S.R. HART 1 , A. HOFMANN and D.E. JAMES. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. (USA). Received April ...
Earth and Planetary Science Letters, 32 (1976) 51-61 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
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R b - S r MANTLE ISOCHRONS FROM OCEANIC REGIONS C. BROOKS D~partement de G~ologie, Universit~ de Montreal, Que. (Canada} and Department o f Terrestrial Magnetism, Carnegie Institution o f Washington, Washington, D.C. (USA} S.R. HART 1 , A. HOFMANN and D.E. JAMES Department o f Terrestrial Magnetism, Carnegie Institution o f Washington, Washington, D.C. (USA) Received April 1, 1976 Revised version received June 7, 1976
Existing data for 87Sr/a6Sr and Rb/Sr ratios of basalts from oceanic islands and mid-ocean spreading ridges show significant positive correlations on a Rb-Sr isochron diagram (when data are averaged by island group). Furthermore, tholeiites and alkali basalts occupy distinct non-overlapping fields on this plot. The tholeiite correlation is interpreted as a mantle isochron, and the agreement of this age (1.6 ± 0.2 b.y.) with that reported for Pb-Pb isochrons from oceanic basalts lends strong support to the use of such isochrons for tracing mantle evolution. Oceanic basalts are apparently sampling a mantle in which chemical heterogeneities have persisted for at least 1.52.0 b.y. The data support a kinematic model for the mantle in which a relatively uniform and non-radiogenic asthenosphere is penetrated by, and mixed with, blobs or plumes derived from an isolated (1.5-2 b.y.) and chemically heterogeneous mesosphere.
1, Introduction Tatsumoto [ 1 ] showed that basalts from oceanic areas preserved a correlation between U/Pb ratio and Pb isotopic ratio, and concluded that this correlation, presumably inherited from the mantle source o f the basalts, suggests a mantle in which U/Pb heterogeneities have existed for a billion years or more. A similar general correlation was noted between aTsr/86Sr and Rb/Sr for oceanic basalts [2,60], for arc volcanics from Peru [3], and for continental alkalic rocks [4]. Recently, this systematic R b - S r isochron correlation was emphasized b y Sun and Hanson [5] who
I Now at: Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Mass. 02139.
showed that Sr isotope data for alkali basalts (and nephelinites) from fourteen oceanic islands defined a correlated array which, if interpreted as an isochron, suggested an age for mantle heterogeneities o f about 2000 m.y. We were impressed with these relationships and u n d e r t o o k a careful screening o f published data to look at the R b - S r systematics o f both tholeiitic and alkali basalts from oceanic regions. We found striking regularities in almost all cases and present these results because o f their importance in constraining the space and time evolution o f mantle chemistry. Similar relationships exist for mantle-derived volcanics o f the continents [6] and the observations from b o t h oceanic and continental regions can be integrated into a single model which links the roles played b y lithosphere, asthenosphere and mesosphere in surface-reaching volcanism.
52 2. Compilation of data
The data were compiled from the literature using only those analyses of relatively high-precision which could be confidently normalized to an Eimer and Amend Standard value of 0.7080. For some islands the published 87Sr/a6Sr was not accompanied by Rb[ Sr data. In these cases powders of the original sample were requested (and are here acknowledged with thanks), and the Rb/Sr ratio measured using conventional X-ray fluorescence techniques. In other cases, unpublished data were made directly available to us, and these are also gratefully acknowledged. During the oceanic island compilation, we attempted to retrieve data for basaltic rocks only, because differentiation effects might distort the Rb-Sr systematics. This screening, together with the identification of association (tholeiitic or alkali basalt) was based on published assignments; for some islands earlier literature was searched for chemical analyses and descriptions as an aid to proper evaluation. The rock types from some islands were classed as transitional, possessing chemical compositions between tholeiite and alkali basalt. Such rocks were grouped with the alkali basalts in order to keep the potentially "primitive" tholeiitic association as uncluttered as possible.
3. Results and treatment STSr/a6Sr-Rb/Sr analyses were retrieved for 167 alkali basalts and 109 tholeiites. There are three principal ways in which correlations between Rb/Sr and a 7Sr/86Sr can be evaluated: (1) On a local scale using individual data. Wherever the number of samples permits, intra-island (or island group) relationships can then be examined. (2) On a regional scale using individual data. The islands from a given ocean can be grouped and the relationships between the individual data examined. (3) On a regional scale using averaged data. The data for each island (or island group) can be averaged to give a single representative data point, and the relationships between these average data points examined. Obviously treatments (2) and (3) will provide the same result if and only if each island is represented by a comparable number of individual analyses. How-
ever, many islands are represented by a few analyses and few islands by many analyses, and it is the latter that then dominate the relationships obtained by use of treatment (2). This is well portrayed in a trial comparison of the Pacific and Indian Ocean tholeiites. The averaged island data are well correlated (r = 0.90) and form a 1100-m.y. isochron. The individual data, however, are uncorrelated (r = 0.12) and do not define an isochron. In this example, the difference is due to the sample weight of the Hawaiian Ialands (N = 30), an island group that displays no internal correlation between Rb/Sr and a7Sr/86Sr on even an intra-island scale. The manner in which the data are treated is essential to the discussion that follows, and we are confident that the average island treatment (3) provides an insight to the regional Rb-Sr systematics of mantle-derived volcanics that otherwise would be lost in the more conventional individual-data treatments. Consequently we will use only treatments (1) and (3) in our appraisal of the data. The averaged data for the oceanic islands (or island groups) are given in Table 1, together with the number of samples and the appropriate source(s) of the data. When plotted on a Rb/Sr versus STSr/a6Sr (isochron) diagram (Fig. 1) the averaged data show two striking features: (1) tholeiites and alkali basalts occupy distinct and essentially non-overlapping fields, and (2) each field indicates a pronounced positive correlation between 87Sr/a6Sr and Rb/Sr. The tholeiite data appear more closely correlated than the alkali basalt data, as there are four alkali basalt data points which appreciably extend that field toward higher Rb/Sr ratios. We note that two of these island points (Gough and Rodriguez) contain only 1 and 2 individual analyses respectively and that the Jan Mayen point may be anomalous because of the possibility that the island is built on continental crust. When examined on an individual sample basis, only two islands, Kerguelen and Samoa, show an intra-island correlation with a positive slope (Fig. 2). The other islands show either horizontal data arrays (constant aTSr/S6Sr, variable Rb/Sr, e.g. Iceland, Tristan da Cunha, the Azores), equidimensional fields with significant variation in both directions (e.g. St. Helena, Hawaii), or correlations with a negative slope (Canary Islands, Jan Mayen, Cape Verde Islands). We find it interesting that the only two islands which internally show positively correlated data are the two
53 TABLE 1 Average Rb/Sr and 87 Sr/S 6St for basalts from oceanic islands (or island groups) and the ocean floors Location
Rb/Sr
87 Sr/86Sr
N*
Oceanic island tholeiites Bouvet Hawaiian Islands Iceland Kerguelen Kolbeinsey Reunion Samoa St. Pauls
0.016 0.017 0.014 0.033 0.009 0.033 0.049 0.039
0.70369 0.7038 0.70307 0.7048 0.70290 0.7042 0.7053 0.7045
4 30 13 5 1 8 5 7
1 2 3 4 5 6 7 8
Ocean floor tholeiites Indian Pacific Atlantic
0.015 0.Gll 0.008
0.70314 0.70265 0.70264
8 15 13
9 10 11
19,20 19,21 19,A
0.7039 0.7028 0.7035 0.7033 0.7031 0.7041 0.7030 0.7038 0.7033 0.7033 0.7033 0.7032 0.7036 0.7052 0.7031 0.7038 0.7054 0.7030 0.7042 0.7052
4 4 26 7 4 2 7 2 1 4 32 10 9 4 1 2 14 18 9 7
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
14 7,22 7,23,24 7,11 25 14 26,C 27,C 22 26,C 10,11,12,D 7,13,A 7,28 14 11 15,B 17,29 16,30 26,C 7,11
Oceanic island alkali basalts Amsterdam 0.045 Ascension 0.066 Azores 0.046 Canary 0.032 Cape Verdes 0.028 Crozet 0.060 Easter 0.019 Eniwetok 0.048 Gough 0.059 Guadeloupe 0.024 Hawaiian Islands 0.028 Iceland 0.040 Jan Mayen 0.074 Kerguelen 0.069 Madeira 0.036 Rodriguez 0.077 Samoa 0.067 St. Helena 0.034 Tahiti 0.047 Tristan Da Cunha 0.062
Graph No. **
Data Sources ***
7 8,9,10,11,12 7,13,A 14 7 14,15,B 16,17 14,18
* N -- number of samples averaged. ** Reference number for figures, *** Data sources listed in references except for the following unpublished data sources: A = S.R. Hart; B = C. Brooks; C = C.E. Hedge; D r- M.A. Lanphere, G.B. Dalrymple and D.A. Clague.
which have the highest average a 78r/86Sr (and Rb/Sr) and we shall return to this point. To decide if the positive correlations shown in Figs. 1 and 2 are statistically significant, the data o f Table 1 have been appraised by (1) calculating the correlation coefficient (r), and (2) testing the slope o f the fitted line for significance against zero. The
regression treatment used to obtain slopes and intercepts was that o f York [31 ]; the errors in the "ag e" and initial ratio are calculated solely from the data scatter and are given at 1 standard deviation (following recommendations o f Brooks e t al. [32]). While it is convenient to express the slopes o f these correlated arrays as ages, we emphasize here that these
54 TABLE 2 Mantle isochrons of Rb/Sr versus present-day s 7Sr/S 6 Sr for basalts of oceanic regions Location identity
Age (m.y.) *
(STSr/S6Sr) *
N **
r ***
600 1190 1430 1570 485 248 355 466 584 510
0.7025 0.7017 0.7025 0.7023 0.7041 0.7045 0.7042 0.7044 0.7039 0.7041
20 16 8 11 5 4 9 5 14 19
0.56(3) 0.91(4) 0.96(4) 0.97(4) 0.80(1) 0.99(2) 0.83(2) 0.74(1) 0.65(1) 0.62(2)
Oceanic islands Alkali basalts - all averaged island data Alkali basalts - all data except points 3, 20, 24 and 27 Tholeiites - all averaged island data Tholeiites - all averaged island data plus MORB Kerguelen - tholeiites - individual data Kerguelen - alkali basalts - individual data Kerguelen - all basalts - individual data ,Samoa - tholeiites - individual data Samoa - alkali basalts - individual data Samoa - all basalts - individual data
-+ 210 +- 150 +- 180 -+ 150 -+ 210 ± 20 ~ 90 ± (250) -+ 200 ± 150
+- 4 +- 3 -* 2 -+ 2 +- 4 -+ 1 -+ 2 -+ 3 +- 6 +- 4
* Errors given at la. ** Number o f samples regressed.
*** Pearson correlation coefficient. Value in parenthesis indicates at what level of confidence the fitted regression line differs from zero (given in increments o f standard deviation).
"ages" need not have time significance. This point is discussed in considerable detail later in the paper. The statistical parameters relevant to oceanic islands are presented in Table 2. The significance levels of the correlation coefficients range from 95% (e.g. for Samoa and Kerguelen) to greater than 99.99% (e.g. for the complete tholeiite suite including ridge basalts, and for the high-density portion of the al-
~,~ 0.707 CO
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0.705
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0.704
o
0.703
I
I
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0.04
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012
Samoa co 0.706 28
co ~o 0.705 0.706 0.704
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(Rb/Sr) Fig. 1. s 7 Sr/S 6 S r - R b / S r isochron plot o f tholeiites (filled circles) and alkali basalts (open circles) from oceanic islands. Each point represents an average o f available data o n individual samples from a given island or island group. Numbers refer to the data index in Table 1.
Ker~uelen
0,7030
I 0,02
L 0.04
I 0.06
I
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00~
0.10
0.12
(Rb/Sr) Fig. 2. Isochron plot o f data for individual samples of tholeiite (filled circles) and alkali basalt (open circles) from t h e Samoa Islands and Kerguelen I s l a n d . Best-fit regression lines and slope values are shown.
55 kali basalt field). We find these correlations remarkable and suggest that this data provides important and hitherto unused constraints relating to processes of oceanic magmatism.
4. Discussion Possible causes for the positive isochron correlations presented in the previous section include (1) surficial weathering and contamination by crust, sediments or seawater, and (2) inheritance of these isochron parameters from a heterogeneous and aged mantle. We believe direct weathering is ruled out because of the fresh and unaltered nature of most of the sampies involved. Other contamination processes might include mixing of low -s 7Sr/86Sr magma of direct mantle derivation (ridge basalts) with more radiogenic material represented by sediments or oceanic crust which has interacted with seawater. Oceanic sediments may have s 7 Sr/S 6Sr values as high as 0.717 and Rb/Sr values greater than 1.0 [33-35]. Only limited data exist for altered oceanic crust, but material has been analyzed with Rb/Sr ratios of 0 . 1 5 0.25 [36,37], and presumably STSr/S6Sr ratios could rise to values as high as seawater (0.709). Thus, both sediments and altered oceanic crust could provide a mixing component which would produce positive correlations o f the kind shown in Fig. 1. This mechanism has been described in detail in earlier papers and generally dismissed as a significant effect in the petrogenesis of oceanic basalts [1,10,22,30]. The fact that the Pb isotope regression line for oceanic basalts is distinctly different from the regression line for oceanic sediment (and seawater) also argues against such a contamination mechanism [5,38-40]. Furthermore, the marked uniformity of S7Sr/S6Sr values for some islands (e.g. Iceland [7,13]) and the general lack of a positive isochron (mixing-line) correlation between individual intra-island samples also argues against a sediment-crust contamination model. As we have discussed, only Samoa and Kerguelen show intraisland "isochrons", whereas these might be expected to be ubiquito~ts under the operation of a contamination model. Some islands do show correlations of negative slope as would be expected from contamination processes involving carbonate.
The conventional interpretation of oceanic island Sr (and Pb) isotope variations ascribes these to derivation from an isotopically heterogeneous mantle (a review of this model is given by Hofmann and Hart [41]). We consider this the most tenable model at present, and advance it here to explain the "isochron" correlations shown in Fig. 1. We view these positive "isochron" correlations as being inherited by oceanic basalt during derivation from a heterogeneous suboceanic mantle.
Rb-Sr coherence during basalt genesis. Fig. 1 shows that oceanic island tholeiites and alkali basalts have a similar range of aTSr/a6Sr values and suggests that the separation of data fields is due to higher Rb/Sr ratios in the alkali basalts. On the average the alkali basalts of Table 1 have Rb/Sr ratios about a factor of 2 higher than the tholeiites (whereas the average tholeiite STSr/a6Sr, 0.7037, is essentially identical with the average alkali basalt STSr/S6Sr, 0.7038). We believe this is a significant observation and propose that the observed Rb/Sr and Sr isotope ratios of the tholeiites are directly representative of the mantle source, but that the alkali basalts are displaced toward higher Rb/Sr ratios relative to the mantle source. The following discussion is an attempt to justify this proposition. While it is generally, though not universally, accepted that a basalt will inherit the Sr isotopic composition from its source [41 ], it is less obvious that a basalt will also preserve the Rb/Sr ratio of that source (because of the effect of partial melting and fractional crystallization processes). This will happen only if the partial melting process leaves no residual mantle phase which is a significant carrier of Sr and Rb, and if all fractional crystallization occurring after melting involves only phases which are poor in Rb and Sr, or which do not fractionate Rb relative to Sr. In the case of tholeiites, which involve large degrees of melting, phases that carry Rb or Sr (such as phlogopite and amphibole) will probably not persist after melting so we would expect the partial melt to have the same Rb/Sr ratio as the original source material. Phlogopite has been shown to have a field of stability well above the wet solidus of peridotite [42-45]. However, the phlogopite content of the mantle must be quite small (probably less than 1%
56 [41]), and phlogopite will be consumed during a large degree of melting even if its upper stability limit exceeds the temperature at which melting occurs. Amphibole may also be stable during hydrous melting of peridotite [45,46]. However, it is less effective than phlogopite at fractionating Rb/Sr during large degrees of melting. For example, for 25% melting of amphibole peridotite, where 1/2 of an original 20% of amphibole persists after melting, the Rb/Sr ratio of the liquid will be only slightly higher (6%) than that of the starting material (using partition coefficients from Philpotts and Schnetzler [47]). For clinopyroxene, the partition coefficients of Rb and Sr are sufficiently small (DSr ~0.08, DRb "43.003 [24,49]) that residual clinopyroxene produces negligible (
