geo dina mica

Click here to load reader

Post on 26-Sep-2015

218 views

Category:

Documents

0 download

Embed Size (px)

DESCRIPTION

GEODINAMICA EE

TRANSCRIPT

  • 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 incr