Geol 2312 Igneous and Metamorphic Petrology Lecture 11A Ocean Island Basalts (OIB) February 24, 2016

Global Variability of MORB Thermal Structure of the Mantle

Intraplate “Hot Spot” Volcanism Figure 14.1. Map of relatively well-established hotspots and selected hotspot trails (island chains or aseismic ridges). Hotspots and trails from Crough (1983) with selected more recent hotspots from Anderson and Schramm (2005). Also shown are the geoid anomaly contours of Crough and Jurdy (1980, in meters). Note the preponderance of hotspots in the two major geoid highs (superswells).

Ocean Island Basalts (OIB) Plume-influenced Volcanism

Mantle Plumes Photograph of a laboratory thermal plume of heated dyed fluid rising buoyantly through a colorless fluid. Note the enlarged plume head, narrow plume tail, and vortex containing entrained colorless fluid of the surroundings. After Campbell (1998) and Griffiths and Campbell (1990). Winter 2010, Fig. 14.20

TWO PRINCIPAL MAGMA SERIES Types of OIB Magmas TWO PRINCIPAL MAGMA SERIES Tholeiitic (dominant type) Parent: ocean island tholeiitic basalt (OIT) Similar to MORB, but some distinct chemical and mineralogical differences Alkaline series (subordinate) Parent: ocean island alkaline basalt (OIA) Two principal alkaline sub-series Silica undersaturated Slightly silica oversaturated (less common) Modern volcanic activity of some islands is dominantly tholeiitic (for example Hawaii and Réunion), 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 Volcanism Staging of Alkaline and Tholeiitic Magma Series

Effect of Variable Degrees of Partial Melting Alkaline Basalts Tholeiitic Basalts

Major Element Chemistry of OIB Variable alkalinity likely reflects variable depths and degrees of partial melting in the plume and variable degrees of mixing and re-equilbration as magma rises through the mantle plume to the ocean crust. Winter (2001) Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

REE Compositions of OIB Alkali basalts strongly LREE-enriched  low degrees of partial melting Similar to E-MORB, but stronger depletion of HREE for both tholeiitic and alkali magmas series  deep garnet-bearing source for both with re-equilibration at shallower depths Lack of Eu anomaly indicates no significant fractionation of plagioclase

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

A Model for Ocean Magmatism based on Isotope Geochemistry Chondritic (undepleted) mantle From recycled ocean crust Mantle Reservoirs (based on radiogenic isotopes) Low Sr87/Sr86 mantle Low Nd143/Nd144 mantle Previously melted mantle U-enriched mantle Winter (2001) Figure 14-10. Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993). Chondritic (undepleted) mantle