424 research outputs found

    Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

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    The Rum Eastern Layered Intrusion (ELI; Scotland) is an open-system layered intrusion constructed of 16 macro-rhythmic units. Each of the macro-rhythmic units consists of a peridotite base and a troctolite (+/- gabbro) top, previously attributed to the fractional crystallisation of a single magma batch. This classic paradigm has been challenged, however, with evidence presented for the emplacement of peridotite sills in Units 9, 10, and 14, such as cross-cutting relationships, upward-oriented apophyses, and lateral discontinuities. To test whether the other major peridotites within the ELI represent sills, we have carried out new field, petrographic, and mineral chemical analyses of the peridotites in Units 7, 8 and 9. The peridotites display large- and small-scale cross-cutting relationships with the overlying troctolite, indicative of an intrusive relationship. The peridotites also show large-scale coalescence and lateral spatial discontinuities such that the ELI unit divisions become arbitrary. Harrisite layers and Cr-spinel seams found throughout Units 7, 8, and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma. Our observations allow for distinct subtypes of peridotite to be defined, separated by intrusive contacts, allowing for their relative chronology to be determined. Older, poikilitic peridotite, rich in clinopyroxene, is truncated by younger, well-layered peridotite, containing abundant harrisite layers. In addition to the new peridotite subtypes defined in this study, we find strong evidence for laterally oriented metasomatism within clinopyroxene-rich wehrlites at the top of the Unit 8 peridotite. The wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene-rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions. The observations presented in this study from the ELI cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre-existing feldspathic cumulate (gabbro or troctolite). The evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism, forming many of the unusual (often clinopyroxene-rich) lithologies that surround the sills. The broad-scale formation of the layered peridotites via incremental sill emplacement, suggested by the occurrence of upward-oriented apophyses, coalescence, and lateral discontinuity, could be applied to much larger ultramafic intrusions, which might have formed by similar mechanisms

    Modifications of Gait as Predictors of Natural Osteoarthritis Progression in STR/Ort Mice

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    OBJECTIVE: Osteoarthritis (OA) is a common chronic disease for which disease-modifying therapies are not currently available. Studies to seek new targets for slowing the progress of OA rely on mouse models, but these do not allow for longitudinal monitoring of disease development. This study was undertaken to determine whether gait can be used to measure disease severity in the STR/Ort mouse model of spontaneous OA and whether gait changes are related to OA joint pain. METHODS: Gait was monitored using a treadmill-based video system. Correlations between OA severity and gait at 3 treadmill speeds were assessed in STR/Ort mice. Gait and pain behaviors of STR/Ort mice and control CBA mice were analyzed longitudinally, with monthly assessments. RESULTS: The best speed to identify paw area changes associated with OA severity in STR/Ort mice was found to be 17 cm · seconds(−1). Paw area was modified with age in CBA and STR/Ort mice, but this began earlier in STR/Ort mice and correlated with the onset of OA at 20 weeks of age. In addition, task noncompliance appeared at 20 weeks. Surprisingly, STR/Ort mice did not show any signs of pain with OA development, even when treated with the opioid antagonist naloxone, but did exhibit normal pain behaviors in response to complete Freund's adjuvant–induced arthritis. CONCLUSION: The present results identify an animal model in which OA severity and OA pain can be studied in isolation from one another. The findings suggest that paw area and treadmill noncompliance may be useful tools to longitudinally monitor nonpainful OA development in STR/Ort mice. This will help in providing a noninvasive means of assessing new therapies to slow the progression of OA

    Is the Bankruptcy Code an Adequate Mechanism for Resolving the Distress of Systemically Important Institutions?

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    The President and members of Congress are considering proposals that would give the government broad authority to rescue financial institutions whose failure might threaten market stability. These systemically important institutions include bank and insurance holding companies, investment banks, and other large, highly leveraged, and interconnected entities that are not currently subject to federal resolution authority. Interest in these proposals stems from the credit crisis, particularly the bankruptcy of Lehman Brothers. That bankruptcy, according to some observers, caused massive destabilization in credit markets for two reasons. First, market participants were surprised that the government would permit a massive market player to undergo a costly Chapter 11 proceeding. A very different policy had been applied to other systemically important institutions such as Bear Steams, Fannie Mae, and Freddie Mac. Second, the bankruptcy filing triggered fire sales of Lehman assets. Fire sales were harmful to other non-distressed institutions that held similar assets, which suddenly plummeted in value. They were also harmful to any institution holding Lehman\u27s commercial paper, which functioned as a store of value for entities such as the Primary Reserve Fund. Fire sales destroyed Lehman\u27s ability to honor these claims. Lehman\u27s experience and the various bailouts (of AIG, Bear Steams, and other distressed institutions) have produced two kinds of policy proposals. One calls for wholesale reform, including creation of a systemic risk regulator with authority to seize and stabilize systemically important institutions. Another is more modest and calls for targeted amendments to the Bankruptcy Code and greater government monitoring of market risks. This approach would retain bankruptcy as the principal mechanism for resolving distress at non-bank institutions, systemically important or not. Put differently, current debates hinge on one question: is the Bankruptcy Code an adequate mechanism for resolving the distress of systemically important institutions? One view says no, and advances wholesale reform. Another view says yes, with some adjustments. This Essay evaluates these competing views: Section II discusses the current structure of the Bankruptcy Code and its limited ability to protect markets from failing systemically important institutions. Section III outlines policy responses. In Section IV, I conclude that the Code is indeed inadequate for dealing with failures of systemically important institutions. A systemic risk regulator is needed because a judicially administered process cannot move with sufficient speed and expertise in response to rapidly changing economic conditions
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