22. basaltic hyaloclastites from hole 396b, … · 22. basaltic hyaloclastites from hole 396b, dsdp...

22. BASALTIC HYALOCLASTITES FROM HOLE 396B, DSDP LEG 46

Hans-Ulrich Schmincke, * 2 Paul T. Robinson,3 Walter Ohnmacht,1 and Martin F. J. Flower1

ABSTRACT

During Leg 46 a 90-meter-thick sequence of loose, sand-size, glassybasaltic debris (hyaloclastite) with some associated broken-pillow brecciawas encountered beneath 175 meters of pillowed basalt. The presence ofbedding in some of the hyaloclastites, the high ratio of sideromelane tocrystalline basalt (up to 60%), and the high degree of brecciation andvesiculation of overlying and underlying pillow breccias all indicate thatthe clastic unit is a sedimentary deposit that existed before drilling. Thisconclusion is confirmed by the high rate of bit penetration in this intervaland by the geophysical log of the hole.

The high ratio of glass to crystalline basalt and the relative chemicalhomogeneity of the glass in the bedded hyaloclastite suggest that theclastic processes were roughly synchronous with eruption, and that theparticles were not transported far from the site of brecciation. Spalling ofpillow rinds to produce "spallation" shards is believed to be the majorprocess of formation of sideromelane shards, but implosion of pillowsand granulation of lava may also have occurred. The process of fragmen-tation of tachylite shards is unknown. Sideromelane in the sediments isextremely fresh, suggesting that the clastic unit was largely sealed offfrom percolating sea water and hence remained relatively dry since for-mation about 13 m.y. ago. This interpretation is supported by downholetemperature measurements.

INTRODUCTION

Hole 396B was drilled approximately 150 km east of theMid-Atlantic Ridge at 22°59.14'N and 43°30.90'W. Basal-tic basement, approximately 13 m.y. old, was penetratedbeneath 255 meters of sediment. Pillow basalts make up theupper 175 meters of basement; the lower 90 meters consistof largely unconsolidated sand-size clastic sideromelane andslightly crystallized basalt particles with rare induratedhyaloclastite and pillow fragments. The clastic material hasbeen briefly described in a preliminary account (ShipboardScientific Crew, 1976).

The clastic sequence, the thickest ever encountered dur-ing oceanic basement drilling, poses many questions regard-ing styles of submarine eruptions, present crustal models,and alteration processes. In our study we have attempted toanswer the following specific questions: Is the clastic debrisprimary or a result of drilling? Was it concentrated andaccumulated by sedimentary or by igneous processes? Canthe distance to the source and the lateral extent of the layerbe determined? What is the importance of this sequence interms of constructional processes of mid-ocean ridges?What is the significance of the extraordinary freshness ofthe sideromelane shards, which are about 13 m.y. old?

'Institut für Mineralogie der Ruhr-Universitat, 4630 Bochum, West Ger-many.

2Sabbatical address: Department of Geological Sciences, University ofCalifornia, Santa Barbara, California.

department of Earth Sciences, University of California, Riverside,California.

STRATIGRAPHY AND THICKNESS

The shipboard party divided the clastic sequence into fourlithologic units (5 to 8). Unit 5 (Cores 23 through 26) iscarbonate-cemented breccia and sparsely phyric pillowbasalt; Unit 6 (Cores 26 through 31) is predominantly loosesand-size clastic debris with rare pillow fragments; Unit 7(Core 32) is predominantly pillow fragments of porphyriticbasalt; and Unit 8 (Core 33) is again loose clastic material,mostly sand-size basalt. In the shipboard description and ina preliminary note (Shipboard Scientific Crew, 1976) theterm gravel was used for the loose clastic debris. The term ismisleading, however, unless it is understood purely in termsof grain size. Moreover, means of nearly all clastic samplesstudied by us are in the sand-size range, so in this paper weuse the term hyaloclastite, or indurated hyaloclastite, for theclastic material.

We have divided the clastic sequence into the followinglithologic units: upper pillow breccia (Cores 23 through 29),upper hyaloclastite (Cores 30 and 31), lower plagioclase-phyric pillow breccia (Core 32), and lower hyaloclastite(Core 33). Recovery was very low in most cores (zero inCores 27, 29, and 31; less than 1% in Cores 25, 26, and 28;and from 7.4% to a maximum of 20% in all other cores).

The upper pillow breccia (especially Cores 23 and 24)differs markedly from the overlying pillow basalts, eventhough these latter basalts are more vesicular, more brec-ciated, and more altered than basalts higher in the section.The upper pillow breccias in Cores 23 through 27 are almostaphyric; the lower units contain more phenocrysts, espe-cially plagioclase, but in irregularly varying amounts. The

341

H-U. SCHMINCKE, P. T. ROBINSON, W. OHNMACHT, M. F. J. FLOWER

most highly phyric unit is the bedded hyaloclastite of Core30. Sand-size hyaloclastites predominate in Cores 30 and33; Cores 23, 24, and 32 contain a significant portion ofbroken-pillow breccia. It is difficult to say how much fine-grained hyaloclastite is present in the intervals representedby Cores 23 to 29 and 31 to 32. Until Core 30, a liner wasnot used in the core catcher to retrieve the loose sand-sizeclastic debris. As a result, much loose debris may have beenlost, and we do not know whether the entire 90-meter-thicksection or only the lower 20 to 30 meters is made up largelyof sand-size particles.

We have investigated six samples, each about 25 grams,of loose particles of different grain size from Core 30 (Sam-ples 131, 133, and 134) and Core 33 (Samples 138, 139,and 140) and a slice of the larger of two indurated hyaloclas-tite fragments from Core 30. To compare shapes of shards,we also studied sand-size basaltic debris formed by steamblasting during drilling in Makaopuhi lava lake (Hawaii).

CLASTIC COMPONENTS OF THEUNCONSOLIDATED HYALOCLASTITES

Clastic components of the hyaloclastites were identifiedby binocular microscope examination, by thin-sectionexamination of grain mounts of different size grades, and byscanning electron microscope examination of selected sam-ples. Six main components were distinguished: (1)sideromelane; (2) spherulitic tachylite to variolitic basalt;(3) crystalline basalt; (4) cemented aggregates; (5) singlecarbonate, zeolite, or chalcedony (?) particles; and (6) crys-tal fragments of plagioclase and olivine.

Sideromelane is the main component in Core 30 (Samples131, 133, and 134) and the second most common compo-nent in Core 33 (Samples 138, 139, and 140) (Table 1; Plate1, Figures 1 and 2). The glass is typically clear and transpa-rent, and contains rare spherulites and very rare small deco-rated vesicles (Plate 2, Figure 3). Small skeletal crystallitesof plagioclase and/or olivine, as well as sparse phenocrystsof plagioclase and olivine, are also present. Particles areblocky, equant, and angular, and become somewhat moreelongate with decreasing grain size (Plate 2, Figures 1 and2; Plate 3, Figures 1-4). This change in shape with degree offragmentation can be predicted from incipient fractures thattend to split blocky particles into more tabular slivers.

Most sideromelane fragments show no microscopic signsof palagonitization (Table 1). Very faint discolored rims arepresent on a few fragments, but more extensive palagoniti-zation is rare.

Spherulitic tachylite to variolitic basalt fragments makeup the most abundant component in Core 33 (Plate 1, Figure1) and the second most abundant component in Core 30(Plate 1, Figure 2). This material ranges from completelyblack, unstructured tachylite to spherulitic and varioliticbasalt in which small plagioclase microlites are set in abrown devitrifled base. Fragments are generally blocky toequant, but with more irregular edges than those ofsideromelane clasts. Vesicles are also slightly more abun-dant than in sideromelane. Although a few fragments aretransitional between completely clear sideromelane andcompletely opaque tachylite, distribution is distinctlybimodal and it is easy to assign particles to one of thegroups. Such a population of particles would be expected

from fragmentation of the outer part of a pillow, where thetransition between the sideromelane rind and the underlyingspherulitic and tachylitic zone is usually quite abrupt.

Completely crystalline basalt particles are rare (Table 1).The crystalline fragments are fine-grained basalt composedof plagioclase, clinopyroxene, olivine, and titanomagnetite.

Nearly all the investigated samples contain one or moreaggregates of small, generally sliver-like sideromelane par-ticles. Such aggregates are probably more abundant in theoriginal material—they would be easily destroyed by siev-ing. Most aggregates are irregular in shape and less than 2mm in diameter. They contrast with the loose particles, inthat none contain tachylite. Carbonate is the predominantcement in the aggregates. It not only binds the particlestogether, but fills small fractures and, in a few cases, par-tially replaces sideromelane particles. The next most com-mon cement is birefringent "palagonite" (smectite?) whichoccurs as a coating around the glass particles. Iron hyd-roxide, phillipsite, and chalcedony (?) are other rare ce-ments .

The cementing minerals in the aggregates may also oc-cur, though rarely, as isolated mineral particles in the loosematerial. Crystal fragments of plagioclase and olivine, thecommon phenocryst phases in the sideromelane, are alsopresent.

GRAIN-SIZE DISTRIBUTION OFUNCONSOLIDATED HYALOCLASTIC MATERIAL

Grain-size analyses of the six samples of unconsolidatedmaterial were done by hand sieving. Medians range fromabout 3 to 1 mm, and most fall in the coarse sand range(Figure 1). Thus, the loose debris, if cemented, would becalled coarse basaltic sandstone or hyaloclastite. Sorting inall analyzed samples is very good. It must be remembered,however, that these data reflect only the grain-size distribu-tion in the plastic liner of the core catcher. Fine-grainedparticles may have been lost during pull-out of the corebarrel, and the clastic material may have been somewhatcontaminated by "drilling debris," as discussed below.

Samples from Core 30 are finer grained than those fromCore 33 (Figure 1). Samples from both cores show systema-tic changes in composition with grain size (Tables 1 and 2;Figures 2 and 3). In all samples, the abundance of spheruli-tic tachylite and variolitic basalt decreases with decreasinggrain size. Sideromelane increases with decreasing grainsize, although at grain sizes below about 0.125 mm it maydecrease again because of the increasing proportion ofplagioclase and olivine crystal fragments concentrated in thefinest material.

INDURATED HYALOCLASTITE

A slightly indurated but friable hyaloclastite fragment,about 6 cm in maximum diameter, was recovered within theloose material of Core 30. It consists of angular particles;the median size of most material is in the range of fine tomedium sand. Particles larger than 1 mm are rare, as aresilt-size particles; thus, sorting is excellent. The fragmentconsists of three main layers (Plate 3, Figure 1). The coar-sest layer, 1.5 cm thick, is strongly graded from coarsesand, with a few elongate fragments up to 3 mm long, touniform fine sand. On the basis of this grading, we interpret

342

BASALTIC HYALOCLASTITES FROM HOLE 396B

TABLE 1Petrographic Constituents of Hyaloclastite in Different Size Grades

Sample Numberand Grain

Size Interval(mm)

131,1-2131,0.5-1131,0.25-0.5133, 1-2133,0.5-1133,0.25-0.5134, 1-2134,0.5-1139, 1-2139,0.5-1140, 2-4140, 1-2

FreshSidero-melane

45851612788

258257911451549

Siderome-lane withIncipient

Palagonit-ization

67_7

1529

48442-

Tachylite

3748683252

15523452478965

Tachylitewith

PlagioclaseMicrolites

99

2312133414

910

472030

CompletelyCrystalline

Basalt

_

_5114_111_-

CementedHyalo-clastite

431524112414

SingleCarbonateParticles

_

32_1511-4.--

SingleOlivineand Pla-gioclaseCrystals

_

13—15_1-4--

TotalNumber

of GrainsCounted

99156263

84173494

6814552

18747

158

RatioSideromelane/

Sideromelane &Tachylite

0.530.620.640.440.610.600.440.620.310.280.370.34

100

90

80

70

60

50

40

a 3020 -

10 -

1

-

-

-

-

1

1 1

134

138-

139—J

140 /i

JJJ]

i Λ

IIIull - L11J 1

jL-131,133

/

1 1 I

1

-

-

-

-

1

32 16 8 4 I 0.5 0.250 0.125 0.06mm

Figure 1. Cumulative grain-size distribution of six samplesof loose clastic basaltic debris from Cores 30 and 33,showing the coarser grain size of Core 33 material.

the fine sand to be the top of the layer. The graded layer isoverlain, with sharp contact, by a very fine grained layer, 5to 8 mm in thickness, which has a slightly coarser centralzone. This layer forms what appears to be a load cast about1 cm wide and 3 mm deep into the underlying layer (Plate 3,Figure 1). The fine-grained layer is in turn overlain, withsharp contact, by relatively uniform fine- to medium-grained hyaloclastite.

Vesicles are nearly absent in the sideromelane shards inthe bedded material. Plagioclase phenocrysts, as singlecrystal fragments or enclosed in sideromelane, make upabout 10 to 15 volume per cent. These exhibit oscillatoryzoning (Amβ -ββ), and many contain inclusions of transpa-rent sideromelane. Olivine makes up 1 to 2 modal per cent,and biogenic debris, such as foraminifers, makes up lessthan 1 percent. Tachylitic, spherulitic, variolitic, or crystal-line basalt fragments are absent from the bedded hyaloclas-tite.

The sideromelane particles in the bedded hyaloclastitehave different shapes, depending on their grain size; mostshards above about 1 mm are subequant and angular, buthave round corners except where freshly broken (Plate 3,Figures 2 and 3). With decreasing grain size, the shardsbecome increasingly flaky or sliver-like with length/widthratios up to about 20 (Plate 3, Figures 2 and 4). Many of thelarger equant fragments have curved fractures, suggestingthat some such fragments have broken down into flakyshards.

A similar relationship between grain shape and grain sizeexists for the mineral fragments. Plagioclase crystals largerthan about 1 mm are nearly equant, but they become in-creasingly flaky with decreasing grain size. Generally,however, crystals and glass are separated from each other,indicating that the interface between them is a plane ofweakness. Curved fractures in the glass commonly start atthe interface between sideromelane and crystals; thissuggests that the boundary is a preferred zone of breakage.

We completed 17 microprobe analyses for major ele-ments on 12 glass shards within the bedded hyaloclastite.Up to three analyses were done initially on single shards, totest for homogeneity. We found that the shards arehomogeneous within analytical error, so did only oneanalysis on the remaining shards. All analyses fall into one oftwo chemically similar but distinct groups, called Zl andZ2 (Table 3). Both are more similar to each other in mostelements than to the other chemical groups of Leg 46 glasses(Flower et al., this volume), especially with respect to Siθ2AI2O3. The most pronounced differences between Zland Z2 are in Tiθ2 and CaO and, less noticeably, in FeO,AI2O3, and K2O. Group Zl is the most common and includesthe larger shards, although data are still sparse. The petrologicsignificance of these chemical data is discussed elsewhere(Flower et al., this volume). Here, we emphasize that theglass shards in the bedded hyaloclastite are derived from twodistinct but similar basaltic magma batches.

Less than 10 per cent of the sideromelane shards arealtered to a relatively homogeneous yellow, isotropic"palagonite." Palagonitization of particles has generallygone to completion, and partly spherulitic particles appearto have been preferentially palagonitized over the smallerparticles. In some palagonitized shards, the interiors are

343


TABLE 2Grain-Size Analyses and Composition of Hyaloclastites, Cores 30 (Samples 131, 133,

134) and 33 (Samples 138,139, 140)

Grain Size(mm)

Sample 1312-41-20.5-10.25-0.50.125-0.25<0.125

Sample 1332-41-20.5-10.25-0.50.125-0.25

Sample 134

2-41-20.5-10.25-0.50.125-0.25<0.125

Sample 1384-82-41-20.5-10.25-0.50.125-0.75

Sample 1394-82-41-20.5-10.25-0.50.125-0.250.125

Sample 140

4-82-41-20.5-1

WeightPercentage

16236_1

-

26037-1

-

4050

9_

1-

54050

3_—

74748

2_1

-

104840

2

Sideromelane

557075806055

455560606050

5045465555-

154340405060

40

3832305040

10303540

CrystallineBasalt

2527221320

5

4037343410-

4045464040-

755052504530

55_60606033-

70606550

Aggregates

101

—-—-

1052

—

-

106

——_-

10758

_2

5_—2

_—-

1010105

Carbonate

101125

10

22225

10

_—122-

_—2123

__—352

10

_--2

SingleCrystals

-125

1530

—224

2545

133

-

—_1135

__-35

1550

———3

dissolved completely, whereas the margins have been trans-formed into crystalline phases.

It is unclear why some particles are completelypalagonitized whereas the majority of shards are completelyfresh. Incipient palagonitization can be seen along somecracks and crystal/glass boundaries, but most such bound-aries are unaffected. Preservation of delicate features ofsome palagonitized shards indicates that palagonitizationmust have occurred after deposition. Plagioclase and olivinecrystal fragments are completely fresh.

DISCUSSION

We will discuss the origin of the hyaloclastic unit in termsof the questions posed in the introduction:

1) Is the clastic debris primary in origin or is it a productof drilling? One of the first interpretations of the clasticdebris made on board ship—and indeed a suggestion madecommonly when similar debris has been encountered onother DSDP legs—is that reaming by the drill bit causescomminution of basalt, particularly when a new drill bit isforced through a hole that is tapered downward. It is thusimportant to establish reliable criteria for distinguishing be-tween such "secondary" comminution processes and thosethat occur during formation of the oceanic crust.

Several lines of evidence convincingly demonstrate thatmost of the clastic debris recovered during Leg 46 is prim-ary, although we cannot exclude the possibility that a smallportion of the material is a result of drill reaming: (a) The

344


othercomponents SIDEROMELANE

2 I 0.5 0.25Grain size in mm

0.125 2 I 0.5 0.25

Grain size in mm

0.125

Figure 2. Histograms of four samples of unconsolidated clastic basaltic debris, showing the difference in grain size amongthe samples and the increase in the sideromelane/tachylite ratio in the finer grained samples. Note the higner per cent ofcrystals in the finest grained material.

presence of bedded, partly cemented hyaloclastite (Plate 3,Figures 1 through 4) is proof that at least some of the mate-rial was a clastic deposit before drilling. Moreover, smallpieces of slightly cemented material occur commonlythroughout Cores 30 and 33. (b) The drilling rate greatlyaccelerated when the clastic layer was encountered, anddrilling rates were similar to those commonly encounteredin "sand" (in the driller's terminology) (see ShipboardSummary, this volume), (c) The logging profile (ShipboardSummary, this volume) conclusively shows an interval ofhigh porosity coinciding with the clastic layer, andporosities in this interval are much higher than in the pillowbasalt drilled above the clastic layer, (d) The high propor-tion of glass relative to crystalline basalt is good evidencethat the clastic debris did not result from simple fragmenta-tion of pillows by drill reaming. Pillows are typically about50 cm in diameter in the pillowed units, a size fairly rep-resentative of pillows in general. The pillows have only thinsideromelane rinds, about 0.5 cm thick, in agreement withdata by Moore (1975). Thus, fragmentation of an entire

pillow would yield about 2 per cent glass. This contrastsstrongly with the hyaloclastites from Hole 396B, which con-tain between 30 and 60 per cent glass (Table 1; Plate 1,Figures 1 and 2). It is unlikely that drill reaming would havepreferentially fragmented glassy pillow rinds. Moreover,the sideromelane of pillow rinds from the overlying pillowsis always partially palagonitized, in strong contrast to thefresh sideromelane shards in the hyaloclastite. (e) The oc-currence of small pillows, less than 10 cm in diameter,within the loose debris is also circumstantial evidence for aprimary origin. Similar pillows were not found in the over-lying sequence, and this suggests differences in eruptiveprocesses between the two intervals. Finally, beginningwith Core 23, coarser grained, cemented, pillow-rind brec-cias are abundant compared to the overlying pillow units,providing strong evidence that eruptive and/or depositionalprocesses had profoundly changed.

From the above evidence we conclude that the clasticmaterial of Cores 30 and 33 is definitely primary in origin,i.e., it constituted a clastic unit in the basaltic oceanic crust

345


TABLE 3Average Chemical Compositions, WithStandard Deviations, for Each Oxide

of Glass Groups 1 and 2 From the BeddedHyaloclastite

0.125 0.25 0.5 1.0 2.0Grain size interval in mm

Figure 3. Plot of grain size versus sideromelane/(sidero-melane + tachylite) ratio for unconsolidated samples.Note increase in sideromelane with decreasing grain sizein samples from Core 30 (131, 133, 134) and slightdecrease in sideromelane with decreasing grain size insamples from Core 33 (139, 140). Note also the highersideromelane I tachylite ratios in samples from Core 30,compared with those from Core 33.

before drilling. The clastic material may have been con-taminated by basaltic debris comminuted during drilling,but we think this is minor based on the above evidence.

2) Are the primary clastic processes associated witheruptions or are they due to faulting, mass wasting (such asfault scarp debris), or erosion? What distance were theparticles transported? That the sideromelane shards in thebedded hyaloclastite belong to only two different but similarchemical groups strongly suggests that these shards werewinnowed out of one, or at the most two, volcanic piles,and probably formed during the eruption. We reason that, inview of the great chemical variability in the ocean crust bothvertically and laterally within a few tens to a few hundredsof meters, material winnowed out of fault scarp debriswould probably be chemically and mineralogically muchmore heterogeneous than the material in the bedded hyalo-clastite. All the glassy shards and some of the basaltic parti-cles are very angular and have sharp edges, indicating anearby source and a lack of weathering prior to transport.The high proportion of glass to crystalline basalt is strongevidence for comminution of glassy pillow rinds duringeruption. Faulting, just as drill reaming, should producedebris with only a small glass fraction.

The almost total absence of crystalline basalt in the bed-ded hyaloclastite and its relative abundance in the loosedebris presents an unsolved problem. Part of the differencemay be a consequence of the difference in the grain size be-tween the loose and consolidatd material. We have noted

BO Number

Core-Section

Interval, cm

ShipboardChem. UnitGlass Group

SiOT i 0 2A 12°3FeO*MnOMgOCaONa2OK2OP2°5Cr2O3

NiOBaOSO3

Total

No. ofAnalyses

1350 a 1350 b

30-1

84-86

C

Z 1

49.39 ±0.371.52 ±0.01

16.15 ±0.199.18 ±0.080.15 ±0.048.04 ±0.04

11.17 ±0.023.08 ±0.020.18 ±0.020.21 ±0.080.06 ±0.02

--

0.13 ±0.03

99.26

11

C

Z 2

49.63 ±0.401.43:±0.02

16.38 ±0.168,90 ±0.100.14 ±0.048.27 ±0.15

11.38 ±0.062.98 ±0.080.16 ±0.010.20 ±0.040.07 ±0.03

--

0.12 ±0.03

99.66

6

above that the ratio of glassy to crystalline basalt increaseswith decreasing grain size, and that the bedded hyaloclastitehas a much smaller median size than the loose debris. Evenin samples of comparable grain size, however, the propor-tion of crystalline basalt in the loose debris is much higher.Perhaps the crystalline basalt is a contaminant produced bydrill reaming of the overlying pillow and breccia units. Suchan origin would explain why some of the crystalline basalt isaltered whereas the glass is extremely fresh. The close as-sociation of the hyaloclastic material with extensively brec-ciated basalt layers is further evidence of a primary origin.

3) What is the nature of the submarine clastic processes?We have presented evidence that the hyaloclastites areprimary deposits formed by submarine eruptions. Four mainmechanisms have been suggested to explain the formationof volcaniclastic material under sub-aquatic conditions:(a) Vesiculation caused by expansion of magmatic gases;(b) steam explosions; (c) spalling of pillow rinds; and(d) direct granulation. We can exclude the first two process-es because the probable depth of extrusion of the lavas(over 3000 m below sea level) was such that the confiningpressures were too high to allow vaporization of sea waterand significant exsolution of magmatic volatiles. This con-clusion is supported by the paucity of vesicles in both theglass and crystalline basalt, although a few particles mayhave formed by vesiculation (Plate 2, Figure 3). Rittmann(1960) proposed that ash-size basaltic particles can be pro-duced in situ by breakage of pillow rinds during submarineextrusion, and he coined the term hyaloclastite for suchmaterial. In many pillow sequences the uppermost part ofglassy pillow rinds if often partly flaked off or wedgedapart, and the fractures are wholly or partly filled with cal-

346


cite or zeolites. Although this wedging apart may have oc-curred after accumulation of the pillows, the brittle fracturesin the quenched rind most likely formed during extrusion.

Another process that may form nonvesicularsideromelane shards when lava is erupted under, or flowsinto, water is granulation caused by thermal shock. Obser-vations from lava flows entering water show that granula-tion may occur under special conditions. Moore et al.(1973) observed that only very thin lava streams becomegranulated upon entering the water. Some clastic disruptioncan also occur in larger lava flows, but in these cases steameruptions are involved. Flake-like shards may also formduring quenching of basalt lava by water (Plate 1, Figures 3and 4) (see also Carlisle, 1963).

Unfortunately, no criteria are available at present thatpermit us to distinguish between flaky sideromelane shardsformed from spalling of glassy pillow rinds and those pro-duced by granulation. That feldspar crystal fragments in thebedded hyaloclastite have flake-like shapes similar to thoseof sideromelane shards suggests that cracking was so rapidthat fractures were propagated through glass and crystalsalike. The relative abundance of tachylite indicates that pro-cesses other than spalling and granulation have been opera-tive, and it is possible that some of the tachylite is attributa-ble to drill-bit reaming.

Regardless of the exact mechanism of formation, theabundance of hyaloclastic material in Hole 396B suggestssomewhat unusual eruptive processes. Drilling data,ocean-bottom photographs, and direct observations by sub-mersibles have shown that pillow lavas are by far the mostcommon mode of extrusion of basalt lava on the sea floor.We suggest that the hyaloclastite material can be accountedfor by very rapid extrusion of lava onto the sea floor, build-ing up steep-sided piles of pillow basalt. Frequent slumpingof pillows on the sides of the mounds, followed by periodicspurts of lava, could lead to rapid spalling of pillow rindsand granulation of glassy material. Normal currents, orconvection currents caused by heating of sea water duringeruption, could winnow the ash-size spallation and granula-tion shards and deposit them nearby, either on the flanks ofsuch piles—as the presence of small pillows mightsuggest—or a short distance away. The great thickness ofthe clastic sequence might be explained by either of twohypotheses: (a) the clastic deposit may represent a moundof granulated material with some small pillows whichformed as a constructional feature around a local vent, or(b) the clastic material may have been winnowed from anearby pillow lava pile and deposited in front of a pillowpile or in an adjacent basin or depression. We are unable toexplain the absence of crystalline basalt fragments in theindurated hyaloclastite and their common presence in theloose material. Two likely possibilities are caving of mate-rial from the overlying breccia and reaming by the drill bit,as discussed above. It is also possible that some crystallinematerial may have been supplied from a different source orby different currents. The presence of foraminifers in thebedded hyaloclastite, and the bedding itself, are good evi-dence for the presence of currents.

4) What is the significance of the hyaloclastite layer inthe formation of oceanic Layer 2? The occurrence of a90-meter-thick layer of rather loose clastic volcanic material

of low density in the upper oceanic crust may help accountfor the low seismic velocities found in Layer 2a. Althoughwe suspect that this layer has a limited lateral extent, webelieve that similar deposits occur elsewhere, and so theoverall volume of clastic material in Layer 2a may be large.Similar clastic basaltic material has been found in other drillholes (Legs 37 and 49); this suggests that clastic volcaniclayers may make up a significant portion of the oceaniccrust.

If our suggestion is correct that unusually rapid extrusionproduced the thick pile of clastic material, we may speculatethat such events are episodic. Study of the oceanic crustduring the past few years by drilling and by submersibleoperations has revealed that constructional processes aretypically episodic, in terms of volcanic and tectonic activ-ity. The data presented here suggest that intensity of vol-canic activity or eruptive mechanisms may also vary sig-nificantly. Whereas most extrusive activity on the sea floorresults in pillow lava and associated breccias and rarerflows, volcanic activity may sporadically be so intense thatsignificant amounts of sand-size particles are produced. Sofar, nothing has been discovered to suggest that the post-ulated high intensity was caused either by unusual chemicalcomposition or by high content of volatile material in themagmas; such factors are often responsible for very intenseactivity in subaerial volcanoes. The intense activity maythus have been controlled by tectonic activity along themedian rift. If so, such activity may have been relativelywidespread along the rift at this particular time.

The striking freshness of the sideromelane and the minorcementation by secondary minerals contrasts strongly withthe overlying pillow sequence, even though reactive sur-faces, porosities, and permeabilities are much higher in thehyaloclastic unit. This general absence of palagonite may bea consequence of any of several possible causes:(a) Sideromelane may have originally had palagonite rimswhich were broken off by winnowing during drilling ortransport. This hypothesis can be excluded becausepalagonitization always tends to round off corners, and thesideromelane shards have sharp corners and sides.Moreover, particles in the indurated hyaloclastites do notdiffer in angularity and freshness from the loose particles.(b) Palagonitization may have been prevented by an earlyphase of carbonate cementation which was later dissolved.This is also unlikely, because carbonate is generally thelatest cementing phase and it tends to replace sideromelaneas well as phenocryst phases, (c) The glass shards havebeen sealed off from sea water. If the glass shards had beenin contact with sea water either on the sea floor or afterburial within the crust, they should all have become margi-nally palagonitized (Moore, 1966; Bryan and Moore,1977). The absence of palagonite must thus mean an essen-tially dry environment (except for local spots) for the last 13m.y. Such an interpretation sounds quite implausible, but itis the only one presently compatible with the evidence.Such a dry space could have been caused by rapid burial ofthe clastic material which thus sealed it off from percolatingwater. This sealing off may have resulted from rapidpalagonitization and cementation of the inter-pillow spacesin the overlying lavas, or it may reflect rapid sedimentation,or both.

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If such sealing can be shown to have occurred, it wouldhelp explain one of the major finds of several DSDP cruises,i.e., that alteration does not increase downward as post-ulated in some models for the oceanic crust. These modelscall for wholesale downward percolation of sea waterthroughout the crust, except at a few places of upwellinghydrothermal cells. It may be that rapid sedimentation andthorough cementation of the upper 100 to 200 meters of theoceanic crust may form an impermeable cap under whichlittle alteration takes place. Supporting evidence for thesuggestion that the hyaloclastites remained dry comes fromtemperature measurements in Hole 396B. Erickson (thisvolume) concludes that cold sea water was moving down-ward into the hyaloclastite layer because of the rather lowtemperatures encountered.

ACKNOWLEDGMENTSOur work was supported by grants (Schm 250/16-18) from the

Deutsche Forschungsgemeinshaft, which we gratefully acknow-ledge. The manuscript was written while H.-U.S. was supportedby a stipend from the VW foundation. We thank D. Pierce for helpwith the scanning electron photomicrographs. D. A. Swanson

supplied us with a sample of Makaopuhi "steam blast" drillingdebris. This is contribution No. 80 from the Biogeology CleanLab, University of California, Santa Barbara.

REFERENCES

Bryan, W.B. and Moore, J.G., 1977. Compositional variations ofyoung basalts in the Mid-Atlantic Ridge rift valley near lat36°49'N, Geol. Soc. Am. Bull., v. 88, p. 556-570.

Carlisle, D., 1963. Pillow breccias and their aquagene tuffs,Quadra Island, British Columbia, J. Geol., v. 71, p. 48-71.

Moore, J.G., 1966. Rate of palagonitization of submarine basaltadjacent to Hawaii. U.S. Geol. Survey Prof. Paper 550-D,p. D163-D171.

Moore, J.G., 1975. Mechanism of formation of pillow lava, Am.Scientist, v. 63, p. 269-277.

Moore, J.G., Phillips, R.L., Grigg, R.W., et al., 1973. How oflava into the sea, 1969-1971, Kilauea Volcano, Hawaii, Geol.Soc. Am. Bull., v. 84, p. 537-546.

Rittmann, A., 1960. Vulkane und ihre Tàtigkeit. Enke, Stuttgart,p.335.

Shipboard Scientific Crew, 1976. Glass-rich basaltic sand andgravel within the oceanic crust at 22°N, Nature, v. 262, p.768-770.

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22. basaltic hyaloclastites from hole 396b, … · 22. basaltic hyaloclastites from hole 396b, dsdp...

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