Evidences for geochemically distinct mantle components

Transcription

1 Evidences for geochemically distinct mantle components 1

2 Mantle Array Oceanic basalts, including seamounts, oceanic islands and middle ocean ridge basalts, were used. 2

3 Binary All analyses fall between two reservoirs as magmas mix Simple Mixing Models Ternary All analyses fall within triangle determined by three reservoirs Figure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 3

4 Sr - Nd Isotopes 4 Silicate Figure Data from Ito et al. (1987) Chemical Geology, 62, ; and LeRoex et al. (1983) J. Petrol., 24, High values of 87Sr/86Sr and low values of 144Nd/143Nd are associated with mantle enrichment 4

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

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7 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 After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989). 7

8 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 After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989). 8

9 3. EMI = enriched mantle type I has lower 87 Sr/ 86 Sr (near primordial) 4. EMII = enriched mantle type II has high 87 Sr/ 86 Sr (> 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 (< ) Figure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989). 9

10 5. PREMA (PREvalent MAntle) Also not a mixing endmember 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 PREMA signature suggests that it may be a distinct mantle source Figure After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989). 10

11 Pb Isotopes Pb produced by radioactive decay of U & Th 238 U 206 Pb 235 U 207 Pb 232 Th 208 Pb 11

12 Pb is quite scarce in the mantle Low-Pb mantle-derived melts susceptible to Pb contamination of U-Th- Pb rich reservoirs U, Pb, and Th are concentrated in continental crust (producing high radiogenic daughter Pb isotopes) 204 Pb non-radiogenic: 208 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 206 Pb/ 204 Pb increase as U and Th decay Oceanic crust also has elevated U and Th content (compared to the mantle) as Sediments derived from oceanic and continental crust Pb is a highly sensitive measure of crustal (including sediment) components in mantle isotopic systems 99.3% of natural U is 238 U, so 206 Pb/ 204 Pb will be most sensitive to a crustal-enriched component 12

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14 NHRL: Th/U=4.0; NHRL a secondary isochrone(?): 1.77 b.y. Above NHRL Dupal rocks. 14

15 Figure After Wilson (1989) Igneous Petrogenesis. Kluwer. 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 15 The remaining mantle reservoir: HIMU (high mu) proposed to account for this great radiogenic Pb enrichment pattern

16 Figure After Wilson (1989) Igneous Petrogenesis. Kluwer. Data from Hamelin and Allègre (1985), Hart (1984), Vidal et al. (1984). 206 Pb/ 204 Pb 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) 16

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19 PRIMA: Primitive Mantle BSE: Bulk Silicate Earth PUM: Primitive Upper Mantle FOZO: Focal Zone (Depleted Lower Mantle) 19

20 Mantle components For the mantle isotope zoo at least four components are needed: DMM = depleted MORB mantle HIMU = High U/Pb component EM I = Enriched mantle I (low Nd) EM II = Enriched mantle II (high Sr) Please, memorize these and let s see again the plots! Isotopically enriched reservoirs (HIMU, EM I, EM II) are too enriched for any known mantle process, and they correspond to crustal rocks and to sediments. 20

21 Origin of the four mantle components DMM is ambient upper mantle, depleted ~2 Ga ago by extraction of the continents. However, MORB can be polluted by influence of nearby plumes, so not all MORB plot right at DMM (E-MORB): Isotopic composition of mid-atlantic ridge samples near the Azores hotspot Questions: How well mixed is DMM reservoir? Is even pure DMM recharged with a flux from somewhere? How does the upper mantle stay fertile over time? 21

22 Origin of the four mantle components HIMU (µ = 238 U/ 204 Pb /evaluate uranium enrichment/) very high 206 Pb/ 204 Pb ratio, high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios, low 87 Sr/ 86 Sr ratio Old enough (> 1 Ga) observed isotopic ratios Source: high in U, not enriched in Rb (ratio of 87 Sr/ 86 Sr): Subducted and recycled (ancient altered) oceanic crust (± seawater), which suffered extraction of Pb from the basaltic part of slab during alteration and at subduction zones leaving behind a high U/Pb residual component, which will evolve to high 206 Pb/ 204 Pb with time. It is necessary that Rb (and other alkalis) also be removed relative to Sr during alteration and subduction. [Others think HIMU is a component of metasomatically altered continental lithospheric mantle, or metasomatically enriched oceanic lithosphere (but difficult to documents such metasomatic fluids).] [Localized mantle lead loss to the core.] Note that HIMU-DMM mixing arrays are linear, which implies Sr/ Pb and Nd/Pb ratios are similar in these end members. 22

23 Origin of the four mantle components EM I slightly enriched in 87 Sr/ 86 Sr, but not in Pb, very low 143 Nd/ 144 Nd High 87 Sr/ 86 Sr require high Rb & long time to 87 Sr Source: it correlates with continental crust (or sediments derived from it), but oceanic crust and sediment are other likely candidates, but it is problematic, all kinds of ideas were in play... Recycling of delaminated subcontinental lithosphere Recycling of subducted ancient pelagic sediments (because of their high Th/U and low U,Th/Pb ratios) It also resembles lower continental crust, from xenoliths and granulite terranes. Perhaps EM I and EM II are distinguished by intracontinental differentiation, and EM I is recycled by delamination, whereas EM II is recycled by erosion and subduction. [H 2 O-rich and CO 2 -rich fluids mobilize trace elements differently. It is possible that HIMU and EM I could be complementary products of migration of CO 2 -rich fluids from continental lithospheric mantle into lower continental crust.] 23

24 Origin of the four mantle components EM II more enriched, especially in 87 Sr/ 86 Sr and radiogenic Pb High 87 Sr/ 86 Sr require high Rb & long time to 87 Sr Source: isotopic composition of young pelagic sediment is a pretty good match for EM II isotopes; continents (and hence also continent-derived sediments) have very high Pb concentrations, hence U/Pb is not very high and EM II does not evolve to especially enriched 206 Pb/ 204 Pb, but Th/U is high (due to remove of Th from seawater), so 208 Pb/ 204 Pb increases faster; Recycled oceanic crust and small amount of subducted terrigenous sediment Recycling of melt-impregnated oceanic lithosphere [Even though sediment signature is transferred to arc basalts at subduction zones, some sediment or some sediment-derived trace elements must be subducted, to arise elsewhere in OIBs.] Because Sr/Pb and Nd/Pb ratios are lower than in other components, mixing arrays towards EM II should be strongly curve in isotope ratio-ratio space, as observed. 24

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26 A Model for Oceanic Magmatism Continental Reservoirs DM OIB EM and HIMU from crustal sources (subducted OC + CC seds) Figure Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993). 26

27 The Upper Mantle as an Open System For Pb, we can prove that there is continuing input to the upper mantle-ocean crust system from some other longlived reservoir, probably the lower mantle. If this input balances Pb flux to continents at arcs, the upper mantle might be in steady-state for incompatible elements(?). This argument is based on Th/U ratios: the continental crust has a chondritic Th/U ratio (3.9), but the MORB source has a much lower Th/U ratio (2.5). If input to upper mantle is chondritic in Th/U, and output to continents is chondritic, upper mantle could be in steady state, even with a different Th/U ratio, but this requires a short residence time of Pb in upper mantle. 27