3. Results
The computed solution compositions for the Type 2 inclusions are high ionic strength 7.8 – 8.2
mol, slightly acidic pH approximately 5.4 brines with water activities of approximately 0.7 Table
1. Total dissolved solids account for approxi- mately 20 – 40 of the mass of the solutions.
Fig. 2. Comparison of computed concentrations of eclogite fluid inclusions ‘eclogite brines’ with seawater ‘seawater’, crossed box and destructively analyzed fluid inclusions ‘fluid inclusions’, small filled circles; Roedder, 1972. a Na, b K, c Ca and d Mg
vs. Cl. The ‘eclogite brines’ symbols are, shaded circles — analyses in Table 1 for the three different HCO
3 −
concentrations; crossed open circle — brine with daughter crystals added to the 100 mgl HCO
3 −
brine in Table 1; open circle — brine with 20 water added to the 100 mgl HCO
3 −
brine in Table 1. The computed compositions for Cl, Na and K represent the most concentrated solutions within this data set. Even so, the computed eclogite fluid inclusion compositions fall along compositional variation trends
suggested by the analyzed fluid inclusion data set.
Fig. 3. Computed molality of Na squares, K circles, Ca triangles, and Mg diamonds, vs. Cl, for the eclogite fluid
inclusion compositions solid symbols, and for the modeled seawater – basalt system connected shaded symbols. The
numbers by the shaded symbols along the Na trend are the number of milligrams of water remaining from 1 kg of water,
during the H
2
O removal stage of the simulated seawater – basalt evolution. Only the last five steps in the reaction pro-
gress are enumerated. The initial fluid composition used in the H
2
O removal process is that resulting from interaction of 0.04 mol of anorthite and 0.12 mol of potassium feldspar with
seawater.
Regardless of the effects of initial assumed HCO
3 −
concentration, these are predominately Na- and K-rich brines, with subordinate Ca and
Mg. The approximate Na:K molar ratio is 2.1 – 2.5, over the range of HCO
3 −
concentrations, while
the Na:Ca:Mg
ratio varies
between 1:0.2:0.05 – 1:0.003:0.0001. Charge balance is pri-
marily maintained
by the
very high
Cl concentrations.
The elemental ratios and aqueous species activi- ties of these brines do not bear any systematic
relationship to those in seawater. Enrichments of elements, and the respective activities of the dom-
inant aqueous species, relative to seawater, are K 100 × , 300 × , Cl 10 × , 10 × , and Na 6 × ,
30 × . The relative concentrations and activities for Ca are either at or above that of seawater, and
for Mg are greater than or less than that of seawater, depending upon HCO
3 −
concentration, which is unknown. Sulfate maintains a dramatic
depletion, relative to seawater, at low HCO
3 −
values, but is enriched at high HCO
3 −
.
4. Discussion
Kategori