Alexander, CMO'D

Bland, PA

Huss, GR

Palme, H

Wasson, JT

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GENETIC RELATIONSHIPS BETWEEN CHONDRULES, RIMS AND MATRIX.  G. R. Huss1, C. M. O’D.Alexander2, H. Palme3, P. A. Bland4 and J. T. Wasson5, 1Department of Geological Sciences, Arizona StateUniversity, Box 871404, Tempe, AZ 85287-1404, USA [gary.huss@asu.edu], 2Department of TerrestrialMagnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, D.C. 20015, USA,3Universität Köln, Institut für Mineralogie und Geochemie, 50674 Köln, Germany, 4Department of Earth Scienceand Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK, 5Institute ofGeophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA.Introduction: The most primitive chondrites arecomposed of chondrules and chondrule fragments,various types of inclusions, discrete mineral grains,metal, sulfides, and fine-grained materials that occur asinterchondrule matrix and as chondrule/inclusion rims.Understanding how these components are related isessential for understanding how chondrites and theirconstituents formed and were processed in the solarnebula.  For example, were the first generations ofchondrules formed by melting of matrix or matrixprecursors?  Did chondrule formation result inappreciable transfer of chondrule material into thematrix?  Here, we consider three types of data: 1)compositional data for bulk chondrites and matrix, 2)mineralogical and textural information, and 3) theabundances and characteristics of presolar materialsthat reside in the matrix and rims.  We use these data toevaluate the roles of evaporation and condensation,chondrule formation, mixing of different nebularcomponents, and secondary processing both in thenebula and on the parent bodies.  Our goal is to identifythe things that are reasonably well established and topoint out the areas that need additional work.Compositional Data:  The bulk compositions ofchondrites exhibit systematic elemental fractionationscompared to CI chondrites, the meteorites that mostclosely represents the bulk composition of the solarsystem.  The most important are: 1) volatility-controlleddepletions of volatile and moderately volatile elements,2) variations in refractory element abundances, perhapssited in CAIs, 3) variations in Mg/Si ratios, and 4)metal-silicate fractionation [1].There are data to suggest that fractionations of thefirst three types occurred in large part prior tochondrule formation [e.g., 2, 3].  Incomplete nebularcondensation has long been considered a primarymechanism for these fractionations [e.g., 1, 4, 5], butdifferential sublimation of the dust inherited from thesun’s parent molecular cloud, which in many ways isindistinguishable from incomplete condensation, mayhave played a substantial role [e.g., 6].  Chondruleformation superimposed additional fractionations.Volatile elements were lost during chondrule melting,although the time scale for chondrule melting was mostlikely too short to permit complete loss [7, 8].  Volatileelements lost from chondrules may have recondensedonto chondrule surfaces [e.g., 3] or in the matrix [8].There is a complimentary relationship in thecompositions of matrix and chondrules in CR and CVchondrites, which suggests that the two componentsoriginated in the same material reservoir [8-10].  Metalmay be largely a by-product of chondrule formation,either due to reduction of FeO during melting andejection of the immiscible metal melt from thechondrule, or through evaporation of iron andrecondensation as metal [e.g., 3, 8].  This implies that atleast some metal-silicate fractionation could have post-dated most chondrule formation.Mineralogy and Petrology of Matrix and Rims:Matrices and fine-grained rims on chondrules and CAIsdiffer significantly in mineralogy among the meteoriteclasses, in large part due to parent-body processing.Aqueous alteration and/or thermal metamorphism hasdestroyed or significantly modified the primarymineralogies of rims and matrix in most chondrites, butin a few, such as Bishunpur (LL3.1) and ALH77307(CO3.0), there is an abundance of amorphous andnanocrystalline material [11, 12].  Amorphous materialis very susceptible to alteration and recrystallization, soits preservation suggests that some of the primarymaterial has survived in these meteorites. Theamorphous material could be chondrule glassfragments, nebular/chondrule condensates or interstellarmaterial accreted directly into the meteorites.Rims are generally thought to have accreted ontochondrules in the nebula, although their low porosity(~10%) remains problematic.  In some instances, itseems that the rims accreted while the host chondruleswere still hot [13].  If so, then at least some fine-graineddust was present during or shortly after chondruleformation, and it may have acted as an important sitefor recondensation of material that evaporated fromchondrules.  However, the majority of rim and matrixmaterial cannot have experienced the high temperaturesof chondrule formation, because it is in them thatpresolar materials (circumstellar grains and interstellarorganics) are found [e.g., 14].Abundances and Characteristics of PresolarGrains:  The known types of presolar materials exhibita broad range of chemical and thermal resistance.Thus, their relative abundances provide a means ofprobing the conditions that they and any accompanyingWorkshop on Chondrites and Protoplanetary Disk (2004) 9078.pdfmaterial experienced, both on the meteorite parentbodies and prior to accretion [6, 14].  There are clearcorrelations between the volatile abundances ofprimitive chondrites and the complex of presolar grainsthat are present in their matrices [6].  The CI chondritesand CM matrices have the highest abundances of mostpresolar components and these components show thewidest range of thermal stability.  The most primitivemembers of the other classes show varying depletionsof labile components in the order CI+CM<OC<CO<CR<CV.  These depletions can best be understood in termsof thermal processing of the mixture of componentsrepresented by CI chondrites and CM matrices [6].Insoluble organic material (IOM) constitutes thebulk of the organic material in chondrites.  The matricesof all chondrite classes seem to have started out withroughly the same CI-like IOM abundances [15].However, unlike for the presolar grains, the mostprimitive IOM is preserved in the CR chondrites [16].The IOM in aqueously altered meteorites (type 1-2)appears to have undergone low temperature oxidation,perhaps associated with the melting of irradiated ices.The IOM in other meteorites of petrologic type ≥3 haveexperienced varying degrees of thermal maturation.Discussion:   In order to successfully evaluatequestions like those posed in the introduction, it isimportant to consider the extent to which parent-bodyprocessing has altered the original record.  In CMchondrites, aqueous alteration has altered both matrixand chondrules, significantly changing the mineralogyand moving elements between them [e.g., 17].  Theoxidized CV chondrites also show considerableevidence of elemental mobilization [e.g., 18].  Even themost primitive UOCs show evidence of exchange ofalkalis between matrix and chondrules [19].  The natureand extent of these processes must be understood beforewe can get a clear picture of the original state of thevarious types of chondrites.  CR chondrites, CO3.0chondrites, and the most primitive UOCs may be thebest samples to work with.Returning to our original questions:  Were the firstgenerations of chondrules formed by melting of matrixor matrix precursors?  Compositional studies indicatethat much of the chemical fractionation that producedthe various classes of chondrites from bulk solar-systemmaterial occurred prior to chondrule formation [e.g., 2,3].  Data for presolar grains in the least metamorphosemembers of each class combined with bulkcompositional data for the host meteorites indicate thatthe precursors of matrix and chondrules experienced thesame nebula processing [6].  And the complementarityof the compositions of matrix and chondrules,particularly in CR chondrites, indicates that theyformed from a common reservoir [8-10].  Theseobservations suggest that the answer may be yes, butthere are problems.  The melting temperature ofchondrules is ~600 K above the evaporationtemperatures of chondritic silicates, and matrix heatedto these temperatures will have experienced extensiveevaporation [20].  Also, the bulk compositions of rimsand matrix in most chondrite classes do not appear to beconsistent with a simple exchange model and the IOM,particularly in CR chondrites, seems to have escapedsignificant nebular heating [13, 16].Did chondrule formation result in appreciabletransfer of chondrule material into the matrix?  Thisquestion is difficult to answer because redistribution ofelements by parent body processes can mimic thistransfer.  However, metal and silicates in CR and OCchondrites make a relatively convincing case thatmaterial evaporated from chondrules during meltingpartially recondensed on fine-grained matrix precursorsand on the surfaces of chondrules [3, 8-10, 13].  Amajor challenge for the future will be to understand thedetails of the processes and material transfers thatgenerated the observed effects.References: [1] Palme H. (2001) Phil. Trans. R.Soc. 359, 2061-2075. [2] Grossman J. N. (1996) InChondrules and the Protoplanetary Disk (Ed. R. H.Hewins, R. H. Jones, and E. R. D. Scott), CambridgeUniv. Press, pp. 243-253. [3] Connolly H. C., Jr., HussG. R. and Wasserburg G. J. (2001) GCA 65, 4567-4588.[4] Larimer J. W. and Anders E. (1967) GCA 31, 1239-1270. [5] Alexander C. M. O’D., Boss A. P. andCarleson R. W. (2001) Science 293, 64-68. [6] Huss G.R., Meshik A. P., Smith J. B. and Hohenberg C. M.(2003) GCA 67, 4823-4848. [7] Yu. Y and Hewins R.H. (1998) GCA 62, 159-172. [8] Kong P. and Palme H.(1999) GCA 63, 3673-3682. [9] Klerner S., Palme H.(1999) MAPS 34, A64-A65. [10] Klerner S. and PalmeH. (1999) LPSC XXX (CD-ROM, #1272. [11]Alexander C. M. O., Barber D. J., and Hutchison R. H.(1989) GCA 53, 3045-3057.  [12] Brearley, A. (1993)GCA 57, 1521-1550.  [13] Alexander C. M. O’D.(1995) GCA 59, 3247-3266.  [14] Huss G. R. and LewisR. S. (1995) GCA 59, 115-160.  [15] Alexander C. M.O’D., Russell S. S., Arden J. W., Ash R. D., Grady M.M. and Pillinger C. T. (1998) MAPS 33, 603-622.  [16]Cody, G. D., and Alexander C. M. O’D. (2004) GCA,in press. [17] McSween H. Y., Jr. (1979) GCA 43,1761-1770. [18] Krot A. N., Scott E. R. D., andZolensky M. E. (1995) Meteoritics 30, 748-775. [19]Grossman J. N. (2004) MAPS, in press. [20] Wasson J.T. and Trigo-Rodriguez J. M. (2004) LPS XXXV,abstract #2140.Supported by NASA grants NAG5-11543 (GRH),NAG5-12887 (JTW), NAG5-13040 (CMO’DA).Workshop on Chondrites and Protoplanetary Disk (2004) 9078.pdf

Genetic relationships between chondrules, rims and matrix.

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