Anthropogenic and impact spherules: Morphological similarity and chemical distinction – A case study from India and its implications Ambalika Niyogi 1 , Jayanta K Pati 1,∗ , Suresh C Patel 2 , Dipak Panda 3 and Shiv K Patil 4 1 Department of Earth and Planetary Sciences, University of Allahabad, Allahabad 211 002, India. 2 Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai 400 076, India. 3 EPMA Laboratory (PLANEX), Physical Research Laboratory, Thaltej Campus, Ahmedabad 380 009, India. 4 Dr. K.S. Krishnan Geomagnetic Research Laboratory, Leelapur Road, Hanumanganj, Allahabad 221 505, India. ∗ Corresponding author. e-mail: [email protected]This paper provides first report of silica-rich anthropogenic spherules of varying colour, shape, size, surface texture and chemical composition found in road-deposited sediments (RDS) of Allahabad city, Uttar Pradesh, India. Morphological details and lithophile elemental composition of the silica-rich spherules are compared to microtektites and impact spherules from India to demonstrate their striking morphological similarities and chemical variability. This study suggests the need to use spherule data carefully while assigning an impact origin to spherule-finds or spherule-bearing lithological horizons. 1. Introduction The evidences of oldest impact event on Earth due to the collision of large extraterrestrial objects come from Archean spherule layers exposed in South Africa and Australia (∼3.5–2.5 Ga; Glikson 2005; Simonson et al 2009 and references therein). The recent report about the catastrophic spread of coal fly ash spherules along with other particulate matter from the Lake Buchanan section, Sverdrup Basin, Canadian High Arctic (Grasby et al 2011) is believed to be related to/associated with the lat- est Permian extinction (LPE). However, spherules are formed in several ways in nature (Simonson et al 2009; French and Koeberl 2010; Saragnese et al 2010; Weiss et al 2010; Yang et al 2010). They can be of terrestrial (biogenic, industrial, diagenetic and volcanic), extraterrestrial (Fredriksson 1956) and they may also form due to the impacts of crater-producing meteorites (Raukas 2000; Osawa et al 2003; Kofman et al 2010). Early work on cos- mic spherules was done by Fredriksson (1956) and it was shown that spherules as ablation products of meteorites as they pass through the Earth’s atmo- sphere leading to the melting of interplanetary dust particles in the upper atmosphere as they get heated above their liquidus due to atmospheric friction. The anthropogenic source of spherules can be varied. This includes the combustion of fos- sil fuel (e.g., in diesel engines) contributes to the formation of spherules and their aggregates (Kim et al 2009). In addition, anthropogenic spherules can also originate from industrial and/or domestic heating systems (Goddu et al 2004; Gautam et al 2005). Coal combustion due to volcanic activity, thermal power plants and steam generation units Keywords. Spherule; road dust; meteoritic impact; microtektites; fly ash. J. Earth Syst. Sci. 120, No. 6, December 2011, pp. 1043–1054 c Indian Academy of Sciences 1043
Transcript
Anthropogenic and impact spherules: Morphologicalsimilarity and chemical distinction – A case study
from India and its implications
Ambalika Niyogi1, Jayanta K Pati
1,∗, Suresh C Patel2,
Dipak Panda3 and Shiv K Patil
4
1Department of Earth and Planetary Sciences, University of Allahabad, Allahabad 211 002, India.2Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai 400 076, India.
3EPMA Laboratory (PLANEX), Physical Research Laboratory, Thaltej Campus, Ahmedabad 380 009, India.4Dr. K.S. Krishnan Geomagnetic Research Laboratory, Leelapur Road, Hanumanganj, Allahabad 221 505, India.
This paper provides first report of silica-rich anthropogenic spherules of varying colour, shape, size,surface texture and chemical composition found in road-deposited sediments (RDS) of Allahabad city,Uttar Pradesh, India. Morphological details and lithophile elemental composition of the silica-richspherules are compared to microtektites and impact spherules from India to demonstrate their strikingmorphological similarities and chemical variability. This study suggests the need to use spheruledata carefully while assigning an impact origin to spherule-finds or spherule-bearing lithologicalhorizons.
1. Introduction
The evidences of oldest impact event on Earthdue to the collision of large extraterrestrial objectscome from Archean spherule layers exposed inSouth Africa and Australia (∼3.5–2.5 Ga; Glikson2005; Simonson et al 2009 and references therein).The recent report about the catastrophic spread ofcoal fly ash spherules along with other particulatematter from the Lake Buchanan section, SverdrupBasin, Canadian High Arctic (Grasby et al 2011)is believed to be related to/associated with the lat-est Permian extinction (LPE). However, spherulesare formed in several ways in nature (Simonson et al2009; French and Koeberl 2010; Saragnese et al2010; Weiss et al 2010; Yang et al 2010). They canbe of terrestrial (biogenic, industrial, diageneticand volcanic), extraterrestrial (Fredriksson 1956)
and they may also form due to the impacts ofcrater-producing meteorites (Raukas 2000; Osawaet al 2003; Kofman et al 2010). Early work on cos-mic spherules was done by Fredriksson (1956) andit was shown that spherules as ablation products ofmeteorites as they pass through the Earth’s atmo-sphere leading to the melting of interplanetarydust particles in the upper atmosphere as they getheated above their liquidus due to atmosphericfriction. The anthropogenic source of spherulescan be varied. This includes the combustion of fos-sil fuel (e.g., in diesel engines) contributes to theformation of spherules and their aggregates (Kimet al 2009). In addition, anthropogenic spherulescan also originate from industrial and/or domesticheating systems (Goddu et al 2004; Gautam et al2005). Coal combustion due to volcanic activity,thermal power plants and steam generation units
gives rise to significant emission of particulate mat-ter and pollutants (Dayal and Sinha 2005; Sharmaet al 2005; Bhanarkar et al 2008; Grasby et al 2011;http://elements.geoscienceworld.org/content/vol6/issue4/#ARTICLES). Unless evidence for mete-orite impact is demonstrated, it is nearly impossi-ble to differentiate between anthropogenic spherules,microtektites and impact spherules based on theirmorphology and/or geochemistry alone (Marini2003; Buchner et al 2009; French and Koeberl2010). However, the detection of shock metamor-phic effects in mineral grains such as quartz orzircon as constituents of spherule layers providesunequivocal evidence for impact, as in the case ofthe K/Pg boundary layer related to the ∼66 MaChicxulub impact on the Yucatan peninsula,Mexico (e.g., Bohor 1990; Schulte et al 2010), thedistal ejecta layer in the United Kingdom derivedfrom the ∼215 Ma Manicouagan impact struc-ture, Quebec, Canada (Walkden et al 2002) orworldwide-distributed Upper Eocene impact ejectain connection with the ∼35 Ma Popigai impact inSiberia (Montanari and Koeberl 2000; Whiteheadet al 2000). Nevertheless, some putative impact
ejecta layers still lack convincing evidence of shockmetamorphic features, such as the spherules of theBarberton Greenstone Belt in South Africa earlierattributed to impact (Hofmann et al 2006).
As a result, in some instances anthropogenicspherules have been misidentified as microtektites(Marini 2003). At times magnetic properties ofspherules have been used to decipher their extrater-restrial origin, whereas many consider them as con-taminated terrestrial objects (Buchner et al 2009).A large database exists on the morphology and geo-chemistry of impact-generated spherules includ-ing tektites but similar data on anthropogenicspherules is semi-quantitative and scarce (Blaha etal 2008). In India, microtektites have been reportedfrom the Indian Ocean sea floor based on their mor-phology (Prasad and Sudhakar 1998; Pattan et al2010) and recently a study has been undertaken onthe impact spherules from the basalt-hosted Lonarstructure, India (Misra et al 2009) to understandthe projectile chemistry. In case of Ramgarh struc-ture, the morphological similarity and chemistryof ‘spherules’ have also been cited as impact diag-nostic criteria (Sisodia et al 2006) although this
Figure 1. (a) The inset map of India showing the state of Uttar Pradesh and Allahabad city. (b) The traffic network mapof Allahabad city with sample locations and number of spherules recovered per location shown with different symbols:Allahabad Railway Station (ARS), Cantonment (CANTT), High Court (HC).
Anthropogenic and impact spherules 1045
claim has been refuted later (Reimold et al 2006).However, the spherical shape of spherules is nei-ther an indicator of impact cratering nor of melting(French and Koeberl 2010).
The present study provided the first reportof silica-rich spherules (SS) from road-depositedsediments (RDS; after Thorpe and Harrison 2008)of Allahabad city, U.P., India (figure 1) of anthro-pogenic origin having excellent morphologicalsimilarity with impact-related spherules and micro-tektites known from the Indian subcontinent. Thiswork also reports the chemical composition ofanthropogenic glass spherules from Allahabad areafor the first time and compares them with pub-lished data on microtektites from the Indian Oceansea floor and spherules from the Lonar structure toestablish their respective similarity and differencesin terms of morphology, size, shape, colour andchemistry. This is intented to carefully evaluatethe growing number of reports of probable/possibleimpact craters, microtektites and impact spherulesfrom the Indian subcontinent mainly based onspherule morphology and chemistry.
spherules per location showing the spatial variabil-ity in their number (figure 1). The RDS collectedfrom different locations across Allahabad city, ingeneral, contain/comprise angular to sub-roundedclasts of quartz and feldspar, flakes of mica, gyp-sum flakes, lumps of cow dung, bones, brokenglass, sugar cane husks, seeds, plastic and metallicfragments and spherules (both metallic and silica-rich). The spherules are glassy in nature, whichhas been verified on the basis of standard pet-rographic techniques and X-ray diffraction study.This study is primarily confined to characterizethe SS in the RDS identified in meso- to micro-scopic scale observations. About 10 gm of sampleswere air-dried and used for microscopic observa-tion and separation of different constituents. Thesamples were divided into two grain size fractionsusing a 35 mesh/500 μm sieve (ASTM; Ameri-can Society for Testing Materials). Spherules werehandpicked under a Leica stereozoom microscope.The abundance of spherules is less than 1 vol% ofRDS samples. The locations of 50 samples, num-ber of spherules observed in each sample and mor-phology and colour of spherules are summarized intable 1.
3. Shapes and surface featuresof the spherules
The specific properties of spherule shape have beenused as an important criterion to identify spherulesof impact origin (Simonson 2003) and shapes ofimpact spherules may vary from more angular totypically spherical types (Simonson et al 2000).Although spherical shapes of melt particles arethe most common (aerodynamically-shaped) typeobserved, certain spherule shapes (teardrops anddumbbells) can be very common to a particularimpact structure (e.g., Chicxulub, Mexico; Smitet al 1992). Sometimes the nonspherical shapes(ovoid, elongated, teardrop and dumbbell) are con-sidered to have developed due to the breakupand spin of melt droplets as they get trans-formed from melt to glass (Simonson 2003). Agglu-tinated shapes have formed due to collision ofmolten spherules in flight but the exact pro-cesses giving rise to various external shapes ofspherules is not fully answered (Simonson 2003).The microtektites reported from Indian Ocean aremostly spherical, however dumbbell shapes arealso observed (Prasad and Sudhakar 1996) and‘impact spherules’ from Lonar structure includeshapes like spheres, teardrop, cylinders, dumb-bells and spindles (Misra et al 2009). In addition,cratered and pitted surfaces of microtektites havebeen attributed to their impact origin (Prasad andSudhakar 1996; Margolis et al 1971).
1046 Ambalika Niyogi et al
Table
1.
Loc
ation
det
ails
of50
sam
ple
s,num
ber
ofsp
her
ule
spe
r10
gmofsa
mple
and
morp
holo
gyand
colo
ur
ofsp
her
ule
sba
sed
on
mes
o-
and
mic
rosc
opic
studie
s.Tra
ffic
implies
vehic
ula
rtraffi
c.C
olo
urles
ssp
her
ule
sare
transp
are
nt.
Sl.
no.
Sam
ple
loca
tion
(Lat.
;Long.)
Des
crip
tion
ofsa
mpling
loca
tion
No.ofsp
her
ule
sM
orp
holo
gy
Colo
ur
1.
Anglo
Ben
gali
Colleg
e,C
ivil
Lin
esC
om
mer
cialare
a;lo
wtr
affi
c17
Spher
icalto
oval
Colo
url
ess,
gre
y,w
hit
e,
(25◦ 2
7′ 0
0.5
8′′N
;81◦ 5
0′ 2
4.6
6′′E
)re
ddis
hbla
ck
2.
Purv
anch
alV
idhut
Vit
ran
Nig
am
Lim
ited
,C
om
mer
cialare
a;hea
vy
traffi
c5
Spher
icalto
oval
Colo
url
ess,
gre
y
Civ
ilLin
es(2
5◦ 2
6′ 4
8.2
2′′N
;81◦ 5
0′ 2
0.8
2′′E
)
3.
Ben
eath
Railw
aydat
kapool,
Com
mer
cialare
a;hea
vy
traffi
c7
Spher
icalto
oval
Colo
url
ess,
gre
y,
nea
rN
iranja
nTalk
ies
reddis
hyel
low
(25◦ 2
6′ 3
9.1
9′′N
;81◦ 5
0′ 1
6.7
7′′E
)
4.
Ghanta
ghar
Inte
rsec
tion,C
how
kD
ense
lypopula
ted
resi
den
tialand
3Spher
ical
Colo
url
ess
(25◦ 2
6′ 2
6.3
3′′N
;81◦ 5
0′ 0
6.4
5′′E
)co
mm
erci
alare
a;hea
vy
traffi
c
5.
Churc
h,C
how
kC
om
mer
cialare
a;hea
vy
traffi
c5
Spher
icalto
oval
Gre
y,purp
le
(25◦ 2
6′ 1
9.4
5′′N
;81◦ 5
0′ 0
2.1
3′′E
)
6.
Atu
lC
hair
House
s,C
how
kC
om
mer
cialare
a;m
oder
ate
traffi
c5
Spher
ical
Gre
y
(25◦ 2
6′ 1
5.1
7′′N
;81◦ 5
0′ 0
3.6
1′′E
)
7.
Kotw
ali
Bahadurg
anj
Com
mer
cialare
a;m
oder
ate
traffi
c5
Spher
icalto
oval
Colo
url
ess,
whit
e,
(25◦ 2
6′ 1
3.4
3′′N
;81◦ 5
0′ 0
2.0
2′′E
)light
yel
low
8.
Chandra
lok
Chaura
ha
Com
mer
cialand
resi
den
tialare
a;
25
Spher
icalto
oval
Gre
y,bro
wn,w
hit
e
(nea
rR
aja
rshiM
andapam
),M
utt
hig
anj
hea
vy
traffi
c
(25◦ 2
6′ 1
3.2
3′′N
;81◦ 5
0′ 0
4.3
3′′E
)
9.
Bata
sha
Mandi,
Chow
kC
om
mer
cialare
a;hea
vy
traffi
c1
Ova
lLig
ht
yel
low
(25◦ 2
6′ 1
4.4
5′′N
;81◦ 5
0′ 0
2.1
3′′E
)
10.
Ram
bagh,dot
pool,
Eid
-Gaah
Com
mer
cialare
a,auto
-ric
ksh
awst
and;
10
Spher
ical
Gre
y,bro
wn,w
hit
e,
(25◦ 2
6′ 1
9.3
5′′N
;81◦ 5
0′ 1
0.3
7′′E
)hea
vy
traffi
clight
yel
low
,co
lourl
ess
11.
Kid
gang,dat
kapool
Com
mer
cialare
a;hea
vy
traffi
c2
Spher
ical
Gre
y
(25◦ 2
6′ 0
8.5
6′′N
;81◦ 5
0′ 4
2.3
8′′E
)
12.
Ary
aK
anya
Deg
ree
Colleg
e,M
utt
hig
anj
Den
sely
popula
ted
resi
den
tial
3Spher
ical
Purp
lish
bla
ck,gre
y
(25◦ 2
5′ 5
4.5
5′′N
;81◦ 5
0′ 4
0.1
3′′E
)and
com
mer
cialare
a
13.
Kaja
lC
inem
a,K
idganj
Com
mer
cialare
a;m
oder
ate
traffi
clo
ad
5Spher
ical
Gre
y,co
lourl
ess,
(25◦ 2
6′ 0
6.2
3′′N
81◦ 5
0′ 5
4.7
4′′E
)bro
wn,w
hit
e
14.
RaniW
ati
ka,K
rish
na
Nagar,
Kid
ganj
Res
iden
tialand
com
mer
cialare
a8
Spher
ical
Gre
y
(25◦ 2
6′ 0
4.3
4′′N
;81◦ 5
0′ 5
8.2
2′′E
)
15.
Allahabad
Deg
ree
Colleg
e,K
idganj
Com
mer
cialare
a;hea
vy
traffi
c15
Spher
ical,
oval
Colo
url
ess
(25◦ 2
6′ 0
2.1
2′′N
;81◦ 5
0′ 5
9.1
1′′E
)and
dum
bbel
l
16.
Paw
ansu
tH
anum
an
Mandir
Com
mer
cialare
a;hea
vy
traffi
c22
Spher
ical
Colo
url
ess,
gre
y,w
hit
e
(nea
rH
igh
Court
)and
elongate
d
(25◦ 2
7′ 0
4.2
8′′N
;81◦ 4
1′ 1
7.5
6′′E
)
17.
Pet
rolP
um
p(n
ear
Hig
hC
ourt
)C
om
mer
cialare
a;hea
vy
traffi
c10
Spher
icalw
ith
pit
sC
olo
url
ess,
gre
y
(25◦ 2
7′ 0
4.4
9′′N
;81◦ 4
1′ 1
8.6
9′′E
)and
crack
s
Anthropogenic and impact spherules 1047
18.
Churc
h,Path
ar
Gir
ja,C
ivil
Lin
esC
om
mer
cialare
a;lo
wtr
affi
c60
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess
(25◦ 2
7′ 0
3.9
3′′N
;81◦ 4
8′ 1
7.3
8′′E
)
19.
Axis
Bank,V
inay
ak
Tow
er,C
ivil
Lin
esC
om
mer
cialare
aw
ith
litt
legre
ener
y16
Spher
ical,
ovaland
dum
bbel
lC
olo
url
ess,
gre
y
(25◦ 2
6′ 0
1.3
5′′N
;81◦ 5
0′ 0
3.9
6′′E
)
20.
Gov
ernm
ent
Gir
lsIn
ter
Colleg
eC
haura
ha,
Colleg
eare
a;lo
wtr
affi
c59
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess,
gre
y,w
hit
e
Civ
ilLin
es(2
5◦ 2
7′ 0
3.5
0′′N
;81◦ 4
9′ 4
3.6
4′′E
)
21.
Bank
ofB
aro
da,C
ivil
Lin
esC
om
mer
cialare
a;m
oder
ate
traffi
c13
Spher
ical
Colo
url
ess,
gre
y
(25◦ 2
5′ 0
3.2
8′′N
;81◦ 5
1′ 1
9.4
7′′E
)
22.
Subash
Chandra
Bose
Chaura
ha,
Res
iden
tialand
com
mer
cialare
a;
15
Spher
icaland
fuse
dty
pe
Colo
url
ess
Civ
ilLin
es(2
5◦ 2
5′ 0
4.5
8′′N
;81◦ 5
1′ 3
4.4
6′′E
)hea
vy
traffi
c
23.
Indir
aB
haw
an
Gate
,C
ivil
Lin
esC
om
mer
cialare
a;hea
vy
traffi
c16
Spher
ical
Colo
url
ess,
gre
y
(25◦ 2
7′ 0
3.1
2′′N
;81◦ 5
1′ 1
7.3
4′′E
)
24.
Atu
lC
hair
House
s,C
how
kC
om
mer
cialare
a;hea
vy
traffi
c2
Dis
tort
edov
al
Colo
url
ess
(25◦ 2
6′ 1
5.1
7′′N
;81◦ 5
0′ 0
3.6
1′′E
)
25.
Shiv
Chara
nLalR
oad
Chaura
ha,
Com
mer
cialare
a;m
oder
ate
traffi
c2
Spher
ical
Gre
y,w
hit
e
Bahadurg
anj
(25◦ 2
6′ 1
3.2
4′′N
;81◦ 5
0′ 0
3.1
1′′E
)
26.
Zer
oR
oad
(Chandra
lok
Talk
ies)
Hea
vy
traffi
clo
ad,re
siden
tialare
a7
Spher
ical
Colo
url
ess,
gre
y,
(25◦ 2
6′ 1
3.4
5′′N
;81◦ 5
0′ 8
.13′′E
)and
com
mer
cialare
asi
lver
gre
y
27.
Ram
Kri
shna
Sew
ash
ram
,M
utt
hig
anj
Den
sley
popula
ted
resi
den
tialare
a;
5Spher
icaland
fuse
dC
olo
url
ess,
gre
y
(25◦ 2
5′ 5
3.4
5′′N
;81◦ 5
0′ 3
8.3
4′′E
)hea
vy
traffi
c
28.
E.C
.C.G
ate
(Gaughat)
Res
iden
tialand
colleg
eare
a13
Spher
icalto
oval
Colo
url
ess,
gre
y
(25◦ 2
6′ 4
3.3
4′′N
;81◦ 5
0′ 4
4.0
6′′E
)
29.
K.L
.K
atj
uC
haura
ha,K
idganj
Com
mer
cialare
a;m
oder
ate
traffi
c7
Spher
ical,
hem
i-sp
her
ical
Colo
url
ess,
gre
y,w
hit
e,
(25◦ 2
6′ 0
5.4
1′′N
;81◦ 5
0′ 5
7.7
7′′E
)and
fuse
dty
pe
bro
wn,light
yel
low
30.
Tuls
ianiP
laza
,C
ivil
Lin
esC
om
mer
cialare
a;lo
wtr
affi
c16
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess,
gre
y
(25◦ 2
7′ 0
3.2
8′′N
;81◦ 5
1′ 7
.44′′E
)
31.
Big
Baza
ar,
Civ
ilLin
esC
om
mer
cialare
a;hea
vy
traffi
c7
Spher
icalto
hollow
spher
ical
Colo
url
ess,
whit
e
(25◦ 2
7′ 0
0.6
6′′N
;81◦ 5
0′ 1
0.7
7′′E
)
32.
Tuls
iC
haura
ha
Hea
vy
traffi
cw
ith
litt
legre
ener
y13
Spher
icalto
oval
Colo
url
ess,
gre
y
(nea
rH
anum
an
Mandir
),C
ivil
Lin
es
(25◦ 2
6′ 5
8.5
9′′N
;81◦ 5
0′ 2
3.4
4′′E
)
33.
Super
inte
ndant
Chandra
Shek
har
Park
,Lush
gre
enare
a;lo
wtr
affi
c14
Spher
ical,
elongate
dC
olo
url
ess,
gre
y
Civ
ilLin
es(2
5◦ 2
6′ 5
6.5
7′′N
;81◦ 5
0′ 4
1.2
3′′E
)and
fuse
dty
pe
34.
C.A
.V.In
ter
Colleg
e,C
ivil
Lin
esC
olleg
eare
a;lo
wtr
affi
c11
Spher
ical
Colo
url
ess,
gre
y,
(25◦ 2
6′ 5
1.7
9′′N
;81◦ 5
0′ 5
7.3
1′′E
)w
hit
e,bro
wn
1048 Ambalika Niyogi et al
Table
1.
(Continued
.)
Sl.
no.
Sam
ple
loca
tion
(Lat.
;Long.)
Des
crip
tion
ofsa
mpling
loca
tion
No.ofsp
her
ule
sM
orp
holo
gy
Colo
ur
35.
C.A
.V.In
ter
Colleg
e,C
ivil
Lin
esC
olleg
eare
a;lo
wtr
affi
c3
Spher
icalto
Colo
url
ess,
gre
y
(25◦ 2
6′ 5
1.7
9′′N
;81◦ 5
0′ 5
7.3
1′′E
)hem
ispher
ical
36.
Alo
pib
agh
Tir
aha,A
lopib
agh
Den
sly
popula
ted
are
a;hea
vy
traffi
c4
Spher
icalto
oval
Colo
url
ess,
gre
y
(25◦ 2
6′ 3
8.8
8′′N
;81◦ 5
2′ 0
9.7
2′′E
)
37.
New
Canto
nm
ent
Res
iden
tialare
aand
gre
enbel
tzo
ne;
60
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess,
(25◦ 2
6′ 5
5.4
2′′N
;81◦ 4
8′ 5
4.4
1′′E
)lo
wtr
affi
cpale
yel
low
38.
New
Canto
nm
ent
Res
iden
tialare
aand
gre
enbel
tzo
ne;
40
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess
(25◦ 2
7′ 0
4.8
1′′N
;81◦ 4
8′ 5
4.5
4′′E
)lo
wtr
affi
c
39.
New
Canto
nm
ent
Res
iden
tialare
aand
gre
enbel
tzo
ne;
48
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess
(25◦ 2
7′ 1
8.7
4′′N
;81◦ 4
8′ 5
7.1
1′′E
)lo
wtr
affi
c
40.
New
Canto
nm
ent
Res
iden
tialare
aand
gre
enbel
tzo
ne;
60
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess
(25◦ 2
7′ 1
9.6
8′′N
;81◦ 4
8′ 5
8.5
4′′E
)lo
wtr
affi
c
41.
New
Canto
nm
ent
Res
iden
tialare
aand
gre
enbel
tzo
ne;
60
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess
(25◦ 2
7′ 2
5.9
4′′N
;81◦ 4
8′ 5
9.4
3′′E
)lo
wtr
affi
c
42.
46,C
annon
Offi
cers
Mes
s,N
ewC
anto
nm
ent
Res
iden
tialare
aand
gre
enbel
tzo
ne;
60
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess
(25◦ 2
7′ 4
9.0
4′′N
;81◦ 4
9′ 0
0.0
3′′E
)lo
wtr
affi
c
43.
Ash
waniD
ronach
ary
a,N
ewC
anto
nm
ent
Res
iden
tialare
aand
gre
enbel
tzo
ne;
10
Spher
ical,
ovaland
fuse
dty
pe
Colo
url
ess
(25◦ 2
8′ 0
2.4
8′′N
;81◦ 4
8′ 3
8.2
9′′E
)lo
wtr
affi
c
44.
Ash
waniD
ronach
ary
a,N
ewC
anto
nm
ent
Res
iden
tialare
aand
gre
enbel
tzo
ne;
13
Spher
icaland
fuse
dty
pe
Colo
url
ess
(25◦ 2
8′ 0
2.6
9′′N
;81◦ 4
8′ 3
8.5
8′′E
)lo
wtr
affi
c
45.
Ash
waniD
ronach
ary
a,N
ewC
anto
nm
ent
Res
iden
tialare
aand
gre
enbel
tzo
ne;
8Spher
icaland
hem
ispher
ical
Colo
url
ess
(25◦ 2
8′ 0
0.4
8′′N
;81◦ 4
8′ 3
7.1
6′′E
)lo
wtr
affi
c
46.
Alo
pShanka
riM
andir
,A
lopib
agh
Res
iden
tialare
aand
gre
enbel
tzo
ne;
3Spher
ical
Colo
url
ess
(25◦ 2
6′ 3
6.6
4′′N
;81◦ 5
2′ 1
2.2
4′′E
)lo
wtr
affi
c
47.
Dis
tric
tE
duca
tion
Tra
inin
gC
entr
e,C
ivil
Lin
esLit
tle
gre
ener
y;hea
vy
traffi
c20
Spher
ical
Colo
url
ess
(25◦ 2
6′ 5
8.6
7′′N
;81◦ 5
0′ 2
3.2
4′′E
)
48.
Tri
konia
,Sabji
Mandi,
Madhw
apur
Den
sely
popula
ted
are
a;hea
vy
traffi
c3
Spher
icaland
teard
rop
Colo
url
ess
(25◦ 2
6′ 3
4.3
3′′N
;81◦ 5
1′ 4
6.1
5′′E
)
49.
Tuls
iC
haura
ha
(nea
rH
anum
an
Mandir
),Lit
tle
gre
ener
y;hea
vy
traffi
c12
Spher
icalto
oval
Colo
url
ess
Civ
ilLin
es(2
5◦ 2
6′ 5
8.5
7′′N
;81◦ 5
0′ 3
.54′′E
)
50.
New
Canto
nm
ent
are
aR
esid
enti
alare
aand
gre
enbel
tzo
ne;
60
Spher
ical,
ovaland
fuse
dC
olo
url
ess
(25◦ 2
7′ 3
5.6
4′′N
;81◦ 4
8′ 5
4.1
1′′E
)lo
wtr
affi
c
Anthropogenic and impact spherules 1049
4. Spherule sizes and their spatialdistribution
The size range of most Archean to Palaeoprotero-zoic impact spherules is constrained between 60and 2000 μm, although some can measure up to 15–20 mm across (Simonson 2003). The spherules fromLonar crater (Misra et al 2009) are sub-millimeter(0.3–1 mm) to centimeter-sized (up to ≈30 cm)and the microtektites from Indian Ocean range insize between 110 and 980 μm. The spherules cantravel several hundreds of km suspended in air fromvarious sources. In case of impact structures, theirspatial distribution is divided into proximal anddistal ejectas. Australasian tektites strewn field areknown to spread over >1 million km2 area (Glassand Wu 1993).
5. Colour variation in spherulesand their occurrences
The colour of the anthropogenic spherules (presentstudy) varies from colourless (transparent) to grey,opaque white, brown, yellow and dark grey. Thepresence of colourless (transparent) spherules isdominated in the city while dark grey ones areobserved in the southern part, i.e., industrial area,Naini of the city. On the contrary, the impactspherules are mostly coloured (orange, greenish,amber, brownish and dark grey; Weiss et al 2010)and microtektites are yellowish, yellowish white,white, greenish yellow and green in colour (Pattanet al 2010). The anthropogenic spherules are gen-erally colourless as their silica content is very highand near absence of transition metal elements.
6. Chemistry of the spherules
In the absence of diagnostic evidence, the origin ofspherules has always been a matter of debate. How-ever, the geochemical analysis of spherules and/orspherule layers in a number of cases (based onPGEs abundances and relevant isotopic concentra-tions) has helped to resolve the problem pertainingto their origin (Lowe et al 1989; Kyte et al 2003;Simonson et al 2009). Since the projectile composi-tion is not always chondritic, PGEs concentrationtoo has limitations. Similarly microtektites alsoshow wide chemical variation (basaltic to nearlypure silica in composition). The composition ofanthropogenic spherules also depends on the chem-istry of raw material(s) and at times practicallyimpossible to distinguish them from spherules ofimpact origin. The nature of particulate pollutantscan be predicted from the study of fly ash from thecoal-based industrial heating systems (Jordanova
et al 2006). The particles mainly comprise elementslike, Si, Al, S, Ca, and Fe since raw coal is known toconstitute kaolinite, quartz, siderite, calcite, pyrite,gypsum and sulphur (Xie et al 2009 and referencestherein). Many features, characteristic of Indian flyash, are considerably different from the correspond-ing features of fly ash generated elsewhere (Sarkaraet al 2005). However, there is currently no compre-hensive dataset on the systematic geochemistry ofanthropogenic spherules in the literature.
7. Results and discussion
The present study is based on the detailed anal-ysis of 167 spherules from 50 representative sam-ple sites. The shapes of the silica spherules fromAllahabad RDS are mostly spherical to ovoid. Thespecific shapes observed include elliptical, dumb-bell, cylindrical, tear-drop and spindle (figure 2a–e).Nearly 84% of the spherules are spherical and16% of the spherules show the agglutinated form(adherence and coalescence features). Most or allof these aggregates tends to have appeared fromthe crystallization of the original melts (e.g., Scallyand Simonson 2005; Sweeney and Simonson 2008)(figure 2f). The surface textures of anthropogenicspherules generally vary from smooth to pittedtypes having majority of irregular pits with somecircular and jagged ones showing their resem-blance to that of impact spherules. About 10–50%of the surface area of the bigger (200–1000 μm)spherules show pitted appearance (‘cratered sur-face’) whereas the smaller ones are relativelysmooth (figure 2g, h). However, ring-like featuresreported by Prasad and Sudhakar (1998) aroundthe pits in Indian Ocean microtektites have notbeen observed in silica spherules from the Alla-habad RDS. The presence of bubbles and multiplebubble trails inside SS is occasionally noted.
The spherules present in Allahabad RDS mea-sure from 100 to 200 μm in size (in the caseof 35 mesh-sized sieved dusts). However, in theunsieved samples, the size varies between >200 and1000 μm. The spherules in the size range of 100–200 μm are predominantly present in the vicin-ity of main industrial area (Naini; figure 1) andare mainly grey to dark grey in colour whereasthe colourless spherules (>200 up to 1000 μm) canbe seen both in residential and industrial areas ofthe city (figure 2a, b). The anthropogenic spherulesobserved in different parts of the Allahabad cityexhibit a size range between 0.55 and 1.67 mm(figure 3). The size of the spherules shows apolymodal distribution with maximum number ofspherules lying between 0.8 and 1 mm and theirsize range is similar to impact spherules (table 2).The silicate glass spherules are most abundant in
1050 Ambalika Niyogi et al
Figure 2. SEM images showing different shapes of spherules from Allahabad RDS and of fly-ash sample. (a) Sphericalspherule with smooth surface and a few micropits. (b) Fused-type spherule showing two sub-spherules attached to themain one. (c) Elongated spherule with many micropits. (d) Teardrop spherule. (e) Oval spherule with numerous micropits.(f) Dumbbell spherule with a narrow neck. (g) Spherical spherule showing numerous micropits. (h) Fly ash sample fromIFFCO Plant, Phulpur, Allahabad, showing many spherules.
the northern part of the city and their numberdecreases towards the southern part.
The 66% of all the anthropogenic spherulesobserved in the unsieved dust samples from Alla-habad (RDS) is colourless to pale coloured (i.e.,some spherules which have been noticed are
light yellow coloured). The coloured anthropogenicspherules’ of various hues include light grey, cream,brown and black-coloured. The metallic and darkgrey spherules of the present study are predomi-nantly observed in the Naini industrial area, Alla-habad. The colourless spherules are observed both
Anthropogenic and impact spherules 1051
Figure 3. The size of 167 anthropogenic spherules collectedfrom different parts of Allahabad city show a polymodaldistribution with maximum numbers lying between 0.8 and1 mm similar to impact spherules.
in residential as well as industrial areas of the city(table 1). The yellow coloured material present onthe external surface of the spherule being analyzedis sulphur.
Compositional range of the SS is given in table 2.The compositional ranges of morphologically sim-ilar spherules reported from Lonar impact struc-ture (LS), Maharashtra, India (Misra et al 2009)and from microtektites (MT) in surficial sedimentsof the Indian Ocean (Prasad et al 2003) are givenin table 2 for comparison. The SiO2 content ofSS is between 67.96 and 85.59 wt% with an aver-age value of 72.09 wt%. MT and LS have lowerSiO2 contents with average values of 68.59 and48.31 wt%, respectively. SS are comparatively richin alkali component, Na2O (0.19–6.79 wt%) andK2O (0.26–1.04 wt%). The total iron content of SSexpressed as FeO is low (0.07–0.40). Al2O3 contentof the SS varies between 1.21 and 3.63 wt% andis much lower than that of MT (10.63–21.12 wt%)and LS (11.76–15.23 wt%). Both NiO and Cr2O3
content are low in the case of SS (0.22 and 0.1 wt%,respectively). In the case of TiO2, the values mea-sure up to 0.07 wt%. The compositional limits ofthe three spherule types (SS, MT and LS) are dis-tinct as illustrated in the binary, Al2O3 vs. SiO2
plot (figure 4a) and ternary, SiO2–(CaO + MgO +FeOT)–Al2O3 plot (figure 4b) and implied origin.Surficial coating of yellow granules occurring at theneck of two attached spherules is analyzed to be
Table 2. A comparison between microtektites, impact spherules and anthropogenic spherules (present study) based on theirshape, size, colour and lithophile elemental composition.
Spherule type Shape Size Colour Chemical composition
Figure 4. (a) Binary Al2O3 vs. SiO2 plot showing dis-tinct domains of SS (SS; open circle) from AllahabadRDS, microtektites (MT; shaded circle) from Indian Ocean,and spherules from Lonar impact structure, Maharash-tra, India (LS; solid circle). (b) Ternary SiO2–(CaO+
MgO + FeOT)–Al2O3 diagram showing distinction among
the three spherule types. FeOT represents total ironexpressed as FeO.
sulphur. Compositional similarity between largerspherules and the smaller ones adhering to themsuggests their common origin. Presence of vesicleswithin SS suggests the presence of a volatile phasein the system.
The BSE-SEM images and EPMA analysisof a fly-ash sample from the steam generationof the IFFCO (Indian Farmers Fertilizer Co-operative Limited) fertilizer plant at Phulpur(25◦32′10.64′′N; 82◦03′47.09′′E), located 40 kmNE of the Allahabad city shows the presenceof spherules (figure 3h) and their chemistryis similar to RDS spherules of Allahabad city,respectively.
8. Conclusions
The present study suggests that the shape, size,surface features and chemistry of spherules are notdiagnostic of impact cratering process and can-not distinguish microtektites and impact spherulesfrom the coal fly-ash spherules produced fromnatural wildfires and thermal power plants.
The various shapes (spherical and agglutinated)of SS suggest their formation at high temperatureand adherence close to their respective liquidustemperature.
The composition of SS can be defined in a SiO2–(CaO + MgO + FeOT)–Al2O3 ternary since theirbulk composition in terms of the three end mem-bers lies between 80 and 90 wt%. The microtektitesare slightly better defined by the three end mem-bers (96–98 wt%) compared to impact spherules(93–96 wt%).
The morphology and chemistry of fly ashspherules from the coal-based thermal power plant,Phulpur, Allahabad are identical with SS observedin RDS samples suggesting their derivation fromthe coal-based NTPC thermal power plant situatedat Phulpur.
The spatial distribution of SS indicates theirtransportation to different parts of Allahabad cityand its adjoining areas by wind as suspendedparticulate matter.
The abundance of SS is high in the northern partof the Allahabad city and their numbers per loca-tion decrease southward. However, the size of thespherules observed during the present study doesnot show any correlation with distance.
Hence, it is necessary to adopt a multi-prongedmethod to evaluate the spherules before assigningtheir origin as they can be carried over hundreds of kmin air from various sources prior to their deposition.
Acknowledgements
Dr Martin Schmieder, University of Stuttgart, Ger-many is immensely thanked for his excellent pre-submission review of the manuscript. JKP thanksthe PLANEX Programme, Physical Research Lab-oratory (Department of Space), Ahmedabad foranalytical support and field equipments. The DST,New Delhi is thanked for providing a researchmicroscope used in the present study through grantno. ESS/16/195/2003. AN thanks the Universityof Allahabad for CRET Scholarship and this workforms part of her D.Phil. thesis. Some of the sam-ples used in the present study are collected duringMr. R K Gautam’s M.Sc. dissertation project. Ananonymous reviewer and Dr Rajat Mazumder arethanked for their comments and editorial handling,respectively.
Anthropogenic and impact spherules 1053
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MS received 22 April 2011; revised 5 July 2011; accepted 23 July 2011
