2Ocean islands and seamounts Commonly associated with hot spots More enigmatic processes and less voluminous than activity at plate marginsNo obvious mechanisms that we can tie to the plate tectonic paradigmAs with MORB, the dominant magma type for oceanic intraplate volcanism is basalt, which is commonly called ocean island basalt or OIB41 well-established hot spots Estimates range from 16 to 122Figure After Crough (1983) Ann. Rev. Earth Planet. Sci., 11,
3Ocean islands and seamounts Commonly associated with hot spots
4Two principal magma series Types of OIB MagmasTwo principal magma seriesTholeiitic series (dominant type)Parental ocean island tholeiitic basalt, or OITSimilar to MORB, but some distinct chemical and mineralogical differencesAlkaline series (subordinate)Parental ocean island alkaline basalt, or OIATwo principal alkaline sub-seriessilica undersaturatedslightly 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)
5Tholeiitic 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
6Cyclic pattern to the eruptive history Hawaiian ScenarioCyclic pattern to the eruptive history1. Pre-shield-building stage somewhat alkaline and variable2. Shield-building stage begins with tremendous outpourings of tholeiitic basaltsThis stage produces 98-99% of the total lava in HawaiiEarly, pre-shield-building stage that is more alkaline and variable, but quickly covered by the massive tholeiitic shieldsRecent 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 developmentThis stage produces 98-99% of the total lava in Hawaii
7Hawaiian Scenario3. Waning activity more alkaline, episodic, and violent (Mauna Kea, Hualalai, and Kohala). Lavas are also more diverse, with a larger proportion of differentiated liquids4. 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 basanitesThe two late alkaline stages represent 1-2% of the total lava outputThe two late alkaline stages represent 1-2% of the total lava outputNote all three OIB series are represented in HawaiiIs this representative of all islands? Probably not
8Evolution in the Series Tholeiitic, alkaline, and highly alkalineFigure After Wilson (1989) Igneous Petrogenesis. Kluwer.
9Alkalinity is highly variable Alkalis are incompatible elements, unaffected by less than 50% shallow fractional crystallization, this again argues for distinct mantle sources or generating mechanismsThe 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
10Trace ElementsHigh Field Strength Elements (HFS or HFSE) elements (Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) are also incompatible, and are enriched in OIBs > MORBsRatios of these elements are also used to distinguish mantle sources. For example:The Zr/Nb ratioN-MORBs are generally quite high (>30)OIBs are low (<10)
11Thus all appear to have distinctive sources Trace ElementsThe 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 MORBsThe ratios of incompatible elements have been employed to distinguish between source reservoirsN-MORB: the K/Ba ratio is high (usually > 100)E-MORB: the K/Ba ratio is in the mid 30’sOITs range from 25-40, and OIAs in the upper 20’sThus all appear to have distinctive sources
12Trace Elements: REEsNote that ocean island tholeiites (OITs represented by the Kilauea and Mauna Loa samples) overlap with MORB and are not unlike E-MORBThe alkaline basalts have steeper slopes and greater LREE enrichment than the OIT’s. Some fall within the upper MORB field, but most are distinctFigure After Wilson (1989) Igneous Petrogenesis. .
13Isotope GeochemistryIsotopes 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
14mFigure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
15Mantle 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 characteristicsFigure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
162. 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 mantleSeveral 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 reservoirsFigure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
17Both 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 sourcesSince the Nd-Sr data for OIBs extends beyond the primitive values to truly enriched ratios, there must exist an ENRICHED mantle reservoirBoth EM reservoirs have similar enriched (low) Nd ratios (< )Figure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
185. PREMA (PREvalent MAntle) Also not a mixing end-memberPREMA represents another restricted isotopic range that is very common in ocean volcanic rocksAlthough 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 sourceFigure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
19Simple Mixing Models Ternary Binary All analyses fall within triangle determined by three reservoirsBinaryAll analyses fall between two reservoirs as magmas mixFigure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
20Note 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 After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).
21Pb is quite scarce in the mantle Low-Pb mantle-derived melts susceptible to Pb contaminationIncompatibles U, Pb, and Th are concentrated in continental crust204Pb is non-radiogenic.208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb increase as U and Th decayOceanic crust also has elevated U and Th content (compared to the mantle)So are sediments derived from oceanic and continental crustSo Pb is a sensitive measure of crustal (including sediment) components contaminating mantle isotopic systems93.7% of natural U is 238U, so 206Pb/204Pb will be most sensitive to a crustal-enriched componentMantle-derived melts are susceptible to contamination from U-Th-Pb-rich reservoirs which can add a significant proportion to the total PbU, Pb, and Th are concentrated in sialic reservoirs, such as the continental crust, which develop high concentrations of the radiogenic daughter Pb isotopes204Pb is non-radiogenic, so 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb increase as U and Th decayOceanic crust has elevated U and Th content (compared to the mantle) as will sediments derived from oceanic and continental crustPb is perhaps the most sensitive measure of crustal (including sediment) components in mantle isotopic systemsSince 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 componentU 234U 206PbU 207PbTh 208Pb
22Figure 14-7. After Wilson (1989) Igneous Petrogenesis. Kluwer. 207Pb/204Pb vs. 206Pb/204Pb data for Atlantic and Pacific ocean basaltsGeochron = 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 systemsThe remaining mantle reservoir: HIMU (high mu) proposed to account for this great radiogenic Pb enrichment pattern207Pb/204Pb vs. 206Pb/204Pb data for Atlantic and Pacific ocean basaltsGeochron = 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
23m = 238U/204Pb (evaluate uranium enrichment) HIMU reservoir: very high 206Pb/204Pb ratioSource with high U,yet not enriched in Rb (has modest 87Sr/86Sr)Old enough (> 1 Ga) to ® observed isotopic ratiosHIMU 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”
24EMI and EMIIHigh 87Sr/86Sr require initially high Rb & long time to ® 87SrCorrelates with continental crust (or sediments derived from it)Oceanic crust and sediment are other likely candidates
25HIMU is also 208Pb enriched, so this reservoir is enriched in Th as well as U Dupré and Allègre (1983),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 UThis highly enriched EM component has been called the DUPAL component, named for Dupré and Allègre (1983), who first described it207Pb/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)
26Other isotopic systems that contribute to our understanding of mantle reservoirs and dynamics He IsotopesNoble gases are inert and volatile4He is an alpha particle, produced principally by a-decay of U and Th, enriching primordial 4He3He 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 denominatorCommon reference is RA (air) = 1.39 x 10-6
27He IsotopesN-MORB is fairly uniform at 8±1 RA suggesting an extensive depleted (degassed) DM- type N-MORB sourceFigure 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).
28He IsotopesOIB 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 signatureOIB 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).
29He IsotopesPHEM (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).
30He Isotopes SummaryShallow mantle MORB source is relatively homogeneous and depleted in HeOIBs have more primordial (high) 3He/4He, but still degassed and less than primordial ( RA) 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
31Re/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
32Os 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 → 187OsFigure 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).
33Other Mantle Reservoirs FOZO (focal zone): another “convergence” reservoir toward which many trends approach. Thus perhaps a common mixing end-memberFOZOFigure After Hart et al., 1992).
34Deeper 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 sedimentsEMISlightly enrichedDeeper continental crust or oceanic crustEMIIMore enrichedSpecially in 87Sr (Rb parent) and Pb (U/Th parents)Upper continental crust or ocean-island crustIf the EM and HIMU = continental crust (or older oceanic crust and sediments), only ® deeper mantle by subduction and recyclingTo remain isotopically distinct: could not have rehomogenized or re-equilibrated with rest of mantle
35The 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.
36Pick any two points on an equilibrium curve Mantle QuestionsIs 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 curvedDG = 0 = DVdP - DSdTThusdPdTSV=DClapeyron Eq.
37No Effect Retards Penetration Enhances Penetration → 2-Layer Mantle Model→ Whole-Mantle mixingFigure Effectiveness of the 660-km transition in preventing penetration of a subducting slab or a rising plume
38Case 1: dP/dT at 660 km is negative Figure Schematic diagram of a 2-layer dynamic mantle modelThe 660 km transition is a sufficient density barrier to separate lower mantle convectionOnly significant things that can penetrate this barrier are vigorous rising hotspot plumes and subducted lithosphereSubducted 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 GradientRange for bothHighest at Surfacewater and cold surfaceIn the future we will often use average values rather than the ranges
39Case 2: dP/dT at 660 km is positive Continental Gradient higher than Oceanic GradientRange for bothHighest at Surfacewater and cold surfaceIn the future we will often use average values rather than the rangesFigure 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) .
402-layer Model for Oceanic Magmatism ContinentalReservoirsDMOIBEM and HIMU from crustal sources (subducted OC + CC seds)Figure Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).
41Whole 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 Schematic model for oceanic volcanism. Nomenclature from Zindler and Hart (1986) and Hart and Zindler (1989).