2. DOWNHOLE VARIATIONS IN GRAIN SIZE AT HOLE 504B

Erzinger, J., Becker, K., Dick, H.J.B., and Stokking, L.B. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 137/140

2. DOWNHOLE VARIATIONS IN GRAIN SIZE AT HOLE 504B: IMPLICATIONS FOR RIFTINGEPISODES AT MID-OCEAN RIDGES1

Susumu Umino2

ABSTRACT

The maximum grain sizes of plagioclase and magnetite in the groundmass of the sheeted dike complex drilled at Hole 504Bhave been measured. Downhole variations through a 440-m-long section show a crude zig-zag pattern consisting of a gradualdecrease or increase followed by an abrupt jump. The gradual decrease or increase in grain size extends over many lithologic units,and hence, does not reflect variations in grain size within a single dike. Such a zig-zag pattern is well explained by grain-sizevariations through multiple dikes. By using the observed inclination of sheeted dikes of 81° ± 2.5°, thickness of the multiple dikesvaries from 0.7 to 8.5 m and averages to 4 ± 1 m. The average thickness of individual dikes forming multiple dikes is 0.8 m. Weexpect such multiple dikes to be formed during rifting events beneath mid-oceanic spreading ridges. If the average expansion atrifting episodes is twice as wide as the average width of the multiple dike units, the full spreading rate of 7.2 cm/yr of Cocos Ridgegives 112 ± 33 yr for a time interval of the rifting.

A simple one-dimensional conductive cooling model is applied to solidification of multiple dikes. Numerical simulations showthat the grain-size variations observed through the drill hole are more consistent with a model where a new injection of a dikeoccurs periodically with a constant time interval rather than one where the next dike intrudes just after the solidification of theprevious one. Grain-size variations within simple dikes from Iritono, Japan, and those for Makaopuhi lava lake, Hawaii, show thatsquare root of crystallization time is linearly correlated with the logarithm of plagioclase size. By using an empirically derivedrelationship between these two variables, the variations of plagioclase size through Hole 504B are directly compared with thecalculated times for crystallization. Each rifting episode at the Costa Rica Rift lasts for several years, and periodic injection of anew dike occurs into the center of a previously solidified multiple dike at time intervals varying from 1 to 12 months.

INTRODUCTION

The majority of Earth's magmas are produced through astheno-spheric upwelling and partial melting processes beneath divergentplate boundaries, and hence, mid-ocean ridges are the loci of intensescientific research. Geophysical, structural, and petrological studies onmid-ocean spreading centers and ophiolites have revealed that mostmagmas solidify in the crust without erupting. At deeper levels in thecrust, magmas solidify as cumulate layers, sheets, or pods, while atshallower levels, they form sheeted dike swarms. Thus, intrusion ofdikes plays a fundamental role in crustal accretion at spreading centers.Recent progress in geomorphological studies on volcanic landformsand tectonic features suggests that volcanic processes similar to thoseat mid-ocean ridges also operate at Iceland; volcanic landforms com-prise major volcanic edifices probably fed through long, continuousfissures, and smaller volcanic cones covering the former edifices(Kong et al., 1989; Smith and Cann, 1992). Historical andprehistoricalrecords of earthquakes and land deformations, as well as volcanic erup-tions in Iceland, suggest that such extensive lava shields are formed bylarge eruptions taking place at intervals of several hundred year, whilecentral volcanoes are foci of magma intrusions at rifting episodes every100-150 yr (Sigurdsson and Sparks, 1978). Seismological and geo-detic data on a recent rifting episode at Krafla, northern Iceland, showthat plate accretion is accomplished by repeated dike intrusions from alocal magma source through a narrow rift zone, followed by crustalmovements during post-rifting stress relaxation (Tryggvason, 1984;Foulger et al., 1992). Rifting at mid-oceanic ridges is likely to occur ina way similar to that in Iceland. However, deep oceans exceeding 3000m prohibits direct observation of volcanic eruptions and shallow intru-sive events at mid-oceanic spreading centers.

The previous Ocean Drilling Program (ODP) Legs 83 and 111deepened Hole 504B through the lava/dike transition zone down to

1 Erzinger, J., Becker, K., Dick, H.J.B., and Stokking, L.B. (Eds.), 1995. Proc. ODP,Sci. Results, 137/140: College Station, TX (Ocean Drilling Program).

2 Institute of Geosciences, Shizuoka University, Ohya 836, Shizuoka 422, Japan.

the sheeted dikes (Adamson, 1985; Becker, Sakai, et al., 1988). ODPLeg 140 further deepened Hole 504B to 2000.4 mbsf and cored 47.69m of sheeted dikes in a section with a total length of 378.9 m. Throughthese sections of the drill hole, dikes are found to be inclined north-ward to the ridge axis with inclinations steepening downhole (Ship-board Scientific Party, 1992). On the basis of grain-size variation inthe sheeted dikes, I estimate time intervals and duration of dike intru-sions, and discuss the 5.9-Ma rifting events at Costa Rica Rift.

SHEETED DIKES IN HOLE 504B

The first appearance of dikes in Hole 504B is at 846 mbsf, wherea chilled contact against upper pillow lava is observed. The numberof dikes increases below 960 mbsf and the last pillow is recovered at1055 mbsf. Below this point, all the rocks recovered are predomi-nantly massive and are interpreted to be sheeted dikes (Adamson,1985; Becker, Sakai, et al., 1988; Shipboard Scientific Party, 1992).The upper part of the dike section is composed of coarse-grainedbasalt or fine-grained diabase; however, the lower section drilledduring Leg 140 is composed of fine- to coarse-grained diabase ex-cluding the chilled margins. These dikes are also steeply inclinednorthward to the ridge axis of Costa Rica Rift. Intrusive contactsobserved on Leg 83 dip 50°-60° from horizontal (Adamson, 1985),whereas Becker, Sakai, et al. (1988) estimated the average dike dip tobe 68°, with a significant proportion of the dikes measuring 82° basedon paleomagnetic measurements. They inferred a tilting of the olderdikes before the intrusion of the steeper dipping dikes. After expand-ing the recovered cores to fit the cored interval, widths of dikes drilledduring Legs 83 and 111 are calculated, assuming that each lithologicunit forms a single dike (Fig. 1). Similar calculations are made on thesheeted dikes of Leg 140, using an average dip of 81.5° on the basisof the recovered chilled contacts.

The most frequent dike width is less than 0.2 m, which is far lessthan that of 0.4-0.6 m estimated by Becker, Sakai, et al. (1988). Asmall peak between 1.2 and 1.6 m is in the range of another peak fre-quency of the dike width of 1.2-1.8 m by the previous study. Because

19

S. UMINO

I

25

20

15

10

Leg 1400.83 m

Leg 83 and 1111.49 m

Dike width (m)

Figure 1. Frequency of dike widths drilled during Legs 83 and 111 (Becker, Sakai, et al., 1988) and Leg 140. The average dike width is 1.49 m for Legs 83 and111, whereas it is 0.83 m for Leg 140.

unit boundaries are placed where an abrupt change in lithology isobserved, some boundaries may separate a finer grained margin anda coarser grained core of the same dike. The frequency of smallerdikes may be overestimated to some extent.

PETROGRAPHY

Except for chilled margins of the dikes, all samples are medium-to coarse-grained diabase. Phenocrysts are of plagioclase, olivine,and augite, but some samples lack olivine or augite. Cr-spinel is occa-sionally included in plagioclase phenocrysts and less commonly inolivine; it rarely occurs as discrete, large crystals in the groundmass.

Groundmass plagioclase is generally elongated and frequentlyacicular in shape. Some are skeletal and have hollow cores. Someplagioclases have euhedral, tabular cores rimmed with skeletal over-growths, which probably began to crystallize as microphenocrysts.There are, thus, two or three stages of nucleation and growth ofplagioclase crystals. Plagioclase without any apparent zonation underan optical microscope probably crystallized at the latest stage and areidentified as a groundmass mineral. These crystals often broadentoward one end or both and sometimes are curved, subophiticallyenclosed in groundmass clinopyroxene. Secondary sulfide mineralsalso occur in the groundmass, but these have equant, euhedral shapeswith clear outlines, and can be readily distinguished under transmittedlight from magnetites which is generally decomposed to an aggregateof skeletal ilmenite and titanite.

All the analyzed samples have whole-rock compositions of normalmid-ocean-ridge basalts (N-MORBs) but are in the most depletedextremity in incompatible elements (Shipboard Scientific Party, 1992).They are moderately evolved and have limited ranges in MgO from 7.7to 10.1 wt% and in Mg/(Mg + Fe) ratio from 0.61 to 0.72. Chemicalvariations within individual lithologic units exceed more than half ofthe spectrum of the chemical variations of the whole sections.

DOWNHOLE GRAIN-SIZE VARIATIONIN HOLE 504B

The maximum grain size of plagioclase and magnetite in thegroundmass was measured under a microscope as described in Ship-

board Scientific Party (1992). Twelve circular areas of approximately23.3 mm2 were chosen from each thin section. Selection of measuredareas was made so as not to overlap each other, and to minimize thearea occupied by phenocrysts. The length and width of the largestcrystals of plagioclase and magnetite were measured. The longestextensions of plagioclase and magnetite are measured as the length,and the widths are measured perpendicular to elongation at the widestportion of the crystals. Tails attached to some euhedral plagioclase arenot measured. Measurement was repeated in even' circle, and thelargest and smallest values among the 12 measurements were ex-cluded; the remaining 10 values were then averaged as the maximumgrain size. Very fine-grained or aphanitic samples taken from chilledmargins of dikes were not measured. The results of the measurementare given in Table 1.

Figure 2 shows crystal size distribution (CSD; Marsh, 1988) of arepresentative diabase sample in order to assess the maximum grainsize measured as such. Although the data are rather scattered, the CSDshows an almost linear variation as is often observed in volcanic rocks(Cashman and Marsh, 1988). Because of a limited size of a thinsection of an individual specimen, the number of crystals measured issmaller than usually used for crystal size distribution plots, which donot give an accurate number for larger crystals. However, the changein the slope reflects change in growth rate or crystallization time ofplagioclase crystals and the inflection at length = 0.16 cm correspondsto the maximum length of plagioclase probably crystallized duringthe groundmass stage. This value is almost equal to the maximumplagioclase length of 0.14 cm measured as described above.

Lengths of plagioclase vs. magnetite, and diameters of circles withequivalent areas (EQD) to the plagioclase and magnetite crystals, areplotted in Figures 3A and 3B. The lengths and EQD of plagioclaseand magnetite are roughly correlated.

Variation in the grain size of plagioclase and magnetite with depthfrom 894 to 2000 mbsf for Leg 140, together with the previous Legs83 and 111, are plotted in Figure 4. The maximum plagioclase lengthgradually decreases through the lava/sheeted dikes transition zonedown to 1360 mbsf, although EQD remains almost constant. Thismeans that plagioclase crystals are very narrow and elongated throughthe transition zone and in the shallower parts of the dike swarm,presumably due to rapid crystal growth under a larger rate of cooling.

20

DOWNHOLE VARIATIONS IN GRAIN SIZE

5.5

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0.05 0.1 0.15 0.2 0.25

Plagioclase length (cm)

0.3 0.35

Figure 2. Plagioclase length distribution of dike Sample 140-504B-227R-1, 83-85 cm. 2021 plagioclase crystals in 163 mm2 of a thin section were measured, andthe number of crystals per unit length per unit area were raised to the 3/2 power as described in Cashman and Marsh (1988). The regression line for the data pointswith length smaller than 0.16 cm has the y-intercept of 1.49 × 105, and the slope is-16.08 cm"1. The maximum plagioclase length, determined as described in thetext, is 0.14 cm, which is slightly smaller than the inflection of the slope at length = 0.16 cm.

Ei *

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Plagioclase length (mm)

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0 0.1 0.2 0.3 0.4 0.5 0.6Plagioclase EQD (mm)

0.7 0.8

Figure 3. Correlation between sizes of groundmass plagioclase and magnetite. A. Plot of the maximum lengths. B. Plot of diameters with equivalent area (EQD)to the crystals.

The most noticeable feature shown by the grain-size variationbelow 1570 mbsf is a crude zig-zag pattern, which is best shown bythe variation in the maximum plagioclase length (Fig. 5A, -B). Themaximum plagioclase length consists of gradual decreases and abruptjumps downhole from 1570 to 1750 mbsf. Such a systematic variationis not observed deeper, but there are some jumps in the grain size whichdisrupt gradual increases or decreases. As shown in Figure 5B, the

zig-zag pattern is an inter-unit variation except that from 1709 to 1718mbsf, which belongs to a single unit 232.

Grain size in a dike is controlled by the melt composition (andhence, the phase relations of crystallizing phases), the cooling rate andthe temperature contrast between the magma and the wall rock. Thestudied samples have similar chemical compositions (SiC>2 50 ± 1wt%), and the growth rate of plagioclase is almost constant in the

21

S. UMINO

800

1000

1200

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2000

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Grain size (mm)

Figure 4. Downhole variation in the size of plagioclase and magnetite in thegroundmass with depth below seafloor for the dikes near the base of theextrusive layers and through the sheeted dike sections drilled on Legs 83, 111,and 140. Plagioclase length = solid circles; plagioclase EQD = open circles;magnetite length = solid triangles; magnetite EQD = open diamonds.

compositional ranges from basaltic and andesitic magmas (Cashman,1990). The cooling rate of a dike is a function of the melt composition,the temperature contrast and the width of the dike. Figure 6 is a plotof the maximum lengths of plagioclase and magnetite vs. dike width.The grain size does not show any systematic increase with dike width.The temperature of the wall rocks at the time of dike injection in-creases downward, so that the coarsest sample from each lithologicunit should ideally increase in the grain size with the depth. However,the inter-unit variations of the grain size exhibit a zig-zag patternconsisting of steady increases or decreases and abrupt jumps. Thus,the downhole variation in the grain size is apparently not due to thedifference in the melt compositions, dike width, nor the rise in thewall rock temperature.

The shipboard scientific party attributed these grain-size varia-tions to those arising through a series of cross-cutting multiple dikes(Shipboard Scientific Party, 1992). The existence of multiple dikes isinferred by a few contacts between adjacent dikes and within-unitgrain size variations, which show only one-way chilling and coarsen-ing toward the other direction. Multiple dikes are well documented inthe sheeted dikes of some ophiolites (e.g., Troodos Ophiolite; Baragaret al., 1990) and continental flood basalt provinces (Gautneb andGudmundsson, 1992), which were formed under extensional regionalstress fields. A multiple dike is formed when new batches of magmaintrude into the center of the previous dike. A few chilled contactsrecovered on this leg are either sharp, rigid, or irregular and wavy,indicating that later dikes are emplaced at or below the solidus tem-perature. Some cores contain deformed cognate inclusions apparentlyderived from semisolidified wall rocks, but such cores are limited tocoarse-grained central portions of multiple dike units. This suggeststhat repeated injection of dikes would have, in effect, thermally insu-lated and reduced the cooling rate of the later emplaced dikes. As a

result, the multiple dikes are coarser grained toward the center of thedike units and form apparent single cooling units (Fig. 7). The steadyincreases and decreases in the inter-unit grain size are variationsthrough such multiple dikes, which are interrupted by another set ofmultiple dike units, resulting in abrupt jumps in the grain size. Thisindicates that the site of dike intrusion jumps from time to time and anew injection of dikes sometimes disrupts or destructs older multipledike units. Thus, the downhole sections do not always have sets ofcomplete multiple dikes but only aggregates of partly destroyed,incomplete multiple dike units.

Through the 430-m-long sections drilled during Legs 137 and140, at least 16 cooling units are identified, on the basis of jumps andcontinuity in grain size (i.e., decrease in the maximum plagioclaselength is gradual from lithologic Units 193 to 211, but interrupted byan abrupt jump at Unit 213). Although there is no measurement on athin section of the lithologic Unit 212, the measurement on the coredsample under a binocular microscope show no abrupt jump in plagio-clase size. Therefore, the cooling unit boundary is defined here. FromUnit 213 to 228, the plagioclase length again appears to decreasecontinuously. However, Units 214-217 are so fine grained that theyare not measured. Thus, Unit 213 is not included in the same coolingunit as the lower units. As already mentioned, whole-rock composi-tions of the cored samples are moderately uniform and there is noapparent difference between the cooling units. Among the 16 coolingunits, only the fifth and the tenth units from the top show well-definedgrain-size variations (Table 2). Cooling Unit 5 consists of six litho-logic units, although the grain size of lithologic Unit 225 is notdetermined (Fig. 8A). Within-unit variations are observed in Units223 and 227; however, the largest size in each unit steadily increasesfrom Unit 227 to 222. Variations in the cooling Unit 10 is not so goodas in the cooling Unit 5 (Fig. 8B). Unit 242 is a very fine-grained rockwhich is apparently taken from a chilled margin. Unit 243 is alsosmaller than neighboring units on both sides. This may be due to aproblem in sampling. The sample may not have been taken from thecoarsest part of the Unit 243. Besides these two, the largest grain sizein each unit gradually increases from Unit 239 to 244.

MODELING OF SOLIDIFICATION OF MULTIPLEDIKES

To simulate solidification of multiple dikes, a numerical approachwas made to a simple one-dimensional conductive cooling model. Iassume that magma is emplaced and solidifies at a constant tempera-ture Tm, and latent heat is released at this temperature. The host rockhas the initial temperature ‰, which is modified by the injection ofmultiple dikes. Because almost all samples have phenocrysts of pla-gioclase + augite ± olivine and crystallized plagioclase and augite asthe groundmass minerals, they are already saturated with the lattertwo phases at the time of emplacement. The magma continues tocrystallize until the solidification front passes through into the interiorof the dike. Thus, the time required for crystallization is equal to thatfor solidification. In the following calculations, Tm is taken to be1000°C, and latent heat of crystallization and heat capacity are as-sumed to be 320 kJ/kg and 1.2 kJ/kg × °K, respectively (Turcotte andSchubert, 1982). The time for crystallization calculated as such tendsto be underestimated than a model including kinetic effects and thedeviation becomes larger as the wall rock temperature is closer to thatof the magma (Spohn et al., 1988). The crystallization time may beunderestimated by up to 35% when TJTm = 0.35. However, as will bediscussed later, TJTm of the Hole 504B dikes is approximately 0.25and the deviation from the model of Spohn et al. is not significant. Theamount of underestimation will be only on the order of 10%.

For crystallization of a simple dike, this is the same as the problemfor which Stefan first obtained the solution. Figure 9 illustrates tem-perature profiles through a simple dike and an adjacent wall rock. Ina simple dike, the first 1 m solidifies in 10 days, but it takes 40 daysfor solidification of 2 m, and 91 days for 3 m.

I


A B1550r 190

210

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230

250

270

2050

3m •*•

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Figure 5. Downhole variation in the size of plagioclase in the

0 0.5 1 1.5 2Plagioclase length (mm)

0.5 1.5 2 groundmass drilled in Legs 137 and 140, plotted against depth (A)> and lithologic units (B). Symbols are as in Figure 4. Cooling units

Plagioclase length (mm) . * ,Lα σ v are shown by curves on (A).

1.60

1.40

1.20

1.00

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0.40

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Dike width (m)

2.5

Figure 6. The maximum plagioclase (solid squares) and magnetite (open squares) lengths plotted against width of dikes. The crystal length is rather scattered anddoes not show any systematic correlation with dike width except for thinner than 0.5 m.

Because dikes follow the pathway of least work, the center of theprevious dike would be preferred for thermal reasons. Therefore, Iassume that new pulses of magmas are injected at the center of theprevious dikes. Crystallization of the second dike is different from thefirst one because the latter is now a part of the wall rock which isheated up by the second injection, and the temperature distributionthrough the new wall rock differs from the initial temperature profilethrough the original wall rock. As the injection is repeated, the timerequired for the crystallization for the identical thickness of the laterdikes is prolonged compared to that for the first one. However, thelater dikes do not always take longer time to crystallized than the

previous one because the crystallization time of the individual dikeswithin a multiple dike unit depends on the temperature profile whenthe dike was emplaced.

For the simulation of multiple injections, two cases are consid-ered: In the first case, new magma injection occurs when the previousdike has solidified and cooled to a constant temperature slightlybelow the solidus. Some chilled contacts recovered on Leg 140 showplastic deformations and engulfments of the host dikes. Such featuressuggest that the new injection occurs before the complete solidifica-tion or just after the solidification of the old dikes when the wall rockis still plastic. Figures 10A and 10B are the results of solidification of

23

S. UMINO

Table 1. Results of grain size measurements from Hole 504B.

Core, section,interval (cm)

83-504B-77-2, 123-12678-2,79-8197-2, 26-29118,29-33127-1,50-53129-1,35-39130R-2, 9-12132-1,96-100

111-504B-142R-1,6-8142R-2, 57-59143R-1, 105-107145R-1,96-98145R-3, 44^16147R-1,39-41147R-1,96-98147R-2, 44-46

137-504B-173R-1,7-8173R-1, 103-106174R-1,50-53174R-2, 85-88175R-1, 19-21176R-1, 126-127180M-2. 35-39181M-l,3-5181M-1,90-92181M-2, 45-17

140-504B-185R-186R-186R-186R-189R-189R-189R-190R-190R-191R-191R-192R-192R-193R-193R-193R-194R-194R-194R-196R-197R-197R-198R-198R-198R-199R-199R-199R-200R-200R-200R-200R-200R-.200R-200R-200R-200R-^202R-202R-203R-2O3R-204R-204R-205R-205R-205R-206R-207R-208R-208R-208R-208R-208R-;208R-:209R-209R-:209R-;210R-210R-211R-211R-212R-213R-213R-213R-214R-214R-

, 15-17, 13-14, 53-57,81-84,13-16

>, 11-13I, 83-86, 75-79, 95-98,48-51.60-62, 1-3, 13-14,7-10.28-31,44^6,5-9,95-96, 115-118, 39-42, 131-138

>, 33-38, 21-25,61-64,70-71,71-76,106-110, 121-123, 19-23, 86-88,105-107

>, 26-315, 13-16I, 59-63J, 106-1085, 125-1301, 23-27,25-29! 41-15,39-12,57-59, 12-14, 28-32. 14-17, 64-67. 87-90, 47-50.0-4,0-3,30-33, 50-53, 85-87

.,61-651, 7-10,98-102

>, 9-11!, 30-33, 36-39, 73-77,48-51, 129-131,4-7,3-6,32-34, 45-47,24-28, 126-129

Piece

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13

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2118

143

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928

66

16

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151153154

163163163

193194195195195200205206210210

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2272272272272272272272272272272272292*02322.32232232232233

235

237237239

23924024024024124124124124124224324.*244244

Depth(mbsf)

894.3904.1

1057.91196.21254.01270.41282.51296.0

1352.91354.91360.41379.31381.81397.81398.41399.0

1570.11571.51576.81578.61586.01596.61619.91620.41621.31622.3

1624.61626.71627.81628.61651.21653.51654.61659.11660.11664.51665.31671.11672.21675.41677.51679.11680.91687.41688.91702.21709.51711.71714.21717.51718.21723.51725.51726.21729.01730.21730.51731.91734.31735.21736.01736.41737.31747.91748.31753.81755.91756.61756.81757.51757.71759.61760.51768.41778.01779.01779.61780.71784.61787.61790.01791.41791.91795.71796.41801.11805.41806.61812.71813.91814.41819.01820.6

Pl-length

1.371.321.111.140.860.940.960.86

0.730.660.640.870.741.021.061.12

1.301.301.39

.120.790.980.900.840.920.72

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Pl-EQD

0.420.360.410.400.270.360.370.27

0.260.220.230.360.32

0.470.49

0.62

0.58

0.330.430.340.320.350.32

0.260.280.650.640.530.560.470.470.460.620.470.440.510.460.580.480.470.480.400.510.460.450.280.350.370.360.260.280.380.300.310.330.300.270.310.300.300.330.360.660.620.640.600.55

0 2 50.270.510.22

0.370.370.370.410.450.320.340.360.410.420.500.500.210.370.400.5!0.47

Mt-length

0.110.090.130.260.170.150.130.08

0.090.150.100.210.180.190.200.21

0.240.240.280.240.110.230.160.190.220.16

0.140.100.280.260.180.190.190.200.210.180.180.180.180.200.170.210.180.200.220.21

0.170.190.170.260 21( U 60.180.220.180.140.150.140.150.150.120.130.200.280.320.27

0.300.300.130.130.090.3.*0.130.26H240.200 ?()0220.170.190.220.210.280.260.290.300.100 ''30.2!0.210.23

Mt-EQD

0.100.090.120.250.160.150.130.07

0.080.150.100.200.170.180.200.20

0.210.210.260.230.100.210.150.180.220.15

0.130.090.250.260.180.190.170.190.190.170.170.180.170.180.150.200.160.180.210.190.170.160.190.160.24

0.150.170.210.160.150.140.130.130.1 30.120.120.190.240.270.27

0.26

0.120.12

0.300.120.230.220.190.180.200.150.180.200.190.260 °5(λ270.260.100.210.200.200.21

Coolingunit

2!

4444444444444445555

5

11777

99

1010

10

10101010101010101010101(

Interval

(m)

8.378.167.387.116.024.451.000.930.800.66

0.310.000.000.125.575.245.074.404.253.603.492.63

2.00.69.45.18

0.23

5.194.113.783.422.93

2.041.751.631.231.051.00

0.440.310.190.130.000.060.000.000.310.420.440.540.420.140.000.00

.42

0.000.160.74

.18

.54

.74

.822.372.483.183.823.984.vi5.065.145. S3


Table 1 (continued).

Core, section,interval (cm)

214R-1, 130-132214R-2, 29-32214R-2, 88-90214R-2, 100-103215R-1,0-4216R-1, 58-60216R-1, 66-68217R-1, 0-2217R-217R-218R-220R-221R-221R-222R-222R-222R-222R-223R-224R-224R-225R-225R-225R-

,4-6, 23-26, 24-27, 27-30, 11-15, 4 0 ^ 41,0-4, 17-19, 37-39, 129-132,20-23,21-24

I, 59-62, 4 0 ^ 4[,59-61L137-139

225R-2, 42-44226R-1, 16-20226R-1, 61-63226R-2, 77-79227R-1, 83-85227R-2, 0-5228R-1, 55-57229R-1, 34-37233R-1, 4-7236R-1, 0-4238R-1, 22-25

Piece

257

2022

11314116774

101IE6

2461

14101434

73

121010

11310218

Lithologicunit

244244244244244245245

245246247252254254

254256258258258259260260260260260260260260262265266269269

Depth(mbsf)

1820.71821.41822.31822.51823.11831.21831.61837.81839.21846.01855.61874.11877.21882.01884.71885.91887.31897.01900.21906.81912.11913.61914.21916.81918.71920.31920.91923.41929.81933.21943.21950.71961.71980.82000.2

Pl-length

1.161.181.191.411.260.810.810.960.950.850.921.141.861.871.531.741.511.570.840.660.800.810.711.011.241.231.131.511.391.351.121.140.551.031.11

Pl-EQD

0.480.420.430.570.440.290.280.400.370.310.340.420.670.780.600.610.600.640.340.240.300.280.270.400.430.570.460.600.520.550.380.410.210.450.48

Mt-length

0.230.210.210.31

0.190.190.200.170.130.200.250.290.340.22

0.250.190.180.160.160.160.210.270.190.19

0.220.230.160.170.210.23

Mt-EQD

0.210.190.200.280.240.180.170.190.160.120.180.230.260.300.200.220.260.230.160.180.150.150.140.190.250.180.180.220.220.200.210.150.170.190.21

Coolingunit

10101010101111111111111213131313131314141414

151515151515151515151616

Interval(m)

6.076.176.316.346.433.61

2.642.431.420.000.002.932.221.811.651.440.001.981.000.220.007.026.636.366.126.035.664.724.222.741.630.000.002.87

Notes: PI = plagioclase; Mt = magnetite; EQD = diameter of a sphere with an equivalent area. Length and EQD are in millimeters.Intervals are calculated on the basis of the average dip of 81.5°.

Intrusion of multiple dikes

Figure 7. Schematic model of multiple dike formation. Numbers indicate orders of intrusion. The site of multiple intrusion shifts within a certain range of thevolcanic zone, destroying the previous multiple dike units. Although the grain size increases toward the center of each multiple dike, occasional jumps in grainsize are seen in the section where older multiple dike units are disrupted by later dikes.

such a multiple dike. Each dike has a width of 1 m, and solidifies andcools to 800°C before the injection of the next dike. The first dikesolidifies in 2 days, and cools to 800°C in 76 hr. The second dikesolidifies in 7 days and 10 days after the emplacement of the first dike.The third dike solidifies in 9 days. Thus, the solidification time isprolonged for later dikes due to heating up by repeated injection.

Figure 11 shows the time required for solidification of the simpledike and those for solidification and cooling of the multiple dike with

each expanding by 1 m. The solidification time for every 0.5-m widthof the simple dike becomes progressively longer as solidification pro-ceeds. On the contrary, a notable increase in the solidification time isrecognized only for the first three dikes of the multiple dike unit. Laterdikes do not have as prolonged a solidification time as the simple dike.Instead, the time to cool to 800°C becomes significantly longer as theintrusion of the dike is repeated. This is because in the first model thewall-rock temperature is assumed to be constant whenever the new

25

S. UMINO

1.40

1.30

"? 1.20

I 1.00

I 0.90

0.80

0.70

Unit 227

• •• i

µ

•

• •

ID

F

Uni

t 22

1

•

•

Unit 223

• !

|_

•

11j

I Unit 222

r- r i

IIII

-L J

1Ti

1

0.00 1.00 2.00 3.00 4.00

Distance from rim (m)

5.00 6.00

Figure 8. Variation of the maximum length of plagioclase through cooling Unit 5 (A) and Unit 10 (B). Although the grain size is slightly scattered, the largest grainsize in individual lithologic units monotonously increases with distance from the rim of the multiple dike unit. An exception is lithologic Unit 242 in cooling Unit10 (B), which is too fine-grained and is apparently taken from a chilled contact.

0 day

1000 - r Temperature (°C)

-5

Wall rock

-4 -3 -2 -1Distance (m)

Figure 9. Solidification of a single dike emplaced in the host rock with the same physical properties as the dike. Temperature profiles at various times through thedike (left) and adjacent wall rock (right) are shown.

injection occurs. As exemplified by cooling Units 5 and 10, the grainsize is coarser in the inner dikes than in the outer ones. This suggeststhat the crystallization time becomes longer toward the center of themultiple dikes. Therefore, the assumption of constant temperatureprior to new injection is not appropriate when the progressive increasein grain size toward the center of the multiple dike units is observed.

Dike intrusions observed in fissure swarms beneath active volca-noes such as Krafla, Iceland, and Kilauea, Hawaii, show that suchintrusions continue for as long as one week and are repeated severaltens of times with intervening quiescent periods ranging from 1 to 18months. So, let us consider a case in which dike intrusion occursperiodically at a constant time interval.

26


8TO

a.

1.50

1.40

1.30

1.20

1.10

1.00

0.90

0.80

0.70

0.60

0.50

Unit 239

• m

m

unit 240

•

•

J L

_

Unit 241

•

•

•

,

i

cY

< ^ •

Uni

t

illed m

T•

ifI t\J

Uni

t

! •1

1

j

grgin

;

••

I

i

i:ii

i

i

!

" "1 "11i i

Unit244

0.00 1.00 2.00 3.00 4.00


Figure 8 (continued).

5.00 6.00 7.00

Table 2. Width of cooling units at minimum, average, and maximum dike dips.

Coolingunit

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Lithologicunit

193212213

217218221

228229231

232

235236238239

245250

253254256257259260266267269

Depth(mbsf)

1570.101627.281627.281632.001632.001651.061651.061697.341697.341747.201747.201752.811752.811758.831758.831768.401768.401779.521779.521829.221829.221865.501865.501876.831876.831897.241897.241916.401916.401976.101976.102000.40

Average

Minimum dip 79°

Width(m)

10.91

0.90

3.64

8.83

9.51

1.07

1.15

1.83

2.12

9.48

6.92

2.16

3.89

3.66

11.39

4.64

5.13

Width × 2(m)

21.82

1.80

7.27

17.66

19.03

2.14

2.30

3.65

4.24

18.97

13.84

4.33

7.79

7.31

22.78

9.27

10.26

Averag

Width(m)

8.45

0.70

2.82

6.84

7.37

0.83

0.89

1.41

1.64

7.35

5.36

1.68

3.02

2.83

8.82

3.59

3.98

;e dip 81.5°

Width × 2(m)

16.90

1.40

5.64

13.68

14.74

1.66

1.78

2.83

3.29

14.69

10.72

3.35

6.03

5.67

17.65

7.18

7.95

Maximum dip 84°

Width(m)

5.98

0.49

1.99

4.84

5.21

0.59

0.63

1.00

1.16

5.20

3.79

1.18

2.13

2.00

6.24

2.54

2.81

Width × 2(m)

11.95

0.99

3.99

9.67

10.42

1.17

1.26

2.00

2.32

10.39

7.58

2.37

4.27

4.01

12.48

5.08

5.62

Notes: Time interval of rifting: Minimum dip = 10.26 ÷ 0.07 = 147 yr, average dip = 7.95 ÷ 0.07 = 114 yr;maximum dip = 5.62 ÷ 0.07 = 80 yr.

Figure 12 shows the temperature at the dike center for intrusion of1.5-m-wide dikes at an interval of 3 months. The first dike solidifiesas soon as that of the first model but it has a longer time for coolingdown to 250°C before the next injection. The second dike intrudesand solidifies as soon as the first dike, but it cools to only 400°C. Asthe intrusion is repeated, temperature just prior to the new injectionbecomes higher, and the crystallization time becomes longer.

When the multiple injection occurs at an interval of 1 month, thehost rock temperature increases more rapidly than in the case for the3-month injection because the time for cooling is much shorter thanthe latter. Although the second, third, and fourth dike intrudes after

the solidification of the previous dikes, the fifth dike intrudes shortlyafter the solidification of the fourth one. Finally, the sixth and laterdikes intrude before the solidification of the previous ones, and thusnew injection of magma merely expands the width of the dike and nomore chilled margins are formed.

Crystallization time also depends on temperature of the wall rock.Figure 13 shows the time for crystallization and cooling of multipledikes periodically intruding at a 6-month interval with the initial wall-rock temperatures of 0°, 250°, and 500°C. The crystallization time forthe first dike with the initial wall-rock temperature of 0° and 250° is120 and 160 hr respectively, but it increases only 400% and 580%

27

S. UMINO

A

7 days. 3jdays

10 days\\ V14 days — \ >

Multiple dike unit

1000

900

^ 800

60C&

500

400

3 0 0

2 0 0

100

θ—

"Temperature (°C)

-

-

\ ^ v Wall rock

-2 -1

Distance (m)

Figure 10. Solidification of a multiple dike with each expansion of 1 m. New injection occurs when the center of the multiple dike cools 800°C. Variation oftemperature profiles is shown from the center of the multiple dike through the host rock. Temperature profiles in the second (A) and third dikes (B) are shown.

Time (hr)

Time for solidification

Time for cooling to 800°C

Simple dike solidification

1800

1600

1400

1200

1000

800

600

400

200

-6 -5 -4 -3

Distance from margin (m)

-2 -1

Figure 11. Time required for solidification and cooling of a multiple dike with 1-m-thick injections. The multiple intrusion occurs when the center of the multipledike cools to 800°C. Time required for solidification or cooling of every 0.5 m of a simple dike is also shown for comparison.

28


Bi f\ Hauc1O Udyb

23 days ^ *

-^27 days

.33 days

Multiple

S~ M4 days

dike unit

1OOO

900

800

^ 7 0 0

500

400

3 0 0

2 0 0

100

θ—

"Temperature (°C)

^ ^ ^ Wall rock

-2 -1Distance (m)

Figure 10 (continued).

5000 10000 15000

Time (hr)

20000 25000

Figure 12. Variation in temperature at the center of a multiple dike intruding every 3 months at initial wall-rock temperature of 0°C. As the intrusion is repeated,the temperature of the center of the multiple dike steadily increases.

after nine repetitions of the intrusion. That for the first dike with theinitial wall-rock temperature of 500°C is 257 hr, while it largelyincreases more than twice for the fourth dike, and up to 12 times forthe tenth dike. Thus, both the decrease in the intrusive time intervaland the increase in the initial wall-rock temperature largely increasesthe crystallization time of the multiple dike units.

Relationships Between Crystallization Timeand Grain Size

Relationships between the grain size and the crystallization timedepend on the nucleation and growth rate, which are functions of tem-perature, pressure, and chemical composition of the magma, and also

29

S. UMINO

Time (days)

200

175

150

125-

100 •

50

25

0°C

Time (hr)-r 4500

-– 4000

-– 3500

-– 3000

-– 2500

2000

-– 1500

-– 1000

-– 500

-8 -7 -6 -5 -4 -3


-2 -1

Figure 13. Time for crystallization (lower three curves) and cooling between dike injections (upper three curves) of multiple dikes. An individual dike has thewidth of 1.5 m and is emplaced every 6 months at initial wall-rock temperatures of 0°C, 250°C, and 500°C. Symbols connected with a line represent injection ofthe first, second, third, etc., dikes from the right.

on physical properties of the magma and the wall rock, such as thermalconductivity, thermal diffusivity, heat capacity, and latent heat of crys-tallization. Numerical simulations of crystallization of a dike of a hypo-thetical binary magma have been presented by Spohn et al. (1988).They showed that (1) logarithm of grain size is linearly correlated withthe wall-rock temperature, and (2) the difference in square root of thecrystallization time is correlated with the wall-rock temperatures:

log pm = 0.44 θ-1.19

Θ

(i)

(2)

(when Av = 2 × 105) where pm is non-dimensional crystal size, xc isthe crystallization time, QR is the initial wall rock temperature, and Avis Avrami number defined as

Av = σ/£/ 3/(D 2/K) 4,

(σ: Shape factor) which is a function of nucleation (/) and growth (U)rates, width of the dike (D), and thermal diffusivity (K). By combiningEquations 1 and 2, we get

Vx7=Clogpm + D (3)

Vr7=CΔlogp m (4)

(C, D: constant).

Thus, the difference in square root of the crystallization time islinearly correlated with the difference in logarithmic grain size. Figure14 shows relationships between the crystallization time for basalticandesite dikes from Iritono, Northeast Japan (Ikeda, 1977), andMakaopuhi lava lake, Hawaii (Kirkpatrick, 1977), together with thebest fit lines. Except for the smallest two data points from the Iritonodikes, both Iritono and Makaopuhi data sets show crude linear relation-ships with similar inclinations. The inclination depends on the system,such as composition of the magma, phase relations, and physical prop-erties of the magma and the wall rock. The Iritono dikes are basalticandesite that intruded into granitic plutons at depth, while theMakaopuhi lava lake is basalt ponded on the East Rift Zone of Kilauea.Although these two largely differ in conditions for crystallization, bothshow similar inclinations. Therefore, we may assume a similar inclina-tion for the Hole 504B dikes so that we can correlate variations in thegrain size directly with those in the crystallization time.

Figure 15 shows variations in maximum plagioclase length rela-tive to that of the outermost dike plotted against distance from themargin of the multiple dike units. Also plotted on the diagram areestimated relative grain sizes of multiple dikes, each with an expan-sion of 1.5-m width and a time interval of 6 and 3 months, usingEquation 4. Cooling Units 5 and 10 are plotted around the data pointsof a 3-month interval and with the initial host rock temperature of0°C, and those of a 6-month interval and 0°C host rock temperature.The first three points of cooling Unit 1 are plotted higher than the6-month, 500°C variation curve, but the outer three points (second,

30


6000

5000

4000

3000

2000

1000

0

• Iritono dikes

ffl Makaopuhi lava lake

VT=-116O5 + 7339!og(r

=-16527+ 8838 log(r )

0.5 1 1.5 2log(r) (µm)

Figure 14. Square root of crystallization time plotted against logarithm of plagioclase size in Iritono dikes, northeast Japan, and in Makaopuhi lava lake, Hawaii.Data points show linear relationships between square root of time and logarithm of size, although two data points of Iritono for smaller size do not fit the line.

third, and fourth dikes) are close to the curve. If the initial wall-rocktemperature was higher than 500°C, the outer dikes in the coolingUnit 1 would be much coarser. This is probably because the first dataof cooling unit 1 is not the coarsest sample in the first dike but wastaken from near the fine-grained margin, or the first four dikes in-truded successively in a short period.

Natural multiple dike units are composed of dikes with variouswidth emplaced at uneven time intervals. Figure 16 shows a relativevariation in maximum plagioclase length of the cooling Unit 5. Alsoshown are calculated grain-size variations for injection at 6-monthintervals and 0°C initial wall-rock temperature and for 3-month inter-vals at 250°C. In both calculations, the widths of dikes are adjusted tothe real dike width. Both lines for 3- and 6-month intervals are plottedslightly above or below the natural data points, but do not fit well.This is undoubtedly due to variations in the time interval of the dikeemplacement. The best fit solution for cooling Unit 5 is also shown inFigure 16. The second dike intrudes 39 days after the emplacement ofthe first dike, and the third dike intrudes 56 days after the second one.The fourth and fifth injection of dike occurred 7 months later andfurther 6 months later.

Because of uncertainties arising from sampling, even the best fitsolutions for the natural cooling units do not reproduce the precise timeintervals and the initial wall-rock temperatures when the multiple dikeintrusions took place, but give the most plausible estimates of thesevariables; individual dikes of the multiple dike units would have beenemplaced at intervals of 1-7 months and initial wall rock temperatureof 0°-250°C, which are comparable to the dike intrusions observed inKrafla and Kilauea (Tryggvason, 1984; Yang et al, 1992).

RIFTING EPISODES AT DIVERGENT PLATEBOUNDARIES

The downhole grain-size variation shows monotonous increasesor decreases segmented by occasional jumps. It does not show anysymmetrical grain size variations expected for those through thewhole section of a single dike or a multiple dike unit, and hencecomplete sections through single dikes or multiple dike units arescarcely seen in the drill hole. Therefore, the width of an individualcooling unit gives the minimum of half of the width of a correspond-

ing multiple dike unit. The average width of the 16 cooling unitsbelow 1570 mbsf is 5.13 m for the minimum dip of 79° and 2.81 mfor the maximum dip of 84°, and 3.98 m for the average dip of 81.5°.Although there is no direct observation of volcanic eruptions andtectonic movements in mid-ocean spreading axes, it is plausible thatrifting in oceanic ridges is also an episodic event associated with suc-cessive intrusion of dikes in a short period which forms a multipledike unit and that individual rifting events are intercalated with com-paratively long quiescent periods. Thus, each cooling unit representsa minimum amount of crustal extension through individual riftingepisodes. The average maximum expansion of the rifting would be atleast twice as much as the average width of the cooling units, rangingfrom 5.6 to 10.3 m and 8.0 m on the average. As the full spreadingrate of the Costa Rica Rift is 7.2 cm/yr, the time interval of riftingwould have been 114 + 33 yr. As deduced from the grain-size vari-ations, each rifting episode forms a multiple dike unit through inter-mittent dike intrusions repeated at 1- to 12-month intervals and last-ing for at least 2-3 yr.

Historical records of major fissure eruptions and tectonism inIceland are thought to be manifestations of such rifting events. Theseinclude fissuring and eruptions of the Krafla swarm in 1724-1729 andin 1975-1981, fissure eruption of Suveinagja and Askja caldera in1874-1875, and the great Laki eruption and volcanic activity inGrimsvötn central volcano in 1783. Each rifting event lasted 1-6 yrduring which intermittent dike intrusions and fissuring took place(Sigurdsson and Sparks, 1978 ). During the latest spreading episodein the Krafla system, a number of dike intrusions were inferred byrapid deflations in the caldera area and earthquakes along the fissureswarm, each of which lasted for only several hours and were sepa-rated by long quiescent periods of 2-10 months. The expansion at asingle intrusion was 0.5-3 m and the total widening was 3.5 m on theaverage for the whole 80-km-long fissure swarm. These time inter-vals between dike intrusions and the duration of the rifting episodeare comparable to those at the Hole 504B dike swarm.

If spreading episodes in Iceland occur every 100-150 yr, the fullspreading rate of 1.9 cm/yr gives an average expansion of 1.9-2.9 mduring a single rifting. The 3.5-m expansion of the Krafla event is84%-21% higher than the expected widening by the NUVEL platevelocity model. At divergent plate boundaries the crust is approxi-

31

S. UMINO

Clog(pl/pl*) & ΔVtime

2500

2000

1500

1000

500

-500

6 months, 500°C

Λ

, 3 months, 0°C

6 months, 250°C

6 months, 0°C


1 0

Figure 15. Variations in relative length of plagioclase with distance from the margin of cooling Units 1 (open triangles), 5 (solid circles), and 10 (open circles).Curves are simulated plagioclase length variations for multiple intrusions with 1.5-m-wide dikes injected every 3 and 6 months at initial wall-rock temperaturesofO°C, 250°C, and500°C.

Best fit for cooling Unit 5

2 months, 0°C

3 months, 250°C

3 months, 0°C

1600

1400

1200

1000

800

600

400

200

0

-200

Figure 16. Variation in relative plagioclase length of cooling Unit 5 compared with simulations. Three lines plotted below or above cooling Unit 5 are calculatedvariations for dikes with the same width as Unit 5 but emplaced at constant intervals of 2 and 3 months in host rocks with initial temperature of 0°C and 250°C.The best fit solution indicates that the second dike of cooling Unit 5 was emplaced 39 days after the intrusion of the first dike. The third, fourth, and fifth dikesintruded at intervals of 56, 216, and 181 days, respectively.

mated by a brittle plate underlain by the molten asthenosphere and amagma chamber. Then, the frequency of dike intrusions is given by

If=Ev/σu, (Gudmundsson, 1990)

where E is Young's modulus, v is full spreading rate, σ is tensile stressand u is the width of a volcanic zone under stress. Gudmundsson(1988) estimates the static Young's modulus at the bottom of the crust

in Iceland to be 51 GPa and the tensile strength to be 3 MPa. Theneovolcanic zone in northeastern Iceland is 45-80 km in width andcomprises four partly overlapping fissure swarms aligned en echelon.Thus, the width of one fissure swarm is approximately 10 km, and theaverage spreading rate per each is 0.5 cm/yr. By substituting E = 51GPa, v = 0.5 cm/yr, σ = 3 MPa, and u = 10 km, Gudmundsson (1990)obtained the dike injection frequency in Iceland (If) to be 0.0085/yr,that is, one dike injection every 118 yr, which is within the range of

32


the rifting episode deduced from historical records. Although we donot know the exact area of recent fissuring at the Costa Rica Rift, theinner valley floor would be the most plausible location. The spreadingcenter is indicated by inward-facing steep scarps about 50 km aparton top and a 700-m-deep valley. The valley floor is approximately 20km wide, which would be subjected to recent intense fissuring anddike intrusions. Young's modulus for the crustal layers of the Semailophiolite is 67 GPa (Gudmundsson, 1990). If Young's modulus is51-67 GPa and the tensile strength at the roof of the subcrustal magmachamber is 3 MPa, the time interval of the spreading episode at CostaRica Rift is 12-16 yr. This value is only 8%-20% of that estimatedfrom the multiple dike units. This discrepancy partly comes from anoverestimation of the width of the multiple dike units. Because of poorrecovery of the cores and still rare recovery of the contact betweentwo lithologic units (dikes), misidentification of a multiple dike unitmay arise when it intrudes into the host rock consisting of fine-grainedold dikes. Then the fine-grained host may be mistaken for part of themultiple dike unit and hence the width of the whole unit is overesti-mated. The other possibility is that the number of dikes decreasesupward and the width of dikes tends to increase and becomes shorterupward (Gudmundsson, 1990). This is because the length-to-widthratio of a dike and the intrusion frequency are proportional to Young'smodulus, which is higher in the deeper part of the crust.

Geodetic measurements such as very long baseline interferometry(VLBI) have detected relative plate movements in only a year-longobservation, which is consistent with the average rates of the platemotion determined by the stripes of magnetic anomalies (Heki et al.,1987). If the rifting at spreading ridges is an episodic event occurringevery 10-100 yr, the crustal movements during a rifting episode mustdiffer from large-scale plate movements. The last expansion of theKrafla fissure swarm was 18 November 1981, and no fissuring anddike intrusion have occurred since then. Recently, geodetic measure-ments using the global positioning system (GPS) were conducted in1987 and 1990 in the Krafla system (Foulger et al., 1992). The GPSmeasurement showed the maximum extension up to 15 cm, which isthree times larger than that expected from the full spreading rate inIceland. Foulger et al. (1992) have explained it as stress relaxation ofthe crust adjacent to the fissure swarm. Because dike intrusions ex-pand the crust in a short period, the strain rate is so high that the crustadjacent to the fissures cannot catch up with the deformation and arestrongly compressed. Then, relaxation of stress occurs during a longperiod after the intrusions and the deformation diffuses to the sur-rounding crust. A numerical simulation of such a diffusive stressrelaxation show that repeated expansions of 5-6 m at intervals of 100yr would be observed as a continuous crustal movement at a rate of1-2 cm/yr at distances beyond 100 km, and could well explain theaverage spreading rate at Iceland (Heki et al., 1993).

Thus, the regional crustal movements are time-integrated, aver-aged diffusion processes of deformation, intermittently taking placealong plate boundaries such as ridge systems and trenches. In con-trast, spreading at diverging plate boundaries is an episodic expansionof the fissure swarm lasting for only several years, which is separatedby quiescent periods of 10 to several hundred years.

ACKNOWLEDGMENTS

I thank all the shipboard scientists, technicians, and crew, espe-cially those who decided to make dense sampling and many thinsections, which made all this work possible. Careful and fine reviewsby Katharine V. Cashman, Bruce D. Marsh, and Henry J.B. Dick aregratefully acknowledged.

REFERENCES*

Adamson, A.C., 1985. Basement lithostratigraphy, Deep Sea Drilling ProjectHole 504B. In Anderson, R.N., Honnorez, J., Becker, K., et al., Init. Repts.DSDP, 83: Washington (U.S. Govt. Printing Office), 121-127.

Baragar, W.R.A., Lambert, M.B., Baglow, N., and Gibson, I.L, 1990. Thesheeted dyke zone in the Troodos Ophiolite. In Malpas, J., Moores, E.M.,Panayiotou, A., Xenophontos, C. (Eds.), Ophiolites: Oceanic CrustalAnalogues. Cyprus Geol. Surv. Dep., Min. Agr. Nat., Res., 37-51.

Becker, K., Sakai, H., et al., 1988. Proc. ODP, Init. Repts., I l l : College Station,TX (Ocean Drilling Program).

Cashman, K.V., 1990. Textural constraints on the kinetics of crystallization ofigneous rocks. In Nicholls, J., and Russell, J.K. (Eds.), Modern Methodsof Igneous Petrology: Understanding Magmatic Processes. Mineral. Soc.Am., 259-314.

Cashman, K.V., and Marsh, B.D., 1988. Crystal size distribution (CSD) inrocks and the kinetics and dynamics of crystallization, II: Makaopuhi lavalake. Contrib. Mineral. Petrol, 99:292-305.

Foulger, G.R., Jahn, C.-H., Seeber, G., Einarsson, P., Julian, B.R., and Heki,K., 1992. Post-rifting stress relaxation at the divergent plate boundary inNortheast Iceland. Nature, 358:488^90.

Gautneb, H., and Gudmundsson, A., 1992. Effect of local and regional stressfields on sheet emplacement in West Iceland. J. Volcanol. Geotherm. Res.,51:339-356.

Gudmundsson, A., 1988. Effect of tensile stress concentration around magmachambers on intrusion and extrusion frequencies. /. Volcanol. Geotherm.Res., 35:179-194.

, 1990. Dyke emplacement at divergent plate boundaries. In Parker,A.J., et al. (Eds.), Mafic Dykes and Emplacement Mechanisms. Proc. 2ndInt. Dyke Conf. Adelaide, South Australia, 47-62.

Heki, K., Foulger, G.R., Julian, B.R., and Jahn, C.-H., 1993. Plate dynamicsnear divergent boundaries: geophysical implications of postrifting crustaldeformation in NE Iceland. /. Geophys. Res., 98:14279-14297.

Heki, K., Takahashi, Y, Kondo, T, Kawaguchi, N., Takahashi, F., and Kawano,N., 1987. The relative movement of the North American and Pacific platesin 1984-1985, detected by the Pacific VLBI network. Tectonophysics,144:151-158.

Ikeda, Y, 1977. Grain size of plagioclase of the basaltic andesite dikes, Iritono,central Abukuma Plateau. Can. J. Earth Sci., 14:1860-1866.

Kirkpatrick, R.J., 1977. Nucleation and growth of plagioclase, Makaopuhi andAlae lava lakes, Kilauea Volcano, Hawaii. Geol. Soc. Am. Bull., 88:78-84.

Kong, L.S.L., Detrick, R.S., Fox, P.J., Mayer, L.A., and Ryan, W.F.B., 1989.The morphology and tectonics of the MARK area from Sea Beam andMARC 1 observations (Mid-Atlantic Ridge 23°N). Mar. Geophys. Res.,10:59-90.

Marsh, B.D., 1988. Crystal size distribution (CSD) in rocks and the kineticsand dynamics of crystallization, I: Theory. Contrib. Mineral. Petrol.,99:277-291.

Shipboard Scientific Party, 1992. Site 504. In Dick, H.J.B., Erzinger, J.,Stokking, L.B., et al., Proc. ODP, Init. Repts., 140: College Station, TX(Ocean Drilling Program), 37-200.

Sigurdsson, H., and Sparks, R.S.J., 1978. Lateral magma flow within riftedIcelandic crust. Nature, 274:126-130.

Smith, D.K., and Cann, J.R., 1992. The role of seamount volcanism in crustalconstruction at the Mid-Atlantic Ridge (24°-30°N). J. Geophys. Res.,97:1645-1658.

Spohn, T., Hort, M., and Fischer, H., 1988. Numerical simulation of thecrystallization of multicomponent melts in thin dikes or sills, 1: the liquidusphase. J. Geophys. Res., 93:4880-4894.

Tryggvason, E., 1984. Widening of the Krafla fissure swarm during the1975-1981 volcano-tectonic episode. Bull. Volcanol., 47:47-69.

Turcotte, D.L., and Schubert, G., 1982. Geodynamics: Applications of Con-tinuum Physics to Geological Problems: New York (Wiley).

Yang, X., Davis, P.M., Delaney, P.T., and Okamura, A.T., 1992. Geodeticanalysis of dike intrusion and motion of the magma reservoir beneath thesummit of Kilauea Volcano, Hawaii: 1970-1985. J. Geophys. Res.,97:3305-3324.

Abbreviations for names of organizations and publications in ODP reference lists followthe style given in Chemical Abstracts Service Source Index (published by AmericanChemical Society).

Date of initial receipt: 24 February 1994Date of acceptance: 5 July 1994Ms 137/140SR-002

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2. DOWNHOLE VARIATIONS IN GRAIN SIZE AT HOLE 504B