Ultramafic Rocks

See http://plate-tectonic.narod.ru/petrographyigneouslinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html
http://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L15-Ultra.html

I. Komatiites

A. Def: ultramafic lavas

-Known only from Precambrian: composition: high MgO (18-33%); first described from Komati River, South Africa - also found in Canada, Australia, Finland

-Part of greenstone belts = arcuate zones of ancient metamorphosed sedimentary + igneous rocks

B. Typical flow consists of several layers:

-Top is highly fractured

-Next is spinifex zone = texture consisting of long criss-crossing blades of olivine crystals that radiate or fan out downward into flow

-Sharp boundary with fine- to medium-grained peridotite, which develops knobby weathering in lowest part

C. Experimental work

-Komatiite liquidus >1650oC => quench crystals likely to form

-High T also => magms likely caused partial melting and assimilation on the way up

-As a result, komatiites have highly variable concentrations of trace elements (contamination)

D. Archean geotherm

-High temperature of komatiites => geotherm was higher/steeper in Archean: today mantle melting generates basalts, would need >50% partial melting to get high Mg of komatiites; experimental work shows melting at high T & P (i.e, deeper) can produce magnesian melts with lower % melting; also, most igneous rocks in greenstone belts are basalts

-Conclusion: basaltic melts formed <100 km depth, komatiitic melts at 150-200km

-Aftter the Archean, geotherm intersected solidus at depths too shallow to make the latter

II. Kimberlites

A. Def: unusual alkaline rock, often containing diamonds

-Found in cratons

-Ages = Precambrian - Cenozoic => stable source for these magmas.

B. Composition

-Has SiO2 = 30% and MgO = 23%

-Unlike other ultramafics, these have high TiO2, K2O, P2O5 & other incompatibles => derived by low % melting of fertile mantle

-Also, they have high Ni & Cr => no significant fractionation of olivine during ascent

C. Presence of diamonds + xenoliths suggest:

-Deep origin

-Emplaced rapidly

D. Occur in diatremes = pipelike body of breccia composed of fragments of country rock and mantle xenoliths, surrounded by kimberlite

-Diatremes are believed to be feeders to small shallow volcanic craters (maars) which form from gas explosions with little or no lava.

-Grades downward into massive dikes.

-"The emplacement of a diatreme resembles a cleanly drilled hole connecting the top of a dike or intersecting dike set with the surface of the Earth, the drilling products being left to fill the hole."