1. Intraplate magmatism

2. Intraplate magmatism Hotspots
Rift zones (often associated with hotspots)
Intra-oceanic plate: Tholeitic to alkaline series; mostly basalts (OIB = Oceanic Islands Basalts), some differenciated alkaline terms
Intra-continental plate:
either large tholeitic basaltic provinces (CFB = Continental Flood Basalts), occasionally bimodal (ass. with rhyolites)
or smaller, alkaline to hyper-alkaline, differenciated intrusions/volcanoes (syenites/phonolites; carbonatites; kimberlites; and more…)

3. Ocean islands and seamounts Commonly associated with hot spots
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 122
Figure After Crough (1983) Ann. Rev. Earth Planet. Sci., 11,

4. Oceanic islands

5. Hotspots

6. Mantle convection and mantle plumes

7. Two principal magma series
Types of OIB Magmas
Two principal magma series
Tholeiitic series (dominant type)
Parental ocean island tholeiitic basalt, or OIT
Similar to MORB, but some distinct chemical and mineralogical differences
Alkaline series (subordinate)
Parental ocean island alkaline basalt, or OIA
Two principal alkaline sub-series
silica undersaturated
slightly silica oversaturated (less common series)
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)

8. Cyclic, pattern to the eruptive history
Hawaiian Scenario
Cyclic, pattern to the eruptive history
1. Pre-shield-building stage somewhat alkaline and variable
2. Shield-building stage begins with tremendous outpourings of tholeiitic basalts
Early, pre-shield-building stage that is more alkaline and variable, but quickly covered by the massive tholeiitic shields
Recent studies of the Loihi Seamount encountered a surprising assortment of lava types from tholeiite to highly alkaline basanites.
Shield-building: Kilauea and Mauna Loa (the two nearest the hot spot in the southern and southeastern part of the island) are presently in this stage of development
This stage produces 98-99% of the total lava in Hawaii

9. Hawaiian Scenario
3. Waning activity more alkaline, episodic, and violent (Mauna Kea, Hualalai, and Kohala). Lavas are also more diverse, with a larger proportion of differentiated liquids
4. A long period of dormancy, followed by a late, post-erosional stage. Characterized by highly alkaline and silica-undersaturated magmas, including alkali basalts, nephelinites, melilite basalts, and basanites
The two late alkaline stages represent 1-2% of the total lava output
Note all three OIB series are represented in Hawaii
Is this representative of all islands? Probably not

10. Evolution in the Series
Tholeiitic, alkaline, and highly alkaline
Figure After Wilson (1989) Igneous Petrogenesis. Kluwer.

11. Thus all appear to have distinctive sources
Trace Elements
The LIL trace elements (K, Rb, Cs, Ba, Pb2+ and Sr) are incompatible and are all enriched in OIB magmas with respect to MORBs
The ratios of incompatible elements have been employed to distinguish between source reservoirs
N-MORB: the K/Ba ratio is high (usually > 100)
E-MORB: the K/Ba ratio is in the mid 30’s
OITs range from 25-40, and OIAs in the upper 20’s
Thus all appear to have distinctive sources

12. Trace Elements
HFS elements (Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) are also incompatible, and are enriched in OIBs > MORBs
Ratios of these elements are also used to distinguish mantle sources
The Zr/Nb ratio
N-MORB generally quite high (>30)
OIBs are low (<10)

13. Trace Elements: REEs
Note that ocean island tholeiites (represented by the Kilauea and Mauna Loa samples) overlap with MORB and are not unlike E-MORB
The alkaline basalts have steeper slopes and greater LREE enrichment, although some fall within the upper MORB field
Note also that the heavy REEs are also fractionated in the OIB samples (as compared to the flat HREE patterns in N- and E-MORB). This indicates that garnet was a residual phase
These melts must have segregated from the mantle at depths > 60 km
Figure After Wilson (1989) Igneous Petrogenesis. Kluwer.

14. MORB-normalized Spider Diagrams
OIBs are enriched in incompatible elements over MORB (values > one)
Broad central hump in which both the LIL (Sr-Ba) and HFS (Yb-Th) element enrichments increase with increasing incompatibility (inward toward Ba and Th)
The pattern we would expect in a sample that was enriched by some single-stage process (such as partial melting of a four-phase lherzolite) that preferentially concentrated incompatible elements.
The pattern is regarded as typical of melts generated from non-depleted mantle in intraplate settings
Figure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

15. Generation of tholeiitic and alkaline basalts from a chemically uniform mantle
Variables (other than X)
Temperature
= % partial melting
Pressure
Fig indicates that, although the chemistry may be the same, the mineralogy varies
Pressure effects on eutectic shift
Figure 10-2 After Wyllie, P. J. (1981). Geol. Rundsch. 70,

16. Pressure effects: Ne Fo En Ab
SiO2
Oversaturated
(quartz-bearing)
tholeiitic basalts
Highly undesaturated
(nepheline - bearing)
alkali basalts
Undersaturated E
3GPa
2Gpa
1GPa
1atm
Volatile-free
Figure 10-8 After Kushiro (1968), J. Geophys. Res., 73,
Increased pressure moves the ternary eutectic minimum from the oversaturated tholeiite field to the under-saturated alkaline basalt field
Alkaline basalts are thus favored by greater depth of melting

17. Tholeiites favored by shallower melting
25% melting at <30 km ® tholeiite
25% melting at 60 km ® olivine basalt
Tholeiites favored by greater % partial melting
20 % melting at 60 km ® alkaline basalt
incompatibles (alkalis) ® initial melts
30 % melting at 60 km ® tholeiite

18. Isotope Geochemistry
Isotopes do not fractionate during partial melting of fractional melting processes, so will reflect the characteristics of the source
OIBs, which sample a great expanse of oceanic mantle in places where crustal contamination is minimal, provide incomparable evidence as to the nature of the mantle

19. Simple Mixing Models Ternary Binary
All analyses fall between two reservoirs as magmas mix
Ternary
All analyses fall within triangle determined by three reservoirs
Figure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

20. Figure 14-6. After Zindler and Hart (1986), Staudigel et al
Figure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

21. Mantle Reservoirs 1. DM (Depleted Mantle) = N-MORB source
Figure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
Shows low values of 87Sr/86Sr and high values of 144Nd/143Nd as well as depleted trace element characteristics

22. 2. BSE (Bulk Silicate Earth) or the Primary Uniform Reservoir
Figure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
Reflects the isotopic signature of the primitive mantle as it would evolve to the present without any subsequent fractionation i.e. neither depleted nor enriched…just plain old mantle
Several oceanic basalts have this isotopic signature, but there are no compelling data that require this reservoir (it is not a mixing end-member), but falls within the space defined by other reservoirs

23. 3. EMI = enriched mantle type I has lower 87Sr/86Sr (near primordial)
4. EMII = enriched mantle type II has higher 87Sr/86Sr (> , well above any reasonable mantle sources
Figure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
Since the Nd-Sr data for OIBs extends beyond the primitive values to truly enriched ratios, there must exist an enriched mantle reservoir
Both EM reservoirs have similar enriched (low) Nd ratios (< )

24. 5. PREMA (PREvalent MAntle)
Figure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
Also not a mixing end-member
PREMA represents another restricted isotopic range that is very common in ocean volcanic rocks
Although it lies on the mantle array, and could result from mixing of melts from DM and BSE sources, the promiscuity of melts with the PRIMA signature suggests that it may be a distinct mantle source

25. 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?
Figure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

26. Pb Isotopes Pb produced by radioactive decay of U & Th
238U  234U  206Pb
235U  207Pb
232Th  208Pb
Pb isotopes also characterize the different reservoirs (see paper presentation Hart 1984)
All three elements are LIL
Fractionate into the melt (or a fluid) phase (if available) in the mantle, and migrate upward where they will become incorporated in the oceanic or continental crust

27. Figure 14-8. After Wilson (1989) Igneous Petrogenesis. Kluwer
Figure After Wilson (1989) Igneous Petrogenesis. Kluwer. Data from Hamelin and Allègre (1985), Hart (1984), Vidal et al. (1984).
Note that HIMU is also 208Pb enriched, which tells us that this reservoir is enriched in Th as well as U
This highly enriched EM component has been called the DUPAL component, named for Dupré and Allègre (1983), who first described it

28. Kellogg et al. (1999)

29. A Model for Oceanic Magmatism
Continental
Reservoirs DM
OIB
EM and HIMU from crustal sources (subducted OC + CC seds)
Figure Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).

30. “Marble cake” model for mantle convection & mixing

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

32. Trapp volcanism

33. LIPs (Large Igneous Provinces)

35. CFB’s Associated to major continental break-up
… or/and to plume head impact

37. Figure Flood basalt provinces of Gondwanaland prior to break-up and separation. After Cox (1978) Nature, 274,

38. Figure Relationship of the Etendeka and Paraná plateau provinces to the Tristan hot spot. After Wilson (1989), Igneous Petrogenesis. Kluwer.

39. Geochemistry
Deccan traps basalts

40. Bimodal magmas Basalts and rhyolites Secondary melting?
Effect of the two eutectics?

41. Figure Condrite-normalized rare earth element patterns of some typical CRBG samples. Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper and Hawkesworth (1993) J. Petrol., 34,

42. Melting within a heterogeneous plume head (initial stages of the Yellowstone hot spot).
The plume head contains recycled stringers of recycled oceanic crust that melts before the peridotite, yielding a silica-rich basaltic magma equivalent to the main Grande Ronde basalts and leaves a garnet-clinopyroxene residue.
The large plume head stalls and spreads out at the base of the resistant lithosphere and the basaltic magma ponds (underplates) at the base of the crust, where it melts some crust to create rhyolite.
Basalt escapes along a northward trending rift system to feed the CRBG.
Figure A model for the origin of the Columbia River Basalt Group From Takahahshi et al. (1998) Earth Planet. Sci. Lett., 162,

43. LIPs and mass extinctions