Bruce R. Doe

Petrologic evolution of the San Juan volcanic field, southwestern Colorado: Pb and Sr isotope evidence

Abstract Two distinct suites of igneous rocks occur within the San Juan volcanic field: an Oligocene suite of predominantly intermediate-composition lavas and breccias, with associated silicic differentiates erupted mainly as ash-flow tuffs, and Neocene-Pliocene bimodal suite of silicic rhyolites and mafic alkalic lavas. The Oligocene volcanism, probably related to subduction along the western margin of the American plate, has chemical and isotopic characteristics indicative of complex interactions with Precambrian cratonic lithosphere. It also appears to record the rise, differentiation, and crystallization of a large composite batholith beneath the San Juan field. The earliest intermediate-composition lavas and breccias have major- and minor-element compositional patterns indicative of high-pressure fractionation and are relatively nonradiogenic in both Pb and Sr, suggesting significant interaction with lower crust of the American plate. The more silicic ash-flow tuffs show compositional evidence of low-pressure fractional crystallization and are more radiogenic in Pb and Sr — features thought to indicate significant shallow residency for the magmas and interaction with upper crust. Especially radiogenic Pb-isotope compositions of some of these rocks may reflect interactions between the magmas and convecting meteoric water rich in leached Pb, a process thought to have been even more important in forming associated hydrothermal ore deposits. Ore leads tend to be more radiogenic than associated rock leads. Many of the Miocene-Pliocene basaltic lavas seem to be mantle-derived lavas, similar to those of oceanic islands, but some anomalous xenocrystic basaltic andesites, containing relatively nonradiogenic lead, may have been slightly contaminated by lower crustal components. Rhyolitic lavas and intrusions of the bimodal suite are also nonradiogenic in Pb and Sr, in comparison with the Oligocene rhyolites, and do not appear to have interacted with Precambrian upper crust, probably because they erupted largely through the subvolcanic batholith. The Miocene-Pliocene rhyolites are best interpreted as partial melts of lower crust, with the thermal energy to initiate magma generation provided by concurrent basaltic volcanism.

EVOLUTION OF THE ISOTOPIC COMPOSITION OF URANIUM AND THORIUM IN SOIL PROFILES.

Mass-spectrometric and alpha-spectrometric analyses for the isotopic composition of U 238 , U 235 , U 234 , Th 232 , and Th 230 have been made on several soils and soil profiles in glacial-derived parent material, volcanic rock, and shale. Three deep profiles from Minnesota represent typical well-developed soils on till and loess of Wisconsin age continuously sampled to depths of 6, 7, and 10 feet, respectively. Results on individual soil horizons, in these three profiles, show similar trends in the variation of U 234 /U 238 and Th 230 /U 238 ratios, the predominant features being excess Th 230 and deficient U 234 compared to U 238 . The other soils and soil profiles have different variations which can be correlated by use of a model to describe uranium migration. The data collected provided information for interpreting the causes of uranium migration in soils and for constructing a tentative model to explain the isotopic evolution of uranium and thorium in soil profiles. The proposed model indicates that (1) uranium was leached at depth in the profile, (2) preferential leaching of U 234 was continuous in the soil, and (3) upward capillary migration of a fraction of the uranium with above-normal U 234 /U 238 ratio tended to make uranium of high U 234 ratio available for isotopic exchange in upper soil horizons and for assimilation in organic complexes in surface soils which are rich in organic matter. Thus some of the organic-rich surface soils, which have had considerable time to develop, contain uranium with excess U 234 compared to U 238 . Interpretations from this model suggest that remixing of radioisotopes by geochemical processes contributes significantly, along with the usual physical processes of radioactive growth and decay, toward producing an isotopic composition near radioactive equilibrium.

The potential source of lead in the Permian Kupferschiefer bed of Europe and some selected Paleozoic mineral deposits in the Federal Republic of Germany

New lead isotopic compositions have been measured for Paleozoic bedded and vein ore deposits of Europe by the high precision thermal emission (triple filament) technique. Eleven samples have been analyzed from the Upper Permian Kupferschiefer bed with representatives from Poland to England, three samples from the Middle Devonian Rammelsberg deposit and one from the Middle Devonian Meggen deposit, both of which are conformable ore lenses and are in the Federal Republic of Germany (FRG); and also two vein deposits from the FRG were analyzed, from Ramsbeck in Devonian host rocks and from Grund in Carboniferous host rocks. For Kupferschiefer bed samples from Germany, the mineralization is of variable lead isotopic composition and appears to have been derived about 250 m.y. ago from 1700 m.y. old sources, or detritus of this age, in Paleozoic sedimentary rocks. Samples from England, Holland, and Poland have different isotopic characteristics from the German samples, indicative of significantly different source material (perhaps older). The isotopic variability of the samples from the Kupferschiefer bed in Germany probably favors the lead containing waters coming from shoreward (where poor mixing is to be expected) rather than basinward (where better mixing is likely) directions. The data thus support the interpretation of the metal source already given by Wedepohl in 1964. Data on samples from Rammelsberg and Meggen tend to be slightly less radiogenic than for the Kupferschiefer, about the amount expected if the leads were all derived from the same source material but 100 to 150 m.y. apart in time. The vein galena from Ramsbeck is similar to that from Rammelsberg conformable ore lenses, both in rocks of Devonian age; vein galena from Grund in Upper Carboniferous country rocks is similar to some bedded Kupferschiefer mineralization in Permian rocks, as if the lead composition was formed at about the same time and from similar source material as the bedded deposits. Although heat has played a more significant role in the formation of some of these deposits (veins and Rammelsberg-Meggen) than in others (Kupferschiefer), there is no indication of radically different sources for the lead, all apparently coming from sedimentary source material containing Precambrian detritus. One feldspar lead sample from the Brocken-Oker Granite is not the same in isotopic composition as any of the ores analyzed.

Primitive and contaminated basalts from the Southern Rocky Mountains, U.S.A.

Basalts in the Southern Rocky Mountains province have been analyzed to determine if any of them are primitive. Alkali plagioclase xenocrysts armored with calcic plagioclase seem to be the best petrographic indicator of contamination. The next best indicator of contamination is quartz xenocrysts armored with clinopyroxene. On the rocks and the region studied, K2O apparently is the only major element with promise of separating primitive basalt from contaminated basalt inasmuch as it constitutes more than 1 % in all the obviously contaminated basalts. K2O: lead (> 4 ppm) and thorium (> 2 ppm) contents and Rb/Sr (> 0.035) are the most indicative of the trace elements studied. Using these criteria, three basalt samples are primitive (although one contains 1.7% K2O) and are similar in traceelement contents to Hawaiian and Eastern Honshu, Japan, primitive basalts.Contamination causes lead isotope ratios, 206Pb/204Pb and 208Pb/204Pb, to become less radiogenic, but it has little or no effect on 87Sr/86Sr. We interpret the effect on lead isotopes to be due to assimilation either of lower crustal granitic rocks, which contain 5–10 times as much lead as basalt and which have been low in U/Pb and Th/Pb since Precambrian times, or of upper crustal Precambrian or Paleozoic rocks, which have lost much of their radiogenic lead because of heating prior to assimilation. The lack of definite effects on strontium isotopes may be due to the lesser strontium contents of granitic crustal rocks relative to basaltic rocks coupled with lack of a large radiogenic enrichment in the crustal rocks.Lead isotope ratios were found to be less radiogenic in plagioclase separates from an obviously contaminated basalt than in the primitive basalts. The feldspar separate that is rich in sodic plagioclase xenocrysts was found to be similar to the whole-rock composition for 206Pb/204Pb and 208Pb/204Pb whereas a more dense fraction probably enriched in more calcic plagioclase phenocrysts is more similar to the primitive basalts in lead isotope ratios.The primitive basalts have: 206Pb/204Pb ∼ 18.09–18.34, 207Pb/204Pb ∼ 15.5, 208Pb/204Pb ∼ 37.6–37.9, 87Sr/86Sr ∼ 0.704–0.705. In the primitive basalts from the Southern Rocky Mountains the values of 206Pb/204Pb are similar to values reported by others for Hawaiian and eastern Honshu basalts and abyssal basalts, whereas 208Pb/204Pb tends to be equal to or a little less radiogenic than those from the oceanic localities. 87Sr/86Sr appears to be equal to or a little greater than those of the oceanic localities. These 206Pb/204Pb and 208Pb/204Pb ratios are distinctly less radiogenic and 87Sr/86Sr values are about equal to those reported by others for volcanic islands on oceanic ridges and rises.

Zinc, copper, and lead in mid-ocean ridge basalts and the source rock control on Zn/Pb in ocean-ridge hydrothermal deposits☆

The contents of Zn, Cu, and Pb in mid-ocean ridge basalts (MORB) and the MORB source-rock control on Zn/Pb in ocean-ridge hydrothermal deposits are examined. The values of Zn, Cu, and Pb for submarine mid-ocean ridge basalts (MORB) are, respectively (in ppm): average MORB—75, 75, and 0.7; West Valley, Juan de Fuca Ridge (JFR)—87, 64, and 0.5; southern JFR—120 and 0.5; and 21°N, East Pacific Rise (EPR)—73, 78, and 0.5. Values of Zn/Pb range from about 100–240 and Cu/ Pb from 100–156. In this study, Zn is found to correlate positively with TiO2 + FeO (mean square of weighted deviates, MSWD, of 1.6 for JFR basalt), and inversely with Mg number (MSWD of 3.5). Therefore, contrary to statements in the literature that Zn should be compatible in MORB, Zn is a mildly incompatible element and must be enriched in the glass phase relative to olivine as Zn does not fit into the other major phenocryst phase, plagioclase. In the source of MORB, Zn likely is most enriched in oxides: spinel, magnetite, and titanomagnetite. Copper generally does not correlate well with other elements in most MORB data examined. When differentiation is dominated by olivine, Cu has a tendency to behave incompatibly (e.g., at Mg numbers > 70), but, overall, Cu shows some tendency towards being a compatible element, particularly along the Mid-Atlantic Ridge, a behavior presumably due to separation of sulfides in which Cu (but not Zn) is markedly enriched. Copper thus may be in dispersed sulfides in the source of MORB. Ocean ridges provide important data on source-rock controls for sulfide deposits because, in sediment-starved ridges, much is known about the possible source rocks and mineralization is presently occurring. In contrast to Zn/Pb ~5 in continental hot Cl-rich brines, Zn/Pb in the hottest sediment-starved ridge black smoker hydrothermal fluids at 21 °N, EPR is about 110, similar to local MORB (145), but Cu/Pb is closer to 30, possibly due to subsurface deposition of Cu. At the JFR, the best value of Zn/Pb in the hydrothermal fluids is about 175, again similar to local MORB (240), but Cu is very low in the fluids that are at temperatures less than 300°C. The large MORB-like Zn/Pb in the hottest black-smoker fluids suggests a source-rock control for the metals that prohibits significant galena in the black-smoker deposits of sediment-starved ridges. In contrast, exhalative deposits on sediment-swamped ridges have significant galena; its presence is suggestive of Pb derivation from sediments, an origin supported by Pb isotope studies of LeHuray and colleagues in 1988.

Variations in Lead-Isotopic Compositions in Mesozoic Granitic Rocks of California: A Preliminary Investigation

Six alkali feldspar and two whole-rock samples of granitic rocks from the Sierra Nevada batholith and adjacent Klamath Mountains were analyzed for their lead-isotope compositions. The samples represented each of three 87 Sr/ 86 Sr groupings ( 0.706) for granitic rocks north of the Garlock fault in California. The isotopic compositions of lead in the samples from the Sierra Nevada batholith range from 18.73 to 19.37 for 206 Pb/ 204 Pb, 15.61 to 15.71 for 207 Pb/ 204 Pb, and 38.44 to 39.10 for 208 Pb/ 204 Pb. A crude parallel correspondence was found between lead and strontium isotopes, in that the specimens with the most radiogenic strontium also tend to have the most radiogenic lead similar to the previously studied Boulder batholith of Montana. A parallel correspondence is thought to imply characteristics of the source rocks for the plutons rather than consequences of partial melting or natural contamination. Lead-isotopic compositions for the Sierra Nevada batholith and the Boulder batholith differ, average values of 206 Pb/ 204 Pb being at least 18.8 for the Sierra Nevada batholith and about 18 for the Boulder batholith. In the Late Cretaceous part of the Sierra Nevada batholith, the secondary isochron “age” for the lead data in these rocks is about 2,900 m.y., far older than known Precambrian in California. Sources are proposed for these plutons from the lower continental crust and upper continental mantle or dominantly recycled continental materials, probably of intermediate composition and possibly carried down to the zone of melting by subduction. This source material may have been formed in Pre-cambrian times but did not undergo a Precambrian metamorphism greater than upper amphibolite facies which would have reduced the values of 238 U/ 204 Pb in the source rocks and resulted in Mesozoic leads like those found in the Boulder batholith and elsewhere in the Rocky Mountain region. A trondhjemite from the Klamath Mountains has a lead-isotope composition ( 206 Pb/ 204 Pb, 18.57; 207 Pb/ 204 Pb, 15.50; 208 Pb/ 204 Pb, 38.08) similar to that of oceanic volcanic rocks, particularly like those of island volcanics on oceanic ridges. Derivation of this trondhjemite from an oceanic mantle or recycled mantle material is indicated by this observation and supports the conclusion of Kistler and Peterman (1973) based on its alkali abundances and 87 Sr/ 86 Sr value.

Lead and strontium isotopes in rocks of the Absaroka volcanic field, Wyoming

The Absaroka volcanic field is comprised of predominant andesitic volcaniclastic rocks and less abundant potassium-rich mafic lavas (shoshonites and absarokites). Strontium and lead isotopic variations preclude a simple derivation from an isotopically uniform source: Sr87/Sr86, 0.7042 to 0.7090; Pb206/Pb204, 16.31 to 17.30; Pb208/Pb204, 36.82 to 37.64. We postulate that these rocks were derived from a lower crust or upper mantle which underwent a preferential loss of uranium relative to lead approximately 2800±200 m.y. ago. Variations in lead and strontium isotopic compositions are thought to reflect small inhomogeneities in U/Pb and Rb/Sr ratios in the source.

Lead and strontium isotopic evidence for crustal interaction and compositional zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of the Coso volcanic field, California

The isotopic compositions of Pb and Sr in Pleistocene basalt, high-silica rhyolite, and andesitic inclusions in rhyolite of the Coso volcanic field indicate that these rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values (87Sr/86Sr=0.7036,206Pb/204Pb=19.05,207Pb/204Pb=15.62,208Pb/204Pb= 38.63). The (initial) isotopic composition of typical rhyolite (87Sr/86Sr=0.7053,206Pb/204Pb=19.29,207Pb/204Pb= 15.68,208Pb/204Pb=39.00) is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic end-member compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.

The past is the key to the future

Abstract A new major frontier of geological research, which was initiated in the 1970s, involves predicting future geologic trends or events through study of the present and past, rather than trying to understand the past, often using what one knows about the present. Like most scientific frontiers, this one began from practical considerations—environmental concerns. The lack of formal recognition of this frontier results from fragmentation among many Federal agencies and highly focused mission-oriented programs ( e.g. , earthquake prediction, CO 2 , nuclear-energy safety, etc.). Most programs aim to predict only the next 50–100 years, but much longer periods of the past need to be studied to do this. Nuclear-waste disposal has sometimes been considered in terms of the next million years, a period of time permitting significant and broad geologic changes. Decreasing public interest in environmental concerns relegates many questions from the realm of applied research back to that of basic research. Most of these questions are so fascinating, however, that the frontier is still worth pursuing. Such questions include whether a phenomenon will or will not take place and the rates at which it can develop ( e.g. , how fast do rifts form, how fast can a caldera event begin, and how quickly can a glacial maximum arrive?). Common elements of all studies include the historic record, trends in the Quaternary, analogues in various periods of the geologic time scale, and allowance for phenomena never experienced before. Other examples of studies include the Cretaceous as a period of a climatic extreme, an especially interesting time period; establishing the amount of paleocloudiness, a particularly challenging and important research area; acid rain as a possible new phenomenon. Geochemistry has much to contribute to this frontier science.

Zinc, copper, and lead geochemistry of oceanic igneous rocks - ridges, islands, and arcs

Variations in the abundances of Zn, Cu, and Pb are found to be useful in identifying tectonic regimes and separating oceanisland basalts into enriched- and depleted-source categories. The average Zn, Cu, and Pb contents of normal mid-ocean ridge basalts (N-MORB) are 84, 70, and 0.35 ppm, respectively. Differences in average Zn contents for various ridges reflect more the varying degrees of differentiation than variations of Zn content in the source rocks. At a Mg# of 70, or Mg#70, which is taken to represent primitive MORB, many MORB sequences converge at a Zn content of 58 ± 6 ppm, which is close to the value for primitive mantle (50 ppm) and ordinary chondrites (∼55 ppm). Values of 0.1 to 0.15 ppm Pb in MORB at Mg#70, best defined at the superfast-spreading Southern East Pacific Rise, are similar to estimates of Pb in the primitive mantle (0.12 to 0.18 ppm). They also are near the lower end of the range for ordinary chondrites. The very slow spreading Southwest Indian Ocean Ridge has a sequence with hig...