Ocean Intraplate Volcanism

(Chapter 14)

last update:10/11/05

Ocean Intraplate Volcanism

Ocean islands and seamounts

Commonly associated with hot spots

 

Types of OIB Magmas (OIB/MORB=9/1)

Two principal magma series

Tholeiitic series (dominant type)

Parental ocean island tholeiitic basalt, or OIT

Similar to MORB, but some distinct chemical and mineralogical differences

Alkaline series (subordinate)

Parental ocean island alkaline basalt, or OIA

Two principal alkaline sub-series

silica undersaturated

slightly silica oversaturated (less common series)

 

Hawaiian Scenario

Cyclic, pattern to the eruptive history

1. Pre-shield-building stage somewhat alkaline and variable (low degrees of partial melt)

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, post-erosional stage. Characterized by highly alkaline and silica-undersaturated magmas, including alkali basalts, nephelinites, melilite basalts, and basanites

 

Evolution in the Series

Tholeiitic, alkaline, and highly alkaline

 

Alkalinity is highly variable

Alkalis are incompatible elements, unaffected by less than 50% shallow fractional crystallization, this again argues for distinct mantle sources or generating mechanisms

 

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 MORBs

The 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 30’s

OITs range from 25-40, and OIAs in the upper 20’s

Thus all appear to have distinctive sources

La/Yb (REE slope) correlates with the degree of silica undersaturation in OIBs

Highly undersaturated magmas: La/Yb > 30

OIA: closer to 12

OIT: ~ 4

(+) slopes ® E-MORB and all OIBs ¹ N-MORB (-) slope and appear to originate in the lower enriched mantle

 

 

 

Isotope Geochemistry

Isotopes do not fractionate during partial melting of fractional melting processes, so will reflect the characteristics of the source

A Model for Oceanic Magmatism

OIB magma is likely from a deep source, but distinct from E-MORB

The conversion of basalt/gabbro to dense eclogite (pyroxene-garnet rock) may facilitate sinking of ocean crust until the 660 km depth (or maybe lower)