The formation of MORB vs Ophiolites

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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

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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?

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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

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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

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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 lt15 partial melting

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EFFECT OF VOLATILES

• H2O CO2 marked influence on peridotite melting
• MORB virtually anhydrous
• Hydrous melting causes more silicious rocks

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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

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• 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.

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FORMATION OF OPHIOLITES

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

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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

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THE END
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MAIN FIELDS DETERMINED FOR PERIDOTITES
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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
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CHEMICAL COMPOSITION OF PERIDOTITES