Eplanation of this presentation

The object is to:

1. Link major element chemical variations to a measure of crystallization progress (Mg’).

2. To link major elements and crystallization progress to the crystal fractionation sequence indicated from the phenocryst assemblages.

3. To show that kinks in the major element trends indicate step changes in the fractionating mineral assemblages

4. To illustrate how trace elements can be used to estimate crystallization progress.

The following presentation is based on a set of Kilauea lava analyses presented in Table 13-1 of Anthony Philpotts, 1990, Principles of igneous and metamorphic petrology, Prentice Hall. It also includes an adaptation and extension of his analysis of the data on pages 237 to 241 of that same text, and his figures 13.1 to 13.5.

Hawaii basalt data set analysesCrystallization sequence: Olivine

AugitePlagioclase

EnstatiteMagnetite

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

SiO2 48.05 48.43 47.92 48.21 49.16 49.20 49.71 50.10 50.37 50.56 50.74 50.85 50.92 51.24 53.42 56.07

TiO2 2.04 2.00 2.16 2.24 2.29 2.57 2.68 2.71 3.09 3.16 3.35 3.36 3.61 3.74 3.36 2.97

Al2O3 10.33 10.70 10.75 11.37 13.33 12.77 13.65 13.78 14.02 13.92 13.57 14.02 13.80 13.60 13.75 13.78

Fe2O3 1.34 1.15 1.08 1.50 1.31 1.50 1.19 1.89 1.88 1.78 1.36 1.90 1.85 1.87 1.96 1.93

FeO 10.19 10.08 10.65 10.18 9.71 10.05 9.72 9.46 10.07 10.18 10.63 10.44 10.71 11.19 10.45 9.78MnO 0.17 0.17 0.18 0.18 0.16 0.17 0.17 0.17 0.17 0.18 0.18 0.18 0.19 0.18 0.18 0.18MgO 17.39 16.29 15.43 13.94 10.41 10.00 8.24 7.34 6.75 6.33 6.16 5.68 5.46 5.12 3.92 2.70CaO 8.14 8.67 9.33 9.74 10.93 10.75 11.59 11.46 10.39 10.24 9.94 9.71 9.45 9.03 7.75 6.54

Na2O 1.66 1.71 1.79 1.89 2.15 2.12 2.26 2.25 2.35 2.61 2.69 2.77 2.80 2.81 3.34 3.86

K2O 0.36 0.35 0.44 0.44 0.51 0.51 0.54 0.57 0.62 0.64 0.67 0.74 0.75 0.83 1.10 1.36

P2O5 0.19 0.18 0.23 0.22 0.20 0.25 0.25 0.27 0.32 0.33 0.37 0.38 0.40 0.41 0.59 0.77

Total 99.86 99.73 99.96 99.91 100.16 99.89 100.00 100.00 100.03 99.93 99.66 100.03 99.94 100.02 99.82 99.92

Molar Mg/(Mg+Fe2+) 0.75 0.74 0.72 0.71 0.66 0.64 0.60 0.58 0.54 0.53 0.51 0.49 0.48 0.45 0.40 0.33A 7 7 8 9 12 12 13 14 15 16 17 18 18 18 24 29F 34 35 38 38 43 44 47 48 51 52 53 53 54 56 56 55M 59 57 55 53 46 44 40 37 34 32 31 29 28 26 21 15

% liquid remaining (K2O) 100% 103% 82% 82% 71% 71% 67% 63% 58% 56% 54% 49% 48% 43% 33% 26%

% liquid remaining (P2O5) 100% 106% 83% 86% 95% 76% 76% 70% 59% 58% 51% 50% 48% 46% 32% 25%

Volume norms and calculated valuesQuartz 0 0 0 0 0 0 0 1.3 2.8 2.4 2.6 3.2 3.9 4.9 7.6 10.7Plagioclase 39.3 40.7 40.9 43.1 49.4 47.9 50.8 51.0 51.9 52.7 52.1 53.3 52.8 52.0 53.3 54.1Orthoclase 2.6 2.5 3.2 3.2 3.6 3.7 3.8 4.0 4.4 4.5 4.7 5.2 5.3 5.8 7.6 9.3Diopside 14.9 16.4 19.0 19.6 20.8 21.1 22.8 22.0 17.7 18.2 18.0 16.5 16.0 15.0 11.3 8.1Hypersthene 19.8 19.7 13.8 14.8 13.3 16.9 15.1 16.2 17.2 16.0 16.5 15.2 15.1 15.2 13.2 11.2Olivine 19.2 16.8 19.0 14.8 8.5 5.4 2.7 0 0 0 0 0 0 0 0 0Ilmenite 2.6 2.5 2.7 2.8 2.8 3.2 3.3 3.3 3.8 3.9 4.1 4.1 4.4 4.6 4.0 3.5Magnetite 1.2 1.0 0.9 1.3 1.1 1.3 1.0 1.6 1.6 1.5 1.2 1.6 1.6 1.6 1.6 1.6Apatite 0.4 0.4 0.5 0.5 0.5 0.6 0.6 0.6 0.7 0.7 0.8 0.8 0.9 0.9 1.3 1.6

Mg/(Mg+Fetotal) in rock 73.1 72.3 70.3 68.3 63.0 61.0 57.6 54.0 50.6 48.9 48.1 45.5 44.0 41.5 36.4 29.5

Mg/(Mg+Fe2+) in norm silicates 79.6 78.5 76.6 76.4 72.0 71.1 67.9 67.3 64.5 62.7 60.4 60.0 58.8 56.0 50.9 42.8Ca/(Ca+Na) in rock 73.0 73.7 74.2 74.0 73.7 73.7 73.9 73.8 71.0 68.4 67.1 66.0 65.1 64.0 56.2 48.4Ca/(Ca+Na) in norm plag. 56.9 57.2 55.4 55.6 56.6 55.6 55.7 56.1 55.1 51.0 48.8 48.7 47.6 46.6 39.1 31.9Differentiation Index (norm) 42.0 43.2 44.1 46.3 53.0 51.6 54.6 56.3 59.1 59.7 59.5 61.7 62.0 62.7 68.5 74.0Calculated density, g/cc 3.14 3.13 3.14 3.12 3.08 3.09 3.07 3.07 3.06 3.05 3.05 3.04 3.05 3.05 3.00 2.95

From Table 13-1 of Philpotts, 1990, Principles of igneous and metamorphic petrology, Prentice Hall.

Hawaii phenocryst compositions

Weight % of the major oxidesOlivine Plagioclase Augite Enstatite Magnetite

SiO2 40.01 51.50 51.20 54.10 0.10

TiO2 0.04 0.12 1.11 0.27 22.70

Al2O3 1.13 29.50 2.62 1.66 1.44

Fe2O3 0.30 0.06 0.81 0.89 24.37FeO 12.33 0.84 9.19 10.80 46.37MnO 0.17 0.00 0.19 0.19 0.76MgO 44.77 0.09 17.10 29.40 3.18CaO 1.21 13.70 17.80 1.98 0.00Na2O 0.00 3.46 0.08 0.00 0.00

K2O 0.00 0.13 0.00 0.00 0.00

P2O5 0.00 0.00 0.00 0.00 0.00Total 99.96 99.40 100.10 99.29 98.92

Molar Mg/(Mg+Fe2+) 0.87 0.16 0.77 0.83 0.11A 0 79 0 0 0F 22 19 35 27 94M 78 2 65 73 6

These are typical phenocryst compositions for Hawaii tholeiitic basalts.

Hawaii: differentiation

index 1

Mg’ is the measure of magma evolution used here. It increases almost linearly with the differentiation index as mafic phases are fractionated from the liquid.

Mg’ = molar Mg/(Mg+Fe2+)

Hawaii: differentiation

index 2

Mg’ is the measure of magma evolution used here. It increases almost linearly with the differentiation index as dominantly mafic phases are fractionated from the liquid.

Mg’ = molar Mg/(Mg+Fe2+)

Primiti

ve

Evolve

d

Magma evolution proceeds by crystal fractionation.

Hawaii: norms 1

With fractional crystallization of olivine, the amount of olivine component in the liquid (normative olivine) decreases until there is none, after which there is normative quartz. Then, ilmenite crystallization continues the trend.

Hawaii: norms 2

Kink is an artifact of the norm calculations

With fractional crystallization of olivine, the amount of olivine component in the liquid (normative olivine) decreases until there is none, after which there is normative quartz. Then, ilmenite crystallization continues the trend.

Hawaii: Mg-Al-Ca 1

Plot of three major oxides vs. Mg’, the index of magma evolution. Data do not define straight lines and so indicate changes in the crystal fractionation assemblage.

Hawaii: Mg-Al-Ca 2

Plot of three major oxides vs. Mg’, the index of magma evolution. Data do not define straight lines and so indicate changes in the crystal fractionation assemblage.

Olivine

OlivineAugite

OlivineAugite

Plagioclase

AugitePlagioclase

EnstatiteIlmenite

Hawaii: Ti-Na-K-P 1

Plot of four rather incompatible oxides, generally having less response to changes in the fractionating mineral assemblage modes.

Hawaii: Ti-Na-K-P 2

Plot of four rather incompatible oxides, generally having less response to changes in the fractionating mineral assemblage modes.

Olivine ± Augite ± Plagioclase

Enstatite+ Augite

+ Plagioclase + Ilmenite

Hawaii: AFM diagram 1

Hawaii: AFM diagram 2

Olivine ±Augite ±

Plagioclase

Enstatite +Augite +Plagioclase +Ilmenite

Hawaii: incompatible elements 1

The major phases:

Olivine:(Mg,Fe)2SiO4

AugiteCa(Mg,Fe)Si2O6

Plagioclase(Ca,Na)(Al,Si)4O8

Enstatite(Mg,Fe)SiO3

IlmeniteFeTiO3

Which major elements are incompatible??

Hawaii: incompatible elements 2

The major phases:

Olivine:(Mg,Fe)2SiO4

AugiteCa(Mg,Fe)Si2O6

Plagioclase(Ca,Na)(Al,Si)4O8

Enstatite(Mg,Fe)SiO3

IlmeniteFeTiO3

Which major elements are incompatible??

Slight drop in TiO2 because of Ti in pyroxenes? Large drop later due to ilmenite

Substantial drop in Na2O due to Na in plagioclase.

No change in P2O5-K2O ratio, because neither are included in major phases and so both are highly incompatible.

Hawaii: percent liquid remaining 1

The % liquid remaining can be calculated:

%l = 100*Io/In

Io: Original concentration of an incompatible element in the parent magma (sample 1).

In: concentration of the same element in any derived magma.

Hawaii: percent liquid remaining 2

The % liquid remaining can be calculated:

%l = 100*(Io/In)

Io: Original concentration of an incompatible element in the parent magma (sample 1).

In: concentration of the same element in any derived magma.