THE RICE INSTITUTE
The Distribution of Thorium and Uranium in Sedimentary Rocks and the Oxygen Content of the Pre-Cambrian Atmosphere
by
Richard Pliler
A THESIS
SUBMITTED TO THE FACULTY
IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Master of Arts
Houston, Texas July 1956 3 1272 00081 1362
&L (U 7 ACKNOWLEDGMENTS
The author Is deeply grateful to Dr. John A. S. Adams for his guidance and most generous assistance throughout the research. The staff members of the Department of
Geology have contributed many suggestions which are appreciated.
The author is indebted to the Shell Development Company, Houston, Texas, for its valuable aid in accumula¬ ting the calibration data presented in Table III and for its placing at the author*s disposal many useful rock samples. For their contribution of samples, thanks are also due to R. H. Nanz, Jr., Shell Development Company, Houston, Texasj A. M. Macgregor, Salisbury, Rhodesia; and E. S. Barghoorne, Department of Biology, Harvard University. This research was generously supported by the Robert A. Welch Foundation. TABLE OP CONTENTS
page
Introduction 1 Procedure 11
Discussion and Interpretation of Data ... 16 Conclusions 22 Bibliography ..... 37 Appendix A, Index to Samples LIST OP TABLES AND FIGURES
Table page
I The Development of the Atmosphere .... 2
II Solubilities of the Fluorides of Uranium and Thorium 5
III Summary of Calibration Data ...... 23
IV Summary of Data on New Material , . . , . 24
Figure
I Thorium to Uranium Ratios in Igneous Rocks 33
II Thorium and Uranium Concentrations in Sea Water Versus Geologic Time .... 34
III Calibration Curve , . 35
IV Thorium to Uranium Ratios in Chemical Precipitates 33
V Thorium to Uranium Ratios in Shales ... 36
VI Thorium to Uranium Ratios in Bentonites 36 INTRODUCTION
The purpose of this research is to study the use of the actinides uranium and thorium as atmospheric oxygen indica¬ tors and thereby to gain more knowledge of the oxygen con¬ tent of the pre-Cambrian atmosphere. A second and broader objective is to determine the relative thorium and uranium distribution in sediments as a function of depositional environment.
Though little understood, the chemical evolution of the earth and its atmosphere during geologic time has aroused the curiosity of earth scientists for many years. The pre¬ sent atmosphere with its high oxygen content of about twenty per cent and its low hydrogen content of 0.5 parts per mil¬ lion is peculiar in that there is no available evidence in the rest of the solar system of a similar lack of hydrogen and surplus of oxygen (Kuiper, 195*0. Investigations have revealed that the universe appears to be over 99 per cent hydrogen and helium with free oxygen a great rarity. These and other observations have led to the conclusion that the early atmosphere of the earth was probably similar to the present atmosphere on the other planets of the solar system in being composed essentially of water, ammonia, methane, hydrogen and carbon dioxide (Urey, 1952).
Table I summarizes the major compositional changes that are believed to have taken place in the original terrestrial atmosphere. Radioactive decay, as shown in Table I, has -2- released both argon and helium to the earth’s atmosphere throughout geologic time. Almost all of the present atmo¬ sphere’s argon content, 0.93 volume per cent, is believed to
TABLE I
The Development of the Atmosphere
Primordial Present Plants Outer Rocks Atmosphere Atmosphere & Rocks Space
radioactive K 40 decay ^ A 40(0.93^) + Ca t°0
He 4 Th 232 radioactive Pb 206 U 235 He 4(5.2x 10“V) +• He 4 decay Pb 207 +• U 238 Pb 208
HpO C COp photosynthesis^ 0p(20,95$) . COo NHo decomposition ? N2(78.09^j "+” organic -t~ H2 CH4 material have been formed by the radioactive decay of potassium-40
(Rankama, 1954). A similar material balance in the helium produced by the radioactive decay of thorium and uranium in¬ dicates that the present atmosphere contains only a trace,
0.5 parts per million, of the total helium-4 released into it (Mayne, 1956). As very ordinary conditions in the earth's atmosphere give an helium-4 atom sufficient velocity to escape the earth's gravitational field, most of the helium produced is thought to have escaped into interplane¬ tary space. Any molecular hydrogen produced from the 3- decoraposition of the original constituents of the primordial atmosphere will escape into outer space even more readily than helium; any molecular oxygen or nitrogen produced in the same decompositions would remain because these gases are too heavy to attain the earth's escape velocity. As more free molecular oxygen accumulated in the atmosphere, less and less hydrogen could escape to outer space before it was combined with oxygen in fires and electrical discharges.
This thesis is concerned with the third major change illustrated in Table I, i.e,, the accumulation of oxygen in the atmosphere and the resulting change from a non-oxidizing to an oxidizing atmosphere. Two limits have been proposed for the time, tQ , when free oxygen began to accumulate in X the atmosphere. One limit, based upon a single study of ferric to ferrous ratios in a suite of Finnish rocks, indi¬ cates that the first accumulation of free oxygen occurred at least two billion years ago (Rankama, 1955). Another tenta¬ tive limit, based upon the biological oxidation and frac¬ tionation of sulfur isotopes, indicates that the first accumulation occurred at least 700 million years ago (Ran- kama, 1948).
To determine t0 previous workers have studied ferrous x to ferric ratios in ancient rocks, taking a high ratio to indicate less oxidizing conditions than a low ratio. This method assumes that the rock material was in equilibrium with the atmosphere at the time of deposition and that there has -4- been no subsequent change In the oxidation state of the iron.
The last assumption is all too frequently not the case, and, therefore, the study of ferrous to ferric ratios can be applied only to a few select geologic situations.
As alternative geochemical oxygen indicators the actinides thorium and uranium may be useful. The use of thorium and uranium in this connection is based upon their rather unique chemical properties and geochemical distribu¬ tions. Seaborg and Katz (1954) have established thorium and uranium as members of an actinide series. This actin¬ ide series is fully analogous to the lanthanide or rare earth series in having an inner, unfilled electron shell and an identical outer electron configuration. Thus, the members of such a series have very similar chemical proper¬ ties, such as solubilities, ionic radii, and so forth. The fluorides of uranium and thorium may be used to exemplify some of these similarities and also a very important difference. Table II illustrates the marked similarity in the solubilities between thorium tetrafluoride and uranium tetrafluoride. This table also demonstrates a very fundamental difference between thorium and uranium.
Uranium has two additional valence electrons in the inner unfilled electron shell. As a result, uranium can have a valence state of plus six as well as plus four, whereas thorium can only have a valence of plus four under geochemi¬ cal conditions. Uranium hexafluoride is radically different -5- frora the thorium and uranium tetrafluorides. It is a gas under ambient conditions and is unstable in the presence of water, with which it reacts to form the very soluble uranyl fluoride.
TABLE II
Solubilities of Fluorides of Uranium and Thorium
Gms/Liter at 25° C
TO1F4 (Gmelin, 1955) < 0.1
UFj| (Katz and Rabinowitch, 1951) < 0.1
UFg(g) unstable in H20, forms U02F2 (Katz and Rabinowitch, 1951)
U02F2 (Katz and Rabinowitch, 1951) c/ 2000
The uranyl ion is formed from the tetravalent uranium ion at an oxidation potential of -0.62 which is near that of the ferrous to ferric oxidation potential of -0.77 (Latimer,
1952). Also, experiments show that ferrous iron reduces the uranyl ion almost quantitatively at pH’s of four and above and over a wide range of ferrous ion concentrations (McKelvey et al, 1956). On the basis of these data it seems likely that if the earth's atmosphere changed from non-oxidizing to oxidizing most of the iron exposed to weathering would have to be oxidized to the ferric state before any tetravalentur- anium could be oxidized to the uranyl form. It is important to note that under present atmospheric conditions uranium in -6- the form of uranyl Ions is strongly fractionated from the insoluble tetravalent thorium. If the early atmosphere could not oxidize tetravalent uranium to the more soluble uranyl form, this fractionation probably would not have occurred to such a marked extent.
In discussing the fractionation of thorium and uranium the role of Igneous rocks Is dominant, because they consti¬ tute about 95 per cent by volume of the earth’s crust. Ig¬ neous rocks represent material which has crystallized deep within the earth's crust out of contact with any free oxygen that may be present in the atmosphere. Because of their great abundance in the crust, igneous rocks must be consi¬ dered the ultimate source material for all other rock types. Most of the uranium in igneous rocks Is in the tetravalent state and most of the iron is in the ferrous state. Figure
I summarizes the data gathered from the literature on the relative abundance of thorium and uranium in igneous rocks. Represented in this figure are the average thorium to uranium ratios found in over 400 determinations on more than 1900 samples of igneous rocks. Some of the determinations were made on aggregate samples of a number of igneous rocks.
The data were gathered by different workers using various analytical methods. The ratios range from 2.5 to 6 with most of the values clustered in the range of three to four. The average thorium to uranium ratio of these samples is about 3.5. Thus, the source material for all the other -7- rock types has a thorium to -uranium ratio of between three and four.
Although there are no quantitative data on the cosmic abundances of thorium and uranium, they have been estimated on the basis of uranium and thorium determinations on meteorites, the only available extra-terrestrial material. The thorium to uranium ratio of meteorites is estimated to be between three and four (Urey, 1956). This is very close to the terrestrial (igneous rocks) ratio and the two may be identical. Under present conditions, igneous rocks are exposed to chemical weathering by the combined action of oxygen, water and carbonic acid. The soluble ions are leached out of the rock and are ultimately transported in solution to the oceans. It is during the leaching process that uranium is separated from thorium under present atmospheric conditions.
The uranium is oxidized to the very soluble uranyl form and is leached, whereas the quite insoluble tetravalent thorium is, for the most part, left behind in the resistates and hydrolyzates. This separation of uranium and thorium in the rock cycle is more fully discussed by MeKeIvey et al (1955).
Sea water today, according to the measurements of Koczy (1956) and others, contains about 2 X 10mg U/ml and about 4 X 10-^ mg Th/ml. These values indicate an ex¬ tremely low thorium to uranium ratio of 0.0002 in sea water and reflect a very effective separation of thorium and -8- uranium under the present oxidizing atmosphere. The sea is obviously very far from being saturated with
the uranyl ion and it may be expected that any chemical pre¬ cipitate from the sea water fixes uranium and thorium mainly by occlusion. One would therefore expect that deposits of precipitates, such as limestones and phosphate rock, would tend to have about the same thorium to uranium ratio as the
sea water from which they were deposited.
Leached, fine-grained elastics and clay material are also transported in suspension to the oceans where they generally accumulate to form clay deposits and ultimately shales. The shales deposited under the present atmosphere should reflect any leaching of uranium by having higher thorium to uranium ratios than their source rocks.
Uranium is also removed from sea water by fixation in bottom clay deposits which are rich in organic matter and other reduetants. It is not known exactly how uranium is fixed in this material which eventually may form black shales. The fixation may be by adsorption of the uranyl ion; it may be by tetravalent uranium being precipitated under the more reducing conditions of this environmentj or it may be by some uranium-organic complex. Regardless of the fixation process marine black shales show a markedly low thorium to uranium ratio and an unusually high uranium concentration.
Continental shales and resistates, such as bentonites and bauxites, formed under an oxidizing atmosphere may be -9- expected to show a high thorium to uranium ratio, for it is very likely that the uranium initially present has been oxidized to hexavalent uranium and leached as the soluble uranyl ion.
Very different mechanisms from those just described would be expected to operate under the postulated non- oxidizing atmosphere of the early pre-Cambrian. At this time the uranium would not be oxidized to the more soluble hexavalent state. Because of the very similar solubilities of tetravalent uranium and thorium, the two elements would accompany each other throughout the rock cycle in essential¬ ly the fundamental ratio of three to four found in igneous rocks. Figure II is a hypothetical plot of the uranium and thorium concentration in sea water versus geologic time.
Considering that thorium would not be appreciably affected by any change in the composition of the atmosphere, the thorium content in the sea has throughout geologic time probably remained near its present concentration of 4 X 10"14 mg Th/ml (Koczy, 1956). It is hypothesised that before tn the thorium to uranium ratio in the sea was about the same as the terrestrial ratio; that is, uranium was present in a concentration of approximately one third that of thorium. After tn the uranium concentration of sea water ux would build up to its present value of about two parts per billion. This build up is shoxvn in Figure II as being very -10- rapid, because the uranium in the present ocean could be replaced by river water in about 300,000 years (Koczy,
1954). If Figure V is correct, rocks formed in the oceans prior to t0 should reflect a poor fractionation of thorium and uranium by having a thorium to uranium ratio near the fundamental ratio of three to four. Furthermore, such rocks should also have very low uranium concentrations comparable to those of thorium found in equivalent rocks after t . x The testing of the above hypotheses regarding the fractionation of thorium and uranium is the main purpose of this thesis. The data for this investigation were gathered both from the literature and from new analyses. The data from the literature are presented in Figures II through IV, and the new data are presented In Table III. Descriptions and loca¬ tions of the new samples analyzed for this work are given in Appendix A. -11-
PROCEDURE
The uranium contents of the samples studied were determined by the fluorimetrie uranium procedure described by Adams and Maeck (195*0. One gram samples were generally used and the fluorimetrie determinations were carried out on a Galvanelc-Morrison fluorimeter, Model JA 2600, manufactured by the Jarrel-Ash Company, Using this method, uranium in concentrations of over 0.05 parts per million can be mea¬ sured ivith an error of less than±15 per cent of the actual uranium present. The precision of the uranium determina¬ tions was normally found to be less than t15 per cent. For a complete listing of the uranium determinations see Table
IV.
There is as yet no routine and reliable method for determining the small amounts of thorium, generally less than 10 parts per million, found in common rocks. However, as thorium and uranium are the only common alpha-emitters found in nature, it has been found that the relative thorium to uranium ratio of a sample in radioactive equilibrium may be estimated by the ratio of total alpha activity to fluori- metrically determined uranium content (Adams, 195*0.
The relative alpha-particle activity of each sample, ground to pass through 100 mesh silk bolting cloth, was measured with a low background proportional counter, Model
PC-2B, manufactured by the Nuclear Measurements Corporation. -12-
The instrument was the argon-flow type with 2 A geometry. The general principles of this instrument are discussed by
Sharpe (1955) and Wilkinson (1950). The counting of each
sample was continued until at least 100 counts and reproduci ble one hour totals were obtained. The majority of samples were then systematically recounted after intervals of
several weeks to check the stability of the instrument and the precision of replicate loadings of the sample dishes. The counts were made with the sample in a two-inch diameter stainless steel dish, giving a sample area of 20.2 cm2. The samples were loaded into the dishes and carefully smoothed flush with the tops of the containers. As the dishes were several millimeters deep, the samples were essentially in¬ finitely thick alpha sources. Background counts were taken with the sample dish loaded with ordinary granulated sugar, which was found to have a very low Intrinsic alpha-particle activity. The background was measured in overnight counts throughout the period during which the samples were counted.
The background usually varied between one and three alpha counts per hour, although it occasionally was as high as five. A National Bureau of Standards alpha-particle standard,
#166-5* was also counted periodically to check the operation of the proportional counter. This standard's alpha activity consistently averaged about 25 counts per second, whereas the National Bureau of Standards reported 23.7 ± 2^ alpha -13- counts per second of this standard when counted In two A geometry. Back-scattering and differences in instrument
sensitivities are believed to cause the difference. On the basis of the standard and the reproducibility of the re¬ counted samples the error of the alpha counting procedure was found to be considerably less than ± 20 per cent for most of the samples counted. For a complete listing of the alpha particle flux measurements, see Table IV. The Shell Development Company, Houston, Texas, analyzed nine samples for their uranium and thorium content by a gamma-ray scintillation spectrometer method similar to that described by Hurley (1956). The Shell Development Company achieved the determination of thorium in three of these sam¬ ples by a colorimetric method similar to that of Byrd and
Banks (1953). The alpha activities of these nine samples along with two standard samples, A and B, from the National
Bureau of Standards, were measured by the procedure discussed previously, and their uranium contents were determined fluorimetrically at The Rice Institute. Table III summarizes the data obtained on these samples.
Figure III Is a plot of the alpha radiation per hour to uranium content in parts per million versus the thorium to uranium ratio determined by gamma-ray spectral analysis (see Table III). Two other samples, A and B, of known thorium and uranium content are Included in this figure. The uranium value used on the abscissa are those determined by the -14- fluorimetric method, while both the thorium and uranium values used on the coordinate were those determined by the gamma-ray method, except for samples A and B which are National Bureau of Standards thorium and uranium ore standards.
All but one of the points fall on or near a linear curve, proving that both methods are measuring the thorium to uranium ratio on samples in or near radioactive equili¬ brium. The one major discrepancy is the position of sample
S-3 which is known to be weathered and therefore may be out of radioactive equilibrium. The gamma-ray scintillation method for determining thorium and uranium is actually a measure of a thorium daughter, thallium-208, and a uranium daughter, bismuth-214, present in the sample. The amounts of these nuclides are calculated into equivalent amounts of the parents thorium and uranium in secular equilibrium with their daughters. Consequently, if the rock sample had pre¬ viously been leached of some of its uranium, the gamma-ray method would report more uranium than actually was present, and such may be the case regarding S-3.
Using the curve represented in Figure III as a calibration curve, it is possible to determine the approxi¬ mate thorium to uranium ratio of a sample solely on the basis of its alpha-activity and uranium content without actually determining the thorium content directly. The dashed curves shown in Figure III represent the maximum and minimum limits -15- of the curve on the basis of a maximum error of ± 20 per cent in the alpha counting technique and t15 per cent in the fluorimetric uranium determinations. With the exception of S-3 all the points on the graph fall well within these very conservative limits. An examination of this graph re¬ veals that on the basis of the alpha particle flux to uran¬ ium content of a sample it is possible to estimate the thorium to uranium ratio of a sample to about one of the actual value. At the far left of the curve, the ratios be¬ low one can be measured with an error of less than ±0.5.
This accuracy is sufficient for the geochemical problem under consideration. All of the thorium to uranium ratios shown in Table IV were determined by this method.
The errors in both the alpha and in the gamma radiometric methods could probably be reduced by improved instrumentation and by counting greater numbers of events. The alpha method, however, has the advantage of requiring less than ten grams of sample, whereas the gamma method requires at least 600 grams. DISCUSSION AND INTERPRETATION OP DATA
The data obtained from the literature on the thorium to uranium ratios in sediments are summarized in Figures IV, V, and VI. The data on new material are listed in Table IV. The alpha activity of each sample Is recorded in Table IV as alpha counts per hour per 20.2 cm^, and the uranium content is reported as parts per million uranium. The maximum, minimum, and average estimates of the thorium to uranium ratio of each sample is also given.
An examination of the data gathered on nine samples definitely known to be continental in origin and Paleozoic or younger in age reveals that the thorium to uranium ratios of these shales, resistates, and hydrolyzates are much higher than the fundamental ratio of three to four. The ratios for these rocks range from 6 to 23. This indicates that in the oxidizing depositional environment of these rocks the uranium was effectively fractionated from thorium, probably as the very soluble uranyl ion. In this connection it is useful to examine thoroughly leached igneous material such as bentonites. Bentonites represent fine igneous debris that has been thoroughly ex¬ posed to the atmosphere. The present data on the relative thorium and uranium content of bentonites are recorded in Figure VI. Over 90 per cent of the thorium to uranium ratios of bentonites fall between 7 and 40, and are -17- appreciably higher than those found in primary igneous rocks.
In contrast to the continental sediments are the marine black shales. The thorium to uranium ratios of the black shales recorded in Figure V and Table IV range from less than 0.1 to 1.1 with a majority of the ratios being less than one. This large difference between the thorium to uranium ratios of continental deposits and those of marine shales deposited slowly in a locally reducing environment demonstrates the fractionation of uranium from thorium under an oxidizing atmosphere.
A very low ratio was also noted in two samples of the
Green River Shale, RP-104 and S-23. Although deposited in a large body of fresh water, these samples represented material very similar to that of the marine black shales.
The available data on thorium and uranium ratios in limestones are shown in Figure IV and Table IV. These data are meager, but the low ratios found in limestones of low clastic mineral content do correspond to the low value ex¬ pected in limestones deposited under the present atmospheric conditions. Because the clastic material may contain such stable uranium and thorium minerals as zircon and monazite, a high clastic content in a limestone may be expected to affect the thorium to uranium ratio of the rock. Analyses of 23 aggregate samples of Paleozoic limestones in another laboratory confirm the low thorium to uranium ratio in lime¬ stones (Dr. J. A. S. Adams, personal communication). -18-
The continental shales, and so forth, on one hand and the marine black shales and carbonates on the other repre¬ sent two extremes of depositional environments and are readily recognized and interpreted. The sediments which are deposited in any number of the transition environments betxveen these two extremes are much more difficult to in¬ terpret and recognize at present. Any interpretation of the thorium and uranium content of these sediments would require more complete knowledge of the mode and rate of de¬ position, the clastic mineral content, and the local source material before any significant conclusions may be reached.
Any of these factors could plausibly cause significant fluctuations in the thorium and uranium content of a sedi¬ ment. A study of these factors is beyond the scope of this research, although such an investigation could prove inter¬ esting and useful. The data obtained on the Paleozoic and younger marine shales, exclusive of black shales, illus¬ trate the great fluctuations in the relative thorium and uranium contents found in these transitional sediments.
These thorium to uranium ratios ranged between 1.1 and 11.2.
It is interesting to note that the majority of the ratios determined on twenty-three marine shales are well above the fundamental ratio of three to four.
The data gathered on the pre-Cambrian sediments are difficult to interpret because of the scarcity of appro¬ priate samples presently available. -19-
The slates designated by the prefix "N" in Table IV are Huron!an in age and are from the southern portion of the Canadian Shield. They are believed to represent rapidly accumulated deltaic material derived from the graywacke and volcanic deposits which are so characteristic of that area (Nanz, 1950). Most of these samples show a thorium to uranium ratio between two and three. This possibly repre¬ sents the ratio in the source material of these sediments which has probably not been subjected to extensive weather¬ ing before deposition (Nans, 1953). Two notable exceptions in this sequence are samples N-2b and N-3b. These two sam¬ ples exhibited high carbon contents comparable to those found in more recent marine black shales. They also yielded high uranium contents and low thorium to uranium ratios of
1.5 and 0.2 which are similar to those found in the more recent black shales. According to the working hypothesis, this indicates that the sequence of sediments represented by these Huronian samples were deposited under an atmosphere similar to that of the Paleozoic and later.
Two other black pre-Cambrian shales were analyzed,
RP-105 and RP-108. These samples had a thorium to uranium ratio of 5.5 and 2.2, respectively. These ratios vary slightly from the fundamental ratio but not enough to draw any decisive conclusion.
Only three pre-Cambrian limestones were available for examination. One of these, S-38, which contained abundant -20- clastic material, shoxved a thorium to uranium ratio of 4.5.
This sample is very late pre-Cambrian in age and its high ratio may be explained by its high clastic content. The other two samples were ideal for this problem in being rela¬ tively free from elastics and in having been accurately dated. The limestone RP-97 was found to have a ratio of
0.1. This value is similar to those of the younger lime¬ stones investigated, The strata including this Central
Texas limestone is believed to be equivalent to the Huronian of the Canadian Shield, and has been dated at about 900 million years (Hutchinson, et al, 1954). According to the previous estimates of tQ , this limestone was deposited X after the accumulation of free oxygen in the atmosphere, and, therefore, it could be expected to have a ratio similar to those of the younger limestones. The third pre-Cambrian limestone, RP-106, which is from a dated sedimentary sequence of Southern Rhodesia, does not fall into the expected pattern. This limestone’s age has been estimated as being over 2,7 billion years (Ahrens,
1955) and, consequently, it would be expected to reflect deposition under the reducing atmosphere. The thorium to uranium ratio of RP-106, however, was found to be 0.3J which is similar to those ratios found in limestones deposited under an oxidising atmosphere. This deviation from the ex¬ pected fundamental thorium to uranium ratio can be explained in one or more ways: the thorium to uranium method for some 21- presently unknown reason is not applicable to chemical precipitates,* there has been some post-depositional change in the uranium content of the limestone; or this sample is not representative of the carbonates deposited at that time.
An examination of a number of other appropriate limestones is essential before any conclusions on the significance of this sample can be reached. Because of the difficult question of their origin and manner of deposition, only limited significance can be presently applied to the thorium to uranium ratios found in the analyzed pre-Cambrian shales, phyllltes, and schists. The ratios in samples of these rocks ranged from two to five. It is interesting to note that the range of these ratios is much smaller than the range, 1.1 to 11.2, found in the younger marine shales. -22-
CONCLUSIONS
The principal conclusions and results of this research
are: 1) It has been established that the thorium to uranium ratio of a sample may be determined on the basis of its alpha activity and uranium content. 2) New data corroborate and extend the literature values showing the marked fractionation of uranium from thorium under present atmospheric conditions. This frac¬ tionation is reflected by the low uranium content and high thorium to uranium ratio of continental sediments on the one hand, and by the high ■uranium content or low thorium to uranium ratio of marine black shales and carbonates on the other.
3) The data indicate that it would be fruitful to study in detail the effect of depositional environments on the thorium and uranium contents of transitional marine sediments. 4) According to the working hypothesis, the accumulation of free oxygen in the atmosphere occurred in pre-Huronian time.
5) A much more detailed sampling of pre-Cambrian limestones and other sediments is required to establish the exact applicability of the working hypothesis. -23-
p OJ JZfr o\ CO as b~ oo Ch O o \ * ♦ * • o o oo CO •=t oo o OO d in CM rH CM ■S
S A A D— in p • ♦ rH rH x CO o\ 00 CTN VO ON vo O OO rH CM P vo in vo -=t oo vo o rH 00 A rH H rH o 3 42 A o in 00 oo in CO OO O CO in O OO b- oo Ov o ON GO 00 -3" rH b- V in OJ H 00 o b- rH bO rH t> cd < P cd d « o S r* a A rH vo vo ON CM P O A « • ♦ • 1 i o * o •H vo in CM rH H CM 1 > rH o £ ^r %— P <+• vo CM w' H cd p >> H cd H P Pt A rH cd a cd s o a> g § 43 S i i -=t o 1 1 I i o cd EH A o e» i i rH rH oo r 1 * i H bO 0 ^ A !>> i>v S 1 A §i *0 <3> s r*JM i A b~ 00 oo 00 vo oo CM CQ A • • • • • • £ in OO rH CM CM vo b~ Vo CO oo 0 _ w P 0 rH H s 00 o •=t \£t • • » • • • « P g IS CO s vo cvi vo H H rH 0 p S in cii cvi cu OJ cvi • i i i i p a in tn iiii E*1 ri 00 CVI cvi cvi cvi a* cvi p s « * t t i i o ro i i i i s o A 0 H AhQ U p ctl CT\ vo co t>- o rH O P \SH in vo vo in o in av 0 XI RP-100 0.29; 0.22; 7.5 25.8 3.8 2.0 2.6 (arenaceous) 0.36 Avg - 0.29 rH «P •H Q 1 2 2 • • •p M CQ cd 2 i>a CVJ CVJ VO «=t m oo 00 O CVJ on H rH in in o I » 1 1 i • • Uppm cxT cph/Uppm Th/U Th/U Th/U Sample (fluorimetric) <>rcph (average) (maximum) (minimum) (average) «P a CQ & c 0) 3 O $ CO •=fr V0 -=t • o CO rH o H OJ o > o rH X PH • ntA **v &Q 1 • * • * VO < < W l 04 04 Cl! CO o CO CO CO rH OJ CO m OJ CO t i I 1 ! $ 1 ! 1 t • VO o -=t fl CO o in rH OJ Os OJw CO 1 * » & ctf to o u 0) 0 U $H £2 •d rH s cd 02 to 0) O •H co 1 a> £ -=t vo rH H in OJ H 1 2 • * CO rH OJ ov in as ITS OJ O rH rH CO 2 1 • * 04 OJ 04 «=t o OJ 04 CO o rH co rH lilt II!! ! 1 1 2 1 * 00 CO vo VD O G\ H rH in H CO 2 i » » -=t -=fr -3* rH CO OJ rH ON rH CO 2 1 * 04 -=t VO .=* 00 in CO as O rH 04 o co I 1 2 I * • 04 OJ -=f -=t 00 CO in tn CO CO rH 04 in co 1 1 1 1 i • • Uppm <*cph/Uppm Th/U Th/U Th/U Sample (fluorimetric) <*cph (average) (maximum) (minimum) (average) cy cucn CM Oi ■g; CM inV0 t- HroinCVJvo is* CM CO OJ00Oi'h s- H CM00 CO I t i J t I t i t co co co Ch H CM CO 1 oo CO 4OJ O b-VO © in co 1 CM CM RP-108 1.8; 1.6; 1.9J 42; 47 23.6 3 1.5 2.2 2.3 Avg - 45 Avg - 1.9 •P Q) (D CO £ m O ■=* *0-=*. VO * OJ ojoi H G\ rH o O OJ < in bo rH 0% O < Is o rH H ro60 irun CM **> **■» »n) 1 * # * ♦ rH . •> * o\ I oo m VO vp CO(M on V OJ o o on rH o • o’ OJGO OrH rH o «$ OHrl hO oooo rH 0) _ w p ctf CO CT\ .=* o CO -=fr vo vo » • • « • • « • p s o CV3 00 OJ m 00 N\*rH £ C, OJ OJ OJ OJ 03 01 6*t Os OJ 03 OJ o o 00 «=t oo VO •=t oo «=t OJ 00 fig S e^ P*Q> CO OO 00 CO t- 03 VO VO 03 VO OJ 03 OO Ol OJ >sp<> O ctf >fw o in oo « oo OJ in a •^00 CD G\ ft in 0 CO o OO 1 1 1 'if ♦n 50 O bo oT bO in > CO OO in > 03 O > rH CO cn< VO rH 00 rH E>- vo •n * *\ **\ «n o -=t o\ VO 03 CO b- rH ♦ • f * * * • u rH rH rH rH 03 OJ OO p s g> •n OJ **svo *«\ in ♦n(\! •n O • *\ N~ *•> VO P< s rH 00 • in OO * 00 tn « AH • rH • rH • rH • rH • 00 • Ol • 03 P P rH rH rH rH 00 03 OJ O 1 1 l 1 1 1 1 3 ♦ ** • n **> H O H bO j>- bD Os OJ is- 50 rH 50 COCO 50 00 M3 60 HCO 50 * * > • > * N • * *J> * £ • • J> • • > rH fH 10 5 9 8 4 7 6 - 3 J 5 - Th/U Th/U 4 - Z~ l • • o 3 _ • • ? - 0.9 .8 J .6 I .5 FIGURE I Igneous Rocks FIGURE IV Chem. Precipitates 438 Analyses 6 Analyses 1919 Samples 53 Samples • Average of 3 or more • Average of one or more analyses analyses ^ Over-all average (Evans & Goodman, 1941; (Breger, 1955; Osmond, 1954; Keevil, 1938; Keevil, 1944; Evans & Goodman, 1941.) Senftle & Keevil, 1947; Rankama & Sahama, 1950.) Th and U Concentrations in Sea Water Versus Geologic Time FIGURE II Geologic Time in Years • CD oc cph U, 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thj/U, FIGURE III - Calibration Curve -36- 100 O 80 60 40J • o 20- • • • • Th/U Th/U 10- 8- 6- 4- 3- 2- I FIGURE V Shales FIGURE VI Bentonites • 12 Analyses 15 Analyses 144 Samples 15 Samples • Black shales • One analysis ° 'Oxidized* shales • O Over-all average (Breger, 1955; Osmond, 1954; (Osmond, 1954.) Koczy, 1949; Rankama & Sahama, 1950.) -37- BIBLIOGRAPHY Adams* J.A.S. (1954), Uranium and thorium contents of volcanic rocks: in Nuclear Geology* H. Faul* edit.* John Wiley and Sons* Inc.* New York. Adams* J.A.S.* and Maeck* W.J. (195*0> Fluorimetric and colorimetric microdetermination of uranium in rocks and minerals: Analytical Chemistry* Vol. 26* p. 1635. Ahrens, L.H. (1955)* Oldest rocks exposed: Geological Society of America* Special Paper 62* pp. 155-168. Breger* I.A. (1955)* Radioactive equilibrium in ancient marine sediments: Geochimica et Cosmochimica Acta* Vol. 8* pp. 63-73. Evans, R.D.* and Goodman* Clark (1941), Radioactivity of rocks: Geological Society of America Bulletin* Vol. 52, PP. 459-490. Hutchinson* R.M.* Jaffe* H.W.* and Gottfried, D. (1954), Magmatic trends and absolute age determinations of pre- Cambrian intrusions of central Texas: Geological Society of America Bulletin* Vol. 65* p. 1266. Keevil* N.B. (1938)* Thorium-uranium ratios of rocks and their relation to lead ore genesis: Economic Geology, Vol. 33* PP. 685-696. (1944)* Thorium-uranium ratios in rocks and minerals: American Journal of Science* Vol. 242* pp. 309-321. Koczy, F.F. (1949)* The thorium content of Cambrian alum shales of Sweden: Sveriges Geol. Undersok.* Ser. C* Avhandl. och Uppoat. no. 509* $rsbok. 43* 12 pp. (1954)* Geochemical balance in the hydrosphere: in ffuclear Geology* H. Faul* edit.* John Wiley and Sons* Inc.* New York. (1956)* Geochemistry of the radioactive elements in the oceans: Deep Sea Research* Vol. 3* p. 93. Kuiper* G.P, (ed.) (1954), The Earth as a Planet: Univer¬ sity of Chicago Press, Chicago Illinois. Latimer* W.M. (1952)* Oxidation Potentials: Prentice-Hall* Inc.* New York. 38- Mayne, K.I. (1958), Terrestrial helium: Geochimica et Cosmochimiea Acta, Vol. 4, pp. 74-Sl. MoKeIvey, V.E., Everhart, D.L,, and Garrels, R.M. (1955), Origin of uranium deposits: in Economic Geology, Fiftieth Anniversary Volume, 1905-1955, A. M, Bateman, edit. (1958), Summary of hypothesis of genesis of uranium cTeposits: in Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Vol. 6, Geology of uranium and thorium, United Nations Publication, New York. Nanz, R.H., Jr. (1950), Composition and abundance of fine¬ grained pre-Cambrian of the Southern Canadian Shield: Ph.D, Thesis, University of Chicago. .,(1953)* Chemical composition of pre-Cambrian slates: Journal of Geology, Vol. 61, pp. 51-64. Osmond, J.K. (1954), Radioactivity of bentonites: Master's Thesis, Uhiversity of Wisconsin. Rankama, Kalervo (1948), New evidence of the origin of pre- Cambrian carbon: Geological Society of America Bulletin, Vol. 59, pp. 389-416. (195*1-) , A calculation of the amount of weathered igneous rock: Geochimica et Cosmochimiea Acta, Vol. 5, pp. 81-84. (1955)* Geologic evidence of the chemical composition of the pre-Cambrian atmosphere: Geological Society of America, Special Paper 62, pp. 651-664. and Bahama, Th.G. (1950), Geochemistry: Uhiversity of Chicago Press, Chicago, Illinois, 912 pp. Reiche, Parry (1949)# Geology of Manzanita and North Manzano Mountains, New Mexico: Geological Society of America Bulletin, Vol. 60, pp. 1183-1212. Seaborg, G.T., and Katz, J.J. (1954), The Actinide Elements: McGraw-Hill Book Company, Inc., New York. Senftle, F.F., and Keevil, N.B. (1947)# Thorium-uranium ratios in the theory of lead ores: Transactions of the American Geophysical Union, Vol. 28, p. 732. -39- Sharpe, J. (1955), Nuclear Radiation Detectors: John Wiley and Sons, Inc., New York. Stark, J.T., and Dapples, E.C. (1946), Geology of the Los Pinos Mountains, New Mexico: Geological Society of America Bulletin, Vol. 57, pp. 1121-1172. Urey, H. (1952), The Planets: Yale University Press, New Haven, Connecticut. , and Suess, Hans E. (1956), Abundances of the elements Reviews of Modern Physics, Vol. 28, p. 53. Wilkinson, D.H. (1950), Ionization Chambers and Counters; Cambridge University Press, London, England. Wilson, E.D. (1939), Pre-Cambrian Mazatzal revolution in central Arizona: Geological Society of America Bulletin Vol. 50, pp. 1113-1164. APPENDIX A Index to Samples RP-2 Ferruginous phyllite from the Alder Creek Series of the central Mazatzal Mountain area of Arizona; road cut on the north side of the Bush Highway approxi¬ mately one half mile west of the Ord Mine, Slate Creek, Arizona. Archean (?). RP-13 Ferruginous phyllite from same formation as RP-2, but from road cut on the north side of the Bush Highway approximately one mile west of the Ord Mine, Slate Creek, Arizona. Archean (?). RP-22 Ferruginous phyllite from same formation and local¬ ity as RP-13 but from the south side of the Bush Highway 50 yards further west. Archean (?). RP-38 Chloritic quartz schist from the Maverick Shale formation of the southern Mazatzal Mountain region of Arizona; west slope of Four Peaks Mountain, Arizona. Archean. RP-45 Ferruginous phyllite from the Lower Metaclastic Series of the Manzanita Mountains region of New Mexico; sample from the north wall of Hell Canyon, Isleta Indian Reservation, New Mexico. Lower Proterozoic (?). RP-48 Ferruginous phyllite from same formation and local¬ ity as RP-45. Lower Proterozoic (?). RP-6G Phyllite from the Lower Metaclastic Series of the Manzano Mountains region of New Mexico; T7N R5E Section 35* one and one half miles southwest of Fourth of July Spring, Tajique, New Mexico. Upper Proterozoic (?). RP-76 Phyllite from the Upper Metaclastic Series of the Manzano Mountains region of .New Mexico, T6N R5E near Tajique, New Mexico. Upper Proterozoic (?). RP-79 Chloritic muscovite schist from the Blue Springs Schist formation; railroad cut on A. T. & S. F. railroad two miles northeast of Blue Springs, New Mexico. Archean (?). RF-80 Chloritic muscovite schist from the same formation and locality as sample RP-79. Archean (?). RP-97 White crystalline limestone member of the Pack- saddle schist; road cut 15.8 miles south of Llano, Texas on Ranch Road 93. Pre-Cambrian (approxi¬ mately 900,000,000 years old). RP-100 Glauconitic limestone from the Morgan Creek forma¬ tion at the Honey Creek-Llano River locality, Llano, Texas. Upper Cambrian. RP-101 Green fossiliferous shale from the Point Peak formation at the Honey Creek-Llano River locality, Llano, Texas. Upper Cambrian. RP-102 Carbonaceous shale from the Smithwich Shale forma¬ tion at the Honey Creek-Llano River locality, Llano, Texas. Pennsylvanian. RP-103 Dark grey shale from the Smithwich Shale formation at the Mormon Mills locality near Marble PallB, Texas. Pennsylvanian. RP-104 Oil shale from the Green River formation; Rifle Oil Shale Mine, Rifle, Colorado. Eocene. RP-105 Carbonaceous shale sheared from horizon between lava flows in Keewatin formation; Hollinger Mine, 92 vein, 425-foot level, Timmins, Ontario. Keewa¬ tin. (Sample furnished by E. S, Barghoorn, De¬ partment of Biology, Harvard University). RP-106 Graphitic limestone from the Shamvaian Series; Huntsman Quarry, Inyati, 33 miles northeast of Bulawayo, Southern Rhodesia. Early Pre-Cambrian. (Sample supplied by A. M. MacGregor, Salisbury, Rhodesia). RP-108 Carbonaceous argillite member of the Paclcsaddle Schist; road cut fifteen miles south Llano, Texas on Ranch Road 93. Pre-Cambrian. RP-113 High grade bauxite from Surinam. (Sample supplied by H. S. McQueen, Salt Dome Production Company, Houston, Texas). RP-114 Arkansas pisolitic bauxite, (Sample supplied by H. S. McQueen). (Samples designated by the letter "N" were furnished by Dr. R. H. Nanz, Jr., of the Shell Development Company, Houston, Texas. Some of these samples are discussed by Dr. Nanz in a paper published in the JOURNAL OP GEOLOGY, Vol. 61, pp. 51-64, 1953.) N-2a Michigamme slate from the Footwall strata of the Marquette district, Michigan; road cut one quarter mile west of U. S. 2 south of Crystal Falls, Michigan. Upper Huronian. N-2b Michigamme slate (graphitic phase) from the same locality as N-2a. Upper Huronian. N-2c Michigamme slate (sideritic phase) from the same locality as N-2a. Upper Huronian. N-3b Michigamme slate (spangled slates) from the Foot- wall strata of the Marquette district, Michigan; Judson Mine dump, S. E. one quarter of N. W. one quarter, Section 13, T42N, R33W, Michigan. Upper Huronian. N-5 Michigamme slate (magnetic phase) from the Hanging Wall strata of the Marquette district, Michigan; Monongahela locality, Michigan. N-6 Michigamme slate from the Hanging Wall strata of the Crystal Falls area, railroad cut of the Mil¬ waukee Line on the north edge of Crystal Falls, Michigan. Upper Huronian. N-T Michigamme slate from the Hanging Wall strata of the Crystal Falls area; 200 feet southeast of the Crystal Falls Mine, Crystal Falls, Michigan. Upper Huronian. N-8 Slate from the Paint Slate formation; road cut on U. S. l4l one half mile north of southern boundary of Baraga County, Michigan. Upper Huronian. N-lOa Slate from the paint Slate formation; road cut on U. S. l4l 1.6 miles north of southern boundary of Baraga County, Michigan. Upper Huronian. N-ll Slate from the Paint Slate formation; road cut on U. S. l4l 4.4 miles north of southern boundary of Baraga County, Michigan. Upper Huronian. N-12 Slate from the Paint Slate formation; road out on U. S. l4l 8.3 miles north of southern boundary of Baraga County, Michigan. Upper Huronian. N-13 Quartz slate from the Afibik formation; from pit 1800 feet east of the northeastern edge of Teal Lake near Negaunee, Michigan. Middle Huronian. N-15 Slate from Misnard quartzite formation; road cut on U. S. l4l five miles west of Marquette, Michigan. Lower Huronian. N-l6 Slate from Slamo slate formation; from west dump of Northlake Mine three miles northwest of Ishpeming, Michigan. Middle Huronian. N-20 Quartz slate from the Palms formation; one mile southeast of Upson, Wisconsin on the Potatoe River Middle Huronian. N~23a Slate from the Tyler slate formation; railroad cut of the Soo Line one mile north of Montreal, Wisconsin. Upper Huronian. N-25 Shale from the Nonesuch shale formation; exact locality unknown. Keweenawan. (Samples designated by the letter "S" were supplied by the Shell Development Company, Houston, Texas.) S-l Chattanooga black shale; marine; Highland Rim, Tennessee. Late Devonian. S-2 Black shale; marine; southeastern Kansas. Pennsylvanian. S-3 Green sandy shale; near shore environment; south¬ eastern Kansas. Pennsylvanian. S-4 Red shale; continental; southeastern Kansas. Pennsylvanian S-5 Oak Hall potassium bentonite; Pennsylvania. Ordovician. S-6 Red shale; continental; Venezuela. Post-Eocene. S-7 Greenish grey shale; continental; Venezuela. Post Eocene, S-8 Red shale; continental; Venezuela, Post-Eocene. S-9 Yellow shale; continental; Venezuela. Post-Eocene S-10 Grey shale; marine; Calgary, Alberta. Upper Jurassic. S-ll Grey shale; marine; Calgary, Alberta. Upper Jurassic. S-12 Yellow silty shale; continental; Calgary, Alberta. Lower Cretaceous, S-13 Yellow silty shale; continental; Calgary, Alberta. Lower Cretaceous. S-l4 Yellow silty shale; continental; Calgary, Alberta. Lower Cretaceous. S-15 Morrison shale; marine; Calgary, Alberta. Jurassic. S-l 6 Dark grey shale; estuary environment; Calgary, Alberta. Cretaceous. S-17 Shale; continental; Calgary, Alberta. Cretaceous. S-18 Morrow black shale; deltaic; Oklahoma. Pennsylvanian. S-19 Shale; continental; Colorado. Cretaceous. S-20 Shale; marine; Colorado. Cretaceous. S-21 Black shale; lagoonal; Colorado. Cretaceous. S-22 Black shale; lagoonal; Colorado. Cretaceous. S-23 Green River shale; lacustrine; Salt Lake, Utah. Eocene. S-24 Green River shale; lacustrine; Salt Lake, Utah. Eocene. S-25 Light grey shale; marine; Ventura, California. Miocene. S-2 6 Light grey shale; continental; Ventura, California. Miocene S-27 Light grey shale; marine; Ventura, California. Miocene. S-28 Dark green shale; marine; California. Eocene. S-29 Grey shale; marine; California. Eocene. S-30 Dark grey shale; marine; California. Eocene. S-31 Dark grey shale; marine; California. Eocene. S-32 Dark grey shale; marine; California. Eocene. S-33 Dolomitic limestone; Saskatchewan. Mississipplan. S-34 Dolomite; Saskatchewan. Mississipplan. S-35 Dolomite; Saskatchewan. Mississipplan. S-36 Dolomitic limestone; Saskatchewan. Mississipplan. S-37 Phyllite; North Carolina. Upper Ocoee, Late Pre-Cambrian. S-38 Dark grey limestone; North Carolina. Upper Ocoee, Late Pre-Cambrian. The following samples were obtained from the National Bureau of Standards, Washington, D. C.; A Uranium Ore Standard; 0.001$ uranium in a mixture of 0.0022$ pitchblende and 99.978$ dunite. B Thorium Ore Standard; 0.001$ thorium and 0.00004$ uranium in a mixture of 0.012$ monazite and 99.988$ dunite.