Modeling arc chemistry with ADIABAT_1ph

Gelu COSTIN

James GIRARDI

1. Why need modeling?

2. Forward modeling

3. How can we do it? ADIABAT_1ph

4. Limitations of the program

5. Example

-different processes and mechanisms similar effects

(e.g. modify the composition of a magma to obtain increasing of SiO2)

-fractional crystallization of a basic magma-different % of crustal assimilation of a basic magma-different % of partial melting of crustal rocks

•experiments partial melting should leave behind important amounts of residuum with high densities

Arc models

a) need quantifying to better constrain geologic models

a) Modeling need constrains to be realistic

1. Why need modeling

2. Forward modeling

We start from unknown (or guessed), trying to arrive to what we know

Try end error method

What do we know?

-several plutons with different composition, age etc we can estimate an average composition

-from the exposed area we can do some estimations of the volume of plutons

-scarce knowledge of the plutons development and composition at greater depths

Models can work on:

-individual protoliths, plutons, residual solids etc (at local scale)

-averaged compositions of the protoliths, plutons, residual solids etc (at arc scale)

Program used for modeling compositions and physical properties

ADIABAT_1ph

Smith, P. M., and P. D. Asimow (2005)Smith, P. M., and P. D. Asimow (2005), Adiabat_1ph: A new public front-end to the MELTS, pMELTS, and pHMELTS models, Geochem. Geophys. Geosyst., 6, art. no. Q02004, doi:10.1029/2004GC000816. 

 It uses the MELTS family of algorithmsIt uses the MELTS family of algorithms   Ghiorso, M.S., and R.O. Sack, Chemical Mass-Transfer in Magmatic Processes IV. A Revised and Internally Consistent Thermodynamic Model for the Interpolation and Extrapolation of Liquid-Solid Equilibria in Magmatic Systems at Elevated-Temperatures and Pressures, Contributions to Mineralogy and Petrology, 119 (2-3), 197-212, 1995. Asimow, P.D., and M.S. Ghiorso, Algorithmic modifications extending MELTS to calculate subsolidus phase relations, American Mineralogist, 83 (9-10), 1127-1132, 1998.

• calculates equilibrium assemblages from a given bulk composition of multicomponent systems

• anhydrous, water-undersaturated, or water-saturated systems

• options of buffering oxygen fugacity

• control on water activity

• subsolidus or suprasolidus calculations

• melting and crystallization models may be batch, fractional, or continuous.

• can simultaneously calculate trace element distributions.

• can calculate along a thermodynamic path set by the user

ADIABAT_1ph version 1.6

4. Limitations of the program

• the compositions of liquids are not realistic above 30-35 kb

•TiO2 overestimate the stability of pyroxene over other solids

• MnO overestimates the stability of liquid and olivine over other phases

• The amphibole stability field is underestimated

• pMELTS routine is to be used for ultrabasic compositions only

The compositions of melts and solids, as well as the phase proportions, are dependent on:

-small variation of H2O content-initial composition of the system (SiO2, Al2O3 etc...)-T-P-Thermodynamic type of calculation (isobaric, isentropic, fractional crystallization .....)

More variables Need simplifications:

e.g. keep some variable = ct

e.g. P=ct

Assumptions:

-isobaric processes at different depths according to a geologic model

need a geological model before starting quantifying

-plutons with heterogeneous compositions, forms, ages, depths etc

-only limited parts of the arcs are exposed

-no direct exposure of the lower levels of the arcs

EXAMPLE – ADIABAT runs for the a pluton from BC

10-15 kb

[3548] BEARD B. L.(1995)

samp. IN9220H-3

BASIN AND RANGE-GREAT BASIN / SOUTHWESTERN GREAT

BASIN / CALIFORNIA / BIG PINE VOLCANIC FIELD

LherzoliteLherzolite

Runs at 30 kb

pMELTS routine

EXAMPLE – ADIABAT runs starting at 1500 ºC

Partial melting of lherzolite (+ H2O) to produce Partial melting of lherzolite (+ H2O) to produce basaltic meltbasaltic melt

Basaltic melts in Basaltic melts in MASHMASH zone zone andesitic basaltandesitic basalt

Acid melts + residueAcid melts + residue

Runs at 15 kb

MELTS routine

Runs at 4 kb

MELTS routine

Compare the result with the composition of plutons

30 kb -Lherzolite

3% H2O

Ts ~1180 ºC

small amount of melt (~ 1 % melt at ~ 1200 º C with SiO2~ 35 %

not enough melt

not “normal” basalt composition

not realistic!!!

Phase proportions for 3% H2O

0

10

20

30

40

50

60

70

1100 1150 1200 1250 1300 1350 1400 1450 1500

T (C)

% vo

l

liquid_0

olivine

garnet

orthopyroxene

clinopyroxene

biotite

spinel

water

Liquid composition for 3% H2O

0

5

10

15

2025

30

35

40

45

1100 1150 1200 1250 1300 1350 1400 1450 1500

T (C)

% wt

SiO2

TiO2

Al2O3

Fe2O3

FeO

MgO

CaO

Na2O

K2O

H2O

Assuming a basic melt arrived at MASH zone

By assimilation and homogenization crystallization

andesitic basalt or basaltic andesite

Protolith for future melts

Runs at 15 kb

Results of different runs are compared with :

From Georoc databaseAndesitic basaltAndesitic basalt

Average composition of the Average composition of the plutonpluton

Estimated composition of composition of the the residueresidue

Initial composition: 1, 2..... X .....

...slightly modifying composition, water content etc....

untill...

...we get an acid melt similar with our pluton...

then...

The guessed initial composition protolith

..... and we can also estimate:

1. proportion of liquid and solid residue

2. temperature where the composition of pluton is valid

3. chemical and petrographic composition of residual solid

4. Estimates on the mineral chemistry of phases of the residual solid

5. density of melt and residual solid

SiO2 53.1TiO2 1.14Al2O3 18.3Fe2O3 2.85FeO 6.8CaO 9.62MgO 3.59MnO 0.19K2O 1.33Na2O 3.21P2O5 0.22H2O 0.59

Andesitic basalt as starting composition (from Georoc database)

CENTRAL AMERICAN VOLCANIC ARC / HONDURAS / SEGMENT 4 / BOQUERON / PACIFIC OCEAN [4231]

Samples averaged

SiO2 63.11

TiO2 0.71

Al2O3 16.83

FeO 5.1

Fe2O3 0.20

MnO 0.08

MgO 2.28

CaO 4.99

Na2O 4.22

K2O 1.67

P2O5 0.24

H2O 0.47

Averaged

Great Tonalite Sill

GJP-12

Great Tonalite

SillEarly

Tertiary

GJP-13

Great Tonalite

SillEarly

Tertiary

GJP-14

Great Tonalite

Sill 61 Ma

GJP-79

Great Tonalite

SillEarly

Tertiary

GJP-84

Great Tonalite

SillEarly

Tertiary

GJP-85

Great Tonalite

Sill 59 Ma

GJP-83

Great Tonalite

SillEarly

Tertiary

Andesitic basalt

Adiabat_1ph

Liquid composition

0

10

20

30

40

50

60

70

80

700 800 900 1000 1100 1200 1300 1400 1500

T (C)

% w

t

SiO2

TiO2

Al2O3

Fe2O3

FeO

MgO

CaO

Na2O

K2O

H2O

Phase proportions at 15 kb

0102030405060708090

100

700 800 900 1000 1100 1200 1300 1400 1500

T (C)

% m

as

s

liquid_0

garnet

clinopyroxene

clinoamphibole

biotite

feldspar

feldspar

quartz

kyanite

T solidus ~ 700 ºC

T liquidus ~ 1420 º C

composition similar with pluton averaged at

~ 1080 º C ( with ~ 15% melt)

Melt is 17.91 %

The residueresidue is 68.4% cpx + 13.69 %grt

P = 15 kb

~45 km depth

Composition ~ pluton

SiO2 52

TiO2 0.1

Al2O3 13

Fe2O3 0.9

FeO 8.5

CaO 12.2

MgO 10.1

K2O 0.5

Na2O 3.0

H2O 0.5

Starting composition

(Protolith)SiO2 63.66

TiO2 0.16

Al2O3 16.18

Fe2O3 0.02

FeO 6.51

MgO 0.44

CaO 5.50

Na2O 6.27

K2O 0.05

H2O 1.22

Liquid composition at T=1180ºC

Residue composition at

T=1080ºC

63.11

0.71

16.83

0.2

5.1

2.28

4.99

4.22

1.67

0.47

calculated Real average

ResidueResiduemass %mass % Estimated formulaEstimated formula

garnetgarnet 13.69 (Ca0.07Fe''0.45Mg0.48)3Al2Si3O12

clinopyroxeneclinopyroxene 68.40 Na0.16Ca0.65Fe''0.23Mg0.65Fe'''0.04Al0.41Si1.85O6

SiO2 49.12

TiO2 0.09

Al2O3 10.44

Fe2O3 1.15

FeO 9.79

MgO 12.99

CaO 14.99

Na2O 1.44

K2O 0.00

H2O 0.00

Densities (g/cm3)

solid 3.345

liquid 2.586

At T=1180 º C

Densities

00.5

11.5

22.5

33.5

4

700 800 900 1000 1100 1200 1300 1400 1500

T (C)

den

sity

(g

/cm

3)

Further Constraints on the Composition of Deep Crustal Rocks• Using the output modeling programs we

can calculate seismic properties of rocks of that composition.

• We can compare the calculated seismic properties to what we see in the batholith.

• How is this possible?

Mineral Physical Properties

• Database of mineral physical properties (Hacker et al. 2003).

• Hackers’ spreadsheet is an Excel workbook which includes database and a macro which will calculate rock physical properties (Hacker and Abers 2004).

• Input into this spread sheet is Vol% of minerals in rock.

CIPW norms

• A norm is a synthetic mineralogy calculated by apportioning chemical components into hypothetical (but hopefully realistic) minerals.

• Mode is the actual mineralologic composition of a rock, volume percentage of minerals.

• Using CIPW norms we can convert the chemical composition attained from the output of modeling programs such as Adiabat, and input the mineral assemblage into Hackers spreadsheet

Calculating CIPW norms

• What was once a The process of calculating CIPW norms can be done in Excel.

The volume % of normative minerals can be input into Hackers spreadsheet to calculate the seismic properties of a rock with that composition.

input

Input into Hackers’ Spreadsheet

An example from CMB

Calculated properties for average Great Tonalite Sill and residue.