Major and Minor Elements 

Magma series


I. Chemical analysis

A. Text tells history of technological developments

-Relatively easy now to obtain whole-rock analyses = bulk chemical composition, traditionally expressed as oxides

-X-ray fluorescence, instrumental neutron activation analysis, mass spectrometry used for whole-rock analyses

-Electron microprobe used to obtain chemical composition within individual mineral grains

-Detection limit = lowest concentration that can be accurately determined

II. Variation diagrams

A. Def: graphical way of presenting data - good for recognizing trends

-No one correct way to do this

-May use either X-Y or triangular plot

B. Harker diagrams

-Various oxides plotted vs. SiO2 for Crater Lake samples - trends obvious but not perfect; lines are best-fit to data; smooth curves => genetic relationship among samples

-Terminology: primary magma = one derived from partial melting of source rock; evolved magma = one that has undergone differentiation; parental magma = least evolved from a suite

-Change in liquid composition = liquid line of descent

-Fractional crystallization considered most likely process: tests for F.C. hypothesis: trends (LLD) should ~ experimental phase diagram; more evolved samples should be younger than less evolved; harker diagrams can support a particular process such as F.C. but can''t prove it

C. AFM diagram

-Triangular plot : A = Na+K; F = total Fe; M = Mg

III. Modeling magmatic evolution

A. Pearce element ratios

-Used to test whether a particular mineral is fractionating from a liquid

B. Graphical analysis

IV. Magma series

A. Def: a group of rocks that shows smooth trends on variation diagrams, which implies a genetic relationship among them

-Three main groups - figure : Alkaline (Sodic/Potassic/Other); Subalkaline (lower in alkalis, higher in silica - commonly oversaturated = qz present)(Tholeitic/Calc-alkaline/Peralkaline )

-These seem to express some underlying physical & chemical reality about magmas.

B. The basalt tetrahedron

-Quaternary system:Ne-Di-Fo-Qz; binary compounds En and Ab

-Di-Ab-Fo triangle divides tetrahedron into two parts = critical plane of silica undersaturation: left has lower Si and foids appear, right has more Si-rich phases.

-Di-Ab-En triangle = plane of silica saturation: to the right, Q appears as a separate phase

Tholeiitic magma series
Calc-alkaline magma series

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