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

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?

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 EXPERIMENT

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 ZONING OF MANTLE

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 SiO 2 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 RESULTS OF MELTING

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

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 MORB VS OPHIOLITE

BUT: MORB = high Alumina Olivine Tholeiite  Shallow segregation and melting is impossible MORB segregates at 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.

Magma with high SiO 2 and high CaO/Na 2 O ratio Second stage melting of refractory peridotite diapir at shallow depth FORMATION OF OPHIOLITES

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 CONCLUSION

THE END

MAIN FIELDS DETERMINED FOR PERIDOTITES

Layer 3 more complex and controversial Believed 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

CHEMICAL COMPOSITION OF PERIDOTITES