nature of sub-volcanic magma chambers, deccan province, india: evidence from quantitativetextural...

Nature of Sub-volcanic Magma Chambers,Deccan Province, India: Evidence fromQuantitativeTextural Analysis of PlagioclaseMegacrysts in the Giant Plagioclase Basalts

MICHAEL D. HIGGINS1* AND D. CHANDRASEKHARAM2

1SCIENCES DE LA TERRE, UNIVERSITE¤ DU QUE¤ BEC A' CHICOUTIMI, CHICOUTIMI, QUE. G7H 2B1, CANADA2DEPARTMENT OF EARTH SCIENCES, INDIAN INSTITUTE OF TECHNOLOGY, MUMBAI 400076, INDIA

RECEIVED SEPTEMBER 27, 2006; ACCEPTED FEBRUARY 5, 2007ADVANCE ACCESS PUBLICATION MARCH 8, 2007

Sub-volcanic magma chambers might be a widespread component of

flood basalt provinces, and their presence can be revealed in some cases

by plagioclase megacrystic basalts. In at least four levels within the

Deccan flood basalt sequence the generally low abundance of small pla-

gioclase crystals increases to 5^25%, with some as large as 30 mm

long.These Giant Plagioclase Basalt (GPB) flows were formed by

mixing of megacryst-rich and megacryst-poor magmas. The crystal

size distributions (CSD) of these megacrysts mostly plot as almost

straight lines on a classic CSD diagram. For a plagioclase growth rate

of 10�10 mm/s steady-state magma chamber models and simple contin-

uous growth suggest residence times of 500^1500 years. However, the

lack of crystals smaller than 2mm suggests that coarsening may have

been involved and crystal shape can help define the environment where

this happened. Plagioclase megacrysts are very tabular and commonly

form clusters of sub-parallel crystals, characteristics that are also found

in the plagioclase of anorthosites formed by flotation at the top of

shallow magma chambers and crystallization in a high Peclet number

regime (e.g. Skaergaard; Sept I“ les). A possible history is as follows. (1)

Plagioclase megacrysts crystallize in a convecting magma chamber just

below the lava pile. (2) Currents sweep the crystals to the top of the

chamber, where they accumulate as a result of their buoyancy.The crys-

tals coarsen in response to the continuous supply of hot magma. (3) New

magma sweeps through the plagioclase mush, mingles and mixes, then

erupts to form the GPBs.The residence time recorded by the megacrysts

in the GPBs is that of the magma chamber where the megacrysts

formed, not that of the magmas that make up the megacryst-poor part

of the GPBs or the other megacryst-poor lavas. Lavas with

megacrysts similar to the GPBs are uncommon but widespread

(Galapagos, Surtsey, etc.), and suggest the presence of sub-volcanic

magma chambers elsewhere.

KEY WORDS: texture; microstructure; continental basalt; megacryst;

plagioclase; crystal shape

I NTRODUCTIONThe Deccan traps are one of the most important continen-tal flood basalt provinces (Sen, 2001; Chandrasekharam,2003; Jerram & Widdowson, 2005). They erupted about 65Myr ago, synchronous with the K^T boundary, and it hasbeen suggested that they might have caused or contributedto the major mass extinction event at that time (Duncan &Pyle,1988). However, the effect of magmatism on climate iscritically dependent on eruption rate. It is not easy todetermine the eruption age of the Deccan Traps, as mostof the rocks are hydrothermally altered. Hofmann et al.(2000) considered that the best ages are from 40Ar/39Ardating of mineral separates, mostly plagioclase. Theydetermined the age of lavas at the top of the WesternGhats pile to be 65�4� 0�7Ma and that of the base as65�2�0�4Ma. Hence, the main eruptive period was prob-ably less than 1 Myr. Nevertheless, magmatism may haveoccurred in a much shorter period or episodically, neitherof which can be resolved by current isotopic techniques.It has been suggested that the duration of magmatism of

the Deccan group can also be determined from rock

*Corresponding author. E-mail: [email protected]

� The Author 2007. Published by Oxford University Press. All rightsreserved. For Permissions, please e-mail: [email protected]

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textures (Sen, 2001, 2002). Most formations in the lowerpart of the Deccan group terminate in an evolved magmathat contains plagioclase megacrysts up to 50mm long(Giant Plagioclase Basalts; GPBs). If these crystals grewcontinuously in the magma storage zone and their growthrate could be estimated then the duration of each magmaticcycle could be determined. If there were no major hiatusesbetween the magmatic cycles then the total duration couldbe determined, or at least a minimum value. Using thismethod, Sen et al. (2006) proposed that the duration of mag-matism may have been as little as 22800 years. Obviously,such a rapid rate of magmatism would have perturbed theclimate considerablyandcould have influencedbiotic evolu-tion. In this paper we will examine quantitatively thetextures (¼microstructure) of plagioclase in the GPB flowsandpropose amodel for their formation.

DECCAN GEOLOGY AND THEGIANT PLAGIOCLASE BASALTSIntroductionThe basalts of the Deccan province were erupted frommany centres, but the most important is a shield-volcano-like structure in theWestern Ghats, near Mumbai (Fig. 1)(Subbarao et al., 1994). Rifting and erosion have exposed a1�7 km thick section of flows, which is the focus of thisstudy. Aphyric or almost aphyric tholeiitic basalts domi-nate the section. It has been divided into sub-groups usingfield geology, petrography and geochemistry (Beane et al.,1986). The lowest sub-group, the Kalsubai, contains anumber of horizons of lavas with abundant, large tabularplagioclase crystals termed megacrysts: these are theGPBs (Beane et al., 1986; Hooper et al., 1988). Megacrystsare crystals that are strikingly larger than other crystalsin the rock, but there is no genetic implication as towhether such crystals are phenocrysts or xenocrysts.Similar GPB flows have also been reported from the north-ern Deccan province but are not considered here(Chandrasekharam et al., 1999). In the western Deccan,GPBs cap each formation, have the highest total Fe and Ticontents, and are considered to be the most chemicallyevolved.‘Plagioclase-phyric’ flows are similar to the GPBs,but with less and/or smaller plagioclase megacrysts; theyare combined in this study with the GPBs. The tops ofsome flows are covered by ‘red boles’ça fine-grainedmaterial rich in haematite thought to have formed byweathering of the basalts and hence indicating a period ofvolcanic repose (Fig. 2).

Giant Plagioclase Basalt flowsIndividual GPB flows are 5^10m thick and occur at dis-tinct horizons within the eruptive sequence (Beane et al.,1986). However, the extent of individual flows is unclearand hence it is impossible to say if the same flow occurs at

different places or if it is just another flow of the same pack-age. GPB flows were mostly sampled in road cuts and quar-ries, where the flows are relatively well exposed (Table 1).At these locations it is clear that the GPBs are not homo-geneous, but are composed of at least two magmatic com-ponents: one aphyric or sparsely phyric and another rich inplagioclase megacrysts (Fig. 3). The fabric defined by

20°

100 km

22°

16°

70°

Mahabaleswar

Ambol i

Mumbai(Bombay)

Area ofinterest

Girnar

NasikDhule

Lonar

BuldanaShirpur

Shahada

ToranmalToranmal

Mhow JabalpurNarmada-rift

Tapi-rift

Cam

bay-rift

Arabian SeaBay of Bengal

78°74°

Fig. 1. The Deccan volcanic province. The samples are from theregion east of Mumbai, where the basalt section is thickest (circled).

00

10 20

0.5

1.0

1.5

Hei

ght (

km)

Phenocrysts (%)

Thalghat GPB

Kashele GPB

Tunnel Five GPB

Deccan-Western Ghats section

‘Red boles’

Fig. 2. Part of the Western Ghats section of the Deccan series. Thephenocrysts are dominated by plagioclase. GPB, Giant PlagioclaseBasalt. ‘Red boles’ are red fine-grained material rich in haematitethought to represent periods of erosion. Modified from Sen (2001).

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plagioclase crystals within the megacryst-rich componentis extremely variable in direction and intensity. The spatialarrangement of these components and their fabric suggeststhat they originated by incomplete mixing (mingling) oftwo components. This may have occurred by simultaneouseruption of two magmas into the flow, or it could also havehappened at depth in the feeder and been preserved duringtransport. This effect is seen in rivers, where water from atributary may be clearly distinguished far from the junc-tion of the streams. All GPB flows examined in this studyhad similar inhomogeneous structures.

PetrographyThe GPB lavas consist of plagioclase megacrysts in a fine-grained matrix. The plagioclase megacrysts are 2^50mmlong. They have compositions ranging from An61 to An64(Hooper et al., 1988) and are generally weakly zoned,except for a narrow rim. Many crystals appear to have a

narrow hollow centre, which is filled with fine-grainedmaterial similar to the matrix. This is assumed to betrapped melt. Plagioclase in the matrix is much smallerthan the smallest megacrystsçtypically 0�25^1mm long.Hence, it is generally easy to distinguish megacrystic andmatrix plagioclase.The texture of the plagioclase megacrysts was examined

both qualitatively and quantitatively (see below). In bothcases the first step was the creation of a binary image ofplagioclase distribution. Rock samples 10^20 cm squarewere slabbed normal to the foliation, if present, andpolished. In most samples the plagioclase was sufficientlyaltered that it stood out clearly from the matrix. However,in some very fresh samples it was not possible to distin-guish clearly the plagioclase megacrysts. In these samplesthe polished surface was painted with hydrofluoric acid,which generally accentuated the contrast. Slabs were thenscanned using a conventional document scanner andthe images transferred to a vector drafting program(CorelDraw). There the crystals were outlined on thescreen using a mouse. Crystals that intersect in the planeof the section, but are clearly two separate crystals wereoutlined and translated to separate them. The crystal out-lines were then filled and exported as tiff files for furtherprocessing (Fig. 4). Large thin sections (50mm by 75mm)from the same samples were also cut, scanned and digi-tized in the same way.Most plagioclase megacrysts are not distributed

homogeneously, but are clustered in touching groups(Fig. 4). An important qualitative aspect of this texture isthe angular relationship of crystals within these clusters.In some of the samples with the coarsest textures plagio-clase crystals occur as clusters of sub-parallel crystals(Fig. 4). In samples with smaller plagioclase crystals someof the clusters may have a radiating texture. We havenot yet found a way to quantify this aspect of texturalvariation.

5 cm5 cm

Megacryst-richcomponent

Megacryst-richcomponent

Megacryst-poorcomponent

Megacryst-poorcomponent

Fig. 3. Variations in the plagioclase megacryst content within a singleGPB flow. The field of view is 40 cm.

Table 1: Sample locations

Sample no. Latitude (N) Longitude (E) Altitude (m) Location Notes

MH-04-04 19841�55 73829�81 422 Nasik road Thalghat

MH-04-05A 19856�45 73843�85 697 Pandavlena quarry Kashele

MH-04-05B 19856�43 73843�83 730 Pandavlena quarry Kashele

MH-04-05C 19856�43 73843�73 725 Pandavlena quarry Kashele

MH-04-06 19856�30 73843�87 676 Near Pandavlena quarry Kashele

MH-04-07 19856�68 73843�56 675 Near Pandavlena quarry Kashele

MH-04-10 19858�40 73826�57 425 Trimbak–Jawhar road Thalghat

MH-04-12 19859�01 73826�68 503 Trimbak–Jawhar road Thalghat

MH-04-14 19811�61 73851�36 868 Shivaneri Fort Manchar

MH-04-16 19825�00 73815�00 775 Bote hill Manchar

HIGGINS & CHANDRASEKHARAM SUB-VOLCANIC MAGMACHAMBERS

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MH-04-05a Kashele

MH-04-05c Kashele

20 mm

20 mm

20 mm

MH-04-16 Manchar

20 mm

MH-04-12 Thalghat

20 mm

MH-04-04 ThalghatMH-04-04 Thalghat

20 mm20 mm

MH-04-05b Kashel eMH-04-05b Kashele

MH-04-06 Kashele

20 mm20 mm

MH-04-07 Kashele

20 mm

MH-04-10 Thalghat

20 mm

MH-04-14 Manchar

20 mm

Plagioclasemegacrysts

in GPB

Fig. 4. Textural variations of plagioclase megacrysts in GPBs. Samples MH-04-05a, 05b and 05c are from the same flow, a few metres apart.Other samples are from other GPB flows. MH-04-12 is from a plagioclase-phyric flow just above theThalghat GPB. Images were produced bymanual digitizing of crystal outlines in polished slabs.


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QUANTITAT IVE TEXTURALMEASUREMENTSIntroductionThe binary plagioclase images described above were quan-tified using the program ImageJ, a java version of the well-known program NIHImage. This program calculatesdimensions of a best-fit ellipse to the crystal outlines andits orientation and position. Intersection size data wereconverted to true crystal size distributions (CSDs) usingCSDCorrections 1.3 (Higgins, 2000, 2002). All CSDs werecalculated for a shape of 1:5:5 (see below) and a roundnessparameter of 0�2 (close to parallelepiped). CSD calcula-tions are not very sensitive to the weak fabrics observedhere, especially for sections cut orthogonal to the foliation;hence a massive fabric was used. Intervals with fewer thantwo crystals were eliminated from the diagrams, as theyare not precise.

Crystal size distributionCSDs were determined in 10 samples from threeGPB groups (Table 2). Three samples from a single flow ofthe Kashele GPB had CSDs that were almost straight on aclassic CSD diagram [S-type CSD of Higgins (2006b)],but lacked small crystals (Fig. 5a). This was a real effect,and not a measurement artefact, as size data were com-bined from one or more slabs and large thin sections foreach sample. Two samples, MH-04-05a and MH-04-05c,were sub-parallel, but separated by 1�5 ln(populationdensity)(mm�4) units.The mean size of the largest intervalwhose population density could be measured was 12mm,and there were no plagioclase megacrysts in intervalssmaller than 2mm. Much smaller plagioclase crystals arepresent in the matrix of these samples, but were not

measured in this study. The third sample, MH-04-05b,was also straight, but extended to larger crystals, in the20mm interval. There were no crystals smaller than3�5mm. The slope of this sample was significantly lessthan that of the other samples. Samples MH-04-06 andMH-04-07 were taken from the same flow as sample MH-04-05, about 1km away. They both have CSDs that aresimilar to those of samples MH-04-05a, MH-04-05b andMH-04-05c. However, sample MH-04-06 continues tolarger crystals than samples MH-04-05a and MH-04-05c.The Thalghat GPB was sampled at two locations.

Sample MH-04-04, from the Nasik road section, also hasan almost straight CSD, the largest crystals of any samplestudied and the shallowest slope. An attempt was made toquantify plagioclase megacrysts in the field at this locationby measuring crystals on fresh surfaces with a ruler.The CSD for larger crystals followed that determinedfrom the slabs of sample MH-04-04, but the lower cutoffsize in the field was only 10mm, compared with 2mm inthe slabs. Because of the importance of crystals smallerthan 10mm this method was not pursued further.Samples MH-04-10 and MH-04-12 were taken from theTrimbak^Jawhar section.There is a considerable differencein elevation for these two samples, but both are consideredto be part of the Thalghat GPB unit. The CSD of sampleMH-04-12 was straight and steep, whereas MH-04-10 wascollinear with MH-04-04, but did not extend to such largecrystals.

The Manchar GPB was sampled at two locations.Both CSDs are approximately straight and not dissimilarto CSDs from the other GPBs. The two CSDs are parallelexcept for the largest size interval, which is not welldetermined.

Table 2: Number of crystals per unit area

Sample: MH-04-04 MH-04-05A MH-04-05B MH-04-05C MH-04-06 MH-04-07 MH-04-10 MH-04-12 MH-04-14 MH-04-16

Area (mm2): 17090 39000 27000 17200 33800 10900 23100 11860 10100 20130

Size bin range (mm)

39�8–25�1 5 0 0 0 1 1 0 0 1 0

25�1–15�8 33 0 11 0 5 7 0 4 4 17

15�8–10�0 39 13 35 8 18 19 23 21 35 31

10�0–6�31 55 25 77 34 27 32 49 62 55 49

6�31–3�98 63 43 83 60 41 25 52 99 77 48

3�98–2�51 35 51 64 59 39 33 39 119 76 59

2�51–1�58 32 43 20 76 51 34 29 94 61 47

1�58–1�00 12 25 9 38 28 29 11 90 52 38

1�00–0�631 9 6 2 16 24 9 1 27 16 16

0�631–0�398 1 4 2 5 9 2 1 11 5 6

0�398–0�251 0 5 0 2 1 0 0 2 1 1


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Because all the megacrysts have CSDs close to S-type(Higgins, 2006b) they can be reduced to a characteristiclength (Cl¼�1/slope) and intercept. Higgins (2002)showed that closure produces a correlation betweenintercept and characteristic length, even if the volumetricphase proportion is variable. Hence, the easiest wayto summarize the CSD data is with a graph of characteris-tic length against modal plagioclase (Fig. 6a).The characteristic length of a CSD says nothing about the

largest crystal in the rock.This can be determined preciselyonly by 3D methods, but can be estimated from the largestintersection (Fig. 6b). In general, there is a good correlationbetween the largest intersection and the total amount ofplagioclase, with the exception of sampleMH-04-06.

Crystal shapeThe relationship between the shape of crystals in rocks andtheir petrogenesis has been examined for a very long time

(e.g. Kostov & Kostov, 1999; Higgins, 2006a). Shape is bestexamined in three dimensions, but for regular crystalswith more or less uniform shapes intersection data can beused to estimate overall shape. Shape is generallyexpressed in terms of the ratio of short:intermediate:long(S:I:L) for a bounding parallelepiped or best-fit ellipsoid.For parallelepipeds (Higgins, 1994) and triaxial ellipsoids(Higgins, 2006a) the mode of intersection width/intersec-tion length (2D aspect ratio) is equal to the ratio S/I.Euhedral plagioclase crystals do not fit either of thesemodels but are close enough for this simple criterion to beused to estimate accurately this aspect of their overallshape.

It is not easy to determine precisely the ratio I/Lfrom intersection data of randomly oriented crystals(Higgins, 2006a). I/L has been estimated from the statis-tical parameters of the intersection length/width

−14

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−10

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−4

0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35

Size (mm) Size (mm)

ln (

popu

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)(m

m−4

)

ln (

popu

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)(m

m−4

)

ln (

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MH-04-05a

MH-04-05b

MH-04-05c

MH-04-06

MH-04-07

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0 5 10 15 20 25 30 35

MH-04-14

MH-04-16

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MH-04-04

MH-04-10

MH-04-12

Kashele GPB Thalghat GPB

Manchar GPB

Data point

Termination of CSD(no smaller or larger crystals)

Size (mm)

(a)

(c)

(b)

Fig. 5. Crystal size distributions in the Kashele (a),Thalghat (b) and Manchar (c) Giant Plagioclase Basalts plotted following the conventionsof Higgins (2006a). All CSDs terminate to the left; that is, there are no megacrysts smaller than 2mm. However, there are very small plagioclasecrystals in the matrix that were not measured in this study.


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distribution (Higgins, 1994; Garrido et al., 2001) and bymodelling (Morgan & Jerram, 2006). However, none ofthese methods give precise values, especially for tabularcrystals (I/L close to one). Morgan & Jerram (2006)developed a method where the whole intersection length/width distribution is fitted to model distributions, thusthey can obtain S/I and I/L. However, the method doesnot take into account the much better precision withwhich the S/I values can be determined, compared withI/L. In this study a more pragmatic approach is usedwith sections of crystals with special orientations(Higgins, 2006a): plagioclase tablets are commonlyflattened along {010} (Fig. 7a). A good cleavage parallelto (010) ensures that the tabular face of plagioclase crystalsis sometimes exposed on broken surfaces (Fig. 7b).This can be used with the observation that crystalsviewed in the (010) plane are equant to establishthat I¼L. Hence only the S/I (or more conveniently I/S)

ratio will be used to characterize the shape of theplagioclase crystals.Although plagioclase in all samples is strongly tabular

the I/S ratio is variable, from 4 to 13 (Fig. 8). All Kasheleand Manchar GPB samples have I/S of 4^7. Two of theThalghat samples also have similar values of I/S, butsample MH-04-10 stands out with I/S¼13. There is aweak correlation between the I/S ratio and the character-istic length of the CSDs.For the purposes of calculating the CSDs an aspect ratio

of S:I:L¼1:5:5 was used for all samples so that they couldbe readily compared. It should be remembered thatchanges in the ratio I/L will change the size scale of theCSD, but different values of S/I will change only the tail-ing corrections (Higgins, 2000).

Crystal orientationThe orientations of crystal outlines were also measured.Orthogonal sections were not measured, hence the true 3Dorientation cannot be determined. The sections were cutparallel to the foliation, therefore the only useful parameteris the dispersion of orientations, which is a measure of the‘quality of foliation’. Here, we used the normalized lengthof the direction vector as the quality parameter (Higgins,2006a): for perfectly aligned crystals it has a maximumvalue of one; massive rocks give a value of zero.The orienta-tion quality of the 10 samples studied here does not appearto correlate significantly with crystal shape (Fig. 9) or anyother textural parameter; however, the sample with thelowest quality foliation, MH-04-10, also has one of thelowest plagioclase contents and the most tabular crystals.

DISCUSS IONEquilibrium crystallization of phasesThe composition of the magma from which the plagioclasemegacrysts of the GPB crystallized is of fundamental inter-est for the calculation of equilibrium crystallization.Ideally, this composition could be determined from meltinclusions in the plagioclase megacrysts; however, this is

16

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Plagioclase (%)

Cha

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ngth

(m

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0

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400

500

800

1000

1200

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Res

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me

(yrs

)R

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e (y

rs)

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Plagioclase (%)

Larg

est i

nter

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ion

(mm

)(a)

(b)

Fig. 6. (a) Characteristic length vs plagioclase abundance. (b)Maximum intersection size vs plagioclase abundance. Residencetimes were calculated for a growth rate of 10�10mm/s.

(010)

(101)(101)

(110) (110)

(001)

(001)

(a) Model plagioclase tablet (b) ‘Giant’ phenocryst

Fig. 7. (a) Ideal shape of a plagioclase crystal. Here, the growth rateof {010} is 0�2 times that of the other faces. This figure was producedusing the programWinXMorph (Kaminsky, 2005). (b) Broken surfaceof a GPB sample with a plagioclase crystal showing the (010) face.Thecrystal is 20mm wide.


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beyond the scope of this paper. Sano et al. (2001) examinedthe compositions of melt inclusions in olivine phenocrystsfrom a regular Deccan basalt that lacked megacrysts.They were able to determine all the more abundant

elements, as well as the water content of the magma,which is essential for modelling crystallization. Weused the mean composition of glass inclusions in theirsample IC15 to model crystallization using the PELE

14

16

0 0.2 0.4 0.6 0.8 1

04

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12

2D aspect ratio

Fre

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0 0.2 0.4 0.6 0.8 1

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0 0.2 0.4 0.6 0.8 1

2D aspect ratio

0 0.2 0.4 0.6 0.8 1

2D aspect ratio

Fre

quen

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ty Kashele GPB Thalghat GPB

Manchar GPB(d)

Blocks-0.2 rounding

(e)Summary

Characteristic length (mm)

Sha

pe I/

S

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1412

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(a)F

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4

0

(c)

(b)

Fig. 8. Intersection shape distributions for samples from (a) Kashele GPB, (b) Thalghat GPB and (c) Manchar GPB.The frequency density isthe number of crystals in an interval of 2D aspect ratios divided by the width of the interval and the total number of crystals (see Appendix).(d) Intersection shape distributions of parallelepiped tablets with a rounding factor of 0�2. (e) Summary of I/S shape ratio vs characteristiclength. These values were determined from the mode of the intersection shape distributions. Schematic cross-sections of crystals with I/S¼ 4and 12 are shown at the left of the diagram.


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program (Boudreau, 1999), which is a version of MELTS(Ghiorso & Sack, 1995). Plagioclase was the first phase tocrystallize when oxygen fugacity was buffered at FMQ(fayalite^magnetite^quartz). The density of this plagio-clase, 2670 kg/m3, is considerably less than that of themagma, 2730 kg/m3, confirming that plagioclase floatsand hence can accumulate at the top of a magma chamber.It is likely that the original magma from which the GPBmegacrysts grew was similar in composition to IC15 meltinclusions and hence plagioclase also crystallized first andfloated in the GPB magma.

Crystal sizeIn igneous petrology we are concerned commonly with thebalance between kinetic and equilibrium processes(Higgins, 2006a). This is expressed by the kinetic processesof nucleation and growth, which are controlled by thedegree of undercooling (or supersaturation) of themagma. If the crystallization driving force is reducedthen the texture of the magma or rock will adjust to mini-mize the overall energy of the system.This can be achievedby coarsening, also known as Ostwald ripening or texturalequilibrium. Mechanical processes can also further modifyboth kinetic and equilibrium textures. Finally, increases inundercooling may rejuvenate kinetic processes. It is notalways easy to distinguish kinetic and equilibrium textures,especially as perfect equilibrium is never achieved.We will start with a simple kinetic model for crystal sizedevelopment.A simple kinetic model for the GPB is crystallization

from a single magma in a chamber that is continuously

filled and drained. Marsh (1988) showed that in such asteady-state system the CSDs are S-type and that themean residence time of a crystal¼ characteristic length/growth rate. The choice of growth rate is clearly veryimportant. Cashman’s (1993) compilation of data for basal-tic systems suggested a growth rate of 10�9mm/s for a cool-ing time of 3 yearsçperhaps typical of thicker lava flows.For a cooling time of 300 years she suggested a growth rateof 10�10mm/s. This is similar to what has been estimatedfor the Palisades sill (Cashman, 1993). Growth rates forplagioclase in the Makaopuhi lava lake were slightlygreater at (3�5^6�5)� 10�10mm/s [data from Cashman &Marsh (1988), recalculated by Higgins (2006a)].Crystallization at greater depths would have occurred atlower degrees of undercooling and hence slow growthrates: if the relationship between cooling time and growthrate continues to be linear then a cooling time of 30 000years would give a growth rate of 10�11mm/s. A plagio-clase growth rate of 10�10mm/s is chosen here as coolingmust have been beneath a lava pile, at the very least, andhence slower than the Makaopuhi lava lake. This shouldprobably be considered a maximum value, as crystalliza-tion may have occurred at greater depths. It should benoted that Sen et al. (2006) used a growth rate of10�9mm/s in their study of the Deccan GPB.If plagioclase megacrysts crystallized in a steady-state

system with a growth rate of 10�10mm/s, then residencetimes vary from 600 to 1500 years (Fig. 6a). Anotherkinetic model involves simple, continuous growth of a crys-tal. For the same growth rate the largest crystal, as esti-mated from the largest intersection, would grow in500^1000 years (Fig. 6b). The two methods correlateloosely, except for two points. The CSD method is morerobust as it depends on the whole population of crystals,whereas the second method depends only on one crystal,the largest.The volume of the magma chamber where the mega-

crysts grew can also be loosely estimated: it equals the resi-dence time multiplied by the refilling rate. If all Deccanmagmas passed through the magma chamber at this timethen the refilling rate equals the eruption rate. The erup-tion rate of a volcanic province equals the volume dividedby the duration of volcanism. In the Deccan boththese parameters are poorly known. The most recent com-pilation of eruption rates (White et al., 2006) proposeda value of 0�9 km3/year for flood basalt provinces, whichwould give a chamber volume of 450^1350 km3.These values would be proportionally lower if magmabypassed the megacryst-bearing magma chamber. Thelower limit is only about twice that of the Skaergaardintrusion, Greenland, recently estimated at 280 km3

(Nielsen, 2004). This intrusion was emplaced beneathflood basalts and has a roof with plagioclase crystalssimilar to those seen in the GPB (Naslund, 1984).

4

05a

05b

05c

6

710

12

14

16

0.0

0.1

0.2

0.3

0.4

0.5

Shape I/S

Orie

ntat

ion

Dis

pers

ion

(qua

lity)

Mas

sive

Fo

liate

d

4 6 8 10 12 14

Fig. 9. Orientation dispersion (quality of foliation) vs crystal shape(I/S). Orientation dispersion varies from zero for massive rocksto 1�0 for parallel crystals. Schematic cross-sections of crystals withI/S¼ 4 and 12 are shown at the bottom of the diagram.


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Larger volume chambers are also possible: Higgins (2005)has proposed that the Sept I“ les intrusion, Canada, with avolume of 35 000 km3 was emplaced beneath flood basalts.It, too, has a roof that contains large, tabular plagioclasecrystals similar to the GPB megacrysts. However, such alarge intrusion would probably produce a distinctive grav-ity and magnetic anomaly, which is not observed in theDeccan. Sen et al. (2006) have proposed that the eruptionduration was only 23000 years, which would give an erup-tion rate of 87 km3/year. The chamber volume would thenexpand to 50 000^130 000 km3, which seems to be unrealis-tically large and would certainly be evident from geophysi-cal measurements.The steady-state and simple growth models presented

above produce a single CSD. Clearly, the different GPBshave different CSDs and hence the situation must be morecomplex. There are a number of processes that can changethe CSD. Compaction of a crystal mush involves loss ofintercrystal fluid, whereas mixing with an aphyricmagma (dilution) increases the amount of intercrystalfluid; hence the two processes are texturally similar. Inneither case is Cl changed, hence the analysis discussedabove is unchanged. On a plot of Cl vs volumetric phaseproportion compaction will displace the points to theright, whereas dilution will produce a vector towards theleft. Within each packet of GPB flows, or even within asingle GPB flow, the variation of Cl exceeds analyticalerror, hence dilution and compaction cannot account forall the textural variation. Clearly, another mechanismmust be invoked that can change the Cl. If the overall crys-tal growth rate increases, then the Cl will also increase.However, we would then expect to see a correlationbetween Cl and the volumetric abundance of plagioclase,which is not observed. Hence, pure kinetic steady-statemodels do not seem to be able to account for texturalobservations. Before we leave pure kinetic models theapproach of Sen et al. (2006) must be discussed.Crystals grow not only in chambers, but also in conduits.

Sen et al. (2006) proposed that plagioclase megacrysts grewin a ‘chicken-wire’ mesh across a conduit, bathed by aquasi-continuous flow of magma. They applied theJohnson^Mehl^Avrami equation and found a value of1580 years, for a growth rate of 10�9mm/s. If they hadused a growth rate of 10�10mm/s, as above, then theywould have found a growth time of 15 800 years. It seemsunlikely that a dyke would have been continuously activefor such a long period of time. Also, this model does notfit the idea of Sen (2001) that the GPBs develop duringtimes of magmatic repose. Finally, it will be shown thatthe textures of the megacrysts do not seem to support sucha model.The lack of small megacrysts in the GPB magmas sug-

gests that the system is returning towards an equilibriumtexture by coarsening. This is the process by which small

crystals dissolve at the same time as larger crystals aregrowing, so that the overall surface energy is minimized(Voorhees, 1992). It occurs when the temperature of thesystem is maintained close to the mineral liquidus. Underthese conditions the nucleation rate is zero, but the growthrate is significant. Higgins (1998) showed that coarseningfollowing the Communicating Neighbours model ofDehoff (1991) will lead to increases in characteristiclength. If the system is allowed to be open (i.e. material isadded from circulating fluids), then the volumetric phaseproportion can also increase. The phase proportion canalso increase if enthalpy is withdrawn from the system atlow degrees of undercooling. Either process will give adiagonal vector on a diagram of characteristic length vsvolumetric phase abundance. The distribution of datapoints in Fig. 6 suggests that we are seeing the combinedeffect of coarsening of a megacryst-rich magma andmixing with an aphyric magma.We will now consider theshape of the megacrysts and what it can tell us about thecrystallization environment.

Crystal shapeThe shape of plagioclase crystals has been determinedquantitatively in a number of studies, as an estimate ofshape is necessary for the calculation of CSDs from inter-section lengths. Higgins (2006a) has summarized thesestudies and concluded that tabular crystals occur in twoenvironments: as microlites in lavas and as much largercrystals in laminated cumulate rocks such as anorthositesand troctolites. He proposed that tabular growth occurs

Chemical potential gradient

Fac

e gr

owth

rat

e

Rate{010}

Rate{001}

(010

)

(001)

(010

)

(001)

Fig. 10. Schematic illustration of the response of plagioclase crystalfaces to the chemical potential gradient around the growing crystal.It is assumed that the growth rates of the {110}, {101} and {1 �10}faces are the same as that of the {001} faces. The overall size of theillustrated crystal reflects both growth rates and crystallization time.


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in situations where there is a high chemical potential gra-dient around the growing crystal (Fig. 10). This is becausethe growth rate of the {010} faces responds less than theother faces to increases in the chemical potential gradient.It should be noted that experimental studies of plagioclasemorphology vs growth have been primarily concernedwith growth forms at large degrees of undercooling, suchas the transitions from laths to skeletal forms to spherulites(Lofgren, 1974). Here we are concerned with variation ofthe shape of euhedral crystals, which form at much lowerdegrees of undercooling where time constraints limitexperiments.The chemical potential gradient around a growing crys-

tal is controlled by the addition of crystal nutrients and theremoval of unwanted components.These occur in responseto diffusion and advection (relative movement between thecrystal and the growing medium). The ratio of the timescales of mass transfer by advection and diffusion iscalled the mass transfer Peclet number (PeMT) and isdefined as

PeMT ¼vl

D

where v is the velocity of the growing medium with respectto the crystal, l is the length scale (size of crystal), and D isthe chemical diffusivity (diffusion coefficient). If the Pecletnumber is high then advection will renew the supply ofnutrients around the crystal and the chemical potentialgradient will be greater than that which is possible by dif-fusion alone. In the case of microlites a high growth rateensures a high Peclet number because the ends of the crys-tal outpace the growth of the depleted zone around thecrystal. Mechanical movement of magma (‘stirring’) byconvection currents can also ensure a high Peclet numberand has been proposed as an explanation for tabular crys-tals (Kouchi et al., 1986; Higgins, 1991). Such conditionsmay occur near the margins of magma chambers, wherethere is a significant velocity gradient.The mass transfer Peclet number in a magma chamber

can be estimated for a basaltic magma under typical con-ditions. Chemical diffusivity varies with magma composi-tion, temperature and element (Chakraborty, 1995). Orderof magnitude values for a mafic magma at 13008C are10�11m2/s for network-modifiers such as Ca and Mg,10�9 m2/s for Na and K, and 10�12m2/s for network formerssuch as Si. If we take a typical value of D of 10�11m2/s, alength scale equal to the length of a typical crystal, 10mm,then a convection velocity of 1mm/s would give a Pecletnumber of 106. Hence, advective transport is much moreimportant than diffusive transport where crystals areexposed to convection currents.The thermal Peclet number (PeT) can also be used to

determine if transport of latent heat away from a growingcrystal is controlled by advection or diffusion. Here theequation is

PeT ¼vl�c

k

where v and l are defined as before, � is the density, c is thespecific heat and k is the thermal conductivity. If the ther-mal Peclet number is high then latent heat will be removedby advection rather than conduction. As before, the ther-mal Peclet number can be estimated for a basaltic magmaunder typical conditions where the density is 2730 kg/m3

(from calculations using PELE; see section 4�1), the ther-mal conductivity is 1�4W/m per K (Horai & Susaki, 1989)and the specific heat is 840 J/kg per K (Clarke, 1966).A crystal size of 10mm and an advection velocity of1mm/s (as above) would give a thermal Peclet numberof 16. Hence, under these conditions heat is removed byadvection and not conduction.The shape of the crystals can therefore give an idea of

the environment of crystallization. The most tabularplagioclase crystals must have grown in an environmentwith the strongest advection, that is shearing or stirring.

Crystal orientation and clusteringIf the crystal mush was transported from the magmachamber by laminar flow then the relative orientation ofthe crystals could be preserved. We would then expectthat well-foliated samples formed in a strongly shearedenvironment and hence would have the most tabularcrystals. The absence of a significant correlation betweencrystal shape and quality of foliation suggests thattransport was turbulent.Although quantitative measures of the overall orienta-

tion of crystals do not appear to be very helpful, qualitativeobservations of short-range order may help clarify the pet-rogenesis of the GPB. In many samples tabular plagioclaseoccurs as clusters of sub-parallel crystals. This structurehas been termed synneusis (‘swimming together’) and isconsidered to form during flow (Schwindinger, 1999).The driving force is the minimization of the surfaceenergy of the crystal aggregate (Ikeda et al., 2002), as inthe processes of coarsening, evidenced here by the shapeof the CSDs.Some samples have radiating clusters of crystals, rather

than sub-parallel aggregates (e.g. MH-04-12; Fig. 4). Itcould be significant that these samples are poorly foliatedand have CSDs with the lowest slopes.This suggests that insome samples a later phase of crystal growth may haveoccurred rapidly in a static environment.We will now dis-cuss the occurrence of plagioclase megacrysts in otherigneous rocks, to allow the creation of a plausible modelfor the origin of GPB.

Plagioclase megacrysts in other volcanicand plutonic rocksBasaltic lavas with plagioclase megacrysts similar to theGPBs are uncommon but widespread. They occur in


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oceanic basalts in a wide variety of settings: spreadingridges, intraplate hotspots, aseismic ridges and fracturezones (Cullen et al., 1989). Plagioclase megacrysts occur incertain phases of the Surtsey volcano and sporadically inthe Eldfel lavas (Furman et al., 1991). They are also welldocumented on the northern Galapagos Islands (Cullenet al., 1989). Here, the plagioclase megacrysts are heteroge-neously distributed and have a texture very similar to theGPBs: plagioclase crystals are tabular and occur in sub-parallel glomerocrysts. The cores of the megacrysts aremore calcic than the rims, which are in equilibrium withthe host magma, as in the GPBs.Many mafic intrusions contain layers or zones of rocks

rich in plagioclase, parts of which may be laminated. Forinstance, the upper border zone of the Skaergaard intru-sion, Greenland, is partly composed of well-laminatedleucogabbro (Naslund, 1984). The position of this unitnecessitates that it formed by accumulation of plagioclaseby flotation. This reflects the greater density of evolved,iron-rich, magmas with respect to plagioclase (Scoates,2000). The upper part of the Sept I“ les intrusive suite,Canada, is dominated by anorthosite, some of which iswell laminated with tabular crystals (Higgins, 1991).Recently, Higgins (2005) has proposed that these rockswere also formed by flotation of plagioclase. Partial disrup-tion of these plagioclase cumulates by granitic fractionatesshows that such accumulations remained very loose andpoorly cemented, even at high crystal concentrations.In both cases convection currents would have broughtplagioclase primocrysts up from lower levels and their lowdensity would have allowed them to remain in the upperborder zone. These currents would have produced a localenvironment with high Peclet numbers and hence the pla-gioclase crystals would have tended to grow with a tabularhabit. It is now possible to integrate field data, quantitativetextural measurements and the occurrence of megacrystsin other environments in the form of an emplacementmodel for the GPBs.

Emplacement model of GPB flowsWe propose that a cycle of magmatism started with theeruption of ‘normal’ basalts, with few or no megacrysts(Fig. 11a). Such magmas were derived from the mantle,but in most cases must have been stored deep in the crust,where there was differentiation and possibly contaminationby the lower crust. No large magma chambers are envi-saged, but more localized swellings in the conduits.Magmas rise to the surface along faults, probably relatedto rifting (Hooper, 1990). Such a cycle may end when themagma is unable to penetrate the lava pile or perhapswhen the production rate of magma wanes. At this pointerosion of fresh lavas may produce a regolith, nowpreserved as the ‘red boles’.The next stage of the cycle starts with plutonism and

may continue directly without a pause. Magmatism

continues, perhaps at a lower rate, but the magma nowstarts to accumulate at depth. The most likely place forthis is along the interface between the lava pile and thebasement, where there is likely to be a density contrast(Fig. 11b). The magma may wedge its way out between thebase of the lava pile and the basement (right side ofFig. 11b), or tectonic forces may produce a rift (centralpart of Fig. 11b). The chamber will be filled graduallywith hot, new magma, ensuring that vigorous convectionoccurs. Crystallization of plagioclase and mafic mineralsproduces dense, iron-rich magmas and basal cumulates.Plagioclase will float if its density is less than that of themagma, but the density difference is not usually large.Plagioclase crystals may nucleate and start to grow atdepth and be wafted by convective currents up to the roof,where some crystals will tend to remain, or they maynucleate and grow at the top of the chamber. Passage ofconvection currents will keep the crystals bathed in hotmagma, such that they remain close to their liquidus tem-perature and coarsen. Such currents will also ensure a highPeclet number environment conducive to the crystalliza-tion of tabular crystals. The upper border zone of themagma chamber will comprise a loose crystal mush thatis easily remobilized. Mafic minerals will be denser thanthe magma and will continue to accumulate along thebase of the magma chamber. This situation could continueuntil the magma has solidified completely as a pluton, or isdisrupted by tectonic forces.The next stage is the transport of the megacryst-bearing

magma to the surface. Reopening of a channel from thepartially solidified magma chamber to the surface mightresult from rejuvenation of existing faults, or by the initia-tion of new faults (Fig. 11c). Once a conduit has formedmagma will be drained from the chamber and feed flowsof GPB basalts. The magma drawn from the uppermostlayer will be rich in megacrysts and may mix turbulentlywith more evolved magmas devoid of crystals drawn fromlower levels of the magma chamber. Some magma maycontinue to crystallize in higher-level static staging areas,producing radiating clusters of crystals. Once magma isdrained from the chamber the cycle of normal basaltflows may recommence.Sen et al. (2006) have proposed two models for the origin

of the GPB, the first of which somewhat resembles the modelproposed above. In their first model the plagioclase mega-crysts form statically on the walls of a magma chamber inthe advancing solidification front (Marsh, 1996). However,we have suggested above that the plagioclase megacrystspreserve evidence of accumulation by flotation and crystal-lization in a dynamic regime, neither of which processes isincluded in the model of Sen et al. Their second model,which they developed quantitatively, proposes that themegacrysts grew in a magma conduit, where they formed a‘chicken-wire’ network. Magma flowing along the conduit


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fed the crystals, accounting for their great size. Subsequentflow disrupted the network and incorporated the megacrystsinto a low-crystallinity magma. This model has the advan-tage that the crystals grew in a dynamic environment, butcannot produce the observed textures: there is no reasonwhy plagioclase should occur as groups of aligned crystals.It also seems unlikely that a chicken-wire network couldexist for the 1538 years that Sen et al. (2006) calculated.Finally, the theory does not account for the presence ofsimilar plagioclase crystals at the top of sub-volcanicmagma chambers such as Skaergaard.

CONCLUSIONSPlagioclase megacrystic basalts are a widespread, but vari-able component of flood basalt provinces. They should beviewed as evidence of magma storage in the crust.Plagioclase is the most common mineral to occur asmegacrysts in these rocks because it is the only mineralthat can float in evolved basaltic magmas. Hence, thepresence of plagioclase megacrysts indicates thatsub-volcanic magma chambers were an important compo-nent of the magmatism. The Skaergaard intrusion,Greenland, is probably a good model for the magma

Magma

conduit

(a) Eruption of ‘normal’ flood basalts

Faults

Magma

conduit

Magma chamber

(b) Surface weathering and sub-volcanic plutonism

(c) Draining of magma chamber and eruption of Giant Phenocryst Basalts

Red bole

Hot, risingmagma

Coarsening

Magma

conduit

Magma chamber Plagioclase

flotation

cumulates

Plagioclase

flotation

cumulates

Mafic cumulates

Fig. 11. Model for the formation of Giant Plagioclase Basalt flows, Deccan.


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chamber in which the megacrysts of the GPB grew. Largersub-volcanic intrusions, such as Sept I“ les, Canada, couldproduce the megacrysts, but would produce a significantgeophysical anomaly, which is not observed.Quantitative studies of plagioclase megacrysts can pro-

vide information on the duration of crystallization andpossibly the size of such magma chambers, but cannotgive an idea of the total eruption duration, as has beenproposed by Sen et al. (2006). This is because there isno evidence that all magmas have passed through thesame magma chamber. It is likely that sub-volcanicmagma chambers existed intermittently, just before theGPBs formed. At other times magma passed almostunchanged from the lower crust or the mantle source tothe surface.

ACKNOWLEDGEMENTSThis research was partly funded by operating grants fromthe Natural Science and Engineering Research Council ofCanada to M.D.H.We would like to thank Hazel Jenkins,Melroy Borges, Gautam Sen and Ayaz Alam. Themanuscript was improved by the insightful reviews ofDougal Jerram, James Scoates and Dick Naslund.

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Voorhees, P. W. (1992). Ostwald ripening of two-phase mixtures.Annual Review of Materials Science 22, 197^215.

White, S. M., Crisp, J. A. & Spera, F. J. (2006). Long-term volumetriceruption rates and magma budgets. Geochemistry, Geophysics,

Geosystems 7, Q03010.

APPENDIX : CALCULAT ION OFFREQUENCY DENSITYThe modal value of the 2D aspect ratio is not easy to deter-mine precisely for strongly skewed distributions, as we havehere. The conventional approach is to use bins of fixedwidth and compile a simple frequency diagram(e.g. Higgins, 1994). If too many bins are used then the fre-quency diagram is very irregular, whereas too few do notmake it possible to define exactly the modal value. A newdiagram of frequency density is proposed here, which isanalogous to simple population density diagrams forCSDs (Higgins, 2006a). The 2D aspect ratios of the inter-sections (� values) are sorted into ascending order anddivided into a number of bins, each with approximatelythe same number of intersections. Ten bins were used here,so each bin had 20^40 intersections. The frequency densityis the number of intersections in each bin divided by thedifference between the upper and lower 2D aspect ratiobounds and the total number of intersections. This dia-gram has the advantage that the 2D aspect ratio bins arenarrowest around the modal value and hence can define itprecisely (Fig. A1). An example of the calculation is showninTable A1.

0

1

2

3

4

5

6

0 0.2 0.4 0.6 0.8 1

Fre

quen

cy d

ensi

ty

Fixed width bins

Fixed number bins

2D aspect ratio

Fig. A1. Comparison of 2D aspect ratio diagrams compiled withfixed width and fixed number bins. The peak is defined by two inter-vals for fixed width bins but six for fixed number bins.


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Table A1: Calculation of frequency density for sample MH-04-04

� values of crystal intersections � mean � range Number of

intersections

Frequency

density

0�0371, 0�0427, 0�0566, 0�0594, 0�0737 0�0883� 0�0371¼ 0�0512 28 2�186

0�0631, 0�0653, 0�0664, 0�0676,

0�0687, 0�0732, 0�0736, 0�0745,

0�0745, 0�0761, 0�0765, 0�0779,

0�0794, 0�0795, 0�0801, 0�0822,

0�0831, 0�0842, 0�0844, 0�0862,

0�0872, 0�0877, 0�0880, 0�0883

0�0892, 0�0914, 0�0942, 0�0954, 0�1025 0�1100� 0�0892¼ 0�0208 28 4�568

0�0973, 0�0982, 0�0987, 0�0992,

0�1005, 0�1010, 0�1014, 0�1015,

0�1020, 0�1029, 0�1034, 0�1042,

0�1042, 0�1050, 0�1051, 0�1056,

0�1059, 0�1064, 0�1077, 0�1087,

0�1097, 0�1105, 0�1107, 0�1100

. . . . . . . . . . . . . . .

� is the 2D aspect ratio of measured crystal intersections. Ten bins of 28 or 29intersections made the total of 284 intersections.


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nature of sub-volcanic magma chambers, deccan province, india: evidence from quantitativetextural...

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