H. Craig

Isotopic Variations in Meteoric Waters

The relationship between deuterium and oxygen-18 concentrations in natural meteoric waters from many parts of the world has been determined with a mass spectrometer. The isotopic enrichments, relative to ocean water, display a linear correlation over the entire range for waters which have not undergone excessive evaporation.

Isotopic standards for carbon and oxygen and correction factors for mass-spectrometric analysis of carbon dioxide

Abstract Seventy-four mass spectrometric analyses have been made on eight isotopic standards for carbon and oxygen. The standards reported on are the National Bureau of Standards limestone, water, and graphite reference samples, the Solenhofen standard used by Niee for absolute measurements, and the carbonate standards used by the Stockholm, Basel, Wellington and Chicago laboratories. The basic measurements are reported as δ values relative to the Chicago PDB standard and as absolute isotope ratios. The absolute ratios are based on new figures for Niers standard, derived from a reevaluation of his data and new measurements on atmospheric oxygen. The correction factors for instrumental effects and for the nature of the mass spectra have been studied. Simple but precise equations for the mass spectra effects have been derived, which give the correction factors as functions of the measured differences between sample and standard. The oxygen isotopic composition of carbon dioxide produced by combustion of carbon has been studied and the derived correction factors for carbon measurements were checked experimentally by varying the oxygen-18 content of carbon dioxide.

THE GEOCHEMISTRY OF THE STABLE CARBON ISOTOPES

Abstract Several hundred samples of carbon from various geologic sources have been analyzed in a new survey of the variation of the ratio C13/C12 in nature. Mass spectrometric determinations were made on the instruments developed by H. C. Urey and his co-workers utilizing two complete feed systems with magnetic switching to determine small differences in isotope ratios between samples and a standard gas. With this procedure variations of the ratio C13/C12 can be determined with an accuracy of ±0.01% of the ratio. The results confirm previous work with a few exceptions. The range of variation in the ratio is 4.5%. Terrestrial organic carbon and carbonate rocks constitute two well defined groups, the carbonates being richer in C13 by some 2%. Marine organic carbon lies in a range intermediate between these groups. Atmospheric CO2 is richer in C13 than was formerly believed. Fossil wood, coal and limestones show no correlation of C13/C12 ratio with age. If petroleum is of marine organic origin a considerable change in isotopic composition has probably occurred. Such a change seems to have occurred in carbon from black shales and carbonaceous schists. Samples of graphites, diamonds, igneous rocks and gases from Yellowstone Park have been analyzed. The origin of graphite cannot be determined from C13/C12 ratios. The terrestrial distribution of carbon isotopes between igneous rocks and sediments is discussed with reference to the available meteoritic determinations. Isotopic fractionation between iron carbide and graphite in meteorites may indicate the mechanism by which early fractionation between deep seated and surface terrestrial carbon may have occurred.

Standard for Reporting Concentrations of Deuterium and Oxygen-18 in Natural Waters

A standard, based on the set of ocean water samples used by Epstein and Mayeda to obtain a reference standard for oxygen-18 data, but defined relative to the National Bureau of Standards isotopic reference water sample, is proposed for reporting both deuterium and oxygen-18 variations in natural waters relative to the same water. The range of absolute concentrations of both isotopes in meteoric-waters is discussed.

Primordial neon, helium, and hydrogen in oceanic basalts

A primordial neon component in neon from Kilauea Volcano and deep-sea tholeiite glass has been identified by the presence of excess20Ne; relative to atmospheric neon the20Ne enrichments are 5.4% in Kilauea neon and about 2.5% in the basalts. The20Ne anomalies are associated with high3He/4He ratios; the ratio in Kilauea helium is 15 times the atmospheric ratio, while mid-ocean ridge basalts from the Atlantic, Pacific, and Red Sea have uniform ratios about 10 times atmospheric. Mantle neon and helium are quite different in isotopic composition from crustal gases, which are highly enriched in radiogenic21Ne and4He. The21Ne/4He ratios in crustal gases are consistent with calculated values based on G. Wetherills18O (α,n) reaction; the lack of20Ne enrichment in these gases shows that the mantle20Ne anomalies are not radiogenic.21Ne enrichments in Kilauea neon and “high-3He” Pacific tholeiites are much less than in crustal neon, about 2 ± 2% vs. present atmospheric neon, as expected from the much lower4He/Ne ratios. Neon concentrations in two Atlantic tholeiites were found to be only 1–2% of the values obtained by Dymond and Hogan; helium concentrations are slightly greater and our He/Ne ratios are greater by a factor of 150. The large Ne excess relative to solar wind and meteoritic gases is thus not confirmed. Pacific and Atlantic basalts appear to be quite different in He/Ne ratios however, and He and Ne may be inversely correlated. He concentration variations due to diffusive loss can be distinguished from variations due to two-phase partitioning or mantle heterogeneity by the effects on3He/4He ratios. The He isotopic and concentration measurements on “low-3He” basalts are consistent with diffusive loss and dilution of the 3/4 ratio by in-situ radiogenic4He, and may provide a method for dating basalt glasses. Deuterium/hydrogen ratios in Atlantic and Pacific tholeiite glasses are 77% lower than the ratio in seawater. The inverse correlation between deuterium and water content observed by Friedman in erupting Kilauea basalts is consistent with a Rayleigh separation process in which magmatic water is separated from an initial melt with the same D/H ratio as observed in deep-sea tholeiites. The consistency of the D/H ratios in tholeiites containing primordial He and Ne components indicates that these ratios are probably characteristic of primordial or juvenile hydrogen in the mantle.

Atmospheric oxygen: isotopic composition and solubility fractionation.

Atmospheric oxygen has been found to be enriched in oxygen-18 by 23.5 � 0.3 per mil relative to average ocean water (SMOW). Oxygen dissolved in seawater is further enriched in oxygen-18 by 0.85 per mil at 0�C. The temperature dependence of the solubility enrichment is given by ε (per mil) = 0.85— 0.010 t (�C). This result is in good agreement with earlier measurements of the solubility effect in distilled water.

The composition of the stone meteorites and the origin of the meteorites

Abstract A study has been made of some 350 chemical analyses of stone meteorites available in the literature, and criteria have been applied in an attempt to select those which are more reliable. It is found that the chondrites fall into two distinct groups with regard to both the total iron content and the oxidation state of the iron. These groups cannot be related by any simple mixing process and thus indicate that the meteorites have been derived from two rather different parent objects rather than from a single body. It appears that preferential volatilization of silicates and retention of nickel-iron operating to a different extent during a high temperature stage in the evolution of the asteroids may explain the different compositions of the two groups of chondrites. The implications of these data with respect to the cosmic abundances of the elements are discussed, and it is concluded that Goldschmidt s abundances for iron and nickel are too high. It is postulated that the parent asteroids went through a low temperature accumulation process, a high temperature stage of partial melting and evaporation, a stage of collisions with smaller objects which produced brecciated and tuffitic agglomerations of material and chondrules; and finally a collision of two such objects compacted the material into the meteorites, preserving however the essential differences in composition between the two groups.

Excess 3He in the sea: Evidence for terrestrial primodal helium

We have found a large enrichment of 3He in deep Pacific water, relative to the atmospheric 3He/4He ratio. A sample from 1737 m shows 21.8 ± 2.1% excess 3He, and deeper samples range from 5.8 ± 1.9% to 12.2 ± 2.0%, whereas a surface sample and one from 866 m show identical 3He/4He as compared to air within experimental error. We have also measured total helium contents and find excess amounts of −3 ± 3% to 8 ± 3% relative to solubility data. The results for total helium are in agreement with recent work of Bieri, Koide and Goldberg, and our previous measurement from Carrousel Expedition. There appears to be a rough correlation between 3He excess and 4He excess indicating that there are two components in varying proportions, one with an atmospheric 3He/4He ratio, and another with a considerably higher ratio. We conclude that the excess 3He is due to leakage into ocean water of a remnant of the earths primodal 3He — there appears to be no other possible mechanism. The effects we see could be accounted for by a leak rate of about 2 atoms cm−2 sec−1, a value of the order of the amount required to balance the difference between the production rate of 3He from various other sources and the escape rate by thermal and non-thermal processes.

Excess 3He in deep water on the East Pacific Rise

Abstract Helium isotope measurements show that water on the crest and flanks of the East Pacific Rise has the highest enrichment in 3He so far observed in the oceans; the 3He/4He ratio anomaly relative to atmospheric helium is + 32% at the mid-depth maximum in the profiles. The corresponding 3He solubility anomaly relative to saturation with atmospheric helium is +50%. These data indicate that active sea-floor spreading sites on the crests of the mid-ocean rises are the sources of primordial helium injected into the ocean from the earths interior. The 3He/4He ratio in this flux is approximately 1.6 × 10−5, about 11 times the atmospheric ratio of 1.4 × 10−6. The total flux of 3He into the atmosphere is 4.6 atoms cm−2 earth-surface sec−1, most of which (4.0 atoms cm−2 sec−1) is supplied by the oceanic flux. The corresponding atmospheric residence time for 3He is 106 years, which, within the large uncertainties of supply and demand (thermal escape), is consistent with the requirement for a steady state.

Binary mixing of enriched and undegassed (primitive?) mantle components (He, Sr, Nd, Pb) in Samoan lavas

We have measured He, Sr, Nd, and Pb isotope ratios and Rb, Sr, Sm, and Nd concentrations in stratigraphically controlled lavas from the Pago shield volcano on Tutuila, American Samoa. We interpret these lavas as products of mixing between two isotopically extreme mantle constituents. The first is a highly enriched component with very high Sr isotope ratios, low ^3He/^4He ratios, and high “Δ(7/4)” and “Δ(8/4)” Pb isotopic characteristics. This is probably recently recycled ( 24R_A) and intermediate Sr, Nd, and Pb isotopic ratios. This material was derived from a largely undegassed mantle source, and, in conjunction with data from several other ocean islands, provides strong evidence for the existence of a high ^3He/^4He ratio mantle end member (primitive helium mantle, PHEM) with consistent Sr, Nd, and Pb isotopic characteristics: near bulk-earth ^(87)Sr/^(86)Sr (0.7042–0.7052) and ^(143)Nd/^(144)Nd (0.51265–0.51280,e_(Nd) = +0.2to+3.2), and radiogenic Pb (^(206)Pb/^(204)Pb∼ 18.5–19.0, ^(207)Pb/^(204)Pb∼ 15.5–15.57, ^(208)Pb/^(204)Pb∼ 38.4–39.2). Although this material cannot have been derived from a reservoir completely closed to elemental fractionation for the full 4.55 Ga duration of Earths history, it may indicate the presence of a highly primitive mantle source. Erratic temporal variations in the isotopic composition of individual flows indicate sporadic and variable mixing of these two sources. We interpret these results using a model in which high ^3He/^4He ratio plume material, rising intermittently from a lower-mantle source, intercepts and melts recycled crustal matter in the upper mantle or lithosphere and erupts as a binary mixture of PHEM and the so-called “EM” components derived from this recycled material.