Principles of Thin Film X-Ray Microanalysis

(This paper is dedicated to the memory of Paul Pellas) ) is a ubiquitous component in the matrix of Krymka (LL3.1) as well as in other highly unequilibrated chondrites (ordinary and carbonaceous). In Krymka, the fayalitic olivine has an unusual anisotropic platy morphology that occurs in at least five types of textural settings that can be characterized as: (1) isolated platelets, (2) clusters of platelets, (3) euhedral to subhedral crystals, (4) overgrowths of platelets on forsteritic olivine, and (5) fluffy (porous) aggregates. From transmission electron microscope (TEM) investigation, the direction of elongation of the platy olivine overgrowths on forsteritic olivine substrates is along the c axis and in most cases it corresponds with the c axis of the substrate olivine, which suggests that the fayalitic olivine grew in this unusual morphology and is not a replacement product of preexisting material. The fayalitic olivine in the matrix of Krymka is compositionally similar to olivine with platy morphology in the matrix of some CV3 chondrites and both have similar Fe/Mn ratios, but important morphological differences indicate that their relationship needs to be explored further. Textural and compositional data indicate that the fayalitic olivine in the matrix of Krymka, as well as in some other unequilibrated ordinary chondrites, formed prior to final lithification of the meteorite and probably prior to parent body accretion. We find that formation of the fayalitic olivine by vapor-solid growth provides the best explanation for our observations and data and is the only feasible mechanism for the formation of fayalitic olivine in the matrix of Krymka. We propose that the fayalitic olivine formed by vaporization and recondensation of olivine rich-dust, during a period of enhanced dust/gas ratio in the nebula.

Section: Methodsmentioning

confidence: 99%

Fayalitic olivine in matrix of the Krymka LL3.1 chondrite: Vapor‐solid growth in the solar nebula

Weisberg¹,

Zolensky²,

Prinz³

1997

“…In the final method a fragment of Tagish Lake embedded in EPON8l2 epoxy was vacuum impregnated at 60°C for 1 h, and then polymerized at 60°C for 48 h. After sectioning by ultramicrotomy, thin films were collected on Cu grid coated with collodion and carbon. We observed the samples using a JEOL 2000FX scanning transmission electron microscope (STEM) equipped with a LINK energy dispersive x-ray spectrometer (EDX) analysis system at JSC, and a JEOL 2010 with a Kevex Sigma system EDX at the University of Tokyo, both operating at 200 kV We used natural mineral standards and in-house determined k-factors for reduction of compositional data; a Cliff-Lorimer thin-film correction procedure was employed (Goldstein, 1979). The TEM-EDX analyses are precise to within 1-3% relative.…”

Section: Sample Preparation and Analytical Proceduresmentioning

confidence: 99%

Mineralogy of Tagish Lake: An ungrouped type 2 carbonaceous chondrite

Zolensky

Nakamura

Gounelle

et al. 2002

222

319

Abstract-In this paper we describe the recovery, handling and preliminary mineralogical investigation ofthe Tagish Lake meteorite. Tagish Lake is a type 2 carbonaceous chondrite which bears similarities to CIl and CM chondrite groups, but is distinct from both. Abundant phyllosilicates as well as chondrules (however sparse) and common olivine grains in the matrix preclude any other classification.The bulk density of Tagish Lake (1.67 glee) is far lower than CI or CM chondrites (2.2-2.3 and 2.6-2.9 glee, respectively), or any other meteorite for that matter. We have identified two lithologies: a dominant carbonate-poor lithology and a less-abundant carbonate-rich lithology. The meteorite is a breccia at all scales.We have noted similarities between Tagish Lake and some clasts within the enigmatic meteorite Kaidun; possibly there are genetic relationships here worth exploring. In the paper we describe a clast ofCMl material within Tagish Lake which is very similar to a major lithology in Kaidun.

“…We used natural mineral standards and in-house determined k-factors for reduction of compositional data; a Cliff-Lorimer thinfilm correction procedure was employed (Goldstein, 1979). In the case of phyllosilicates, mineral identifications were made on the basis of both composition and electron diffraction data whenever possible.…”

Section: Samples and Analytical Proceduresmentioning

confidence: 99%

Secondary calcium‐iron‐rich minerals in the Bali‐like and Allende‐like oxidized CV3 chondrites and Allende dark inclusions

Krot

Petaev

Zolensky

et al. 1998

Abstract-We have characterized Ca-Fe-rich silicates (salite-hedenbergite pyroxenes ( F s~o -~o W O~~-~O ) , andradite (Ca3Fe2Si3012), kirschsteinite (CaFeSiOd), and wollastonite (Ca3Si309)) in the type I chondrules and matrices in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions. In type I chondrules in the Bali-like CV3 chondrites, metal is oxidized to magnetite; magnetite-sulfide nodules are replaced by Ca-Fe-rich pyroxenes with minor andradite and pure fayalite. We infer that Ca-Fe-rich pyroxenes, andradite, fayalite, magnetite, and phyllosilicates (which occur in mesostases) formed at relatively low temperatures (<300 "C) in the presence of aqueous solutions. Thermodynamic analysis of phase relations in the Si-Fe4a-0-H system and large 0 isotopic fractionation of the coexisting magnetite and fayalite (-20%0) (Krot et at., 1998) are consistent with this interpretation.In type I chondrules in the Allende-like CV3 chondrites and dark inclusions, magnetite-sulfide nodules are replaced by Ca-Fe-rich pyroxenes and ferrous olivine; iow-Ca pyroxene and forsterite phenocrysts are rimmed and veined by ferrous olivine. It appear that the Ca-Fe-rich pyroxenes predate formation of ferrous olivine; the latter postdates formation of talc and biopyriboles (Brearley, 1997). The Allende dark inclusions are crosscut by Ca-Fe-pyroxene-andradite veins and surrounded by Ca-rich rims that consist of Ca-Fe-rich pyroxenes, andradite, wollastonite, and kirschsteinite. Calcium-rich veins and rims formed after aggregation and lithification of the dark inclusions. The rimmed dark inclusions show zoned depletion in Ca, which is due to a lower abundance of Ca-Fe-rich pyroxenes close to the rim. Calcium was probably leached from the inclusions and redeposited along their edges. We infer that the Allende-like chondrites and dark inclusions experienced similar aqueous alteration to the Bali-like chondrites and were metamorphosed subsequently, which resulted in loss of aqueous solutions and dehydration of phyllosilicates.We conclude that Ca-Fe-rich silicates in the oxidized CV3 chondrites and Allende dark inclusions are secondary and resulted from aqueous fluid-rock interactions during progressive metamorphism of a heterogeneous mixture of hydrous (ices?) and anhydrous materials; the latter were possibly mineralogically similar to the reduced CV3 chondrites. IN T R 0 DUCT I 0 NOn the basis of the opaque mineralogy, McSween (1977) divided CV3 chondrites into reduced, with Ni-poor metal and troilite, and oxidized, with magnetite, Ni-and Co-rich metal and Ni-rich sulfides, subgroups. In our review paper (Krot et al., 1995), we showed that Ca-Al-rich inclusions (CAls), chondrules and matrices in the oxidized subgroup experienced alteration to various degrees, whereas all components in the reduced subgroup largely escaped it. The alteration features observed in the oxidized CV3 chondrites include: (a) ferrous olivine rims (Fa35-50) around isolated forsterite grains and forsterite phenocrysts in chondrules; (b) fer...