Abstracts of Papers to be Published in the January 2002 Issue

Please contact the correspondence author for reprints of all published articles

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Nitrogen and argon release profiles in Luna 16 and Luna 24 regolith samples:  The effects of regolith reworking

S. S. Assonov*, I. A. Franchi, C. T. Pillinger, A. S. Semenova, Yu. A. Shukolyukov, A. B. Verchovsky and A. N. Iassevitch

*Correspondence author's address:  Max Planck Institute für Chemie, J.J.Becher Weg 27, 55128, Mainz, Germany, author's e-mail:  assonov@mpch-mainz.mpg.de

Abstract–Fines, microbreccias and agglutinates from the Luna 16 mature regolith 1635 and fines from the immature/submature Luna-24 regolith have been analysed for N and argon isotopes in order to understand the origin of isotopically distinct N released at different temperatures.  All high-resolution runs reveal a similarity in the release of ³⁶Ar, ⁴⁰Ar and N over a wide temperature interval.  The similarity in the ⁴⁰Ar and ³⁶Ar releases and the near coincidence in the 1635 agglutinates implies that the implanted species were redistributed and homogenised during regolith processing such that, regardless of the huge difference in ion implantation energy between solar ³⁶Ar and non-solar ⁴⁰Ar, their present distribution and their release temperatures are now essentially equal.  A small amount of ⁴⁰Ar released in the lower temperature steps with elevated ⁴⁰Ar/³⁶Ar is considered to be trapped after reworking.

While such mixing and homogenisation may also be expected for N components of different origins, to date all known stepped runs regularly demonstrate a reproducible variation in ¹⁵N, suggesting no homogenisation.  We consider regolith N to be a mixture of several components trapped at different times, and some nitrogen that was not involved in the reworking.  Relatively heavy N released around 500 °C appears to be the most pure form of the component trapped after reworking, probably from accreted meteoritic matter.  Middle temperature isotopically lighter N appears to be a mixture of solar and non-solar N largely homogenised, and therefore solar N can not be seen in its pure form.  Bulk ¹⁵N as well as formally deconvoluted ¹⁵N thermal profiles imply that the non-solar N has a variable ¹⁵N value.  Several non-solar N sources are considered with their input resulting in increasing regolith ¹⁵N with time.  Because N from meteorites and IDPs appear to be dominant, a mechanism is required to reduce the C/N ratio typical of meteoritic matter to that approaching the low value observed in the lunar regolith.  Preferential loss of methane appears to be a viable explanation, following generation either by proton sputtering or in reducing vapour plumes.

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Zoned chondrules in Semarkona: Evidence for high- and low-temperature processing

Jeffrey N. Grossman, Conel M. O'D. Alexander, Jianhua Wang and Adrian J. Brearley

*Correspondence author's address:  US Geological Survey, 954 National Center, Reston, Virginia 20192, USA; author's e-mail address:  jgrossman@usgs.gov

Abstract–At least 15% of the low-FeO chondrules in Semarkona (LL3.0) have mesostases that are concentrically zoned in Na, with enrichments near the outer margins.  We have studied zoned chondrules using electron microprobe methods (x-ray mapping plus quantitative analysis), ion microprobe analysis for trace elements and hydrogen isotopes, cathodoluminescence imaging, and transmission electron microscopy in order to determine what these objects can tell us about the environment in which chondrules formed and evolved.

Mesostases in these chondrules are strongly zoned in all moderately volatile elements and H (interpreted as water).  Calcium is depleted in areas of volatile enrichment.  Titanium and Cr generally decrease toward the chondrule surfaces, whereas Al and Si may either increase or decrease, generally in opposite directions to one another;  Mn follows Na in some chondrules but not in others;  Fe and Mg are unzoned.  D/H ratios increase in the water-rich areas of zoned chondrules.  Mesostasis shows cathodoluminescence zoning in most zoned chondrules, with the brightest yellow color near the outside.  Mesostasis in zoned chondrules appears to be glassy, with no evidence for devitrification.

Systematic variations in zoning patterns among pyroxene- and olivine-rich chondrules may indicate that fractionation of low- and high-Ca pyroxene played some role in Ti, Cr, Mn, Si, Al, and some Ca zoning.  But direct condensation of elements into hot chondrules, secondary melting of late condensates into the outer portions of chondrules, and subsolidus diffusion of elements into warm chondrules cannot account for the sub-parallel zoning profiles of many elements, the presence of H₂O, or elemental abundance patterns.

Zoning of moderately volatile elements and Ca may have been produced by hydration of chondrule glass without devitrification during aqueous alteration on the parent asteroid.  This could have induced structural changes in the glass allowing rapid diffusion and exchange of elements between altered glass and surrounding matrix and rim material.  Calcium was mainly lost during this process, and other nonvolatile elements may have been mobile as well. Some unzoned, low-FeO chondrules appear to have fully altered mesostasis.

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Leonard Medal Address:  The rocks of Mars, from far and near
 
Harry Y. McSween, Jr.

Author's address:  Department of Geological Sciences, University of Tennessee,
Knoxville, Tennessee 37996-1410, USA; e-mail address:  mcsween@utk.edu

Abstract–The age, structure, composition, and petrogenesis of the martian lithosphere have been constrained by spacecraft imagery and remote-sensing.  How well do martian meteorites conform to expectations derived from this geologic context?  Both data sets indicate a thick, extensive igneous crust formed very early in the planet's history.  The composition of the ancient crust is predominantly basaltic, possibly andesitic in part, with sediments derived from volcanic rocks.  Later plume eruptions produced igneous centers like Tharsis, the composition of which cannot be determined because of spectral obscuration by dust.  Martian meteorites (except ALH84001) are inferred to have come from volcanic flows in Tharsis or Elysium, and thus are not petrologically representative of most of the martian surface.  Remote-sensing measurements cannot verify the fractional crystallization and assimilation that have been documented in meteorites, but subsurface magmatic processes are consistent with orbital imagery indicating thick crust and large, complex magma chambers beneath Tharsis volcanoes.  Meteorite ejection ages are difficult to reconcile with plausible impact histories for Mars, and oversampling of young terrains suggests either that only coherent igneous rocks can survive the ejection process or that older surfaces cannot transmit the required shock waves.  The mean density and moment of inertia calculated from spacecraft data are roughly consistent with the proportions and compositions of mantle and core estimated from martian meteorites.  Thermal models predicting the absence of crustal recycling, and the chronology of the planetary magnetic field agree with conclusions from radiogenic isotopes and paleomagnetism in martian meteorites.  However, lack of vigorous mantle convection, as inferred from meteorite geochemistry, seems inconsistent with their derivation from the Tharsis or Elysium plumes.  Geological and meteoritic data provide conflicting information on the planet's volatile inventory and degassing history, but are apparently being reconciled in favor of a periodically wet Mars.  Spacecraft measurements suggesting that rocks have been chemically weathered and have interacted with recycled saline groundwater are confirmed by weathering products and stable isotope fractionations in martian meteorites.

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

A critical evaluation of oxidation versus reduction during metamorphism of L and LL group chondrites,and implications for asteroid spectroscopy

Heather K. Gastineau-Lyons, Harry Y. McSween, Jr.* and Michael J. Gaffey

*Correspondence author's address:  Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410, USA; e-mail address:  mcsween@utk.edu

Abstract–Modal mineralogies of individual, equilibrated (petrologic type 4-6) L and LL chondrites have been measured using an electron microprobe mapping technique, and the chemical compositions of coexisting silicate minerals have been analyzed.  Progressive changes in the relative abundances and in the molar Fe/Mn and Fe/Mg ratios of olivine, low-Ca pyroxene, and diopside occur with increasing metamorphic grade.  Variations in olivine/low-Ca pyroxene ratios (Ol/Px) and in metal abundances and compositions with petrologic type support the hypothesis that oxidation of metallic iron accompanied thermal metamorphism in ordinary chondrites.  Modal Ol/Px ratios are systematically lower than normative Ol/Px ratios for the same meteorites, suggesting that the commonly used C.I.P.W. norm calculation procedure may not adequately estimate silicate mineral abundances in reduced chondrites.  Ol/Px ratios calculated from VISNIR reflectance spectra of the same meteorites are not in agreement with other Ol/Px determinations, possibly because of spectral complexities arising from other minerals in chondrites.  Characteristic features in VISNIR spectra are sensitive to the proportions and compositions of olivine and pyroxenes, the minerals most affected by oxidative metamorphism.  This work may allow spectral calibration for the determination of mineralogy and petrologic type, and thus may be useful for spectroscopic studies of asteroids.
 

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Anorthite-rich chondrules in CR and CH carbonaceous chondrites: Genetic link between Ca,Al-rich inclusions and ferromagnesian chondrules

Alexander N. Krot* and Klaus Keil

*Correspondence author's address:  Hawai'i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, Honolulu, Hawai'i 96822, USA; e-mail address: sasha@higp.hawaii.edu

Abstract–Anorthite-rich chondrules in CR and CH carbonaceous chondrites consist of magnesian low-Ca pyroxene and forsterite phenocrysts, FeNi-metal nodules, interstitial anorthite, Al-Ti-Cr-rich low-Ca and high-Ca pyroxenes, and crystalline mesostasis composed of silica, anorthite and high-Ca pyroxene.  Three anorthite-rich chondrules contain relic Ca, Al-rich inclusions composed of anorthite, spinel, ±Al-diopside, and ±forsterite.  A few chondrules contain regions which are texturally and mineralogically similar to magnesian (Type I) chondrules and consist of forsterite, low-Ca pyroxene and abundant FeNi-metal nodules.  Anorthite-rich chondrules in CR and CH chondrites are mineralogically similar to those in CV and CO carbonaceous chondrites, but contain no secondary nepheline, sodalite or ferrosilite.  Relatively high abundances of moderately-volatile elements such as Cr, Mn and Si in the anorthite-rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only. We infer instead that anorthite-rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors (olivine, pyroxenes, FeNi-metal) mixed with the refractory materials, including relic CAIs, composed of anorthite, spinel, high-Ca pyroxene and forsterite.  The observed mineralogical and textural similarities of the anorthite-rich chondrules in several carbonaceous chondrite groups (CV, CO, CH, CR) may indicate that these chondrules formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated.  This may explain the relative enrichment of anorthite-rich chondrules in ¹⁶O compared to typical ferromagnesian chondrules (Russell et al., 2000).

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Major element fractionation in chondrites by distillation in the accretion disk of a T Tauri Sun?

Robert Hutchison

Author's address:  Mineralogy Department, The Natural History Museum, London SW7 5BD, United Kingdom; e-mail address: robh@nhm.ac.uk

Abstract–Redistribution or loss of batches of condensate from a cooling protosolar nebula is generally thought to have led to the formation of the chemical groups of chondrites.  This demands a nebula hot enough for silicate vaporization over 1–3 AU, the region where chondrites formed.  Alternatively, heating of a protosolar accretion disk may have been confined to an annular zone at its inner edge, about 0.06 AU from the protosun.  Most infalling matter was accreted by the protosun, but a proportion was heated and carried outwards by an x-wind.  Shu et al. (1996; 1997) proposed that larger objects such as chondrules and Ca-, Al-rich inclusions (CAIs) were returned to the disk at asteroidal distances by sedimentation from the x-wind.  Fine dust and gas were lost to space.  The model implies that solids were not lost from the cold disk.  The chemical compositions of the chondrite groups were produced by mixing different proportions of CAIs and chondrules with disk solids of CI composition.  Heating at the inner edge of the disk was accompanied by particle irradiation, which synthesized nuclides including ²⁶Al.

The x-wind model can produce CAIs, not chondrules, and allows survival of presolar grains >0.06 AU from the protosun.  Normalization to Al and CI indicates that non-carbonaceous chondrites (CCs) may be disk material that gained a Si- and Mg-enriched fraction.  CCs are different; they appear to be CI that lost lithophile elements more volatile than Ca.  Five CC groups also lost Ni and Fe but the CH group gained siderophiles.  Elemental loss from CI is incompatible with the x-wind model.  Silicon and CI normalization confirms that the CM, CO, CK and CV groups may be CI that gained refractories as CAIs.  The Si-, Mg-rich fraction may have formed by selective vaporization followed by precipitation on grains in the x-wind.  This fractional distillation mechanism can account for lithophile element abundances in non-CC groups, but an additional process is required for the loss of Ca and Mn in the EL group and for fractionated siderophile abundances in the H, L and LL groups.  Heated and recycled fractions were not homogenized across the disk so the chondrite groups were established in a single cycle of enhanced protosolar activity in <10⁴ yr, the time for a mm-sized particle to drift into the Sun from 2–3 AU, due to gas-drag.

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

The halite-bearing Zag and Monahans (1998) meteorite breccias: Shock metamorphism, thermal metamorphism and aqueous alteration on the H-chondrite parent body

Alan E. Rubin*, Michael E. Zolensky and Robert J. Bodnar

*Correspondence author's address:  Institute of Geophysics and Planetary Physics University of California, Los Angeles, California 90095-1567, USA; e-mail address: aerubin@ucla.edu

Abstract–Zag and Monahans (1998) are H-chondrite regolith breccias comprised mainly of light-colored metamorphosed clasts, dark clasts that exhibit extensive silicate darkening, and a halite-bearing clastic matrix.  These meteorites reflect a complex set of modification processes that occurred on the H-chondrite parent body.   The light-colored clasts are thermally metamorphosed H5 and H6 rocks that were fragmented and deposited in the regolith.  The dark clasts formed from light-colored clasts during shock events that melted and mobilized a significant fraction of their metallic Fe-Ni and troilite grains.  The clastic matrices of these meteorites are rich in solar-wind gases.  Parent-body water was required to cause leaching of chondritic minerals and chondrule glass; the fluids became enriched in Na, K, Cl, Br, Al, Ca, Mg and Fe.  Evaporation of the fluids caused them to become brines as halides and alkalies became supersaturated; grains of halite (and, in the case of Monahans (1998), halite with sylvite inclusions) precipitated at low temperatures ( <100 °C) in the porous regolith.  In both meteorites fluid inclusions were trapped inside the halite crystals.  Primary fluid inclusions were trapped in the growing crystals; secondary inclusions formed subsequently from fluid trapped within healed fractures.

 

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