Olivine Transformation in SAH 293: Constraints on Shock Conditions

1. Olivine Transformation in SAH 293: Constraints on Shock Conditions C. Fudge, J. Hu and T. G. Sharp ASU/NASA Space Grant

2. 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

3. 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 inSAH 293 • Use high pressure mineral assemblage to estimate P-T shock conditions • Understand why wadsleyite occurs in this sample. Wds Rw 200 µm

4. 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

5. SAH 293 Classification • Olivines are 25-26% fayalite, • consistent with L or LL chondriticclassification • Highly shocked (S6) • Shock-induced melt vein matrix • associated ringwoodite, wadsleyite and maskelynite

6. Shock Veins • 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 a Rw b Shock Vein Wds

7. Ringwoodite a • 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 Rw b Rw lamellae

8. Wadsleyite a • Wadsleyite (colorless to pale green) • SEM: olivine completely recrystallized to wadsleyite • Contrast variation indicate slight variation in Fe content Wds b

9. 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 Mg-pv+Peri Peri+St Aki+Peri Maj+Peri Rwd Wad Fo

10. 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

11. 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