A SYSTEMATIC CLASSIFICATION OF METEORITES
PART VI

PART I: CHONDRITES, METACHONDRITES
PART II: PRIMITIVE ACHONDRITES, ACHONDRITES, STONY-IRONS, IRONS
PART III: MARTIAN METEORITES—GEOCHEMICAL CLASSIFICATION
PART IV: DIOGENITES—IUGS TAXONOMY
PART V: ENSTATITE (E) CHONDRITES—SUBGROUPS

STONY METEORITES—METAMORPHIC FACIES SERIES

An improved classification method by Tomkins et al., MAPS, Early View (2020)
'A review of the chondrite–achondrite transition, and a metamorphic facies series for equilibrated primitive stony meteorites'

To gain a better understanding of the chondrite–achondrite transition, Tomkins et al. (2020) conducted a study of numerous meteorites including equilibrated chondrites classified as types 6 and 7, both grouped and ungrouped primitive achondrites, and both aubrites and ungrouped enstatite achondrites, while employing optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. They advocate for a new set of criteria for determining the dividing point between chondrites and (primitive) achondrites. This division should be based on the onset of silicate melting which is more accurately constrained compared to the more petrographically ambiguous onset of FeNi–FeS melting commonly used as a means today. Tomkins et al. (2020) identified minerals that are present at different metamorphic stages and assessed other petrographic characteristics to derive an equation by which the initial melting temperature relative to each meteorite can be calculated: Tm = 0.035Fa2–3.51Fa + 1109 (in °C, where Fa is the proportion of fayalite in olivine). They also presented a new metamorphic facies series representing three distinct metamorphic stages within the current classification scheme as follows (also see composition vs. temperature diagram below):

Unequilibrated Meteorites: equivalent to petrologic type 3–4; exhibiting only minimal or no equilibration under sustained low temperature conditions (500–700°C)

1. Plagioclase Facies: equivalent to petrologic type 5–6; exhibiting extensive chemical equilibration by diffusion at subsolidus temperatures; plagioclase attains larger grain-sizes and can form 120° triple junctions with other grains

2. Sub-Calcic Augite Facies: equivalent to petrologic type 7/primitive achondrites; exhibiting incipient silicate melting (initiation temperature dependent on oxidation state) inducing low-Ca exsolution lamellae in augite; intergranular plagioclase wetting evolves towards interconnected plagioclase networks

3. Pigeonite Facies: high-temperature conditions marking the appearance of pigeonite; thermally evolving towards inverted pigeonite, extraction of metallic and silicate melts, and elemental fractionation

Metamorphic Facies Diagram for Equilibrated Stony Meteorites
standby for temp-comp facies diagram
click on image for a magnified view

Diagram credit: Tomkins et al., MAPS, Early View (2020)
'A review of the chondrite–achondrite transition, and a metamorphic facies series for equilibrated primitive stony meteorites'
(https://doi.org/10.1111/maps.13472)

Tomkins et al. (2020) contend that both decay of short-lived radionuclides like 26Al and severe impact bombardment were occurring at the same time in the early Solar System. Therefore, they propose that the transition from chondrite to (primitive) achondrite should be defined on the basis of the highest temperature attained throughout a sample rather than making a distinction based on whether thermal metamorphism involved radiogenic heating followed by slow cooling on the one hand, and rapid impact-generated melting followed by slow cooling on the other. By adherence to this methodology the classificational terms 'type 7' and 'primitive achondrite' would become inclusive or synonymous terms. The following table brings some visual clarity to this issue:

standby for meteorite classification table
click on image for a magnified view

Diagram credit: A. Tomkins (2020)
See related paper in Tomkins et al., MAPS, Early View (2020)
'A review of the chondrite–achondrite transition, and a metamorphic facies series for equilibrated primitive stony meteorites'
(https://doi.org/10.1111/maps.13472)


PART I: CHONDRITES, METACHONDRITES
PART II: PRIMITIVE ACHONDRITES, ACHONDRITES, STONY-IRONS, IRONS
PART III: MARTIAN METEORITES—GEOCHEMICAL CLASSIFICATION
PART IV: DIOGENITES—IUGS TAXONOMY
PART V: ENSTATITE (E) CHONDRITES—SUBGROUPS


You must collect things for reasons you don't yet understand.
Daniel J. Boorstin – Librarian of Congress


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