Chapter 14 Ocean Intraplate Chapter 14 Ocean Intraplate Volcanism (from Plumes)Volcanism (from Plumes)

Ocean islands and seamountsOcean islands and seamounts

Commonly associated with Commonly associated with hot spotshot spots

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

Ocean islands and seamountsOcean islands and seamounts

Commonly associated with Commonly associated with hot spotshot spots

Types of OIB MagmasTypes of OIB MagmasTwo principal magma seriesTwo principal magma series

TholeiiticTholeiitic series (dominant type) series (dominant type) Parental ocean island tholeiitic basalt, or Parental ocean island tholeiitic basalt, or OITOIT Similar to MORB, but some distinct chemical Similar to MORB, but some distinct chemical

and mineralogical differencesand mineralogical differences AlkalineAlkaline series (subordinate) series (subordinate)

Parental ocean island alkaline basalt, or Parental ocean island alkaline basalt, or OIAOIA Two principal alkaline sub-seriesTwo principal alkaline sub-series

silica undersaturatedsilica undersaturated slightly silica oversaturatedslightly silica oversaturated ( (less common less common

series) series)

Tholeiitic and Alkaline examplesTholeiitic and Alkaline examples Modern volcanic activity of some islands is Modern volcanic activity of some islands is

dominantly dominantly tholeiitictholeiitic (for example Hawaii (for example Hawaii and Réunion).and Réunion).

Other islands are more Other islands are more alkalinealkaline in character in character (for example Tahiti in the Pacific and a (for example Tahiti in the Pacific and a concentration of islands in the Atlantic, concentration of islands in the Atlantic, including the Canary Islands, the Azores, including the Canary Islands, the Azores, Ascension, Tristan da Cunha, and Gough)Ascension, Tristan da Cunha, and Gough)

Hawaii data, both tholeiitic and alkalineHawaii data, both tholeiitic and alkaline

Hawaiian ScenarioHawaiian ScenarioCyclic pattern to the eruptive historyCyclic pattern to the eruptive history

1. Pre-shield-building stage1. Pre-shield-building stage somewhat somewhat alkaline and variablealkaline and variable

2. Shield-building stage 2. Shield-building stage begins with begins with tremendous outpourings of tremendous outpourings of tholeiitic tholeiitic basaltsbasalts

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

Hawaiian ScenarioHawaiian Scenario3.3. Waning activity more Waning activity more alkalinealkaline, episodic, and , episodic, and

violent (Mauna Kea, Hualalai, and Kohala). violent (Mauna Kea, Hualalai, and Kohala). Lavas are also more diverse, with a larger Lavas are also more diverse, with a larger proportion of differentiated liquidsproportion of differentiated liquids

4.4. A long period of dormancy, followed by a A long period of dormancy, followed by a late, late, post-erosional stagepost-erosional stage. Characterized by . Characterized by highly alkalinehighly alkaline and silica-undersaturated and silica-undersaturated magmas, including alkali basalts, nephelinites, magmas, including alkali basalts, nephelinites, melilite basalts, and basanitesmelilite basalts, and basanites

The two late alkaline stages represent 1-2% of The two late alkaline stages represent 1-2% of the total lava outputthe total lava output

Evolution in the SeriesEvolution in the SeriesTholeiiticTholeiitic, alkaline, and , alkaline, and highly alkalinehighly alkaline

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

Table 14-4. Alkali/silica ratios (regression) for selected ocean island lava suites.

Island Alk/Silica Na2O/SiO2 K2O/SiO2

Tahiti 0.86 0.54 0.32Principe 0.86 0.52 0.34Trinidade 0.83 0.47 0.35Fernando de Noronha 0.74 0.42 0.33Gough 0.74 0.30 0.44St. Helena 0.56 0.34 0.22Tristan da Cunha 0.46 0.24 0.22Azores 0.45 0.24 0.21Ascension 0.42 0.18 0.24Canary Is 0.41 0.22 0.19Tenerife 0.41 0.20 0.21Galapagos 0.25 0.12 0.13Iceland 0.20 0.08 0.12

Alkalinity is highly variableAlkalinity is highly variable Alkalis are incompatible elements, unaffected by less Alkalis are incompatible elements, unaffected by less

than 50% shallow fractional crystallization, this again than 50% shallow fractional crystallization, this again argues for argues for distinct mantle sourcesdistinct mantle sources or generating or generating mechanismsmechanisms

Trace ElementsTrace 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)

Trace ElementsTrace Elements The large ion lithophile (The large ion lithophile (LILLIL) trace elements (K, Rb, ) trace elements (K, Rb,

Cs, Ba, PbCs, Ba, Pb2+2+ and Sr) are incompatible and are and Sr) are incompatible and are all all enriched in OIB magmasenriched in OIB magmas with respect to MORBs with respect to MORBs

The The ratiosratios of incompatible elements have been of incompatible elements have been employed to distinguish between source reservoirs employed to distinguish between source reservoirs N-MORB: the K/Ba ratio is high (usually > 100)N-MORB: the K/Ba ratio is high (usually > 100) E-MORB: the K/Ba ratio is in the mid 30’sE-MORB: the K/Ba ratio is in the mid 30’s OITs range from 25-40, and OIAs in the upper 20’sOITs range from 25-40, and OIAs in the upper 20’s

Thus all appear to have distinctive sourcesThus all appear to have distinctive sources

Trace Elements: REEsTrace Elements: REEs

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

Note that ocean island Note that ocean island tholeiites (OITs tholeiites (OITs represented by the represented by the Kilauea and Mauna Loa Kilauea and Mauna Loa samples) overlap with samples) overlap with MORB and are not MORB and are not unlike E-MORBunlike E-MORB

The alkaline basalts The alkaline basalts have steeper slopes and have steeper slopes and greater LREE greater LREE enrichment than the enrichment than the OIT’s. Some fall within OIT’s. Some fall within the upper MORB field, the upper MORB field, but most are distinctbut most are distinct

Isotope GeochemistryIsotope 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

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

Mantle ReservoirsMantle Reservoirs

1. DM (Depleted Mantle) = N-MORB source

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

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

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

5. PREMA (PREvalent MAntle)

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

Simple Mixing ModelsSimple Mixing Models

BinaryAll analyses fall between

two reservoirs as magmas mix

TernaryAll analyses fall within

triangle determined by three reservoirs

Figure 14.7. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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

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

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

9-20 238U 234U 206Pb9-21 235U 207Pb9-22 232Th 208Pb

Figure 14-7. After Wilson (1989) Igneous Petrogenesis. Kluwer.

207207Pb/Pb/204204Pb vs. Pb vs. 206206Pb/Pb/204204Pb data for Atlantic and Pacific ocean basaltsPb data for Atlantic and Pacific ocean basalts

GeochronGeochron = simultaneous evolution of = simultaneous evolution of 206206Pb and Pb and 207207Pb in a rock/reservoirPb in a rock/reservoir

= line on which all modern single-stage (not disturbed or reset) Pb isotopic systems, such as BSE = 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. (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. Nor do they fall within the EMI-EMII-DM triangle, as they appear to do in the Nd-Sr systems. The remaining mantle reservoir: The remaining mantle reservoir: HIMUHIMU (high (high ) proposed to account for this great radiogenic ) proposed to account for this great radiogenic Pb enrichment patternPb enrichment pattern

= 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)

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

Figure 14.10 After Wilson (1989) Igneous Petrogenesis. Kluwer. Data from Hamelin and Allègre (1985), Hart (1984), Vidal et al. (1984).

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)

HIMU is also HIMU is also 208208Pb enriched, so Pb enriched, so this reservoir is enriched in Th this reservoir is enriched in Th as well as Uas well as U

Dupré and Allègre (1983), Dupré and Allègre (1983),

He IsotopesHe IsotopesNoble gases are inert and volatile4He is an alpha particle, produced principally by -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 denominator

Common reference is RA (air) = 1.39 x 10-6

Other isotopic systems that contribute to our understanding of mantle reservoirs and dynamics

He IsotopesHe 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 IsotopesHe Isotopes

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).

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

Low R/RA has higher 4He due to recycled (EM-type?) U and Th.

He IsotopesHe Isotopes

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).

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.

He Isotopes SummaryHe 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 IsotopesRe/Os system and Os Isotopes187Re → 187Os

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 FOZOOs Isotopes plus the FOZO

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).

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 Crust is high (with little (with little overlap to overlap to peridotites).peridotites).

187 R

e →

187 O

s

FOZO (focal zone): another “convergence” reservoir toward which many trends approach. Thus perhaps a common mixing end-member

Other Mantle ReservoirsOther Mantle Reservoirs

Figure 14.15. After Hart et al., 1992).

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 MantleThe 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.

Mantle QuestionsMantle 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

dG = 0 = VdP - SdT

ThusdP

dT

S

V

Clapeyron Eq.Clapeyron Eq.

Figure 14.16. Effectiveness of the 660-km transition in preventing penetration of a subducting slab or a rising plume

No Effect Retards Penetration Enhances Penetration→ 2-Layer Mantle Model → Whole-Mantle mixing

•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).

Case 1: dP/dT at 660 km is negative

Figure14.17. Whole-mantle convection Whole-mantle convection model with geochemical heterogeneity preserved as blobs of fertile mantle in a host of depleted 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, 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) .probably rising from a discontinuous D" layer of dense “dregs” at the base of the mantle. After Davies (1984) .

Case 2: dP/dT at 660 km is positive

2-layer Model for Oceanic Magmatism2-layer Model for Oceanic Magmatism

DMDM

OIBOIB

ContinentalContinental

ReservoirsReservoirs

EM and HIMU from EM and HIMU from crustalcrustal sources (subducted OC + CC seds) 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 MagmatismWhole Mantle Model for Oceanic Magmatism

Figure 14.19. Schematic model for oceanic volcanism. Nomenclature from Zindler and Hart (1986) and Schematic model for oceanic volcanism. Nomenclature from Zindler and Hart (1986) and Hart and Zindler (1989). Hart and Zindler (1989).