Open system melting and temporal and spatial ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH,

VOL. 97, NO. B6, PAGES 9219-9241, JUNE 10, 1992

OpenSystemMeltingandTemporalandSpatialVariationof Peridotite and Basalt at the Atlantis II Fracture Zone KEVIN T. M. JOHNSON1 Department of Earth.Atmospheric, andPlanetaryScie•wes, Massachusetts Instituteof Technology, Cambridge and.Department of Geologyand Geophysics, WoodsHole Oceanographic Institution,WoodsHole, Massachusetts HENRY

J. B. DICK

Departmentof Geologyand Geophysics, WoodsHole Oceanographic Institution,WoodsHole, Massachusetts In situ ion microprobeanalysis of trace and rare earth elementsin discrete diopsidesin abyssal peridotitesfrom nine transformdredgehaulsfrom the AtlantisII FractureZone (AII FZ) showsthat these sampleshavea widerangeof traceelementcontents closeto thetotal rangefoundfor the entireSouthwest Indian Ridge. Thoughthe spreadin analysesis large,the averagecompositionof the peridotitesis close to that reportedfor the AII FZ by Johnsonet al. (1990) and lies at the relativelyundepletedend of the spectrumfor SW Indian Ridge residualmantle peridotites. A sharp break in peridotite diopside compositionand modalmineralogyoccursacrossthe transform,suggesting that it actsas a boundaryfor different melting regimesand initial mantle compositions.The differencein peridotitecompositionsis mirrored in spatiallyassociatedbasalts,which lie on separateparallel liquidus trends in the normative ternary pyroxene-olivine-plagioclase. Basaltsfrom the east side of the transformhave higher nonnative plagioclasecontents,indicatingthat they may be productsof lower degreesof mantlemeltingthanbasalts from the westernside, consistentwith greaterdepletionof peridotitesfrom the westernwall or a more depletedinitial composition.Basaltsfrom the easternwall alsohave consistentlylower FEB.0and higher Na8.0 than basaltsfrom the westernwall and lie parallelto the global along-ridgeFe8.0 - NAB.0 trend (Klein and Langmuir,1987) and orthogonalto the local meltingpathsof Klein and Langmuir(1989). Our data provide strongevidencefor segmentation of the melting regime, with major mantle discontinuities occurringat transformoffsetsat slow spreadingridges. Peridotitesanalyzedalongthe easternwall of the fracture zone also showa systematicchangein compositionwith latitude and, with the older peridotites from the mediantectonicridge, definea systematicchangein the degreeof melting of the mantle occurring beneaththe paleoridgeaxis over the last 11 m.y. Emplacement of mantleshowingthe lowestdegreeof melting, or the least depleted parental mantle composition, correspondsroughly to the time of crystallizationof OceanDrilling Programsite 735B gabbros. Melting is modeledas a non-steadystate, discontinuous processwith 0.1-0.5 vol % aggregated melt retainedin the porousresidue(open system melting). The rangein degreeof opensystemmeltingfor the combinedsuiteof AII FZ peridotitesis 820%. Such a large systematicvariation would appearto require a dynamicallysignificantchangewith time, either in the initial temperatureand/or a large compositionaldifferenceof the mantle beneaththe paleoridgeaxis. This in turn suggests that in the relativereferenceframeof the ridgeaxis,mantleflow was non-steadystate. This could reflectepisodicmantlediapirismbeneaththe ridge axis or, alternatively,that the ridge axis has moved over a zoneof enhancedupflow in the underlyingmantle that was fixed in the absolutehotspotmantlereferenceframe.

INTRODUCTION

Systematicsampling along the walls of fracture zoneswith well documentedspreadinghistories, and hence age constraints In this study,we presentanalysesof peridotitesand basalts on the relative ages of the samples dredged, should permit dredgedfrom the walls of Atlantis II Fracture Zone and the evaluationof the third dimensionof geochemicalvariability in

adjacentrift valleysof the SW IndianRidge duringcruise27-9 the ocean crust - the dimension of time. Fracture zone walls of theR/V Robert Conrad. We find considerable variabilityof the peridotite and basalt compositionsalong and acrossthis frequentlyexposea wide variety of crustaland shallowmantle transformfault. Due to its position far from the influenceof material emplaced at the ridge axis close to a single any hotspot, the peridotite and basalt suites from the Atlantis lithosphericflow line over many millions of years, recording the chemical

evolution

of the crust and shallow

mantle

at a

II FractureZone (FZ) are idealfor comparison to thepreviously documentedlong-wavelengthessentiallyzero age chemical singlepoint in the plate referenceframe with time. Our initial variations along ocean ridges found in earlier studies at a investigationof Atlantis II FZ peridotites and basaltsshows that not only does the fracture zone separatetwo independent variety of scales [e.g., Schilling, 1973; Hart et al. ,1973; Melson and O'Hearn, 1979; Schilling et al., 1983; Dick et al., 1984; Klein and Langrnuir , 1987; Brodholt and Batiza, 1989; Michael and Bonatti, 1985; Dick, 1989; Johnson et al., 1990].

walls of the fracture

zone.

This latter variation

is consistent

with major temporal changesin the depth and natureof mantle flow in response to chemical heterogeneities and/or local mantle plumes in the asthenosphere.Althoughour resultsare preliminary and need substantiation through additional sampling, this study suggeststhat the responseof shallow

1Nowat BishopMuseum, Honolulu, Hawaii Copyright1992by the AmericanGeophysical Union Papernumber92JB00701. 0148-0227/92/92JB-007

mantle flow regimes, each tapping mantle of significantly different compositionbut that there may also exist systematic variations in the chemistry of the residual mantle along the

01 $05.00 9219

9220

mantle

•OHN•ON AND DICK: TEM•O•.•L VARIATIONS IN ]V[ANTLEIV[ELTINO

convection

and crustal formation

to movement

of the

accretionaryplate boundariesin the absolutereferenceframe

produced in the vicinity of transform faults at very slow spreadingridges is "normal crust" in theseregions.

over the lower mantle can be tracked in time.

TECTONIC SETITNGOF THE ATLANTIS II

TI-IE NATURE OF FRACTURE ZONE CRUST

FRACHSRE ZONE

Most studieson ridge segmentationconcentratedon alongaxis compositional variability in the basalts, while few have looked in detail at the variability along mantle flow lines perpendicularto the ridge axis [see Bougault et al., 1985].

The SouthwestIndian Ridge forms one leg of the ridgeridge-ridgeIndian Ocean triple junction (Figure 1). Since80

Ma, this triplejunctionhasbeenmigratingrapidlyto the east, with extensionof the SouthwestIndianRidge forminga series of new ridge segmentsand numerous,long (100-250 km) very slow compositions,melting regimes, and ridge segmentationon a fracturezones. Becauseof the characteristically variety of scales, there is a clear need for more temporal spreadingrates along the SouthwestIndian Ridge (1.5-1.6 informationon crustal evolution at oceanridges. An obvious cm/yr full rate), the seafloortopographyis muchrougherthan place to conductsucha studyis at a fracturezone,wherehigh that associatedwith the faster spreadingCentral and Southeast relief and continued slumping frequently exposerelatively Indian ridges [Sclater et al., 1976; Tapscott et al., 1980; While the resultsof thesealong-axisstudieshaveprovideda wealth of information about variability in basalt

fresh unweatheredrocks and expose the deeper levels of the crust and shallow

mantle

as well.

Sclater et al., 1981; Fisher and Sclater, 1983]. The Atlantis II FZ is a 210-km, 20-m.y., left-lateral offset formed near the

Fracture zones, however, have been largely avoided by Indian OceanTriple Junctionduringridge extensionat 58 (+2) geochemists,except to study effects they may have on crustal Ma [Dick et al., 1991]. The spreadingrate determinedfrom evolution.Because of thispostulated transform "coldedge" magneticanomaly lineationsflanking the active transformto effect on the mantle thermal structureand subsequent chemical the east and west is ~1 cm/yr (half rate). The anomaly evolution of magmas near ridge-transform intersections identifications of 11.5 Ma on the easternwall agreevery well [Hekinian and Thompson, 1976; Fox and Gallo, 1984; with a zircon Pb isotopicage of 11.3 Ma on a trondhjemite Langmuir and Bender, 1984; Bender et al., 1984], it hasbeen from the OceanDrilling Program(ODP) drill hole at site735B arguedthat crustproducedneartransformfaultsis "anomalous" [Stakeset al., 1991]. Thus, the calculatedtotal spreadingrate and that crustproducedaway from fracturezonesis "normal". of 1,6 cm/yr in this region [Tapscott et al., 1980] requires This may sometimesbe true, but various authorsargue that asymmetricspreadingat both the eastern and westernridges, there are no differences between basalts erupted in fracture with ratesof 0.6 cm/yr on the platesspreadingaway from the the AtlantisII FZ is growingat a zones and ridge segmentsalong the SouthwestIndian and transform.As a consequence, American-Antarcticaridges [leRoexet al., 1983, 1985]. Still, rate of 0.4 cm/yr with a slip rate of 1.2 cm/yr (Figure2). The abundantevidence showsthat slow and fast spreadingridges are inactive trace of the fracture zone to the north strikes punctuatedby a variety of axial discontinuitiesthat can have approximatelyN10øE, comparedto a nearlydue N-S strikeof a major 10ø changein subtle morphological expressions but profound chemical the activetransformfault, suggesting effects [Bryan et al., 1981; Christie and Sinton, 1981, 1985; spreadingdirectionat 17 Ma. On the basisof the small proportionof gabbroin dredge Langmuir et al., 1986;Batiza et al., 1988]. Many of the above effects, however, are ephemeral or recovery statistics for fracture zones along the Southwest related to the local influence of propagatingrifts. More to the Indian Ridge, Dick [1989] inferred that crustal thicknessesat point, however, is the growing realization that magmatism very slow spreadingridges are lower than thoseat mediumand along ocean ridges is punctuated,and that individual magmatic fast spreadingridges. However, the proportionof gabbros cells may have a complex internal Structure[Whiteheadet al., recoveredin the AtlantisII FZ is higherthanin othertransform 1984; Crane, 1985; Macdonald et al., 1988]. Thinner crust, faults along the SouthwestIndian Ridge [Dick et al., 1991], often with large differencesin chemistryand structureto that implying that magmaproductionrates and crustalthickness,at for.med near the midpoint of a magmatic center, is likely to least locally, may be greaterthan those estimatedfor the exist at the distal portionsof such cells [Dick, 1989]. This, to Southwest (SW)IndianRidgeasa whole. a fair degree, makes the concept of "normal crust" moot, The high-resolutionSea Beam bathymetricmap of the particularly for slow and very slow spreadingridges. Since AtlantisII FZ [Dick et al., 1991] showsan average reliefof fracturezonesare commonlybelievedto boundmagmaticcells, 4500 m with total relief of 6000 m. The transform walls are it is'reasonable to suspect thatmanypreviously observedsteepon both sides,but the east wall is generallysteeperand differences between "normal" and fracture zone crust may higherthan the west wall. Slopeson the eastwall rangefrom simply be the differencebetweenmidpoint and distal end of a 11ø to 32ø, averaging23ø, while slopeson the westernwall are from 11ø to 24ø, averaging 19ø. Nodal basins at both the magmatic segment. In the case of long-lived transforms, it is reasonable to southernand the northernridge-transformintersectionsare the assume that whatever

variations

exist in the thermal

structure

of the mantle adjacentto it, a steadystate has been reached. Thus, chemical contrastsacrossor along the fracturezone will directly reflect variations in the regional compositionof the mantle and perturbationsin the mantle flow patternwith time. Moreover, as large transformfaults (50-250 km) generally occur every 30 to 100 km along very slow spreadingridges [Schoutenet al., 1985] (e.g., the SouthwestIndian Ridge), most of the lithosphereaccretedin suchregionsforms under the influence of transform faults. Thus, in any case, crust

deepestpointsin the surveyedarea. Crustspreading awayfrom the axes of the neovolcaniczone parallel to the transformis upliftedto form large inside-corner highsat the ridge-transform

intersection,while crust spreadingaway in the opposite direction forms relatively low rift mountains. Nearly continuousuplift of successive crustalblocks at the ridgetransformintersectionscreatedthe high flanking transverse ridgesandthe steepwalls of the AtlantisII transformvalley. The fracture zone has a broad transform valley in its northernhalf with a 1.5-km-high median tectonicridge down

JOHNSONAND DICK: TEMPORAL VARIATIONS IN MANTLE MELTING

9221

Mascare•e

Plain •

20øS

,Mauritius Is, -

Reunion

Madagascar

o

o

.

ß ß

Madagascar Basin

Mozambique Basin

30 ø

• CrozetBasin

40 ø

40øE

50 ø

60 ø

70 ø

80 ø

Fig. 1. Regionalbathymetric map of the Southwest IndianRidgeshowingthelocationof the AtlantisII FractureZone.

its center. The inferred present-daytransformfault is inferred the inferredpresent-daytransformfault zone, and thereforenow to lie on the western side of this ridge. In the south, the lie on the African plate. On the basis of their geochemical inactive western half of the valley joins the eastern half and characteristics, however, we believe that these rocks were becomes the active transform. transportedto their present position by turbidity currentsand The majorityof the samplesstudiedcamefrom the walls of debris flows from the eastern wall of the transform. the transformvalley and their agescan be determineddirectly The median tectonicridge lying on the floor of the Atlantis

to within a few hundredthousandyears by simple tectonic reconstruction.Dick et al. [1991] ran 13 magneticlinesalong these walls at a 5-km spacing parallel to the axis of the transform,producinga very densely surveyedmap of the magneticanomalies. They found a well-developedpatternof linear anomaliesrunning parallel to the SW Indian Ridge. These anomalies extend over the crest and down the walls of the

II transform is similar to those at other slow slipping transformslike the Kane [Karson and Dick, 1983; Pockalnyet al., 1988] and the Vema FZs [MacDonald et al., 1986]. These transforms are all floored by relatively broad transtensional basins believed to have formed due to amagmaticextension

acrossthe transform during plate readjustmentfollowing a recent spreading direction change [van Andel et al., 1971;

ridges flanking the transformvalley. The continuityof these Roest and Collete, 1986; Dick et al., 1991]. The median anomaliesover the transformwalls precludetectonicshuffling tectonic ridges are believed to reflect shallow mantle flow, of the crust from one wall of the transform to the other. Thus, hydration, and serpentine diapirism adjacent to the active the age of the rocksdredgedfrom the walls is a direct function transformfault as a consequence of extreme crustalthinning of pastspreadingrate and their presentposition. Thoughmost during transtension. of the materialwas undoubtedly dredgedfrom talusslopesand In the caseof the AdantisII FZ the surficialrocks.exposed debris flows, down slope transportdue to mass wastageis along the ridge appearto be largely turbidiresuplifted from the largely orthogonal to the fracture zone and parallel to the floor of the transform. Seismic lines acrossthe valley floor anomaliesand thus introduceslittle additionalerror in the age show a minimum of a half kilometer of sediment With an estimates. unusualseismiccharacter. Pistoncores in the valley iden.tify Peridotiresfrom two critical dredge hauls discussedin this these as turbiditic gravels and sands. Moreover, where the paper, however, come from the median tectonicridge which linescrossthe mediantectonicridge in the south,they suggest lies along the floor of the transformvalley. Dredges23 and 25 that it is covered with a disrupted sequenceof these same were situatedon the west and east walls of this ridge, west of sediments.The averagecontentsof the four dredgehauls from

9222

IOHNSO• AND DICK: TEMPORALVARIATIONSIN MANTLE MELTING

SpreadingGeometryof the AtlantisII FractureZone the transform walls, and could have been depositedby a I

0.6 cm/yr

turbidity flow from either wall. Though there is a somewhat greater likelihood that it came from the western wall, dependingon the uplift history of the ridge in this area, this would be contrary to our geochemical evidence. Since the dredge hauls represent surficial turbidites, their age and paleopositionmust be closeto that of the rockson the adjacent transform

fault walls.

SAMPLES ANDANALYTICALTECItNIQUES

Dredgelocations,depths,and inferredagesare presentedin Table 1 and Figure 3. Completedredgerecoverystatisticsare given by Dick et al. [ 1991].

1 cm/yr !

Peridotites

210 km

Of 24 dredgesfrom the transformwalls, approximately 43% were peridotites[Dick et al., 1991]. They are typicalof dredged oceanic residual peridotites from other locations and include protogranular, porphyroclastic, and mylonitic varieties. Protogranular textured peridotites, characterized by a predominanceof coarse-grainedtexture with smooth curved interlocking dentate and cuspate grain boundaries, were preferred for analysis. Such samplesare inferred to be little affected by the late stagerecrystallizationand reequilibration

1 cm/yr

associated with deformation

and the formation

of brittle-ductile

shear zones accompanyingshallow level emplacementof the peridotite. Though they are the least alteredsamplesavailable for each dredge haul, the peridotites selected for analysis

0.6 cm/yr •( i

Fig. 2. Cartoon depicting the spreadingand slip vectors for the Atlantis II FZ ridge system.

contain 50% to 100% serpentineand clay pseudomorphing olivine, orthopyroxene, and clinopyroxene. Virtually all pyroxenes exhibit exsolution lamellae, but based on examination by transmitted light microscope and electron microprobe, exsolution in all the samples is estimated to be

the median tectonicridge are virtually identical to the average