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

2. Global Variability of MORB Thermal Structure of the Mantle

3. Intraplate “Hot Spot” Volcanism
Figure 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).

4. Ocean Island Basalts (OIB) Plume-influenced Volcanism

5. 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

6. 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)

7. Hawaiian Volcanism Staging of Alkaline and Tholeiitic Magma Series

8. Effect of Variable Degrees of Partial Melting
Alkaline Basalts
Tholeiitic Basalts

9. 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 After Wilson (1989) Igneous Petrogenesis. Kluwer.

10. 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

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

12. 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 Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).
Chondritic (undepleted) mantle