18. CRYSTAL MORPHOLOGIES IN BASALTS FROM DSDP SITE 39523° N, 46° W, MID-ATLANTIC RIDGE

James H. Natland, Deep Sea Drilling Project, Scripps Institution of Oceanography, La Jolla, California

INTRODUCTION

At DSDP Site 395, a sequence of basaltic pillowlavas, thin basaltic flows, and intrusive basalts wasdrilled in two holes, along with minor gabbroic andultramafic rocks (Chapter 7, this volume). Both aphyricand strongly phyric basalts were recovered. The latterinclude both plagioclase-olivine phyric and plagioclase-olivine-clinopyroxene phyric basalts. The aphyric andphyric basalts were designated types A and P, respec-tively, on the basis of chemical and magneticstratigraphy (Chapter 7, this volume). Three aphyric (A2through A4) and five phyric (Pi through P5) basalt typeswere defined. Their general stratigraphy is shown inFigure 1, together with the correlation between Holes395 and 395A. Originally, an additional aphyric basalttype, Ai, was defined on the basis of one chemicalanalysis (shipboard data, Bougault et al., this volume);but shore-based analyses later demonstrated that similarrocks occur sporadically throughout Unit A2, so the twotypes were combined (Chapter 7, this volume). In addi-tion, intrusive rocks deeper in Hole 395A are designatedtype P4 ' , indicating close chemical similarity to Unit P4higher in the hole. The two units also have the samemagnetic inclination (Figure 1).

Each chemical unit can consist of glassy or near-glassy pillow fragments, coarser grained pillow interiorsand, in some cases, thicker flows (e.g., Unit P3, Figure1). Some units contain fracture breccias or glass breccias(P5, A3, and A4). All are altered and fractured to greateror lesser degrees. The one common characteristic ofeach unit, however, is not its lithologic variability, butits internal chemical coherence (Chapter 7, this volume;Bougault et al., this volume).

The texture of basalts in any given unit can vary con-siderably. The range of crystal morphologies dependslargely on what portion of a pillow rim or interior is ex-amined, but it also depends on chemistry. Near theglassy rims of pillows, spectacular spherulitic to skeletalcrystals can be seen in thin sections, a consequence ofthe high degrees of undercooling when the molten lavaswere extruded as pillows or thin flows into the frigidwater of the deep sea floor. Within the pillow or flow in-teriors, the rocks are more holocrystalline, and mostcrystals have euhedral, tabular, or granular forms.

This paper is essentially a pictorial display, with com-mentary, of silicate crystal morphologies in Site 395basalts. By means of photomicrographs, I will illustratethe cooling history of the basalts, the dependence ofcrystal morphologies on undercooling and on surpris-ingly small differences in chemistry, and the significance

of phenocrysts in the phyric basalts on theirpetrogenesis. For discussion of opaque minerals, seeJohnson (this volume), and for spinels, Graham et al.(this volume).

Much of the interpretation in this paper dependscritically on the chemical, mineralogical, and ex-perimental work of other contributors to this volume. Ihope that these photomicrographs will extend andclarify many of their conclusions. The terminology forcrystal morphologies adopted herein is primarily that ofLofgren (1971, 1974) for spherulitic forms in generaland Donaldson (1976) for olivines. Some of the terms ofBryan (1972) are also used.

The emphasis here is not so much on particularminerals or particular crystal morphologies as onmineral interrelationships. Samples from deep-penetration DSDP sites are particularly suited for thistype of study because stratigraphic control is superb,because all ranges of crystal morphologies of particularchemical types occur, and because a good variety ofchemical types closely related in time and space can beexamined.

CRYSTAL MORHPOLOGIES IN APHYRICBASALTS

The type A2 aphyric basalts, present in both Hole395 and Hole 395A, are pillows, each commonly with avariolitic zone, a vesicular zone, and a chilled, but rarelyglassy, rind. Textures range from extremely fine grainedto coarsely microlitic, but very little true glass wasfound.

The reason for this appears related to the unusuallylarge (up to 1 cm) variolites abundant in these basalts."Variolites" here is a term used in much its originalsense: "spherical bodies appearing on the weatheredsurfaces or marginal portions of certain diabases"(quoted from Lofgren, 1974, his table 2, after Johann-sen, 1939). A typical piece of basalt from aphyric UnitA2 is shown in Figure 2, along with representativephotomicrographs showing textures and crystal mor-phologies with increasing distance away from inferredquench margins. The variolites themselves are mats orsworls of extremely fine Plagioclase needles, dotted withopaques, and criss-crossed with altered linked-chain(terminology of Donaldson, 1976) or "lantern andchain" olivine (terminology of Bryan, 1972). These areshown in Figure 2B.

The material surrounding the variolites is buff in col-or, apparently the result of oxidation of glass ortitanomagnetite to clays and iron oxyhydroxides.Altered linked-chain olivines are still present, but the

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Figure 1. Basement stratigraphy, Site 395.

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0.5 mmFigure 2. Crystal morphologies in basalts of Unit A2• Unit A2, aphyric basalt, is a series of pillow lavas with abundant evi-

dence for extensive undercooling under water. The large photograph shows a single piece with large "variolites" (dark graypatches) surrounded by much finer grained basalt. The increase in grain size toward the cooling-unit interior is shown bythe photomicrographs, all to the same scale. Photo A shows extremely elongate linked-chain olivines in a matrix of acicularPlagioclase microlites. Photo B, the interior of one of the variolites, is somewhat coarser grained, and has abundant titano-magnetite in addition to acicular Plagioclase. Photo C, taken away from the variolitic zone, shows ornamented "hopper cry-stal" olivines, less elongate than in A orB, in a matrix of acicular Plagioclase and titanomagnetite. Photo D represents aboutthe coarsest grain size recovered in this unit. The central crystals are granular or skeletal olivines. They are surrounded byskeletal Plagioclase enclosing clinopyroxene, titanomagnetite, or glass. Sample numbers: Photos A andB: 395A-9-1, 128-138 cm (#3). Photo C: 395A-7-1, 75-77 cm (#3). Photo D: 395-15-2, 130-136 cm (#3B). Basalt piece is Sample 395-16-2, 37-45 cm (#7). Top is shown by arrow on piece.

fine Plagioclase needles in the matrix are so fine as to bebarely discernible (Figure 2A). The variolites thereforeappear to be centers of nucleation which formed atslightly lesser undercoolings than the extreme edges ofthe cooling units.

Coarser grained portions of cooling units containvery elongate, possibly dendritic, Plagioclase microlites.Olivine crystals are better formed, fresh, and not soelongate (Figure 2C). They conform to the "hoppercrystal" morphology of Donaldson (1976).

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Titanomagnetite is skeletal and abundant. Clinopyrox-ene, if present, is in the spaces between the Plagioclasecrystals; its morphology cannot be discerned.

The coarser grained basalts of aphyric Unit A2 con-tain subhedral to skeletal olivine up to 0.5 mm long,discernible in hand specimens (Figure 2D). These are ina matrix of stubby to elongate skeletal Plagioclasecrystals, each with one or more hollow interior spacesfilled with probably dendritic augitic clinopyroxene andskeletal titanomagnetite. This material also surroundsthe exterior of the Plagioclase crystals. The Plagioclasehas tended to nucleate on the larger olivine crystals,from which it grows in radial sworls. For most sworls,however, no nucleating agent can be seen.

The contrasts between aphyric Unit A2 and aphyricUnits A3 and A4 is subtle chemically (Chapter 7, thisvolume; Bougault et al., this volume), but distinctpetrographically. Figure 3 shows a series ofphotomicrographs, paralleling the sequence of Figure 2,from glassy rims to coarser interiors. Units A3 and A4are quite similar petrographically, and are here de-scribed together. Unlike Unit A2, in which virtually noglass was found, Units A3 and A4 contain abundantglass, including a thick glass breccia zone (lithologicUnit 19 of Hole 395A, table 3 in Chapter 7, thisvolume). The glass is clear and pale brown, with tinycrystallites of olivine that have tiny swallow-tail armsradiating from the corners (Figure 3A). The crystallitesare surrounded by fine feathery spherulitic halos whichare more readily seen in cross-polarized light (Figure3B). About 0.5 to 1 cm farther toward cooling unit in-teriors, these are joined by dark brown to blackspherulites about 0.05 mm in diameter. These often arearranged in curved rows, paralleling the glassy margin(Figure 4). About 0.5 cm farther into cooling units,these spherulites become predominant, and contain intheir centers tiny Plagioclase microlites which often haveswallow-tail arms merging with the brown spheruliticovergrowths. Fibers within the spherulites can be den-dritic (Figure 3C). Between about 2 and 5 cm fromglassy edges, Plagioclase becomes distinctly skeletal andelongate, having grown radially from nucleation centerssuch as olivine (Figure 3D). But it does not reach the ex-treme c-axis elongation of Plagioclase in aphyric UnitA2. Clinopyroxene forms spherulitic patches betweenthe radiating Plagioclase grains, and also occurs withinthe elongate axial hollows of the skeletal Plagioclase.Titanomagnetite, which is not evident in the glassy andspherulitic zones of typical A3 and A4 basalts, first oc-curs here.

In the coarser grained rocks (which are finer grainedthan any basalts in Unit A2), stubby euhedral toelongate skeletal plagioclases are intergrown withgranular olivine, and surround later-grown pale brownaugitic clinopyroxene, skeletal titanomagnetite, andpatches of brown glass (Figure 3E).

The striking contrast in crystal morphologies be-tween coarser grained microlitic basalts of aphyric UnitsA2 and A3 can be seen in Figure 5, which compares twosamples with well-developed skeletal Plagioclase. TheA2 basalt is distinctly coarser grained than the A3 basalt

(we recovered no A3 basalts much coarser grained thanthat shown in Figure 5), yet the skeletal Plagioclasehabit and c-axis elongation is much more pronounced.

Table 1 summarizes the sequence of crystallization ofminerals and their morphologies with increasingdistance into cooling units. The morphologic changes ingeneral conform to those predicted experimentally forboth olivine (Donaldson, 1976) and Plagioclase(Lofgren, 1974) at decreasing undercoolings. The dif-ferences between the two types of aphyric basalt appearto arise primarily because the aphyric basalt Units A3and A4 are more fractionated than Unit A2 (Bougault etal., this volume; Rhodes et al., this volume). A2 basaltshave both more Fe (as FeO*) and more MgO than A3 andA4 basalts, so are more prone to crystallize olivine.Olivine is not only more abundant in the A2 basaltsthan in the others, but its apparent abundance isenhanced at higher undercoolings (note abundant "hop-per" olivines in Figure 2C). The higher total iron mayalso explain why the titanomagnetite/clinopyroxenecrystallization sequence is reversed at higher undercool-ings in the A2 basalts, and why titanomagnetite occurs somuch closer to cooling unit boundaries than in the A3and A4 basalts. A similar reversal of crystallization se-quence between Plagioclase and ilmenite was obtainedat high undercoolings in experiments on a lunar picriticbasalt by Walker et al. (1976). There is no completelyobvious reason why crystal morphologies at about thesame distance from cooling unit boundaries seem to dif-fer between the A2 and A3-A4 basalts (linked-chainolivine nearest A2 cooling unit boundaries versusswallow-tail olivine in a similar zone in the A3 and A4basalts, for one example, and extreme c-axis elongationin the A2 basalts but not in the A3 and A4 basalts, foranother). The causes are probably compositional, butcould also be related to melt viscosity (a function in partof volatile contents), or PQ2 . Some combination of fac-tors limited the diffusion rate at all levels of undercool-ing in the A2 basalts more effectively than in A3 and A4basalts, inhibiting formation of polyhedral crystals inthe coarser grained basalts, and of all but needle-likecrystals in the finer grained basalts. No well-definedcenters of nucleation could develop in the finer grainedA2 basalts, so the large variolites grew near the chilledmargins. All these morphologies appear to result fromhigh undercooling, since a range of crystal mor-phologies similar to that in A3 basalts was produced ex-perimentally at cooling rates between 2°/hr and 30°/hrby Dungan et al. (this volume).

CRYSTAL MORPHOLOGIES IN PHYRIC BASALTS

Phyric basalts in Hole 395 can be divided into twotypes (Figure 1): an upper plagioclase-olivine-clinopyroxene phyric group (Core 18, chemical type Pi)and a lower plagioclase-olivine phyric group (Cores 19and 20, chemical type P2). Pi is not present in Hole395A, but P2 extends from Core 13 to Core 16.Chemical type P3 (Cores 17 through 22) is once againplagioclase-clinopyroxene-olivine phyric, as is P4 (Cores22 through 27). Phyric Unit P5 (Cores 28 through 33)contains mainly Plagioclase and olivine phenocrysts, but

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Figure 3. Crystal morphologies in basalts of Units A3 andA4 (A3 illustrated). Photos A through E represent typical texturesfrom glassy margins to cooling-unit interiors. Glasses typically contain tiny crystallites of olivine (A) with bright "halos"in cross-polarized light (B). Incipient Plagioclase crystallites form the interiors of the darker spherulites in (B). and becomecrowded and less spherical toward the cooling-unit interior (C). The individual fibers of spherulites in (C) can be dendrites.Still farther into cooling units, radiating acicular Plagioclase bundles emenate from olivine nuclei and enclose incipient clino-pyroxene (D). The coarsest grained rocks show granular olivine and skeletal Plagioclase, and partially skeletal clino-pyroxene and titanomagnetite. Note contrast in behavior of crystals closest to quench margins between Units A2 (Figure2) and A3. Sample numbers: Photos A and B: 395A-56-3, 94-96 cm (#11). Photo C: 395A-55-2, 137-138 cm (#8).Photo D: 395A-17-1,102-107cm (#12). Photo E: 395A-55-2, 72-75 cm (#3D).

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palagonite

Figure 4. Sample 395A-56-3, 94-96 cm. Aligned Plagioclase spherulites in glassy rim ofUnit A ̂ pillow. Transmitted light. Width of figure at top is 1.4 cm. Note palagoniteat edge of glass and small Plagioclase crystal to left of center.

Cores 29, 31, and 33 have minor clinopyroxenephenocrysts.

A general description of these rocks was given inChapter 7 (this volume) as follows: "In general, thesephyric units are characterized by an abundance ofeuhedral Plagioclase phenocrysts, up to 25 per cent inparts (visual estimate), with about 7 per cent olivine andminor clinopyroxene. A number of thin sections, madefrom hand specimens in which pale emerald greenclinopyroxenes were seen, either with the unaided eye orusing a binocular microscope (× 6), have no clinopyrox-ene phenocrysts when examined microscopically,because the distribution of clinopyroxene as aphenocryst phase is patchy. In Cores 13 through 15,none was visible either in hand specimen or in thin sec-tion. These fine-grained phyric basalts contain asphenocrysts about 20 per cent Plagioclase, An75 (0.3 to0.5 cm across), and 7 per cent olivine (~Fo85, 2V~90°)0.3 cm across, set in a holocrystalline groundmass ofclinopyroxene (2V~60°), olivine, lath Plagioclase, andtitanomagnetite. The feldspar phenocrysts show bothnormal and oscillatory zoning, and often contain inclu-sions of brown glass as blebs in their cores. In Cores 16through 22, clinopyroxene is present as a phenocrystphase, and rare, rounded, dark brown spinel occurs" (italso occurs in Units P4 and P5).

There are two generations of Plagioclase crystals inall these phyric basalts. Large phenocrysts (0.3 to 0.5cm long) have cores with compositions of around An86-83(Dungan et al., this volume) and rims of An73-62 Theyoften show simple albite twinning, and the cores almost

always have abundant glass inclusions (Figure 6A). Inthe coarser grained rocks, these inclusions are finelycrystalline (Figure 6B), but nearer cooling unit boun-daries they are still glass. The more sodic rims of thesecrystals have only minor glass inclusions (Figure 6C andD). In the coarser grained rocks (sub-ophitic to gabbroictextures), the rims can have extremely irregular termina-tions (Figure 6A), and the sides of some crystals areplated with smaller Plagioclase crystals which apparent-ly grew by heterogeneous nucleation (Figure 7). Inspherulitic and glassy basalts, the sodic rims have no in-clusions, but under conditions of high undercooling(upon extrusion of pillows from larger flows?), theyare dendritic (Figures 6C and D). In these basalts themore sodic rims also tend to have more "squared-off"morphologies than the crystals they surround (Figure6E). The surrounded crystals can have roundedoutlines, suggesting that they were first partially re-sorbed (Figure 6E).

The more sodic rims are compositionally identical tothe range of compositions of the second generation ofPlagioclase phenocrysts in the phyric basalts, and aretherefore part of that second generation. Crystal mor-phologies of single second-generation Plagioclasephenocrysts are simple, and tend to be square or rec-tilinear in glassy rocks (Figure 8A), with at most a singletwinning plane within them. Some are skeletal (Figure8B). In some basalts there are glomerophyric clumps ofPlagioclase with more complex twinning (Figure 8C),but these too belong to the more sodic generation ofphenocrysts. The crystal clumps can have feldspars

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Figure 5. (A) Sample 395A-5-1, 100-102 cm (#16D), Unit A2• Transmitted light. Long dimension= 3 cm. (B) Sample 395A-55-2, 72-75 cm (#4), Unit A3. Transmitted light. Long dimension =2.9 cm.

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TABLE 1Sequence of Crystallization and Crystal Morphologies in Site 395 Aphyric Basalts

as They Vary With Distance From Cooling Unit Boundaries

A 2 A3"A4

0-1 cm 1) Olivine (linked-chain)2) Plagioclase needles and sworls

Rare glass (glassiest zones have spheru-litic Plagioclase and ironoxyhydroxides)

~I-IO cm 1) Olivine (linked-chain to hopper crystals)2) Plagioclase needles (in variolites)3) Titanomagnetite

> 10 cm 1) Olivine (granular)2) Plagioclase (skeletal and large)3) Clinopyroxene (tiny and dendritic(?))4) Titanomagnetite (skeletal)

1) Olivine (swallow-tail and feathery)Glass

1) Olivine (swallow-tail to granular)2) Plagioclase (spherulites to radial clumps)

1) Olivine (granular)2) Plagioclase (skeletal to euhadral)3) Clinopyroxene (dendritic)4) Titanomagnetite (skeletal)

Note: Numbers by each mineral indicate order of crystallization.

stacked upon each other sideways (Figure 8D), or theycan be randomly oriented (Figure 8E). The first typemay have grown by heterogeneous nucleation, perhapspromoted by flow of the basalt (note parallel orienta-tion of many Plagioclase phenocrysts in Figure 9). Themore randomly oriented kind may have grown within amagma chamber where magma movements were eitherminor or turbulent, and in any case not laminar.Magma movements may also be indicated by second-generation Plagioclase with fine oscillatory zoning(Figure 8F).

Of the two types of Plagioclase phenocrysts, the moresodic variety is perhaps an order of magnitude moreabundant numerically. But because they are so muchsmaller (0.1 to 1 mm) than the calcic types, the volumeproportions of the two types are probably about thesame (Figure 9).

Olivine and clinopyroxene phenocrysts are much lessabundant than Plagioclase phenocrysts, and it is notalways clear with which generation of Plagioclase theymight have crystallized. Most olivine apparently belongswith the older generation of plagioclases. Dungan et al.(this volume) found the olivine in phyric basalts to bedistinctly more forsteritic than olivine grown underequilibrium conditions at low pressure from whole-rockstarting compositions. (They also noted that Plagioclasegrown in the same experiments was more sodic than thecores of the large phenocrysts.) Glass inclusion com-positions from both olivine and Plagioclase phenocrystsare also less evolved than the glass from quickly chilledrims of the phyric basalts (Fujii et al., this volume;Rhodes et al., this volume). This complex of data sug-gests to Rhodes et al. (this volume) and Dungan et al.(this volume) that the first (more calcic) generation ofPlagioclase phenocrysts, as well as the olivine, grew in amore primitive melt than that represented by the bulkcomposition of the phyric basalt types or theirphenocryst-free glasses. They propose that thisprimitive magma, charged with phenocrysts, invadedone or more magma chambers containing substantiallymore fractionated basalts, and that the phyric basalttypes therefore are a hybrid of at least two distinctmagma types.

Because spinel and clinopyroxene compositions inthe experiments conformed with those in the basalts,Dungan et al. (this volume) thought them to be"probably not exotic to these rocks." Spinels andclinopyroxene therefore appear to belong for the mostpart to the second generation of phenocryst minerals.

In one respect, the experiments oversimplify inter-pretation of the growth of phenocrysts in the phyricbasalts. The experiments were done on whole-rock com-positions, not on either of the inferred end-members,"primitive" or "evolved," of which the whole rocks ap-pear to be hybrids.

The experiments were also done on a sample fromUnit P4, which has a more fractionated compositionthan Units Pi and P3, and differs from them in thatclinopyroxene phenocrysts are scarce or absent (Chapter7, this volume). The experiments therefore apply mainlyto the more evolved Plagioclase and olivine phyric unitsP2, P4, and P5. There appear to be, in fact, two differenttypes of olivine phenocrysts and several different typesof clinopyroxene phenocrysts in the phyric basalts(Figure 10).

In some basalts, olivine can occur as large wholemegacrysts with fractured and cracked edges (Figure10A). These appear to be "first-generation" olivine. Atthe time of extrusion, they were not growing in their sur-roundings, and were too magnesian to have beenundergoing resorption (assuming they were "mixed" in-to a more fractionated liquid than that in which theyoriginally grew). The edges, however, appear to bemechanically disintegrating. Also, in sinking or other-wise moving through their originally molten surround-ings, many have "collected" clusters of other crystalsaround them, as in Figure 10A. In other basalts, olivinephenocrysts can be euhedral, and enclose Plagioclasecrystals.

Other olivines are distinctly skeletal, conforming tothe "granular" or "hopper-crystal" terminology ofDonaldson (1976). These crystals are considerablylarger than their morphological equivalents in theaphyric basalts. Some, indeed, are several millimeters indiameter, and contain "cavernous" round or oval glassor groundmass inclusions (Figure 10B). Others have

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1.0 mm 0.1 mm

0.1 mm 0.1 mm

0.1 mm

Figure 6. (A) Sample 395-18-1, 37 cm (#1), Unit Pj. Crystal showing internal skeletal structure(glass inclusions) indicative of initial period of high undercooling, a zone of stable growth (noglass inclusions), and a zoned dendritic rim, grown at high undercooling (pillow extrusion) Planelight slightly cross-polarized to show twinning. (B) Same sample as in A, showing detail ofspheru-litic crystal growth within inclusions in Plagioclase phenocryst. Crossed nicols. (C) Same sam-ple as in A, showing "staircase" dendritic overgrowth in sodic rim of sharply zoned phenocryst.Note how merging of dendritic overgrowths results in skeletal inclusions of groundmass. Alsonote thin twin lamellae paralleling shorter faces of overgrowth. Crossed nicols. (D) Sample395A-17-1, 131-135 cm (#15D), Unit P2• "Staircase" dendritic crystal margin and skeletal in-clusions in sodic Plagioclase microphenocryst. Flared tips of overgrowths indicate even higherundercoolings as the rock was rapidly chilled upon extrusion. Plane Ugh t. (E) Sample 395-19-1,36-38 cm (#4), UnitP2. Sharply zoned Plagioclase phenocryst with spherulitic inclusion at bot-tom. The rounded edges of the inner zone suggest that resorption was followed by overgrowthof a more sodic rim, first at relatively small undercoolings, then at larger undercoolings, to pro-duce the "squared-off" crystal boundary, which trapped inclusions of groundmass material.

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Figure 7. Sample 395A-15-1, 13-17 cm (#1D), Unit P2. Large Plagioclase crystal in center is 3.2 mmat its widest part. The negative image was taken by putting the thin section directly in an enlarger,and exposing print paper directly. Extremely fine details, such as strings of very tiny melt in-clusions in the Plagioclase, can be resolved in this way. Core of large Plagioclase has compositionAn88; rim is An7g. Olivine at bottom is FQ9Q (J. Natland, unpublished data). Large dark glass in-clusions in center of Plagioclase are here prominently white. The crystal is plated with smaller secondgeneration Plagioclase crystals.

outlines which suggest the more typical dome orprismatic forms of olivine (Figures IOC and 10D). Theseare invariably associated with second-generationPlagioclase, similar in size and even in morphology,tending to be simple euhedral to skeletal crystals (Figure10D). Rarely, small euhedral olivines can be seen, somewith sector zoning (Figure 10E).

The various types of clinopyroxene phenocrysts areexemplified by Figure 11 A, which shows a roundedphenocryst with a rim of intergrown Plagioclase andclinopyroxene; a large second-generation Plagioclasephenocryst is at the upper right. Figure HB showsdetails of a similar clinopyroxene-plagioclase rim on alarger phenocryst. The clinopyroxene in the rim hasgrown dendritically from the core, but the attachedPlagioclase crystals have a somewhat random orienta-tion. The material outside the crystal is extremely finegrained, and crystallized at fairly high undercoolings.The intergrowth of clinopyroxene and Plagioclase at thephenocryst rims suggest that, at the high undercoolingsin which the dendritic rim formed, crystallization of theclinopyroxene locally enriched the residual liquid in thecomponents of Plagioclase, which therefore grew inwhatever remaining space it could find.

A smaller version of a two-stage clinopyroxene isshown on Figure HC. This clinopyroxene shows sectorzoning and has a distinct rim. Rims of clinopyroxene

such as this are both normal and reversely zoned withrespect to the crystal interiors (Fujii and Fujioka, thisvolume).

Figure 1 ID shows one final example of a clinopyrox-ene phenocryst. This one is axially intergrown with aPlagioclase phenocryst. It also shows strong radial com-positional zoning about its intersection with thePlagioclase grain. This almost certainly reflects com-positional gradients induced by the strong partitioningof Ca into the Plagioclase.

There appear, then, to be several different stages ofcrystallization represented by olivine and clinopyrox-enes in the phyric basalts. The olivines represent at leasttwo stages. The first consists of the more forsteriticvariety, which occurs as granular megacrysts,sometimes with tattered edges, sometimes euhedral.These correspond in many cases to the first generationof Plagioclase phenocrysts. The second consists of theskeletal olivine "hopper" crystals, or sector-zonedcrystals, which grew simultaneously with the more sodicsecond-generation plagioclases. Among the clinopyrox-enes occur compositionally uniform granular pheno-crysts with dendritic edges, normal- and reversely-rimmed, sector-zoned phenocrysts, and unrimmedradially-zoned phenocrysts intergrown with late-stagePlagioclase. Clinopyroxene is also a major groundmassphase. In the more fractionated phyric basalts (Units

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CRYSTAL MORPHOLOGIES

0.1 mm 0.5 mm

1.0 mm

1.0 mm

\

4

E

8k^jj|, 1 l 1

I*mmmnm

i| ; 1 jy

1.0 mm 0.5 mm

Figure 8. (A) Sample 395A-17-1, 131-135 cm (#15D), Unit P5. Plane polarized light. Tabular plagio-clase crystals joined lengthwise in spherulitic groundmass. Note thin dendritic projections at cornersof the crystals. (B) Sample 395A-17-1, 131-135 cm (#15D), Unit P3. Skeletal second-generationPlagioclase crystal in same sample as A. Plane polarized light. (C) Sample 395-20-1, 32-36 cm (#5),Unit P2. Crossed nicols. Composite photomicrograph of a large Plagioclase glomerocryst. (D) Sam-ple 395-18-2, 38 cm (#7), Unit P2. Plane polarized light. Synneusis cluster of tabular second-generation Plagioclase microphenocrysts. (E) Sample 5'95A-15-1, 93-94 cm (#10), Unit P2. Crossednicols. Central portion of a large second-generation Plagioclase glomerocryst in which most crystalsare at irregular angles to one another. (F) Sample 395A-23-1, 64-68 cm (#4), Unit P4. Oscillatoryzoning in Plagioclase microphenocryst.

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Figure 9. Sample 395A-17-1, 84-89 cm, Unit P3. Full thin-section view in transmittedlight. Section width about 2.4 cm. Note sub-parallel alignment of many Plagioclase cry-stals (tabular, elongate, many with glass inclusions), and in the center the large irregularglomerocryst without inclusions. Clumps of smaller crystals are second-generationPlagioclase and clinopyroxene.

P2, P4, and P5), no clinopyroxene is definitely in-tergrown with a first-generation Plagioclase, but olivineis (in the dolerite). But olivine and clinopyroxene can beintergrown with second-generation Plagioclase. Clino-pyroxene, but not olivine, is zoned and shows dendriticmargins, reflecting clinopyroxene growth at the fairly

extreme undercoolings represented by the typical fine-grained to spherulitic groundmass. Olivine is not pre-sent in the groundmass of the finer grained rocks.

Based on mineral relationships and the experimentsof Dungan et al. (this volume), there were at least threestages of phenocryst growth in Units P2, P4, and Ps. The

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0.1 mm 0.1 mm

0.5 mm 0.1 mm

0.5 mm

Figure 10. (A) Sample 395-18-2, 38 cm (#7), Unit Pj. Crossed nicols. Collection of second-generationPlagioclase crystals along one side of an olivine megacryst which has a broken, irregular edge. (B) Sample395-18-2, 38 cm (#7), Unit Pj. Crossed nicols. Large embayed olivine crystal partially enclosing plagio-clases with skeletal interiors. (C) Sample 395-18-2, 38 cm (#7), Unit Pj. Crossed nicols. Skeletal oli-vine crystal. (D) Sample 395-18-2, 38 cm (#7), Unit Pj. Plane polarized light. Associated skeletal oli-vine and Plagioclase crystals. (E) Sample 395A-l5-1, 55-58 cm, Unit P2. Crossed nicols. Sector-zonedolivine partially enclosing second-generation Plagioclase crystal.

first occurred in a more "primitive" melt, and re-sulted in calcic Plagioclase and forsteritic olivine withabundant glass inclusions. Even at this stage, some crys-tal growth was skeletal, resulting in the glass inclu-sions. At this point, many of the plagioclases underwentpartial resorption (Figures 6E and 12). In the model ofDungan et al. (this volume), these melts and their in-

cluded phenocrysts were then mixed with more frac-tionated liquids. This apparently produced strong zon-ing on the feldspars and fractured, fragmented rims onolivine phenocrysts. At this stage, euhedral sodicPlagioclase, granular clinopyroxene, and skeletal olivinegrew. The small crystal size and the skeletal olivines sug-gest that this stage involved fairly substantial undercool-

435

J. H. NATLAND

E - ̂

• \ "

1 - s' .*

1 v . • .

0.5 mmB 0.1 mm

0.5 mmD 0.5 mm

Figure 11. (A) Sample 395A-23-L 45-46 cm (#1), UnitP4. Crossed nicols. Clinopyroxene microphenocryst with den-dritic rim intergrown with tiny plagioclases. (B) Sample 395A-23-1, 120-121 cm (#12), Unit P4. Crossed nicols.Detail of dendritic overgrowth on clinopyroxene microphenocryst, showing intergrown plagioclases. Note "staircase. "morphology of overgrowth, resembling that of overgrowths on plagioclases in Figures 6C and 6D. (C) Sample 395A-16-1, 90-92 cm (#4), UnitP^• Crossed nicols. Sector-zoned clinopyroxene with an additional zoned rim on one sector.Growth of Plagioclase at right may have influenced formation of sector zones. (D) Sample 395A-23-1, 64-48 cm(#4), Unit P4. Crossed nicols. Bow-tie intergrowth of Plagioclase and clinopyroxene, apparently producing strongradial zoning in the clinopyroxene

1.0 mm 1.0 mm

Figure 12. Sample 395-18-1, 37 cm (#1), Unit Pj. Crossed nicols. Strongly zoned Plagioclase phenocryst in fairly well cry-stallized groundmass. Pitting at edge of inner zone, and its generally rounded outline, suggest that it was partially resorbedbefore growth of the outer zone. (A) Plane polarized light. (B) Crossed nicols.

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CRYSTAL MORPHOLOGIES

ings and occurred in a fairly shallow magma reservoir(more on this later). A fair amount of heterogeneousnucleation occurred, producing the glomerocrysts. Thefinal stage was extrusion of the basalts as thin flows andpillows.

Because it is possible that these basalts representhybrids of two different magma types, one "primitive"and the other "evolved," both containing phenocrysts,the order of crystal growth that can be deduced from thephenocryst varies from rock to rock. At one atmos-phere, Dungan et al. (this volume) produced olivine asthe second mineral to crystallize (after Plagioclase) in anequilibrium crystallization experiment on a Unit P4 whole-rock composition. Clinopyroxene and titanomagnetitecrystallized next after the olivine. In some basalts, in-deed, olivine phenocrysts partially enclose euhedralPlagioclase, and thus formed after Plagioclase. In someglomerocrysts, however, olivine appears to have formedbefore Plagioclase. In other rocks, Plagioclase andclinopyroxene are euhedral or granular, and olivine isskeletal, suggesting that it was the third mineral toform. As we shall shortly discuss, Plagioclasespherulites are ubiquitous in glassy zones, and arejoined in slightly coarser grained basalt by clinopyrox-ene.

This suggests that the two "end members" whichmixed to produce these basalts had fundamentally dif-ferent equilibrium mineral assemblages. The "primi-tive" component crystallized first olivine thenPlagioclase. The "evolved" components crystallizedfirst Plagioclase, then clinopyroxene, but no olivine(and in this way are similar to Fe- and Ti-rich oceanridge basalts). One might then see in the same hybridrock evidence in glomerocrysts that first-generationolivine had crystallized before first-generation Plagio-clase, and that skeletal olivine is associated withgranular clinopyroxene and euhedral second-generationPlagioclase in the surrounding rock. The groundmassmight have only Plagioclase and clinopyroxene. On theother hand, if the hybrid were predominantly a first-generation composition, there would be no clinopyrox-ene phenocrysts, and no skeletal olivine. Zoning onfirst-generation plagioclases would be minor, with lessdifference in compositions.

These crystal relationships, as complex as they are,apply mainly to the more evolved phyric basalt com-positions, Units P2, P4, and P5 (Chapter 7. thisvolume). The least fractionated phyric basalt units (Piand P3) contain abundant clinopyroxene phenocrysts inaddition to olivine and Plagioclase. In these basalts,some clinopyroxene appears to be a first-generationmineral. The one-atmosphere experiments of Dungan etal. (this volume) do not apply to these basalts. Thoughthey are interbedded with the more evolved phyricbasalts, the contrasting phenocryst assemblages betweenthe two phyric basalt types imply a complex evolution ifthe two types are co-magmatic. The "primitive" endmembers inferred to have mixed with more fractionatedcompositions in Units P2, P4, and P5 carried primarilyPlagioclase and olivine phenocrysts and glomerocrysts.Yet Units Pi and P3 represent still other "primitive"

compositions, with three phenocryst phases, and hadtheir own history of hybridization.

A typical sequence of crystallization of minerals inphyric basalts at high undercooling is depicted in a seriesof photomicrographs showing the development ofgroundmass textures from the edges to the interior oftypical cooling units (Figure 13). Within about 1 cm ofglassy rims there are brown spherulites about 0.1 mm indiameter (Figure 13A). These are set in a clear, palebrown glass. Some have tiny central crystallites ofPlagioclase. They differ from Plagioclase spherulites ofthe A3 and A4 basalts in being more perfrectly circular inplan, larger, and lighter brown, and in having a distinctrim about 0.02 mm in diameter. This same brownspherulitic material nucleates both on tiny crystallitesand on Plagioclase phenocrysts of any size (Figures 13Band 13C). No olivine spherulites occur in any phyricbasalt.

About 2 cm from cooling unit boundaries,Plagioclase growth is sheaf- (or bow-tie) spherulitic(Figure 13D). Tiny titanomagnetites are concentrated atthe ends of the fan-like sheafs. Clinopyroxene is prob-ably present, but is masked by the brown of thespherulitic bundles.

Still farther into cooling units (probably > 10 cm),groundmass crystal morphologies are skeletal forPlagioclase, spherulitic and dendritic for clinopyroxene,and skeletal for titanomagnetite (Eisenach, this vol-ume), as shown on Figures 13E and 13F. In the largercooling units (thick flows and the dolerite intrusions),Plagioclase becomes euhedral and tabular, but clino-pyroxene is still largely dendritic (Figure 13G). Atdistances several meters from cooling-unit boundaries,textures are sub-ophitic, and Plagioclase encloses gran-ular clinopyroxene and skeletal titanomagnetite (Figure13H). In some, but not all, coarse-grained rocks,granular olivine can be seen in the groundmass.

Olivine, then, does not appear to have grown in thegroundmass at any condition of undercooling repre-sented by extrusive pillow lavas or thin flows, but doesoccur in thicker flows and in the fairly shallow doleriticintrusives (originally about 300 m below the sea floor,assuming time equivalence to chemically similar phyricUnit P4). This contrast with the aphyric basalt units canpartly be explained by the higher CaO of phyric glassescompared with aphyric glasses (Melson, this volume).Many phyric basalt glasses also have lower Na2O andTiO2. Thus, Plagioclase, rather than olivine orclinopyroxene, is the liquidus mineral in these basalts,and grew as euhedral crystals and coarse dendritic rimson phenocrysts much closer to cooling-unit boundariesthan Plagioclase of similar morphologies in the aphyricbasalts. The high CaO also favored uptake of MgO intoclinopyroxene, thus contributing to the suppression ofolivine crystallization.

PHENOCRYSTS AND GLOMEROCRYSTSIN THE DOLERITES

Thin sections from the dolerite sections deep in Hole395A (lithologic Units 20 and 22) demonstrate clearlythat there are two intrusive bodies, both with fine-

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J. H. NATLAND

0.5 mm 0.5 mm

Figure 13. Sequence of photomicrographs showing range of groundmass textures in avariety of phyric basalts. Glassiest basalts contain only pale brown Plagioclase spheru-lites (A through C). The same spherulitic brown fibrous material rims phenocrysts(B, C) A few centimeters farther into pillows, Plagioclase fan-spherulites are welldeveloped (D). Probably many of the fibers here are clinopyroxenes. As rocks be-come coarser (E through H), Plagioclase changes from skeletal (E, F) to tabular (G, H),and clinopyroxene from spherulitic (E) to dendritic (F, G) to granular, sub-op hiticallyintergorwn with Plagioclase and surrounding an olivine crystal (H). (A) Sample 395A-17-1, 131-132 (#15D), UnitP3; (B)Sample 395A-16-1, 90-92 (#4), UnitP3; (C)Sam-ple 395A-17-1, 131-133 (#15Dj, Unit P3; (D) Sample 395A-15-1, 55-58, Unit P2;(E) Sample 395A-31-1, 83-87 (#6), Unit P5; (F) Sample 395A-20-1, 142-144 (#1);UnitP3; (G) Sample 395A-32-1, 35-36 (#3), Unit P5; (H) Sample 395A-14-1, 90-100(#12), UnitP2. All pictures were taken in plane polarized light.

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CRYSTAL MORPHOLOGIES

grained (but not glassy) margins and coarse-grained sub-ophitic interiors. Their intrusive nature is inferred from(1) chemical equivalence to phyric Unit P4 higher in thesection (Bougault et al., this volume; Rhodes et al., thisvolume); (2) remagnetization of basalts below the lowerdolerite (Johnson, "Paleomagnetism of igneousrocks," this volume); and (3) oxygen-isotope evidencefor formation of secondary minerals at temperatures

near 15°C above the dolerites, and between 30° and40°C below them (Lawrence et al., this volume).

Several features of the dolerites are worth mention-ing from a petrographic point of view. First, megacrystsof first-generation zoned Plagioclase containing glass in-clusions are both larger and more abundant in the in-terior than at the edges of the dolerites (Figures 14 and15). Second, rare phenocrysts of olivine and clinopyrox-

I

51.

,

Figure 14. Full thin-section photomicrograph of upper chilled margin of lower dolerite. Sample 395A-62-1,80-87 cm (#7B). Transmitted light. Longest dimension is equivalent to 3.2 cm. Rounded crystal incorner is Plagioclase. The larger mafic minerals (olivine = ol, clinopyroxene = cpx) are shown, but manysmaller ones, especially clinopyroxene, occur. Skeletal crystals are all plagioclases.

439

J. H. NATLAND

... _ . . . . . si. : " SM . J ^

• • * . ^

Figure 15. Fw// thin-section view of the interior of the lower dolerite, Sample 395A-63-1, 40-43 cm(#3D). Transmitted light. Longest dimension is equivalent to 3.0 cm. Rare isolated olivines (ol) areindicated. Skeletal crystals are Plagioclase megacrysts.

440

ene occur near the fine-grained margins, but not in theinteriors of the dolerites. Third, large glomerophyric in-tergrowths of Plagioclase and olivine are concentratedtoward the bottom of the intrusives, along with large in-dividual crystals of Plagioclase and olivine (Figure 16).Both the plagioclases and the olivines contain glass in-clusions, and are therefore "first-generation" minerals.

These relations indicate that gravitational settling ofthe glomerophyric clumps was occurring in the dole-rites, so they are not vertical, and that the large Plagio-clase megacrysts were first concentrated toward thecenters of the intrusives, probably by flow differentia-tion. Crystal settling in the dolerites was alsodemonstrated by Propach et al. (this volume).

CRYSTAL MORPHOLOGIES

The extrusive phyric basalt Units P t through P5 in-terestingly enough exhibit some of these same contrasts.One nearly glassy basalt of Unit P4 is charged with largeglomerocrysts (Figure 17), yet in another basalt fromUnit P4 they are absent (Figure 18). It is thereforeevident that some of the size sorting of phenocrystsand glomerocrysts is manifested in the extrusive stage ofthese basalts, and can be ascribed to a combination ofgravitational crystal settling and, possibly, flow dif-ferentiation before extrusion.

One other aspect of the Plagioclase phenocrysts in thedolerites is worth noting. The plagioclases associatedwith olivine in the glomerophyric clumps either are onlyslightly skeletal or are not skeletal at all. By contrast,

Figure 16. Full thin-section view of lower chilled margin of lower dolerite, Sample 395A-64-2,116-122 cm. Transmitted light. Abundant and fairly large individual olivines, and olivineswith Plagioclase in glomerocrysts, are indicated (ol). Note relative lack of skeletal plagio-clases compared with Figure 15, and large dark spherulitic inclusions in the largest olivineand glomerocryst.

441

J. H. N ATL AND

Figure 17. Full thin-section view of portion of pillow in Unit P4 Sample 395A-24-2, 124-130 cm (#2). Transmitted light. Longest dimension equivalent to 3.6 cm. Large olivines(ol) and non-skeletal Plagioclase (pi) in glomerocrysts are shown.

the isolated first-generation plagioclases are riddledwith glass inclusions, indicating skeletal growth. Thisimplies that the more calcic plagioclases, not just thesodic plagioclases, grew at a range of undercoolings. Wemight therefore speculate that at least two fairly large

slowly cooled magma reservoirs were involved in theevolution of the phyric basalts, one allowing glomero-crysts of the more calcic Plagioclase and the forsteriticolivine to form, the other allowing glomerocrysts of themore sodic plagioclases to form. Periodically, magmas

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CRYSTAL MORPHOLOGIES

Figure 18. Full thin-section view of portion of pillow in Unit P4, Sample395A-27-2, 69-78 cm (#4). Transmitted light. Longest dimension equiva-lent to 2.9 cm. Largest olivine (ol) and clinopyroxene (cpx) crystals areshown. Other large crystals are Plagioclase.

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J. H. NATLAND

from the first were injected into the second, and it wasduring the transit between the two that the skeletalcalcic plagioclases formed. Once inside the second reser-voir, more sodic but non-skeletal zones formed aroundthe calcic Plagioclase cores. This effect was most pro-nounced when the proportion of introduced magma wassmall, or the contrast in compositions between the twomagma types was extreme, because in some basalts littlezoning can be seen. The period of transfer between thetwo reservoirs was a period of heightened undercoolingwhich promoted skeletal growth. The transfer couldhave been either lateral or vertical, but the skeletal calcicplagioclases might be evidence for lateral transfer.Jaeger (1968) calculated that the country rock midwaybetween two parallel molten intrusive sheets, one abovethe other, could become considerably hotter than therock at the same distance either below or above thesheets and remain so for a longer time. How hot it gets,and how long it remains hot depends, of course, on thethickness of the sheets and the distance between them.But in a setting of rapid production of magmas andsteep thermal gradients within the crust, a vertical con-duit between two such bodies would not be a likely placeto maintain high cooling rates. Instead, we may herehave an analog of the Kilauea rift system, in whichrelatively unfractionated magmas branch laterally froma main conduit system and encounter pockets of frac-tionated magmas far out along the rift (Fiske andJackson, 1971; Wright, 1971). Supporting this is theidea that the magmas within the inferred second reser-voir were more fractionated than those that invaded it;that some crystals within the invading magma wereskeletal, implying heightened undercooling during thetransfer process; and that phenocrysts andglomerocrysts native to the second reservoir were con-siderably smaller than most of the introducedphenocrysts. In fact, the typical second-generationPlagioclase phenocryst size (either as single crystals orwithin glomerocrysts) is about the same (0.1 to 0.5 mm)as plagioclases which crystallized between 12.8 and 16.5meters depth in Makaopuhi lava lake, Kilauea volcano,Hawaii (Kirkpatrick, 1976). Here, too, nucleation waspredominantly heterogeneous on pre-existing crystals.

From the cooling-rate experiments of Dungan et al.(this volume), one can get some idea of the cooling ratesat which various crystal morphologies were produced inthe phyric basalts. At 30°/hr, skeletal Plagioclaseneedles grew in a spherulitic matrix. And at 2°/hr,Plagioclase laths and subhedral olivine were set in anolivine, Plagioclase, and spherulitic-clinopyroxenematrix. Even at the slowest of these cooling rates, themagmas would be entirely consolidated within a fewdays. The experiments therefore cannot possibly bearon the consolidation of larger magma bodies, given thatMakaopuhi lava lake (about 83 m deep) was producingcrystals similar in size to the phyric basalt second-generation Plagioclase, at a depth of 16.5 meters, fully12 months after formation of the lake. The cooling-rateexperiments, however, are probably pertinent to theconsolidation of pillows and thin flows, which exhibitthis range of crystal morphologies. The interiors of the

thicker flows and the dolerites, though, apparently grewat considerably lower cooling rates than 2°/hr, sincetextures are sub-ophitic, with Plagioclase surroundingwell-formed, not dendritic, clinopyroxene.

The critical question which the cooling-rate ex-periments do not answer is, what cooling rates are im-plied by the skeletal first-generation Plagioclasecrystals? If cooling rates were between 2°/hr and30°/hr, then the transfer conduits must indeed havebeen shallow, probably within the upper few hundredmeters of crust cooled by circulating sea water. Thedolerite intrusions, which were probably overlain byabout 300 meters of basalt at the time of their injection,do not have comparable skeletal textures in second-generation plagioclases even 1 meter from theirmargins. Since they have the composition of phyricbasalt type-P4 extrusives, and contain both first- andsecond-generation phenocrysts, they, too, are evidentlyhybrids. The dolerites thus do not represent conduitsbetween the two inferred magma reservoirs; they aredikes or sills emanating from the second reservoir.

Another problem not completely resolved at this timeis that the Sr contents of the phyric basalts are not con-sistent with their being co-magmatic with each other,nor with any basalt resembling the aphyric basalts. Niand Cr appear to be diluted 15 to 25 per cent, relative toaphyric basalts, by Plagioclase phenocrysts (Chapter 7,this volume; Bougault et al., this volume), but the extentof this dilution is not consistent. It can be more in a rockwhose Ni, Cr, and Zr indicate an unfractionated com-position than in a fractionated rock, and vice versa.

In the mode, Plagioclase phenocrysts range between15 per cent and 25 per cent, and at first sight, this mightexplain the dilution effect. But it is obvious that Plagio-clase is the liquidus phase in these rocks and that thesecond-generation plagioclases in particular are prob-ably not accumulative. Phenocrysts in the coarser grain-ed rocks are all first-generation minerals, and thesecond-generation Plagioclase phenocrysts grew at allstages of undercooling through the extrusion of thebasalts except the consolidation of small pillows. Theythus belong "in the basalts."

In any phyric basalt, first-generation Plagioclasephenocrysts probably amount to no more than half thetotal Plagioclase phenocryst content, and usually less.Although they have been introduced by magma-mixingprocesses, and their concentration in any given phyricbasalt may have been increased by crystal settling(usually in glomerocrysts also containing olivine) andflow-differentiation, it is doubtful that the full magni-tude of the "Sr anomaly" can be explained entirely bythese processes. Two other effects might be responsible.

Sr in the phyric basalts might be concentrated insecondary minerals, such as calcite in cracks and alongmineral boundaries (Bougault et al., this volume). It isnot clear why this would be so much more pronouncedin phyric basalts than aphyric basalts, especially when intheir overall aspect they are just as fresh.

Sr might be fundamentally more abundant in thesebasalts than in the aphyric basalts. Sr, of course, doesnot behave as an incompatible element with respect to

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CRYSTAL MORPHOLOGIES

Plagioclase, either during melting or during crystal frac-tionation. The phyric basalts have Plagioclase alone onthe liquidus, whereas the aphyric basalts have olivine, orolivine closely followed by Plagioclase. This suggeststhat more of the components of Plagioclase were selec-tively incorporated into the parents of these phyricbasalts during melting than into the aphyric basalt"parents." The result might have been higher concen-trations of Sr in the original melts, diluting high-partition coefficient elements such as Ni and Cr. Zr andTiO2 contents are lower with respect to Ni and Cr in thephyric basalts than in the aphyric basalts, suggestingthat the phyric basalts were produced by greater partialmelting in the mantle (Chapter 7, this volume; Bougaultet al., this volume).

A final question to consider is whether the variousmagma types, phyric and aphyric, could represent re-peated eruptions from the same magma reservoir. Onbalance, I believe that at least two magma reservoirsmust have been involved. The three aphyric types differso subtly in composition (Chapter 7, this volume) thatthey could well have come from the same reservoir,replenished episodically from deeper sources to producethe small compositional differences. Sufficient timeelapsed between eruptions (Johnson, Paleomagnetismof Igneous Rocks, this volume) to erase all evidence ofhybridization or internal fractionation. There are essen-tially no phenocrysts, and no internal chemical trendseither, to indicate fractionation or mixing between oldand new chemical types. In every respect, the aphyricbasalts are compositionally discrete.

The phyric basalts, on the other hand, have everypossible mineralogical evidence for magma mixing, andneither group of "primitive" or "evolved" end-members of these hybrid rocks resembles the aphyricbasalts. Substantial differences still exist in contents ofCaO, Na2O, TiO2, and Ni/Zr (Bougalt, et al., thisvolume. Differences also exist in contents of CaO,Na2O, and TiO2 in the glasses (Melson, this volume). Thisprobably reflects at least a basic difference in the extentof melting in the mantle (higher in the phyric basalts;Bougault et al., this volume). In addition, a range insizes of magma reservoirs in an interrelated network ap-pears necessary to explain the variety of phenocrystmorphologies and sizes. Phenocrysts have been concen-trated in the phyric basalts by mechanical processes (ac-cumulation, flow differentiation, etc.). In the aphyricbasalts, they either were never present, or they havebeen eliminated. This alone indicates fundamentally dif-

ferent fluid dynamic processes, which would be critical-ly affected by the size and shape of reservoirs and con-duits, and by the rate of flow of magma. Apart from allof these is the stratigraphic fact of aphyric basalt unitsboth below and above the phyric basalts. The basicplumbing and rate of supply for aphyric basalt typesthus was not disrupted by the interval of phyric basalteruptions. Most probably, then, two independentsource and conduit systems were involved, closelyspaced in the median rift of the Mid-Atlantic Ridge nearSite 395 about 7 million years ago.

ACKNOWLEDGMENTS

I would like to thank Bill Melson for thoughtful and pro-voking discussion about the significance of crystal mor-phologies, on board the Challenger during Leg 45, and JimKirkpatrick for properly introducing me to the subject and forreviewing the manuscript. I am indebted to the fine staff ofmarine technicians on Leg 45 who all participated in makingthe thin sections used for this study.

REFERENCESBryan, W.B., 1972. Morphology of quench crystals in subma-

rine basalts, J. Geophys. Res., v. 77, p. 5812-5819.Donaldson, CH., 1976. An experimental investigation of oli-

vine morphology, Contrib. Mineral. Petrol., v. 57, p.187-213.

Fiske, R.F. and Jackson, E.D., 1972. Orientation and growthof Hawaiian volcanic rifts: the effects of regional structureand gravitational stresses, Phil. Trans. Roy. Soc. LondonProc, v. 329, p. 399-326.

Jaeger, J.C., 1968. Cooling and solidification of igneous rocks,In Hess, H.H. and Poldervaart, A. (Eds.), Basalts, v. 2:New York (Wiley Int er science), p. 503-536.

Johnannsen, A., 1939. A descriptive petrography of the igne-ous rocks: Chicago (Univ. Chicago Press), v. 1, p. 1-318.

Kirkpatrick, R.J., 1977. Nucleation and growth of Plagio-clase, Makaopuhi and Alea lava lakes, Kilauea Volcano,Hawaii, Geol. Soc. Am. Bull., v. 88, p. 78-84.

Lofgren, G., 1971. Spherulitic textures in glassy and crystal-line rocks, J. Geophys. Res., v. 76, p. 5635-5648.

, 1974. An experimental study of Plagioclase crystalmorphology: isothermal crystallization, Am. J. Sci., v. 274,p. 243-273.

Walker, D., Kirkpatrick, R.J., Longhi, J., and Hays, J.F.,1976. Crystallization history of lunar picritic basalt sample12002: Phase-equilibria and cooling-rate studies, Geol.Soc. Am. Bull., v. 87, p. 646-656.

Wright, T.L. and Fiske, R.S. 1971. Origin of differentiationand hybrid lavas of Kilauea Volcano, Hawaii, J. Petrol., v.12, p. 1-65.

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