tephra SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O total Ref Hekla 4 Tephra 72.58 0.13 12.69 1.90 0.08 0.06 1.35 5.03 2.78 96.59 201 Hekla 4 Tephra 72.47 0.08 12.42 1.85 0.08 0.02 1.36 4.63 2.94 95.85 201 Hekla 4 Tephra 71.80 0.05 12.72 1.82 0.06 0.03 1.27 4.74 2.80 95.28 201 Hekla 4 Tephra 67.87 0.29 13.57 4.76 0.11 0.08 2.75 4.98 2.31 96.73 201 Hekla 4 Tephra 64.40 0.41 13.67 6.56 0.26 0.22 3.55 5.36 2.05 96.67 201 ------------------------------------------------------------------------ 201: Dugmore, A.J., Larsen, G., Newton, A.J., and Sugden, D.E. (1992) Geochemical stability of fine-grained silicic tephra layers in Iceland and Scotland. Journal of Quaternary Science 7, 173-183.