Chapter 14 Ocean Intraplate Volcanism (from Plumes)

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.

Ocean islands and seamounts Commonly associated with hot spots

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)

Tholeiitic and Alkaline examples Modern volcanic activity of some islands is dominantly tholeiitic (for example Hawaii and Réunion). 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) Hawaii data, both tholeiitic and alkaline

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 This stage produces 98-99% of the total lava in Hawaii 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 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.

Alkalinity is highly variable Alkalis are incompatible elements, unaffected by less than 50% shallow fractional crystallization, this again argues for distinct mantle sources or generating mechanisms The variation in Na/K among the suites makes the possibility much more likely, and leads us to suspect that the mantle is more heterogeneous than we had previously thought

Trace Elements High Field Strength Elements (HFS or HFSE) 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. For example: The Zr/Nb ratio N-MORBs are generally quite high (>30) OIBs are low (<10)

Thus all appear to have distinctive sources Trace Elements The large ion lithophile (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: REEs Note that ocean island tholeiites (OITs 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 than the OIT’s. Some fall within the upper MORB field, but most are distinct Figure 14-2. After Wilson (1989) Igneous Petrogenesis. .

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

m 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 Shows low values of 87Sr/86Sr and high values of 144Nd/143Nd as well as depleted trace element characteristics Figure 14.8. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

2. BSE (Bulk Silicate Earth) or the Primary Uniform Reservoir 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 Figure 14.8. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

Both EM reservoirs have similar enriched (low) Nd ratios (< 0.5124) 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 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) Figure 14.8. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

5. PREMA (PREvalent MAntle) 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 Figure 14.8. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

Simple Mixing Models Ternary Binary All analyses fall within triangle determined by three reservoirs Binary All analyses fall between two reservoirs as magmas mix Figure 14.7. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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

Pb is quite scarce in the mantle Low-Pb mantle-derived melts susceptible to Pb contamination Incompatibles U, Pb, and Th are concentrated in continental crust 204Pb is non-radiogenic. 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb increase as U and Th decay Oceanic crust also has elevated U and Th content (compared to the mantle) So are sediments derived from oceanic and continental crust So Pb is a sensitive measure of crustal (including sediment) components contaminating mantle isotopic systems 93.7% of natural U is 238U, so 206Pb/204Pb will be most sensitive to a crustal-enriched component Mantle-derived melts are susceptible to contamination from U-Th-Pb-rich reservoirs which can add a significant proportion to the total Pb U, Pb, and Th are concentrated in sialic reservoirs, such as the continental crust, which develop high concentrations of the radiogenic daughter Pb isotopes 204Pb is non-radiogenic, so 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb increase as U and Th decay Oceanic crust has elevated U and Th content (compared to the mantle) as will sediments derived from oceanic and continental crust Pb is perhaps the most sensitive measure of crustal (including sediment) components in mantle isotopic systems Since 93.7% of natural U is 238U, equation 9-20 will dominate over equation 9-21, and thus 206Pb/204Pb will be most sensitive to a crustal-enriched component 9-20 238U  234U  206Pb 9-21 235U  207Pb 9-22 232Th  208Pb

Figure 14-7. After Wilson (1989) Igneous Petrogenesis. Kluwer. 207Pb/204Pb vs. 206Pb/204Pb data for Atlantic and Pacific ocean basalts Geochron = simultaneous evolution of 206Pb and 207Pb in a rock/reservoir = line on which all modern single-stage (not disturbed or reset) Pb isotopic systems, such as BSE (Bulk Silicate Earth), should plot. ~ NONE of the oceanic volcanics fall on the geochron. Nor do they fall within the EMI-EMII-DM triangle, as they appear to do in the Nd-Sr systems The remaining mantle reservoir: HIMU (high mu) proposed to account for this great radiogenic Pb enrichment pattern 207Pb/204Pb vs. 206Pb/204Pb data for Atlantic and Pacific ocean basalts Geochron = simultaneous evolution of 206Pb and 207Pb in a rock/reservoir = line on which all modern single-stage (not disturbed or reset) Pb isotopic systems, such as BSE (Bulk Silicate Earth), should plot. ~Notice NONE of the oceanic volcanics fall on the geochron. Nor do they fall within the EMI-EMII-DM triangle, as they appear to do in the Nd-Sr systems. The remaining mantle reservoir: HIMU (high mu) proposed to account for this great radiogenic Pb enrichment pattern

m = 238U/204Pb (evaluate uranium enrichment) HIMU reservoir: very high 206Pb/204Pb ratio Source with high U, yet not enriched in Rb (has modest 87Sr/86Sr) Old enough (> 1 Ga) to ® observed isotopic ratios HIMU model: Subducted and recycled oceanic crust (± seawater) The similarity of the rocks from St. Helena Island to the HIMU reservoir has led some workers to call this reservoir the “St. Helena component”

EMI and EMII High 87Sr/86Sr require initially high Rb & long time to ® 87Sr Correlates with continental crust (or sediments derived from it) Oceanic crust and sediment are other likely candidates

HIMU is also 208Pb enriched, so this reservoir is enriched in Th as well as U Dupré and Allègre (1983), Figure 14.10 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 207Pb/204Pb data (especially from the N hemisphere) ® ~linear mixing line between DM and HIMU, a line called the Northern Hemisphere Reference Line (NHRL) Data from the southern hemisphere (particularly Indian Ocean) departs from this line, and appears to include a larger EM component (probably EMII)

Other isotopic systems that contribute to our understanding of mantle reservoirs and dynamics He Isotopes Noble gases are inert and volatile 4He is an alpha particle, produced principally by a-decay of U and Th, enriching primordial 4He 3He is largely primordial (constant) The mantle is continually degassing and He lost (cannot recycle back) 4He enrichment expressed as R = (3He/4He) R unusual among isotope expressions in that radiogenic is the denominator Common reference is RA (air) = 1.39 x 10-6

He Isotopes N-MORB is fairly uniform at 8±1 RA suggesting an extensive depleted (degassed) DM- type N-MORB source Figure 14.12  3He/4He isotope ratios in ocean island basalts and their relation to He concentration. Concentrations of 3He are in cm3 at 1 atm and 298K.After Sarda and Graham (1990) and Farley and Neroda (1998).

He Isotopes OIB 3He/4He values extend to both higher and lower values than N- MORBs, but are typically higher (low 4He). Simplest explanations: High R/RA is deeper mantle with more primordial signature OIB 3He/4He values extend to both higher and lower values than N-MORBs, but are typically higher. Fresh glasses (yellow) are generally required to avoid contamination. Some mineral inclusions, either fluid or melt/glass (other symbols), may also be good. Low R/RA has higher 4He due to recycled (EM-type?) U and Th. Figure 14.12  3He/4He isotope ratios in ocean island basalts and their relation to He concentration. Concentrations of 3He are in cm3 at 1 atm and 298K.After Sarda and Graham (1990) and Farley and Neroda (1998).

He Isotopes PHEM (primitive helium mantle) is a He3/He4 mantle end-member reservoir with near-primitive Sr-Nd-Pb characteristics. PHEM no longer exists due to radiogenic increases in 4He. Figure 14.13  3He/4He vs. a. 87Sr/86Sr and b. 206Pb/204Pb for several OIB localities and MORB. The spread in the diagrams are most simply explained by mixing between four mantle components: DM, EMII, HIMU, and PHEM. After Farley et al. (1992).

He Isotopes Summary Shallow mantle MORB source is relatively homogeneous and depleted in He OIBs have more primordial (high) 3He/4He, but still degassed and less than primordial (100-200RA) values, consistent with our deeper mantle ideas. Again, PHEM concept may be like that more primitive mantle reservoir, prior to natural increase in radiogenic He and contamination. Current Lower than PHEM 3He/4He in OIB’s may be due to recycled crustal U and Th

Re/Os system and Os Isotopes Both are platinum group elements (PGEs) PGEs → core or sulfides depending on whether or not they are compatible. Os is compatible during mantle partial melting (goes into → solids as a trace in sulfides, so they don’t leave the mantle), but Re is moderately incompatible (goes into → melts and, eventually, crust silicates) The mantle is thus enriched in Os relative to crustal rocks and crustal rocks. Crustal rocks have higher Re and lower Os and develop a high (187Os/188Os) as Re decays, which should show up if crustal rocks are recycled back into the mantle. Re is Rhenium and Os is Osmium

Os Isotopes plus the FOZO All of the basalt provinces are enriched in 187Os due to high Re decaying, over the values in mantle peridotites and require more than one 187Os-enriched reservoir to explain the distribution. Crust is high (with little overlap to peridotites). 187Re → 187Os Figure 14.13  187Os/188Os vs. 206Pb/204Pb for mantle peridotites and several oceanic basalt provinces. Os values for the various mantle isotopic reservoirs are estimates. After Hauri (2002) and van Keken et al. (2002b).

Other Mantle Reservoirs FOZO (focal zone): another “convergence” reservoir toward which many trends approach. Thus perhaps a common mixing end-member FOZO Figure 14.15. After Hart et al., 1992).

Deeper continental crust or oceanic crust EMII More enriched EMI, EMII, and HIMU: too enriched for any known mantle process...must correspond to crustal rocks and/or sediments EMI Slightly enriched Deeper continental crust or oceanic crust EMII More enriched Specially in 87Sr (Rb parent) and Pb (U/Th parents) Upper continental crust or ocean-island crust If the EM and HIMU = continental crust (or older oceanic crust and sediments), only ® deeper mantle by subduction and recycling To remain isotopically distinct: could not have rehomogenized or re-equilibrated with rest of mantle

The Nature of the Mantle N-MORBs involve shallow melting of passively rising upper mantle → a significant volume of depleted upper mantle (DM which has lost lithophile elements to melts which ended in late fractionation rocks, and which has lost He). OIBs seem to originate from deeper levels. Major- and trace-element data → the deep source of OIB magmas (both tholeiitic and alkaline) is distinct from that of N-MORB. Trace element and isotopic data reinforce this notion and further indicate that the deeper mantle is relatively heterogeneous and complex, consisting of several domains of contrasting composition and origin. In addition to the depleted MORB mantle, there are at least four enriched components, including one or more containing recycled crustal and/or sedimentary material reintroduced into the mantle by subduction, and at least one (FOZO or PHEM) that retains much of its primordial noble gases. MORBs are not as homogenous as originally thought, and exhibit most of the compositional variability of OIBs, although the variation is expressed in far more subordinate proportions. This implies that the shallow depleted mantle also contains some enriched components.

Pick any two points on an equilibrium curve Mantle Questions Is the mantle layered (shallow depleted and deeper non- depleted and even enriched)? Or are the enriched components stirred into the entire mantle (like fudge ripple ice cream)? How effective is the 660-km transition at impeding convective stirring? This depends on the Clapeyron slope of the phase transformation at the boundary! Pick any two points on an equilibrium curve dDG = 0 = DVdP - DSdT Thus dP dT S V = D Clapeyron Eq.

No Effect Retards Penetration Enhances Penetration → 2-Layer Mantle Model → Whole-Mantle mixing Figure 14.16. Effectiveness of the 660-km transition in preventing penetration of a subducting slab or a rising plume

Case 1: dP/dT at 660 km is negative Figure 1.14. Schematic diagram of a 2-layer dynamic mantle model The 660 km transition is a sufficient density barrier to separate lower mantle convection Only significant things that can penetrate this barrier are vigorous rising hotspot plumes and subducted lithosphere Subducted lithosphere sinks to become incorporated in the D" layer where they may be heated by the core and return as plumes). After Silver et al. (1988). Continental Gradient higher than Oceanic Gradient Range for both Highest at Surface water and cold surface In the future we will often use average values rather than the ranges

Case 2: dP/dT at 660 km is positive Continental Gradient higher than Oceanic Gradient Range for both Highest at Surface water and cold surface In the future we will often use average values rather than the ranges Figure14.17. Whole-mantle convection model with geochemical heterogeneity preserved as blobs of fertile mantle in a host of depleted mantle. Higher density of the blobs results in their concentration in the lower mantle where they may be tapped by deep-seated plumes, probably rising from a discontinuous D" layer of dense “dregs” at the base of the mantle. After Davies (1984) .

2-layer 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).

Whole Mantle Model for Oceanic Magmatism The upper mantle is (variably) depleted (DM), and is the source of N-type MORB. The lower mantle is a major source for E-MORB and OIB magmas. Both regions are heterogeneous, containing stirred (stretched and folded) disrupted remnants of “marble-cake” crust, lithospheric mantle, and sediment recycled by subduction (as illustrated in the magnified insert for the upper mantle). The lower mantle contains more of these (denser) constituents: enriched mantle (EM), high-m mantle (HIMU), high-3He/4He (FOZO), and perhaps primitive mantle (BSE) and “prevalent” mantle (PREMA) reservoirs as well. The 660-km seismic discontinuity is tentatively retained as the boundary layer between upper and lower mantle, but is left gradational to allow for other options, such as a deeper boundary near 1700 km or whole-mantle convection with progressively less depleted and more dense deeper mantle containing more recycled material. Figure 14.19. Schematic model for oceanic volcanism. Nomenclature from Zindler and Hart (1986) and Hart and Zindler (1989).