Isotopic constraints on Columbia River flood basalt genesis and the nature of the subcontinental mantle

00167037/84/$3.00

Gmhimica o Carmochimica A& Vol. 48. pp. 2357-2372 8 Fn-gamon Press Ltd. 1984. Printed in U.S.A.

+ I)0

Isotopic constraints on Columbia River flood basalt genesis and the nature of the subcontinental mantle RKHARD

w. ChRLSON

Department of Terre&al Magnetism, Carnegie Institmioa of Washington, 524 1 Broad Branch Road, NW, Washington, D.C. 200 15 (Received April 3, 1984; accepted in revisedjbnn Augusr 17, 1984) Ah&met-Pb, 0, Nd, aad Sr isotopic data for the Columbia River basalts paint a complex picture for the or&in of this flood basalt province. At least 3 distinct mantle soutces appear to have been involved, superimposed upon which am the elTectsof crystal tiactioaatioa aad mass exchange with evolved crustal walho&. To a large degra, the initiation of Columbia River vokxiakxa aad the #cal characteristics of the basalts appear to have been iatlueaced by subduction of the Juaa de Fuca plate beneath the North Ameticaa plate in a meaner analogous to the or@ia of back-arc basins The physical structure of the crust appears to have influenced the late stage evolution of the magmas by direct@ the locus of eruption to the border between the aacieat continental interior aad much younger crust to the south aad west. This proximity to the continental interior also allowed old enriched subcoatiaeatal mantle to become involved in the very late sta8es of Columbia River volcaaism. An important coasequeace of the existence of eariched maatle re8ioas beneath continents is that the combination, crust plus enriched mantle, reouitm mote incompatible elements to have been extracted from the remainder of the mantle than would be the case if ao enriched mantle existed. INTRODUCITON

CONTINENTAL floodbash and their counterparts erupted along ocean ridges differ substantially in their chemical and isotopic characteristics. Though these differences have been recognized for a considerable time, their ultimate cause is still subject to debate (e.g. DEPAOLO, 1983; CAIUON et al.. 1983). For the most part, this debate centers on whether the incompatible element enrichment and more radiogenic Sr and less mdiogenic Nd isotopic compositions of many continental flood basalt.5 requires their mantle source materials to be correspondingly enriched compared to the YepleWY sources of ocean ridge basalts. The difficulty in answering this question arises from the fact that many of the chemical and isotopic characmristics attributed to the volcanic products of an “enriched” source can be mimicked by mixing between materials of the continental crust and primary melts derived from depleted mantle. Thii problem is compounded by the possibility of crustal materials being recycled into the mantle through subduction m (ARAKIBONG, 198 1; HOFMANNand WHITE, 1982). Of the commonly used isotopic tracers, only oxygen, because of its near constant abundance in the crust and mantle, is insensitive to whether the “contamination” occurs in the mantle or crust. Thus, by the combined study of oxygen and radiogenic isotopic variations in a suite of rocks it is potentially possible to determine the relative importance of crustal assimilation as opposed to mantle procemes in controlling the trace-element and isotopic composition of the lavas (TAYLOR, 1980; JAMES 198 1). In this paper, Pb and 0 isotopic data are presented for the flood basalts of the Columbia River group along with Nd and Sr isotopic data for a number of chemical units within the province for which this

information has not been reported previously (e.g. MCDOUGALL, 1976; CARLSON m aI., 1981). These

data serve to define further the procemes involved in the formation of this major basalt province and provide additional information on the characteristics and history of the subcontinental mantle. GEOLOGIC BACKGROUND During the time period 6 to 17 Ma a80 approximately 200,000 Km3 of basaltic lavas wete erupted from aa atea near the Oregon-Washia8toa-Idaho State botders in the northwestern United States (e.g. WATERS,1961; SWANSON and WRIGHT,1979, Fii 1). These Columbia River Basalts (CRB) have been the subject of a large number of field and petrologic studies (BINGHAMaad GROLIER,1966; WRIGHT et al.. 1973; SWANSONet al., 1979; CAMP and HOOPER, 1981; REIDEL,1983; Ross, 1983). To brietly summarize from this work, the CRB have been subdivided into five main stratigraphic formations (SWANSONet al., 1979). The largest of these, the Grande Roade, erupted over a narrow time interval 14-16.5 Ma a8o. contains some 75% of the total volume of besalts in the province. Chemically, Graade Roade basalts dispky the chamcteristics often considered typical of continental thokiites, in particular, high SiQ and iacompstible element contents, with low Mg/Fe and compstibk ekment coacentretions. Some Gmnde Roade bssshs have Sr and Nd isotopic compositions near the assumed values of the bulk-earth. Thii in part, lead to the suggestion that continental flood besalts sre derived from primordial mantle sources (DEPAOLOaad WASSERBURG,1976, DEPAOLO,1983). Compsred to the Gmnde Roade, the partly time correlative picture Gorge, and the slightly older Imashs stratigmphk fomustions, @pear to be less evolved chemicslly (lower SQ, higher M@e etc.). Combined, the Picture Gorkte aad Imashs units coataia 14% of the volume of Columbia River extrusives (SWANSONand WRIGHT,1979). Stratigraphicaily younger than the Grande Ron&, tlows within the Wsaapum folmotoa (5% of the CRB volume) sm, with a fm exceptions, cheaticafly and isotopically simikr to the evolved members of the Graade Ronde sequence. The most aoticeabk distinguishing chanrcteristics

2357

2358

R. W. Carbon

100 km c-(

a

“,

MONT.

the SMB were suggested (CHURCH, 1976:CARLSON etal.. 1981) to have been derived from a mantle source distinct from that involved in the genesis of the main volume of CRB lavas, perhaps from an enriched subcontinental lithospheric section. ANALYTICAL PROCEDURES

With the exception of Sm and Nd, the trace element analyses listed in Table I were performedby instrumental neutron activation at Washington University (following JAc0n.s et al.. 1977). Uncertainties are l-2% except for Ba, Hf, Tb (<5%) and Cs, Th (~10%). Procedures for sample dissolution and Rb, Sr and Nd separation have been described previously (ISHIZAKAand -CARLSON,1983). Rb and Sr concentrations were determined by isotope dilution. Repeat analyses (8) of BCR-I show L__ reproducibiities of = I .O%about a mean of 48.3 ppm for _i ; UTAH L Rb and =0.5% (mean 325.3 ppm) for Sr. ” I I NEV. I Sr isotopic compositions were measured predominantly on a 6’ mass spectrometer with ion currents of @Sr = l-2 H CASCADES X lo-” amps. Rb was monitored at mass 85 but Sr isotopic m COLUMBIA PLATEAU data were not taken until the signal at mass 85 decayed below detection (=10-‘6 A). Results are fractionation cora OREGON-MODOC PLATEAU rected to %r/“Sr = 0.1194 and reported relative to “Sr/ SNAKE RIVER PLAIN %r = 0.70800 for the E and A Sr standard. The cumnt measured value for *‘S$?Sr for this standard is 0.708082 FIG I. Laation of major tectonic and volcanic provinces & 14 (mean and 20, of 21 separate runs) and 0.710279 in the northwestern U.S. Short parallel lines indicate areas + 15 for the NBS 987 Sr standard (mean of 8 separate of feeder dikes for the Grande Ron& (CJ, Chief Joseph runs). dike swarms), Picture Gorge (PG), and Steens Mountain Isotopic compositions for Nd are fractionation corrected (SM) basalt% other major structural features including the to ‘~NdO/“‘NdO = 0.722251 (corresponding to ltiNd/ Blue Mountains Anticline (BMA), Olympic-Wallowa Lin“‘Nd = 0.7219) and thereafter for oxygen based on the eament (OWL), and the Columbia and Snake Rivers are oxygen isotopic composition given by NIER (1950). Sm shown for mference. interference was monitored by normalizing the raw data to “*Nd and comparing the sample’s “‘Nd/“?Vd or “Nd/ “*Nd with values measured for the La JoUa Nd standard. Corrections of 1 part in 10,000 or less to the measured are considerably higher Ti02 and PzOs compared to the “lNd/“‘Nd were required for about one-quarter of the Grande Ronde. samples discussed in this paper. Cc0 or PrG interferences Following the extrusion of these four units, volcanism on greater than 1 part in 10’ were not observed for any sample the Columbia Plateau slowed considerably as the remaining in this study. “‘Nd/“‘Nd data are reported relative to a 2,000 Km’ of basahs of the Saddle Mountains formation value of “%d/“‘Nd = 0.5 11860 for the La JoBa Nd standard. were erupted from 13 to 6 Ma ago. Compared to other The average measured “‘Nd/“‘Nd at DTM for this standard CRB, most Saddle Mountains basahs (SMB) show a very is 0.511837 + 7 (mean and 2a, of 19 separate analyses). wide vziance in chemical composition, and have. anomalous Sr and Nd isotopic compositions (“Sr/s%r h 0.7075, eNd eNd values (cu., = (((‘4JNd/“‘Nd)s.,,,pd(‘43Nd/‘UNd)s,,u-Culh)r& 1) X 10’) are calculated with “Bulk-Earth” ‘*‘Nd/ d -5). Previous isotopic studies of the CRB (MCDOUGALL, ‘Nd = 0.512638 (JACOBSENand WASSERBURG,1980). fNd 1976; CARBON efal., 1981) have shown them to have a values for Columbia River basahs reported by CARLSONef al. (1981) should be lowered by I .4 t units to compare with very large range in Sr and Nd isotopic composition (“Sr/ those discussed here. %r from 0.7035 to 0.7145; cw from +6.4 to -16.9). With Pb was separated for isotopic analysis with two consecutive the exception of the SMB, the remaining stratigraphic HBr-HCI anion exchange columns. Pb blank for these formations show much more limited ranges in isotopic procedures ranged from 0.7 to I .6 ng Isotopic determinations composition. Basahs of the Picture Gorge formation have were made on a IS” mass spectrometer using a silica-gel, the lowest “Sr/%, hi&heat Q+,, and the most restricted phosphoric acid, loading technique. Measured vahtes for the variation in isotopic compositions (e.g. nSr/%r = 0.7035 NBS 981 Pb standard are: x%#“% = 2.1643; 2mPb/‘06Pb to 0.7039) which do not overlap the values measured for = .91419; mPb/zo6Pb = 0.059070; with a fractionation conGrande Ronde and Wanapum basal&s (*‘Srp”sr = 0.7044 to 0.7058). Sr and Nd isotopic comIx’sitions for basahs of trolled reproducibility of 0.01-0.0246 per AMU. The mean 20, uncertainty in measured m”Pb/mPb for the samples the okler Imnaha formation (%r/ysr = 0.7035 to 0.7048) reported here is 0.06%. do, however, span the ranges observed for Picture Gorge Oxygen was extracted by reaction with BrFs at 7OO“Cfor and Grande Rondc+Wanapum basahs. at least 14 hours. After conversion to COz, samples were Based on a generaI correlation between degree of fractionation (e.g. M&Fe, incompatible element contents) and iso- analysed in a triple collector mass spectrometer producing pm&ions for ‘O/‘% better than 0.04%. Reproducibility of topic composition, CARLSONet al. ( 198 I) sug@ed that the 13% based on duplicate measurements is approximately chemical and &topic chamcmristics of the Grande Ronde, Wanapum, and more evolved Imnaha basahs could be 0.2%. Reported 6% values are normalized to 8.45% for the CIT rose quartz standard yielding a measured 6% of 9.55% explained by a combination of tiactional -on and for NBS 28 white quartz. assimilation of Pmcambrian “granitic” wallaks by primary magmas similar to the mote primitive flows of the Picture RESULTS Gorge and lmnaha formations. This model could not, however, account for the widely varied chemical and isotopic Samples from each of the CRB stratigraphic formations compositions of the Saddle Mountains basahs. Therefore, were selected for trace element, Pb and 0 isotopic analysis

1

2359

Isotopic constraints on Columbia River basalt gensis Table

1:

Trace from

element each of

dbta in ppm for samples the mqlor stratipaphic

selected fOr’mat.iOna

___________________-__-_________________________________-______________________________________

Plctwe Oorge

Imnaha

Crande

Saddle

Wanapum

Ronde

SAMPLE’ M-l PGl PO14 L-11 L-41 Yb-1 ORORN,-3 OFU-1 W-2 W-2 _______________________-________________________________________________________________________ SC Cr

37.9 124

39.4 46.8

41.4 179

Ba Rb2

260 14.6

400 12.0

240

40.5

36.3 56.2

38.0 4.12

25.1 0.0

460

710 41.8

500 35.6

625 42.7

525

560 34.4 1.00

550 28.0 0.50 5.60 4.22

3550 525 46.5 53.0 0.79 11.5 6.28 7.12 0.48

7.48 18.9 13.1

1.79 1.10 3.15

1.58 0.90 3.80

3.67 1.38 0.81 2.53

5.92 1.79 1.02 3.17

M@J/4e020.46 WCS 21100

0.38 14600

0.71 14500

0.43 11500

40 ::

32.5 26.5

33.3 33.3

9.91 22.9 16.0 4.52

19 63 18

29.1 56.1

33.5 138

13.8 33.0 23.0 6.00

Ba/La LaKs Ba/Rb

WIH-2

35.5 9.1

0.37 3.02 1.55

EU Tb Yb

WEM-2

35.5 30.2

0.22 4.3 1.93

La Ce Nd2 sm2

WSM-2

34.00 56.4

7.0 0.27 2.51 0.82

CS Hi Th

W-2

31.4 0.82 4.76 3.49 17.4 24.1 41.1

32 ::

26 21 15

0.83 5.20

0.93 4.70

5.30

4.43

25.0 55.6

19.6 44.7

20.3 6.51 1.95 1.20 3.60

24.5 5.02 1.03 1.06 3.27

0.42 16600 28 30 17

0.38 12300 26 21 14

35.7 0.67 4.96 4.21

0.07 5.14 5.26

5.29 4.01

23.6

23.7 51.5 20.2

53.7 31.0 7.46

6.53 1.97 1.18 3.60

2.27 1.31

24.0 56.1

26.9 60.9

30.9 7.26 2.30

36.0 a.33 2.49

1.33 3.06

1.40 4.00

3.55 0.28 17500

26

22

23

0.32 16700 20

27 15

35 15

24 16

46 20

0.20 14700

0.32 11900

47.0 100.8

Mountains

36.7 70.0

52.3 39.4 10.0 a.45 4.40 2.40 1.65 4.4O 0.20 27100

1.35 3.68 0.32 -

650 31.0 7.52 6.08 33.5 76.9 40.0 9.35 2.78 1.67 4.60

76

14

0.29 19

59 76

10

20

190 500 12.6 0.14 6.69 1.00 35.2 79.7 47.9 10.0 3.05 1.75 4.90 0.41 46200 14 251 40

________________-___~~~~~~~~~___~~~-~~~~~~~~_~~~--~~~~~~~~~~~~~~~~~~--~~~~~~~~_~----~~~~~~-~ ‘Semple 2

Data

descriptions from

Carlson

and

et

collection

localities

are

given

in

Carlson

et

al.

(1981).

(1901) .

al.

from those analysed previously for their Nd and Sr isotopic compositions (CARLSONet al., 1981). Traceclement data for these samples are compiled in Table I. Pb and 0 isotopic Table

2:

Pb end

results are given in Table 2. Major and some trace-element data for these sampla were reported in CARSON et al (198 1). A number of additional samples were also anal@ oxygen

isotopic

data

--_--__~---____-------~~~~~-~~~~~--~~~~~~~~~~~___~~~~~~~~_____~___~~~~___~~_______ Sample Number nember 6”O **‘Pb/‘o’Pb *"Pb/"'Pb ="'P8/"'Pb ________--______----~~~~~~~~~~~~~~~~~~~~~~~~_____~~~~~~___________________________ IWAHA : C-l M-l n-13 PICTURE WRGE: PC-1 PG14 ORANDE RONDE: DSTY 73-355 DSTY 75-219 YEN 77-127 L-l 1 L-41 YB-1 DSTY 78-258 ORGR N,-3 BCR- 1 YANAPUI: DSTY 73-322 DSTU 73-296 DSTU 72-152 DSTY 77-321 DSTU 78-400 DSTU 75-9

5.60 6.09 6.40

10.769 19.006 19.040

15.511 15.646 15.610

38.73 30.60

5.64 5.64

10.037

15.555

30.38

10.810 18.978 18.969 10.910 18.925 10.785 18.758 18.801

15.564 15.640 15.603 15.607 15.611 15.601 15.613 15.609

38.66 38.81 38.67 30.74 30.74 30.76 30.93 38.66

19.009 10.090 18.001 10.773

15.504 15.500 15.612 15.625

38.50 38.54 38.75 38.77

10.729

15.558

38.70

10.198 17.736 10.085 17.819 19.751 18.367 19.109 18.626 10.656 10.464

15.629 15.509 15.627 15.507 15.952 15.656 15.777 15.653 15.652 15.654

39.36 38.81 39.17 38.87 40.13 39.74 39.91 39.45 39.44 30.76

19.049 10.000

15.702 15.660

40.60 39.32

7.13 6.62 6.01 6.27 6.91 6.33 6.77 4.94

Robinette Mtn. Dodge Shumaker Eckler Mtn. Frenchman springs PPIest Raplds

&~TAIN~~~ nU-1 uiatim DSTU 72-213 Wilbur Creek DSTU 72-31 Asotln 20-6-63-l nuntzlnger IiSM-2 Esquat ml IfPCtl-1 PODOna HEM-2 Elephant Mtn. DSTW 75-U6 Basin City DSTU 75-114 Martindale DSTH 73-361 Lower Monumental CRUSTAL XENOLITHS : DSTY 73-92OC DSTW 73-l 23B

6.60 6.69 6.82 6.36 6.60 6.47

SAZE

SPmPle @Van

He12

desmiptlons. BY CarlsOn (1900).

et

collection al. (1981);

6.05 8.03 6.37 6.73 6.09 6.09 6.34 5.01 7.41 12.37

localities, Yright et

al.

and major element mp08iti0M (1979, 1980, 1982); and

30.20

2360

R. W. Carlson Table

3:

Nd and Sr

isotopic

data

-------~-~~--_-----_~----___---~~~~--~________--___-_____-____--______________--____ SaPple

Number’

Chemical

Type

Rb2

Sr2

“?,r/‘%3

I. SNd, I. .N*3

3

17.8 15.1 4.6

389 380 406

48.3 49.6

325 311

0.70494t5 0.70500f4 0.70489*5 0.705DQ6 0.70499*3 0.70495*4

0.5126117*24 0.512683*21 0.512643i19 D.512643+19

[email protected] +0.9*0.4 +0.1io.4 l0.1*0.4

3.3 11.6 48.2 49.4

330 362 335 324

0.512769i19 0.512738*19 0.512671*16 0.512664i21 0.512625i26

*2.6ti.4 +2.0&.4 +0.6iO.3 +0.5?0.4 -0.3Kl.5

29.5

321

0.5126Oli23 0.51259Di38

-0.7x).5 -0.9t0.7

39.2 41.2

273 271

0.71047t4 0.71063t3

0.511770t21 0.511801*24 0.51234Oi19

-16.9to.4 -16.3ti.5 -5.aio.4

12.5

252

46.9

244

0.70755*5 0.70884t9 0.71317*14 0.70702t0 0.70764*7

0.511991+27 0.512060r21 0.512312i30 0.512329*19 0.512335f22 0.512333i26 0.512128t16

-12.6i0.5 -11.3i0.4 -6.4eJ.6 -6.0H.4 -5.920.6 -6.OiO.5 -9.9N.3

tNd

_______________________~~__________________________________.________________________

CAANDE DSTU DSTW DSTU DSTW

RCMDE: 72-151 HlSh HS. S1 73-355 High MS. Si 75-219 High HS 70-425 BCR-1 BCR-2 WANAPUH: DSTW 73-322 Robinette Htn. DSTH 73-296 Dodge DSTY 72-152 Shumaker DSTY 77-321 Eckler Ht.“. DSTU 78-408 Frenchman SD. DSTU 72-313 Rosalia DSTW 75-9 Priest Rapids DSTY 71-42 LO10 SADDLE KIUNTAINS: DSTU 72-213 Yilbur Creek DSTU 71-&JUilbur Creek DSTW 72-31 Asoti” DSTU 71-15 ASoti” 20-6-63-l Hu”tzi”Eer DSTY 73-349 Esquatzel BaSl” city DSTW 75-46 DSTU 75-114 Ko'tindale GoOse Island DSTY 75-35 India” Mfmorlal DSTU 75-36 DSTY 73-361 Lower Monumental XMOLITH: DSTU 73-1238

‘Sample descriptlona. Collection by Wright et al. (1979, 1980, 2

Trace

element

3 Uncertainties

contenta

in

12.7

237

18.6

250

0.70438*4 0.70435*4 0.70481 t3 0.70546r4 0.70517*4 0.70507t6 0.70521i18 0.70506*5

0.70780*4 0.71087*6 0.71083+6

75.2

localities 1982).

and major

element

COrnpOSitIOn

give”

pm.

are 20 of the mea" and correspond

from several of the chemical units not studied previously. The major and trace element compositions and collection localities of these samples have been reported by WRIGHT et al. (1979a, 1980, 1982). Nd and Sr isotopic compositions for these samples are presented in Table 3. Chondrite normahxed REE patterns for representatives of the samples listed in Table 1 are shown in Fii 2. Within the individual stm@aphic units, most flows show a relatively limited ran@ in REE abundances As a pneralization, total REE abundances, and the ratio of li&tt to heavy REE, increase in the order picture Gorge, Grande Ronde, Wanapum, Saddle Mountains or roughly in order of stratiknaphy. Notabk exceptions to this are the Robinette Mountain and Dod8e members of the Wanapum group. These two units have lower total REE abundances than other Wanapum flows, near the ran8e observed for Picture Gorse basalts (WRIGHT ef al., 1979a,1980, 1982). However, their La/Yb ratios am s&htly hi8her than found for Picture Gorge basal&. Abundances of the heavy REE vary considerably between, and within. formations, indicating that &arnet was not an important phase controlling the trace element character of these basalts. This suggtsts either that melting pmceeded past the point where all garnet was consumed, or more likely, that final equilibration between melt and solid occurred at relatively shallow depths, above the stability field of garnet. Besides the gcntral enrichment in incompatible elements in the CRB, they are especially enriched in Cs and Ba compared to ocean ridge and ccean island basalts. For example, average K/Cs ( I 5140). La/Cs (32), and Ba/La (26) for the non-Saddle Mountains samphts listed in Table I show more similarity to island arc basahs (avenge 11740; 13; and 27 mapectively; Mortms and HART, 1983) than they do to average ocean island basalts (44310; 145; IO respectively; MORRISand HART, 1983). For the most part, the new Nd and Sr isotopic data (Fig. 3) fall along the trend defined previously (CARLSONef al., 198 1). The most obvious exceptions are data for the Huntx-

to the last

digit.

inger (H in Fi8s. 3-7) and Wilbur Creek (W) members of the SMB formation that continue along the extrapolation of the line defined by the Picture Gorge-Imnaha-Grande Ronde-Wanapum units, rather than curving to hi& “Sr/ “6Sr as observed for the UmatiRa (U), Lower Monumental (L), and most noticeably the Esquatzel (E) units of the SMB. Additional Nd and Sr isotopic data for the Grande Ron& formationminfometheo&rva&mthattheimtopicva&ion of this unit covers the ranlpc from Lo = +3 to -1. With these new data, the gap in w and “Sr/‘%r between the main mass of CRB and the be&ning of data for the SMB

I(

(

,

1

,

,

,

,

,

,

1

,,,I

zoo-

IOO[

ao-

4

_

P P Y

_ 40-

$ ;: 20-

IO-

FIG. 2. Chondrite normaheed REE patterns for basahs from each strati8mpbic forma&m of the CRB. Symbols represent data for the (m) Imlllha, (V) Picture Gor8e; (0) Grande Rondo; (0) Wanapum; and (a) Saddle Mountains formations for this and all remaining figures.

2361

Isotopic constraints on Cotumbk River basalt gensis

0.704

0.706

0.706

0.710

0.712

0.714

87sr/86sr Rtj. 3. Sr and Nd isotopic variation of CRB samples FieIds for mid-ocean ridge and selected ocean island basahs (Wm and HOFMANN, 1982; !WM.Eet uk, 1983) are shown for mference. Binary mixing curves between components C2 and C4, and Q and C4 are presented for component compositions given in Table 4 (large stars denote individual component locations). Dots aIong each curve represent 10% increments in the amount of C4 present in each mixture. Letters above Saddle Mountains basalt data correspond to the fbst letter of the stratigraphic members given in Tables 2 and 3. becomes more distinct (Fii 3). Data for additional members of the-Wanapum formation show that this unit has approximateIy the same degree of isotonic variation as the Grande Ronde. However, these data also may allow for the subdivision of the Wanapum into distinct groups on the basis of isotopic composition as the chemically primitive Robinette Mountain and Dodge flows have higher Q,, and knver s’Sr/ %r than other members of the Wanapum group. Pb isotopic compositions (Fig. 4) for the Picture Gorge, Imnaha, Grande Ronde, and Wanapum basahs show a chrstef of points starting within the array seen for ocean island basahs and extending to bigber w/=Pb and =I%/ *Pb. Pb isotopic compositions for the SMB are dispiaced from the majority of CRB data towards much higher “Pb/ -Fb and x’sPb/~Pb as well as showing a much greater range in 2MPb/r@‘Pb. If the linear trend of the SMB data on the rePb/rc% verrtfs rVb/+b diagram is interpreted as a secondary isochron, the slope corresponds to an age of 2.6 Ga with sample WSM-2 (Esquatxel, E) included, and 2.0 Ga without. Similar observations on the variations in Pb isotopic composition of the CRB previously have been discussed brktly by CHURCH (1976). The SMB utanogenic Pb data fait along the same slope found for basalts from the nearby Snake River Flain (F$. 4, LEEMAN,1974) and YeBowstone (L&E ef al., 1982). Compared to the Snake River PIain and Yellow&one basal&, the Saddle Mountains data are dispIa& to SiitIy higher x+b/scVb values. GxySen isotopic results for the CRB sampks show a rektiveIy wide range in 5% values from +4.9 to +8.0. The unusually low 8% of Grande Ronde sample ORGR Nr3, coupled with the presence of ‘%e in this sample (BROWN et al.,1982). suggests that this flow has been affected by mcent deuteric alteration. Exchrding this sampk. the mmainder of the basalts have oxygen isotopic compositions within, to shghtly above, the range expected for pristine mautie derived melts (e.g. TA~OR, 1968). When plot@ against a Wicgenic isotope mtic (Fig. S), the oxygen data show two distinct groups, one defined by the SMB and the other by the remaining formations of the CRB. Both groups gcfefaify have increasing 8% with decmasing *, and increasing SrPSr, suggesdveof interaction between primary

magmas and high PO ctwtal materials. In the case of the SMB, however, this ‘@hnary” magma appears to have had s’Sr/%r PJ 0.7075 and h fr -6, markedly distinct from the values (6.7035, +7 mspeetively) for the low IVO Picture Gotge basal&.The oxygen isotopic compositions do not d&e linear trendswhen plotteda8ainstradiogenicisotopic composition, possibly reflecting &able amounts of clay alteration in the CRB, a range in isotopic compositions for one of the mixing endmembers, or a partial decoupling of oxygen and trace-ekment contamination. However, none of ~p~~~~~~rn~he~on~sr isotopic composition of these samples as shown by the very good correlation between their Sr and Nd isotopic composition {Fig. 3). DISCUSSION The wide range in isotopic ambition of the CRB requires that more than one isotopically distinct component have been involved in the genesis of these basalts. The question arises again as to whether these components include material assimilated from the overlying crustal section or can be represented solely by heterogeneity within the mantle. The arguments for and against crustal ~n~ination of the CRB based on major and trace element and Sr and Nd isotopic data have been discussed previously (M~D~xJGALs 1976; CARWIN et af., 198 i , 1983; DEPAOLO, 1983). Rather than reiterating these arguments, the foBowing discus&on address primariiy the new Pb and 0 isotopic data for the CRB and attempts to show how these data contribute to the refinement of earlier petrogenetic models. Primordiai mantle Based primarily on the observed clustering of Nd isotopic com~tions of a limited number of con&

R. W. Chrlson

2362

17.8

18.2

18.8

19.0

19.4

19.8

FIG. 4. Pb isotopic compositions measured for CRB samples. (A) symbol represents data for the two crustal xenoliths. Parallel band labeled “oceanic array” encompasses data for MORB and most ocean island basal& (SUN, 1980). Mixing lines between components C2, C3, and C4 are as described for Fig 3. Mixing lines between a mantle source similar to that of the primitive Imnaha basalt C-l and a s&&ted

sediment component (Tab& 4) are alsc shown. On these two lines, the small dots npnaent 1%increments in the sediment foment of the mixture. Tbe ranges of lead isotopic composition for o%bore sediments from the northwestern U.S. are shown by tbe small fields labeled SED (CHURCH,1976). fields labeled

SRP encompasses data for Snake River Plain olivine tholeiites (LEEMAN, 1974).

nentaI fkxxi basalts around eNd= 0 (by definition the value expected for a primordial “chondritic” source), DEPAOLO and WASSERBURG(1976) presented the idea that continental flood basalts are derived from an un~~~ntia~ mantle reservoir. Discussions of the relative merits of this model as it applies to the CRB were presented by CARLSONet al. ( 198 1, 1983) and DEPAOLCI(1983). The Pb isotopic compositions of the CRB with cNdnear zero diverge markedly from values expected for a primordial, undifferentiated source (Fig. 4) which, by definition, should have uranogenic Pb isotopic compositions plotting along the geochron. In contrast, all CRB data (with the exception of two SMB) plot to the tight of the geochron in Fig. 4, a clear indication of multistage evolution of the U-Pb system in these basalts. Unless one appeais to seiective ~ff~ntia~on mechanisms, such as Pb removal to the corn (D~JPREand ALLEGRE, 1980) that affect only the U-Th-Pb system and not the Rb-Sr or Sm-Nd systems, the Pb isotopic composition of the main volume of the CRB requires their mantle source to have been differentiated, perhaps several times, during the history of the Earth. In league with previous arguments, the Pb isotopic data indicate that primordial, undifferentiated, mantle materials wefe not involved in the genesis of the CRB.

On a plot of Pb isotopic composition (Fig. 41, binary mixing between two endmembers will produce a line connecting the two endmembers. This is true even if the binary mixing is accompanied by ftnctional crystallization of the magma, though in this latter case the mixing line may not reach the contaminating endmem~r (e.g. JAMES,1981). Clearly in Pii 4, no single mixing line is defined by the data. The hick of a well defined Pb isotopic trend linking SMB with the remainder of the CRB supports earlier supposition (CHURCH, 1976; CARLWN~~., 198l)thatthe SMB are derived from a distinct mantle source compared to the main volume of CR& However, the Pb isotopic scatter seen for Picture Gorge, imnaha, Grande Ronde, and Wanapum basahs contrasts with the single trend displayed by the Sr and Nd isotopic composition of these formations (Fig. 3). Thus, the Pb isotopic data conflict also with the simple model linking basahs of these four formations by combined fractional crystafliition and assimilation of a single primary magma and a single crustal “contaminant.” Despite the lack of well defined trends on Pb-Pb plots (Fig. 4), when the Pb, Sr, Nd, and 0 isotopic data are considered simu~tan~u~y, systematic variations reflecting the involvement of several distinct

2343

Isotopic constraints on Columbia River basalt gensis I

I

8.0 -’

1

‘“t

1

-16

I

I

I

I

I

.

I

I

I

1

I

I

-12

-8

-4

0

4

8

& Nd FIG. 5. Plot of d’*O versuseNdfor Columbia River basal& Mixing curves shown are between components C2, C3 and contaminants with 6180 values of 8 and 16. Dots along these curves denote 10%increments in the amount of contaminant present in the mixture. A mixing curve between Cl mantle and subducted sediment (Table 4) is shown also. Dots along this curve denote 1%increments in sediment abundance in the mixture.

components

in the formation of these basalts can be Figures 3-7 illustrate these trends when pairs of isotope systems am compared. If these isotopic trends are interpreted as mixing lines between various distinct reservoirs, the following endmember compositions can be identified: Componenf 1. Component 1 (Cl) has cr.,,,h i-6.5, *‘Sr/%Ir 9 0.7035, msPb/m4Pb = 18.8, zo7Pb/204Pb = 15.51, “8Pb/mPb = 38.3, and a’*0 w +5.6. This component most likely represents the participation of an incompatible&ementdepleted mantle teservoir in the production of these basalts, and is best preserved in the one Imnaha sample (C-l) and the Picture Gorge basalts. Picture Gorge basalts have eruptive centers well to the south and west of the feeder dike swarms of other CRR basalts (Fig 1: WRIGHT et al., 1973). This geographic isolation may have played a key role in preserving the signature of the depleted source. That the isotopic composition chosen for Cl does not result from crustal contamination of a more “MORB-w (Le. w h + 10) primary magma cannot be conclusively demonstrated However, a wide variety of basalt types from the northwestern U.S., including the relatively unfractionated, high-Mg/Fe, high-Al basalts of the Oregon Plateau (HART, 1984), also have maximum cNd of less than +8 (CARLSON and HART, 1982, 1983). The lack of basalts in the western identified.

U.S. with cNd> +8 suggests that the isotopic values chosen for Cl do, in fact, represent those of a regionally extensive mantle reservoir rather than a mixture between MORl3 magmas and old continental crust. Component 2. A second, less easily identified, component is best represented by the isotopic characteristics of Imnaha basalt sample M-l. Compared to Cl desctibed above, C2 has higher s’Sr/%r (0.704), “Pb/-Pb ( 19.09), 207Pb/204Pb ( 15.65), ‘O”Pb/“Pb (38.7) and Si80 (>6.0), with lower tNd (+4.5). The need for this component is best illustrated in plots of 206Pb/r”‘Pb vs. ti or s7Sr/s%r (Figs. 6, 7), where it shows up as a “kink” in the trend defined by the Imnaha, Picture Gorge, Grande Ronde, and Wanapum data. One possible interpretation of the geologic significance of C2 is that it is simply a point on a mixing trend between Cl and a crustal material with very radiogenic Pb isotopic composition, such as that represented by the metasedimentary xenolitb 7392GC. Roughly 1oW by weight bulk contamination of sample C-l by 73-92GC will produce isotopic compositions close to that of C2, although the 207Pb/ ?b will be slightly lower, and the 2oBPb/aPb higher in the mixture compared to, for instance, the isotopic composition of sample M-l. Unless accompanied by

2364

R. W. Carlson

I 0.703

I

I 0.705

I 0.708

*

I

l

I 0.712

87sr186sr FIG. 6. Sr versus 206Pb/zo4Pb and 208Pb/204Pb for CRB. Field labeled OIB includes data for the majority of ocean island baralts (e.g. Azores, Canary, Ascension: SUN, 1980, COHENand O'NIONS, 1982; STILLE et al., 1983).The curve eminating from component C I (denoted by the large star) shows the path created by mixing, up to a maximum of 3%, subducted sediment with Cl type mantle (Table 4). Other dotted curves illustrate the mixing relationships between components C2, C3, and C4 as described for Pigs. 3-5.

Note change in scale on X-axis.

fractional crystallization, the assimilation of 10% of a material like 73-92GC would produce only minor shifts in the chemical composition of the mixture. For instance, elements sensitive to contamination, such as SiOs and Rb would increase from 46 to 50 wt.% and 3 to 7 ppm respectively while a compatible element like MgG would decmase in concentration from 8.9 to 8.2 wt% assuming the primary magma was similar in composition to the Imnaha basalt C-l. An alternative explanation for C2 is that it represents a mantle region that initially was similar in isotopic composition to “depleW oceanic mantle (Cl), but was contaminated by material derived from subducted sediments. As shown in Figs. 4-7, only 2% subducted sediment with the Pb isotopic composition measured for North Pacific sediments (CHURCH, 1976; Table 4) mixed with a depleted oceanic type mantle will produce a mixture with Sr, Nd, Pb, and 0 isotopic composition similar to Imnaha basalt M-l. This latter explanation is supported by the following observations: The well defined correla-

tion between 206Pb/204Pb and tNd or s7Sr/86Sr (Figs. 6, 7) for the Grande Ronde basalts extrapolates back to a single point (the C2 endmember) rather than scattering between Cl and the crustal contaminant as would be expected for variable amounts of crustal assimilation by melts of C 1 composition; trace element ratios sensitive to the presence of subducted sediment contamination (i.e. K/Cs, La/Ba, etc.) of the Grande Ronde basalts are more similar to island arc basalt& where a sediment component is oRen inferred, than they are to ocean island basalts; the basalts with isotopic composition near component C2 am only moderately differentiated compared to the majority of Grande Ronde basalt& a feature that can only be reconciled with a crustal contamination model if the assimilation was not coupled strongly with fractional crystallization. A sediment enriched mantle source for the Grande Ronde basalts also may be more easily reconciled with other, mom subjective, observations such as the relative homogeneity of their chemical characteristics, especially their silica saturated nature and degree of incompatible element enrich-

2365

isotopic cwwtraints on calunibia River has& gerlsis

39.5

% 39.0 B 2 38.5 zz RI 38.0

MORB \

-16

-12

-8

-4

0 E

4

tJ

lz

Nd

FIG 7. Comparison of Pb and Nd isotopic variation. Mixing curves are BS described for previous figures. Dots along the curve eminating kom Cl indicate 1%additions of subducted sediment to CI type mantle (Table 4). Dots along other curves give 10% increments in the amount of C4 present in the mixture.

ment, and the lengthy history of subduction beneath the northwestern U.S. Comjwirent 3. C3 is illustrated best in piots of oxygen vs. radiogenic isotopic composition as it bas low S’*O (s 4-4) but radiogenic Sr (“Sr/% = 0.7075) and low cNd (-5). The Pb isotopic composition of C3 is defined poorly as the basaits with Sr, Nd and 0 isotopic compositions closest to C3 have “6Pb/mPb ranging from 18.09 to 19.11 (the Asotin (A) and Elephant Mountain (EM) members respectively). For the purposes of modeling, the isotopic composition of the Martindale (M) sampie was chosen to represent C3 because of the low #‘O measured for this sample. Again, as for C2, several distinct geologic interpre-

tations could be assigned to C3 given its isotopic chawteristics. The first is that it represents a point on a mixing curve between C1 and an old crustal

------------~---------------~---____--~__--~~__________--_~_-_____ ccaponmt ct c2 c4 Cl Subduotsd c3 Mantlr S*di~cnt

CPblPP l'Sr/**sr %I

400 ,*

300 23 10

250 20 5

350 26 15

25 0.6 0.06

150 15 20

0.7035 l.5

0.7036 r4.5

0.7076 -6.0

0.715 -30.

0.7035 as.5

0.708 -2.0

1

***pb/*e.W 18.77 19.09 18.66 17.0 18.77 19.05 15.52 15.51 15.64 r"Fbf'*"Pb 15.51 15.6s 15.63 "*Pb/'**Pb 38.28 39.44 38.5 38.66 61.0 r5.6 ::.;3 l5.a l14. %" l25. -------*-----~~--------~*---~-"_____-__-____-__-____-~_-_-___"_-__

.

.

2366

R. W. Carlson

material with even more radiogenic Sr and Pb and less radiogenic Nd isotopic compositions than inferred for C3, but a 6’sO within the range expected for mantle derived melts. This possibility is considered unlikely. To reach the isotopic composition of C3, starting from Cl, requires about 50% by weight crustal component be present in the mixture even if the crustal material has very evolved Sr and Nd isotopic compositions (CARLSONet al., 1981). Based on major and trace element arguments discussed previously (CARLSON et al., 198 1), a mixture containing 50% crustal material would have a composition dramatically different than actually observed. For example, such a mixture between primitive basalt and a granitic composition crustal component, even assuming that no fractional crystallization accompanied mixing, would have MgO and K20 contents around 5% and 2% respectively while flows of the Asotin member of the SMB have MgO and KzO contents of 8.2% and 0.51% respectively (SWANSON d al., 1979). An alternative to this, more conventional, type of crustal contamination scenario is that proposed by SEMIUN and DEPAOL~ (1983). In their model, the lower crust is remobilized and allowed to mix into the upper mantle to produce a zone of “enriched’ mantle which then melts to produce the isotopically evolved basalts. This model must satisfy at least two observations if applied to explain the origin of the C3 component discussed here. First is that the most easily identified “crustal” component involved in CRB genesis (C4, to be discussed in the next section) has very low 206Pb/204Pbunlike that of C3 (Table 4). Secondly, several voluminous basalt units covering a wide area of the northwestern U.S. such as the SMB, the Snake River Plain basalts (LEEMANand MANTON, 197 1; LEEMAN, 1974; MENUES ef al., 1983), and the Owyhee Uplands high-Al basahs (HART, 1984) also have Sr and Nd isotopic compositions that cluster very near those inferred for C3. To produce this relative Sr and Nd isotopic homogeneity by the mechanism proposed by SEMKIN and DEPAOLO (1983), requires that crust of very similar chemical and isotopic composition be mixed in exactly the same proportions with mantle over the whole geographic area where basalts with the C3 isotopic signature occur. Given the inherent variability of both mixing pmcesses and the composition of the crust over such a wide area, mixing between lower crust and mantle does not seem to be a likely explanation for the origin of C3. The C3 component can not be formed by the addition to more “normal” oceanic type mantle (C 1) of material derived from subducted sediments because of both the low 6**0 of C3, and the observation that no sediments likely to be subducted in this region have Pb isotopic compositions in the range of C3. Perhaps the best source of incompatible element enrichment in C3 is through the addition of incompatible element rich fluids or melts into a stable

subcontinental lithospheric section (e.g. BROOKS et al., 1976). Again, however, this “enrichment” appears to have occurred between 2.0 and 2.6 Ga ago. Thus, C3 most likely corresponds to an enriched subcontinental lithospheric reservoir similar to that proposed earlier as a source for a number of other basalt units in the western U.S. (MARK et al., 1975; LEEMAN. 1975; MENZIESet al., 1983; HART, 1984). Component 4. C4 is identified as a very high “Sr/ %r nonmdiogenic Nd and Pb isotopic component with high 6180. This component is given responsibility for the isotopic variability of the Grande Ronde and the SMB (all except the Elephant Mountain (EM) and Esquatzel (E) units) that have increasing “Sr/ “Sr and decreasing CMand M6Pb/20”Pbwith increasing 6’*0. C4 is perhaps best identified with the crustal contaminant used in the elemental and Nd and Sr isotopic modeling of the CRB discussed in CARLSON ef al. (198 l), with the stipulation that C4 have very low 2osPb/204Pb (< 17.7) but moderate 207Pb/Z04Pb (15.52) and 20sPb/2WPb (38.5). This component is thus distinguished from xenolith 73-92GC which has a very radiogenic Pb isotopic composition. The relatively low *‘Sr/*?Sr and *06Pb/‘04Pbof C4 suggest that this component may represent an old silicic lower crustal rock type with low Rb/Sr and U/Pb caused by granulite facies metamorphism. The conclusions drawn by CARLSON ef al. ( I98 1) regarding the viability of a model of fractional crystallization coupled with assimilation of a crustal component such as C4 as a explanation for the Grande Ronde chemical and isotopic characteristics need be modified only slightly if C2 is considered to be the mantle source for these basalts rather than an oceanic mantle component like Cl. Primarily, the supposition of a sediment contaminated mantle source for the Grande Ronde basahs would reduce the maximum amount of crustal contaminant required to explain their isotopic variation from about 2030% to only lo-201 (Figs. 3-7). However, if a single crustal component is hypothesized to explain the isotopic trends of both the Grande Ronde basalts starting with a primary magma composition like C2, and the contaminated SMB like the Huntzinger (H) and Wilbur Creek (W) tlows. whose parent magma may have been like C3, a crustal component with much lower tNd than that used (-13 to -18) in CARLSON et al. (1981) is required. This is best illustrated in the plots of Fig. 8 which show the radiogenic isotopic composition required for C4 for a given 0 isotopic composition and elemental abundance ratio between contaminant and primary magma. The best match to the data, if a single crustal contaminant is assumed for both the Grande Ronde and those SMB mentioned earlier, is produced by a crustal endmember (C4) with 6”O of +12 to + 16, cNd-Z -20, 87Sr/%r > 0.710 and ‘ObPb/ 2MPb < 17.4. Using the isotopic composition for C4 given in Table 4, about 30% contaminant is required to explain the isotopic composition of the lowest ~~4

Isotopic constraints on Columbia River basalt gensis

16.5-

6

0

IO

12 16

14

16

10

20

60 FIG. 8. Radiogenicisotopic compositions required for

various 6% values for a crustal mixing endmember that will simultaneously produce mixing curves through the Grande Ronde data, starting at component C2, and the Wilbur Creek member of the Saddle Mountains basalts starting with component C3. Numbers along the curves give the element concentration ratios between crustal endmember and starting magma composition.

member of the SMB, the Wilbur Creek flow. This same crustal component, if mixed with a primary magma like C2, would produce the maximum Sr, Nd, Pb, and 0 isotopic shifts observed in the Grande Ronde basalts with only 10% C4 in the mixture (Figs. 3-7). The actual isotopic compositions chosen for crustal component C4 are highly dependent on the assumed concentrations of Nd, Pb, and Sr in both C4 and the primary magma undergoing mixing. The Nd, Sr, and Pb concentrations listed in Table 4 for the C2 and C3 endmembers are approximated from the concentrations of the basalt samples with isotopic composition closest to these endmember components. If these concentrations are lowered to what may be more

2361

reasonable values for unfmctionated PrirWUYmagma& the amount of crustal contaminant required to Produce a given isotopic change in the miXWe can be correspondingly reduced. Alternatively, if the amount of contaminant is held constant, its k0t0pk properties would have to move along the lines shown in Fig. 8 towards less evolved compositions, closer to the primary magma values, to produce the mixing curves shown in the figures. The concentrations of C4 (Table 4) are the average upper crust values of TAYLOR and MCCLENNAN ( 198 1) with the isotopic composition chosen from Fig. 8 to produce the mixing curves shown in Figs. 3-7. If the Nd, Sr, and Pb concentrations of the crustal endmember are changed to better match the expected composition of a granitic melt extracted from the crust (i.e. low Sr/Nd in particular), the *‘Sr/ “Sr of the contaminant must be raised above 0.76 as the Sr content drops below 50 ppm, all else being equal (Fig. 8). Because the isotopic trends seen for Grande Ronde and SMB flows can best be fit by contaminants with elemental concentration ratios (i.e. Sr/Nd, Nd/Pb, Sr/Pb) within the range of common crustal rocks, selective contamination by fluids derived from individual mineral species, which often have very distinct elemental ratios (e.g. K-feldspar, biotite), is not considered a likely alternative to assimilation of melts derived by anatexis of crustal wallrocks. While simple mixing between C2 and C4 can approximate the trends seen for the Wilbur Creek (W) and Huntzinger (H) members of the SMB, it does not work well for other obviously contaminated basalts like the Umatilla (U) and Lower Monumental (L) units that appear to require contaminants with much more radiogenic Sr isotopic compositions than C4. Alternatively, simultaneous mixing and crystallization of a phase concentrating Sr (i.e. plagioclase) would cause the mixing curves between C3 and C4 shown in Figs. 3 and 6 to curve towards higher 87Sr/ %r more closely approximating the Umatilla and Lower Monumental trends (e.g. DEPAOLO, 1981; JAMES, 1981). Combined assimilation and fractional crystallization will have little effect on the shape of the mixing curves not involving Sr because both Nd and Pb would have similar, and very low, distribution coefficients for any phase likely to be removed from primary CRB magmas. Component 5. An additional component may be required to explain the highly radiogenic Pb isotopic composition of the Esquatzel and Elephant Mountain members of the SMB. Again C5 could represent a crustal component distinguished from C4 by a much more radiogenic Pb isotopic composition. To reach the Sr and Nd isotopic composition of the Esquatxel sample starting from an evolved primary magma like C3 would require the addition of upwards of 50% crustal component. The evolved chemical composition of the Esquatxel unit (MgG/FeO = 0.31; SWANSON et al.. 1979) is compatible with a large amount of

2368

R. W. Carlson

crustal contamination. However, the relatively low 6% of sample WSM-2 excludes a metasedimentary material similar to xenolith 7f92GC as the crustal endmember. An alternative explanation for the mdiogenic Pb isotopic compositions of the Esquatzel unit is that its source is a subcontinental mantle reservoir related to, but with slightly hiier Rb/Sr, Nd/Sm, and U,Th/ Pb compared to C3. For example, if at the time of formation of the SMB source 2.6 Ga ago, the source had uniform isotopic compositions equal to those of a l‘bulk+arth” reservoir, the Martim& and E%quatzel units, respectively, would require s’Rh/‘%r = 0.16 and 0.33; “‘Sm/14*Nd = 0.177 and 0.159; and z3*U/ x”Pb = 9.7 and 12.0, in their source regions to explain their present day isotopic compositions. In this model, C3 and C5 are interpreted as a single component, a subcontinental Lithosphere variably enriched in incompatible elements some 2.0-2.6 Ga ago. In either of the cases mentioned ahove, the linearity of the SMB data on the 207Pbf2MPb vs. “O”Pb,“Pb plot suggests that the chemical characteristics of whatever endmembers contributed to the chemical and isotopic variation of these basalts were estabhshed at a common time some 2 to 2.6 Ga ago. Whether this implies that two distinct crustal compositions (C4, C5) were separated from an enriched mantle reservoir (C3) at this time, or that crust formation in the Late-Archean to early-Proterozoic created a crust (C4) and an underlying, variably enriched, mantle (C3, CS) is uncertain.

CARLSON

and HART, 1983: Fig. 1) erupted basalts with isotopic characteristics similar to many oceanic island basalts (e.g. s’Sr/e6Sr = .7036). Though the details of the geographic features of the isotopic variation are constrained only weakly by the available data, an area in the vicinity of the Blue Mountains or Olympic-Wallowa lineament (OWL; RAtSz, 1945: Fig. I) appears to mark the change from low “Sr/ s6Sr rocks to the south and higher “Sr/“Sr basalts to the north and east. This same area is believed to be a major suture zone between older continental crust to the east and newly formed (i.e. mostly Cenozoic) crust to the south and west (SK&HAN, 1966; HILL. 1972; BARRASHet al., 1983). This idea is supported by the isotopic data for the CRB as both Grande Ronde and SMB erupted north of the OWL appear to have interacted with continentai crustal material of Archean or early Proterozoic age. The nearest flood basalts erupted to the south of the OWL (Picture Gorge) show only extremely limited isotopic variation, suggesting either that they did not experience crnstal contamination or that whatever crustal materials the primary magmas encountered had isotopic characteristics similar to the basalts themselves. Besides the influence on the chemical and isotopic composition of the CRB, this structural zone may also have controlled the locus of eruption for these basalts by acting as a zone of weakness (ROSS, 1983) in the crust where dike propagation from a deep magma chamber (e.g. SHAW and SWANSON, 1970) could occur more easily than in surrounding areas. The fractionated chemical compositions (i.e. low Genesis of the CRB: mixing the e~members MgG/FeO, Ni and Cr) of the majority of CRB, Imnaha-Picture Gorge-Grande Ronde. The identiespecially the Grande Ronde sequence, indicate that these lavas are not primary melts of an olivine fication and isolation of possible endmember components involved in the evolution of the CRB places dominated, peridotitic source. Given the isotopic a number of constraints on the origin of this flood evidence for sediment con~mination of the Grande Ronde source, some of the distinct elemental charbasalt province and the nature and history of materials in the s&continental mantle. The isotopic characteracteristics of these basalts may have been controlled by mantle source composition, especially the high istics of component C2, the sediment contaminated Si02, Cs and Ba contents of these lavas. However, mantle, and the importance of C2 for the isotopic the isotopic evidence for crustal contamination of trends of the major volume of the CRB, the Grande the Grande Ronde basalts and the relative success of Ronde formation, implies a relation between recent a combined fractional crystallization-crustal assimisubduction and volcanism on the Columbia Plateau. lation model (CARLXIN et a[., t981) implies that the This signature of a “recycled” sediment component in the mantle source of the Grande Ronde basalts is highly fractionated composition of these magmas results from extensive crystal fractionation during a supportive of the analogy between the CRB and residence period in a crustal storage chamber. Roth back-arc basin basalts (MCDOUGALL, 1976) as similar the lack of subsidence grabens in the Columbia signatnres of recycled sediment are found in some oceanic back-am basalts (e.g. COHEN and O’NIONS, Plateau associated with the emptying of a shallow magma chamber (WRIGHT el al., 1979b) and the 1982). very large volumes of single flows, suggest the magma Though plate interaction may have played a major role in initiating CRB volcanism (e.g. CHRIST~ANSEN storage chambers for these basalts were confined to great depth in the crust or at the cru!+mantle interface and MCKEE, 1978), the structure of the continental (SHAW and SWANSON, 1970; WRIGHT et al., 1979b; crust, and perhaps the presence of an ancient sub Cox, 1980). During this storage, the primary magmas continental lithosphere as well, appears to have influfractionated a crystal assemblage dominated by plaenced the physical and chemical evolution of the gioclase and pyroxene (H&Z, 1980; COX, 1980; CRB magmas. Major eruption localities to the south CAREZONet al., 198 1) while also mixing with isotopiand west of the Grande Ronde dike swarms (Picture tally evolved “granitic” composition materials (C4) Gorge, Steens Mountain (GUNN and WATKINS, 1970:

Isotopic constraints on Columbia River basalt gensis

extracted from the surrounding crustal section. The presence of this crustal component and the possibility of a heterogeneous mantle source explains the difficulty of accounting for Grande Ronde chemical variations by simple fractional crystallixation models alone (e.g. WRIGHT ef al., 1979b; Ross, 1983). Wanapum. Following the cessation of Grande Ronde volcanism, the transition to the Wanapum basalts is marked generally by a distinct increase, in particular, in TiOs, FeO, and PzOS, and a decrease in Si02 (WRIGHT et al., 1973). However, the oldest chemical units of the Wanapum (Robinette Mountain, Dodge) do not show the “TiOr discontinuity” (SIEMS et al., 1974) characteristic of other Wanapum flows. Rather they are relatively primitive chemically (high Mg/Fe, low incompatible element contents) and have the lowest *‘S#%r and highest +,a of the WanapUIXI group. The Pb isotopic composition measured for two samples from the Robinette Mountain and Dodge tlows plot near the “contaminate mantle component (C2) postulated as the source of the Grande Ronde. The fact that the Robinette Mountain member has higher Mg/Fe, A1203, and lower K20, Rb, and Ti4 contents than any other flow of the Grande Ronde and Wanapum formations again suggests that the isotopic compositions of endmember component C2 do in fact represent those of a distinct mantle source and are not the result of crustal contamination of magmas derived from a more oceanic, or MORB like, mantle. Following the eruptions of the Robinette Mountain and Dodge members, the magma compositions shit&d to more characteristically fractionated basalts with low Mg/Fe and high incompatible element contents (e.g. SWANSON et al., 1979). The isotopic compositions of these later Wanapum members (Shumaker, Frenchman Springs, Roxa, etc.) lie towards the high “Srl%r end of the trends defined by the Grande Ronde basal& Presumably, by analogy with the Grande Ronde, this again reflects interaction between primary magmas with isotopic compositions similar to, for example, the Robinette Mountain member, and isotopically evolved materials of the crust. However, the oxygen isotopic compositions of the Wanapum basalts show no correlation with their mdiogenic isotopic composition though all Wanapum samples have elevated 15’*0 above 6.3%. Given the isotopic similarity between the fractionated Wanapum and Grande Ronde flows, the chemical differences between these two units perhaps can be attributed best to different degrees of partial melting of similar, but separate, sediment contaminated mantle sources. If so, the higher TiO* and P205, and lower SiO* of the Wanapum suggest these basalts were produced by fractionation of increasingly more alkalic primary magmas originating by smaller degrees of partial melting during the waning stages of volcanism on the Columbia Plateau. saddle Mountains. AS argued previously, the highly evolved isotopic compositions of the SMB coupled

2349

with the observation that at least some members of this group (Pomona, Asotin) have relatively unfractionated chemical compositions and mantle-like 8’sO values, suggests the generation of these basalts from a distinct, and very old, mantle source. Because the isotopic compositions of primitive SMB are similar to those of relatively unfmctionated basalts from the Snake River Plain (LEEMAN and MANTON, 1971; LEEMAN, 1974, MENZES et al., 1983) and the Owyhee River region (HART, 1984) there is reason to believe that this old subcontinental mantle reservoir is geographically extensive beneath older portions of the northwestern U.S. If this enriched mantle was also present beneath the Columbia Plateau during CRB volcanism, the lack of evidence for its participation in magma genesis during the height of CRR volcanism is surprising. Only during the waning stages of the CRB volcanic episode during eruption of the SMB do the basalt isotopic characteristics reflect the presence of this reservoir. Furthermore, following the final eruption of the Wanapum basalts, the switch to the enriched source for later eruptives is almost complete. No SMB have similar isotopic signatures to Grande Ronde basal& and only a few, isolated, small volume flows from northeastern Oregon have isotopic compositions like Picture Gorge basalts (TAUBENECK, 1979; CARLSON et al., 198 1). This dramatic switch in isotopic characteristics suggests that mantle with the isotopic signature of the source of Grande Ronde and Wanapum basalts was no longer available for magma genesis during Saddle Mountains time. This could be either because it was completely consumed during earlier CRB volcanism, or its melting was no longer being triggered by the subducting plate perhaps due to changes in relative plate motions. The lack of evidence for contribution of the enriched source (C3) in pm-Saddle Mountains CRB conversely may then imply that the enriched mantle was not present beneath the plateau during this time but was brought in from the east by mantle flow caused by the physical and thermal pertubation of the earlier melting activity. If such a source had been present beneath the plateau prior to SMB eruptions, even if it had not been involved in melting, evidence might be expected for mixing between the SMB source and the large volumes of magma which penetrated it to erupt as the older units of the CRB. Such evidence is not apparent in the isotopic data for the main volume of the CRB. Regardless of the mechanism by which this ancient source was introduced beneath the area, its melting gave rise to primary magmas some of which made it to the surface with only moderate fractionation and little, if any, interaction with the crust (e.g. Pomona, Asotin, and Martindale units). Other magmas derived from this source interacted with the crust and underwent fractional crystahixation, perhaps in much the same manner as the Grande Ronde basalts, to produce the variety of chemical and isotopic compositions observed in the Saddle Mountains flows.

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R. W. Carlson

Implications for the nature of the subcontinental mantle 20

Compared to ocean ridge or ocean island basalts, the basahs belonging to a single, geographically and temporally restricted, continental flood basalt locality, the Columbia River province, show a large diversity in isotopic composition. While some of this isotopic variation may, arguably, be attributed to crustal contamination, there is still ample evidence to suggest that the mantle source materials of these basalts were markedly more heterogeneous than the corresponding sources of oceanic volcanic rocks. If the interpretations discussed earlier am correct, beneath a limited geographic area of the northwestern U.S. exists “depleted’ oceanic type mantle, mantle contaminated with material derived from relatively recently subducted sediments, and an enriched mantle formed and isolated some 2.0 to 2.6 Ga ago. Clearly, the combined presence of these three types of mantle reservoirs is related to the position of the Columbia Plateau on the active western border of the Precambrian craton where subduction of the Juan de Fuca plate has been ongoing for more than 100 Ma (e.g. ATWATER, 1970). The recognition of the presence of enriched mantle sources for the CRB, including both that formed by the addition of material from recently recycled continental sediments and the more ancient source, perhaps formed by an analogous process more than 2.0 Ga ago, carries important implications for the geochemical evolution of the subcontinental mantle. If, as proposed by BRCKKS et al. (1976) the subcontinental mantle acts as a “sponge” soaking up fluids or melts rising from deeper in the mantle, the subcontinental lithosphere may contain a significant fraction of the incompatible element inventory of the Earth (e.g. DAVIES, 198 1). Figure 9 shows how the presence of enriched mantle would affect the ratio of depleted to undepleted mantle based on simple mass balance calculations for certain incompatible elements, in this case Nd. Given that seismic velocity differences between subcontinental and suboceanic mantle exist, in areas, to depths exceeding 400 km (e.g. JORDAN, 1975), the mass of subcontinental lithosphere may be some 5 to 10 times that of the continental crust. Depending upon how much of this lithosphere is enriched in incompatible elements and the degree of its average enrichment, the mass of wrrespondingly depleted mantle must increase with respect to the amount of undepleted mantle (DAVIES, 1981). Based on cakxlations discussed by HART (1984), the ancient enriched mantle source involved in the genesis of Snake River Plain basal& and by analogy the SMB, is enriched in Rb, Sr, and Nd by between a factor of 2 to 6 over the abundances for undepleted mantle given by JACOBSENand WASSERBURG( 1979). Figure 9 shows that if all the seismically anomalous subcontinental lithosphere is enriched to this degree, between 50% to 100% of the remaining mantle must be as

-

lo6-

2-

1

11111 0.3

I

I

I

0.5 MD,

0.7 /

I

1 0.9

MM

FIG. 9. Ratio of mass of depleted mantle (MDM)to total mantle (J&) required by mass balance to provide the amount of Nd contained in a given mass of enriched mantle (MuI) plus continental crust (Mc). Elemental concentrations for depleted and undepleted mantle arc from model I of JACOBSEN and W~ssnaaurt~ (1979). Numbers along each curve give the enrichment factor (concentration ratios) of enriched mantle relative to undepkted mantle.

depleted as the MORB source mantle to provide this quantity of incompatible elements. These ligures are considerably more than the -30% depleted mantle calculated if no enriched mantle is assumed (O’NIONS et al., 1979; JACOBSENand WASSERBURG, 1979). Though the calculations mentioned above most likely overestimate the importance of enriched subcontinental mantle, previous mantle-crust mass transport calculations neglecting completely the presence of enriched mantle clearly underestimate the relative abundance of depleted mantle in the Earth (DAVIES, 1981). Acknowledgments-1 am especially grateful to T. Wright and R. Helz for generously supplying well documented samples and for numerous helpfit discussions. Many thanks to M. Lindstrom and R. Bat&a for providing the INAA trace element measurements. D. James, W. Hart, B. Barreiro andRStemprovidedas&anceandadviceinthelaboratory for which I am grat&l. Comments on an early version of this manuscript by W. Hart and J. Morris, and constructive mkws by W.-La&an and J. Mahoney am most appnxiated. This work was supported hy NSF grant EAR 8206708 and the Camegie Institution of Washington.

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