Minor Crystalline Solids and Elements: Minor Crystalline Solids and Elements: Physicochemical Behavior and Geologic Physicochemical Behavior and Geologic

SignificanceSignificance

• Ti4+, P5+ associate with (silica) network-modifying cations, e.g., Mg, Fe

• Strong effect on melt composition and structure, mineral-melt equilibria, liquid line of descent

TiO2-enrichment in silicate melts

Ol+Opx+Ox - saturated liquidsLunar glasses

TiO2 (wt% ) in silicate melt

0 5 10 15 20 25

SiO

2 (

wt%

) in

silic

ate

melt

30

35

40

45

50

55

SiO

2 in m

elt

0.2

0.4

0.6

0.8

1633 K, 1.2 GPa

1803 K, 2.8 GPa

XSiO

2 in m

elt

0.3

0.4

0.5

1633 K, 1.2 GPa

1803 K, 2.8 GPa

XTiO2 in melt

0.00 0.04 0.08 0.12 0.16

aSiO

2 in m

elt

0.2

0.3

1633 K, 1.2 GPa

1803 K, 2.8 GPa

Xirouchakis et al (2001)

TiO2-enrichment in silicate melts

Ol + Px + Ox saturated Liquids

TiO2 (wt.% ) in liquid

(0 - 2 wt.% Na2O + K2O in liquid)

0 5 10 15 20 250.20

0.24

0.28

0.32

0.36

0.40

Ol/

Liq F

eO

-MgO

KD

Longhi et al (1978): 0.5 - 1.5 GPa, 1568 - 1648 K

Longhi et al (1978): in vacuo, 1423 - 1603 K

Wagner & Grove (1997): 1.4 - 1.7 GPa, 1643 - 1693 K

Baker & Stolper (1994): 1.0 GPa, 1543 - 1663 K

Longhi & Pan (1988): in vacuo, 1423 - 1523 K

Xirouchakis et al (2000): 1.2 GPa, 1633 K

Xirouchakis et al (2000): 2.8 GPa, 1803 K

Xirouchakis et al (2000)

aoxi

de

0.2

0.4

0.6MgO: 1633 K, 1.2 GPa

FeO: 1633 K, 1.2 GPa

MgO: 1803 K, 2.8 GPa

FeO: 1803 K, 2.8 GPa

Ol+Opx-saturated liquids

Xoxi

de

0.1

0.2

0.3

XTiO2 in liquid

0.00 0.04 0.08 0.12 0.16 0.20

oxi

de

2

3

4

P5+, geochemical significanceLess of a phosphate mineral may be needed to account for the bulk P2O5 of Solid Earth & Planetary systems (e.g., mantle) as silicate minerals may host P5+

In the absence or prior to saturation with a phosphate mineral, mafic silicate melts may become enriched in P2O5, especially in the geochemically important low melt fraction regime

Interactions between P5+-hosting silicates and accessory phosphates may affect the stability of the latter

P5+-bearing silicates as a source of phosphorus for microorganisms in nutrient-limited, sub-surface environments.

Reports of extreme enrichment in Olivine and Pyroxenes:

2-4 wt.% P2O5 in olivine and pyroxene in Pallasites [Buseck &

Clark 1984]

and in reduced terrestrial basalts [Goodrich 1984]

17 wt.% P2O5 in zoned olivine crystals from ancient slags [Kresten et al. 1998]

Poorly characterized silico-phosphates in Angrites [Mittlefehldt 2002]

Isostructural solid solutions of silicates and phosphates are not common; Si4+, P5+ and Al3+ have similar radii but also different EN

The higher the degree of SiO4-polymerization, the lower the P2O5 content, e.g., Gt > Ol >> Px (Koritnig 1965; Henderson

1968; Anderson & Greenland 1969; Thompson 1975; Bishop et al. 1978; Brunet & Chazot 2001)

Al3+

0.39Å

EN=1.5

P5+

0.31Å

EN=2.1

Si4+

0.26Å

EN=1.8

Phosphorus in silicate minerals

5-9 GPa supersolidus 5-9 GPa supersolidus experimentsexperiments

Sample in graphite capsule

“hard” Al2O3 sleeve

crushable Al2O3

sample

WC cubes

9 GPa, 1775°C

Ol Gt

Opx

All JSC data for P2O5 wt% in orthopyroxene [0.011(6) wt.%], olivine [0.07(3) wt%], and garnet [0.24(5) wt.%] in equilibrium with liquid [0.7-1.5 wt%]

0.15

0.2

0.25

0.3

0.35

0.4

P2O

5 w

t%

Gt

mean P2O

5 wt%

Ol0.02

0.04

0.06

0.08

0.1

0.12

0

0.005

0.01

0.015

0.02

0.025

OpxP

2O

5 wt% in Gt

0.1 0.2 0.3 0.40

5

10

15

20

25

30

35

P2O

5 wt% in Ol

0 0.05 0.10

2

4

6

8

10

12

P2O

5 wt% in Opx

0 0.01 0.020

2

4

6

8

10

12

72 78 84 90 96

Ol mg#

0.0

0.2

P2O

5 w

t%

DX & DSD (2002)Brunet & Chazot (2001)This study

Al2O3 wt% in Ol

0.0 0.2 0.4 0.6

0.0

0.2

P2O

5 w

t%

Na2O wt% in Ol

0.00 0.06 0.120.0

0.2

P2O

5 w

t%

78 84 90

Opx mg#

0.00

0.02

Al2O3 wt% in Opx

1 2 3

0.00

0.02

Na2O wt% in Opx

0.0 0.3 0.60.00

0.02

a b

c d

e f

0 4 8

P (GPa)

0.0

0.2

0.4

0.6

DX & DSD (2002)Thompson (1975)Bishop et al. (1978)

This study

T (oC)

1000 15000.0

0.2

0.4

0.6

P2O

5 w

t% in G

t

mg# in Gt

20 40 60 800.0

0.2

0.4

0.6

P2O

5 w

t% in G

t

SiO2 wt% in Gt

36 40 44 48

0.0

0.2

0.4

0.6

Al2O3 wt% in Gt

16 20 240.0

0.2

0.4

0.6

Na2O wt% in Gt

0.0 0.2 0.40.0

0.2

0.4

0.6

a b

dc

e f

….Partitioning coefficients….

• depend on ionic size and charge of the element

• may depend on the composition and structure of the involved crystalline and liquid phases phase

• may depend on intensive parameters

<< 1

1

liqxlD /

liqxlD /

incompatible element:

compatible element:

phosphorus behaves incompatibly in pyroxenes (chain silicate) and becomes more compatible in olivine and garnet (orthosilicates)

Dxl/ liqP2O5

0.0 0.1 0.2 0.3 0.4

Opx

Ol

GtAll JSC data

Dxl/liq = Cixl/Ci

liq

SiO2 wt% in gl

DP

2O

5

Gt/

Liq

0.0

0.2

0.4

DX & DSD (2002)Thompson (1975a, b)This study

P2O5 wt% in gl

0 1 2

0.0

0.2

0.4

36 42 48 54D

P2O

5 O

l/Li

q

0.0

0.1

0.2

DX & DSD (2002)Libourel et al. (1994)Brunet & Chazot (2001)This study

SiO2 wt% in gl

36 42 48 54

DP

2O

5 O

px/L

iq

0.00

0.02

0.04

0 1 2

0.0

0.1

0.2

ba

dc

P2O5 wt% in gl

0 1 2

0.00

0.02

0.04e f

Si pfu (12 O)

2.9 3.0 3.1 3.2 3.3

Dgt/

liq P

2O

5

0.0

0.2

0.4

This studyDX & DSD (2002)Thompson (1975)

liqOpxOPD

/

52liqGt

OPD/

52

P (GPa)

3 4 5 6 7 8 90.0

0.2

0.4

Dgt/

liq P

2O

5

This studyDX & DSD (2002)Thompson (1975)

2.2 2.4 2.6 2.8 3.2 3.43.0

P (GPa)0.08

0.12

0.16

0.20

0.24

DGt/Liq P2O5: Thompson (1975)

1300oC

1300oC 1325oC

1350oC

1385oC1450oC

1250oC

• Common silicates (Opx, Ol, Gt) can host phosphorus (P)

• Possible positive correlation between P and Na in Gt, but no apparent correlation between P and other elements in Ol and Opx

• P2O5 (wt%) in Gt > Ol > Cpx > Opx > Plag: P incorporation more structure than site specific(?)

• Ol/Liq, Opx/Liq DP2O5 probably constant

• Gt/Liq, Cpx/Liq, Plag/Liq DP2O5 may vary with P and/or T, and liquid and garnet composition