Leon T. Silver

The effects of long alpha-stopping distances on (U-Th)/He ages

A mathematical framework for quantitative evaluation of alpha-stopping effects on (U-Th)/He ages has been developed. Alpha stopping ranges in the ^(238)U, ^(235)U, and ^(232)Th chains vary between ∼10 and ∼30 μm, depending on decay energy and density/composition of the stopping medium. In the case of U- and Th-rich accessory minerals (e.g. apatite, zircon, titanite), the dominant effect of long stopping distances is alpha ejection to adjacent minerals. For grains smaller than a few hundred microns in minimum dimension, ejection effects will cause measured helium ages to substantially underestimate true ages. For example, a sphere of 100 μm radius retains only ∼82% of its alphas. For a homogeneous distribution of parent nuclides, the fraction of alphas ejected is ∼ 1/4 of the mean alpha range multiplied by the crystal surface to volume ratio, independent of geometry. Removal of the outer 20 μm of a crystal prior to dating eliminates the region which has experienced alpha loss, but may lead to erroneous ages when crystals are strongly zoned with respect to uranium and thorium. By careful characterization of four sieved apatite separates from a single sample, we show that it is possible to accurately correct (U-Th)/He ages for alpha ejection even when ejection exceeds 35% of total decays. Our results are useful for identifying the size and shape of grains which are best suited for (U-Th)/He dating and provide the basis for correcting ages when ejection effects are significant. This work underscores that meaningful (U-Th)/He ages require either large crystals, or correction of measured ages for alpha ejection.

Rare earth element distributions among minerals in a granodiorite and their petrogenetic implications

A study of the distribution of lanthanide rare earths in a granodiorite from the eastern Peninsular Ranges batholith, southern California, reveals that a large fraction of the REE in this rock resides in the accessory phases sphene and allanite. The minerals plagioclase, alkali feldspar, biotite, epidote and apatite each contribute approximately 1% or less of each REE to the whole rock, with the exception of Eu for which plagioclase contributes 7%. Sphene and allanite together contain 80% to 95% of each REE. Each of these phases is zoned in REE concentration with substantial decreases from core to margin. Textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma. REE zoning trends in sphene and allanite, and unexpectedly low REE concentrations in largely later crystallizing minerals such as feldspar, indicate that the precipitation of sphene and allanite significantly reduced REE concentrations in residual melts. These results illustrate the potential that sphene and allanite have for controlling the behavior of REE in granitic magmas. Available information collectively suggest that the sampled granodiorite existed as a complete melt, that the REE contained in the assemblage of phases were derived by direct crystallization from the melt, and that the melt behaved essentially as a closed system once crystallization of the phases now present began. Close correspondences between the major and trace element chemistries of the granodiorite and phenocryst-poor lavas from similar tectonic settings support these conclusions. The REE pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism. The high liquidus temperature of a granodiorite melt (~ 1000°C) indicates the importance of mantle-derived components within the sources of batholithic magmas in the Peninsular Ranges.

Helium diffusion and low-temperature thermochronometry of apatite

To investigate the potential of the (U-Th)/He system for low-temperature thermochronometry, we have studied helium diffusion and have measured helium ages on Durango fluorapatite and on apatites from a gabbro and two tonalites from the Peninsular Ranges Batholith. Diffusivity at moderate to very low temperatures (as low as 80°C) was measured to high analytical precision using long duration incremental outgassing experiments. All four apatites displayed remarkably similar helium diffusion behavior. Helium loss apparently occurs via volume diffusion from subgrain domains (<60 μm) which are nearly identical in size in all samples. At temperatures below 290°C, diffusivity obeys a highly linear Arrhenius relationship with an implied activation energy of about 36 kcal/mol. Above this temperature, diffusivity deviates from linearity toward lower activation energies. This transition does not arise from multiple diffusion domains, but rather from a reversible change in the physical mechanism of helium diffusion. For thermochronometric purposes the high-temperature diffusion behavior is largely irrelevant because essentially no helium is retained over geologic time at temperatures above 290°C. Using the results from the low-temperature regime, all samples yield helium closure temperatures in the range 75 ± 7°C. This value is independent of chemical composition and grain size of the apatites, suggesting that a single closure temperature may apply to a wide range of samples. The (U-Th)/He ages of these apatites (17–120 Ma) range from a small fraction to nearly 100% of the crystallization age of their host rocks, and are consistent with a low-temperature thermochronometric interpretation. These results strongly support previous suggestions that (U-Th)/He dating of apatite can provide high precision chronometry of very low temperature geological events.

Petrology of eucrites, howardites and mesosiderites.

The eucrite and howardite calcium-rich achondrites and many mesosiderites are considered as a coherent meteorite assemblage, their silicates consisting essentially of calciumpoor monoclinic and orthorhombic pyroxenes and calcium-rich plagioclase feldspar. The achondrites can be grouped according to their brecciated structure as follows: eucrites—unbrecciated and monomict brecciated achondrites; howardites—polymict brecciated achondrites. Many mesosiderites contain brecciated structures; they are distinguished from the achondrites by their large metallic fraction. The structure and composition of rock fragments in the breccias indicate a complicated sequence of events including magmatic differentiation, brecciation, recrystallization and refragmentation, and ejection from the parent body. Detailed mineralogical and chemical data suggest that the magmatic differentiation proceeded primarily by the separation of pyroxene from an ultrabasic parent material that had a much lower alkali content than ordinary chondrites. Magmatic crystallization took place in environments ranging from extrusive to deep-seated intrusive. Polymict breccias contain fragments with a wide variety of magmatic and recrystallization textures, which suggests that the breccias were formed either in very large or repeated fragmentation events. Monomict breccias contain fragments with a small range of similar magmatic textures, which suggests that these breccias were formed by small or single events. Petrographic evidence suggests that many of the breccias are impact breccias. Either in their original magmatic crystallization sites or in the sites of breccia accumulation, most of these meteorites apparently had a near-surface location prior to ejection from the parent body. Evidence obtained from eucrites, howardites and mesosiderites forms an important part of our understanding of the early evolution of the surface regions of their parent body. Chemical and oxidation conditions were different from those presently found in the Earths crust and upper mantle, but the necessary conditions may have been present in the early history of the Earth. A lunar origin for eucrites, howardites and mesosiderites is proposed, but an asteroidal origin can not be presently excluded.

Uranium and Lead Isotopic Stability in a Metamict Zircon under Experimental Hydrothermal Conditions

Abstract. Disturbance of the uranium-lead isotopic system in a metamict Ceylon zircon has been produced in a 2 molal NaCI solution at 500�C and 1000 bars fluid pressure. Loss of radiogenic lead to the extent of 61 percent in 13 days was the most significant effect. The experimental results support the episodic rather than continuous lead-loss interpretation of natural zircon systems utilized in geochronology.

S-type granites and their probable absence in southwestern North America

Criteria that have been (and still are) used to characterize S-type granites of the Lachlan Fold Belt (LFB) of southeastern Australia are reviewed, and comparisons are made with various peraluminous granites of southwestern North America, some of which have been classified as S-types on the basis of insufficient data. Virtually all of the vast volume of S-type granites in the LFB are near-surface, batholithic granites that are commonly associated with S-type volcanics and are not associated with regional metamorphic rocks and migmatites. They are strongly peraluminous, as shown by the presence of cordierite. Granites with primary muscovite are rare. All are low in Na, Ca, and Sr as a result of chemical weathering during formation of the sedimentary sources. Peraluminous granites of various ages in southwestern North America are distinctly different. They rarely contain cordierite (a mineral characteristic of LFB S-types), but some are highly evolved such that Fe-Mn-rich garnet has crystallized. They are dominantly two-mica granites, indicating crystallization at higher water fugacities and greater depths than most peraluminous granites of the LFB. Cordierite-bearing volcanics (S-types) have not been reported. Sodium is generally high in the peraluminous granites of southwestern North America. Some of these rocks have trondhjemitic affinities; the parent magmas seem more likely to have been produced by partial melting of altered basaltic rocks. Locally, some peraluminous rocks (marginal to metaluminous types) may owe their compositions to high-level contamination of I-types; these are not S-type rocks. No compelling evidence has been presented that any of the peraluminous granites of southwestern North America are S-types.

Late Triassic paleogeography of the southern Cordillera: The problem of a source for voluminous volcanic detritus in the Chinle Formation of the Colorado Plateau region

The Upper Triassic Chinle Formation of the Colorado Plateau contains voluminous volcanic detritus evidently derived from a source to the south. Volcanic rocks exposed in southern Arizona and northern Sonora have been assumed to represent this source terrane, but U-Pb isotopic geochronology and regional stratigraphic correlations indicate that these volcanic rocks are distinctly younger than the Chinle, and thus not a source for the volcanic detritus in the Chinle. Igneous rocks of known or possible Late Triassic age in Nevada, California, or northeastern Mexico are possible sources, but a clearly defined source terrane for the volcanic detritus in the Chinle has not been identified. Tectonic removal of the source terrane by rifting or strike-slip offset, though not proven, is a possibility.

Oxygen isotope studies of minerals in stony meteorites

Oxygen isotope analyses demonstrate the following sequence (as in terrestrial igneous rocks) of increasing O^(18) content for coexisting minerals of stony meteorites: olivine, pyroxene, plagioclase, free silica. Except for the carbonaceous chondrites, the O^(18)/O^(16) ratio of a given mineral is quite uniform in each meteorite class. Differences exist between classes of meteorites, however, as well as between certain meteorites and terrestrial igneous rocks. For example, the O^(18)/O^(16) ratios of meteoritic pyroxenes vary from δ = −0.5 per mil to δ = + 8.6 per mil (relative to SMOW), whereas pyroxenes in 8 terrestrial igneous rocks have δ-values which range only from + 5.5 to +6.6. The oxygen isotope data suggest a separation of the stony meteorites into three groups, as follows: 1. I. Basaltic achondrites, hypersthene achondrites, and mesosiderites—with pyroxene δ-values of 3.7 to 4.4. 2. II. Hypersthene-olivine chondrites, bronzite-olivine chondrites, enstatite chondrites, enstatite achondrites, and nakhlites—with pyroxene δ-values of 5.3 to 6.3. 3. III. Types I, II and III carbonaceous chondrites, and ureilites—with highly variable olivine and pyroxene δ-values. It is suggested that the meteorites within each of these three groups are genetically related to one another, but that the three groups may themselves be only distantly related or unrelated. The chondrites have O^(18)/O^(16) ratios similar to their terrestrial analogs, the ultramafic rocks. However, whereas terrestrial basaltic rocks are 1 to 2 per mil richer in O^(18) than ultramafic rocks, the basaltic meteorites are 0.5 to 1.5 per mil lower than chondrites; thus there are serious restrictions placed on any postulated derivation of the basaltic meteorites from chondrites. In addition, the carbonaceous chondrites cannot be simply normal chondrites which have suffered a low-temperature alteration. The olivine in carbonaceous meteorites is lower in O^(18)/O^(16) and isotopically much more variable than chondrite olivine, implying that it is derived from a distinctly different source material.

Lead-isotope inhomogeneity in Precambrian igneous K-feldspars

Stepwise Pb-removal experiments, using both vacuum volatilization and HF-leaching techniques, on acid-washed K-feldspar concentrates from Precambrian igneous rocks show that all contain some unsupported radiogenic Pb. Two types of radiogenic Pb were recognized. One has a “normal” isotopic composition, with relative abundances of ^(206)Pb, ^(207)Pb and ^(208)Pb consistent with the age and U-Th contents of the rocks. The other type of unsupported radiogenic lead in the feldspars is apparently pure ^(206)Pb, derived from long-term migration and accumulation of radioactive daughter(s) of ^(238)U. This “pure ^(206)pb” lead occupies different sites from the “normal” radiogenic lead, and tends to show a release maximum during vacuum volatilization at about 1150°C. The usefulness of stepwise vacuum volatilization may be limited by the tendency of a small amount of radiogenic lead to concentrate in the least volatile fraction. Stepwise partial HF attacks appear to give at least as good separation of radiogenic from original feldspar lead, and are recommended as a routine procedure for isotopic analyses of Precambrian feldspars. The fact that most of the five K-feldspars examined contained unsupported radiogenic lead implies that caution must be used in applying total-sample lead analyses of Precambrian feldspars to problems of lead-isotope evolution in crustal rocks.

Tertiary metamorphic core complexes in Sonora, northwestern Mexico

Several ranges encompassing more than 35,000 km 2 of Sonora, Mexico, contain distinctly lineated and foliated granitic and metamorphic rocks that constitute the lower plates of metamorphic core complexes. Penetrative deformation is characterized by gently dipping mylonitic foliation across which northeast trending stretching lineation is everywhere developed. Prominent northwest trending fractures, dikes, and normal faults are orthogonal to the lineation. Most kinematic indicators in lower plate mylonitic rocks record top-to-the-southwest sense of shear. Upper plate stratigraphic sequences include Mesozoic supracrustal rocks, Tertiary volcanic and sedimentary rocks, and allochthonous Precambrian basement. Tilted blocks of upper plate strata generally overlie the mylonites along gently dipping detachment faults. Previously published U-Pb and K-At ages from lower plate granitic orthogneisses, upper plate volcanic sequences, and crosscutting dikes constrain the time of mylonitic deformation and detachment faulting in several of these areas to late Oligocene-early Miocene. Partitioning of extensional strain in Sonora was influenced by pre-Tertiary crustal structure. The belt of core complexes developed across two contrasting blocks of continental crust separated by the N60oW striking Mojave-Sonora megashear. Portions of the southern Papago block (northeast of the megashear) consisting of Jurassic magmatic arc rocks and Upper Jurassic- Cretaceous siliciclastic and carbonate strata resting upon a concealed, tectonically fragmented Precambrian basement were especially susceptible to crustal attenuation. Some core complexes of the southern Papago block occur within zones trending northwest that may coincide with Late Jurassic lineaments. In the Caborca block (southwest of the megashear), core complex-related rocks and structures have not been identified where surface exposures of Middle Proterozoic basement and overlying Upper Proterozoic-Paleozoic platform strata are common. However, extensional mylonitic fabrics are locally developed along the margins of a Tertiary two-mica granite batholith. Core complexes on both sides of the megashear appear to be preferentially developed where Tertiary granites have intruded regions of crust with basement disrupted by pre-Tertiary structures. Sonoran core complexes preserve an extensional tectonic history comparable with that described from core complexes farther north in the United States and Canadian Cordillera. The timing of mid crustal extension in Sonora (25-18 Ma) is contemporaneous with the timing of core complex development in Arizona, Nevada, and Utah. Extension occurred later in these areas than in the Pacific Northwest-British Columbia region but earlier than in the