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 Shock Vein 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 lamellae

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+Peri Maj+Peri Rwd Wad 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 – GPa – 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