A long in situ section of the lower ocean crust: results of {ODP} Leg 176 drilling at the Southwest Indian Ridge

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle-ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally =<5% in the bottom kilometer. Hole 735B provides evidence for a strongly heterogeneous lower ocean crust, and for the inherent interplay of deformation, alteration and igneous processes at slow-spreading ridges. It is strikingly different from gabbros sampled from fast-spreading ridges and at most well-described ophiolite complexes. We attribute this to the remarkable diversity of tectonic environments where crustal accretion occurs in the oceans and to the low probability of a section of old slow-spread crust formed near a major large-offset transform being emplaced on-land compared to sections of young crust from small ocean basins

Light Requirement for Seed Germination of Payson Sedge

Payson sedge (Carex paysonis Clokey) is a dominant component of many alpine floras in the western United States. This species appears to be a highly adapted colonizer (early invader) on disturbances such as acidic mine spoils at high elevations, and its rhizomatous growth habit offers promise for revegetation of these sites. The few natural seedlings observed in the field suggest that seed germination of Payson sedge is low compared with that of other alpine colonizers, and seeding trials with this species have met with poor success. Therefore, studies were designed to determine the germination requirements of this species. The effects of 2 levels each of light (visible vs. complete darkness), temperature (constant 25 degrees C vs. variable 25/3 degrees C day/night), and seed position in soil (surface vs. buried) on germination were investigated under controlled conditions in the laboratory. The highest germination percentage (mean = 28.8%) was attained under conditions of complete darkness followed by exposure to light at variable temperatures. There were no significant differences (p<0.05) in germination levels under conditions of light coupled with variable temperature (mean = 21.3%), and complete darkness followed by light at constant temperature (mean = 22.8%). Germination levels were low (mean = 10.0%) under light at constant temperature and seeds subjected to complete darkness alone germinated poorly (mean is lesser than or equal to 1.2%). We recorded low levels of germination (mean is lesser than or equal to 2.8%) from treatments of buried seeds exposed to both light conditions and both temperature levels. A requirement for light coupled with low germination levels of buried seeds suggests that standard revegetation techniques, where seeds are buried beneath the soil surface, may be inappropriate for Payson sedge. We recommend surface seeding in the fall so that extended periods of natural snow cover will promote germination the following spring.This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries.The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform August 202

Effects Of The Contingency For Homework Submission On Homework Submission And Quiz Performance In A College Course

Effects of the contingency for submission of homework assignments on the probability of assignment submission and on quiz grades were assessed in an undergraduate psychology course. Under an alternating treatments design, each student was assigned to a points condition for 5 of 10 quiz-related homework assignments corresponding to textbook chapters. Points were available for homework submission under this condition; points were not available under the no-points condition. The group-mean percentage of homework assignments submitted and quiz grades were higher for all chapters under the points condition than in the no-points condition. These findings, which were replicated in Experiment 2, demonstrate that homework submission was not maintained when the only consequences were instructor-provided feedback and expectation of improved quiz performance

Lower oceanic crust formed at an ultra-slow-spreading ridge; Ocean Drilling Program Hole 735B, Southwest Indian Ridge

Ocean Drilling Program ODP Hole 735B, drilled on Legs 118 and 176, 1508 m of oceanic layer 3 on a transverse ridge adjacent to the Atlantis II Fracture Zone, Southwest Indian Ridge. The cored sequence consists predominantly or olivine gabbro and troctolite and lesser amounts of gabbro, and gabbronorite rich in oxides. The section contains live major blocks of relatively primitive olivine gabbro and troctolite, composed of many smaller igneous bodies. Each Of these composite blocks shows a small upward decrease in Mg# [defined as 100 x Mg/(Mg + Fe 2+)] and contains more fractionated Fe- and Ti-rich gabbros near the top.Small, crosscutting bodies of olivine gabbro and troctolite with diffuse boundaries may represent conduits through crystal mushes for melts migrating upward and feeding individual intrusions. Oxide gabbros and gabbronorites are commonly associated with shear zones of intense deformation, which crosscut the section at all levels, However, oxide-rich rocks decrease in abundance downward and are nearly absent in the lower 500 m of the section. The gabbros and gabbronorites appear to have formed from late-stage, Fe- and Ti-rich, intercumulus melts that were expelled out of fractionating olivine gabbros into the shear zones. The fabrics of the recovered gabbros are consistent with synkinematic cooling and extension of the crustal section in a mid-ocean ridge environment. However, thick intervals of the core have only a weak magmatic foliation. The magmatic foliation is commonly overprinted by a weak, parallel, deformational fabric probably reflecting the transition from a largely magmatic to a largely crystalline state. Deformation in this crustal section decreases markedly downward. Metamorphism and alteration also decrease downward, and much of the core has less than 5% background alteration. Major zones of crystal-plastic (ductile by dislocated creep) deformation in the upper part of the core probably formed under conditions equivalent to granulite-facies conditions when there was little or no melt present. Late-magmatic and hydrothermal fluids produced a variety of plagioclase, amphibole, and diopside veins. Late-stage, low-temperature veins of calcite, smectite, zeolite, prehnite are present in a few intervals. The fact that the cored is unlike ophiolite as defined by the Penrose Conference Participants suggests that no ophiolite representing an ultra-slow-spreading-ridge environment like the Southwest Indian Ridge may be preserved