The influence of ridge geometry at the ultraslow-spreading Southwest Indian Ridge (9º-25ºE) : basalt composition sensitivity to variations in source and process

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2006Between 9º-25º E on the ultraslow-spreading Southwest Indian Ridge lie two sharply contrasting supersegments. One 630 km long supersegment erupts N-MORB that is progressively enriched in incompatible element concentrations from east to west. The second 400 km long supersegment contains three separate volcanic centers erupting EMORB and connected by long amagmatic accretionary segments, where mantle is emplaced directly to the seafloor with only scattered N-MORB and E-MORB erupted. Rather than a major break in mantle composition at the discontinuity between the supersegments, this sharp contrast in geometry, physiography, and chemistry reflects “source” versus “process” dominated generation of basalt. Robust along-axis correlation of ridge characteristics (i.e. morphology, upwelling rate, lithospheric thickness), basalt chemistry, and crustal thickness (estimated from gravity) provides a unique opportunity to compare the influence of spreading geometry and rate on MORB generation. What had not been well established until now is the importance of melting processes rather than source at spreading rates < 20 mm/yr. Along the orthogonally spreading supersegment (14 mm/yr) moderate degrees of partial melting effectively sample the bulk mantle source, while on the obliquely spreading supersegment (7-14 mm/yr) suppression of mantle melting to low degrees means that the bulk source is not uniformly sampled, and thus “process” rather than “source” dominates melt chemistry.The main body of work consisting of major element, trace element, and isotopic data acquisition and interpretation (Chapter 2 & 3) was funded by H. Dick’s grant from the National Science Foundation-OCE 9907630. National Science Foundation-OCE 0137325 supported the U-series work described in Chapter 4. The published work of Chapter 5 was funded by National Science Foundation-EAR 9804891, NSF-OCE 9416620, and NSF-OCE 0096634

Young off-axis volcanism along the ultraslow-spreading Southwest Indian Ridge

Author Posting. © The Authors, 2010. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 3 (2010): 286-292, doi:10.1038/ngeo824.Mid-ocean ridge crustal accretion occurs continuously at all spreading rates through a combination of magmatic and tectonic processes. Fast to slow spreading ridges are largely built by adding magma to narrowly focused neovolcanic zones. In contrast, ultraslow spreading ridge construction significantly relies on tectonic accretion, which is characterized by thin volcanic crust, emplacement of mantle peridotite directly to the seafloor, and unique seafloor fabrics with variable segmentation patterns. While advances in remote imaging have enhanced our observational understanding of crustal accretion at all spreading rates, temporal information is required in order to quantitatively understand mid-ocean ridge construction. However, temporal information does not exist for ultraslow spreading environments. Here, we utilize U-series eruption ages to investigate crustal accretion at an ultraslow spreading ridge for the first time. Unexpectedly young eruption ages throughout the Southwest Indian ridge rift valley indicate that neovolcanic activity is not confined to the spreading axis, and that magmatic crustal accretion occurs over a wider zone than at faster spreading ridges. These observations not only suggest that crustal accretion at ultraslow spreading ridges is distinct from faster spreading ridges, but also that the magma transport mechanisms may differ as a function of spreading rate.This work was supported by the following NSF grants: NSF-OCE 0137325; NSF-OCE 060383800; and NSF-OCE 062705300

MORB generation beneath the ultraslow spreading Southwest Indian Ridge (9–25°E) : major element chemistry and the importance of process versus source

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q05004, doi:10.1029/2008GC001959.We report highly variable mid-ocean ridge basalt (MORB) major element and water concentrations from a single 1050-km first-order spreading segment on the ultraslow spreading Southwest Indian Ridge, consisting of two supersegments with strikingly different spreading geometry and ridge morphology. To the east, the 630 km long orthogonal supersegment (<10° obliquity) dominantly erupts normal MORB with progressive K/Ti enrichment from east to west. To the west is the 400 km long oblique supersegment (up to 56° obliquity) with two robust volcanic centers erupting enriched MORB and three intervening amagmatic accretionary segments erupting both N-MORB and E-MORB. The systematic nature of the orthogonal supersegments' ridge morphology and MORB composition ends at 16°E, where ridge physiography, lithologic abundance, crustal structure, and basalt chemistry all change dramatically. We attribute this discontinuity and the contrasting characteristics of the supersegments to localized differences in the upper mantle thermal structure brought on by variable spreading geometry. The influence of these differences on the erupted composition of MORB appears to be more significant at ultraslow spreading rates where the overall degree of melting is lower. In contrast to the moderate and rather constant degrees of partial melting along the orthogonal supersegment, suppression of mantle melting on the oblique supersegment due to thickened lithosphere means that the bulk source is not uniformly sampled, as is the former. On the oblique supersegment, more abundant mafic lithologies melt deeper thereby dominating the more enriched aggregate melt composition. While much of the local major element heterogeneity can be explained by polybaric fractional crystallization with variable H2O contents, elevated K2O and K/Ti cannot. On the basis of the chemical and tectonic relationship of these enriched and depleted basalts, their occurrence requires a multilithology mantle source. The diversity and distribution of MORB compositions, especially here at ultraslow spreading rates, is controlled not only by the heterogeneity of the underlying mantle, but also more directly by the local thermal structure of the lithosphere (i.e., spreading geometry) and its influence on melting processes. Thus at ultraslow spreading rates, process rather than source may be the principle determiner of MORB composition.This work was originally funded in large part by NSF grants OCE-9907630 and OCE-0526905 and more recently by OPP-0425785

Impact of opioid-free analgesia on pain severity and patient satisfaction after discharge from surgery: multispecialty, prospective cohort study in 25 countries

Background: Balancing opioid stewardship and the need for adequate analgesia following discharge after surgery is challenging. This study aimed to compare the outcomes for patients discharged with opioid versus opioid-free analgesia after common surgical procedures.Methods: This international, multicentre, prospective cohort study collected data from patients undergoing common acute and elective general surgical, urological, gynaecological, and orthopaedic procedures. The primary outcomes were patient-reported time in severe pain measured on a numerical analogue scale from 0 to 100% and patient-reported satisfaction with pain relief during the first week following discharge. Data were collected by in-hospital chart review and patient telephone interview 1 week after discharge.Results: The study recruited 4273 patients from 144 centres in 25 countries; 1311 patients (30.7%) were prescribed opioid analgesia at discharge. Patients reported being in severe pain for 10 (i.q.r. 1-30)% of the first week after discharge and rated satisfaction with analgesia as 90 (i.q.r. 80-100) of 100. After adjustment for confounders, opioid analgesia on discharge was independently associated with increased pain severity (risk ratio 1.52, 95% c.i. 1.31 to 1.76; P &lt; 0.001) and re-presentation to healthcare providers owing to side-effects of medication (OR 2.38, 95% c.i. 1.36 to 4.17; P = 0.004), but not with satisfaction with analgesia (beta coefficient 0.92, 95% c.i. -1.52 to 3.36; P = 0.468) compared with opioid-free analgesia. Although opioid prescribing varied greatly between high-income and low- and middle-income countries, patient-reported outcomes did not.Conclusion: Opioid analgesia prescription on surgical discharge is associated with a higher risk of re-presentation owing to side-effects of medication and increased patient-reported pain, but not with changes in patient-reported satisfaction. Opioid-free discharge analgesia should be adopted routinely