Mineral Textures and Zoning as Evidence for Open System
Processes
Martin J. Streck
Department of Geology
Portland State University
Streck, 2008, RiMG 69:595-619
Outline of presentation:
1. Brief summary of open system processes
2. Mineral evidence
- single xls vs. xls populations
- pheno-, ante-, xenocryst
- general zoning and textures
3. Select zoning and textures and mineral examples
- patchy zoning in plagioclase & pyroxene
- step-zoning in pyroxene
- reverse and oscillatory zoning in plagioclase
- cellular textures: pervasive resorption vs. growth
- break down reactions in amphibole
4. Mineral populations
Streck, 2008, RiMG 69:595-619
1. Open system processes
Magma mixing
- ranges from mixing of strongly compositionally contrasting magmas (e.g.
basalt-rhyolite) to convective self-mixing (e.g. Couch et al. 2001)
- all mixing events may leave mineralogical fingerprints
- mixing can be of melts and/or crystals (e.g. dispersion of quenched mafic
inclusions (e.g. Feeley & Dungan, 1996)
Contamination (Assimilation)
- most commonly viewed as incorporation of country rock
- may range from incorporation of vastly different
country rock lithologies to ‘reworking’ crystalline margins similar to
crystallizing mineral assemblage
- bulk vs. selective assimilation (e.g. Edwards & Russell, 1996) rock vs.
partial melt, or combination of both
Streck, 2008, RiMG 69:595-619
2. Mineral evidence - single xls vs. xls populations
Evidence for open system based on:
- single crystals
- mineral populations
- zoning & textures
- juxtaposition of mineral assemblage
distinguished by various criteria (mineral
type, size, composition, zoning, textures)
cpx
Two distinct plag populations
Single cpx from Arenal volcano
quartz
1
2
min. patchwork
e.g. Carroll et al., this meeting
Fo86 olivine
e.g., Gillam et al., this meeting
e.g., Streck et al., 2002
Streck, 2008, RiMG 69:595-619
Narcondam volcano, this contribution
2. Mineral evidence - pheno-, ante-, xenocryst
Phenocryst (sensu stricto)
Phenocryst (sensu lato)
- larger xls set in finer matrix
- in-situ crystal growth
Antecryst
- cognate crystal originating from a magma genetically related
to the one in which it is found
Xenocryst
- foreign crystal
But: single crystals can - more often than recognized - belong to different categories
phenocrystic rim
(An 60-55)
(An 70-80)
ante-, or
xenocrystic
core
(An 72-60-70)
(An 90-72)
(An 95-92)
e.g., Lunney, 2001
Streck, 2008, RiMG 69:595-619
2. Mineral evidence - general zoning and textures
Styles of zoning and textures of single crystals
Zoning
- normal vs. reverse
- step-zoning vs. progressive
rim
rim
rim
rim
- patchy zoning (vs. concentric)
- oscillatory zoning (vs. monotonous or steady)
- (sector zoning)
rim
rim
Etching (Nomarksi, NDIC) will show zoning as textures
Textures
- spongy cellular/boxy cellular
- dissolution surfaces
- reaction rims (decomposition into mineral assemblage)
- mineral mantles
Streck, 2008, RiMG 69:595-619
3. Select zoning and textures:
- Patchy zoning in plagioclase and pyroxene
opx Mg#
~79
86
Irregular zoning pattern suggestive of
antecrystic or xenocrystric material as
induced by:
1) infilling of skeletal crystals
2) diffusional reequilibration
200 µm
plag
Sharpness of compositional transition (step vs.
progressive) between patches is possible
discriminator between 1) and 2). Step-like
suggests skeletal crystal, progressive rather
diffusion induced
An89
100 µm
An82
Common to: plagioclase, pyroxenes (cpx, opx), also observed in amphibole, apatite,
Streck, 2008, RiMG 69:595-619
Patchy zoning through diffusional reequilibration
Option A:
~homogenous
diffusional exchange
between melt and xls
opx
Option B:
xls with discrete growth zones
growth zones destroyed by
diffusion
cpx
cpx
Streck, 2008, RiMG 69:595-619
3. Select zoning and textures: Step-zoning in pyroxene
Best explained by magma mixing
Normally step-zoned
Reversely step-zoned
Plain light
BSE
cpx
Mg-rich
Fe-rich
Fe-rich
start
cpx
Mg-rich
andesite
distance, µm
Plain light
end
phonolite
X-polars
Mg-rich
start
end
Fe-rich
cpx
cpx
andesite
andesite
distance, µm
Common to: plagioclase, pyroxenes, amphibole, uncommon in olivine
Streck, 2008, RiMG 69:595-619
Arenal cpx minerals preserve evidence for replenishment events
3/70 P1
(picture)
Cr2O3 wt.%
(picture)
BSE
X-nicols
8/93 P1
Streck, 2008, RiMG 69:595-619
3. Select zoning and textures:
- Reverse & oscillatory zoning in plagioclase
(A):
Reverse zoning associated with crossing
dissolution surface (--> saw-toothed pattern)
Can be induced by:
- recharge by higher Ca/Na magma
- heating (hotter magma, latent heat of
crystallization, Blundy et al., 2006)
- higher water content
- What do we call oscillatory zoning?
- At what point does reverse zoning reflect
open system processes?
(B):
Reverse progressive zoning & normally
step-zoned
Streck, 2008, RiMG 69:595-619
- low amplitude oscillations (LAO) vs.
saw-toothed pattern (STR) with >5 mol%
An, Ginibre et al. 2002
75-69
Streck, 2008, RiMG 69:595-619
3. Select zoning and textures:
- Cellular texture: Pervasive resorption vs. rapid growth
Spongy cellular vs. boxy cellular
spongy
100 µm
Spongy
- pervasive resorption texture
Boxy
- rapid growth texture (right)
- pervasive resorption texture
boxy
boxy, resorption
Nakamura & Shimakita, 1998
Common to: plagioclase, pyroxenes, less common to amphibole, in olivine mostly as rapid
growth texture (skeletal xls)
Streck, 2008, RiMG 69:595-619
Spony cellular textures
Coarse grained (vermicular)
200 µm
500 µm
Fine grained (sieved)
Streck, 2008, RiMG 69:595-619
3. Select zoning and textures: Break down reactions in amphibole
- common feature induced by ascent
(Rutherford, this short course)
- however, amph with no reaction rim
next to pseudomorphs suggest
destabilization likely due to open system
processes
400 µm
Very common in amphibole and biotite, seldom in olivine, rare or absent (?) in pyroxene
and plagioclase (as primary feature)
Streck, 2008, RiMG 69:595-619
3. Select zoning and textures: Break down reactions in amphibole
Cpx mantle on pseudomorphs
after amphibole
- without characteristic amph shape
could be mistaken, e.g. for
disequilibrium feature of pyroxene
Streck, 2008, RiMG 69:595-619
4. Mineral populations
Distinguished by:
- crystal sizes (crystal size distributions)
- composition
- texture & zoning
- crystal assemblages
Except crystal sizes, xls populations based on other criteria seldom quantified
except for visual estimates.
Examples for:
- min. populations
- criteria to establish populations
- how to quantify
Streck, 2008, RiMG 69:595-619
Crystal mapping: Pyroxene populations in Mt. Shasta HMA
Spongy+patchy zoned
Low Mg# core+
reversely
step-zoned
Streck et al. 2007; this meeting
Streck, 2008, RiMG 69:595-619
Crystal mapping: Plagioclase populations in Mt. St. Helens dacites
Crystal mapping
• pre-selected survey area across thin sections (blue)
• took images at 25x of survey area
• inspected every plagioclase phenocryst for features
• color coded according to findings
1981
2004
n= number of
n=835 xls’s in mapped area n=989
Streck, 2008, RiMG 69:595-619
Streck et al., 2008
SH304-2a (Nov.‘04)
Streck, 2008, RiMG 69:595-619
resorption surface
resorption surface
opx
Features to distinguish crystal ‘types’
Type I, ‘purple’ type
• resorption surface(s)
• opx inclusions
opx
Type II, ‘green’ type
resorption surface
• resorption surface(s),
not associated with
opx inclusions
resorption surface
Type III, ‘red’ type
opx
opx
• no resorption surface(s),
with or without opx inclusions
Blue ‘type’
• ambiguous
Multiple crystal assemblages in Arenal bas. andesite
Crystallization
Env ironment
Domina nt
Clinopyroxene
Mg#
Al/Ti
Cr 2O3
wt %
71-78
~3.5-7 < 0.12
Orthopyrox ene
Mg#
Al/Ti
Olivine
Fo
Plagioclase
An
Cr#
71-77
3-20
none
60-85
<10
Spinel
TiO 2
wt %
2-7
Al2O3
wt %
2-8
(cpx, opx, plag,
)
Minor I
~78-85
~4-14
~0.7
0.12 0.7
none
none
>78
>75
10-42
<1
10-40
71-78
~4-7?
< 0.12
none
none
<78
~90
<10
2-7
<10?
71-78
>7-12
< 0.12
none
none
?
none?
<10?
2-7
10-20
~60-74
2-~3.5
< 0.12
60-75
2-10
none
40-89
<10
6-13
<5
(cpx, ol, CrAlsp,
g)
Minor II
(cpx, ol, plag, timt)
Minor III
(cpx, timt, ol?)
Microlites
(cpx, opx, timt,
g)
Orthopyroxene includes pigeonite among microlites
Streck et al., 2005
Streck, 2008, RiMG 69:595-619
Thank You