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Como se datava a Terra - Sodré Neto

Como se datava a Terra - Sodré Neto

Segundo o livro "Entendendo a terra" seria de 4,5 bilhões de anos a idade da terra . Esta idade foi assumida desde a publicação de Patterson 1955-56 onde se assumiu certas crenças cosmológicas como pressuposto para então haver tal afirmação. 

Pressupostos e crenças assumidos 

1. The following assumptions are made concerning meteorit,es: they were formed at, the same time: they existed as isolated and closed systems: they originally contained lead of the same isotopic composition; they contain uranium which has the same isotopic composition as that in the earth. On the basis of these assumptions various leads might be expected to evolve as a result of different original U/Pb ,ratios in separate meteorites, and an expression* for any pair of leads derived from such an array is: 4, - 4, (eAIT - 1) R,, - RPb = k(e”zT - i) (1) where R, = Pb*07/Pb204 and R, = Pb206/Pb 204 for leads from different met,eorites a and b, k = U23s/U 235 today (137.8), A1 = U23S decay-constant (9.72 x lo-lo yr-l), A, = U238 decay-constant (1.537 x lo-lo yr-l), and T = age of the array. The isotopic composit,ions of leads isolated from three stone and two iron meteorites are listed in Table 1 (PATTERSON, 1955). 

Os seguintes pressupostos são feitos sobre meteoritos, es: eles foram formados ao mesmo tempo: eles existiam como sistemas isolados e fechados: originalmente continham o chumbo da mesma composição isotópica; eles contêm urânio que tem a mesma composição isotópica do que na Terra. Com base nessas premissas, pode-se esperar que várias derivações evoluam como resultado de diferentes relações U / Pb originais em meteoritos separados e uma expressão * para qualquer par de derivações derivadas dessa matriz é: 4, - 4, ( eAIT - 1) R ,, - RPb = k (e "zT - i) (1) onde R, = Pb * 07 / Pb204 e R, = Pb206 / Pb 204 para derivações de diferentes met, eorites a e b, k = U23s / U 235 hoje (137.8), A1 = constante de decaimento U23S (9.72 x lo-lo yr-l), A, = constante de decaimento U238 (1.537 x lo-lo yr-l) e T = idade de a matriz. O composto isotópico, os íons de fios isolados de três pedras e dois meteoritos de ferro estão listados na Tabela 1 (PATTERSON, 1955).

*** Ponderamos que tais pressupostos são totalmente inválidos devido ao fato de que a própria instabilidade contingencial de proporção de elementos químicos que é gerada no momento de uma colisão de grandes asteroides , não nos fornece nenhuma base para tal afirmação, muito pelo contrário; o cenário anterior tem uma probabilidade de quintrilhões de vezes  nula possibilidade de "mesmas proporções" . 



"De um modo geral, a comunidade científica acredita que a taxa de decaimento de um núcleo radioativo é imutável. No entanto, é possível alterar a taxa de decaimento alterando o ambiente do emissor.... Desta forma, a taxa de decaimento da radioactividade dos materiais é grandemente acelerada e os materiais são assim descontaminados a uma velocidade muito mais rápida do que o normal. O estímulo pode ser aplicado aos materiais radioativos, colocando esses materiais dentro da esfera ou terminal de um gerador de Van de Graaff onde eles são submetidos ao potencial elétrico do gerador, como na faixa de 50 kilovolts a 500 kilovolts, para em pelo menos um período de 30 minutos ou mais. A presente invenção baseia-se no facto de a taxa de decomposição de materiais radioactivos poderem ser aceleradas ou reforçadas e assim ser controlada por um estímulo, tal como um potencial electrostático aplicado. Esse potencial, por exemplo, é incorporado na equação de tunelamento mecânico quântico para o coeficiente de transmissão T * T, incluindo uma energia potencial adicional"
https://patents.google.com/patent/US5076971


Muitos outros trabalhos e pedidos de patente para métodos de aceleração de decaimento radioativo e descontaminação de materiais estão descritos na literatura (An Kinderewitscg, 2003; Gorodezki, 2005) . O aparato de Willian Parker citado precisou de 50-500 kilovolts para gerar aceleração de decaimento e descontaminar assim em apenas 30 minutos, mas quantos milhões de kilovolts geraria a queda de apenas 1 grande bólido? e qual seria as consequências em termos de aceleração de decaimento e envelhecimento de rochas  diversos bólidos caindo, eletrificando, aquecendo e fragmentando a crosta terrestre?

As experiências com fusão nuclear  em inúmeros testes e projetos como de equipamentos de tokamaks , de que , através de sistemas de plasma e temperatura, poder não somente aumentar decaimento mas até alterar núcleo de elementos estáveis (Bosch, 1999; Lee, 2008; Hesslow L et al, 2017; Izumi et al, 2016; Zhang et al, 2016; Xie et al, 2014) , e os efeitos de plasmas e outros aceleradores de partículas durante a queda de grandes bólidos (dos quais temos catalogado apenas em torno de 0,2%), tendo nós mesmos dado uma contribuição relevante para tendência de decaimento acelerado em relação ao diâmetro do bólido (Figura 1), tudo isso nos assegura que podemos simular aqui uma interpretação isenta da dependência  tradicional da geocronologia devido esta não poder mais estar (pelo menos “absoluta”) diante de tais testes e fatos e de muitas outras perspectivas datacionais anacrônicas, sem necessitar portanto de tais inúmeras justificativas ad hoc, podendo simular uma interpretação com os dados como eles simplesmente são e estão.

A interpretação isenta da geocronologia convencional e tradicionalmente ensinada desde dois últimos séculos, economiza por assim dizer, uma série de malabarismos justificadores de anomalias anacrônicas que pululam nas descobertas científicas, mas ela trará novos desafios como por exemplo, se não temos este tempo todo distanciando a queda de bólido do outro, então poderíamos nos aproximar mais de estudos da NASA de múltiplos impactos na história da terra ? (Spray, 1998; Donald R. Lowe, 2004; Bunch, 2012; Witke JH, 2013; Kennett, 2015). A evasão de gases e o isolamento dos raios solares pelas grossas nuvens de fumaça esfriariam rapidamente a superfície da terra, criando a glaciação, em cima de uma crosta  fervendo logo abaixo dos continentes e das águas  em movimento ? Poderia a queda de grandes bólidos envelhecer rochas pela aceleração de decaimento radioativo? poderia transformar alguns elementos estáveis em instáveis arrancando nêutrons de seus núcleos?  Na figura abaixo percebemos uma pista nesta direção:

Figura 1 - Linha de aumento de idade relacionada ao diâmetro de bólidos . Sodré & Lutero, 2017

 

 

 ***Observamos que  há uma linha de tendência em relação a dizer que numericamente diâmetros maiores  dos bólidos possuem maior probabilidade de estarem relacionados a maiores idades,  e numericamente bólidos  pequenos ajuntados em torno de idades menores, possuem proporcionalmente probabilidade de estarem relacionados a idades menores . 

 

A queda de maiores bólidos representam potencial de aceleração maior de partículas e de possível até mesmo surgimento de elementos instáveis (Brown, 2013). As poucas exceções a esta tendencia verificada , podem ter explicações em relação ao terreno do impacto se apresentaria ou não amortecimento do impacto, e o mesmo vale para bólidos de diâmetros pequenos se o terreno impactado teria gerado ou não maiores fatores de aceleração de partículas. 

A queda de um bólido maior implica em maior temperatura conjugada a  maior efeito plasma conjugado a  maior efeito piezoelétrico e a maior onda sonora;  todos estes efeitos são aceleradores de partículas e , em graus cada vez mais elevados,  são mais  capazes de arrancar neutrons até de elementos estáveis, bem como estes neutrôns soltos se agregar a outros elementos criando instabilidade por neutrons a mais e a menos nos novos elementos radioativos gerados.

Se possuem tais potenciais de aceleração de decaimento maior, logo, bólidos maiores teriam a tendencia de ter maiores idades como revela o gráfico e bólidos menores , menores idades como demonstra o gráfico.

Ao observar as idades de rochas defrontamos com as rochas mais velhas do planeta no cinturão de rochas verdes Nuvvuagittuq próxima ao arco de Nastapoka no litoral litoral da costa sudeste da Baía de Hudson , no Canadá . Isso combina perfeitamente com nossas observações das ações de impactos (Beals, 1968; Goodings, 1992;  Bleeker, 2004) no envelhecimento de rochas. 




O LADO OCULTO DA  LUA REPLETO DE CRATERAS

O lado oculto da lua


Um bombardeio de asteroides vindo de uma mesma fonte  em um pequeno período,  explicaria a desigualdade de impactos que estariam mais uniformes em relação aos lados da lua,  caso não houvesse um tempo especial de bombardeio afetando sobretudo o lado exposto e "escuro" (para nós) da lua, tal diferenciação dificilmente existiria. Seria ilógico deduzir isso pois teríamos que assumir algo que não acontece na proporção de meteoros que colidem com a mesma. Esta diferença revela um momento único em que as costas da lua foram castigadas por um bombardeio de milhares de asteroides. 



27 dias, 7  horas e 43 minutos se repetem exatamente e sincronicamente (na rotação em torno de si) e (na revolução da lua em torno da terra), tendo seu lado sempre voltado para a terra e seu lado oposto para objetos que atacassem a terra,  sempre exposto, que no caso estaria exposto especialmente num  momento em que houve uma chuva de asteroides. 



Existe um lado da lua desigual ao outro em termos de queda asteroides. Isso recomenda um período especial que ela recebeu uma chuva de impactos?

No vídeo abaixo se faz uma simulação de formação da lua



Quando retiramos os bilhões de anos , passamos a entender o bloco conjunto de situações onde explicamos porque temos quedas maiores gerando nítida diferenciação de temperatura na queda, o que se harmoniza com queda de corpos maiores em menor quantidade e corpos menores em maior.

Em geral quando ocorre o confronto de uma única vez de dois corpos,  o despedaçamento ocorre formando blocos maiores e centenas de milhares de blocos menores , tendo poucos jogados para bem mais longe que outro (resultado da tensão num ponto). Esta imagem se harmoniza com o presente estudo , pois observamos :

1. O lado escuro e mais "queimado" sendo atingido por corpos maiores, que não deveriam estar sozinhos pois que era resultado de grande colisão.
2. Os efeitos plasma, piezoelétricos , ondas sonoras e aceleradores de partícula vão dar a impressão de que estas quedas foram mais antigas (Na lua onde não temos a dinâmica do mar criando estratos "moles" (não litificados) esta relação (observada por Hector Lutero Siman quando discorríamos o assunto) fica bem mais evidente; observe que a queda dos grandes bólidos lunares são considerados os mais "antigos" .
3. Esta imagem explicara casos mais esporádicos de meteoros mais próximos viajando ainda no espaço alguns provavelmente oriundos de uma  mesma colisão.

 





Geochimica et Cosmochimica Acta, 1956, Yol. 10, pp. 236 to 23i.
file:///C:/Users/06/Desktop/age%20earth.pdf


P~~WIXXI Prers Ltd., London Age of meteorites and the earth CLAIRE PATTERSOX’ Division of Geological Sciences California Institute of Technology, Pasadena. California (Received 23 Janwrry 1956) Abstract-Within experimental error, meteorites have one age as determined by three independent radiometric methods. The most accurate method (Pb*07/Pb*06) gives an age of .i*BS T O,OT x. IO” yr. Using certain assumptions which are apparently justified, one can define the isotopic evolution of lead for any meteoritic body. It is found that earth lead meets the requirements of this definition. It is therefore believed t,het the age for the earth is the same as for meteorites. This is the time since the earth attained its present mass. IT seems we now should admit that the age of the earth is known as accurately and with about as much confidence as the ~oncentratio~l of aluIninium is known in the Westerly, Rhode Island granite. Good estimates of the earth’s age hare been known for some time. After the decay-constant of U235 and the isotopic compositions of common earth-leads were determined by NIER, initial calculations. such as GERLING’S, roughly defined the situation. Approximately correct calculations were made by HOLMES and by HOUTERMAWS 011 the basis of bold assumptions concerning the genesis of lead ores. Subsequent criticism of these calculations created an air of doubt about anything concerning common leads and obscured the indispensable contributio~ls which these investigators made in est~ablishiIlg the new science of the geochemistry of lead isotopes. When the isotopic col~lpositio~l of lead from an iron meteorite was determined, we were able to show that a much more accurate calculation of the earth’s age could be made, but it still was impossible to defend the computation. Now, we know the isotopic compositions of leads from some stone meteorites and we can make an explicit and logical argument for the computation which is valid and persuasive. The most accurate age of meteorites is determined by- first, assuming that. meteorites represent an array of nranium-lead systems with eert,ain properties, and by then computing t’he age of this array from the observed lead pat,tern. The most. accurate age of t,he earth is obtained by denlollst.ra~~i~~g that t,he earth’s urallilln~-lead system belongs to the array of meteoritic uranium-lead syst,ems.* The following assumptions are made concerning meteorit,es: they were formed at, the same time: they existed as isolated and closed systems: they originally contained lead of the same isotopic composition; they contain uranium which has * C. PATTER~~K: S.R.C. Conference on nuclear processes in geologir ncttiitgs. I%.? Scptembcr meeting, I’ennsylvnnia State University. Except for minor disaprecments. this paper IS probably a concrete expression of the attitudes of most investigators in this field. both hew and iii Europt~. The author is grateful to his coih?agues, CLAYTON, INC~HRAX, TILTOX, ~.ws~lmc~rt:, ILHC~ ~ETIIERILL, for their critici&ns which helped ctarify this paper. 230 Age of meteorites and the earth the same isotopic composition as that in the earth. On the basis of these assumptions various leads might be expected to evolve as a result of different original U/Pb ,ratios in separate meteorites, and an expression* for any pair of leads derived from such an array is: 4, - 4, (eAIT - 1) R,, - RPb = k(e”zT - i) (1) where R, = Pb*07/Pb204 and R, = Pb206/Pb 204 for leads from different met,eorites a and b, k = U23s/U 235 today (137.8), A1 = U23S decay-constant (9.72 x lo-lo yr-l), A, = U238 decay-constant (1.537 x lo-lo yr-l), and T = age of the array. The isotopic composit,ions of leads isolated from three stone and two iron meteorites are listed in Table 1 (PATTERSON, 1955). Because the radiogenic and nonradiogenie leads may occur in different mineral environments in a stone meteorite and the sample dissolution procedures may be chemically selective, the lead ratios for the first three meteorites in Table 1 have estimated errors from t,he absolute of about - ,O. The lead ratios for the last two meteorites in Table 1 hare estimated errors *, 0 : from the absolute of about 1%. Table 1. The isotopic compositions of lead in meteorites / I Pb Composition Meteorite _~. 206/%04 207/204 208/‘304 Xuevo Laredo, Mexico I 50.28 34.86 67.95 Forest City, Iowa 19.27 15.95 39.05 Modoc, Kansas 19.48 15.76 38.21 Henbury, Australia 9.55 10.38 29.54 Canyon Diablo, Arizona 9.46 10.34 29.44 These leads cover an extreme range in isotopic composition and satisfy expression (1). yielding, within experimental error. a unique value of T. This is illustrated in Fig. 1, where it is shown that the Pbzo6/Pbzo4 and Pbzo7/Pb204 ratios from meteorite leads lie on a straight line whose slope corresponds to an age of a.55 x lo9 yr. The dotted lines indicate how stone meteorite leads have evolved. It is clear that, the assumpbions of the age method are justified by the data. Errors in the lead data and in the decay-constants contribute about equally to the overall error in the calculated age, which amounts to about 1 i 96. The age for the meteorite array is calculated t,o be 4.+% _C 0.07 x lo9 yr. The assumptions have not been shown t’o bc unique. The data can be explained * Ai similar form of this expression was first wctl hy A. SIER iI1 1!)3!!. F. HO~TERIANS has termed the rspression an “isochron.” IXeferences for the constant,s we’: (k) 31. INGHILUY; Vol. 14 Mallhattan Project Tech. SC-., Div. 2, Gaseous Diffusion Projrct. Chap. I-. p. 33 (l!M6); -4. GHIORSO, nnd R. ~LTNNINGH.A51: ,%1/S. he. 88, tiJ:! (l!).i?). (i,, 1,) E. FLEMING, 231 CLAIRE PATTERSOS by other qualifying or even contradictory assumptions. Most of t,hese can be excluded as improbable. One common criticism should be mentioned: the time of a process of division or agglomeration of meteoritic material (without differentiation) cannot be distinguished by this age method. It seems probable thal any such process of division or agglomeration would be accompanied by chemical differentiabion. Any meteorite which had a differentiat8ion historT after its initial formation would fall off Dhe isochron. The fire meteorites in Table 1 represent a most extreme range of differentiation which occurred during t’he initial process of 1 I I 40- A = 4.6 x 10gyrs. B = 4.5 x IO' yrs. A B 30- x %I g zo% n_ IOI I 1 0 IO 20 30 40 50 Fi:r. 1. The leatl isochron for meteorites and its estimated limits. The outline around each point. indicates measumnent error. formation. This criticism is not serious a.s far as meteorites are concerned. since if it were valid the lead-lead isochron would date the occurrence of different,iation processes : however. it is important with respect to the age of the earth and will be mentioned later. At the present, t,ime, t)he nest most accurate meteorite age is determined by the A40/Ii”o method. The argon ages of six stone meteorites, three of them determined b- WASSERBI-RG and HATDES (1%X), and three of them clet~ermined by THOMSON mcl MATSE (1035). are listed in Table 2. The age of Forest City has been rrdrtermined without, change by RETSOLDS and LIPSOS (IS%). Two sets of ages are calrnlated on the basis of the two reasonable limits of the c-//j- branching ratio. The 0*0S5 branching ratio is the ra.lue obtained by studies of old pot.az;sium minerals dated by uranium-lead techniques. The 0.1% branching ratio is the value obtained by counting techniques and 1,~ direct measurements of the amounts of clecag products. The difference between t,he two values can be accountecl for 1)~ systemat,ic loss of radiogenic argon in the old potassium minerals. If one assumes _ige of meteorites and the earth that a fixed amount of about LO 3 96 of radiogenic argon is lost, from all stone meteorites, i.e. using a branching ratio of about O-10. then t,here is agreement of lead and argon ages for the same st,one and an indication that the stones hare existed aa cold and solid bodies since they were formed. Argon meteorite ages different from the ones mentioned here have been reported by GERIXSG (10.51). and PAVLOVA GERLIXG and RIK (1954). Since errors in the data present,ed by GEXLISG and P_kv~ova cannot be eraluat,ed with anF certainty. we cannot be concerned by differences bet,ween ages calculated by them and ages calculated from ot’her data. Because of logarithmic behaviour, ralues for calculated ages of these old samples are insensitive to changes in the e-l/?- branching ratio. For t,his reason only disagreements of about 15 “/: between A40/I<4’J and Pb2’J’/Pb20G meteoritic ages can be accounted for by a twofold change in the branching ratio. Large age differences must, therefore be reconciled on the basis of other experimental errors. Measurements of the amounts of nonradiogenic argon in radiogenic and nonradiogenic argon mixtures are subject t,o large uncertainties, and for t.he first four meteorites in Table 2. AJo/K40 ages of meteorites Meteorite Age x lo-$ Investigators ’ (e-//3- = 0.085) (e-/p- = 0.1%) 1 - Beardsley, Kansas Holbrook, Arizona Forest City, Iowa Akabu, Transjordan Brenham Township, Kansas Monze, Sorthern Rhodesia 4.8 4.2 4.8 4.' 4.7 4.1 4.4 3,s 4 3 1 I WASSERBURG and HAYDEN ~~.~SSERB~RG~~~~HA~DES IKASSERBURG md HAYDEN IREYNOLDS and LIPSOS THonfsos and MAX-NE THOMPSON; and ~IAYXE 1, Ta.ble 2, nonradiogenic argon corrections were small. For t,he last two meteorites in Table 2, nonradiogenic argon corrections were eskemely large and t,he errors in calculated age are excessive. The isotope dilution det8ermination of potassium, used by MrA4ss~~~u~~ and HAYDEN;? is nearlv an absolut,e method. while the flamephotomet’ric determination of potassium. used by THOMSOS and K~YSE, requires a nat,ural absolute standard which they did not use. The age of meteorites 1ia.s been determined by bhe SrS’/Rb’J’ method. The concentrations of rubidium and st,rontium and the isotopic compositions of strontium have been determined in two stone meteorites by SCHUMACHER (1955). The Rb/Sr ratio in one stone was so low that any change in isotopic composition of st’rontium due bo radioactivity would be within experimental error. The Rb/Sr ratio in the other stone (Forest City. Iowa) was considerably higher and sufficient 233 CLAIRE PATTERSON to cause a loo/ difference in t,he relative abundance of Sr8i when the isotopic compositions of strontium from both stones were compared. The value for the decay-constant of Rbsi Is ’ in question at the present time. Reported values range from 4.3 to 6-i x lOlo yr for the half-life. Part of the difficulty in the counting techniques of measuring the half-life arises from the fact that, the frequency of 8-s at the low end of the energy spectrum increases rapidly with no appearance of a maximum. Measurements of decay products in terrestrial rubidium minerals dated by uranium-lead technique inrolx-e errors of open chemical systems. SCHUMACHER’S experiment probably constitutes an ideal case of the geological measurement of the half-life of Rbsi. since the ages have been det’ermined by lead methods and the possibility of open chemical systems are remot’e. His methods of measurement are at least as accurate as the radiometric methods. One would t’herefore use his data to calculate the half-life of Rbs7. using the Pb-Ph isochron age of meteorites. The half-life of Rbsi, as determined by these data. is 5-l j; lOlo yr, and is probably the most reliable value at, present. The half-life determined by the geological method on terrestrial minerals (5.0 x lOlo yr) agrees well with this.* Because of t’he overwhelming abundance of nonradiogenic helium in iron met,eorites and the large errors associated with the determinabion of the concentrations of uranium and thorium in iron and stone met8eorites. the age of meteorites by the helium method is not accurate to much better than an order of magnitude (PAKETH ef al., 1953; DALTON et al., 1953). It has been reported that iron meteorites and the metal phases of stone meteorites were outgassed of helium as of about 5 >: lOa yr ago, while the silicate phases of stone meteorites were not, (REASBECR and M/lsun-E, 195.5). Such an event would be highly significant and would require detailed evolutionary theory for meteorites. Recent neutron activation (REED and TURKEVICH) and nuclear emulsion (PICCIOTTO) analyses of iron meteorites show that the concentrations of uranium in t,hese bodies are very low. and that’ the uranium concent,rations used for helium a,ge calculations of iron met8eorites may be erroneously high. The question is unresolved at present. but it seems reasonable to believe that, invest,igations of meteoritic helium will become vitally important to cosmic-ray studies and may be decisive in meteorite evolution theory, but cannot be used for accurate meteorite-age calculations at the present time. The Canyon Diablo lead listed in Table 1 was isolated from troilite where the U238/Pb204 ratio was shown by direct analysis to be 0.025 (PATTERSO?; ef al.. 1953). This ratio is accurate to at least an order of magnitude. and it is so small that no observable change in the isot,opic composition of lead could hare resulted from radioactive decay after the meteorite was formed. Since stone meteorites were cold and solid during their lifetime, it is unliliely that lead transport could have occurred between iron and st,one meteoritic phases if they existed in one body. This iron-meteorite lead is therefore primordial and represents the isotopic composition of primordial lead at the time meteorit’es were formed. Using the isotopic composition of primordial lead and the age of meteorites. expressions can be * 4 value rerommendecl by the work of the geochronology laboratory at the Dept. of Terrestrial Magnetism, Carnegie Institute of Washington. 234 Age of meteorites and the earth writt,en for a representative lead which is derived today from any system belonging to the met’eoritic array: Pb2”6/Pb2”4 = 9.50 + l-014 U238/Pb204 (4 Pb207/Pb204 = 10.36 + O-601 U23s/Pb204 (3) If any two of the three ratios above can be independently measured in the earth’s uranium-lead system, and they satisfy expressions (2) and (3). then this system belongs to the meteoritic array and must have it’s age. Two of t,he ratios can be measured in a sample of earth lead. but the problem of choosing such a sample is complex because the ratio of uranium to lead varies widely in different rocks and minerals whose ages are short compared to t’he age of the earth. One approach is to partition the earth’s crust int,o separate chemical syst’ems of uranium and lead and consider t’heir interactlions. Such systems may range from minerals t’o geochemical cycles. Nearly all of the lead-isotope data concerns either minerals in which the uranium-to-lead ratio is very high (uraninites, et,c.) or minerals in which this ratio is essentially zero (galenas). The approsimat,e times of formation of some galenas have been determined! and of these. two dozen or so lately formed galenas may be used as a measure of earth lead (Suclea) Qeobogy (lg.%). IV. Faul, Ed.). The isotopic compositions of lead in some recent oceanic sediments have also been determined (PATTERSOK, GOLDBERG. and IXGHRAM 1953), and these may be used as a measure of earth lead. Any of these samples will be improper or biased if they are derived from a system of uranium and lead which is only partially closed and is subject to slow but appreciable transport from other systems with different I_?238/Pb204 ratios. In this respect, the sample which may represent the system of largest mass is probably t’he more reliable. One sample of oceanic sediment lead probably represents more material than a dozen galenas. The isotopic composition of t,his sediment lead is Pb206/Pb204 = 19-O and Pb207/Pb20f = 15-5, which satisfies expressions (2) and (3) surprisingly well. It is doubtful if these figures are grossly biased, since a few measurements of uranium and the isotopes of lead in rocks with widelv different -CT238 Pb204 I ratios indicate rather good mixing t,o be t,he first-order effect on the isotopic composition of lead in the earth’s crust (PATTERSOS: TILTOX, and ISGHRAM, 1955). Independent of the absolute abundances of lead isotopes, a rough measure of t’he rates of change of the lead-isotope abundances in the earth’s crust may be obt,ained from the isotopic composit’ion of galenas of different ages. These rates of change are defined by the ratios of uranium and thorium to lead in the material from which the galenas are derir’ed. From t,he observed rate of change of Pb206, the U23s/Pb204 ratio in the earth’s crust is found to be 10 (COLLINS, RUSSELL, and FARQCHAR? 1953). This value satisfies expression (2) and (3) for sedimentary lead with unexpectedly good agreement. In Fig. 2 it is shown that oceanic sediment lead (open circle) falls on the meteoritic lead isochron. Most of the lately formed galenas fall within the dotted outline, although a few are widely aberrant. isochron is determined by t(he U23S/Pb204 The position of a lead along the ratio in the system from which the lead 235 CLAIRE PATTERSON ev elves . The arrow indicates the position on the isochron which sediment lead should occupy as predicted by the isotopic evolution of dat’ed ore leads. Independently measured values for all tShree ratios adequa,tely sat,isfy espressions (2) and (3), and therefore the time since the eart.h attained its present mass is 455 f 0.07 x log yr. Fig. 2. The relationship between COM~OI~ earth lea& and the meteoritic lead isochn3u. If the earth is a late agglomeration without differentiation of meteoritic material then it, can have ally age less than meteorit,ic material. Rather thaxi arguiug that such a process would be accompanied by chemical differeilt,iatioI~ (aud a change of the U/Pb ratio), it seems reasonable to beliere instead that such a late agglomeration process would be less probable than one xvltere both n:eteorit,es and the eart,h were formed at the same time. It is a fact that. estreme chemical different’iation occurred during the process which led to the mechauical isolat,ion of the mass of material of which the earth is made, and since changes in this mass were accompanied by chemical differel~tiatioI1, the Pb/Pb met,eorite isochron age properly refers to the time since t.he earth attained it,s present, mass. REFERESCES Age of meteorites and the earth PATTERSOX C., TILTOS, G., and IXGHRAM 11. (1955) Science 121, 69. PICCIOTTO E. Suclear Physics Centre, Universuy of Brussells, manuscript. REYXOLDS J. and LIPSOS J. Epipoleological Societ,y, spring 1955 meeting, U.C.L.A. REASBECK P. and JI~YxE H. (1955) Satwe 176, 186. REED G. and TURKEYITCH A. Inst. Kuciear Studies, Universitp of Chicago, manuscript. SCH~-M.~CHER E. S.R.C. conference on nuclear processes in Geologic Settings (communicated b>- H. UREY and JI. INGHRAV), 1955 Sept. meeting, Penn. State University. (Copies of his manuscript are available). THOMPSON S. and 3I_4YxE E. (1955) Geociiim et Cosmocllim. Acta 7, 169 VT.~SSERB~RG G. and Brly~~s R. (1955) Phys. Rev. 97, S6. IVASSERBURG G. S.R.C. Conference on nuclear processes in Geologic Settings. 1953 Sept. meeting, Penn. State UniversitS. Swlecrr (:coZogy (1951) H. Faul, Ed.. J. Wiley, S.T.