Geochemistry of Pallasite Olivines and the Origin of Main-Group Pallasites

Main-group pallasites (PMG) are mixtures of iron-nickel metal and magnesian olivine thought to have been formed at the core-mantle boundary of an asteroid [1]. Some have anomalous metal compositions (PMG-am) and a few have atypically ferroan olivines (PMG-as) [2]. PMG metal is consistent with an origin as a late fractionate of the IIIAB iron core [2]. Most PMG olivines have very similar Fe/Mg ratios, likely due to subsolidus redox reaction with the metal [3]. In contrast, minor and trace elements show substantial variation, which may be explained by either: (i) PMG were formed at a range of depths in the parent asteroid; the element variations reflect variations in igneous evolution with depth, (ii) the pallasite parent asteroid was chemically heterogeneous; the heterogeneity partially survived igneous processing, or (iii) PMG represent the core-mantle boundaries of several distinct parent asteroids [4, 5]. We have continued doing major, minor and trace elements by EMPA and INAA on a wider suite of PMG olivines, and have begun doing precise oxygen isotope analyses to test these hypotheses. Manganese is homologous with Fe(2+), and can be used to distinguish between magmatic and redox processes as causes for Fe/Mg variations. PMG olivines have a range in molar 1000*Mn/Mg of 2.3-4.6 indicating substantial igneous fractionation in olivines with very similar Fe/Mg (0.138-0.148). The Mg-Mn-Fe distributions can be explained by a fractional crystallization-reduction model; higher Mn/Mg ratios reflect more evolved olivines while Fe/Mg is buffered by redox reactions with the metal. There is a positive association between Mn/Mg and Sc content that is consistent with igneous fractionation. However, most PMG olivines fall within a narrow Mn/Mg range (3.0-3.6), but these show a substantial range in Sc (1.00-2.29 micro-g/g). Assuming fractional crystallization, this Sc range could have resulted from approx.65% crystallization of an ultramafic magma. This is inconsistent with formation at the core-mantle boundary of a single asteroid [4]. One alternative is that the PMG are fragments of several asteroids, and these could have had different initial Sc contents, Mn/Mg and differences in igneous history. Our preliminary O isotope data and those of [6, 7] do not support this, although the coverage of PMG olivines is incomplete. The PMG-as Springwater is not easily fit in any scenario. Its olivine has among the highest Mn/Mg suggesting it is one of the most evolved, but the lowest Sc content suggesting it is the least evolved. The O isotopic composition of Springwater olivine is the same as that of other PMG. Thus there is no indication that it represents a distinct parent asteroid. Our preliminary O isotopic data favor a single PMG parent asteroid. In this case, the olivines are more likely melt-residues, and that the parent asteroid was initially heterogeneous in chemical, but not isotopic, composition

Follow-up of patients with curatively resected colorectal cancer: a practice guideline

BACKGROUND: A systematic review was conducted to evaluate the literature regarding the impact of follow-up on colorectal cancer patient survival and, in a second phase, recommendations were developed. METHODS: The MEDLINE, CANCERLIT, and Cochrane Library databases, and abstracts published in the 1997 to 2002 proceedings of the annual meeting of the American Society of Clinical Oncology were systematically searched for evidence. Study selection was limited to randomized trials and meta-analyses that examined different programs of follow-up after curative resection of colorectal cancer where five-year overall survival was reported. External review by Ontario practitioners was obtained through a mailed survey. Final approval of the practice guideline report was obtained from the Practice Guidelines Coordinating Committee. RESULTS: Six randomized trials and two published meta-analyses of follow-up were obtained. Of six randomized trials comparing one follow-up program to a more intense program, only two individual trials detected a statistically significant survival benefit favouring the more intense follow-up program. Pooling of all six randomized trials demonstrated a significant improvement in survival favouring more intense follow-up (Relative Risk Ratio 0.80 (95%CI, 0.70 to 0.91; p = 0.0008). Although the rate of recurrence was similar in both of the follow-up groups compared, asymptomatic recurrences and re-operations for cure of recurrences were more common in patients with more intensive follow-up. Trials including CEA monitoring and liver imaging also had significant results, whereas trials not including these tests did not. CONCLUSION: Follow-up programs for patients with curatively resected colorectal cancer do improve survival. These follow-up programs include frequent visits and performance of blood CEA, chest x-rays, liver imaging and colonoscopy, however, it is not clear which tests or frequency of visits is optimal. There is a suggestion that improved survival is due to diagnosis of recurrence at an earlier, asymptomatic stage which allows for more curative resection of recurrence. Based on this evidence and consideration of the biology of colorectal cancer and present practices, a guideline was developed. Patients should be made aware of the risk of disease recurrence or second bowel cancer, the potential benefits of follow-up and the uncertainties requiring further clinical trials. For patients at high-risk of recurrence (stages IIb and III) clinical assessment is recommended when symptoms occur or at least every 6 months the first 3 years and yearly for at least 5 years. At the time of those visits, patients may have blood CEA, chest x-ray and liver imaging. For patients at lower risk of recurrence (stages I and Ia) or those with co-morbidities impairing future surgery, only visits yearly or when symptoms occur. All patients should have a colonoscopy before or within 6 months of initial surgery, and repeated yearly if villous or tubular adenomas >1 cm are found; otherwise repeat every 3 to 5 years. All patients having recurrences should be assessed by a multidisciplinary team in a cancer centre

Fluid flow and diffusion in the Waterville limeston, South-Central Maine: constraints from strontium, oxygen and carbon isotope profiles

Oxygen, carbon and strontium isotopic profiles across the margin of the Waterville limestone member are used to investigate advective and diffusive transport during metamorphism of the Waterville Formation in south–central Maine, USA. Rb–Sr isotopic systematics were homogenized on the ~10 cm hand-specimen scale at ages that are within error of the 366±6 Ma Rb–Sr whole-rock age of the syn-metamorphic Hallowell pluton. This is consistent with a plutonic heat source for this low-pressure andalusite- and sillimanite-grade Acadian metamorphic terrane. Advective displacements of all three isotope profiles at the garnet-grade Blue Rock Quarry indicate fluid flow to the east into the limestone, and the oxygen-isotope profile implies a time-integrated fluid flux of 3.2±1.5 m3/m2 (2σ error). This cross layer flux is insufficient to cause the observed reaction progress of the muscovite+ankerite–Quartz to biotite+anorthite (in plagioclase)+calcite reaction in the ~100 m thick Waterville limestone member and much of the fluid flow responsible may have been layer parallel. The isotope profiles indicate advective–diffusive homogenization over distances of 1.5 m (δ13C) to 6 m (δ18O) and such homogenization distances are difficult to reconcile with observations of order of magnitude variations in reaction progression the centimetre scale or less. It is possible that infiltration occurred during events short lived compared with diffusion, that the reactions started at different temperatures dependent on bulk composition or that diffusion of water from layers with less reactants to layers with more reactants was important in driving the biotite-producing reaction. However, variations of fluid composition inferred from the mineral assemblages are apparently inconsistent with diffusion driving reactions progress, and models of precursor assemblages do not indicate significant compositional control of the temperature of the first appearance of biotite in the rocks. Irrespective of the details of flow and diffusive exchange on the centimetre scale, the average reaction progress in the Waterville limestone member requires significant layer-parallel fluid fluxes

Highly-siderophile-element Abundances and Re-Os Isotopic Systematics of Yamato 984028

No abstract available

Highly-siderophile-element Abundances and Re-Os Isotopic Systematics of Yamato 984028

No abstract available