Melt Inclusions & Volatiles in Silicic Magmatic Systems · What can we infer about volatiles in magmas that are parental to rhyolites? • Note that the range of H 2O in arc rhyolites

Melt Inclusions & Volatiles in Silicic Magmatic Systems

Photo by D. Harlow, USGS

Outline

• Generation of intermediate & silicic magma

• Melt inclusions – host crystals & inclusion textures

• Volatile concentrations in silicic magmas

• Vapor saturation & magma chamber configurations

• Processes that cause volatile variations

• Inferring vapor compositions

• Sulfur & chlorine

How are intermediate and silicic magmas formed?

• Continental crust has mafic lower crust and more evolved granite-dominated upper crust

• Mantle-derived magmas in subduction zone settings are basalt to high-Mg andesite in composition

• Processes for forming more evolved magma:

• Differentiation of primary mafic magmas by crystallization

• Partial melting of older crustal rocks

• Partial melting of earlier formed cumulates/plutons in the lower crust

KOS data

Blundy MSH figures

Importance of Mafic Magma• Crustal magmatic systems are fundamentally basaltic – heat & mass

• Basaltic magma transfers volatiles from mantle to crust

• Mantle volatile sources include upper mantle & subduction-recycled components

Hildreth (1981)

Conceptual representation of a deep crustal hot zone(Annen et al., 2006)

• Hydrous basaltic melts intruded into thelower crust as sills

• Heat & H2O from the crystallizing basaltpromote partial melting of the lower crust

• Mixing of residual & crustal melts

• Ascent of H2O-rich melts into the uppercrust

• Degassing & crystallization at shallow depthlead to large increases in viscosity & stallingof magma to form magma chambers

Strong crystal fractionation signature in rhyolites

Halliday et al. (1991)

• Data suggest a genetic link between crystal-poor & crystal-rich rhyolites &I-type granitoids

• But how are viscous silicic melts physically separated from crystals?

Crystal mush model – Bachmann & Bergantz (2004)

Upward migration of low-density, residual melt from a crystallizing boundary layer is a popular, but problematic, hypothesis for shallow magmatic differentiation

• Difficult to explain old, complexly zoned phenocrysts• Large silicic magma bodies have sill-like aspect ratios• Re-entrainment of magma in cap may be relatively rapid

An alternative model for formation of intermediate & silicic magma

Bachmann &Bergantz (2004)

What can we learn from melt inclusions about formation, crystallization & storage of silicic magmas?

Montserrat – photo by B. Voight

Quartz-hosted melt inclusions, Bishop Tuff

Cathodoluminescence images

Wafers for FTIR & microbeam analysis

Rhyolitic melt inclusions in quartz phenocrysts

Secondary electron images of bipyramidalquartz from the 1912 eruption of Katmai showing semi-skeletal growth form & trappingof inclusions (Lowenstern, 1995)

QuickTime™ and a decompressor

are needed to see this picture.

Rhyolitic melt inclusions – vapor bubbles & quench crystals

Wallace et al. (2003)

• Melt inclusions in high-silica rhyolities are typically bubble free (a)• With slower cooling, inclusions develop a darker color (c), bubbles (b), & fine crystals (g, k, l)• In some cases, bubbles nucleate on daughter crystals (g, h, l)• Bubbles can also be caused by cracking (decrepitation) of the host

Rhyolitic melt inclusions in plagioclase



Transmitted light photomicrographsCrater Lake (Bacon et al., 1992)

• Melt inclusions in plagioclase areoften poorly sealed

• Vapor bubbles are common

SEM images of Mount St. Helensplagioclase (courtesy of J. Blundy)

Volatiles in melt inclusions from subduction zone rhyolites

Wallace (2005)

• Agreement between melt inclusion vapor saturation pressures & total pressure constrained by experimental phase equilibria

• Volcanic SO2 and CO2 emissions

• CO2 vs. trace elements in melt inclusions

• Fluid inclusions in phenocrysts in volcanic rocks

Independent Evidence for Vapor Saturation

Pasteris et al. (1994)

Quartz phenocryst, PinatuboPlagioclase phenocryst, Guagua

Pichincha, Ecuador

Photo courtesy of Jake Lowenstern, USGS

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7 8

H2O (wt%)

YellowstoneBishop Tuff Pine

GroveTuff

Pinatubo

0.5 kb

1 kb

2 kb3 kb

N. Chile

Toba

KatmaiMazama

Data sources: Chesner & Newman (1989); Bacon et al. (1992); Lowenstern (1994); Wallace & Gerlach(1994); Gansecki (1998); Wallace et al. (1999); Schmitt (2001); Wallace (unpubl.)

Comparison of subduction zone & other rhyolites

CO

2(p

pm)

Bishop Tuff & Long Valley Caldera

Cathodoluminescence images ofquartz-hosted melt inclusion

Melt Inclusions from the Bishop Tuff, California


Crystalpoor

Crystal rich

Configuration of pre-caldera magma body, Long Valley Caldera, CA

5 km

10 km

North South

middle

late

unerupted mush

initial vent site

ring fractures formedduring eruption andcaldera collapse

no vertical exaggeration

early

Use of melt inclusions to infer magma body configuration




Wark et al. (2007)

Late Bishop Tuff preserves evidence of pre-eruption intrusion of hot, CO2-rich rhyolitic melt into crystal mush in lower part of magma chamber

Crystal-rich mush Intrusion by hotter rhyoliteDissolution of quartz

Quartz overgrowths traphigh CO2 melt inclusions

What processes cause variations in volatile contents?

Decompression – leads mainly to variations in CO2



Tuff of Pine Grove, Utah (Lowenstern, 1994)

2

3

4

5

6

7

8

0.1 1 10 100 10 3

Ba (ppm)

Late BT

Early BT

Middle BT

Sanidine fractionation Yellowstone

N. Chi le

PineGrove

H2O

(ppm

)

What causes variations in H2O content?

• Variation of H2O with Ba suggests fractional crystallization control onincreasing H2O contents

• Magmatic H2O contents increase during vapor-saturated crystallizationif CO2 is present

0

500

1000

1500

2000

2500

0 1 2 3 4 5 6 7

CO

2 (ppm

)

H2O (wt.%)

3 kb

2 kb

1 kb

vapor saturation isobars

CO

2(p

pm)

Vapor–Saturated Crystallization

Vapor–saturatedcrystallization

Anderson et al. (1989)

Exsolution of vapor during closed-system crystallization

<

<

volume

crystal %

vapor mass

volume

crystal %

vapor mass<

initial finalcrystals

exsolved vapormelt

% crystallization(C

O

/ CO

in

itial

) dis

solv

ed in

mel

t2

2

302010000.0

0.2

0.4

0.6

0.8

1.0

2%

3%

4%

7%

1%

2%

2%3%

6%

1%

5%2%1%0.5%0%

initial exsolved vapor (wt.%)

CO2 exsolution during vapor-saturated crystallization

Melt inclusions from clast CHAL-9

Loss of CO2 to Exsolved Vapor During Closed-System Crystallization



0

200

400

600

800

1000

1 10 100

CO

2 (pp

m)

Ba (ppm)

Melt Inclusions from the Bishop Tuff

Late

Early

Middle

Sanidine fractionation

Vaporexsolution

error

CO

2(p

pm)

Loss of CO2 to Exsolved Vapor During Vapor-Saturated Crystallization

What can we infer about volatiles in magmas that are parental to rhyolites?

• Note that the range of H2O in arc rhyolites is similar to that in arc basalts

• How are the two related?

Effects of fractional crystallization & ascent into upper crust

• Parental basaltic magmas must have relatively low H2O, and/or

• Lower crustal melting must involve relatively H2O-poor sources

Wallace (2005)

Kos Plateau Tuff – quartz crystals recycled from mush(?)

Bachmann et al.(in press)

• Crystal rich (≤40 vol%) rhyolitic tuff; eruptive volume >60 km3

• Geologic evidence suggests magma chamber drawdown of ~1 km during eruption

• Crystals may mostly be derived from mush zone beneath & surrounding chamber

Effects of composition & temperature on S contents of magmas



Shinohara (2008)

aa

0

500

1000

1500

2000

2500

3000

3500

0 1 2 3 4 5 6 7 8

H2O (wt.%)

Saturation with H

2O-rich vapor

Bishop Tuff

Pine Grove Tuff

Bandelier (CTR)

2 kbar

Saturation withconcentrated brine Taupo

Cl (

ppm

)

Chlorine in Rhyolitic Melt Inclusions

• Most non-mineralized rhyolites do not have enough Cl to be saturatedwith concentrated brine (hydrosaline melt)

Decompression-driven versus cooling-driven crystallisation

0

1

2

3

4

5

6

7

68 70 72 74 76 78

wt%

H2O

wt% SiO2 (anhyd.)

IV. Syn-eruptivedegassing

III. DecompressionH

2O-saturated

I. IsobaricH

2O-saturated

II. IsobaricH

2O-undersaturated

Inclusion populations record ascent trajectories – time and pressure variations

Blundy & Cashman (2008)

Decompression-driven crystallisation

• Decompression-driven crystallization (H2O loss) is much faster than crystallization driven by cooling

0

1

2

3

4

5

6

7

66 68 70 72 74 76 78 80

1980-81

Apr-10 (plag)Crypto (plag)Crypto (opx)May-18 (plag)May-18 (hbl)May-25 (plag)

Jun-12 (plag)Jul-22 (plag)Aug-7 (plag)Aug-7 (opx)Oct-16 (plag)Oct-16 (hbl)

Dec-27 (plag)Dec-27 (cpx)Jun 1981 (plag)May-18 (gm)1980 (gm)

wt%

H2O

wt% SiO2(n)

rupturedinclusions

Blundy & Cashman (2008)

Modified from Hochstein & Browne (2000)

Silicic magma bodies & hydrothermal systems



Student & Bodnar (2004)

Melt Inclusions from Porphyry Copper Deposits



Key points to remember:

• Melt inclusions in rhyolitic systems preserve a complex record of magmacrystallization, degassing, mixing and storage

• Melt inclusion H2O & CO2 data can be used to infer magma bodyconfiguration and depths of crystallization

• Rhyolitic magmas are typically vapor saturated in the upper crust.Some may be saturated with a hydrosaline melt (brine) phase

• Dissolved sulfur concentrations are low, but there may be considerableS as H2S and SO2 in the coexisting vapor phase

0

500

1000

1500

2000

2500

0 1 2 3 4 5 6 7

CO

2 (ppm

)

H2O (wt.%)

3 kb

2 kb

1 kb

Vapor saturation isobars

Vapor composition isopleths mol% H2O in vapor

10%20%

30%

40%50%

60%70%

80%

90%

Inferring Vapor Composition from Melt Inclusion Data

CO

2(p

pm)

Isobars & isopleths from VolatileCalc (Newman & Lowenstern, 2002)

0.50.40.30.20.10.0300

250

200

150

100early & middle inclusionslocal averagelate inclusions

X in vapor

Pre

ssur

e (M

Pa)

CO2

error

Exsolved Vapor Composition in Pre-Caldera Magma Body

Bishop Tuff


Importance of Mafic Magma

• Crustal magmatic systems are fundamentally basaltic

• Basaltic magma transfers volatiles from mantle to crust

“Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components.”

Hildreth (1981)

“[Stable isotope] data suggest that magmatic fluxes of C and S are dominated by mantle sources”

0

50

100

150

200

250

700 750 800 850 900 950 1000

Temperature (°C)

Fish CanyonPinatubo

Toba

Redoubt

Krakatau

MSH

Ruiz

2 kbar (NNO)FeS saturated

Katmai

BishopTuff

S (p

pm)

Sulfur in Rhyolitic Melt Inclusions

• Many rhyolitic magmas are sulfide saturated• Saturation limit of S increases with temperature & oxygen fugacity

Documents

Melt Inclusions & Volatiles in Silicic Magmatic Systems · What can we infer about volatiles in magmas that are parental to rhyolites? • Note that the range of H 2O in arc rhyolites