Showing papers by "University of Wisconsin-Madison published in 1998"

Embryonic Stem Cell Lines Derived from Human Blastocysts

Abstract: Human blastocyst-derived, pluripotent cell lines are described that have normal karyotypes, express high levels of telomerase activity, and express cell surface markers that characterize primate embryonic stem cells but do not characterize other early lineages. After undifferentiated proliferation in vitro for 4 to 5 months, these cells still maintained the developmental potential to form trophoblast and derivatives of all three embryonic germ layers, including gut epithelium (endoderm); cartilage, bone, smooth muscle, and striated muscle (mesoderm); and neural epithelium, embryonic ganglia, and stratified squamous epithelium (ectoderm). These cell lines should be useful in human developmental biology, drug discovery, and transplantation medicine.

Nonpoint pollution of surface waters with phosphorus and nitrogen

Abstract: Agriculture and urban activities are major sources of phosphorus and nitrogen to aquatic ecosystems. Atmospheric deposition further contributes as a source of N. These nonpoint inputs of nutrients are difficult to measure and regulate because they derive from activities dispersed over wide areas of land and are variable in time due to effects of weather. In aquatic ecosystems, these nutrients cause diverse problems such as toxic algal blooms, loss of oxygen, fish kills, loss of biodiversity (including species important for commerce and recreation), loss of aquatic plant beds and coral reefs, and other problems. Nutrient enrichment seriously degrades aquatic ecosystems and impairs the use of water for drinking, industry, agriculture, recreation, and other purposes. Based on our review of the scientific literature, we are certain that (1) eutrophication is a widespread problem in rivers, lakes, estuaries, and coastal oceans, caused by overenrichment with P and N; (2) nonpoint pollution, a major source of P and N to surface waters of the United States, results primarily from agriculture and urban activity, including industry; (3) inputs of P and N to agriculture in the form of fertilizers exceed outputs in produce in the United States and many other nations; (4) nutrient flows to aquatic ecosystems are directly related to animal stocking densities, and under high livestock densities, manure production exceeds the needs of crops to which the manure is applied; (5) excess fertilization and manure production cause a P surplus to accumulate in soil, some of which is transported to aquatic ecosystems; and (6) excess fertilization and manure production on agricultural lands create surplus N, which is mobile in many soils and often leaches to downstream aquatic ecosystems, and which can also volatilize to the atmosphere, redepositing elsewhere and eventually reaching aquatic ecosystems. If current practices continue, nonpoint pollution of surface waters is virtually certain to increase in the future. Such an outcome is not inevitable, however, because a number of technologies, land use practices, and conservation measures are capable of decreasing the flow of nonpoint P and N into surface waters. From our review of the available scientific information, we are confident that: (1) nonpoint pollution of surface waters with P and N could be reduced by reducing surplus nutrient flows in agricultural systems and processes, reducing agricultural and urban runoff by diverse methods, and reducing N emissions from fossil fuel burning; and (2) eutrophication can be reversed by decreasing input rates of P and N to aquatic ecosystems, but rates of recovery are highly variable among water bodies. Often, the eutrophic state is persistent, and recovery is slow.

Pharmacokinetic/Pharmacodynamic Parameters: Rationale for Antibacterial Dosing of Mice and Men

Abstract: Investigations over the past 20 years have demonstrated that antibacterials can vary markedly in the time course of antimicrobial activity. These differences in pharmacodynamic activity have implications for optimal dosage regimens. The results of more recent studies suggest that the magnitude of the pharmacokinetic/pharmacodynamic parameters required for efficacy are relatively similar in animal infection models and in human infections. However, there is still much to learn. Additional studies are needed to further correlate pharmacokinetic/pharmacodynamic parameters for many antibacterials with therapeutic efficacy in a variety of animal infection models and in human infections. The potential value of using pharmacokinetic/pharmacodynamic parameters as guides for establishing optimal dosing regimens for new and old drugs and for new emerging pathogens and resistant organisms, for setting susceptibility breakpoints, and for reducing the cost of drug development should make the continuing search for the therapeutic rationale of antibacterial dosing of mice and men worthwhile.

Imperfect Oriented Attachment: Dislocation Generation in Defect-Free Nanocrystals

Abstract: Dislocations are common defects in solids, yet all crystals begin as dislocation-free nuclei. The mechanisms by which dislocations form during early growth are poorly understood. When nanocrystalline materials grow by oriented attachment at crystallographically specific surfaces and there is a small misorientation at the interface, dislocations result. Spiral growth at two or more closely spaced screw dislocations provides a mechanism for generating complex polytypic and polymorphic structures. These results are of fundamental importance to understanding crystal growth.

Organizational Memory

Abstract: nowledge management is an essential capability in the emerging knowledge economy. In particular, organizations have a valuable asset in the informal knowledge that is the daily currency of their knowledge workers, but this asset usually lives only in the collective human memory, and thus is poorly preserved and managed. There are significant technical and cultural barriers to capturing informal knowledge and making it explicit. Groupware tools such as E-mail and Lotus NotesTM tend to make informal knowledge explicit, but they generally fail to create an accessible organizational memory. On the other hand, attempts to build organizational memory systems have generally failed because they required additional documentation effort with no clear short term benefit, or, like groupware, they did not provide an effective index or structure to the mass of information collected in the system. This paper explores the design of a project memory system that overcomes the barriers to capturing informal knowledge. The key component of this design is the use of a display system that captures the key issues and ideas during meetings. The emphasis in this approach is on improving K This paper was originally written several years ago. Since then, there have been many important papers and books published including Dialogue Mapping: Building Shared Understanding of Wicked Problems, Jeff Conklin, Ph.D. Link to Dialogue Mapping book.

Development of Depression From Preadolescence to Young Adulthood: Emerging Gender Differences in a 10-Year Longitudinal Study

Abstract: The authors investigated the emergence of gender differences in clinical depression and the overall development of depression from preadolescence to young adulthood among members of a complete birth cohort using a prospective longitudinal approach with structured diagnostic interviews administered 5 times over the course of 10 years. Small gender differences in depression (females greater than males) first began to emerge between the ages of 13 and 15. However, the greatest increase in this gender difference occurred between ages 15 and 18. Depression rates and accompanying gender differences for a university student subsample were no different than for a nonuniversity subsample. There was no gender difference for depression recurrence or for depression symptom severity. The peak increase in both overall rates of depression and new cases of depression occurred between the ages of 15 and 18. Results suggest that middle-to-late adolescence (ages 15-18) may be a critical time for studying vulnerability to depression because of the higher depression rates and the greater risk for depression onset and dramatic increase in gender differences in depression during this period.

Foldamers: A Manifesto

Abstract: Nature relies on large molecules to carry out sophisticated chemical operations, such as catalysis, tight and specific binding, directed flow of electrons, or controlled crystallization of inorganic phases. The polymers entrusted with these crucial tasks, mostly proteins but sometimes RNA, are unique relative to other biological and synthetic polymers in that they adopt specific compact conformations that are thermodynamically and kinetically stable. These folding patterns generate “active sites” via precise three-dimensional arrangement of functional groups. In terms of covalent connectivity, the groups that comprise the active site are often widely spaced along the polymer backbone. The remarkable range of chemical capabilities that evolution has elicited from proteins suggests that it might be possible to design analogous capabilities into unnatural polymers that fold into compact and specific conformations. Since biological evolution has operated under many constraints, the functional properties of proteins and RNA should be viewed as merely exemplifying the potential of compactly folded polymers. The chemist’s domain includes all possible combinations of the elements, and the biological realm, vast and complex though it may be, is only a small part of that domain. Therefore, realization of the potential of folding polymers may be limited more by the human imagination than by physical barriers. I use the term “foldamer” to describe any polymer with a strong tendency to adopt a specific compact conformation. Among proteins, the term “compact” is associated with tertiary structure, and there is as yet no synthetic polymer that displays a specific tertiary structure. Protein tertiary structure arises from the assembly of elements of regular secondary structure (helices, sheets, and turns). The first step in foldamer design must therefore be to identify new backbones with well-defined secondary structural preferences. “Well-defined” in this case means that the conformational preference should be displayed in solution by oligomers of modest length, and I will designate as a foldamer any oligomer that meets this criterion. Within the past decade, a handful of research groups have described unnatural oligomers with interesting conformational propensities. The motivations behind such efforts are varied, but these studies suggest a collective, emerging realization that control over oligomer and polymer folding could lead to new types of molecules with useful properties. The purpose of this “manifesto” is to introduce a large audience to the broad research horizons offered by the concept of synthetic foldamers. The path to creating useful foldamers involves several daunting steps. (i) One must identify new polymeric backbones with suitable folding propensities. This goal includes developing a predictively useful understanding of the relationship between the repetitive features of monomer structure and conformational properties at the polymer level. (ii) One must endow the resulting foldamers with interesting chemical functions, by design, by randomization and screening (“evolution”), or by some combination of these two approaches. (iii) For technological utility, one must be able to produce a foldamer efficiently, which will generally include preparation of the constituent monomers in stereochemically pure form and optimization of heteropolymer synthesis. Each of these steps involves fascinating chemical challenges; the first step is the focus of this Account.

Unto Others: The Evolution and Psychology of Unselfish Behavior

Abstract: Introduction: Bentham's corpse. Part 1 Evolutionary altruism: altruism as a biological concept a unified evolutionary theory of social behaviour adaptation and multilevel selection group selection and human behaviour human groups as adaptive units. Part 2 Psychological altruism: motives as proximate mechanisms three theories of motivation psychological evidence psychological evidence philosophical arguments the evolution of psychological altruism. Conclusion pluralsim.

Rates of Spontaneous Mutation

Abstract: Rates of spontaneous mutation per genome as measured in the laboratory are remarkably similar within broad groups of organisms but differ strikingly among groups. Mutation rates in RNA viruses, whose genomes contain ca. 10(4) bases, are roughly 1 per genome per replication for lytic viruses and roughly 0.1 per genome per replication for retroviruses and a retrotransposon. Mutation rates in microbes with DNA-based chromosomes are close to 1/300 per genome per replication; in this group, therefore, rates per base pair vary inversely and hugely as genome sizes vary from 6 x 10(3) to 4 x 10(7) bases or base pairs. Mutation rates in higher eukaryotes are roughly 0.1-100 per genome per sexual generation but are currently indistinguishable from 1/300 per cell division per effective genome (which excludes the fraction of the genome in which most mutations are neutral). It is now possible to specify some of the evolutionary forces that shape these diverse mutation rates.

Managers as Initiators of Trust: An Exchange Relationship Framework for Understanding Managerial Trustworthy Behavior

Abstract: In many approaches to interpersonal and organizational trust, researchers focus on employees' perceptions that their managers are trustworthy. We turn the tables, however, and examine the antecedents of managerial trustworthy behavior and the challenge of initiating trust. Drawing on agency and social exchange theories, we present an exchange relationship framework that identifies organizational, relational, and individual factors that encourage or constrain managerial trustworthy behavior.

Resident enteric bacteria are necessary for development of spontaneous colitis and immune system activation in interleukin-10-deficient mice.

Abstract: Mice with targeted deletion of the gene for interleukin-10 (IL-10) spontaneously develop enterocolitis when maintained in conventional conditions but develop only colitis when kept in specific-pathogen-free (SPF) environments. This study tested the hypothesis that enteric bacteria are necessary for the development of spontaneous colitis and immune system activation in IL-10-deficient mice. IL-10-deficient mice were maintained in either SPF conditions or germfree conditions or were populated with bacteria known to cause colitis in other rodent models. IL-10-deficient mice kept in SPF conditions developed colitis in all segments of the colon (cecum and proximal and distal colon). These mice exhibited immune system activation as evidenced by increased expression of CD44 on CD4+ T cells; increased mesenteric lymph node cell numbers; and increased production of immunoglobulin A (IgA), IgG1, and IL-12 p40 from colon fragment cultures. Mice populated with bacterial strains, including Bacteroides vulgatus, known to induce colitis in other rodent models had minimal colitis. Germfree IL-10-deficient mice had no evidence of colitis or immune system activation. We conclude therefore that resident enteric bacteria are necessary for the development of spontaneous colitis and immune system activation in IL-10-deficient mice.

Gabapentin for the Symptomatic Treatment of Painful Neuropathy in Patients With Diabetes Mellitus: A Randomized Controlled Trial

Abstract: Context.—Pain is the most disturbing symptom of diabetic peripheral neuropathy. As many as 45% of patients with diabetes mellitus develop peripheral neuropathies.Objective.—To evaluate the effect of gabapentin monotherapy on pain associated with diabetic peripheral neuropathy.Design.—Randomized, double-blind, placebo-controlled, 8-week trial conducted between July 1996 and March 1997.Setting.—Outpatient clinics at 20 sites.Patients.—The 165 patients enrolled had a 1- to 5-year history of pain attributed to diabetic neuropathy and a minimum 40-mm pain score on the Short-Form McGill Pain Questionnaire visual analogue scale.Intervention.—Gabapentin (titrated from 900 to 3600 mg/d or maximum tolerated dosage) or placebo.Main Outcome Measures.—The primary efficacy measure was daily pain severity as measured on an 11-point Likert scale (0, no pain; 10, worst possible pain). Secondary measures included sleep interference scores, the Short-Form McGill Pain Questionnaire scores, Patient Global Impression of Change and Clinical Global Impression of Change, the Short Form–36 Quality of Life Questionnaire scores, and the Profile of Mood States results.Results.—Eighty-four patients received gabapentin and 70 (83%) completed the study; 81 received placebo and 65 (80%) completed the study. By intent-to-treat analysis, gabapentin-treated patients' mean daily pain score at the study end point (baseline, 6.4; end point, 3.9; n = 82) was significantly lower (P<.001) compared with the placebo-treated patients' end-point score (baseline, 6.5; end point, 5.1; n = 80). All secondary outcome measures of pain were significantly better in the gabapentin group than in the placebo group. Additional statistically significant differences favoring gabapentin treatment were observed in measures of quality of life (Short Form–36 Quality of Life Questionnaire and Profile of Mood States). Adverse events experienced significantly more frequently in the gabapentin group were dizziness (20 [24%] in the gabapentin group vs 4 [4.9%] in the control group; P<.001) and somnolence (19 [23%] in the gabapentin group vs 5 [6%] in the control group; P = .003). Confusion was also more frequent in the gabapentin group (7 [8%] vs 1 [1.2%]; P = .06).Conclusion.—Gabapentin monotherapy appears to be efficacious for the treatment of pain and sleep interference associated with diabetic peripheral neuropathy and exhibits positive effects on mood and quality of life.

Understanding Relationships Using Copulas

Abstract: This article introduces actuaries to the concept of “copulas,” a tool for understanding relationships among multivariate outcomes. A copula is a function that links univariate marginals to their full multivariate distribution. Copulas were introduced in 1959 in the context of probabilistic metric spaces. The literature on the statistical properties and applications of copulas has been developing rapidly in recent years. This article explores some of these practical applications, including estimation of joint life mortality and multidecrement models. In addition, we describe basic properties of copulas, their relationships to measures of dependence, and several families of copulas that have appeared in the literature. An annotated bibliography provides a resource for researchers and practitioners who wish to continue their study of copulas. For those who wish to use copulas for statistical inference, we illustrate statistical inference procedures by using insurance company data on losses and expen...

Refining Initial Points for K-Means Clustering

Abstract: Practical approaches to clustering use an iterative procedure (e.g. K-Means, EM) which converges to one of numerous local minima. It is known that these iterative techniques are especially sensitive to initial starting conditions. We present a procedure for computing a refined starting condition from a given initial one that is based on an efficient technique for estimating the modes of a distribution. The refined initial starting condition allows the iterative algorithm to converge to a “better” local minimum. The procedure is applicable to a wide class of clustering algorithms for both discrete and continuous data. We demonstrate the application of this method to the popular K-Means clustering algorithm and show that refined initial starting points indeed lead to improved solutions. Refinement run time is considerably lower than the time required to cluster the full database. The method is scalable and can be coupled with a scalable clustering algorithm to address the large-scale clustering problems in data mining.

Organizational Improvisation and Organizational Memory

Abstract: We define organizational improvisation as the degree to which the composition and execution of an action converge in time, and we examine the theoretical potential of this definition. We then propose that both organizational procedural memory (skill knowledge) and declarative memory (fact knowledge) moderate improvisation's impact on organizational outcomes in distinct ways. We also suggest that improvisation influences organizational memory by (1) generating experiments and (2) permitting the development of higher-level competency in improvisation. Contemporary technological changes related to the nature of organizational memory intensify the salience of these issues.

The effect of finasteride on the risk of acute urinary retention and the need for surgical treatment among men with benign prostatic hyperplasia

Abstract: Background Finasteride is known to improve urinary symptoms in men with benign prostatic hyperplasia, but the extent to which the benefit is sustained and whether finasteride reduces the incidence of related events, including the need for surgery and the development of acute urinary retention, are not known. Methods In this double-blind, randomized, placebo-controlled trial, we studied 3040 men with moderate-to-severe urinary symptoms and enlarged prostate glands who were treated daily with 5 mg of finasteride or placebo for four years. Symptom scores (on a scale of 1 to 34), urinary flow rates, and the occurrence of outcome events were assessed every four months in 3016 men. Prostate volume was measured in a subgroup of the men. Complete data on outcomes were available for 2760 men. Results During the four-year study period, 152 of the 1503 men in the placebo group (10 percent) and 69 of the 1513 men in the finasteride group (5 percent) underwent surgery for benign prostatic hyperplasia (reduction in risk with finasteride, 55 percent; 95 percent confidence interval, 41 to 65 percent). Acute urinary retention developed in 99 men (7 percent) in the placebo group and 42 men (3 percent) in the finasteride group (reduction in risk with finasteride, 57 percent; 95 percent confidence interval, 40 to 69 percent). Among the men who completed the study, the mean decreases in the symptom score were 3.3 in the finasteride group and 1.3 in the placebo group (P<0.001). Treatment with finasteride also significantly improved urinary flow rates and reduced prostate volume (P<0.001). Conclusions Among men with symptoms of urinary obstruction and prostatic enlargement, treatment with finasteride for four years reduces symptoms and prostate volume, increases the urinary flow rate, and reduces the probability of surgery and acute urinary retention.

Effectiveness of Interventions to Improve Patient Compliance a Meta-Analysis

Abstract: Objectives.This article summarizes the results of 153 studies published between 1977 and 1994 that evaluated the effectiveness of interventions to improve patient compliance with medical regimens.Methods.The compliance interventions were classified by theoretical focus into educational, behaviora

Prevalence of Hearing Loss in Older Adults in Beaver Dam, Wisconsin The Epidemiology of Hearing Loss Study

Abstract: There are no recent population-based data on the prevalence of hearing loss in older adults using standard audiometric testing. The population-based Epidemiology of Hearing Loss Study was designed to measure the prevalence of hearing loss in adults aged 48-92 years, residing in Beaver Dam, Wisconsin. Hearing thresholds were measured with standardized protocols using pure-tone air- and bone-conduction audiometry in sound-treated booths. The examination also included an otoscopic evaluation, screening tympanogram, and a questionnaire on hearing-related medical history, noise exposure, other potential risk factors, and self-perceived hearing handicap. Of the 4,541 eligible people, 3,753 (82.6%) participated in the hearing study (1993-1995). The average age of participants was 65.8 years, and 57.7% were women. The prevalence of hearing loss was 45.9%. The odds of hearing loss increased with age (odds ratio (OR) = 1.88 for 5 years, 95% confidence interval (CI) 1.80-1.97) and were greater for men than women (OR = 4.42, 95% CI 3.73-5.24). The male excess of hearing loss remained statistically significant after adjusting for age, education, noise exposure, and occupation (OR = 3.65). These results demonstrate that hearing loss is a very common problem affecting older adults. Epidemiologic studies are needed to understand the genetic, environmental, and sex-related determinants of age-related hearing loss and to identify potential intervention strategies.

Internationalization and Firm Governance: The Roles of CEO Compensation, Top Team Composition, and Board Structure

Abstract: Using the complementary lenses of information-processing and agency theories, this study tests the proposition that the complexity resulting from a firm's degree of internationalization will be acc...

Vagus nerve stimulation therapy for partial-onset seizures A randomized active-control trial

Abstract: Objective: The purpose of this multicenter, add-on, double-blind, randomized, active-control study was to compare the efficacy and safety of presumably therapeutic (high) vagus nerve stimulation with less (low) stimulation. Background: Chronic intermittent left vagus nerve stimulation has been shown in animal models and in preliminary clinical trials to suppress the occurrence of seizures. Methods: Patients had at least six partial-onset seizures over 30 days involving complex partial or secondarily generalized seizures. Concurrent antiepileptic drugs were unaltered. After a 3-month baseline, patients were surgically implanted with stimulating leads coiled around the left vagus nerve and connected to an infraclavicular subcutaneous programmable pacemaker-like generator. After randomization, device initiation, and a 2-week ramp-up period, patients were assessed for seizure counts and safety over 3 months. The primary efficacy variable was the percentage change in total seizure frequency compared with baseline. Results: Patients receiving high stimulation (94 patients, ages 13 to 54 years) had an average 28% reduction in total seizure frequency compared with a 15% reduction in the low stimulation group (102 patients, ages 15 to 60 year; p = 0.04). The high-stimulation group also had greater improvements on global evaluation scores, as rated by a blinded interviewer and the patient. High stimulation was associated with more voice alteration and dyspnea. No changes in physiologic indicators of gastric, cardiac, or pulmonary functions occurred. Conclusions: Vagus nerve stimulation is an effective and safe adjunctive treatment for patients with refractory partial-onset seizures. It represents the advent of a new, nonpharmacologic treatment for epilepsy.

Min-max feedback model predictive control for constrained linear systems

Abstract: Min-max feedback formulations of model predictive control are discussed, both in the fixed and variable horizon contexts. The control schemes the authors discuss introduce, in the control optimization, the notion that feedback is present in the receding-horizon implementation of the control. This leads to improved performance, compared to standard model predictive control, and resolves the feasibility difficulties that arise with the min-max techniques that are documented in the literature. The stabilizing properties of the methods are discussed as well as some practical implementation details.

Force and Velocity Measured for Single Molecules of RNA Polymerase

Abstract: RNA polymerase (RNAP) moves along DNA while carrying out transcription, acting as a molecular motor. Transcriptional velocities for single molecules of Escherichia coli RNAP were measured as progressively larger forces were applied by a feedback-controlled optical trap. The shapes of RNAP force-velocity curves are distinct from those of the motor enzymes myosin or kinesin, and indicate that biochemical steps limiting transcription rates at low loads do not generate movement. Modeling the data suggests that high loads may halt RNAP by promoting a structural change which moves all or part of the enzyme backwards through a comparatively large distance, corresponding to 5 to 10 base pairs. This contrasts with previous models that assumed force acts directly upon a single-base translocation step.

The Convergence of Planning and Execution: Improvisation in New Product Development:

Abstract: The field of marketing strategy often makes the important assumption that marketing strategy should occur by first composing a plan on the basis of a careful review of environmental and firm inform...

Matchmaking: distributed resource management for high throughput computing

Abstract: Conventional resource management systems use a system model to describe resources and a centralized scheduler to control their allocation. We argue that this paradigm does not adapt well to distributed systems, particularly those built to support high throughput computing. Obstacles include heterogeneity of resources, which make uniform allocation algorithms difficult to formulate, and distributed ownership, leading to widely varying allocation policies. Faced with these problems, we developed and implemented the classified advertisement (classad) matchmaking framework, a flexible and general approach to resource management in distributed environment with decentralized ownership of resources. Novel aspects of the framework include a semi structured data model that combines schema, data, and query in a simple but powerful specification language, and a clean separation of the matching and claiming phases of resource allocation. The representation and protocols result in a robust, scalable and flexible framework that can evolve with changing resources. The framework was designed to solve real problems encountered in the deployment of Condor, a high throughput computing system developed at the University of Wisconsin-Madison. Condor is heavily used by scientists at numerous sites around the world. It derives much of its robustness and efficiency from the matchmaking architecture.

Bilateral orchiectomy with or without flutamide for metastatic prostate cancer

Abstract: Background Combined androgen blockade for the treatment of metastatic prostate cancer consists of an antiandrogen drug plus castration. In a previous trial, we found that adding the antiandrogen flutamide to leuprolide acetate (a synthetic gonadotropin-releasing hormone that results in medical ablation of testicular function) significantly improved survival as compared with that achieved with placebo plus leuprolide acetate. In the current trial, we compared flutamide plus bilateral orchiectomy with placebo plus orchiectomy. Methods We randomly assigned patients who had never received antiandrogen therapy and who had distant metastases from adenocarcinoma of the prostate to treatment with bilateral orchiectomy and either flutamide or placebo. Patients were stratified according to the extent of disease and according to performance status. Results Of the 1387 patients who were enrolled in the trial, 700 were randomly assigned to the flutamide group and 687 to the placebo group. Overall, the incidence of tox...

Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts

Abstract: The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the Siberian Platform vary from 4.7 to 5.9& VSMOW. High precision, accurate analyses by laser reveal subtle pipe-to-pipe diAerences not previously suspected. These zircons have distinctive chemical and physical characteristics identifying them as mantle-de- rived megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-diopside, K-richterite, or Mg-rich ilmenite. Several lines of evi- dence indicate that these d 18 O values are unaltered by kimberlite magmas during eruption and represent com- positions preserved since crystallization in the mantle, including: U/Pb age, large crystal size, and the slow rate of oxygen exchange in non-metamict zircon. The aver- age d 18 O of mantle zircons is 5.3&,0:1& higher and in equilibrium with values for olivine in peridotite xenoliths and oceanic basalts. Zircon megacrysts from within 250 km of Kimberley, South Africa have average d 18 Oa 5:32 0:17&Ona 28U. Small, but significant, diAerences among other kimberlite pipes or groups of pipes may indicate isotopically distinct reservoirs in the sub-continental lithosphere or asthenosphere, some of which are anomalous with respect to normal mantle values of 5:3 0:3&. Precambrian zircons (2.1-2.7 Ga) from Jwaneng, Botswana have the lowest values yet measured in a mantle zircon, d 18 Oa 3:4 to 4.7&. These zircon megacrysts originally crystallized in mafic or ul- tramafic rocks either through melting and metasomat- ism associated with kimberlite magmatism or during metamorphism. The low d 18 O zircons are best explained by subduction of late Archean ocean crust that ex- changed with heated seawater prior to underplating as eclogite and to associated metasomatism of the mantle wedge. Smaller diAerences among other pipes and dis- tricts may result from variable temperatures of equili- bration, mafic versus ultramafic hosts, or variable un- derplating. The narrow range in zircon compositions found in most pipes suggests magmatic homogenization. If this is correct, these zircons document the existence of sig- nificant quantities of magma in the sub-continental mantle that was regionally variable in d 18 O and this information restricts theories about the nature of ancient subduction.