Chapter 14 Spring 2010 - Chapter 14: Ocean Intraplate...

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Unformatted text preview: Chapter 14: Ocean Intraplate Volcanism Ocean islands and seamounts Commonly associated with hot spots Figure 14-1. After Crough (1983) Ann. Rev. Earth Planet. Sci., 11, 165- Types of OIB Magmas q Two principal magma series Tholeiitic series (dominant type) Parental ocean island tholeiitic basalt, or OIT 3 Similar to MORB, but some distinct chemical and mineralogical differences 3 q Alkaline series (subordinate) Parental ocean island alkaline basalt, or OIA 3 Two principal alkaline sub-series v silica undersaturated v slightly silica oversaturated (less common series) 3 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 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, posterosional stage. Characterized by highly alkaline and silica-undersaturated magmas, including alkali basalts, nephelinites, melilite basalts, and basanites Hawaiian Scenario Tholeiitic, alkaline, and highly alkaline Evolution in the Series Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer. Trace Elements q q The LIL trace elements (K, Rb, Cs, Ba, Pb2+ and Sr) are incompatible and are all enriched in OIB magmas with respect to MORBs The ratios of incompatible elements have been employed to distinguish between source reservoirs 3 3 3 N-MORB: the K/Ba ratio is high (usually > 100) E-MORB: the K/Ba ratio is in the mid 30's OITs range from 25-40, and OIAs in the upper 20's Thus all appear to have distinctive sources OIB Chemistry q Depletion in HREE indicating relatively deep melting depths (>80 km) q Strong enrichment in highly incompatible trace elements (REE, LILE, HFSE) indicating undepleted (primitive) and/or "enriched" mantle sources Trace Elements q q HFS 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 3 The Zr/Nb ratio v N-MORB generally quite high (>30) v OIBs are low (<10) Trace Elements: REEs These melts must have segregated from the mantle at depths > 60 km q OIBs: 87Sr/86Sr = 0.7035 to 0.710 and Nd = +5 to -5, primitive and/or enriched mantle source(s) Mantle Reservoirs 1. DM (Depleted Mantle) = NMORB source 2. BSE (Bulk Silicate Earth) or the Primary Uniform Reservoir 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 5. PREMA (PREvalent MAntle) 6. HIMU - subducted and recycled oceanic crust HIMU (high ) proposed to account for this great radiogenic Pb enrichment pattern Figure 14-7. After Wilson (1989) Igneous Petrogenesis. Kluwer. OIB Petrogenesis q Adiabatic decompression melting associated with deep upwelling of anomalously hot (+100-200 C) mantle. Partial melting initiated >100 km depth well within garnet stability field (explains common HREE depletion). Relatively enriched trace element and isotope signatures indicate enriched mantle source components. q q OIB Petrogenesis Continental Reservoirs DM OIB EM and HIMU from crustal sources (subducted OC + CC seds) ...
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