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.]