BASALTS

  1. MORB
  2. OIB
  3. CFBs
  4. CABs and IABs

I. Mid-ocean ridge basalts (MORBs)

A. 70% of Earth's surface covered by oceanic basalts, so highly voluminous. Formed at spreading centers, or oceanic ridges which make up incredibly long mountain chains. Pillow basalts, flows

B. Petrography - variable

mineralogy - (modal) ol + plag. Sometimes augite

C. Chemistry - remarkable similarities worldwide (in major elements)

Restricted compositional range (short differentiation sequence) - No rhyolites!

D. Magma generation - Not totally understood

Upwelling asthenosphere, melts as it rises

E. Anomalous MORB - e.g. Iceland

-much more voluminous volcanism, hotspot coincides with spreading axis (only place where ridge is exposed above sea level.

F. More detailed view of this process:

Formation of oceanic crust was at first studied through ophiolites. Now studies seismically and by drilling, so lots of new information.

Review of the ophiolite information:

Ophiolites - oceanic crust exposed on land (obducted - whereas most is subducted)

Best examples: Oman, Newfoundland, Papua, Troodos (Cypress). All show characteristic stratigraphy

 

OLD IDEA:

 

NEW IDEAS:

From seismic data & drilling:

Figures used in lecture

to be added by 5/5

 

II. Oceanic Island Basalt (OIBs) - intraplate

A. Conspicuous expression - chains of volcanic islands in Pacific

Oldest islands at NW end of chain are 80 - 75 Ma. Youngest at SW end are 0 Ma

Also along mid-Atlantic ridge

Also seamounts & fracture zones

B. OIBs vs. MORBs

C. OIBs closest to spreading axes tend to be tholeiitic (e.g. Iceland, Galapagos)

On islands most distant from spreading centers, alkaline rocks are more abundant and strongly differentiated

ex: Hawaii, Tahiti

D. Fractionation trends: NO SILICA OVERSATURATED ROCKS HERE!

1. Si saturated (tholeiites)

Olivine tholeiite -> Fe-rich andesites -> rhyolite or trachyte

2. Si undersaturated to saturated - straddle boundary

Alkali olivine basalt or ankaramite -> Hawaiite -> mugearite -> benmoreite

(benmoreites can then go to Q normative trachytes and pantellerites)

3. Strongly Si undersaturated

Basanite -> mafic phonolite -> felsic phonolite Nephelinite -> mafic phonolite -> felsic phonolite

E. Model

1. Hawaiian Islands

Hawaii - 5 volcanic peaks above sea level

Mauna Loa & Mauna Kea highest mountains on earth (>30000 feet above sea floor)

2. Observations: both tholeiitic and alkaline rocks (tholeiitic most abundant)

Tholeiitic rocks - picritic basalts (ol-rich). Olivine tholeiites

Alkaline rocks - ankaramite basalts (pyroxene-olivine phyric). Alkali olivine basalts

3. Eruptive history:

Pre-shield - Alkali olivine basalts. Example: Loihi

Shield-building stage - development of central caldera over course of 1 Ma. Example: Kilauea

Massive volumes of tholeiite erupt from caldera

(central Kilauea caldera had lava lake in it until 1924)

Magma chamber is at upper levels of volcano

Caldera-filling stage - lavas erupt in caldera, fill it up. Example: Haleakala, Mauna Kea

tholeiitic -> alkaline over time. Ankaramites, hawaiites, benmoreiites

Then quiet erosional stage - 1-2 million years

Post shield/ erosional stage - Example: Diamond Head

Strongly alkaline magmas. Nepheline basanites rich in melilite

Erupts on flanks of volcano

4. Origin: mantle plume

 

III. Continental Flood Basalt (CFBs) -

1. Huge volumes of basalts erupted over short periods of time (individual flows must be >1 km3)

Fissures rather than central vents. So get mazes of dikes

2. Examples:

Provinces

Parana: Paraguay and Brazil, 600,000 km3 Jr

Karoo: S. Africa; 2,400,000 km3 Jr

Keeweenawan: Lake Superior; 400,000 km3 700 ma

Deccan Traps: India; >500,000 km3 K-T bound

Columbia River: E. Wash, Oregon; 200,000 km3 Mio-Pliocene

Columbia River Plateau. Linked geographically with Snake River plain & Yellowstone. Passage of hotspot? Back arc basin?

3. Emplaced rapidly. e.g. Roza member of Columbia River basalts

175 km long x 5 km wide. Two flows, each 700 km3

Tens or hundreds of cubic km per day

Whole flow emplaced within 7 days

1km3/day/1km dike length

4. Magma chambers probably very deep, so no calderas

5. Vents, very unimpressive. Low H2O so no violent eruptions

6. Composition: mostly olivine tholeiites, some quartz tholeiites. No compositional variation with time, not real primitive in terms of Mg. But where did they get differentiated? Where is cumulate pile? Base of crust has high seismic velocity, so deep magma chamber.

7. Hyptheses for origin of flood basalts:

(a) initiation of rifting (continental breakup)

(b) initiation of mantle plume (e.g., Columbia River basalt, Deccan traps)