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TitleA new projection scheme and differentiation index for Cr-spinels A new projection scheme and differentiation index for Cr-spinels
AuthorBedard, J H
SourceLithos vol. 42, 1997 p. 37-45,
Alt SeriesGeological Survey of Canada, Contribution Series 1997037
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
Subjectsmineralogy; igneous and metamorphic petrology; crystallography; chromite; chromium; spinel; differentiation; ophiolites; gabbros; dunites; magmatic differentiation; layered intrusions; isotherms
Illustrationstables; diagrams; plots
AbstractThe Cr# (100Cr/(Cr + Al)) and Fe2# (100Fe2+/(Mg + Fe2+)) of Cr-spinels affected by low-temperature reequilibration with olivine are controlled by isotherms with shallow slopes on the Cr# vs. Fe2# diagram. Consequently, in slowly cooled olivine-rich plutonic rocks, the Fe2# of Cr-spinel is an unreliable measure of the degree of evolution of the magma from which it crystallized. For example, high-Cr# spinels in a reequilibrated dunite could have a higher Fe2# than low-Cr# spinels in a reequilibrated gabbro, despite the fact that the spinel in the gabbro originally crystallized at a lower temperature from a more evolved liquid than did the spinel in the dunite. As a result, the distribution of data points is difficult to interpret when minor elements in Cr-spinel (Ti, V, Fe3+, Mn) are plotted against Cr-spinel Fe2#. A solution to this problem is to project spinel compositional data onto an axis perpendicular to isotherms in Cr# versus Fe2# space. This allows better discrimination of spinel compositional associations and trends. The compositional changes attendant upon down-temperature reequilibration of Cr-spinels also causes problems when comparing Cr-spinels from plutonic rocks with those from volcanic rocks or liquidus-temperature numerical models. A solution to this problem is to recalculate high-temperature spinels to their low-temperature equilibrium compositions before making the comparison. A procedure is outlined allowing higher-temperature spinels to be adjusted down to the 700 °C isotherm.

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