THE δ 1 8 0 AND δ 13C ISOTOPIC COMPOSITION OF SECONDARY CARBONATES FROM BASALTIC LAVAS CORED IN...

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    Eldholm, O., Thiede, J., Taylor, E., et al., 1989Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 104

    2 5 . T H E 1 8 0 A N D 1 3 C I S O T O P I C C O M P O S I T I O N O F S E C O N D A R Y C A R B O N AT E S F R O MB A S A L T I C L AVA S C O R E D I N H O L E 6 4 2 E , O C E A N D R I L L I N G P R O G R A M L E G 1 0 4 1

    D a v i d A . L o v e ,2 , 3 S . K . F r a p e ,2 I a n L . G i b s o n ,2 a n d M . G . J o n e s2

    ABSTRACT

    Hole 642E is located near the outer margin of the Voring Plateau in the Norwegian Sea. The thick pile of basalticlavas penetrated during drilling are variably altered with extensive development ofcalcite, which fills vesicles and fractures along with saponite and celadonite. 513C results, determined by mass spectrometry, show that most carbonatesabove about 1040 m have values between - 2 .5 and -5.5%o (P DB), but a few samples at approximately 1090 m havedepleted 5 13C values down to -26.3%o.Below 1100 m the 8 13C values decrease from -6.0%o to - 12%o. The 5 ls O values range between - 1.9 and - 13.7%o (PDB ), and generally decrease with depth. The results are interpreted as indicating that the calcites were precipitated from cool seawater percolating through the basalt pile at w aterrrock of less than10:1, during seawater incursion at about 54 Ma. The progressive depletion with depth may result from subsequentreequilibration at temperatures below those of forma tion, an d the geothermal gradient on the Wring Plateau appea rs tohave decreased withtime. The very depleted values of8 13C for carbonates around the 1090-m level are probably relatedto organic matter from an underlying volcaniclastic unit.

    I N T R O D U C T I O N

    Hole 642E, drilled during Leg 104 of the Ocean Drilling P rogram, is located near the outer margin of the Wring Plateau inthe Norwegian Sea. One of the primary objectives of the Legwas to examine a thick dipping series of lavas which were erup tedjust before, or during, the initial stages of the formation of theNorwegian Sea (Eldholm, Thiede, and Taylor, 1987; Mutter etal . , 1982; Talwani et al., 1983, Hinz et al., 1984). This objectivewas achieved, and Hole 642E penetrated a 914-m succession oflavas below a 315-m thick cover of Cenozoic hemipelagic sediments. These lavas constitute the easternmost edge and strati -graphically lowest part of the very much thicker Wring Plateaudipping reflector lava pile (Shipboard Scientific Party, 1986).

    The lavas are variably altered with extensive development ofcalcite, which generally fills vesicles and fractures in the flowsand is the most abundant nonsilicate secondary mineral. In anattempt to learn more of the posteruptive history of the lavapile, particularly secondary alteration, we have examined the carbon and oxygen isotopic composition of 20 calcite samples takenthroughout the core. The objective was to determine whether sub-aerial secondary mineralization, comparable to that found inpresent-day Iceland, affected the flows prior to submergence andburial.

    G E O L O G IC A L R E L AT I O N S A N D S A M P L I N G

    The basement lithology is well described in Shipboard Scientific Party (1987), and the following summary is from that accoun t. The volcanic succession cored in Ho le 642E is divided intotwo series: a 763-m thick upper series, comprising approximately122 tholeiitic basalt flows and 53 interca lated v olcaniclastic sediment horizons; and a 143-m thick lower series, containing 16

    1 Eldholm, O., Thiede, J., Taylor, E., et al., 1989.Proc. ODP, Sci. Results, 104:College Station, TX (Ocean Drilling Program).

    2 Dept. of Earth Sciences, University of Waterloo, Waterloo, Ontario, CanadaN2L 3G1.

    3 Current address: Dept. of Geological Sciences, Queen's University, Kingston,Ontario, Canada, K7L 3N6.

    flows of peraluminous andesite and basalt, which are sometimesglassy, 4 dikes, and 7 volcaniclastic layers including a 5-m thickignimbrite. A red-brown eutaxitic tuff and mudstone (Unit S43)which is at least 7 m thick separates the upper and lower series.The upper series has two varieties of flows, thick (average 8 m)fine-grained lavas and thin (average 3.3 m) m edium-grained lavas,which are similar chemically. The fine-grained flows have a distinct textural zonation whereas the medium-grained flows are tex-turally more uniform. The upper series flows are mostly sub-aerial, and red, weathered, brecciated to scoriaceous flow tops arecommon throughout. In contrast, except for the uppermost horizon (Unit S 43), red weathered horiz ons are absen t in the lower series.

    Throughout the upper series, carbonate occurs in vesicles andfractures along with saponite and celadonite. Calcite is the mostabundant open space-filling carbonate mineral. Vesicles are oftenzoned, having saponite rims and calcite cores and an interveningzone of celadonite. Sometimes the celadonite and calcite are in-tergrown. There is distinct zonation of vesicles within finegrained flows, and some zonation of vesicles in medium-grainedflows, but there is no relationship between the presence or abundance of vesicle-filling carbonate minerals and the internal characteristics of the flows. Nor is there any relationship between vesicle-filling carbonate minerals and the red alteration at the tops ofsome flows (Shipboard Scientific Party, 1987). The lowest 50 m ofthe upper series has red-brown staining associated with ferroancalcite in fractures and vesicles.

    In the lower series, alteration is a little more extreme; parts ofsome flows are bleached white, and volcaniclastic rocks containzeolites and secondary silica as well as small amounts of carbonate. Carbonate postdates silica and is less abundant in fractureand vesicles.

    The physical nature of the carbonate in the upper and lowerseries is the same. The carbonate is most often equant, granularcrusts of orthosparite on saponite or celadonite, and very rarelycoats fracture vesicle walls. Many of the vesicles in the upper series are only partly filled, but carbonate-bearing vesicles tend tobe completely filled, although there are a few partially filled oneswhich have isopachous equant granular spar crusts. Smectite-rimmed, unfilled vesicles can occur adjacent to carbonate-filledones. In some places it is obvious that smaller vesicles are com-

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    D. A. LOVE, S. K. FRAPE,I. L. GIBSON, M. G. JONES

    pletely smectite-filled, whereas larger ones have successive smectite and carbonate.

    Fractures that were not broken open during drilling are all completely filled, having saponite rims an d often carbon ate cores. T hecarbonate in fractures varies from massive equant granular sparto fibrous and/or bladed crusts which grewto completely fillthe fracture. It is virtually impossibleto tell whether brokenfractures were completely filled. Minor amountsof bright, lightgreen celadonite, and rarely, dull olive green saponite are inter-

    grown with equant granular sparin the fractures. Some fractures have distinctive coresof gray-green (Mg?) saponite surrounded by carbonate, and rimsof dark green (Fe?) saponite.The mineralogy, chemistry, and iso topic contentsof the secondary clay mineralsin Hole 642E are describedin detail in Des-prairies et al. (this volume) and LeHuray and Johnson (this volume).

    Very rarely, intergranular carbonate is found in the flow matrix; for example, in Sections 104-642E-94-3,34 cm, and-94-3, 101 cm. Sulfides are never foundin vesicles, but occurrarely in fractures; for example, in Sections 104-642E-17-2,77cm, and -94-2, 141 cm.

    Geopetal texturesin vesicles are uncommon but do occurthroughout the upper series.In most cases the spar fills the remaining open space after geopetal smectite. Very rarely does the

    spar itself appear geopetal,as in Sections 104-642E-17-2,28cm; and here, although the sparis certainly not isopachous,itmay not be geopetal.

    Many of the vesicles and fractures sampled for carbonate arerimmed witha dark green smectite, probably saponite.In somecases carbonate and celadonite are intergrown. Carbonate wasscraped from fracturesand vesicles usinga small hand-heldgrinder and stainless-steel dental tools. Care was taken duringsample preparationto include as little of the clay mineralsaspossible in the carbonate sample. The styleof carbonate mineralization is noted in Table 1.

    E X P E R I M E N TA L P R O C E D U R E SCarbon and oxygen as C 02 were extracted from 30- to 50-mg calcite

    samples by treatment of the calcite with 100% phosphoric acid at 50 C

    Table 1. Analytical results and style of mineralization: Section= core -section, interval (cm), V= vesicle-filling,F = fracture-filling.

    6 C P D B 5 O P D B 5 O S M O W Depth

    Sect ion (% 0) (%o) (%o) (mbsf ) S ty le

    Upper se r ies(104-642E-)

    9-2, 1917-1, 3217-1, 12817-2, 7718- 2, 7225- 4, 326- 4, 2841-3, 15

    41-3, 1764-2, 7485-1, 11094-2, 14194-3, 3494-3, 101

    - 5 . 13- 4 . 7 0- 3 . 2 6- 5 . 5 3- 4 . 00- 4 . 8 4- 4 . 31- 2 . 5 3

    - 2 . 5 2- 2 . 4 8-4 .47

    - 16 . 7 2- 2 6. 2 6- 21 . 3 5

    - 3. 25- 4 . 7 1- 6 . 0 0- 4 . 04- 3 . 4 6- 6 . 09- 4 . 8 3- 1. 88

    - 2. 10- 2 . 8 1- 6 . 23- 9. 20- 9. 25- 8. 16

    27. 526. 024. 726. 727. 324. 625. 928. 9

    28. 728. 024.4

    21. 421. 322.4

    - 3 67 . 4 7- 4 24 . 6 2- 4 25 . 5 8- 4 26 . 5 7- 4 35 . 4 2- 4 96 . 5 4- 5 06 . 5 8- 6 44 . 3 0

    - 6 44 . 3 2- 814 .71- 998. 70*

    - 1085. 54- 1085. 97*- 1086. 74

    VV & FV & FFFVFV

    vVVFFV

    Lower series(104-642E-)

    98- 1, 14798- 2, 16102- 1, 126107- 2, 73109- 1, 56110- 1, 39

    - 6 . 3 8- 6 . 02- 5 . 5 8- 9 . 14

    - 11 . 7 9- 11. 3 8

    - 9. 33- 8 . 7 1- 4 . 2 4

    - 1 3. 7 4- 1 2. 3 9- 1 3. 6 5

    21. 821. 926. 516. 718. 116. 8

    - 1117. 27*- 1117. 46- 1154.76- 1193. 63- 1210. 96- 1220. 29

    VFFFVF

    M e a n of two rep l ica te de te rmina t ions

    using a method modified from McCrea (1950). The13C and lgO werethen determined directly usinga semiautomated V.G. Micromass 90triple-collecting mass spectrometer. All the isotopic data are repusing the standard 6 notation relativeto the Peedee belemnite (PDBstandard for carbon and oxygen, and also the SMOW (standard mocean water) standard for oxygen. Isotopic analyses of replicates onstrated that the analytical uncertainty was less than 0 .2 for8 ls Oand 0.1 for6 13C.

    R E S U LT S

    The isotopic results are givenin Table 1, and plots of th edata are shown in Figures 1 and 2 . Table 1 shows th at there systematic difference between the isotopic content of vesiclefracture carbonates.

    The 513C results (Fig. 1 A) sug gesta bimodal grouping of thcarbonates. Most vesicles and fracture fillings in the upper of the core, above about 1040 m, have values between- 2.5 and- 5 . 5 %0 (PDB).At approximately 1085to 1090 m several samples with very depleted8 13C values were found; the most depleted samplesin this group have valuesof -26.3%o. Below1100 m th e8 13C values gradually decrease from -6.0% o to vues closeto - 12%o.

    The 5 l sO values range between- 1.9 and - 13.7%o (PDB),and generally have steadily decreasing values with depth (IB).

    D I S C U S S I O N

    There is little texturalor geological evidence concerning thorigin of the carbonate mineralsin Hole 642. The absenceofany relationship between vesicle-filling carbonate materials the red alterationat the tops of some flows suggests that calcitoccurrence is not directly relatedto subaerial exposure. Geopetal texturesof carbonate vesicle fillings can be interpreted as dicators of precipitation from meteoric waterin the vadosezone, but these textures are very rarein Hole 642E and uncertainty remainsas to whether they are truly geopetalor simplynonisopachous crusts. Partly-filled vesicles having isopachequant granular spar crustson saponite or celadonite persistthroughout the hole, indicating only that mostof the carbonatewas precipitatedin the phreatic zone.

    The ferroan calcitein the lowest 50m of the upper seriesisunusual in that it comes from thin veins bordered on each siby pronounced narrow zonesof red-brown discoloration.Noanalytical work has been attempted here to measure the checal changes involvedin the discoloration process. However, tmost likely explanationis that iron was enrichedin local solut ions, resulting in formation of ferroan calcitein the fracturesand probably ferric hydroxidesin the discolored zone.

    O X Y G E N I S O T O P E S

    Hattori and Muehlenbachs (1982) analyzed the oxygen carbon isotopic compositionsof calcites from a series of boreholes in three areas in Iceland. The results are summarizedinTable 2. Samples are from depthsto 3 km and the rocks pene

    trated by the boreholes range in age from Holocene to abouMa. The Icelandic basalts are,in general, very comparableincompositionto the subaerial flowsof the upper seriesof Hole642E (Shipboard Scientific Party, 1987; Vierecket al., this volume). However, Table2 shows that the Icelandic calcites havemuch lower8 ls O compositions. Hattori and Muehlenbachs (198suggested that these low8 l sO values resulted from the precipitation of calcite at 300 to 400 C from seawaterat Reydarfjordurand Reykjavik and from meteoric waters with low8 1 80 valuesat Krafla. In contrast the8 l sO compositionsof secondary veinand replacement calcites precipitated from seawaterat low temperatures in submarine ocean floor basaltsare comparabletothe results from the subaerial flows from Hole 642E (see, forample, Muehlenbachs, 1977).

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    5 1 80 and 8 13C ISOTOPIC CO MPOS ITION FROM BASALTIC LAVAS

    - 3 0 0 IS . RI

    -1200

    x

    X XX

    D

    C D aDm

    3 0

    A

    - 2 0

    s , 3 c

    -1 0

    PDB

    -1 5

    B

    10 - 5

    S l 8 0 PDBFigure 1. The isotopic contents of contained secondary carbonates compared with the Hthostratigraphy and alteration of the volcanic rocks in 642E: A. plot of8 13C vs. depth (mbsf), and B. plot of5 1 80 vs. depth. Flows: no ornament = fine-grained; right-slanting hatching = medium-grained; left-slanting hatching = mixed. Flow tops: G = gray, R = reddened. Alteration: no ornament = low-temperature mineral replacemright-slant hatching = highly weathered zone; left-slant hatching = zeolitic zone.

    0

    - 5

    CD -10QQ_

    O - ' 5ic

    CO - 2 0

    - 2 5

    -30 L-1 5

    MARINE CARBONATES

    a a

    ' a n ,

    FRESHWATERCARBONATES

    I oco cr.