Lakehead University Geology - Journal Yearbook (Thunder Bay, Ontario Canada)

Page 14 text:

Sam V Chromite opaque grains, within a serpent ini zed ultrabasic lock. In tne cavities of the chromite some lelics of primary olivine and pyroxene may be found. The fractures in the chromite grains indicate a serpentinization of the surrounding rock with a slight increase in volume. (45 N, Mid Atlantic Ridge) Diabase dredged close to the M.A.R. A rough intersertal structure is formed by coarse microlites of plagioalase and interstitial pyroxene. Probably belonging to a thick flow, or a shallow sill. (45 N on the Mid Atlantic Ridge, 1200 fins) This specimen is a serpentinized dunite. A tiny fracture in the specimen was invaded by a fora minifera ooze, and the walls of the fissure are coated by dendritic Mn and Re oxides. (From 45 N, on the Mid Atlantic Ridge, 1300 fms dep) A quickly chilled basalt, with radiating thin microlites of feldspar, within an opaque hematite stained glassy matrix. Small angular grains of olivine are visible. Note the hollow transversal sections of the feldspatic microlite. (From 45 N, Mid Atlantic Ridge, 1200 fms deep] 12

Page 16 text:

Dr. Edward Mercy, B.Sc., Ph.D. , D.I.C. Academic Background Lecturer in Geology: Imperial College of Science and Technology, London, England. Lecturer in Geology, later Senior Lecturer in Geology, University of Edinburgh, Scotland. Professor of Geology and Chairman of the Department, Lakehead University. Research Geochemistry of a Granite Series in Donegal, Ireland. Geochemistry and mineralogy of gamet-peridotites and eclogites from Norway, S. Europe and S. Africa. Geochemistry and mineralogy of spinel-peridotites from the Lac de Lherz region of the Pyrenees. Geochemical studies of plutonic and volcanic rocks in Northwestern Ontario. CURRENT RESEARCH The mineralogical nature of the mantle of the earth can be deduced in three ways - by considering what known earth materials fit the determined geophysical parameters (such as pressure, temperature, density, value of the gravitational constant, the velocities of seismic waves, and others), by relating the known chemical compositions of basaltic magmas to possible mantle compositions which could produce such lavas by partial or complete melting processes, and by study- ing the mineralogy and chemistry of rocks, emplaced at high levels in the crust, which might represent unaltered mantle squeezed up into the crust by tectonic processes or brought up as xenoliths in magmas. There is general agreement that the uppermost part of the mantle and at least some part of the lower crust is made up of the assemblage: olivine + orthopyroxene + clinopyroxene + spinel = spinel lherzolite. This is consider- ed to be representative of granulite facies conditions in contrast to the eclogite facies conditions of the deeper parts of the mantle which is represented by the assemblage: olivine + orthopyroxene + clinopyroxene + garnet = garnet lherzolite. Natural materials which have the appropriate properties are the spinel-lherzolite xenoliths occurring in basaltic lava flows and the garnet- lherzolite xenoliths in the Kimberlite diatremes of South Africa. Another type is represented by the series of small tectonic intrusions of spinel-lherzolite which outcrop in the French Pyrenees . A period of field work based at the beautiful town of Foix enabled me to make a detailed study of what Lacroix described as the type lherzolite. This occurs at a height of 1300 metres o.d. at the Etang de Lers as a tectonic intrusion into tightly folded and strongly metamorphosed Mesozoic rocks. The lherzolite contains 45-85% by volume of olivine, 10-35% orthopyroxene, 5-20% diopside, and 1-6% spinel. These differences in mineralogical composition are caused by the layered structure of the lherzolite. The main point of lherzolite work is an understanding of the chemistry of the lherzolites and particularly the distribution of elements such as nickel and chromium between the various mineral phases. Such data can lead to knowledge of the conditions of equilibration amongst the phases and may enable one, in comparison with a very great amount of published work, to establish models for the chemistry of the upper mantle. 14 Edward Mercy