Potential Generation and Ejection Temperatures of Archean and Proterozoic Komatiitic Lava in Karelia

Thermal regime evolution is an important problem in the understanding of the geological history of Earth in Precambrian time. To determine possible melt ejection and magma formation temperatures of peridotitic komatiites from the Vedlozero-Segozero greenstone belt, where the volcanics have ages of 2.9-3.1Ga, the Hizovaara structure-2.8Ga, the Kostomuksha structure- 2.79 Ga and the Vetreniy Poyas -2.39Ga. Our calculations have shown that the maximum komatiite ejection temperatures were 1560-1600 degrees in the Vedlozero-Segozero greenstone belt [Tab.1]. (Author used a Nisbet method [3])
The above magma outflow temperatures correlate well with those obtained for almost coeval komatiites from the Alexo flow, Ontario, Canada, and for 2.7 Ga komatiites of Reliance Formation, Belingwe greenstone belt, Zimbabwe [3].
Generation and ejection temperatures of komatiitic lava and Max. MgO contents in liquids

Structure	Age Ga	Ejection temperatures degrees C	Generation temperatures degrees C	Max. MgO in liq.
Barberton	3.45	1600-1640	1900	32.97 %
Vedlozero-Segozero	2.9-3.1	1560-1600	1850	30-31 %
Hizovaara	2.8	1560-1580	1800	28-29 %
Kostomuksha	2.79	1540-1580	1800	27-29 %
Belingwe	2.7	1520	1800	25.68 %
Alexo Township	2.7	1560	1800	28.10 %
Vetreniy Poyas	2.39	1480-1520	1650	24-26 %
Gorgona	0.155	1400	1600	20.60 %

It has been shown by correlating the ages of komatiites with their maximum eruption temperatures that the Mg contents of the melts generated tende to decrease. Consequently, their outflow temperatures decline with decreasing age. To determine the potential temperatures of the Archean mantle, komatiite eruption temperature data and relationships betwen komatiite generation conditions and P-T parameters in the sources of melts are used (McKenzie and Bickle 1988) . Calculations for the Barberton, Belingwe and Gorgona komatiites have been made by N.T.Arndt, C.Herzberg, and E.G.Nisbet. They obtained the following temperatures in the mantle sources: 1900 degrees for Barberton, 1800 degrees for Belingwe and 1600 degrees for Gorgona.{Fig.2},[Tab.1],[3]
Calculations made for the Baltic Shield have shown that in the Archean structures komatiite generation temperatures were approximately 1800~1850 in the source, and in the Proterozoic structures- 1600~1650 degrees in the source.[Tab.1] All the temperature values obtained indicate that the potential temperature of the upper Archean mantle was , on the average, 250-300 degrees higher than its present temperature during the 2.8-3.1 Ga period, and 450-500 degrees higher during the 3.45 Ga period (with allowance for differences in depthes and pressures of their formation ).
References:
1. Herzberg C., Ohtani E., Origin of komatiite at high presures // Earth and Planet. Sci. Lett. 1988.Vol.88, p.321-329.
2. Herzberg C. Depth and Degree of Melting of Komatiites// J. Geophys. Res.1992. Vol.97, N 4, p.4521-4540.
3. Nisbet E.G.,Cheadle M.J., Arndt N.T., Bickle M.J. Constraining the potential temperature of the Archaean mantle A review of the evidence from komatiites// Lithos. 1993.Vol.30, p.291-307.