Olivine Transformation in SAH 293: Constraints on Shock Conditions

C. Fudge, J. Hu and T. G. Sharp

ASU/NASA Space Grant

Collision History Improves Understanding of Impacts in the Asteroid Belt

• SAH 293 contains evidence of a large impact event, including high pressure mineral assemblages and shock features– These features provide constraints on P-T

conditions of impact• Collisions are an important geological process in our

solar system. – Virtually every planetary body has experienced

shock as a result of impact processes

Background and Purpose• SAH 293 is an ordinary

chondrite with melt veins and pockets

• Olivine transformed to ringwoodite, wadsleyite.

• Wadsleyite only reported in the Peace River chondrite [Price et al. 1983].

• The purpose of this study is – Classify shock in SAH 293– Use high pressure mineral

assemblage to estimate P-T shock conditions

– Understand why wadsleyite occurs in this sample.

200 µm

Wds

Rw

Methods• Polarized-light microscopy (PLM) and Raman

spectroscopy – Observe deformation effects and melt-vein

mineralogy• Scanning electron microscopy – Characterize melt-vein textures and partial

transformation features in olivine• Electron Microprobe Analysis (EPMA)– Classify chemical composition of SAH 293 olivines

SAH 293 Classification

• Olivines are 25-26% fayalite, – consistent with L or LL

chondritic classification

• Highly shocked (S6)– Shock-induced melt vein

matrix– associated ringwoodite,

wadsleyite and maskelynite

Shock Veins

Shoc

k Ve

in

Rw

Wds

a

b

• High pressure mineral assemblages– constrained within and

along shock melt veins.

• Ringwoodite– high-pressure polymorph

of olivine– primarily occurs along

melt veins and pockets

• Wadsleyite– higher temperature

polymorph of olivine– Entrained within melt

vein matrices

Ringwoodite• Ringwoodite (colorless

to blue)– Raman spectrum =

olivine partially transformed to ringwoodite

– BSE: ringwoodite occurs as lamellae in partially transformed olivine• Some fragments not

subjected to T for complete transformation

b

Rw

a

Rw lamella

e

Wadsleyite

• Wadsleyite (colorless to pale green)– SEM: olivine

completely recrystallized to wadsleyite

– Contrast variation indicate slight variation in Fe content

a

Wds

b

Peri+St

Mg-pv+Peri

Aki+PeriMaj+Peri

RwdWad

Fo

Pressure-Temperature Conditions

• Ringwoodite and wadsleyite– Wadsleyite formed in hotter

regions

• Abundant ringwoodite and rare wadsleyite in L6 chondrites– High shock pressure– Kinetically favorable over

wadsleyite

• SAH 293 parent body shock conditions – 21-23 GPa– 1500-2000 K– Large impact event

L vs. LL Parent Body

• Need further work to better constrain this, results inconclusive

• If SAH 293 is an LL chondrite, mineralogy indicates that the LL parent body experienced a similar impact event as the L parent body

• If SAH 293 is an L/LL chondrite, this signifies that L and LL chondrites originate from a parent body with chemical heterogeneity

Conclusions

• The shock-melt vein and high pressure mineral assemblage in SAH 293 are consistent with highly shocked (S6) classification

• Formation of ringwoodite and wadsleyite reflect temperature heterogeneity during shock– Kinetic effect/high impact pressure explains abundance of

ringwoodite• L vs. LL parent body

– If LL chondrite the parent body could have experienced the same level of shock as the L parent body

– L and LL chondrites originate from identical parent body