Major Elements

Updates/questions Midterm Topics: 3 component systems continued Major element compositions

Liq/total solids = a-m/Liq-a total Di/Fo = m-Fo/Di-m Ternary lever w/ 3 phases Imagine triangular plane X - Di - Fo balanced on bulk a Liq x a Di m Fo Liq/total solids = a-m/Liq-a total Di/Fo = m-Fo/Di-m

Pressure Effects

Major Elements Measure minerals or glasses: = notes provided Electron microprobe Electron beam + element electrons = Xray Measure whole rocks (multiple minerals) XRF (XRay Fluorenscence) Xrays excite inner-shell electrons, on return to ground state emit Xrays ICP-MS (Inductively-Coupled Plasma Mass Spectrometry) Measure by “counting” atoms per mass Few more in book (AA, INAA, Ion Probe) = notes provided

Crustal composition Abundance of the elements in the Earth’s crust

Data example T a b l e 8 - 3 . C h m i c n y s o f r p t v g u k P d B 2 4 6 9 5 7 1 1 5 . 7 4 2 3 9 F e O 6 8 M n g C a N K H + 3 1 . 1 1 . 5 7 T o t a l 9 8 . 7 5 9 9 . 6 9 9 . 3 9 9 . 5 9 9 . 2 3

From oxide % to atom % Mid-Ocean Ridge Basalt

Rocks & mineralogy

Plotting compositions 12 17 22 Al 2 O 3 5 10 MgO FeO* 4 6 Na 15 CaO 45 50 55 60 65 70 75 1 K SiO Based on compiled data by Rick Conrey (unpub)

Effect of crystallization Key: B=bulk, L=liquid, P-S=crystals

Magma Evolution Harker diagram Smooth trends Model with 3 assumptions: 55 65 75 45 2 3 4 8 6 10 14 16 Al2O3 MgO CaO Fe2O3 Na2O K2O Wt. % SiO2 B BA A D RD R Harker diagram Smooth trends Model with 3 assumptions: 1 Rocks are related 2 Trends = liquid line of descent (mineral control) 3 The basalt is the parent magma from which the others are derived B=basalt, BA=basaltic-andesite, A=andesite, D=dacite, RD=rhyo-dacite, R=rhyolite

Magma Series

Alkali vs. Silica -- Hawaiian volcanics: 12 10 8 6 4 2 35 40 45 50 55 60 65 %SiO2 %Na2O + K2O Alkaline Subalkaline Figure 8-11. Total alkalis vs. silica diagram for the alkaline (open circles) and sub-alkaline rocks of Hawaii. After MacDonald (1968). GSA Memoir 116

Ternary Variation Diagrams FeO + Fe2O3 K2O + Na2O MgO Skaergaard Crater Lake

Subalkaline subdivision F A M Calc-alkaline T h o l e i t c

F Tholeiitic Calc-alkaline M A Evolving rocktypes B-A A D R Rhyolite Dacite Andesite Basaltic Andesite Ferro-Basalt Basalt Calc-alkaline Tholeiitic B-A A D R M A

Occurrence of different series After Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer

The Basalt Tetrahedron Di Q En Critical plane of silica undersaturation Ne Ab Plane of silica saturation Fo Ol Ne Ab Opx Q Alkaline field Subalkaline field Dividing line Figure 8-12. Left: the basalt tetrahedron (after Yoder and Tilley, 1962). J. Pet., 3, 342-532. Right: the base of the basalt tetrahedron using cation normative minerals, with the compositions of subalkaline rocks (black) and alkaline rocks (gray) from Figure 8-11, projected from Cpx. After Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548.

Keeping series separated Ol Ne Ab Opx Q Alkaline field Subalkaline field Dividing line Ne Ab Q 1070 1060 1713 Ab + Tr Tr + L Ab + L Ne + L Liquid Ne + Ab Thermal Divide