Variability in mineral assemblage, temperature, and oxygen fugacity in a suite of strongly peralkaline lavas

and tuffs from Pantelleria, ItalyJohn C. White

Eastern Kentucky University

Minghua RenUniversity of Texas at El Paso

Don F. ParkerBaylor University

Pantelleria, Italy• Strait of Sicily

transtensional rift• Type locality for

pantellerite (strongly peralkaline quartz trachyte and rhyolite)

• Cinque Denti caldera and related post-caldera vents (45 to 5.5 ka)

Mah

ood

and

Hild

reth

(19

86)

Bul

l Vol

c 48

: 14

3-17

2

Cuddia Randazzo (CR)98520, 98521,98522, 98531

Monte Gelkhamar (MG)98523, 98526

Green Tuff (GRT)98527

Cuddie di Belizzi (CB)98529

Monte Gelkhamar

Cala Cinque Denti

wt% FeOT

5 6 7 8 9

wt%

Al 2O

3

8

10

12

14 PT

PC

CT 98521-Inc

98520

98529

98526

9853198523

985229852198527

Classification of samples studied, following Macdonald (1974).Key to rock types: C, Comendite; CT, Comenditic Trachyte;P, Pantellerite; PT, Pantelleritic Trachyte.

TiO

2

0.40

0.45

0.50

0.55

0.60

MgO

0.10

0.15

0.20

0.25

0.30

Al 2O

3

8

10

12

14

SiO264 66 68 70 72

FeO

T

6.5

7.0

7.5

8.0

8.5

CaO

0.4

0.6

0.8

1.0

64 66 68 70 72

Alk

ali M

etal

s

4.5

5.0

5.5

6.0

6.5

7.0

7.5Na2O

K2O

98520

98521-Inc

9853198523

98526 98522

9852198529

98527

Rb

80100120140160180200220

Nb

100

200

300

400

Zr600 800 1000 1200 1400 1600 1800 2000

Th

10

20

30

40

Ba

100

200

300

400

Sr

5

10

15

20

25

600 800 1000 1200 1400 1600 1800 2000

Eu

2

3

4

5

98521-Inc

98520

98531

98526

9852198522

98523

98527

98529

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Roc

k / C

hond

rite

(Tay

lor,

198

0)

10

100

1000

98521-Inc

98527 & 98529

98520

98531 &98526

98521

Crystal Fractionation

• An assemblage dominated (~86%) by feldspar (Or35-36) with Na-Cpx, Aenigmatite, Fayalite, Ilmenite, and Apatite.

• F = 76-79%• Civetta et al. (1998), Lowenstern and Mahood (1991),

White and Parker (2000), White et al. (2003).

Or

31

32

33

34

35

36

37

38

An

985209852198521-Inc985229852398526985279852998531

1 2

SanAnr

Alkali feldspar analyses.Sanidine (San) / Anorthoclase (Anr) boundaryfollowing Deer et al. (1966)

Isobaric projection (PH2O = 980 bars) of the liquidus surface of the8.3% ae + 8.3% ns plane in the system Ab-Or-Q.(Carmichael and MacKenzie, 1963)

Or0 10 20 30 40 50 60 70 80 90 100

Q

0

10

20

30

40

50

Ab

50

60

70

80

90

100

X

985209852198521-Inc985229852398526985279852998531Feldspar (Or35)

Minimum(Q40.5Or34.5Ab25)

X820 825

745 740735

730705 700

720

755765

775 780

Minimum

• Rock compositions follow the “Thermal Valley” of Carmichael and MacKenzie (1963), dominated by fractionation of alkali feldspar (Or35-36).

• Comparison with experimental data suggests liquidus temperatures decreasing from >825° to 700°C.

• Most evolved rocks (98527, 98529) located at the minimum (700-705°C) on the cotectic with quartz.

Or0 10 20 30 40 50 60 70 80 90 100

Q

0

10

20

30

40

50

Ab

50

60

70

80

90

100

X

985209852198521-Inc985229852398526985279852998531Feldspar (Or35)

Minimum(Q40.5Or34.5Ab25)

X820 825

745 740735

730705 700

720

755765

775 780

Minimum

Magnetite-Ilmenite

• Equilibrium two-oxide pairs only found in least evolved samples: 98521-Inc (~63% SiO2, A.I. ~ 1.17) and 98520 (~64% SiO2, A.I. ~ 1.33)

• 98521-Inc: 1005°C, -11.5 log fO2

• 98520: 903°C, -13.6 log fO2

• (Andersen and Lindsley, 1988)

Fa0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Clinopyroxene and Olivine

Fs0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Wo

0.0

0.1

0.2

0.3

0.4

0.5

9852098521 (Aegirine-Augite)98521-Inc98522 (Aegirine-Augite)98523 (Augite)9852698527 (Sodic)98529 (Sodic)98531

Augite

Pigeonite

Enstatite Ferrosilite

Diopside Hedenbergite

Clinopyroxene and Olivine analyses.Classification of clinopyroxene following Morimoto et al. (1988)

A.I.

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

Fs

(cpx

)

0.42

0.44

0.46

0.48

0.50

0.52

wt%

Na 2O

(cp

x)

0.5

1.0

1.5

2.0

2.5

Fa

(ol)

0.90

0.92

0.94

0.96

0.98

QUIlF: Ilm + TiMt + Fa + Cpx

• 98521-Inc: 990°C, -11.8 log fO2.

• 98520: 886°C, -13.9 log fO2.

– K-spar liquidus temperature: ~820°C (Carmichael and MacKenzie 1963) to ~860°C (Tuttle and Bowen, 1958).

• Calculated at P = 1000 bars (Lindsley and Frost, 1992; Frost and Lindsley, 1992; Andersen et al., 1993).

QUIlF: Ilm + Fa + Cpx

• 98526: 793°C, -15.2 log fO2

– K-spar liquidus temperature: ~780°C

• 98531: 764°C, -15.9 log fO2

– K-spar liquidus temperature: ~760°C

• 98523: 756°C, -15.8 log fO2

– K-spar liquidus temperature: ~760°C– Putirka et al. (2003) CPX-melt: 745°C

• Calculated at P = 1000 bars.• Note: Aenigmatite is present in 98531 and 98523.

T-fO2 Constraints on other (fa-free) assemblages?

• 98521, 98522: cpx + ilm +ang– 740°C (K-spar) to 700°C (Cpx)– Pyrrhotite microphenocrysts = closer to 740°?

• 98527: cpx + ilm + ang + q + Fe-richterite– 705°C (K-spar) to 678°C (Cpx)– Use QUIlF to try to “recover” fO2 given 700°C and cpx

+ ilm + q assemblage. (-17.0 +/- 0.4)

• 98529: cpx + ang + q– 700°C (K-spar) to 701°C (Cpx)– Above FMQ in “no-oxides” field (Nicholls and

Carmichael, 1969)

mol% Q (normalized Q-Ab-Or)5 10 15 20 25 30 35 40

T (

°C)

650

700

750

800

850

900

950

1000

1050Alkali Feldspar-melt equilibrium(Carmichael and MacKenzie 1963)

Clinopyroxene-melt equilibria(Putirka et al., 2003)

QUIlF equilibria (QUILF95, Andersen et al., 1993)Minimum, Q40.5

(Carmichael and MacKenzie 1963)

Comparison of methods

Temperature – Oxygen Fugacity

T (°C)

400 500 600 700 800 900 1000 1100

log

fO

2

-22

-20

-18

-16

-14

-12

-10

-8 This study (Pantelleria)Bizourd et al. (1980)Carmichael (1967, 1991)Novak and Mahood (1986)Vogel et al. (1989)Mahood (1981)

HM NNO

FMQ WM

Conclusions

• From least-evolved lava (98520) to most-evolved lava (98529):– SiO2 increases from ~64 to ~70 wt%.

– A.I. increases from 1.33 to 1.97– Th: 17 to 42, Nb: 253 to 407, Zr: 822 to 2010– Temperature decreases: 886° to 700°C– Oxygen fugacity decreases: -13.8 to <-17.0

Summary• Increasing SiO2, A.I.; Decreasing T-fO2:

• Feldspar compositions remain nearly constant

(Or35-36)

• Continuous Reaction Series:– Augite to Sodic Augite / Aegirine-Augite

• Discontinuous Reaction Series:– (to ~880°C) Fayalite + Ilmenite + Ti-Magnetite– (to ~780°C) Fayalite + Ilmenite– (to ~760°C) Fayalite + Ilmenite + Aenigmatite– (to ~700°C) Ilmenite + Aenigmatite– (to <700°C) Aenigmatite

Summary

T (°C)

400 500 600 700 800 900 1000 1100

log

fO

2

-22

-20

-18

-16

-14

-12

-10

-8 This study (Pantelleria)Carmichael (1967, 1991)

HM NNO

FMQ WM

Fa + Ilm + TiMt

Fa + Ilm

Fa + Ilm + Ang

Ilm + Ang + Q

Ilm + Ang

T (°C)

400 500 600 700 800 900 1000 1100

NN

O

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

This StudyCarmichael (1967, 1991)

FMQ

WM

98521-Inc

98520

98527

9852698531

98523

3112

?

Summary