Experimental constraints on subduction-related
magmatism :
Hydrous Melting of upper mantle perdotites
Modified after a ppt by Peter Ulmer
(Blumone, Adamello, Italy)
Garnet-peridotite in kimberlite
Topics
Mantle composition? How do we constrain them?
Dry Melting of mantle peridotites
Hydrous Melting: Basic concepts
Hydrous Mantle Melting: P-T-f-x relationships
Conclusion: Arc primary mantle magmas are:
• “basaltic”, representing relatively large melt fractions
• wet (hydrous)
• hot (>1200°C)
• oxidized (NNO – NNO+2)
Arc-Genesis Model I
P-T Lherzolite (Dry) Melting
Evidence for Mass transfer: Metasomatic Peridotites
Carbonatite-globule
phl
olivine
CO2-rich fluid inclusions
phl
olivine
Phlogopite-Peridotite in kimberlite
Mantle-cpx in basanite
Dry Lherzolite Melting
Fundamental Principles (Phase Equilibria)
Pressure effects on melting and composition of
primary melts
Temperature effects on melting and compositions
of primary melts
Fundamentals: Forsterite – SiO2
Pressure < 1.4 kbar
Fundamentals: Forsterite – SiO2
Peritectic
Pressure < 1.4 kbar
Cotectic (thermal max)
Pressure > 1.4 kbar
Anhydrous Peridotite Melting:
Melt fraction as a function of pressure and source composition
Ulmer (2001)
Anhydrous Peridotite Melting: Solidus temperatures and
melt compositions as a function of source composition at 3 GPa
Hirschmann (2003)
Hydrous Lherzolite Melting
Fundamental principles (phase equilibria)
Pressure – H2O effects on melting and
composition of primary melts
Temperature effects on melting and compositions
of primary melts
Geochemical signatures of “Arc” magmas
Diopside – Peridotite – H2O - Melting
H2O – solubility in basalt and albite liquids at
1100°C
Schematic diagram
showing melting phase
relations for a system
containing
Anhydrous minerals (A)
Hydrous mineral (H)
H2O (V)
Important (univariant)
curves:
H2O-saturated solidus (A)
Dehydration solidus (V)
Dry solidus (V) (low right)
Peridotite – H2O
Melting
ACMA: Average
Current Mantle
Adiabat
(diamond symbols:
multiply saturated
primary liquids =
extraction depth?)
olivine
clinopyroxene
orthopyroxene
primitive arc magma
multiple
saturation
(ol+opx±cpx)
of primitive
arc magmas
spinel
peridotite = mantle
Inverse – multiple
saturation
experiments
on primary arc
basalts
Picrobasalt (3 wt.% H2O) phase diagram with multiple saturation
Hydrous Peridotite Melting:
Melt fraction as function of MELT H2O-content
Ulmer (2001)
Effects of small amounts of H2O (in source) on melt-fractions
Parameterization (mostly thermodynamically based, including PMELTS)
Katz et al (2003)
Evidences for hydrous nature of “arc” magmas and
geochemical characteristics of supra-subduction magmas
Violent, explosive, gas-rich (H2O) eruptions typical for
differentiated magmas (andesite – rhyolite)
Melt inclusions (up to >10 wt.% H2O in primitive Olivine
inclusions (e.g. Shasta, Hess, Grove, Sisson and co-workers)
Early amphibole (and biotite) saturation indicating > 4 wt% H2O
at time of crystallization
Geochemical characteristics of supra-subduction magmas
(major and traces) => “Calc-alkaline” and “arc trace element
signature” of magmas and their (metasomatized) mantle sources
high fO2 probably related to oxidation by slab-derived fluids (Feisotopes indicate reduced arc mantle prior to fluid metasomatism)
“Spiderdiagram” of Island Arc Basalts (IAB)
HFSE – depletion, LILE and LREE enrichment, fluid mobile elements, residual
rutile and garnet to retain HFSE and HREE in “slab source” during dehydration
Spiderdiagram of Philippine Mantle Xenoliths
Major Element composition of MORB - IAB
Arcs: Silica enrichment and FeO-suppression due to late plag, early amph and mag
Mantle melting trend to high-SiO2 - low FeO*/MgO is controlled
by reaction relations during ascent to the base of the crust
Opx = Olivine + Liquid (SiO2-component)
Grove et al. (03)
Composition of primitive arc magmas
wt. %
Picro-
Olivine-
SiO2-rich
High-Mg
Basalt
Tholeiite
Tholeiite
Andesite
Boninite
SiO2
46.8
48.5
51.5
56.6
55.0
TiO2
0.7
1.0
1.8
0.9
0.2
Al2O3
12.4
14.4
13.8
17.6
12.5
Fe2O3
2.0
1.0
2.2
1.9
6.6
FeO
7.5
11.9
8.9
5.0
6.5
MgO
17.0
12.4
9.4
6.0
12.0
CaO
10.3
12.9
8.9
8.1
6.5
Na2O
1.2
1.5
2.5
3.4
1.9
K2O
0.4
0.5
0.7
1.0
0.7
xMg
0.77
0.65
0.65
0.68
0.77
30kb
18kb
12kb
7-10kb
ca.10
max. Press:
Oxygen Fugacity from Ol-Spinel oxybarometry
Oxygen Fugacity from volcanic glasses
Mature Island Arc (after Ringwood, 1974)
4 points to remember:
Presence of H2O during melting leads to enormous solidus
depression (function of pressure => solubility)
However, geochemistry (major elements) and experimental
constraints indicate significant melt fractions (10-20%)
generated at conditions close to the mantle adiabat
(>1200°C)
Arc magmas are more siliceous at a given pressure
compared to dry tholeiites (MORB, OIB) => “calc-alkaline*
Arc magmas carry particular signatures (trace elements,
fO2, fH2O) that can be linked to slab-derived components
=> Primary mantle melts are: basaltic, hot, wet, oxidized
Groves chlorite solidus
Grove’s “primitive “ melts???????
Ulmer
Conclusion: Arc primary mantle
magmas are:
• “basaltic”, representing relatively large
melt fractions
• wet (hydrous)
• hot (>1200°C)
• oxidized (NNO – NNO+2)
Grove
Small fractions
Can be as high in silica as 60%
Wet
Low temps (800-820C)
Oxidized
Conclusions for us
Lets assume that wet basalts are in fact
the most common compositions coming
out of the mantle beneath arcs
Will have to test this against observational
data to see
Need to look at primitive basalts from
various deep crustal arc sections.
Deep crustal sections
Salinia
Sierra Valle Fertil
Southern Sierra
Parts of the Cascades
Chapman et al., 2014
Famatinia at Valle Fertil
Tilted section
Down to 30 km
Up to 10 km mafic section
Upper crust: granodiorite
From Walker et al., 2015
CONCLUSIONS
Primitive arc magmas are basalts
NOT andesites if periditotite is the source
Wet basalts
They stall in the mid to lower crust and
fractionate hb, cpx and opx
Basalt fractionation does not lead to
anything more than a quartz diorite
Lifting the SiO2 above 55-58% requires
upper crustal contributions