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On the validity of paleogeotherms



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American Mineralogist Volume 61 pages 638442 1976 On the validityof paleogeotherms determined from xenolithsuitesin basaltsandkimberlites ANrsoNv J InvrNc Departmentof Earth and PlanetarySciences Massachusetts Institute of Technology Cambridge Massachusetts 02 1391 Abstract Considerable uncertaintyattendsthe conceptof determiningpaleogeotherms from xenolith suitesin alkaline basaltsand kimberlites Until further experimentalstudiesare made on complexsystems it is not possibleto specifypressures and temperatures of equilibrationfor thesenatural sampleswith sufficientprecisionto make suchinterpretations Furthermore it can be argued that xenolith suitescannot be used to define valid geothermsbecausethe processes of magmagenerationand xenolithformationinvolveperturbationsof the very geothermswe seekto measure Introduction The conceptof deducingpaleogeotherms by applicationof mineralequilibriato suitesof maficto ultramafic xenolithsin alkalinebasaltsand kimberlites e g Boyd 1973 MacGregor 1974 MacGregorand Basu 1974 Mercierand Carter 1975 hasa number of attendantuncertainties The purposeof thisnoteis to point out theseuncertainties which haveto some extent been overlooked in most treatmentsof the subject The two main themesof this paperare l that we are far from a situation of confidencein pressure temperature estimatesfor individual natural samplesbasedon simplesystem equilibriaand 2 that we cannotrely on xenolithsuitesto define valid geothermsbecausethe processes of magma generationand xenolithformationinvolveperturbations of the very geothermswe seekto measure Uncertaintiesin P 7 estimates for naturalsamples The major element chemicalcompositionsof the constituentphasesof mafic to ultramaficxenoliths are determinedby many variables but the most importantare pressure temperature andbulk chemical phase equilibrium composition Experimental studies of simpleand multicomponent systemshavedemonstrated that for a given bulk chemicalcomposition I Presentaddress The Lunar



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