ABSTRACT

Some subduction-related volcanic arcs demonstrate an increase in potassium (expressed as K2O) content as distance from the trench increases. Geologists have proposed two different mechanisms that may act additively to explain these trends: 1) the degree of partial melting of the mantle decreases as distance from trench increases, and 2) crustal contamination of the ascending magma increases with distance from trench. Insight into these hypotheses can be gained by studying the Mexican Volcanic Belt (MVB), which is unique among the world’s subduction-related volcanic arcs due to its non-parallel nature with the associated trench; instead, the E-W trend of the MVB forms a 15-degree angle with the N-W-trending Middle America Trench.

Crustal thickness increases eastward along the MVB, and over the past several million years the focus of activity in the MVB has migrated southward toward the trench. Accordingly, it should be possible to isolate the contribution of the first mechanism within the MVB by focusing on N-S-trending volcanic chains that have migrated southward over crust of relatively constant thickness.

For this project, three such volcanic chains from the MVB were examined by using a chemical database of 444 volcanic rock samples and focusing on variations of K2O contents versus a variety of other parameters. To aid in the research GIS (Geographical Information System) images of each of the chains were prepared. This research yielded two important results: 1) K2O concentrations for each chain decrease significantly from N to S, supporting the role of decreasing partial melting with increasing trench distance, and 2) K2O content for the three volcanic chains increase from W to E, also consistent with the eastward increase in trench distance, but perhaps also evidence for enhanced eastward contamination by thicker continental crust.

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INTRODUCTION

• Working in circum-Pacific subduction-related island arcs, Dickinson and Hatherton (1967) first noticed a trend of increasing potassium (K) content in rocks with greater distance (d) from the trench. The results of their work, referred to as the K-d relationship, made it plausible to claim temporal, spatial, and volumetric relationships for the world’s generalized arc models

• In 1977 Arculus and Johnson provided evidence from the Lesser Antilles volcanic arc to disprove the generalized relationships suggested by the Dickinson and Hatherton (1967) model, and many other examples were later shown to also counter the generalized K-d model.

• Nonetheless, the K-d relationship has been shown to apply in many subduction-related arcs. Geologists have proposed two mechanisms, which may act additively, to explain these trends: 1) a decrease in the degree of partial melting in the underlying mantle with increased d, and 2) an increase in the extent of crustal contamination of the magma during ascent with increased d (Gill, 1981; Sakuyama and Nesbitt, 1986; Halsor and Rose, 1988).

• This research is centered on the potassium contents of the Cantaro-Colima Fig. 1B), Iztaccíhautl-Popocatépetl (Fig. 1C) and Cofre de Perote-Pico de Orizaba (Fig. 1D) volcanic chains within the Mexican Volcanic Belt (MVB).

	Photo of the S. flank of Colima during an eruption in 1982. The nighttime exposure shows the incandescence of the lava.
	This Dec. 1994 photo of Popocatepétl shows a plume of steam and ash erupting from the summit.
	Pico de Orizaba is México’s highest peak and the highest volcano in N. America.

• The MVB is distinctive for three reasons: 1) the E-W-trending MVB forms a 15 degree angle with the N-W-trending Middle America Trench (Fig. 1A), 2) crustal thickness increases eastward in the MVB (Table 1), and 3) over the past several million years the focus of eruptive activity in the MVB has migrated southward toward the trench (Fig. 1B-D).

• Because each volcanic chain is migrating southward over continental crust of relatively uniform thickness, we can isolate the effect of distance to the trench and test its control on potassium content in the MVB.

•The two mechanisms should work together progressing from west to east across the MVB. Thus, if either, or both are important, the K2O content of the volcanic rocks should increase from west to east.

• Null Hypotheses: #1 The K2O content of each volcanic stage of each chain has not changed significantly. #2 The K2O contents of the three volcanic chains are indistinguishable.

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METHODS

Assessments

• To gauge the differences and similarities, potassium content was plotted against SiO2 and the different volcanic stages from each chain were distinguished.

• To determine possible mechanisms for the concentration of potassium, plots of the K-bearing mineral biotite against each stage were used.

Analysis:

• A reduced major axis line was calculated for each volcanic stage, and volcanic chain. To make comparisons of K2O at a single value, a median SiO2 value of 60% was chosen, the data points were then recalculated about 60% SiO2.

• The Mann-Whitney-U test was run on each set of recalculated data to see if the data were significantly different.

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RESULTS

• Comparing the potassium content of the three chains, and the oldest volcanic stage in each chain to the youngest volcanic stage (Fig. 4A-D) we can reject each null hypothesis and conclude that there are statistically significant increases in each case.

Fig. 4a-d. K2O concentrations plotted against SiO2 wt. %. Solid lines represent the calculated reduced major axis line for each volcanic stage. (CA = Cantaro, NC = Nevado de Colima, CO = Colima, IZ = Iztaccíhautl, PO = Popocatépetl, PE = Perote, LC = Las Cumbres, LG = La Gloria, OR = Orizaba; for example CO II = stage II of Colima’s eruptive history)

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CONCLUSIONS

The data (Fig. 4A-D, Table 2) support the hypothesis that distance from the Middle America Trench has some control of the K2O contents of rocks from the Mexican Volcanic Belt.

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Fig. 2 also supports this hypothesis. Biotite crystallizes when significant quantities of potassium are present in the melt. Elevated K2O correlate with the presence of biotite.

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Fig. 3 suggests that the thickness of crust also may play a role in controlling K2O contents of volcanic rocks in the MVB. With the present data set, we can not devise a means to isolate the influence of crustal contamination. Radiogenic isotopic data, currently only available for the Colima chain, would be useful in this regard.

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SUGGESTIONS FOR FURTHER RESEARCH

• Fig. 4 suggests there may be a correlation between crustal thickness and 87Sr/86Sr in the MVB. Further research could reveal how much crustal contamination the magmas experience during their ascent

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ENTOMOLOGY

Sarah Garrett
"An Inventory of Leafroller Species of Moths of the Great Smoky Mountains National Park"

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