Modeling arc chemistry with ADIABAT_1ph

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Modeling arc chemistry with ADIABAT_1ph
Gelu COSTIN James GIRARDI
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1. Why need modeling?
2. Forward modeling
3. How can we do it? ADIABAT_1ph
4. Limitations of the program
5. Example

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1. Why need modeling

• -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 ?
• need quantifying to better constrain geologic
  models
• Modeling need constrains to be realistic

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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)
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Program used for modeling compositions and
physical properties ? ADIABAT_1ph 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, doi10.1029/2004GC000816.    It
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.
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ADIABAT_1ph version 1.6

• 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

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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

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• 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. Pct
Assumptions -isobaric processes at different
depths according to a geologic model
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EXAMPLE ADIABAT runs for the a pluton from BC
10-15 kb

• 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

need a geological model before starting
quantifying
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EXAMPLE ADIABAT runs starting at 1500 ºC
Lherzolite
3548 BEARD B. L.(1995) samp. IN9220H-3 BASIN AND RANGE-GREAT BASIN / SOUTHWESTERN GREAT BASIN / CALIFORNIA / BIG PINE VOLCANIC FIELD
Partial melting of lherzolite ( H2O) to produce
basaltic melt
Runs at 30 kb pMELTS routine
Basaltic melts in MASH zone ? andesitic basalt
Runs at 15 kb MELTS routine
Acid melts residue
Runs at 4 kb MELTS routine
Compare the result with the composition of plutons
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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!!!

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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
Andesitic basalt
From Georoc database
Results of different runs are compared with
Estimated composition of the residue
Average composition of the pluton
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• 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
• proportion of liquid and solid residue
• temperature where the composition of pluton is
  valid
• chemical and petrographic composition of residual
  solid
• Estimates on the mineral chemistry of phases of
  the residual solid
• density of melt and residual solid

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Andesitic basalt as starting composition (from
Georoc database) CENTRAL AMERICAN VOLCANIC ARC /
HONDURAS / SEGMENT 4 / BOQUERON / PACIFIC OCEAN
4231
Averaged Great Tonalite Sill
Samples averaged
Andesitic basalt
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
GJP-12 Great Tonalite Sill Early Tertiary
GJP-13 Great Tonalite Sill Early Tertiary
GJP-14 Great Tonalite Sill 61 Ma
GJP-79 Great Tonalite Sill Early Tertiary
GJP-84 Great Tonalite Sill Early Tertiary
GJP-85 Great Tonalite Sill 59 Ma
GJP-83 Great Tonalite Sill Early Tertiary
SiO2 53.1 TiO2 1.14 Al2O3 18.3 Fe2O3 2.85 FeO 6.8
CaO 9.62 MgO 3.59 MnO 0.19 K2O 1.33 Na2O 3.21 P2O5
0.22 H2O 0.59
Adiabat_1ph
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T solidus 700 ºC
T liquidus 1420 º C
Composition pluton
? composition similar with pluton averaged at
1080 º C ( with 15 melt) ? Melt is 17.91 ?
The residue is 68.4 cpx 13.69 grt
P 15 kb
45 km depth
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Liquid composition at T1180ºC
Residue composition at T1080ºC
Starting composition (Protolith)
Real average
calculated
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
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
63.11
0.71
16.83
0.2
5.1
2.28
4.99
4.22
1.67
0.47
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
Residue
mass Estimated formula
garnet 13.69 (Ca0.07Fe''0.45Mg0.48)3Al2Si3O12
clinopyroxene 68.40 Na0.16Ca0.65Fe''0.23Mg0.65Fe'''0.04Al0.41Si1.85O6
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At T1180 º C
Densities (g/cm3) solid ? 3.345 liquid ? 2.586
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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?

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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.

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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

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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
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Input into Hackers Spreadsheet
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An example from CMB
Calculated properties for average Great Tonalite
Sill and residue.