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Some second thoughts about the Origin of Basaltic Magma

Some second thoughts about the Origin of Basaltic Magma. Mid-Ocean Ridge Basalts (MORB). Figure 13-1. After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576. . MORB Chemistry. MORB’s are dominantly olivine tholeiites with distinctively low K 2 O (< 0.2%).

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Some second thoughts about the Origin of Basaltic Magma

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  1. Some second thoughts about the Origin of Basaltic Magma

  2. Mid-Ocean Ridge Basalts (MORB) Figure 13-1. After Minster et al. (1974) Geophys. J. Roy. Astr. Soc., 36, 541-576.

  3. MORB Chemistry • MORB’s are dominantly olivine tholeiites with distinctively low K2O (< 0.2%). • All incompatible trace elements (LREE, K, Rb, Cs, Ba, Pb, Sr, Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) are depleted. • No depletion in HREE elements indicating relatively shallow melting within the spinel stability field (<70 depth). • Sr isotope ratios are low and Nd isotope ratios high, indicating a long term (billions of years) depleted upper mantle source.

  4. MORB Chemistry • No depletion in HREE indicating relatively shallow melting depths (<70 km) • Strong depletions in highly incompatible trace elements (REE, LILE) indicating “depleted” mantle source

  5. MORBs: 87Sr/86Sr < 0.7035 and Nd > +5, ® depleted mantle source

  6. MORB Petrogenesis • Decompression partial melting associated with near-adiabatic rise of mantle due to plate separation. • N-MORB melting initiated ~ 60-80 km depth in upper depleted mantle, followed by up to 30-40% melting at shallow depths.

  7. Oceanic Island Basalts (OIB)

  8. Oceanic Island Basalts (OIB)

  9. OIB Chemistry • OIB’s are range from olivine tholeiites to highly alkaline basalts with relatively high TiO2, Na2O, K2O, and P2O5. • All incompatible trace elements (LREE, K, Rb, Cs, Ba, Pb, Sr, Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) are enriched in OIB magmas with respect to MORBs. • HREE elements are depleted indicating deep melting within the garnet stability field (>70-80 depth). • Sr and Nd isotope ratios are relatively enriched with respect to MORBs indicating undepleted (primitive) and/or enriched mantle source components.

  10. Types of OIB Magmas 1. Tholeiitic series (dominant type) • “Shield-building stage” - tremendous outpourings of tholeiitic basalts

  11. Types of OIB Magmas “Post-shield stage” - Waning activity. Lavas are more diverse, with a larger proportion of chemically differentiated magmas 2. Alkaline series (small volumes)

  12. OIB Chemistry • Depletion in HREE indicating relatively deep melting depths (>80 km) • Strong enrichment in highly incompatible trace elements (REE, LILE, HFSE) indicating undepleted (primitive) and/or “enriched” mantle sources

  13. OIB Petrogenesis • Adiabatic decompression melting associated with deep upwelling of anomalously hot (+100-200 °C) mantle. • Partial melting initiated >100 km depth well within garnet stability field (explains common HREE depletion). • Relatively enriched trace element and isotope signatures indicate enriched mantle source components.

  14. OIB Petrogenesis Continental Reservoirs DM OIB EM and HIMU from crustal sources (subducted OC + CC seds)

  15. Continental Flood Basalts (CFB) Large Igneous Provinces (LIPs) Oceanic plateaus Some rifts Continental flood basalts (CFBs) Figure 15-1. Columbia River Basalts at Hat Point, Snake River area. Cover of Geol. Soc. Amer Special Paper 239. Photo courtesy Steve Reidel.

  16. Tectonic Setting of CFBs • Continental hot spots • Columbia River Plateau – Yellowstone • Deccan Traps • Continental rifting • Parana-Entendeka • CAMP – (Central Atlantic Magmatic Province)

  17. Columbia River Basalts

  18. Columbia River Basalts

  19. CFB Chemistry • CFB’s are mostly tholeiitic and similar to OIB • Incompatible trace elements and isotopes are enriched, like OIB BUT show much more variability toward more enriched compositions. • Distinctive enrichments of the most highly incompatible elements (K, Ba, Rb, Th, Pb, and LREE) over typical OIB. Noticeable depletions in HFSE (Nb, Ta) compared to OIB. • Sr and Nd isotope ratios overlap with OIB, but extend to more enriched compositions (higher 87Sr/86Sr and lower Nd).

  20. CFB Chemistry • No depletion in HREE indicating relatively shallow melting depths (<70 km) • Strong enrichments in highly incompatible trace elements (REE, LILE) indicating enriched mantle source(s)

  21. Distinctive ernirchments in Ba and Pb and depletions in HFSE (Nb, Ta) compared to OIB.

  22. CFB Petrogenesis • Basically OIB petrogenesis + added components from continental lithospheric mantle and crust

  23. Figure 15-14. Diagrammatic cross section illustrating possible models for the development of continental flood basalts. DM is the depleted mantle (MORB source reservoir), and the area below 660 km depth is the less depleted, or enriched OIB source reservoir. Winter (20010 An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  24. Mantle Sources and Structure • Upper depleted mantle = MORB source • Lower undepleted & enriched OIB source

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