Chapter 8: Major Elements

1. therefore some TE's will follow similar major E Periodic Table is next slide therefore some TE's will follow similar major E Periodic Table is next slide

4. Not exact, since D varies with the composition of mins & melt Not exact, since D varies with the composition of mins & melt

5. Depends on the minerals involved! Sr -> melt as ol & px separate -> plag (Ca) & not melt if plag is phenocryst phase Commonly standardized to mantle compositions (olivine, pyroxenes, and perhaps garnet) Thus the major elements Mg and Fe would usually be referred to as compatible, while K and Na as incompatible Depends on the minerals involved! Sr -> melt as ol & px separate -> plag (Ca) & not melt if plag is phenocryst phase Commonly standardized to mantle compositions (olivine, pyroxenes, and perhaps garnet) Thus the major elements Mg and Fe would usually be referred to as compatible, while K and Na as incompatible

6. Depends on the minerals involved! Sr -> melt as ol & px separate -> plag (Ca) & not melt if plag is phenocryst phase Commonly standardized to mantle compositions (olivine, pyroxenes, and perhaps garnet) Thus the major elements Mg and Fe would usually be referred to as compatible, while K and Na as incompatible Depends on the minerals involved! Sr -> melt as ol & px separate -> plag (Ca) & not melt if plag is phenocryst phase Commonly standardized to mantle compositions (olivine, pyroxenes, and perhaps garnet) Thus the major elements Mg and Fe would usually be referred to as compatible, while K and Na as incompatible

10. Add to end: 87Sr is stable & non-radiogenic Sr is present in nearly any rock The amount of 87Sr in any rock = original 87Sr + radiogenic 87Sr from 87Rb over time Thus a rock cannot ? an unambiguous age if you don’t know what % or 87Sr is radiogenic Isochron technique use to solve this problemAdd to end: 87Sr is stable & non-radiogenic Sr is present in nearly any rock The amount of 87Sr in any rock = original 87Sr + radiogenic 87Sr from 87Rb over time Thus a rock cannot ? an unambiguous age if you don’t know what % or 87Sr is radiogenic Isochron technique use to solve this problem

12. Primordial mantle ~ chondrite Major melting event about 3 Ga ago ? continental crust (hypothetical simplification) Crust has high Rb/Sr so evolution curve is steep Mantle is depleted in Rb, so lower Rb/Sr and shallower evolution curve If a modern melt is derived from partial melting of the mantle: (87Sr/86Sr)o < 0.706 If derived from old crust: (87Sr/86Sr)o > 0.706Primordial mantle ~ chondrite Major melting event about 3 Ga ago ? continental crust (hypothetical simplification) Crust has high Rb/Sr so evolution curve is steep Mantle is depleted in Rb, so lower Rb/Sr and shallower evolution curve If a modern melt is derived from partial melting of the mantle: (87Sr/86Sr)o < 0.706 If derived from old crust: (87Sr/86Sr)o > 0.706

14. Also requires the isochron technique to solveAlso requires the isochron technique to solve

16. Partial melting selectively depletes the mantle in the daughter (Nd) so Sm ? Nd and radiogenic swamps original Nd Basalt derived from the mantle will have a higher initial 143Nd/144Nd than a magma derived from the crustPartial melting selectively depletes the mantle in the daughter (Nd) so Sm ? Nd and radiogenic swamps original Nd Basalt derived from the mantle will have a higher initial 143Nd/144Nd than a magma derived from the crust

19. Figure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, both of which indicate a depleted mantle source E-MORBs extend the MORB array to more enriched values (higher 87Sr/86Sr and lower 143Nd/144Nd), providing even stronger support for the distinct mantle reservoirs for N-type and E-type MORBs T-MORBs (not shown) exhibit intermediate mixed valuesFigure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, both of which indicate a depleted mantle source E-MORBs extend the MORB array to more enriched values (higher 87Sr/86Sr and lower 143Nd/144Nd), providing even stronger support for the distinct mantle reservoirs for N-type and E-type MORBs T-MORBs (not shown) exhibit intermediate mixed values

22. Mid-Ocean Ridge Basalts (MORB)

23. MORB Chemistry MORBs are chemically distinct from basalts of other petrogenetic associations Glass samples are very important chemically, because they represent liquid compositions, whereas the chemistry of phyric samples can be modified by crystal accumulationMORBs are chemically distinct from basalts of other petrogenetic associations Glass samples are very important chemically, because they represent liquid compositions, whereas the chemistry of phyric samples can be modified by crystal accumulation

25. Strong depletions in highly incompatible trace elements (REE, LILE) indicating “depleted” mantle source Ocean Island Basalt (Hawaiian alkaline basalt) Looks like partial melt of ~ typical mantle Mid Ocean Ridge Basalt (tholeiite) How get (+) slope??Ocean Island Basalt (Hawaiian alkaline basalt) Looks like partial melt of ~ typical mantle Mid Ocean Ridge Basalt (tholeiite) How get (+) slope??

26. MORBs: 87Sr/86Sr < 0.7035 and ?Nd > +5, ® depleted mantle source Figure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, both of which indicate a depleted mantle source E-MORBs extend the MORB array to more enriched values (higher 87Sr/86Sr and lower 143Nd/144Nd), providing even stronger support for the distinct mantle reservoirs for N-type and E-type MORBs T-MORBs (not shown) exhibit intermediate mixed valuesFigure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, both of which indicate a depleted mantle source E-MORBs extend the MORB array to more enriched values (higher 87Sr/86Sr and lower 143Nd/144Nd), providing even stronger support for the distinct mantle reservoirs for N-type and E-type MORBs T-MORBs (not shown) exhibit intermediate mixed values

27. 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 where it inherits depleted trace element and isotope signatures. Continue with: % partial melting increases to ~ 15-40% as diapirs of melting mantle rise toward the surface % melted when reaches top depends on the source depth, temperature, and the rate of rise (and spreading) Melting is terminated by conductive heat loss to the surface near the top of the column, perhaps aided by the consumption of clinopyroxene, which, when gone, will create a discontinuous temperature jump in melting Continue with: % partial melting increases to ~ 15-40% as diapirs of melting mantle rise toward the surface % melted when reaches top depends on the source depth, temperature, and the rate of rise (and spreading) Melting is terminated by conductive heat loss to the surface near the top of the column, perhaps aided by the consumption of clinopyroxene, which, when gone, will create a discontinuous temperature jump in melting

29. Oceanic Island Basalts (OIB) More enigmatic processes and less voluminous than activity at plate margins No obvious mechanisms that we can tie to the plate tectonic paradigm As with MORB, the dominant magma type for oceanic intraplate volcanism is basalt, which is commonly called ocean island basalt or OIB 41 well-established hot spots Estimates range from 16 to 122More enigmatic processes and less voluminous than activity at plate margins No obvious mechanisms that we can tie to the plate tectonic paradigm As with MORB, the dominant magma type for oceanic intraplate volcanism is basalt, which is commonly called ocean island basalt or OIB 41 well-established hot spots Estimates range from 16 to 122

30. 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. MORBs are chemically distinct from basalts of other petrogenetic associations Glass samples are very important chemically, because they represent liquid compositions, whereas the chemistry of phyric samples can be modified by crystal accumulationMORBs are chemically distinct from basalts of other petrogenetic associations Glass samples are very important chemically, because they represent liquid compositions, whereas the chemistry of phyric samples can be modified by crystal accumulation

32. Types of OIB Magmas 1. Tholeiitic series (dominant type) “Shield-building stage” - tremendous outpourings of tholeiitic basalts Modern volcanic activity of some islands is dominantly tholeiitic (for example Hawaii and Réunion), while other islands are more alkaline in character (for example Tahiti in the Pacific and a concentration of islands in the Atlantic, including the Canary Islands, the Azores, Ascension, Tristan da Cunha, and Gough)Modern volcanic activity of some islands is dominantly tholeiitic (for example Hawaii and Réunion), while other islands are more alkaline in character (for example Tahiti in the Pacific and a concentration of islands in the Atlantic, including the Canary Islands, the Azores, Ascension, Tristan da Cunha, and Gough)

33. Types of OIB Magmas 2. Alkaline series (small volumes) Modern volcanic activity of some islands is dominantly tholeiitic (for example Hawaii and Réunion), while other islands are more alkaline in character (for example Tahiti in the Pacific and a concentration of islands in the Atlantic, including the Canary Islands, the Azores, Ascension, Tristan da Cunha, and Gough)Modern volcanic activity of some islands is dominantly tholeiitic (for example Hawaii and Réunion), while other islands are more alkaline in character (for example Tahiti in the Pacific and a concentration of islands in the Atlantic, including the Canary Islands, the Azores, Ascension, Tristan da Cunha, and Gough)

35. Strong enrichment in highly incompatible trace elements (REE, LILE, HFSE) indicating undepleted (primitive) and/or “enriched” mantle sources Ocean Island Basalt (Hawaiian alkaline basalt) Looks like partial melt of ~ typical mantle Mid Ocean Ridge Basalt (tholeiite) How get (+) slope??Ocean Island Basalt (Hawaiian alkaline basalt) Looks like partial melt of ~ typical mantle Mid Ocean Ridge Basalt (tholeiite) How get (+) slope??

36. OIBs: 87Sr/86Sr = 0.7035 to –0.710 and ?Nd = +5 to -5, ® primitive and/or enriched mantle source(s) Figure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, both of which indicate a depleted mantle source E-MORBs extend the MORB array to more enriched values (higher 87Sr/86Sr and lower 143Nd/144Nd), providing even stronger support for the distinct mantle reservoirs for N-type and E-type MORBs T-MORBs (not shown) exhibit intermediate mixed valuesFigure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, both of which indicate a depleted mantle source E-MORBs extend the MORB array to more enriched values (higher 87Sr/86Sr and lower 143Nd/144Nd), providing even stronger support for the distinct mantle reservoirs for N-type and E-type MORBs T-MORBs (not shown) exhibit intermediate mixed values

37. Note that all of the Nd-Sr data can be reconciled with mixing of three reservoirs: DM EMI and EMII since the data are confined to a triangle with apices corresponding to these three components. So, what is the nature of EMI and EMII, and why is there yet a 6th reservoir (HIMU) that seems little different than the mantle array? Note that all of the Nd-Sr data can be reconciled with mixing of three reservoirs: DM EMI and EMII since the data are confined to a triangle with apices corresponding to these three components. So, what is the nature of EMI and EMII, and why is there yet a 6th reservoir (HIMU) that seems little different than the mantle array?

40. Continental Flood Basalts (CFB) Large Igneous Provinces (LIPs) Oceanic plateaus Some rifts Continental flood basalts (CFBs)

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

45. 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). MORBs are chemically distinct from basalts of other petrogenetic associations Glass samples are very important chemically, because they represent liquid compositions, whereas the chemistry of phyric samples can be modified by crystal accumulationMORBs are chemically distinct from basalts of other petrogenetic associations Glass samples are very important chemically, because they represent liquid compositions, whereas the chemistry of phyric samples can be modified by crystal accumulation

49. CFBs: 87Sr/86Sr = 0.7035 to 0.713 and ?Nd = +5 to -10, ® enriched mantle source(s) Figure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, both of which indicate a depleted mantle source E-MORBs extend the MORB array to more enriched values (higher 87Sr/86Sr and lower 143Nd/144Nd), providing even stronger support for the distinct mantle reservoirs for N-type and E-type MORBs T-MORBs (not shown) exhibit intermediate mixed valuesFigure 13-11: 143Nd/144Nd vs. 87Sr/86Sr data for MORBs N-MORBs plot as a relatively tight cluster with 87Sr/86Sr < 0.7035 and 143Nd/144Nd > 0.5030, both of which indicate a depleted mantle source E-MORBs extend the MORB array to more enriched values (higher 87Sr/86Sr and lower 143Nd/144Nd), providing even stronger support for the distinct mantle reservoirs for N-type and E-type MORBs T-MORBs (not shown) exhibit intermediate mixed values

53. Layered mantle Upper depleted mantle = MORB source depleted by MORB extraction > 1 Ga Lower = undepleted & enriched OIB source Boundary = 670 km phase transition Sufficient D density to impede convection so they convect independently It is interesting to note that this concept of a layered mantle was initiated by the REE concentrations of oceanic basalts Later support came from isotopes and geophysicsLayered mantle Upper depleted mantle = MORB source depleted by MORB extraction > 1 Ga Lower = undepleted & enriched OIB source Boundary = 670 km phase transition Sufficient D density to impede convection so they convect independently It is interesting to note that this concept of a layered mantle was initiated by the REE concentrations of oceanic basalts Later support came from isotopes and geophysics