Melt inclusions in basaltic and associated volcanic rocks

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Melt inclusions in basaltic and
associated volcanic rocks
Adam Kent, Oregon State University
Melt inclusions: An introduction
“Parcels” of melt
trapped in igneous
crystals
 Fluid inclusions
Occur in basaltic and
related rocks wherever
they are found:
Arcs, OIB, CFB, MORB,
LMI, ET’s
Silicic and Plutonic Rocks
50 µm
Xenoliths
Saal et al. 1998
Scope: Basaltic and related volcanic rocks
Scope: Basaltic and related volcanic rocks
Nanos gigantum humeris insidentes
Bernard of Charles, 1159
Why study melt inclusions?
Melt inclusions preserve compositions that
are different from those of erupted
lavas/tephra
9°N Mid Atlantic Ridge
Ultra-depleted
Sobolev and Shimizu, 1993
Why study melt inclusions?
Melt inclusions preserve compositions that
are different from those of erupted
lavas/tephra
1. More variable than host and associated lavas
•
•
•
Bulk rock, Matrix glass  “Averages”
Provide larger data sets per rock
Preserve low volume or low survivability melts
•
Primitive Melts
2. Trap volatile elements
•
Compare volatile and non volatile behaviour
3. Provide melt samples in altered rocks
But there’s a catch
Melt inclusions are NOT a universal panacea!
• Specific samples: phyric ± rapidly cooled
• More work/time/money per sample
– Mineral separation, mounting and polishing
• Specialized analysis techniques
– Melt inclusions are small!
– Typically (trace element and isotope) analyses are less
precise
– Isotopic data are limited
• Require significant additional interpretation
Melt Inclusion Variations
Magmatic
• Crystallization
• Assimilation
• Magma mixing
• Source heterogeneity
• Degassing
Inclusion-specific
• Boundary layers
• Post-entrapment
crystallization
• Re-equilibration with
host or external melt
• Non representative
trapping
i.e. things that drive changes in
magma compositions
i.e. things that are unique to
inclusions
Inclusion-Specific Processes
• Re-equilibration between inclusion and host
– Portnyagin et al. 2007, Spandler et al. 2007, Cottrell et al.
2002, Danyushevsky et al. 2000; Gaetani and Watson,
2000, 2002
• Preferential trapping of unusual, nonrepresentative compositions
– Michael et al. 2002, Danyushevsky et al. 2004, Yaxley et
al. 2005
• Trapping boundary layers
– Kohut and Nielsen, 2004; Faure and Schiano, 2005, Baker
et al. 2008. Goldstein and Luth, 2007
• Alteration of inclusions
– Nielsen et al. 1998
Analysis
Major
Elements
EMPA
Yes
SIMS
LA-ICP-MS
Trace
Elements
Isotopes
S*, Cl
Yes
Maybe
Volatiles
C,H,F,S,Cl
Yes
FTIR
Pb, Sr
H*,C
*plus speciation
H, Li, B, Cl,
S, O, Pb
How do melt inclusions form?
The widespread occurrence of melt inclusions
in basaltic rocks shows that their formation is
a normal part of the process of crystallization
in igneous rocks
Melt inclusions form in regions of
relatively slow crystal growth
How do melt inclusions form?
Modified from V.S. Sobolev and Kostyuk 1975; Roedder, 1979, 1984
Faure and Schiano 2005
Do melt inclusion formation
processes fractionate trapped
compositions?
Baker et al. 2008
Not all experimental studies show boundary
layer effects
•Most natural suites do not
show clear indications of
boundary layer effects
•Perhaps we sample larger
inclusions (only significant at
< 30 µm)
•Longer isothermal times in
natural samples
•Are boundary layers static?
•Kinetic experiments
Evolution of melt inclusions after trapping
Important impact on physical appearance and chemical compositions
25 µm
50 µm
25 µm
Evolution of melt inclusions after trapping
Wallace, 2005
1. Venting/breaching/alteration
2. Post-entrapment crystallization
3. Diffusive exchange
Correction for postentrapment
crystallization
Experimental
• Reheat to
(estimated) trapping
temperature
Numerical
• Based on chemical
equilibrium
–Olivine: KDFeO*/MgO =
0.33 ± 0.03
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Loihi Seamount (Kent et al., 1999)
Compatible elements are the least robust after
correction for post-entrapment crystallization
Equilibration between Host and Inclusion
Equilibration more rapid at
• Higher Diffusivity
• Higher Temperatures
• More compatible
• Larger inclusion
• Smaller host
Qin et al. (1992)
Fe Loss
Yaxley et al. 2005
• Negative correlation
between measured FeO* and
Fohost
• Anomalously low FeO* wrt
liquid line of descent
Danyushevsky et al. 2000
Trace element re-equilibration
• The most robust data
sources in melt
inclusions are slow
diffusing and
incompatible elements
– Altered only by
dilution/concentration
– Ratios unchanged
Are incompatible trace elements affected by
diffusional re-equilibration?
Cottrell et al. 2002
REE equilibration with host after 2500 years
Cottrell et al 2002
Slater et al., 2001
Trace element re-equilibration
Spandler et al. 2007
Baffin Island olivine-hosted n = 103
50
Standard Deviation (%)
45
40
35
30
25
20
15
10
Diffusion in basalt melt
5
Diffusion in Olivine
0
-12
-10
-8
-6
Log Do (cm2/sec)
-4
Preserve inter-crystal variations
Driving Force?
The message from melt
inclusions: Variability
• In many basaltic systems it is clear that the primary
control on melt inclusion compositions is the
variability of melts present within the system
– These are sampled by erupted lavas as well, but
are homogenized
– Implies large scale mixing of smaller melt
“batches” is extremely widespread
• Melt inclusions and host lavas related by mixing
• Basaltic melt generation
and transport systems
are variable at scales
smaller than individual
eruptive units (factors of
10’s)
• Phenocrysts
• Melt inclusions sample
this variation
• Some real and apparent
homogenization
(mixing) occurs prior to
eruption
• Rates: Transport >> Reequilibration
Comparison between melt inclusions and host lavas
Baffin Island olivine-hosted
Melt inclusions sample the same population of melts as host lavas
Variability in trace element composition is driven by the same
processes in inclusions and in lavas
Magma
Magma
Melt Inclusion
Kellogg et al. 2002
Baffin Island olivine-hosted
sinclusions
slava 
n
n  90
[T heistareykir: n  30100
Slater et al. (2001)
MacLennan et al. 2003]

Borgahraun, Iceland
Maclennan et al. 2003
Comparison between host and
inclusions provides a means to assess
relationship between inclusions and
magmatic systems
9°N
MAR
Ultra-depleted
Sobolev et al 2000
Sobolev & Shimizu 1993
• Anomalous melt inclusions
– Low volume melts?
– Magma chamber or primary?
– Artifacts of trapping?
“ There is no necessary connexion between the size of an object and
the value of a fact, and…though the objects I have described are
minute the conclusions to be derived from the facts are great ”
Sorby 1858
Geol. Soc. London. Quart. Jour. 14 453-500
[from Roedder (1979) Bull. Mineral.]
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