melt inclusions in basaltic and associated volcanic rocks
DESCRIPTION
Adam Kent, Oregon State University. Melt inclusions in basaltic and associated volcanic rocks. 50 µm. Melt inclusions: An introduction. “Parcels” of melt trapped in igneous crystals Fluid inclusions. Occur in basaltic and related rocks wherever they are found: - PowerPoint PPT PresentationTRANSCRIPT
Melt inclusions in basaltic and Melt inclusions in basaltic and associated volcanic rocksassociated volcanic rocks
Adam Kent, Oregon State UniversityAdam Kent, Oregon State University
50 µm
Melt inclusions: An introductionMelt inclusions: An introduction
Occur in basaltic and related rocks wherever
they are found:
Arcs, OIB, CFB, MORB, LMI, ET’s
Silicic and Plutonic Rocks
Xenoliths
“Parcels” of melt trapped in igneous
crystals
Fluid inclusions
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
Ultra-depleted
Sobolev and Shimizu, 1993
9°N Mid Atlantic Ridge
Why study melt inclusions?
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
Melt inclusions preserve compositions that are different from
those of erupted lavas/tephra
But there’s a catch
• 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 inclusions are NOT a universal panacea!
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
trappingi.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, non-representative 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
Trace Element
s
Volatiles Isotopes
EMPA Yes S*, Cl
SIMS Yes C,H,F,S,ClH, Li, B, Cl,
S, O, Pb
LA-ICP-MS Maybe Yes Pb, Sr
FTIR H*,C
*plus speciation
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
50 µm
25 µm
Evolution of melt inclusions after trapping
25 µm
Important impact on physical appearance and chemical compositions
Evolution of melt inclusions after trapping
Wallace, 2005
1. Venting/breaching/alteration
2. Post-entrapment crystallization
3. Diffusive exchange
Loihi Seamount (Kent et al., 1999)
Correction for postentrapment crystallizationExperimental
• Reheat to (estimated) trapping temperatureNumerical• Based on chemical equilibrium
–Olivine: KDFeO*/MgO =
0.33 ± 0.03
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Compatible elements are the least robust after correction for post-entrapment
crystallization
Equilibration between Host and Inclusion
Qin et al. (1992)
Equilibration more rapid at• Higher Diffusivity• Higher Temperatures• More compatible• Larger inclusion• Smaller host
Fe Loss
• Negative correlation between measured FeO* and Fohost
• Anomalously low FeO* wrt liquid line of descent
Danyushevsky et al. 2000
Yaxley et al. 2005
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
0
5
10
15
20
25
30
35
40
45
50
-12 -10 -8 -6 -4
Log Do (cm2/sec)
Standard Deviation (%)Diffusion in basalt melt
Diffusion in Olivine
Baffin Island olivine-hosted n = 103
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 >> Re-equilibration
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
Comparison between melt inclusions and host lavas
Baffin Island olivine-hosted
Kellogg et al. 2002
Magma
Melt Inclusion
Magma
€
slava =sinclusions
nn ≈ 90
[Theistareykir : n ≈ 30 −100
Slater et al. (2001)
MacLennan et al. 2003]
Baffin Island olivine-hosted
Borgahraun, Iceland
Maclennan et al. 2003
Comparison between host and inclusions provides a means to assess relationship between inclusions and magmatic systems
• Anomalous melt inclusions– Low volume melts?– Magma chamber or primary?– Artifacts of trapping?
Ultra-depleted
9°N MAR
Sobolev et al 2000 Sobolev & Shimizu 1993
“ 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.]