endodakazi. Ngenxa yokuthi ingxenye yokuphila iyimpahla ehlala njalo ngokomzimba ngaphansi kwezimo zemvelo, isilinganiso sezakhi zomzali nendodakazi kumaminerali singasetshenziswa ukubala ubudala baso kusukela ngesikhathi iminerali “ivala” uhlelo lwayo (ukuvalwa), okungukuthi, lapho iminerali iphola futhi izakhi zomzali nendodakazi zingasakwazi ukungena nokuphuma kalula.
Kodwa-ke, kubalulekile ukuqonda ukuthi ukuqoqwa kwe-radiometric ngokuvamile kunquma ubudala bokwakheka noma ukupholisa kwamaminerali, hhayi nje ubudala "bedwala" kunoma yimuphi umongo. Isibonelo, amadwala ashintshashintshayo angabonisa ubudala bokuguqulwa kwesimo, kuyilapho izihlabathi zivame ukuqukethe okusanhlamvu kwamaminerali okudala kunesikhathi sawo sokufakwa.
Indlela ye-Uranium–Lead (U–Pb): Insika Yesikhathi Sasendulo Ukunqunywa
Enye yezindlela ezibaluleke kakhulu zamatshe amadala yindlela ye-Uranium–Lead (U–Pb), evame ukusetshenziswa ku-zircon yamaminerali (ZrSiO₄). I-Zircon iwusizo kakhulu ngoba:
1. Iyakwazi ukufaka i-uranium esakhiweni sayo sekristalu uma yakhiwa.
2. Yenqaba i-lead (Pb) ngesikhathi sokwakheka kwekristalu, ukuze i-Pb elinganisiwe ngokuvamile ibe umphumela wokubola.
3. Ayimelani nokuguquguquka kwesimo sezulu kanye nokuguquka kwesimo sezulu, ngakho-ke ingagcina "amarekhodi esikhathi" amadala kakhulu.
Kuhlelo lwe-U–Pb, kunezindlela ezimbili eziyinhloko zokubola: U-238 kuya ku-Pb-206 kanye no-U-235 kuya ku-Pb-207. Ngama-"clock" amabili ngesikhathi esisodwa, le ndlela ivumela ukuhlola kwangaphakathi. Ukuhlaziywa kwesimanje kusebenzisa amasu afana ne-LA-ICP-MS noma i-SIMS, angalinganisa ubudala esikalini se-micron, okuvumela izindawo ezihlukene zokukhula kwe-zircon ukuthi zinikeze ulwazi mayelana nezehlakalo ze-geological eziphindaphindwayo.
I-U–Pb ibalulekile ekulinganiseni ubudala boMhlaba ngoba ingabala usuku lwamatshe amadala kunawo wonke eMhlabeni futhi, okubaluleke kakhulu, ibala usuku lwezinto zangaphandle komhlaba ezifana nama-meteorite akheka ohlelweni lwelanga lokuqala.
Izindlela ze-Potassium–Argon (K–Ar) kanye ne-Argon–Argon (Ar–Ar)
Indlela ye-Potassium-Argon (K-Ar) isebenzisa ukubola kwe-K-40 kuya ku-Ar-40. Ngenxa yokuthi i-argon iyigesi, ayigcinwa kalula kumaminerali ashisayo, asanda kwakheka. Njengoba idwala liphola, i-argon iqala ukubambeka ethangini lamaminerali. Lokhu kwenza i-K-Ar ibe wusizo ekutholeni isikhathi samadwala entaba-mlilo kanye nama-metamorphic.
Ukwehluka okuyinkimbinkimbi kakhulu, i-Argon–Argon (Ar–Ar), ilinganisa isilinganiso se-argon isotope isebenzisa imisebe ye-neutron, ivame ukunikeza ukunemba okukhulu futhi ivumela ukuhlaziywa kokushisa ngesinyathelo. Le ndlela iwusizo kakhulu ekuqondeni ukuthi idwala liye laphinde lafudunyezwa yini “okusetha kabusha” iwashi lalo le-radiometric.
I-Rubidium–Strontium (Rb–Sr) kanye ne-Samarium–Neodymium (Sm–Nd)
Indlela ye-Rb-Sr isebenzisa ukubola kwe-Rb-87 kuya ku-Sr-87. Le ndlela ivame ukusetshenziswa emadwaleni angenalutho nashintshashintshayo, ikakhulukazi ngendlela ye-isochron, indlela enciphisa isidingo sokucabanga mayelana nokwakheka kokuqala kwezinto eziyindodakazi.
Okwamanje, i-Sm–Nd (Sm-147 iyabola ibe yi-Nd-143) yaziwa ngokuthi imelana kakhulu nezinguquko ze-metamorphic, okwenza kube usizo ekufundeni ukuvela koqweqwe loMhlaba kanye ne-mantle. Ngaphezu kokunikeza ubudala, uhlelo lwe-Sm–Nd luphinde lusize ekuchazeni imithombo ye-magma kanye nomlando wokwehluka koMhlaba, njengoba i-neodymium ingaba uphawu lokwakheka koqweqwe lwezwekazi oluvela ku-mantle.
Ukuqomisana Okuhlobene: I-Stratigraphy kanye ne-Index Fossils
Nakuba ukunquma ubudala boMhlaba kuncike kakhulu ezindleleni ze-radiometric, i-geochronology idinga nohlaka lwe-stratigraphic. Izingqimba zamadwala ezisekelwe odakeni emhlabeni wonke zingahlotshaniswa kusetshenziswa izici ze-lithological, amaphethini okubekwa, kanye nezinsalela.
Izinsalela ze-Index—izinsalela zezinto eziphilayo ezaphila isikhathi esifushane kodwa ezazisakazeke kabanzi—zivumela ukuhlobana phakathi kwezifunda. Ngale ndlela, izazi ze-geology zingakha isikali sesikhathi se-geological (isb., i-Jurassic, i-Cretaceous, i-Paleogene), bese “zivalela” leso sikali esihlobene nenani eliphelele lezingqimba zezintaba-mlilo ezihlanganiswe phakathi kwezinsalela (isb., umlotha wentaba-mlilo ongalinganiswa ne-U–Pb noma i-Ar–Ar).
Iminyaka Yomhlaba Inqunywa Kanjani?
Umbuzo wobudala boMhlaba awunakuphendulwa kuphela ngokususelwa emadwaleni amadala kakhulu ebusweni, ngoba ungqimba loMhlaba luhlala lusetshenziswa kabusha yi-plate tectonics: ungqimba lolwandle luyancishiswa, ungqimba lwezwekazi luyaguqulwa, futhi kwakheka i-magma entsha. Ngakho-ke, ososayensi basebenzisa izindlela eziningana:
1. Amatshe amadala kunawo wonke eMhlabeni: Amatshe amadala kakhulu atholakala kuma-craton amaningana (izingxenye ezizinzile zamazwekazi), njengeCanada, iGreenland, i-Australia, kanye neNingizimu Afrika. Amaminerali e-zircon avela eNtshonalanga ye-Australia, isibonelo, aneminyaka engaphezu kwezigidigidi ezine ubudala, okubonisa ukuthi uqweqwe lwawo lwakhiwa kusenesikhathi kakhulu.
2. Ama-Meteorite nezinto ze-Solar System zakuqala: Ngenxa yokuthi ama-meteorite abhekwa njengezinsalela zokwakheka kwe-Solar System, ubudala bawo bunikeza imingcele eminyakeni yokwakheka koMhlaba. Ubudala be-U–Pb bama-meteorite (ikakhulukazi ama-chondrite) buveza isibalo seminyaka ecishe ibe yizigidigidi ezingu-4,56. Umhlaba wadalwa kusenesikhathi kakhulu, phakathi nalesi sikhathi esifanayo, ngakho-ke lesi sibalo siyinkomba eyinhloko.
3. Amasampula Enyanga: Amatshe eNyanga avela emisebenzini ye-Apollo nawo asiza ekusunguleni ukulandelana kwezikhathi kwasekuqaleni. Ukuqhathaniswa kweminyaka yeNyanga noMhlaba kusekela umbono wokuthi kokubili kwavela ezigabeni zokuqala zeSistimu Yelanga, kufaka phakathi umcimbi omkhulu womthelela okucatshangwa ukuthi wadala iNyanga.
Uma kuhlanganiswa lobu bufakazi, umphakathi wesayensi uphetha ngokuthi uMhlaba uneminyaka engaba yizigidigidi ezingu-4,54 ubudala (ngokungaqiniseki okuncane), hhayi ngokusekelwe kusampula eyodwa, kodwa kunalokho ekuhambisaneni kwezinhlelo eziningi ze-isotopic kanye nezinhlobo eziningi zezinto.
Izinselele Nokuqinisekiswa
Empeleni, ukuqomisana nge-radiometry kubhekene nezinselele ezifana nalezi:
– Uhlelo oluvulekile: izakhi zomzali noma zengane zingangena futhi ziphume ngenxa yesimo sezulu esibi, ukuguquka kwesimo sezulu, noma uketshezi olusebenzisa amanzi ashisayo.
– Ukusetha kabusha: ukuvuselela kunga "setha kabusha" iwashi le-isotopic.
– Ukungcola: ingxube yamaminerali amadala namasha kusampula eyodwa ingafihla imiphumela.
Ngakho-ke, i-geochronology yanamuhla incike ekuqinisekisweni okuhlanganisiwe: ukusebenzisa amaminerali amaningi, izindlela eziningi ze-isotopic, kanye nomongo ocacile we-geological. Ukuhlaziywa okuncane kwama-zircons, indlela ye-isochron, kanye nokuhlanganiswa kwedatha ye-petrological kanye ne-stratigraphic kuqinisa kakhulu imiphumela yokuqopha.
I-Penutup
Izindlela ze-geochronological ziyisihluthulelo sokuqonda ubudala boMhlaba kanye nomlando omude. Ukuqomisana okuhlobene kunikeza ukulandelana kwemicimbi, kuyilapho ukuqomisana kwe-radiometric kunikeza iminyaka eqondile. Uhlelo lwe-U–Pb lwe-zircon, i-K–Ar/Ar–Ar yamaminerali entaba-mlilo, kanye ne-Rb–Sr kanye ne-Sm–Nd yamadwala angenawo ugesi ahlinzeka ngesisekelo sokunquma iminyaka yasendulo. Lapho imiphumela yalezi zindlela iqhathaniswa nama-meteorite namasampula enyanga, kuvela isithombe esifanayo sokuthi uMhlaba uneminyaka engaba yizigidigidi ezingu-4,54 ubudala. Ngakho-ke, i-geochronology akuyona nje ithuluzi "lokulinganisa isikhathi," kodwa iwindi lesayensi endabeni yokuvela kweplanethi okuyinkimbinkimbi kakhulu, kusukela ekwakhekeni kwayo kokuqala kuya ezimweni ezavumela ukuphila ukuba kukhule.
