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Submarine basalts from Kilauea rift, Hawaii: nondependence of trace element composition on extrusion depth
EARTH AND PLANETARY SCIENCE LETTERS 20 (1973) 201-203. NORTH-ttOLLAND PUBLISHING COMPANY
SUBMARINE BASALTS FROM KILAUEA RIFT, HAWAII : NONDEPENDENCE
OF TRACE ELEMENT COMPOSITION
ON EXTRUSION
DEPTH
S.R. HART Department of TerrestrialMagnetism, CarnegieInstitution of Washington, Washington, D.C. 20015, USA Received 7 March 1973 Revised version received 24 July 1973
Four tholeiites dredged from 500 to 5000 meters on the east rift of Kilauea were analyzed for K, Rb, Cs, Sr, Ba, Ni and 87Sr/86Sr. No effects were found which relate to extrusion into sea water at depth.
1. Introduction Mid-ocean ridge basklts (MORB) have characteristic patterns of trace element abundances, particularly for the LIL elements (large-ion-lithophile, such as the alkali, alkaline earth and rare earth elements). These basalts are typically depleted in the LIL elements, show highly fractionated trace element ratios (such as K/Rb and Sr/Ba) and contain less radiogenic strontium compared to most other modern terrestrial basalts [ I - 5 ] . These characteristics are difficult to explain by partial melting-differentiation mechanisms operating on normal mantle, and are usually ascribed to magma derivation from a mantle source previously depleted in the LIL elements [ 2 - 6 ] . While such a model appears quite satisfactory, I have sometimes wondered if the peculiar characteristics of MORB are not primary but are produced by some process related to underwater extrusion. The basalts dredged between 500 and 5000 m on the east submarine rift zone of Kilauea volcano by Moore [7] represent a very suitable suite with which to evaluate this possibility. The results reported here for selected LIL elements are straightforward and give no cause for alarm.
lar in major element composition to the crystallized interior of the pillows, and to subaerial lava flows on the same rift zone. No evidence was found for any major chemical effect related to extrusion into sea water at great depth. Moore kindly supplied samples dredged from depths of 480, 1400, 3420 and 4680 m. K, Rb, Cs and Ba were analyzed by isotope dilution and Sr and Ni were analyzed by XRF. H 2 0 + and CO2 values were determined by gas chromatography. 875r/86 Sr ratios were analyzed with a precision of 0.01% (2a) and are reported relative to standard values of 0.70800 (E & A) and 0.71022 (NBS 987). Details of these techniques may be found in [5]. The results are given in table 1, along with selected major element data from Moore [7]. K, % 35 4 0 :45 i
,o /"
Sr, p p m 250 300 350
'/
K/Rb 4 9 0 500
7,
36 37 f
Ni 0
500 I000
l
L
2.0
i
)
I I 50
i J.
I
I I i
40
2. Samples and results Moore [7] showed that the glassy crust of pillow basalts dredged from the Kilauea rift zone is very simi-
K/Bo 35
5.0
I
I
I
I
I
I
Fig. 1. Variation of trace element contents and ratios with depth of dredging.
&R. Hart, Submarine basalts from Kilauea rift, Hawaii
202
TABLE 1 Trace and major a element composition of Kilauea rift tholeiites Sample No. [7] 1
Depth (meters) K, ppm Rb, ppm Cs, ppm Sr, ppm Ba, ppm Ni, ppm H20 +, % CO~, % 87Sr/86Sr K/Rb K/Cs (× 10 - 3 ) K/Ba Sr/Ba SiO2 A1203 FeO MgO CaO Na20
490 4240 8.48 0.0907 338 118.7 118 0.60 0.33 0.70364
2
4,5
6
1400
3420
4680
3780 7.70 0.0804 292 106.4 450 0.48 0.29 0.70357
3750 7.65 0.0797 261 102.6 700 0.54 0.08 0.70358
3910 8.00 0.0797 284 110.2 620 0.43 0.13 0,70357
Max. range %
Initial liquid b
! MORB c
12 10 13 26 15
3940 7.95 0.0839 310 109,0 385 0.51 0.21 0.70359
1160 1.11 0,016 135 10.5 110 0.43 0.15 0.70255
0.01
501 46.8 35.8 2.85
490 47.0 35.5 2.74
491 47.1 36.6 2.54
489 49.0 35.5 2.58
2.5 4.6 3.1 4.0
496 47.0 36.1 2.84
50.32 13.61 9.18 7.59 10.79 2.34
49.44 12.98 9.16 9.79 10.17 2.24
48.42 10.82 9.94 15.46 8.31 1.91
49.16 11,73 9.90 12,64 9.16 2.06
3.9 23 9.7 70 27 20
49.4 12.8 9.4 10.9 9.8 2.1
1060 70.0 110 12.9 49.92 15.63 8.19 7.65 11.17 2.75
a Data from Moore [7]. b Calculated from crystal fractionation model. c Mid-ocean ridge basalt averages compiled from literature, see [5].
3. Discussion
It is clear that there is no major variation of LIL element composition related to extrusion depth (fig. 1). The Kilauea dredge samples are quite unlike MORB in LIL element composition, and there is no tendency for the deeper Kilauea samples to change in the direction of MORB. In fact, the Kilauea dredge samples are quite similar in most respects to tholeiites from other oceanic islands and island arcs (see table 2, Hart et al. [8]). While there are minor variations ( 1 0 26%) in LIL element concentrations between the four Kilauea samples, various ratios of these elements are remarkably uniform. For several ratios (K/Rb, K/Ba, aTSr/S6 Sr) the variations lie within experimental error. Since one of the most striking characteristics of MORB are their high K/Ba ratios, it is comforting to find this ratio essentially invariant in the Kilauea dredge suite.
Moore [7] noted a tendency for the deeper Kilauea samples to be richer in modal and normative olivine (see, for example, the MgO values, table 1). The large variation in Ni contents, Table 1, is correlated with MgO variation and this olivine effect. The minor variations in LIL element concentrations cannot be totally accounted for by variable dilution with olivine, however, since the Sr concentrations show a wider variation than the other LIL elements. All of the data in table 1 (except CO2 and H20 +) can be successfully reconciled by a simple equilibrium fractionation model involving batch additions or removals of plagioclase and olivine. Samples 1 and 4, 5 represent end members for this model, with No. 1 having the highest LIL element, Al2 03 and CaO contents and the lowest MgO and Ni contents, and Nos. 4, 5 being the opposite. In simplest form this model supposes an initial liquid with a composition lying between No. 1 and Nos. 4, 5; loss of 8% olivine (Fo 90) from this liquid
S.R. Hart, Submarine basalts from Kilauea rift, Hawaii
generates No.l, and a gain of 10% olivine and loss of 5% plagioclase (An 84) generates Nos. 4, 5. It was not possible to use either end member as the initial liquid. For reasonable values of solid/liquid partition coefficients (0.1 and 2 for K and Sr in plagioclase: 0.01, 0.01 and 9.5 for K, Sr and Ni in olivine [9, 10], this model accurately predicts the LIL element and Ni concentration differences between samples 1 and 4,5. If a fractional crystallization (surface equilibrium) model is used for Ni in olivine, a higher Ni partition coefficient must be used (= 16). It should be pointed out that the samples used here were obtained as fragments from Moore and may not beidentical to the powders analyzed by Moore [7]. Some heterogeneity in the fragments was observed, particularly in the distribution of olivine. In a comprehensive treatment of the major element chemistry of Kilauea lavas, Wright and Fiske [11 ] have shown that the submarine lavas can be explained as mixtures of olivine-controlled magmas and differentiated magmas. Their model is difficult to evaluate quantitatively in terms of the data presented here, but it is qualitatively consistent with the data. The main point of the simple model I have presented is to show that the minor chemical variations observed between these dredge samples can be very adequately explained in terms of simple differentiation effects and that there is no evidence for any effects related to the process of submarine extrusion.
203
References [1] A.E.J. Engel, C.G. Engel and R.G. Havens, Chemical characteristics of oceanic basalts and the upper mantle, Bull Geol. Soc. Am. 76 (1965) 719. [2] M. Tatsumoto, C.E. Hedge and A.E.J. Engel, Potassium, rubidium, strontium, thorium, uranium and the ratio of strontium-87 to strontium-86 in oceanic tholeiitic basalt, Science 150 (1965) 886. [3] P.W. Gast, Trace element fractionation and the origin of tholeiitic and alkaline magma types, Geochim. Cosmochim. Acta 32 (1968) 1057. [4] R. Kay, N.J. Hubbard and P.W. Gast, Chemical characteristics and origin of oceanic ridge volcanic rocks, J. Geophys. Res. 75 (1970) 1585. [5] S.R. Hart, K, Rb, Cs, Sr and Ba contents and Sr isotope ratios of ocean floor basalts, Phil. Trans. Roy. Soc. (London) 268 (1971) 573. S.R. Hart, W.E. Glassley and D.E. Karig, Basalts and Sea Floor spreading behind the Mariana Island Arc, Earth Planet. Sci. Letters 15 (1972) 12. [6] A.E. Bence, The differentiation history of the Earth by rubidium-strontium isotopic relationships, in: Variations in isotopic abundances of strontium, calcium and argon and related topics, ed. P.M. Hurley, M.I.T 1381-14, Fourteenth Ann. Prog. Rep. (1966) 35. [7] J.G. Moore, Petrology of deep-sea basalt near Hawaii, Am. J. Sci. 263 (1965) 40. [8] S.R. Hart, C. Brooks, T.E. Krogh, C.L Davis and D. Nava, Ancient and modem volcanic rocks: A trace element model Earth Planet. Sci. Letters 10 (1970) 17. [9] J.A. Philpotts and C.C. Schnetzler, Phenocryst-matrix partition coefficients for K, Rb, Sr and Ba, Geochim. Cosmochim. Acta 34 (1970) 307. [10] T. HaPdi and T.L. Wright, The fractionation of nickel between olivine and augite as a geothermometer, Geochim. Cosmochim. Acta 31 (1967) 877. [ 11 ] T.L. Wright and R.S. Fiske, Origin of the differentiated and hybrid lavas of Kilauea Volcano, Hawaii, J. Petrology 12 (1971) l.
SUBMARINE BASALTS FROM KILAUEA RIFT, HAWAII : NONDEPENDENCE
OF TRACE ELEMENT COMPOSITION
ON EXTRUSION
DEPTH
S.R. HART Department of TerrestrialMagnetism, CarnegieInstitution of Washington, Washington, D.C. 20015, USA Received 7 March 1973 Revised version received 24 July 1973
Four tholeiites dredged from 500 to 5000 meters on the east rift of Kilauea were analyzed for K, Rb, Cs, Sr, Ba, Ni and 87Sr/86Sr. No effects were found which relate to extrusion into sea water at depth.
1. Introduction Mid-ocean ridge basklts (MORB) have characteristic patterns of trace element abundances, particularly for the LIL elements (large-ion-lithophile, such as the alkali, alkaline earth and rare earth elements). These basalts are typically depleted in the LIL elements, show highly fractionated trace element ratios (such as K/Rb and Sr/Ba) and contain less radiogenic strontium compared to most other modern terrestrial basalts [ I - 5 ] . These characteristics are difficult to explain by partial melting-differentiation mechanisms operating on normal mantle, and are usually ascribed to magma derivation from a mantle source previously depleted in the LIL elements [ 2 - 6 ] . While such a model appears quite satisfactory, I have sometimes wondered if the peculiar characteristics of MORB are not primary but are produced by some process related to underwater extrusion. The basalts dredged between 500 and 5000 m on the east submarine rift zone of Kilauea volcano by Moore [7] represent a very suitable suite with which to evaluate this possibility. The results reported here for selected LIL elements are straightforward and give no cause for alarm.
lar in major element composition to the crystallized interior of the pillows, and to subaerial lava flows on the same rift zone. No evidence was found for any major chemical effect related to extrusion into sea water at great depth. Moore kindly supplied samples dredged from depths of 480, 1400, 3420 and 4680 m. K, Rb, Cs and Ba were analyzed by isotope dilution and Sr and Ni were analyzed by XRF. H 2 0 + and CO2 values were determined by gas chromatography. 875r/86 Sr ratios were analyzed with a precision of 0.01% (2a) and are reported relative to standard values of 0.70800 (E & A) and 0.71022 (NBS 987). Details of these techniques may be found in [5]. The results are given in table 1, along with selected major element data from Moore [7]. K, % 35 4 0 :45 i
,o /"
Sr, p p m 250 300 350
'/
K/Rb 4 9 0 500
7,
36 37 f
Ni 0
500 I000
l
L
2.0
i
)
I I 50
i J.
I
I I i
40
2. Samples and results Moore [7] showed that the glassy crust of pillow basalts dredged from the Kilauea rift zone is very simi-
K/Bo 35
5.0
I
I
I
I
I
I
Fig. 1. Variation of trace element contents and ratios with depth of dredging.
&R. Hart, Submarine basalts from Kilauea rift, Hawaii
202
TABLE 1 Trace and major a element composition of Kilauea rift tholeiites Sample No. [7] 1
Depth (meters) K, ppm Rb, ppm Cs, ppm Sr, ppm Ba, ppm Ni, ppm H20 +, % CO~, % 87Sr/86Sr K/Rb K/Cs (× 10 - 3 ) K/Ba Sr/Ba SiO2 A1203 FeO MgO CaO Na20
490 4240 8.48 0.0907 338 118.7 118 0.60 0.33 0.70364
2
4,5
6
1400
3420
4680
3780 7.70 0.0804 292 106.4 450 0.48 0.29 0.70357
3750 7.65 0.0797 261 102.6 700 0.54 0.08 0.70358
3910 8.00 0.0797 284 110.2 620 0.43 0.13 0,70357
Max. range %
Initial liquid b
! MORB c
12 10 13 26 15
3940 7.95 0.0839 310 109,0 385 0.51 0.21 0.70359
1160 1.11 0,016 135 10.5 110 0.43 0.15 0.70255
0.01
501 46.8 35.8 2.85
490 47.0 35.5 2.74
491 47.1 36.6 2.54
489 49.0 35.5 2.58
2.5 4.6 3.1 4.0
496 47.0 36.1 2.84
50.32 13.61 9.18 7.59 10.79 2.34
49.44 12.98 9.16 9.79 10.17 2.24
48.42 10.82 9.94 15.46 8.31 1.91
49.16 11,73 9.90 12,64 9.16 2.06
3.9 23 9.7 70 27 20
49.4 12.8 9.4 10.9 9.8 2.1
1060 70.0 110 12.9 49.92 15.63 8.19 7.65 11.17 2.75
a Data from Moore [7]. b Calculated from crystal fractionation model. c Mid-ocean ridge basalt averages compiled from literature, see [5].
3. Discussion
It is clear that there is no major variation of LIL element composition related to extrusion depth (fig. 1). The Kilauea dredge samples are quite unlike MORB in LIL element composition, and there is no tendency for the deeper Kilauea samples to change in the direction of MORB. In fact, the Kilauea dredge samples are quite similar in most respects to tholeiites from other oceanic islands and island arcs (see table 2, Hart et al. [8]). While there are minor variations ( 1 0 26%) in LIL element concentrations between the four Kilauea samples, various ratios of these elements are remarkably uniform. For several ratios (K/Rb, K/Ba, aTSr/S6 Sr) the variations lie within experimental error. Since one of the most striking characteristics of MORB are their high K/Ba ratios, it is comforting to find this ratio essentially invariant in the Kilauea dredge suite.
Moore [7] noted a tendency for the deeper Kilauea samples to be richer in modal and normative olivine (see, for example, the MgO values, table 1). The large variation in Ni contents, Table 1, is correlated with MgO variation and this olivine effect. The minor variations in LIL element concentrations cannot be totally accounted for by variable dilution with olivine, however, since the Sr concentrations show a wider variation than the other LIL elements. All of the data in table 1 (except CO2 and H20 +) can be successfully reconciled by a simple equilibrium fractionation model involving batch additions or removals of plagioclase and olivine. Samples 1 and 4, 5 represent end members for this model, with No. 1 having the highest LIL element, Al2 03 and CaO contents and the lowest MgO and Ni contents, and Nos. 4, 5 being the opposite. In simplest form this model supposes an initial liquid with a composition lying between No. 1 and Nos. 4, 5; loss of 8% olivine (Fo 90) from this liquid
S.R. Hart, Submarine basalts from Kilauea rift, Hawaii
generates No.l, and a gain of 10% olivine and loss of 5% plagioclase (An 84) generates Nos. 4, 5. It was not possible to use either end member as the initial liquid. For reasonable values of solid/liquid partition coefficients (0.1 and 2 for K and Sr in plagioclase: 0.01, 0.01 and 9.5 for K, Sr and Ni in olivine [9, 10], this model accurately predicts the LIL element and Ni concentration differences between samples 1 and 4,5. If a fractional crystallization (surface equilibrium) model is used for Ni in olivine, a higher Ni partition coefficient must be used (= 16). It should be pointed out that the samples used here were obtained as fragments from Moore and may not beidentical to the powders analyzed by Moore [7]. Some heterogeneity in the fragments was observed, particularly in the distribution of olivine. In a comprehensive treatment of the major element chemistry of Kilauea lavas, Wright and Fiske [11 ] have shown that the submarine lavas can be explained as mixtures of olivine-controlled magmas and differentiated magmas. Their model is difficult to evaluate quantitatively in terms of the data presented here, but it is qualitatively consistent with the data. The main point of the simple model I have presented is to show that the minor chemical variations observed between these dredge samples can be very adequately explained in terms of simple differentiation effects and that there is no evidence for any effects related to the process of submarine extrusion.
203
References [1] A.E.J. Engel, C.G. Engel and R.G. Havens, Chemical characteristics of oceanic basalts and the upper mantle, Bull Geol. Soc. Am. 76 (1965) 719. [2] M. Tatsumoto, C.E. Hedge and A.E.J. Engel, Potassium, rubidium, strontium, thorium, uranium and the ratio of strontium-87 to strontium-86 in oceanic tholeiitic basalt, Science 150 (1965) 886. [3] P.W. Gast, Trace element fractionation and the origin of tholeiitic and alkaline magma types, Geochim. Cosmochim. Acta 32 (1968) 1057. [4] R. Kay, N.J. Hubbard and P.W. Gast, Chemical characteristics and origin of oceanic ridge volcanic rocks, J. Geophys. Res. 75 (1970) 1585. [5] S.R. Hart, K, Rb, Cs, Sr and Ba contents and Sr isotope ratios of ocean floor basalts, Phil. Trans. Roy. Soc. (London) 268 (1971) 573. S.R. Hart, W.E. Glassley and D.E. Karig, Basalts and Sea Floor spreading behind the Mariana Island Arc, Earth Planet. Sci. Letters 15 (1972) 12. [6] A.E. Bence, The differentiation history of the Earth by rubidium-strontium isotopic relationships, in: Variations in isotopic abundances of strontium, calcium and argon and related topics, ed. P.M. Hurley, M.I.T 1381-14, Fourteenth Ann. Prog. Rep. (1966) 35. [7] J.G. Moore, Petrology of deep-sea basalt near Hawaii, Am. J. Sci. 263 (1965) 40. [8] S.R. Hart, C. Brooks, T.E. Krogh, C.L Davis and D. Nava, Ancient and modem volcanic rocks: A trace element model Earth Planet. Sci. Letters 10 (1970) 17. [9] J.A. Philpotts and C.C. Schnetzler, Phenocryst-matrix partition coefficients for K, Rb, Sr and Ba, Geochim. Cosmochim. Acta 34 (1970) 307. [10] T. HaPdi and T.L. Wright, The fractionation of nickel between olivine and augite as a geothermometer, Geochim. Cosmochim. Acta 31 (1967) 877. [ 11 ] T.L. Wright and R.S. Fiske, Origin of the differentiated and hybrid lavas of Kilauea Volcano, Hawaii, J. Petrology 12 (1971) l.