micro-textures in plagioclase from 1994e1995 eruption

8/13/2019 Micro-Textures in Plagioclase From 1994e1995 Eruption

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Research paper

Micro-textures in plagioclase from 1994e1995 eruption, Barren Island Volcano:Evidence of dynamic magma plumbing system in the Andaman subduction zone

M.L. Renjith*

Marine and Coastal Survey Division, Geological Survey of India, 4th Floor, C-Block, Kendriya Bhavan, Kakkanadu, Cochin, Kerala 682037, India

a r t i c l e i n f o

Article history:Received 11 December 2012

Received in revised form

19 March 2013

Accepted 25 March 2013

Available online xxx

Keywords:

Micro-texture

Plagioclase

Magma chamber process

Barren Island Volcano

Andaman subduction zone

a b s t r a c t

A systematic account of micro-textures and a few compositional proles of plagioclase from high-alumina basaltic aa lava erupted during the year 1994e1995, from Barren Island Volcano, NE India

ocean, are presented for the rst time. The identied micro-textures can be grouped into two categories:

(i) Growth related textures in the form of coarse/ne-sieve morphology, ne-scale oscillatory zoning and

resorption surfaces resulted when the equilibrium at the crystal-melt interface was uctuated due to

change in temperature or H2O or pressure or composition of the crystallizing melt; and (ii) morphological

texture, like glomerocryst, synneusis, swallow-tailed crystal, microlite and broken crystals, formed by the

inuence of dynamic behavior of the crystallizing magma (convection, turbulence, degassing, etc.). Each

micro-texture has developed in a specic magmatic environment, accordingly, a rst order magma

plumbing model and crystallization dynamics are envisaged for the studied lava unit. Magma generated

has undergone extensive fractional crystallization of An-rich plagioclase in stable magmatic environment

at a deeper depth. Subsequently they ascend to a shallow chamber where the newly brought crystals and

pre-existing crystals have undergone dynamic crystallization via dissolution-regrowth processes in a

convective self-mixing environment. Such repeated recharge-recycling processes have produced various

populations of plagioclase with different micro-textural stratigraphy in the studied lava unit. Intermit-

tent degassing and eruption related decompression have also played a major role in the

nal stage ofcrystallization dynamics.

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Elsevier B.V. All rights reserved.

1. Introduction

In an open-system volcanic process, the erupted magmatic

products contain mixed crystal populations of xenocryst, antecryst,

phenocryst and microlite (Jerram and Martin, 2008). A mineral

phase, in any of such forms, highly sensitive to the modications in

the volcanic system, and able to record the changes in thermody-

namic equilibria in their textural and compositional zoning

patterns, depending on the process they underwent, will be a po-

wer full tool in understanding the magma process. Specically,

several studies have concluded that texture and chemical zoning in

plagioclase, in particular, may be an efcient tool for conning the

dynamics and kinetics of magmatic process, due to its high sensi-

tivity to changes in physical-chemical conditions (T,P,P(H2O),f(O2),

melt composition) of the system (Stamatelopoulou-seymour et al.,

1990;Blundy and Shimizu, 1991;Stimac and Pearce, 1992;Singeret al., 1995; Tepley et al., 1999, 2000; Ginibre et al., 2002a,b;

Humphreys et al., 2006; Ginibre and Wrner, 2007;Smith et al.,

2009;Viccaro et al., 2010,2012).

High-alumina basalt (HAB) erupted in the form of aa lava from

the Barren Island Volcano (BIV), NE Indian Ocean, during the year

1994e1995, constituted of various physical mixtures and geneti-

cally related phenocrysts and xenocrysts (2e5 mm size) of Ca-rich

plagioclase, Mg-rich olivine and clinopyroxene (Luhr and Haldar,

2006). According to them these phenocrysts and xenocryst have

been entered the magma through contamination of troctolitic

crystal mushes or plutonic xenoliths at depth. This argument is

* Tel.: 91 484 2428937; fax: 91 484 2428940.

E-mail address:[email protected].

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Geoscience Frontiers

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http://dx.doi.org/10.1016/j.gsf.2013.03.006

Geoscience Frontiers xxx (2013) 1e14

Please cite this article in press as: Renjith, M.L., Micro-textures in plagioclase from 1994e1995 eruption, Barren Island Volcano: Evidence ofdynamic magma plumbing system in the Andaman subduction zone, Geoscience Frontiers (2013), http://dx.doi.org/10.1016/j.gsf.2013.03.006
mailto:[email protected]://www.sciencedirect.com/science/journal/16749871http://www.elsevier.com/locate/gsfhttp://dx.doi.org/10.1016/j.gsf.2013.03.006http://dx.doi.org/10.1016/j.gsf.2013.03.006http://dx.doi.org/10.1016/j.gsf.2013.03.006http://dx.doi.org/10.1016/j.gsf.2013.03.006http://dx.doi.org/10.1016/j.gsf.2013.03.006http://dx.doi.org/10.1016/j.gsf.2013.03.006http://www.elsevier.com/locate/gsfhttp://www.sciencedirect.com/science/journal/16749871mailto:[email protected]


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supported by the elevated Ca-rich plagioclase phenocryst per-

centage, high Eu anomaly (>1), Sr (209e227 ppm) and Al2O3(21e23 wt.%) contents when compared to older lavas and imply

the vital role of plagioclase in the petrogenesis of HAB. However,

the mechanism of plagioclase addition has not been well under-

stood yet and, of course, it is a difcult task in separately identi-

fying the foreign derived xenocrysts from co-genetic phenocrysts

of a single lava unit. This paper presents various micro-textures in

plagioclase from this lava unit and based on that various plagio-

clase populations, their growth history and dynamic plumbing

system operated during this aa lava eruption are envisaged

qualitatively.

2. Background information

Barren Island Volcano, the only active subduction-related vol-

cano in the Andaman Sea, NE Indian Ocean (12.29N, 93.85E) is a

circular shaped island covering an area ofw10 km2 with w3 km

diameter, and its cinder cone rises 355 m above the sea level

(Fig. 1a,b). BIV falls within the northern extension of Sumatra vol-

canic chain formed by northeastward oblique subduction of Indo-

Australian oceanic plate beneath the Southeast Asian plate

(Fig. 1a). This active volcano erupted in three major cyclic stagesleaving a small quiescent period in between (Luhr and Haldar,

2006; Sheth et al., 2009; Pal et al., 2010 and the references

therein). First cycle: predate the year 1787 (pre-historic lava) and

has no age record yet; second cycle: between the year 1787 and

1832; and the third cycle: from 1991 to 2009 consists of four stages

of eruption, during the year 1991, 1994e1995, 2005e2006 and

2008e2009. Though, in gist, the erupted products are in the form of

aa and blocky aa lava ows along with volcanoclastic deposits

including tephra fallout, lahars and surge deposits (Sheth et al.,

2009).

3. Analytical technique

A total of 106 samples were randomly collected from aa lavaow unit of 1994e1995 eruption during the eld visit to Barren

Island, Andaman sea, way back in December 2004 (Fig. 1b). About

146 thin sections were made and petrographic observations with

special emphasize on micro-textures in plagioclase were carried

under polarizing microscope. The identied micro-textures are

presented as microphotographs (Figs. 2e9) as well as schematic

diagrams (Table 1) for a better visual understanding. Four plagio-

clase grains displaying maximum micro-textural diversity were

selected for determining the compositional prole by micro-probe

analysis and the data obtained are presented in Table 2. Plagioclase

phenocryst was analyzed along the core-rim transect with steps of

w5e10mm. Electron Probe Micro Analysis (EPMA) carried out with

a CAMECA SX100 (Central Petrological Laboratory, Geological Sur-

veyof India, Kolkata) operated at 15 kV,12 nA using natural mineralstandards and results were corrected with a PAP matrix correction

program. The standards were analyzed at regular intervals to check

the precision of sample analysis. Fe2 and Fe3 were distributed

based on stoichiometry.

4. Present study

Present study is conned to the lava unit erupted during

1994e1995 (Fig. 1b). They are black coloured massive looking

blocky aa lava with rugged surface. They erupted in different pulses

in strombolian style and have high-alumina basaltic composition

(SiO2: 51e52 wt.%) with unusual An-rich plagioclase, cpx and

olivine phenocryst and xenocrysts (Luhr and Haldar, 2006).

Morphologically they show remarkable similarities with viscous

tooth paste lavas reported from Hawaii, Paricutin and Etna volcanicelds (Sheth et al., 2011). Hand specimen shows salt and pepper

appearance due to the presence of whitish-grey plagioclase phe-

nocrysts in a ne-size to glassy black matrix. Under microscope all

samples show porphyritic texture developed by the unusually large

euhedral plagioclase (>97%), euhedral olivine (2e3%) and cpx

(0e4%) set in a background mass consisting of glass and microlites

of same phenocryst phases. Plagioclase, the dominant modal phase

in all samples, found in three sizes such as small (


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4.1. Micro-textures in plagioclase

Sieve-texture, a frequently found micro-texture in the studied

lava, is the glass inclusions (previously melt) in plagioclase. These

inclusions behave like an opaque under plane polarized light and

impart porous appearance to the crystals. They composed of glass,

microlites and Fe-Ti oxides and found in coarse (coarse-sieve; CS)

(Fig. 2a,b; T1, Table 1) or ne sizes (ne-sieve; FS) (Fig. 2c,d; T2,

Table 1). Their size difference is well visualized inFig. 2d. In general

CS and FS are not found superimposed on each other; rather they

occur as a discrete zone preferred manner in a single grain (Figs.2d

and 5). Coarse-sieve morphology is largely amoeboidal in shape

and frequently found at the core of large plagioclase (Fig. 2a,b,d). In

some grains their size gradually reduces towards the rim (Fig. 5).CS

found as isolated or interconnected manner; isolated CS are

randomly distributed melt inclusion patches without any inter-

connectivity and do not display any distribution pattern (Fig. 2a).

Interconnected CS are found as (i) elongated and run parallel to the

cleavage planes and cut across laterally to get interconnected with

adjacent one (Fig. 2b) or (ii) forming a ring shape leaving a less

sieved inner core (Fig. 9b). CS seldom carry microlite of cpx or

olivine as trapped inclusions along with melt rather than poiki-

litically enclosed phases (Figs.2a and5). Fine-sieves (FS) are veryne-size glass inclusions uniformly distributed in a zone preferred

manner (Figs.2c,d,4 and5; T2,Table 1). Their closely spaced and

high density (total area/number of inclusions) distribution gives a

dusty appearance (Fig. 2d). They are well interconnected and occur

in an organized way. In large grains they occur towards the rim

(Figs.2d,4and5) and are devoid at the core whereas in medium/

small-size grains they occur as multiple zones of varying thickness

or overprinted on ne-scale growth zones.

Fine-scale oscillatory zones (FOZ) (T3,Table 1), another impor-

tant micro-texture identied in plagioclase, is often found in

medium-size crystals and rarely at the outer rim of large plagio-

clases (Figs. 2a,c and 3aec). FOZ occur as sets comprising 5e15

individual zones of less than 4 mm thick. There are up to ve zone-

sets that can be recognized from a single grain. Each zone-set isseparated by drastic change in composition (optically different)

(Fig. 3b) or ne-sieve zones or major resorption surfaces (Figs. 4

and 5). Individual zones are characterized by curved corners

(Fig. 3b; T4,Table 1), varying thickness (Fig. 3b) and wavy rather

than a straight margin (Fig. 3a,c).

Resorption surfaces (RS) are the horizons, like an unconformity

surface marking a boundary between two textural domains or

growth zones (Figs. 4 and 5; T5, Table1). Forexample, in Fig. 5 a FOZ

domain (D-3) rapidly changes over to FS domain leaving a thin

melt-rich zone in between. The irregular surface at the outer edge

of the FOZ domain represents a major RS (thick arrow in Fig. 5).

Similarly many examples like a grain inFig. 4show a gap between

two sets of FOZ domain (D-iv and v); a CS dominated core showing

oval shaped boundary in Fig. 9b; and an irregular boundary of aclear-plagioclase core in Fig. 9d is the major RS marking the

boundary between two textural domains in a phenocryst.

Many plagioclase crystals, particularly the ones with a large size,

have developed diverse and multiple micro-textural domains. An

example inFig. 4shows a plagioclase that has six textural domains,

separated by irregular resorption surfaces from its core to rim. A

domain of CS textured core (D-i) ends towards the rim with a well

interconnected FS domain (D-ii) which again wrap up with a poorly

interconnected FS domain (D-iii). Over these FS domains, two sets

of FOZ domains (D-iv and D-v) are formed. At the rim a thin zone of

clear plagioclase (D-vi) forms an outer shell. Similarly in another

example, part of a large plagioclase shown in Fig. 5has six distinct

textural domains. At the core two euhedral growth zones with

isolated CS pattern (D-1 and D-2) are characterized by their distinct

Table 1

Schematic representation and interpretation of micro-textures in plagioclase.

Texture Description Interpretation

T1 Coarse-sieve Dissolution by varying

rate of adiabatic

decompression of

H2

O-undersaturated

magma

T2 Fine-sieve Partial dissolution due

to reaction with more

Ca-rich melt

T3 Fine-scale oscillatory

zoning

Convection driven small

scale physical-chemical

perturbation at the

crystal-melt interface

T4 Rounded zone corner Minor dissolution when

the crystals move across

the magmatic gradient

T5 Resorption surface Intense and prolonged

dissolution while reacting

with more primitive

magma

T6 Synneusis Turbulent magmatic state

related convection

T7 Glomerocrysts Suturing of spatially closer

resorbed crystals

T8 Swallow-tail Rapid growth due to

undercooling may be

related to eruption process

T9 Microlites Degassing or water

exsolution driven

undercooling may be

related to eruption process

T10 Broken crystal Decompression related

forceful aerial eruption

M.L. Renjith / Geoscience Frontiers xxx (2013) 1e14 3



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all the samples. Particularly, the plagioclase microlites with tabularhabit are more abundant than prismatic habit (T9, Table 1). In many

samples, small-size plagioclase shows peculiar features of both

euhedral and rugged grain margins indicating that they are broken

crystals (Fig. 8b; T10, Table 1). Euhedral face represents the

remnant of a pre-existed euhedral grain which broken apart

through the crystal interior so the crystal core region, growth zone

and sieve zones get directly in contact with ground mass (Fig. 8b).

Over the broken surface no regrowth features are noted.

4.2. Plagioclase compositional proles

Four plagioclase crystals, three are from large-size group

(Fig. 9aec) and one from medium-size group (Fig. 9d) showing

maximum textural diversity were selected for the core to rim

compositional proling by EPMA technique. The data presented inTable 2are graphically presented as anorthite (An) and FeO proles

along with corresponding microphotographs of each grain inFig. 9.

FollowingViccaro et al. (2010), it is referred here that concordant

behaviorwhen the An content increases and FeO increases or vice

versa for each spot analysis along the same direction independent

of the absolute extent of the variation. Similarly the discordant

behavioris referred when An increases and FeO decreases or FeO

remains constant.

A plagioclase in Fig. 9a (Grain-I) has three textural domains: (i) a

broad clear-plagioclase core; (ii) a thick FS zone; and (iii) a thin FOZ

domain at the outer rim. Along the prole AeA0 almost uniform An

content (An86e89) at the core part after crossing FS zone slightly

increases (from An86 toAn89) and then suddenly dropped to An78 at

the rim. Correspondingly FeO varies abruptly between 0.38 and

Figure 3. Photomicrograph of plagioclase showing features of FOZ textures (T3). (a) Arrow points to the wavy surface of micro-compositional layer in a FOZ domain. Note the

textural diversity as CS at the core, FOZ domain and clear plagioclase at the rim. Thin stripped line marked over a thin FS zone. (b) Growth zones with varying thickness and curved

corners (T4) (arrow). (c) Undulation surface of growth zone in a FOZ dominated outer shell (arrow). Thin stripped line demarcates textural domains.




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0.65 wt.% at the core part and then shows discordant behavior

(0.77 wt.%) at the rim. In Grain-II, CS dominated core and FOZ

domain outer shell are separated by major resorption surface

(Fig. 9b). Along the prole BeB0 composition gradually drops from

An93to An87at the core part, then after crossing CS domain a slight

enrichment gradually dropped at the RS (Fig. 9b). FOZ domain

shows a general normal zoning with oscillatory behavior. However,

within each growth zone reverse zoning, i.e., An content increase

towards the outer edge is witnessed. In therst zone An89 increasesto An91and then in the next zone a sudden drop of An77gradually

increases to An82. At the crystal edge outer zone shows a very high

drop to An62. All along the prole FeO behaves coherently with An

content. But discordance is noticed at the An enrichment ( Fig. 9b).

Grain-III (Fig. 9c) shows three textural domains: (i) CS domi-

nated clear-plagioclase core; (ii) two FS domains; and (iii) a thick

FOZ at the crystal edge. From core to rim An content decreases with

oscillatory behavior (Fig. 9c). At the CS core domain An content

shows oscillatory low amplitude variation between An80and An90.

Subsequently a sudden increase from An74 to An80 as reverse

zoning within the FS domain drops to An 61at the beginning of FOZ

domain. This drop gradually increases to An65 towards the rim.

Likewise FeO behaves discordantly in the CS core domain and then

predominantly coherent in the FS and FOZdomains (Fig. 9c). Fourth

plagioclase grain (Fig. 9d), representing small-size group, has twodistinct textural domains: (i) a resorbed clear plagioclase at the

core; and (ii) a FOZ domain up to the crystal edge characterized by

the presence of two bright zones rich in melt inclusions. From core

to rim An content shows oscillatory high amplitude variation. A

normally decreasing An content (An76e72) at the core suddenly

increases to An83 within the inclusions rich zone. This increase

again dropped to An66eAn57 towards the rim. Variation of FeO is

moderately discordant except for the core and melt inclusion-rich

domain.

5. Discussion

5.1. Interpretation of micro-textures and compositional proles

Plagioclase develops sieve-texture during the decompression

process (Nelson and Montana,1992) or reactionwith hotter Ca-rich

melt (Tsuchiyama, 1985). In decompression process, when an H2O-

undersaturated magma ascend at fast rate, P(H2O) of the system

increases and reduces the stability of plagioclase causing dissolu-

tion (Nelson and Montana,1992; Blundyand Cashman, 2001, 2005).

Voids created by dissolution are lled with entrained melt which

later entrapped inside by the subsequent regrowth of that crystal

and developed as sieve-texture in crystals. Whereas, the reaction

with a hotter Ca-rich melt plagioclase undergoes partial dissolution

and generates a ne crystal-melt mixture at the crystal edges as

sieve morphology (Tsuchiyama, 1985). It is worth noting that sieve-

texture produced by decompression event is coarser in size (refer

Fig. 1 inNelson and Montana, 1992) whereas superheating creates

Figure 4. Photomicrograph shows part of a plagioclase grain developed CS dominated core (D-i), two FS zones (D-ii, D-iii), two sets of FOZ (D-iv, D-v) and thin clear plagioclase rim

(D-vi) separated by resorption surfaces as marked with stripped lines. The inset photograph at the right side shows the CS dominated core region of the D-i domain of the same

grain shown in the left side photo.

Figure 5. Photomicrograph of a plagioclase with multiple textural domains. Clino-

pyroxene (cpx) occurs as a trapped inclusion within sieve morphology. Stratigraphy of

textures is given inTable 3.

M.L. Renjith / Geoscience Frontiers xxx (2013) 1e148



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ne-size sieves (Tsuchiyama, 1985). Coarse and ne-sieve mor-

phologies in the studied plagioclase mimic the respective magma

processes in their genesis (Fig. 2a,d; T1,Table 1). CS morphology is

conned only at the cores of large plagioclase phenocrysts

(Fig. 2a,d). Optical continuity (excluding the area occupied by CS

morphology) and absence of growth zoning at these cores indicate

that phenocrysts cores have grown in a quiescent equilibrium

magmatic environment followed by an ascent related decompres-

sion driven dissolution (e.g.,Nelson and Montana, 1992). Although,

unusually high An content (wAn93e85) (Fig. 9aec) at these cores

provides clue about their parental magma from which it had

crystallized. Many experimental investigations have proved that

growth of An-rich plagioclase is favored in high temperaturemagmas at lower pressure (


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to near-equilibrium condition (Pearce, 1994;Ginibre et al., 2002a)

(Kinetic Model). In the present samples, FOZ domain exhibits minor

resorption features like curvy zone corners (Fig. 3b; T4, Table 1) and

wavy rather than a straight zone margin (Fig. 3a,c). These features

indicate that during their growth minor dissolution has occurred

(e.g., Ginibre et al., 2002a). Although, FOZ domain has high

amplitude An content variation and discordant or concordantbehavior of FeO and An (Fig. 9d) support a strong involvement of

uctuating magmatic parameters like temperature or P(H2O) or

composition during their growth (e.g., Viccaro et al., 2010). The

evidences of intermittent dissolution-regrowth features and high

amplitude (>2%) An variation in the FOZ domains strongly support

the dynamic model (e.g., Ginibre et al., 2002a). In dynamic model

crystals need to be repeatedly move across a magmatic gradient (in

the form of temperature orP(H2O) or composition) prevailed in the

chamber under the inuence of convection or turbulence and cause

the growth ofne oscillatory zones (e.g.,Singer et al., 1995;Ginibre

et al., 2002a). Repeated occurrences of plagioclase laths within the

FOZ as synneusis attachment, support the dynamically active state

of the magma during the growth of oscillatory zones (Fig. 6a; T6,

Table 1) (e.g., Vance, 1969). Synneusis is a process of drifting

together and mutual attachment of crystals in a liquid-rich magma

(Vance, 1969; Dowty, 1980; Schwindinger and Anderson, 1989;

Schwindinger,1999). For synneusis a mechanismsuch as shear ow

or turbulent mixing is needed to rotate the crystals into alignment

even though it can happen in quiescent settling in a liquid

(Schwindinger, 1999). The close association of synneusis texture,

particularly its repeated occurrence with FOZ (Fig. 6) implying that

during their growth crystals were repeatedly in motion may be due

to dynamically active convection or turbulence of the crystallizing

magma.

In many plagioclase phenocrysts two distinct textural domains

are seperated with a major resorption surface as noticed in be-

tween CS dominated core and FOZ (Fig. 2d), FOZ and FOZ (Fig. 4),

FOZ and FS (Fig. 5) and clear-plagioclase core and FOZ (Fig. 9d).

Thus RS represent a major episode of dissolution creating a large

information gap between two textural domains. Major resorption

surface in plagioclase represents a prolonged or intense dissolution

event generally attributed to profound changes in temperature,

pressure, melt composition, and water content in the magma at

large scale caused, for example, by magma recharge ( Nixon and

Pearce, 1987; Davidson and Tepley, 1997). After the dissolution

phenocrysts regain equilibrium at the crystal-melt interface and

regrow in the newly equilibrated magma.Glomerocrysts are the aggregate of two or more plagioclase

formed when the partially resorbed crystals get spatially closer.

While resorption intergranular melt is produced along the grain

boundaries which act as binding material for the similar partially

resorbed crystals together through slow overgrowths (Hogan,

1993). In the studied glomerocrysts individual grains with CS

morphologies suggest that before they get sutured phenocrysts

have undergone partial dissolution (Fig. 7a,b). Such dissolution

might have created a boundary layer melt around the grains which

subsequently cooled and bind the closely spaced similar pheno-

crysts together. In addition their interlocking nature also implies

that the interplay of interfacial energies and large dihedral angle

(q > 60) at a solid-liquid-solid triple junction subsequent to the

crystal contact probably enhance the formation of glomerocryst(e.g., Ikeda et al., 2002). After they weldedtogether many clots have

further grown as single grain by developing common growth zone

with FOZ (Fig. 7a) or FS or clear plagioclase (Fig. 7b) along the outer

rim.

The ground mass microlites in the present samples imply that

lava has undergone effective cooling (DT) (e.g., Lofgren, 1974,

1980) or an increase in the liquidus temperature due to water

exsolution, degassing and vesiculation induced by magma syn-

eruption related decompression (e.g., Hammer et al., 1999;

Cashman and Blundy, 2000; Hammer and Rutherford, 2002;

Martel and Schmidt, 2003; Szramek et al., 2006; Suzuki et al.,

2007; Toramaru et al., 2008). Many experimental studies have

shown that as the degree of undercooling increases crystal habit

changes from euhedral tabular to spherulitic through prismatic,hopper, skeletal and dendritic habits (e.g., Hammer and

Rutherford, 2002; Suzuki and Fujii, 2010). Tabular and prismatic

microlites in the studied samples imply that they have grown

under low rate of undercooling. Similarly the swallow-tailed H-

shaped skeletal texture at the crystal edge (Fig. 8a) is also

developed by the rapidly-increased growth rate due to under-

cooling related to eruption (e.g., Kirkpatrick, 1977; Kirkpatrick

et al., 1979; Lofgren, 1980; Viccaro et al., 2010). Many broken

crystals in the studied lava imply that violet eruption causes them

to break apart (Fig. 8b), because when the erupting magma

decompressed, the gas-rich vesicles entrapped at high pressures

as inclusions in the phenocrysts try to escape from them by blew

them apart (e.g., Best and Christiansen, 1997; Miwa and Geshi,

2012).

Figure 8. (a) Plagioclase showing H-shaped crystal edge (thick arrow) known as

swallow-tail (T8); (b) broken plagioclase crystal (T10). Planar crystal phases at the left

end (stripped arrow) and rugged irregular surface (thick arrow) through which the

inner of the grain is in direct contact with ground mass.




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5.2. Textural stratigraphy

The identied micro-textures can be grouped into two cate-

gories: (i) Growth textures in the form of CS, FS, FOZ and RS were

formed when the equilibrium at the crystal-melt interface was

uctuated due to change in temperature or H2O or pressure or

composition of the magma; and (ii) Morphological textures, like

glomerocryst, synneusis, swallow-tailed crystal, microlite and

broken crystals are the products of dynamic magma processes

such as convection, degassing and explosive eruption. Each texture

has developed under a particular magmatic environment.

Construing their stratigraphy can provide valuable information

about the sequence of magma process involved from deep source

to areal eruption. Phenocryst size can reveal some clue about the

Figure 9. (aed) Plagioclase grains with multiple micro-textures and their corresponding compositional proles (An% and FeO wt.%). Filled circle: An%; open circle: FeO wt.%.




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order of crystallization (e.g.,Marsh, 1998;Yu et al., 2012). Among

the identied three phenocryst size-groups, small (


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and Gardner, 2004); acicular and hollow crystals of amphibole and

plagiolcase (e.g., Gerbe and Thouret, 2004) and crystal clots (e.g.,

Seaman, 200 0) preclude the possibility of end-member magma

mixing in the studied aa lava. Thus it is convincing that FS are the

product of superheating by recharge event. Such recharge events

could be like acryptic-mixing process in which the shallow chamber

magma experience repeated addition of small pulses of primitive

and hotter magma of similar composition but different in f(O2) or

H2O contents (e.g., Humphreyset al., 2006). Whenever the recharge

event brought new pulses of primitive magma, the pre-existed

crystals in the shallow chamber interacted with it causing partial

dissolution in the form of FS morphology. After each partial

dissolution they re-equilibrated with new Ca-rich magma and were

re-grown as An-rich plagioclase as it recorded the anorthite jump

within or just after FS domain (Fig. 9aec). However, when there

was a profound change in magmatic parameters by a recharge

event, the crystals have undergone intense dissolution which

marked as major resorption surfaces (Fig. 5) to equilibrate with

more primitive magma.

In addition to the superheating, crystals in the shallow chamber

also experienced repeated movement across the magmatic gradi-

ents by convection or turbulence as evidenced from the FOZ do-

mains (Fig. 3) and synneusis (Fig. 6) in plagioclase (e.g., Singeret al.,1995;Ginibre et al., 2002a). Work byPal et al. (2010)also pointed

out that present studied lava unit was erupted from a magmatic

diapir having compositional gradient in the form of hydrous core

and anhydrous rim. Therefore the evidences like FS, FOZ and syn-

neusis strongly suggest that at the shallow chamber crystals were

undergoing repeated dissolution-regrowth process in a convective

recycling magmatic environment (Fig. 10). That means aself mixing

environmentenhanced by the recharge of hotter magma at the base

of the magma chamber (e.g., Huppert et al., 1982; Couch et al.,

2001)(Fig. 10). During the self-mixing process the magma cham-

ber might have experienced undercooling by degassing or water

exsolution followed by violent aerial eruptionproducing microlites,

broken and swallow-tail crystals (Fig. 10).

Luhr and Haldar (2006)suggested that the 1994e

1995 lava unitcontains various physical mixtures of melt, genetically related

crystals and xenocrysts. This study also observed that (i) modal

proportion of plagioclase with disequilibrium textures and unusu-

ally large An-rich phenocrysts varies from sample to sample; (ii)

phenocrysts with different textural sequence occur side by side in a

thin section; and (iii) lack of common textural sequence among

plagioclase phenocrysts, suggesting that this lava unit is heteroge-

neous in grain to outcrop-scale in terms of multiple crystal popu-

lation. This heterogeneity may be developed due to many factors

like:(i) at the shallow chamber magma,processeswere notuniform

or pervasiveenough to produce a uniform crystal population; or (ii)

the self-mixing magma process was more like a chaotic dynamic

condition capable of producing textural heterogeneity (e.g., Perugini

et al., 2003); or (iii) due to the long resident time of magma in theshallow chamber it was allowed to mix with multiple pulses of new

crystal-rich magma from the deeper level, because the studied lava

erupted after a major quiescence period of about 153 years. During

this quiescent period, shallow chamber must have experienced

multiple recharge events which brought a new pulse of crystal-rich

magma from the deep chamber and recycled at the lower chamber.

6. Conclusion

This is the rst systematic study of micro-textural and An%-FeO

zoning in plagioclase phenocrysts from the aa lava erupted from

Barren Volcano, Andaman sea. Both the identied growth related

and morphological micro-textures in the studied lava unit are the

strong records of complex magma process involved during its

journey from source to areal eruption. Each texture is formed under

a specic magmatic environment. The deduced micro-textural

stratigraphy helped to draw the picture of progressive and sys-

tematic sequence of magma processes involved. Accordingly arst

order magma plumbing model is proposed for the Barren volcanic

eruption. At a shallow magma chamber crystallization was mainly

constrained by the dissolution-regrowth process in convective self-

mixing environment and the repeated recharge of crystal-rich

magma from the lower chamber. The intermittent degassing and

violent eruption also contributed the development of varying

population of plagioclase in the nal erupted product.

Acknowledgements

Author is grateful to the Deputy Director General (B.K. Saha),

Geological Survey of India (GSI) for giving an opportunity to take part

in the eld excursion to Barren Island Volcano way back in the year

2004 (December).The eldtripwas conductedby theResearchVessel

Samudra Manthan (GSI) and supported by Indian Navy and Indian

Coast Guard. Dr. S. Nirmal Charan is grateful for valuable suggestions,

providing facilities for making thin sections and microphotographs at

NGRI,Hyderabad.Authoracknowledges the helpof B. Chattopadhyay,S.K. Sengupta and S. Nandi of EPMA laboratory, GSI, Kolkata for EPMA

study.Director General (A. Sundaramoorthy)and Director(Dr.Thomas

Mathai) of GSI are thanked for administrative approval for this paper.

Dr. Faruk Aydin and Dr. Jiandong Xu are thanked for their helpful re-

views. Author thanks Prof. M. Santhosh for the pre-submission

comment, Dr. Yener Eyuboglu for editorial suggestions and Dr. Lily

Wang for editorial handling of this manuscript.

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micro-textures in plagioclase from 1994e1995 eruption

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