Ocean islands and seamounts

Commonly associated with hot spots

Chapter 14: Ocean Intraplate Volcanism

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

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

Types of OIB Magmas

Two principal magma series

Tholeiitic 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 Runion), 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)

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.

Alkalinity is highly variableAlkalis 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

The 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 reservoirs N-MORB: the K/Ba ratio is high (usually > 100)E-MORB: the K/Ba ratio is in the mid 30sOITs range from 25-40, and OIAs in the upper 20s

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 > MORBsRatios of these elements are also used to distinguish mantle sourcesThe Zr/Nb ratioN-MORB generally quite high (>30)OIBs are low (

Trace Elements: REEs

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

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

Trace Elements: REEs

La/Yb (REE slope) correlates with the degree of silica undersaturation in OIBs

Highly undersaturated magmas: La/Yb > 30OIA: closer to 12OIT: ~ 4(+) slopes E-MORB and all OIBs N-MORB (-) slope and appear to originate in the lower enriched mantle

In Chapter 10, I argued that MORB tholeiites probably originated in the depleted upper mantle, and alkali basalts in the enriched lower mantle reservoir. Now it appears that E-MORB and ocean island tholeiites also have a source in the deeper reservoir (high % PM can still tholeiite from an enriched source)

MORB-normalized Spider Diagrams

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

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

Isotope Geochemistry

Isotopes do not fractionate during partial melting of fractional melting processes, so will reflect the characteristics of the sourceOIBs, 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

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.

Sr - Nd Isotopes

Figure 13-12. Data from Ito et al. (1987) Chemical Geology, 62, 157-176; and LeRoex et al. (1983) J. Petrol., 24, 267-318.

High values of 87Sr/86Sr and low values of 144Nd/143Nd are associated with mantle enrichment

OIBs show considerably more variation than MORB as seen on next frame

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

m

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 enrichedjust 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

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 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?

Pb Isotopes

Pb produced by radioactive decay of U & Th

9-20238U 234U 206Pb

9-21235U 207Pb

9-22232Th 208Pb

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

Pb is quite scarce in the mantle

Mantle-derived melts susceptible to contaminationU, Pb, and Th are concentrated in continental crust (high radiogenic daughter Pb isotopes)204Pb 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 a sensitive measure of crustal (including sediment) components in 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 component

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, 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 m) proposed to account for this great radiogenic Pb enrichment pattern

m = 238U/204Pb (evaluate uranium enrichment)HIMU reservoir has a very high 206Pb/204Pb ratio, suggestive of a source with high U, yet not enriched in Rb, and old enough (> 1 Ga) to develop the observed isotopic ratios HIMU models: subducted and recycled oceanic crust (possibly contaminated by seawater), localized mantle lead loss to the core, and Pb-Rb removal by those dependable (but difficult to document) metasomatic fluidsmu = 238U/204Pb, and is used to evaluate uranium enrichmentThe HIMU reservoir is quite distinctive in the Pb system, having a very high 206Pb/204Pb ratio, suggestive of a source with high U, yet not enriched in Rb, and old enough (> 1 Ga) to develop the observed isotopic ratios by radioactive decay over timeSeveral models have been proposed for this reservoir, including subducted and recycled oceanic crust (possibly contaminated by seawater), localized mantle lead loss to the core, and Pb-Rb removal by those dependable (but difficult to document) metasomatic fluidsThe similarity of the rocks from St. Helena Island to the HIMU reservoir has led some workers to call this reservoir the St. Helena component

The high Sr ratios in EMI and EMII also require a high Rb content and a similarly long time to produce the excess 87SrThis signature correlates well with continental crust (or sediments derived from it)Oceanic crust and sediment are other likely candidates for these reservoirsThe 207Pb/204Pb data, especially from the northern hemisphere ~ a linear mixing line between DM and HIMU, a line called the Northern Hemisphere Reference Line (NHRL)

The data from the southern hemisphere, particularly from the Indian Ocean departs from this line, and appears to include a larger EM component (probably EMII)

This should be apparent from Fig. 14-7, which shows the 208Pb/204Pb data

Figure 14-8. After Wilson (1989) Igneous Petrogenesis. Kluwer. Data from Hamelin and Allgre (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 Dupr and Allgre (1983), who first described it

Can map the geographic distribution of the isotopic data

Figure 14-9. From Hart (1984) Nature, 309, 753-756.

Contours are for D8/4 which is a quantitative way of estimating the distance that an isotopic data set for 208Pb and 204Pb plots above the NHRL

Why this enriched anomaly extends as a band across the southern hemisphere at about 30o S is still a mystery

Isotopically enriched reservoirs (EMI, EMII, and HIMU) are too enriched for any known mantle process, and they correspond to crustal rocks and/or sedimentsEMI (slightly enriched) correlates with lower continental crust or oceanic crustEMII is more enriched, especially in radiogenic Sr (indicating the Rb parent) and Pb (U/Th parents) correlates with the upper continental crust or ocean-island crust

If the EM and HIMU reservoirs represent continental crust (or possibly older oceanic crust and sediments) they could only be introduced into the deeper mantle by subduction and recycling

To remain isotopically distinct, however, they could not have fully rehomogenized or re-equilibrated isotopically with the rest of the mantle

A Model for Oceanic Magmatism

DM

OIB

Continental

Reservoirs

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).

Table 14-4

. Alkali/silica ratios (regression) for selected

ocean island lava suites.

Island

Alk/Silica

Na

2

O/SiO

2

K

2

O/SiO

2

Tahiti

0.86

0.54

0.32

Principe

0.86

0.52

0.34

Trinidade

0.83

0.47

0.35

Fernando de Noronha

0.74

0.42

0.33

Gough

0.74

0.30

0.44

St. Helena

0.56

0.34

0.22

Tristan da Cunha

0.46

0.24

0.22

Azores

0.45

0.24

0.21

Ascension

0.42

0.18

0.24

Canary Is

0.41

0.22

0.19

Tenerife

0.41

0.20

0.21

Galapagos

0.25

0.12

0.13

Iceland

0.20

0.08

0.12