Physics
Scientific paper
Mar 1973
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1973gecoa..37..651h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 37, Issue 3, pp.651-666
Physics
25
Scientific paper
The stability of the amphibole pargasite [NaCa 2 Mg 4 Al(Al 2 Si 6 ))O 22 (OH) 2 ] in the melting range has been determined at total pressures ( P ) of 1.2 to 8 kbar. The activity of H 2 O was controlled independently of P by using mixtures of H 2 O + CO 2 in the fluid phase. The mole fraction of H 2 O in the fluid ( X H 2 O * fl ) ranged from 1.0 to 0.2. At P < 4 kbar the stability temperature ( T ) of pargasite decreases with decreasing X H 2 O * fl at constant P . Above P 4 kbar stability T increases as X H 2 O * fl is decreased below one, passes through a T maximum and then decreases with a further decrease in X H 2 O * fl . This behavior is due to a decrease in the H 2 O content of the silicate liquid as X H 2 O * fl decreases. The magnitude of the T maximum increases from about 10°C (relative to the stability T for X H 2 O * fl = 1) at P = 5 kbar to about 30°C at P = 8 kbar , and the position of the maximum shifts from X H 2 O * fl 0.6 at P = 5 kbar to X H 2 O * fl 0.4 at P = 8 kbar . The H 2 O content of liquid coexisting with pargasite has been estimated as a function of X H 2 O * fl at 5 and 8 kbar P , and can be used to estimate the H 2 O content of magmas. Because pargasite is stable at low values of X H 2 O * fl at high P and T , hornblende can be an important phase in igneous processes even at relatively low H 2 O fugacities.
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