Igneous - Tholeiitic vs Alkaline

Question 1

Mantle summary

Answer 1

• Upper-most 200 km of mantle is lherzolite
  (Ol-Opx-Cpx + an aluminous phase).
• MORB melting occurs by decompression; chemistry controlled by
  1. Fractional crystallisation, depends on the phases present on the solidus
  2. Extent of melting, controlled by mantle potential temperature
  3. Mantle heterogeneity, revealed by radiogenic isotopes
• Rb-Sr and Sm-Nd isotopes indicate that the MORB source in the mantle has experienced long-term depletion of incompatible elements
  Mantle has unradiogenic Rb ratio

Question 2

OIBs summary

Answer 2

• Persistent, stationary volcanism over 107 to 108 years.
• Basalts of variable composition, generally smaller degree, deeper melts than MORB.
• Variable trace element and isotopic composition, with OIBs derived from both an incompatible element depleted (like MORB) and enriched source.
• Minor part of oceanic crust – arise from deep mantle showing detail on that part of the mantle

Question 3

Two types of OIB Magmas

Answer 3

• Tholeiitic series (dominant type)
o Parental ocean island tholeiitic basalt
o Similar to MORB
o e.g. Hawaii, Réunion
o Forms at high pressure and with higher melt fractions
o Highly incompatible  Smaller degree of melting 

Distinctive feature of OIB
• Alkaline series (subordinate)
o Parental ocean island alkaline basalt
o e.g. Tahiti, Canary Islands, the Azores
o Forms only at high pressure and with small melt fractions

Question 4

OIB Melting problem:

Answer 4

• Deeper melting, so higher temperature and would expect more melting than MORB but there is actually less

Possible explanations:
• Limited upwelling (oceanic crust limits this)
• Limited material to melt

Question 5

Tholeiitic basalt crystallisation

Answer 5

o Will crystallize OPX and with extensive fractional crystallization and will eventually crystallize quartz
 Cool it  Olivine crystalizes  Orthopyroxene  Ortho + Clinopyroxene  Quartz
Dominant basalt is Tholeiitic as its made through greater degrees of melting

Question 6

Alkali basalt crystallisation

Answer 6

o Will not crystallize OPX or Quartz
 Olivine  Olivine + Clinopyroxene
o Alkali basalt is formed at high pressure

Question 7

Trace elements during melting

Answer 7

• Highly incompatible elements are greatly concentrated in the initial small fraction of melt that is produced by partial melting, and subsequently get diluted as F increases
• D=0.001 – Low melt fractions – highly concentrated in melt

Question 8

Mantle Reservoirs:

Answer 8

1. BSE (Bulk Silicate Earth)/Primary Uniform Reservoir (Chondritic Composition)
   a. Composition of Earth’s mantle before it started melting
2. DM (Depleted Mantle)
   a. Mantle – MORB source
   b. Mantle which has been melted to make MORB
   c. Long term depletion
3. EMI (enriched mantle type 1)
   a. Lower 87SR/86SR
   b. - EMI (slightly enriched) thought to correspond with lower continental crust
4. EMII (Enriched Mantle Type 2)
   a. Higher 87SR/86SR
   b. Well above any reasonable mantle sources
   c. More enriched, especially in radiogenic Sr (indicating enriched in Rb parent) corresponds with the upper continental crust
5. Both - Radiogenic 87Sr/86Sr requires high Rb/Sr & long time to decay 87Sr + Unradiogenic 143Nd/144Nd due to low Sm/Nd
6. PREAM (PREvalent Mantle)
   a. FOZO
   b. Where trends converge
7. HIMU
   a. High U/Pb = high m

Question 9

Variation in composition of continental crust:

Answer 9

• Lower enriched in mafic rock types

* Upper enriched in silica

Question 10

FOZO:

Answer 10

• Another “convergence” reservoir toward which many trends approach

Question 11

Pb isotopes:

Answer 11

• U, Pb, and Th are concentrated in both oceanic and continental crust (U & Th highly incompatible)
• Oceanic crust has elevated U and Th content (compared to the mantle), all isotopes more radiogenic
• Continental crust is generally older than oceanic, so higher 207Pb/204Pb from the decay of 235U
• So 206Pb/204Pb and 207Pb/204Pb will both increase due to the presence of recycled oceanic crust, and 207Pb/204Pb will be relatively higher when recycling continental crust
• Overtime crust should evolve to more radiogenic composition than mantle

Question 12

HIMU reservoir isotopes

Answer 12

very high 206Pb/204Pb ratio
• Source with high U, (too high for any mantle process).
• Old enough (> 1 Ga) to generate observed isotopic ratios
• Yet not enriched in Rb
• Old subducted and recycled oceanic crust

Question 13

EM reservoirs isotopes

Answer 13

high 207Pb/204Pb ratio relative to a given 206Pb/204Pb.
• Source with higher 235U, must be old relative to DM and HIMU
• Old recycled continental crust?

Question 14

Crustal recycling in the mantle

Answer 14

• EMI, EMII, and HIMU: too enriched for any known mantle process…must correspond to crustal rocks and/or sediments
• If the EM and HIMU = continental or oceanic crust, only at deeper mantle by subduction and recycling
To remain isotopically distinct: could not have rehomogenized or re-equilibrated with rest of the mantle