The formation of MORB vs Ophiolites

1. The formation of MORB vs Ophiolites Anhydrous Melting of Peridotite at 0-15 Kb Pressure and the Genesis of Tholeiitic BasaltsA.L. Jaques and D.H. Green Anneen Burger

2. INTRODUCTION • Basaltic magmas from peridotitic upper mantle • P,T, volatile content influence type of Basalt formed • Direct melting studies necessary to determine where tholeiitic Basalts form. • What is a tholeiite?

3. EXPERIMENT • Two peridotites melted at 0-15 kBar. (Anhydrous) Results are broadly tholeiitic • Two peridotites similar in major elements, but two end members of compositional spectrum with regard to incompatible elements • Pyrolite – enriched mantle-oceanic island volcanism • Tinaquillo peridotite – depleted mantle - MORB

4. ZONING OF MANTLE • Mantle shows areas of enrichment and depletion of incompatible elements • There is evidence of chemical zoning in the mantle • Lower part of lower velocity zone (LVS) depleted; upper part of LVZ enriched due to upward migration of small melt fractions • Pyrolite – upper zone • Tinaquillo peridotite –lower zone

5. RESULTS OF MELTING • Melting increases rapidly above solidus but then settles and increases linearly with temperature • Three main stability fields determined for both peridotites from nearest the solidus: • Ol + Opx + Cpx + Cr – Spinel + L • Ol + Opx + Cr-Spinel + L • Ol + Cr – Spinel + L • Possible to have aluminous phase near solidus (Plag) • Melts at low pressure are generally SiO2 oversaturated but become Olivine normative at high pressure • Tholeiites derived from relatively large degree of partial melting (20-30%) • Alkali basalts from <15% partial melting

6. EFFECT OF VOLATILES • H2O + CO2 marked influence on peridotite melting • MORB virtually anhydrous • Hydrous melting causes more silicious rocks

7. MORB VS OPHIOLITE • Popular models equate Ophiolites with present day MORB = COGENETIC • Oceanic and Ophiolite layer 3 regarded as accumulation of phases involved in low pressure crystal fractionation of overlaying basaltic lavas in a magma chamber below an axial zone of crystal dilation • Because of the ophiolite model it is inferred that magma melting and segregation happened at shallow depth

8. BUT: MORB = high Alumina Olivine Tholeiite • Shallow segregation and melting is impossible • MORB segregates at 60-70 km depth • Major discrepancy in nature of near liquidus phases for Olivine tholeiites and cumulate sequences in ophiolites • MORB not able to yield cumulate sequence with much Mg Opx and much calcic plag as in a number of ophiolites.

9. FORMATION OF OPHIOLITES • Magma with high SiO2 and high CaO/Na2O ratio • Second stage melting of refractory peridotite diapir at shallow depth

10. CONCLUSION • Mantle is chemically diverse • Amount of partial melting of the source along with PT conditions determines which basalt forms • Presence of volatiles also influences composition of basalts • MORB can not be formed under the same conditions that ophiolites form under

11. THE END

12. MAIN FIELDS DETERMINED FOR PERIDOTITES

13. Layer 3 more complex and controversialBelieved to be mostly gabbros, crystallized from a shallow axial magma chamber (feeds the dikes and basalts) Layer 3A= upper isotropic and lower, somewhat foliated (“transitional”) gabbros Layer 3B is more layered, & may exhibit cumulate textures

14. CHEMICAL COMPOSITION OF PERIDOTITES