Cenozoic Volcanoes

Abstract

The Transantarctic Mountains of northern and southern Victoria Land contain large volcanoes of Cenozoic (late Tertiary) age which have erupted lavas that are alkali-rich and silica undersaturated. In addition, the lavas contain inclusions of granulite from the deep crust and ultramafic rocks from the lithospheric ­mantle. The Cenozoic volcanoes in the Transantarctic Mountains are located on the East Antarctic rim of the West Antarctic rift system (Section 15.5.5). The opposite side of this rift in Marie Byrd Land also contains Cenozoic volcanoes in the Flood Range, the Erven Nunatak, the Executive Committee Range, and in the USAS Escarpment, all of which appear on the geologic map of the area by Wade (1969). The Executive Committee Range is so named because it consists of a linear array of four towering, equally-spaced volcanoes starting with Mt. Sidley (4,181 m) in the south and continuing with Mt. Hartigan (2,815 m), Mt. Cumming (2,615 m), and Mt. Hampton (3,223 m) in the north for a distance of 65 km.

Appendices

Appendices

1.1 Average Chemical Compositions of the Granulite Inclusion from the Deep Crust Beneath the Transantarctic Mountains and the Ross Embayment (Kalamarides and Berg1991)

Table 4

1.2 Isotopic Compositions of Strontium and Sulfur in Soil Salts on Ross Island Including the Summit of Mt. Erebus (Jones et al.1983; Faure and Jones 1989)

Table 5

1.3 Isotopic Compositions of Two-Component Mixtures (Faure and Jones1989)

The salts in the soil of Ross Islands contain strontium and sulfur both of which are mixtures of two components that have different isotope compositions. Such mixtures are described by an equation that was derived by Langmuir et al. (1978) and presented by Faure (1986). The procedure is to calculate the ⁸⁷Sr/⁸⁶Sr ratio and δ³⁴S value separately for the same mixing parameters. The results are the coordinates of points on the Sr-S mixing hyperbola which can be constructed by means of a smooth curve through the points calculated from the mixing equations:

$${R}_{M}^{X}=\frac{{R}_{A}^{X}{X}_{A}{f}_{A}+{R}_{B}^{X}{X}_{B}(1-{f}_{A})}{{X}_{A}{f}_{A}+{X}_{B}(1-{f}_{A})}$$

(16.2)

R_M = the isotope ratio of an element X in the mixture M of components A and B,

X_A, X_B = the concentrations of element X in components A and B,

f_A = weight fraction of component A in any mixtures of A and B (i.e., f_A = A/A + B).

The curvature and attitude of the mixing hyperbola depend on the concentration ratios of the two elements X and Y which are related by the equation:

$${K}=\text{}{\left(\text{X}/\text{Y}\right)}_{\text{A}}/{\left(\text{X}/\text{Y}\right)}_{\text{B}}$$

(16.3)

where X_A and Y_A are the concentrations of elements X and Y in component A and similarly for component B.

The hyperbolas in Fig. 16.24 are based on values of the parameter chosen from the data (Appendix 16.8.2). For the marine component:

(⁸⁷Sr/⁸⁶Sr)_m = 0.70906; δ³⁴S_m = +20.0‰

For the volcanic component:

(⁸⁷Sr/⁸⁶Sr)_v = 0.70340; δ³⁴S = +0.0‰

In the construction of Fig. 16.24 the concentrations of Sr and S were chosen such that K = 10 for one of the hyperbolas and K = 0.10 for the other. The ­concentration of S is fixed by the stoichiometry of the sulfate ion, which means that only the concentration of Sr is assumed to vary. Note also, that the mixtures form a straight line when K = 1.0

The ⁸⁷Sr/⁸⁶Sr ratio of a mixture of components of A and B is calculated from Eq. 16.2 for selected values of the mixing parameters f_A and K. Assume that f_A = 0.40 and the K = 10. In that case:

$${\left(\text{Sr}/\text{S}\right)}_{\text{A}}=\text{1}0\text{}{\left(\text{Sr}/\text{S}\right)}_{\text{B}}$$

Let the marine component be A and the volcanic component B. If S_A = S_B, then

$$ {\text{Sr}}_{\text{A}}=\text{1}0{\text{Sr}}_{\text{B}}$$

Therefore, we set Sr_A = 100 ppm and Sr_B = 10 ppm and substitute appropriate values into Eq. 16.2 to calculate the ⁸⁷Sr/⁸⁶Sr ratio:

$$ {\left(\frac{Sr}{Sr}\right)}_{M}=\frac{0.70906\times 100\times 0.4+0.7034\times 10\text{\hspace{0.22em}}(1-0.4)}{100\times 0.4+(1-0.4)}$$

$$ {\left(\frac{Sr}{Sr}\right)}_{M}=\frac{28.3624+4.2204}{40+6}=0.70832$$

Similarly for sulfur:

$$ {d}^{34}{S}_{M}=\frac{20.0\times {S}_{A}\times 0.4+0\times {S}_{B}\text{\hspace{0.22em}}(1-0.4)}{{S}_{A}\times 0.4+{S}_{B}\text{\hspace{0.22em}}(1-0.4)}$$

Since S_A = S_B,

$$ {d}^{34}{S}_{M}=\frac{20\times 0.4}{0.4+0.6}=8.0\text‰$$

Therefore, the coordinates of the point on the mixing hyperbola at f_A = 0.4 are:

$$ {{\text{(}}^{\text{87}}\text{Sr}{/}^{\text{86}}\text{Sr)}}_{\text{M}}=\text{}0.\text{7}0\text{832}$$

$$ {d}^{\text{34}}{\text{S}}_{\text{M}}=\text{8}.0‰$$

Additional points on the mixing hyperbola in Fig. 16.24 for K = 10 were calculated by varying f_A stepwise from 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0.

1.4 Isotope Compositions of Strontium and Neodymium of Volcanic Rocks from the Mt. Melbourne Volcanic Field (Wörner et al.1989)

Table 6

1.5 Isotope Compositions of Strontium of the Cenozoic Lavas of Northern Victoria Land and Adjacent Islands

Table 7