Intraplate magmatism

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…)

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 14-1. After Crough (1983) Ann. Rev. Earth Planet. Sci., 11, 165-193.

Oceanic islands

Hotspots

Mantle convection and mantle plumes

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)

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

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

Evolution in the Series Tholeiitic, alkaline, and highly alkaline Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

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

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)

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 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

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 14-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

Generation of tholeiitic and alkaline basalts from a chemically uniform mantle Variables (other than X) Temperature = % partial melting Pressure Fig. 10-2 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, 128-153.

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, 619-634. 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

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

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

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 14-5. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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

Mantle Reservoirs 1. DM (Depleted Mantle) = N-MORB source Figure 14-6. 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

2. BSE (Bulk Silicate Earth) or the Primary Uniform Reservoir Figure 14-6. 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

3. EMI = enriched mantle type I has lower 87Sr/86Sr (near primordial) 4. EMII = enriched mantle type II has higher 87Sr/86Sr (> 0.720, well above any reasonable mantle sources Figure 14-6. 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 (< 0.5124)

5. PREMA (PREvalent MAntle) Figure 14-6. 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

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

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

Figure 14-8. After Wilson (1989) Igneous Petrogenesis. Kluwer Figure 14-8. 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

Kellogg et al. (1999)

A Model for Oceanic Magmatism Continental Reservoirs DM OIB EM and HIMU from crustal sources (subducted OC + CC seds) Figure 14-10. Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).

“Marble cake” model for mantle convection & mixing

Continental Flood Basalts 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.

Trapp volcanism

LIPs (Large Igneous Provinces)

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

Figure 15-2. Flood basalt provinces of Gondwanaland prior to break-up and separation. After Cox (1978) Nature, 274, 47-49.

Figure 15-3. Relationship of the Etendeka and Paraná plateau provinces to the Tristan hot spot. After Wilson (1989), Igneous Petrogenesis. Kluwer.

Geochemistry Deccan traps basalts

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

Figure 15-7. 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, 1203-1246.

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 15-13. A model for the origin of the Columbia River Basalt Group From Takahahshi et al. (1998) Earth Planet. Sci. Lett., 162, 63-80.

LIPs and mass extinctions