Jökull


Jökull - 01.12.1987, Side 40

Jökull - 01.12.1987, Side 40
concentration of N2 in the parent hot water was taken to be 0.71 mmoles/kg which corresponds to saturation in water at 5°C in contact with the atmosphere (the mean annual temperature in Krísuvík) using solubility data from Weiss (1970). N2 saturation at 25°C gives the fol- lowing expression: t = 135.9 + 63.14 • Qcn + 6.241 • Qcn- - 1.813 • Qcn' (2a) Equations (2) and (2a) can be used as a gas geother- mometer. Steam condensation in the upflow, if it occurs by conductive heat loss, does not affect the C02/N2 ratio significantly unless well over 95% of the steam condens- es. Therefore, discrepancy between the N2 c concentra- tions according to equation (1) and the C02/N2 geother- mometry estimates can be used to evaluate condensa- tion in the upflow, if it is assumed that this condensation occurs by conductive heat loss. For each C02/N2 temper- ature there is a corresponding value for N2 c (see equa- tion 1). From conservation of mass of nitrogen we have: N| = N: m(l—Zc) (3) and Zc = (N: m - N: C)/N: m (3a) where Zc is the fraction of steam which condensed (1—Zc is, therefore, the remaining steam fraction), N2 m is the measured N: concentration in the steam and equation (1) defines N2 c. The temperature which is to be inserted into equation (1) is derived from the CO:/N2 ratio (equa- tion 2). When steam condensation occurs by mixing with cold water the situation is more complicated than is the case with conductive heat loss. The cold water will contain dissolved N2 that will be transferred to the remaining steam and the N2 content of the initial steam will depend on the temperature of the parent water as well as on the temperature of the steam when the mixing occurs. Fet us assume that condensation occurs by mixing steam with cooler water under adiabatic conditions, that the masses of H20, CO: and N: are conserved, that negligible fraction of the CO: dissolves in the steam heated water and that the cold water is quantitatively degassed with respect to N:. Expressing the mass of water and steam as fractions gives: Xs, + Xw2 = 1 and X,.f = xs„ - Xc. (5) xw, = xw, + Xc (6) for the steam and water phases, respectively. The sub- scrips s and w note steam and water phases, respectively; i and f indicate the initial and final mass fractions and Xc is the mass fraction of steam of the total mass which condenses. The fraction of steam which has condensed, Zb, out of the initial steam fraction is thus defined as zb = Xjxs, (7) From conservation of heat, C02 and N2 we have: hs • Xs, + hw • Xw, = hs • Xs, + hf • Xw, (8) CO: c • Xs i = CO, m • Xs f (9) N2.c • Xs, + N2 k • Xw, = N: m • Xs, (10) h is the enthalpy of the steam and water phases. The subscripts s, w, i and f have the same notation as in equations (4) to (6); k indicates the N2 concentration in the cold water and m indicates the measured gas concen- tration in the steam; c designates calculated gas concen- trations from equation (1) in the case of N2. For CO: the CO: gas geothermometry function of Arnórsson and Gunnlaugsson (1985) was used: t = -44.1 + 269.25 • Qc - 76.88 • Qc2 + 9.52 • Qc3(U) t is in °C and Qc = logCO: in mmoles/kg. From equa- tions (4), (6) and (8) it is seen that C02 c = CO: m(l-Zb). Combining equations (4) to (7) into equation (10) and eliminiation of Xc, Xw, and Xs, yields: XS,(N2, + N: m(l—Zb) - N: c) = N:, (12) Similarly combination of equations (4) to (7) into equa- tion (8) gives: Xs,(Zb(hs-hf) + h, - hw) = h, - hw (13) or Zb = (N:.m - N2 c)/(N2 m + F) (14) (4) where F = N2, • (hs—hf)/(hf—hw). If condensation of 100°C steam occurs in 5°C water and N2, = 0.71 mmoles/ kg (the amount of N2 dissolved in water at 5°C — the 38
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