A mathematical approach to the assessment of the conodont colour alteration index (CAI)

GUSTAVO G. VOLDMAN; RAÚL A. BUSTOS MARÚN; GUILLERMO L. ALBANESI

Cagary

Simposio; International Conodont Symposium (ICOS 2009); 2009

University of Calgary

    The conodonts have become a widely used tool in integrated
basin analysis and in the exploration of oil and gas because of
its coupled use as precise biostratigraphic markers and thermal
maturity indicators. When subjected to heat, conodont elements
experience progressive and irreversible chemical transformations
of the organic matter interspersed with phosphatic lamellae in the
crown ultrastructure. Unaltered conodonts present a pale yellow
and a smooth surface with silky brightness (CAI 1). Gradually
increasing temperature results in successive carbonization pro-
cesses of conodont elements that outcome in the colour sequence
light through dark brown (CAI 1.5–4) to black (CAI 5). Subsequent
colour changes towards grey (CAI 6), white (CAI 7) and finally
translucent (CAI 8) are consequences of oxidation of organic
matter, release of constitutional water and recrystallization. The
different CAI stages, which range in temperature from 50 to >
600ºC, are directly related to temperature and duration of heat-
ing, conforming to the Arrhenius reactions (Epstein et al., 1977;
Rejebian et al., 1987). From these pioneer works, much research
has been conducted in order to quantify the conodont colour alter-
ation index (CAI), employing electron spin resonance, organic
geochemistry, spectral reflectance, and colour image analysis,
among other techniques.
    The Arrhenius equation is a rather simple, but precise formula
for the temperature dependence of the rate constant, therefore, for
the rate of chemical reaction. To evaluate this equation a pre-ex-
ponential factor, A (which is independent of temperature in the
range of temperatures experienced by sedimentary basins), and the
activation energy, E, are required. One of the major disadvantages
in using the Arrhenius equation to model conodont thermal matu-
ration directly is the selection of suitable values for these kinetic
parameters (cf., Snowdon, 1979). Alternatively, the CAI values
can be predicted directly from a time-temperature Arrhenius
graph (Epstein et al., 1977; Rejebian et al., 1987). However, the
application of this plot to complex geological histories is difficult
because under natural conditions the heating occurs at variable
temperatures, hampering the precise determination of paleotem-
peratures or geothermal paleogradients from CAI data (cf., García
López et al., 2001).
    We present a mathematical approach for determining the
progress of conodont colour alteration by means of integration of
time and temperature, based on the Arrhenius graphs of Epstein
et al. (1977) and Rejebian et al. (1987). Although geochemical
alteration of organic matter occurs through a series of complex,
competing, parallel and/or successive reactions, the overall pro-
cess of conodont alteration can be reduced to a series of simple
pseudo-first-order reactions, from dominantly carbonization to
oxidation of organic matter within the CAI 1.5–6 interval. Using
the data from the published Arrhenius plots, we work out from the
Arrhenius equation the dependance of the CAI values to tempera-
ture and time as:
CAI = 7 - exp(-1.615E11 * t * exp(-13623.0311 / T)) - exp(-
1.80335E12 * t * exp(-17147.2317 / T)) - exp(-1.35857E14 * t *
exp(-22188.1258 / T)) - exp(-7.91407E18 * t * exp(-32769.4825 / T))
- exp(-1.31948E27 * t * exp(-51334.4557 / T)) - exp(-2.50159E29 *
t * exp(-64379.3110 / T))
where T is temperature in Kelvin and t is time expressed in
years.
    We compared the values of calculated CAI with the data of
Epstein et al. (1977) and Rejebian et al. (1987) and found a standard
deviation of only 0.1 up to CAI 6, much lower than the resolution
of the visual CAI determination technique and without the need
to access to the Arrhenius plot. Our approach is especially ame-
nable to a spreadsheet program. Because maturation effects on the
organic material are additive and irreversible, the total maturity
for a given stratigraphic level results from the sum of the maturi-
ties acquired in each successive time-temperature interval.
    The preliminary equation will derive in a definitive form,
which will be presented in the final manuscript. The objective is to
apply a CAI formula to optimize thermal history models by com-
puting profiles of CAI values with depth and time; thus providing
a new tool for quantitative CAI paleothermometric analysis.
References
Epstein, A.G., Epstein, J.B. and Harris, L.D., 1977. Conodont
    colour alteration - An index to organic metamorphism.
    Geological Survey Professional Paper 995: 27 pp.
García López, S., Bastida, F., Aller, J., and Sanz López, J., 2001.
    Geothermal palaeogradients and metamorphic zonation from
    the conodont colour alteration index (CAI). Terranova 13(2):
    79–83.
Rejebian, V.A., Harris, A.G. and Huebner, J.S., 1987. Conodont
    colour and textural alteration: An index to regional metamor-
    phism, contact metamorphism, and hydrothermal alteration.
    Geological Society of America Bulletin 99: 471-479.
Snowdon, L.R., 1979. Errors in Extrapolation of Experimental
    Kinetic Parametres to Organic Geochemical Systems. AAPG
    Bulletin 63(7): 1128-1138.