Fractionation of Ca isotopes during calcite precipitation

[LukeMM]

Dear David and User Forum

I am attempting to model the fractionation of a calcium isotope (44Ca) during evaporation of a saline-hypersaline lagoon and precipitation of calcite. Our measurements show heavy Ca isotope composition (d44/40Ca, up to about 2‰) in the water during evapo-concentration. We estimate about about 35 to 40% of dissolved Ca2+ had to be removed into CaCO3 during evaporative concentration to explain the heavy d44/40Ca signatures that we observed in the waters. I have tried to set up the model but am running into a few difficulties as this is the first time I have used the isotope functionality of PHREEQC. In particular am unsure how to incorporate the calcite phase and 44Ca fractionation of the phase into the model. We estimate the fractionation factor between seawater and calcite is about 0.9‰.

Below is what I have tried to set up for the model so far, it is not running correcting at present, it is giving a warning on alkalinity but I am not clear if this is main issue. I would greatly appreciate your assistance.

Thanks very much,

Luke

DATABASE C:\Program Files (x86)\USGS\Phreeqc Interactive 3.1.7-9213\database\iso.dat


TITLE Calcium isotope fractionation during seawater evaporation

SOLUTION 1 "Seawater" Composition in Lagoon
    temp      20
    pH        8.38
    pe        8
    redox     pe
    units     mg/L
    density   1
    -water    1 # kg
    Alkalinity 133.98 gfw 50.04
    Ca        382.9
    Cl        19174.3 charge
    Mg       1275.1
    K         338.7
    Na        10377.8
    S(6)      2693.8
    [44Ca]   1.92   # permil average seawater Fantle and Tipper 2014

SOLUTION_MASTER_SPECIES

[44Ca]        [44Ca]+2        0       [44Ca]   44

SOLUTION_SPECIES

[44Ca]+2 = [44Ca]+2
     log_k           0

PHASES # Unsure how to implement 44Ca fractionation between water and calcite!
# Calcite
[44Ca]CO3(s)
     [44Ca]CO3(s) = CO3-2 + [44Ca]+2
     -log_k           -8.48      #adapted dissolution reaction and log K for calcite from PHREEQC.dat                   
           
ISOTOPES
Ca
   -isotope      [44Ca]   permil   0.021518       #NIST SRM 915a standard 44Ca/40Ca absolute ratio

USER_GRAPH
 -head H2O SC 44Ca
 -init false
 -axis_scale x_axis 0.5 3.5
 -axis_scale y_axis 0 150
 -axis_scale sy_axis 0 6 0.2 100
 -axis_titles "Evapo-concentration factor" "Specific EC (mS/cm, 25C)" "Moles 44Ca"
 -chart_title "Conductivity versus 44Ca during evaporation"
 -start
 10 graph_x 1 / tot("water")
 20 graph_y SC / 1e3       
 35 graph_sy TOT("[44Ca]")
 80 if STEP_NO > 20 THEN PRINT "x", "Specific EC"
 90 if STEP_NO > 20 THEN PRINT 1 / tot("water")
 -end
REACTION
 H2O -1; 0 9*4 6*1 2*0.25 0.176 4*0.05 5*0.03
INCREMENTAL_REACTIONS true

END
 

[dlparkhurst]

It takes a lot to get all of the definitions together. I think this is close, but there could easily be mistakes. The fractionation factor is defined in NAMED_EXPRESSIONS (as 1000ln(alpha)), but I never know whether I  have the sign right.

NAMED_EXPRESSIONS
Log_alpha_44Ca_Calcite/Ca+2
     -ln_alpha1000  0.9

You will have to take it from here.

[LukeMM]

Thank you very much David, that is a huge help! Will let you know how we get on comparing to our Ca isotope measurements and repost any refinements to the draft model.

Best wishes and thanks again,

Luke