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Correlations of Mid-Ocean Ridge Basalt chemistry with the geoid
Earth and Planetary Science Letters 153 Ž1997. 37–55
Correlations of Mid-Ocean Ridge Basalt chemistry with the geoid Pascal Lecroart a b
a,),1
, Francis Albarede ` b, Anny Cazenave
a
G.R.G.S r U.M.R 39, 18 aÕenue E. Belin, F-31055 Toulouse Cedex, France
´ Ecole Normale Superieure de Lyon, 46, allee ´ ´ d’Italie, F-69364 Lyon Cedex 07, France Received 10 September 1996; revised 2 July 1997; accepted 1 August 1997
Abstract The major-element composition of Mid-Ocean Ridge basalts ŽMORB. shows geochemical variations controlled by the temperature, pressure and source composition produced by convection in the mantle beneath the spreading centers. The long-wavelength signal of the geoid reflects deep mantle processes. In this paper, we characterize the correlations between the major-element chemistry of MORB and the geoid anomalies at different wavelengths to determine the extent to which the major-element composition of the MORB reflects deep mantle circulation. The chemical effects of the low-pressure fractionation have been corrected using a new method based on the FeOtrMgO ratio. The fractionation trends modeled by a linear trend in the geochemical parameter space normalized by the MgO denominator converge to a common point with ; 8% MgO. The separation vector d between the fractionation trends and this common point defines a new parameter whose sodium component shows a negative correlation significant at the 99% level with the axial depth for both the Mid-Atlantic Ridge ŽMAR. and the East Pacific Ridge ŽEPR. and with the geoid only for the MAR. The latter correlation suggests that the major-element composition of the MORB is clearly related to deep mantle convection by material and thermal coupling between the lower and upper mantle. The lack of correlation at long or medium wavelengths between the mantle enrichment inferred from the K 2 OrTiO 2 ratio and the geoid indicates that either source composition does not influence MORB chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle. q 1997 Elsevier Science B.V. Keywords: geoid; major elements; chemical composition; mid-ocean ridge basalts
1. Introduction The major-element composition of Mid-Ocean Ridge basalts ŽMORB. is the result of complex melting and mineral fractionation processes con) ´ Corresponding author. Ecole Normale Superieure de Lyon, ´ Laboratoire des Sciences de la Terre, 46, allee ´ d’Italie, F-69364 Lyon Cedex 07, France. Fax: q33-472-72-80-80. E-mail: [email protected] 1 ´ Now at: Ecole Normale Superieure de Lyon, 46, allee ´ ´ d’Italie, F-69364 Lyon Cedex 07, France.
trolled by the temperature, pressure and composition of the mantle beneath the ridges w1x. These parameters largely depend on the mantle convection regime which also influences other geophysical fields such as the local geoid height and the axial depth. A recent concern shared by geochemists and geophysicists is to identify possible relationships between the geophysical fields and geochemical variables that can be used as constraints on quantitative models of mantle convection. Klein and Langmuir w2x showed a global correlation of the sodium content in MORB with the axial depth at which the basalts were erupted.
0012-821Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 1 2 - 8 2 1 X Ž 9 7 . 0 0 1 4 0 - 4
Likewise, Humler et al. w3x and Zhang et al. w4x found that the sodium content in MORB is also correlated with the local seismic velocity anomalies at shallow depth. These observations were interpreted as suggesting deeper onset of melting in the upwelling mantle being associated with more extensive melting, thicker oceanic crust, and shallower bathymetry. Concurrently, regional correlations between geochemical parameters and the geoid Žthe equipotential surface of the Earth gravity field. have been sought at medium wavelength: Investigating the CrrŽCr q Al. ratio in spinel from abyssal peridotites as a proxy for the extent of partial melting, Dick et al. w5x found this ratio to be correlated with the Azores geoid height. Along the Mid-Atlantic Ridge, Bowin et al. w6x found some correlation between the geoid anomalies and the LarSm ratios which they believe reflect crustal thickness or density variations induced by mantle plumes. The long-wavelength signal of the geoid is dominant over that of medium and short wavelengths and is usually ascribed to deep mantle processes w7x. Along ridge axes, the long-wavelength geoid anomalies due to long-wavelength shallow density contrasts have an amplitude lower than 10 m w8x and are assumed to be insignificant. In the present study, we analyse the correlations between the major-element chemistry of MORB and the geoid anomalies at different wavelengths in order to assess the extent to which the major-element composition of the MORB may reflect some features associated with deep mantle circulation. The focus of this work is on the Mid-Atlantic Ridge ŽMAR. and the East Pacific Rise ŽEPR. because a high-quality chemical data-set exists for these ridges and because they have rather distinctive and reasonably well-understood geodynamic characteristics. After correcting for the chemical effects of low-pressure fractionation using a new method, we identify new geochemical variables and analyse their correlations with the geoid and the axial depth.
cal fields such as the geoid height. A relatively dense coverage exists, however, along the MAR and the EPR that makes it easier to investigate the detailed variations of the major-element compositions. More than 3400 glasses and whole-rock analyses of zeroage MORB have been compiled from the literature w9–42x, the Smithsonian Institution w43x, and the Joint Oceanographic Institution w44x data bases ŽFig. 1.. No data are available for the MAR segment between 108 and 208S, whereas the FAMOUS zone Ž; 368N. is overrepresented with more than 250 samples. Oxide abundances are recalculated to volatile-free 100 weight percent. Geoid anomalies represent the relative elevation of the gravity field equipotential surface, which coincides with the mean sea level, with respect to a reference ellipsoid. That reference is either a best-fit ellipsoid with the same flattening as the real Earth Žgeometric ellipsoid., or an ellipsoid having the flattening of a rotating fluid planet with the same radial density distribution as the Earth Žhydrostatic ellipsoid.. We will hereafter refer to the geoid anomalies as the geometric and hydrostatic geoid, respectively. The geoid anomaly with respect to the hydrostatic ellipsoid is dominated by a flattening term not accounted for by the Earth’s rotation which is believed to represent the overall dynamic signal of the mantle circulation w45x. The geoid data were compiled from Geosat, Topex–Poseidon, and ERS-1 altimetric data w46x. They are dominated by long-wavelength anomalies ŽFig. 1.. The geometric geoid presents maxima over the Iceland and the Azores hotspots which isotopic and trace-element studies have shown to influence MORB chemistry Že.g., w47x.. For the axial ridge depth, we used the bathymetric ETOPO5 data-set provided by the National Geophysical Data Center in Boulder, Colorado. The geoid and the axial depth were interpolated at the location of geochemical analyses from the two data-sets defined above and averaged over ridge segments of one-degree latitude.
2. The data-set
3. Correcting fractional crystallization and standardizing the data
Because each sample must be individually fetched and analyzed, the spatial coverage of geochemical data remains sparse and uneven in comparison with the wealth of data accumulated on global geophysi-
One of the critical steps that made Klein and Langmuir’s w2x approach very successful was the correction of mineral fractionation and accumulation
Fig. 1. Ža. The hydrostatic geoid and Žb. the geometric geoid map for the Atlantic and the East Pacific regions. Units are meters. Black lines represent the ridge axes, white circles the location of the sites for which major-element composition has been used in this study. The coverage along the two spreading centres is correct except along the southern Mid-Atlantic Ridge and the southern East Pacific Rise.
effects using the major-element patterns. The assumptions that Ž1. all MORB have experienced the removal of a cumulate with approximately the same mineralogy and Ž2. standardizing the data to a common MgO content of 8% would leave a residual signal that could essentially be assigned to the melting processes turned out to be very powerful. It was shown w48x, however, that Na 8.0 is still significantly correlated to the FeOrMgO ratio, taken as a differentiation index, and that fractionation and accumulation are not perfectly taken into account by a single cumulate mineralogy w48,49x. Shen and Forsyth w49x found that the Na content calculated for the parent magmas at adequate reference values of a differentiation index ŽMgO s 8 wt% or FeOrMgOs 1.2. is nearly independent of the slope assumed for the differentiation trend. They found, however, that this is no longer true for their Fe content. A filter was designed to detect the outliers, which we interpret as related to either analytical error or entrainment of a low-pressure mineral assemblage of olivine, clinopyroxene and plagioclase. Both ridge axes were subdivided into regions separated by either major transform faults with offset ) 100 km or major geochemical boundaries Že.g., Charlie–Gibbs Fracture Zone w43x.. For each sample, we first computed the chi-squared variable a defined by
a s Ž x y x m . Vy1 Ž x y x m .
T
Ž 1.
where x is the vector of a major-element composition, x m the mean vector and V the covariance matrix. Selecting a fraction f of the samples having the largest a values should identify the probable outliers in a normal population Žpractically, f is taken as being equal to 5% or 10%.. The filter defined by Eq. Ž1. was applied to each region individually. Fig. 2 shows the FeOtrMgO vs. Na 2 OrMgO for two segments of the MAR with filtering at the 10% level. The filter, however, can eliminate samples with extreme values that are not necessarily fraught with analytical problems. To take this problem into account, filters at 0%, 5% and 10% level were considered concurrently. Corrections for low-pressure crystallization were based on the FeOtrMgO differentiation index. This ratio is a reasonably well-understood fractionation parameter of basaltic magmas w48x. Crystallization of
Fig. 2. FeOt rMgO vs. Na 2 OrMgO for MORB from two regions in the Mid-Atlantic Ridge: solid circles are MORB analyses from 638N, and solid squares from 338S. Open symbols represent samples excluded by the f s10% filtering. Arrows are the fractionation of various minerals from an arbitrary initial composition Ž solid triangle ..
all major minerals but plagioclase is expected to fractionate the FeOtrMgO ratio relative to the parental melt. Only moderately fractionated lavas with a FeOtrMgO ratio in the range 0.5–3.0 were considered for further processing. In order to keep mixing, subtraction and accumulation relationships linear while removing the strong correlations induced by the closure conditions, MgO was used as a common denominator. We have considered six major-element ratios X irMgO Žwhere X i stands for SiO 2 , Al 2 O 3 , FeOt , CaO, Na 2 O, and TiO 2 . which define the major-element multispace subsequently referred to as R6 . In order to average the geochemical data over a manageable scale, each spreading centre was subdivided into segments covering one degree of latitude. The major-element variability observed for these segments has at least three components w2x ŽFig. 2.: A large part of the variability results from the lowpressure fractionation of olivine, plagioclase and clinopyroxene. As illustrated in the Fig. 2, this process can be accounted for by a linear trend in the six-dimensional space R6 with the most conspicuously variable ratio FeOtrMgO as the master variable. In addition, global systematics are observed for a given value of the differentiation index FeOtrMgO.
For example, for the same FeOtrMgO ratio, the Na 2 OrMgO ratio is larger for the 338S segment than for the 638N segment. This variability has been explained by variable temperature, pressure and composition of the mantle source w2x. Finally, individual data points scatter around the fractionation lines, which may reflect either small-scale heterogeneities in the source or small differences in the melting regime or an artefact of the ridge segmentation by one degree divisions. The interlaboratory
bias, which also contributes to the scatter of the geochemical analyses w1x, is handled as noise. In order to assess which part of the variability reflects mantle source properties, geochemical analyses have been corrected for low-pressure fractional crystallization and small-scale scattering eliminated. A fractionation line in the multispace R6 defined above is assigned to each one-degree ridge segment. Correlation coefficients were computed between the different variables. Only the segments for which a
Fig. 3. Variations of the FeOtrMgO ratio vs. Na 2 OrMgO ratio for Ža. and Žb. the Mid-Atlantic Ridge, and Žc. and Žd. the East Pacific Rise. Ža. and Žc. the data-set. The geochemical analyses diverge from a common composition. Žb. and Žd. Fractionation lines computed from each data-set with f s 5% and c s 90%.
fractionation line could be fitted through the data at a requested confidence level c w50x were kept. We found that 28 MAR segments comply with that criterion at the 90% confidence level, and 41 at 80%. We assume that, for the segments with no significant correlation, the fractionation process is more complex. For the segments which straddle major faults, correlation tend to stand out as relatively poor. In the multispace R6 , the fractionation lines appear to converge towards a common composition, a feature which is particularly clear for the MAR ŽFig. 3.. This common point composition is distinct enough from the average MORB composition that a statistical artifact can be ruled out. The estimate y of the major-element composition vector y of this common point M is determined as the point which lies closest to each fractionation straight line. For the jth ridge segment, the fractionation line d j is characterised by a mean point A j and a vector l j which was chosen as the direction of maximum spread, i.e., the eigenvector associated with the largest eigenvalue of the sample covariance matrix in R6 . The orthogonal projection matrix Pj of any point onto the fractionation line d j w51x is therefore defined as Pj s
l j l jT
Ž 2.
l jT l j
If M j represents the projection of M onto the straight line d j then, by virtue of the projection property yj y a j s Pj Ž y y a j .
Fig. 4. Sketch of the different computed parameters. The bold lines represent the fractionation trends for different ridge segments. The lower figure zooms the surrounding box of the upper figure. ssslope of the fractionation trend; M s the nearest point of the different fractionation trends; dsdistance between the fractionation trend of the Ž jq2.th ridge segment and the point M.
where n is the number of fractionation lines. Using Eq. Ž4. and the property that a projector projects onto itself Žthe projector Pj is said idempotent., we get n
Ss
Ž 6.
js1
Ž 3.
where yj and a j represent the composition vector of M j and A j , respectively. The residue vector d j connects the estimate M to its projection M j onto d j in the multispace R6 ŽFig. 4., therefore
T
Ý Ž a j y y . Ž I y Pj . Ž a j y y .
Making the differential of S equal to zero minimises the sum of squared deviations as n
dSs
Ý 2d yT Ž I y Pj . Ž a j y y . s 0
Ž 7.
js1
d j s yj y y s Ž I y Pj . Ž a j y y .
Ž 4. or
where I represents the identity matrix. The coordinate vector y of the common point M is found by minimizing the sum S of its squared-distances to each fractionation line, namely n
Ss
n
T
Ý d j2 s Ý Ž yj y y . Ž yj y y . js1
js1
Ž 5.
n
ys
y1
n
Ý Ž I y Pj .
Ý Ž I y Pj . a j
js1
js1
Ž 8.
Errors are propagated by repeating a large number of calculations in which each measurement is treated as a random deviate ŽMonte Carlo method. and a 2 s
Fig. 5. Computed common point M Ž circle . and fractionation trends Ž solid lines . for Ža. the MAR and Žb. the EPR in X irMgO vs. FeOtrMgO diagrams. f s 5% and c s 10%. Errors were propagated by Monte Carlo method Ž100 iterations.. The ellipses correspond to the 90% confidence level.
Fig. 5. Žcontinued.
Table 1 Composition Žwt%. of point M computed from different values of f and c f Ž%.
c Ž%.
Selected data Ž%.
Selected segments Ž%.
SiO 2
Al 2 O 3
66 76 64 76 67 76
29 40 28 41 28 40
50.64 50.68 50.58 50.66 50.56 50.63
15.41 15.50 15.45 15.54 15.60 15.64
85 87 90 93 91 93
42 51 47 56 48 57
50.87 50.88 50.85 50.84 50.93 50.91
14.76 14.84 14.98 14.99 14.88 14.89
FeOt
MgO
CaO
Na 2 O
TiO 2
9.79 9.82 9.75 9.71 9.54 9.56
8.54 8.41 8.58 8.47 8.64 8.52
11.94 11.85 11.99 11.93 12.05 11.97
2.38 2.44 2.37 2.42 2.36 2.43
1.28 1.31 1.28 1.28 1.24 1.26
10.85 10.68 10.32 10.35 10.46 10.49
7.36 7.45 7.64 7.62 7.52 7.51
11.60 11.65 11.83 11.82 11.78 11.77
2.81 2.80 2.75 2.76 2.77 2.77
1.76 1.71 1.62 1.63 1.66 1.67
Mid-Atlantic Ridge 0 0 5 5 10 10
90 80 90 80 90 80
East Pacific Rise 0 0 5 5 10 10
90 80 90 80 90 80
ellipse was drawn w52x for each rectangular diagram ŽFig. 5.. Finally, using the closure equation 6
100 s MgO
Ý is1
Xi MgO
Ž 9.
the X irMgO components of y and the X jirMgO components of yj are converted into major-element compositions X i and X ji , respectively. 4. Results The composition of the point M computed as the nearest point to all fractional lines is too poor in MgO to represent a mantle melt. The convergence of many fractionation lines towards M indicates the existence of a rather ubiquitous set of processes which take place prior to low-pressure fractionation. We therefore interpret M as representing the average composition of the liquids emerging from the upper mantle into shallow magma chambers. Although the number of segments for which a fractionation line can be found at a given significance level depends somewhat on the values of the filter f and confidence level c, the composition of the common point M remains remarkably stable along the same ridge
axis ŽTable 1.. Nearly parallel fractionation lines along the EPR define a rather elongated error ellipse ŽFig. 5b. and the FeOtrMgO ratio of the associated point M is consequently poorly constrained. Only the cases with f s 5% and c s 10% will therefore be discussed. We found that MgO s 8.6 wt% ŽFeOr MgO s 1.14. and MgO s 7.6 wt% ŽFeOr MgO s 1.35. for the composition of the common point M on the MAR and the EPR, respectively. These values provide an independent support for the choice of the standardization values adopted by Klein and Langmuir w2x ŽMgO s 8 and 7.3 wt% for the MAR and the EPR, respectively. and by Shen and Forsyth w49x ŽFeOrMgOs 1.2.. The shift of each fractionation line with respect to the common point M is evaluated by the residual d ji between the ith component of the jth projection M j and the point M. They define objective geochemical parameters that depend on the regional pressure, temperature, and composition variations of the mantle source. Subsequently, dNa 2 O will be used for d Na 2 O. The large proportion of ridge segments excluded ŽTable 1. mostly reflects hiatuses in data coverage. No systematic association is observed between the segments excluded and major transform faults.
5. Discussion 5.1. Correlations of MORB compositions with geoid and axial depth Fig. 6 shows that a negative correlation significant at the 99% level exists for both the MAR and the EPR between dNa 2 O and the axial depth which is essentially equivalent to the global correlation of Klein and Langmuir w2x. A negative correlation significant at the 99% level is also observed between
Fig. 6. Observed correlations between the sodium component ŽdNa 2 O. of the distance between the point M and the points M j and Ža. the axial depth or Žb. the geometric geoid. Closed symbols refer to MAR, open squares to EPR. The correlation coefficients are computed Ža. for the MAR and Žb. for both MAR and the EPR. Other symbols are used for near-hotspot segments.
dNa 2 O and the geometric geoid on the MAR. A shallow ridge segment is therefore associated with both a geoid high and a low dNa2O. This correlation does not hold with the hydrostatic geoid. Along the MAR ŽFig. 7., a significant correlation exists between dFeOt and the axial depth, while both axial depth and the geoid are weakly correlated with dCaO. No correlation is visible with the other elements and the dCaOrdAl 2 O 3 ratio. Along the EPR, chemistry, axial depth, and the geoid are essentially constant. The slopes of the fractionation lines correlate with neither the axial depth nor the geoid. No significant correlations are observed between the other components of the distance d, the dCaOrdAl 2 O 3 , the geoid and the axial depth. Geoid anomalies result from uneven density distributions within the Earth. The wavelength of the geoid anomalies strongly depends on both the vertical and lateral extent of local density heterogeneities. Geoid anomalies induced by short-scale density contrasts are filtered out by the mantle unless they are located at shallow depth. Long-wavelength geoid variations must be due to deep-mantle heterogeneities of mass distributions. The observed correlation between dNa 2 O and the geoid height at long wavelength Ž) 7000 km, Fig. 8. suggests a coupling between deep mantle circulation and the extent of melting, which controls the sodium component of the melt. Coupling may be due to massive mass transfer from the deep mantle to the upper mantle along the ridge axis such as in whole mantle convection regime. Alternatively, coupling in layered-mantle convection is essentially thermal which does not require that material from the deep mantle massively crosses the transition zone. Unfortunately, mantle tomography models do not in general have enough vertical resolution to help resolve the dilemma. Humler et al. w3x and Zhang et al. w4x compared the MORB element chemistry and the seismic velocity anomalies attributed to thermal or chemical heterogeneity. The RG5.5 tomographic model w53x used by these authors is however limited to depth - 500 km and does not sample deep-mantle anomalies. The medium-wavelength signal Ž; 2000–7000 km. of both the geoid anomalies and axial depth over the ridges and hotspot swells can be related to upper-mantle density contrasts determined by either local and regional thermal anomalies or crustal thick-
Fig. 7. Correlations of dFeOt , dCaO and dTiO 2 with the geoid and the axial depth along the MAR. Same symbols as Fig. 6.
ness variations w54x. The geometric geoid, the axial depth, and the residual d i data on the MAR have been filtered using a Gaussian filter, the errorweighted least-squares method described in Ref. w55x
and a bandpass of 2000–7000 km. The final results are nearly insensitive to the actual choice of a priori errors. Once the wavelengths in excess of 7000 km are filtered out, the hydrostatic and geometric geoids
Fig. 8. Observed Ž solid lines . and long-wavelength Ž) 7000 km. signal Žbold lines. along the MAR for the geometric geoid Župper panel., the sodium component of the points M j Žmiddle panel. and the axial depth Žlower panel..
Fig. 9. Residual medium-wavelength Ž2000–7000 km. geoid Ž solid lines ., axial depth Ž dashed lines . and d i Žsolid circles. on the MAR as a function of the latitude. Each panel shows the different component of the distance d.
are essentially identical so that they will both thereafter simply be referred to as the ‘‘geoid anomalies’’. The residual axial depth mimics the residual geoid north of 208N, whereas the signal is virtually flat along the Southern MAR ŽFig. 9.. The residual geoid highs appear to be associated with hotspots such as Iceland, Azores and Tristan da Cunha and a significant negative correlation shows up between dNa 2 O and dTiO 2 on the one hand and the geoid on the other hand. The dCaO residuals correlate with the geoid north of the equator ŽFig. 9.. Not enough ridge segments with a consistent fractionation pattern may explain the overall lack of correlation in the southern hemisphere. The lack of geochemical data between 108 and 208S does not make it possible to understand the transition between the North and South Atlantic segments. Near the Iceland hotspot, the signal associated with the dFeOt residuals is shifted southward by ; 108 with respect to that of dNa 2 O, dCaO, dTiO 2 , and to the geoid and the axial depth. A southward dAl 2 O 3 shift is also observed around the Azores hotspot. The origin of these shifts is not known. The residuals dSiO 2 with modulus - 0.2 wt% are barely significant. A negative correlation is visible between dNa 2 O and the medium-wavelength band Ž7000–2000 km. of the geoid, which is created by density anomalies in the upper mantle and the crust. Positive and negative correlations between the geoid, the axial depth, dCaO, and dTiO 2 also show up in that band while they are not normally present at ) 7000 km. The correlation of the geoid with dCaO and dTiO 2 therefore indicates that large-scale chemical variations in MORB chemistry and density anomalies may have a similar origin. 5.2. Mantle heterogeneities Langmuir et al. w1,2x argued that, for normal ridges, global variations of Na 8.0 and Fe 8.0 are largely controlled by the temperature field over a relatively homogeneous mantle. However, indications that part
of the major-element variations is inherited from mantle heterogeneities are rather strong. Natland w56x ascribes sodium variations in MORB to mantle heterogeneities. Albarede ` pointed out that Ži. once standardized to a same FeOrMgO, ocean island basalts are overall depleted in sodium with respect to MORB Žsee fig. 6 in Ref. w48x., and Žii. both Na 8.0 and isotopic anomalies correlate with the axial depth of mid-ocean ridges. Shen and Forsyth w49x find a significant correlation between the K 2 OrTiO 2 ratio and Fe 8.0 , which indicates that mantle enrichment influences the iron component of the MORB composition corrected for low-pressure fractionation. The K 2 OrTiO 2 ratios computed from the present data-set were averaged by one degree of latitude intervals ŽFig. 10a.. Besides the identified hotspots that stand out with relatively high ratios, a few ridge segments with a prominent linear crystallization trend have a K 2 OrTiO 2 ratio higher than 0.2w49x. These segments presumably contain a significant hotspot component. No significant correlation exists between the components of the residual vector d j and the K 2 OrTiO 2 ratio, whether the segments affected by enrichment ŽK 2 OrTiO 2 ) 0.2. and potentially contaminated by an OIB component are excluded ŽFig. 10b and c. or not. Then, the K 2 OrTiO 2 ratio has been filtered as discussed above ŽFig. 11.. South of the equator, the dominant signal is at short wavelength. At northern latitudes, the medium-wavelength K 2 OrTiO 2 signal shows no correlation with the geochemical residuals d i. Such a pattern may result from three non-exclusive causes: Ž1. source composition does not influence the major-element chemistry of mantle melts produced in the same conditions of T, P, and extent of melting, which is supported by the broad resemblance of magmas produced by ridge-axis hotspots and mid-ocean ridges w48x, or Ž2. hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle. The K 2 OrTiO 2 ratio appears less sensible to mantle enrichment than the isotopic composition.
Fig. 10. Ža. K 2 OrTiO 2 variations along the Mid-Atlantic Ridge segments of 18 of latitude Ž solid line .. Horizontal line represents the limit K 2 OrTiO 2 s 0.2 above which the ridge segments are assumed to be affected by mantle heterogeneities w49x. Circles: K 2 OrTiO 2 values of the ridge segments for which MORB composition has been corrected for low-pressure fractionation by the method based on the FeOtrMgO ratio. Žb. dFeOt and Žc. dNa 2 O expressed as a function of K 2 OrTiO 2 for the segments where K 2 OrTiO 2 is lower than 0.2.
Fig. 11. Medium-wavelength variations Ž2000–7000 km. of K 2 OrTiO 2 ratios Ž solid lines . and residual components of d i Ž solid circles . expressed as a function of the latitude along the MAR.
Anomalously high dTiO 2 and high K 2 OrTiO 2 ratios suggest the presence of a mantle heterogeneity ŽFigs. 9 and 10. near 148N, where previous trace-element and isotopic investigations clearly identified a prominent geochemical anomaly w12,19x. The local geoid and bathymetry show however low associated disturbance which suggests that the 148N ‘‘hotspot’’ is a narrow and shallow feature. In contrast with other hotspots ŽBouvet, St. Paul, Iceland., the Azores do not seem to be located in the neighborhood of any dFeOt high at medium-wavelength ŽFig. 9., which possibly reflects the presence of a fluid in their mantle source. Comparison of experimental data on anhydrous and hydrous melts indicates that, for a similar extent of melting, hydrous melts have less FeOt w57x. A wet mantle instead of a hotspot below the Azores was previously suggested from different lines of evidence, such as the volatiles content in basalts w58x and the lack of high values of the mantle temperature inferred from geothermometers w59x.
6. Conclusions The major-element composition of Mid-Ocean Ridge basalts is corrected for low-pressure fractionation using local cumulate control lines. In the space of major elements, these control lines are observed to converge to a common point with ; 8% MgO, which supports the overall premises of Klein and Langmuir’s w59x correction scheme. The distance of each fractionation line to the common point defines the geochemical residuals such as dNa 2 0, dFeO, etc., which are suggested to reflect melting conditions and source effects. We reproduce the correlation between dNa 2 0 and axial depth of Klein and Langmuir w59x. At long wavelength Ž) 7000 km., we also find that dNa 2 0 correlates with the geoid which supports some material or thermal coupling between the lower and the upper mantle. In contrast, the K 2 OrTiO 2 ratio which traces the contribution of plume melts does not correlate with the geoid, indicating that either source composition does not influence MORB chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle.
Acknowledgements We thank E.M. Klein, R. Batiza and an anonymous reviewer for thoughtful comments on the manuscript. This work was supported by the Institut National des Sciences de l’Univers through the program ‘‘Terre profonde’’ and by the Centre National ´ d’Etude Spatiale. The authors are particularly grateful to A. Briais for helpful discussions. [FA]
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Correlations of Mid-Ocean Ridge Basalt chemistry with the geoid Pascal Lecroart a b
a,),1
, Francis Albarede ` b, Anny Cazenave
a
G.R.G.S r U.M.R 39, 18 aÕenue E. Belin, F-31055 Toulouse Cedex, France
´ Ecole Normale Superieure de Lyon, 46, allee ´ ´ d’Italie, F-69364 Lyon Cedex 07, France Received 10 September 1996; revised 2 July 1997; accepted 1 August 1997
Abstract The major-element composition of Mid-Ocean Ridge basalts ŽMORB. shows geochemical variations controlled by the temperature, pressure and source composition produced by convection in the mantle beneath the spreading centers. The long-wavelength signal of the geoid reflects deep mantle processes. In this paper, we characterize the correlations between the major-element chemistry of MORB and the geoid anomalies at different wavelengths to determine the extent to which the major-element composition of the MORB reflects deep mantle circulation. The chemical effects of the low-pressure fractionation have been corrected using a new method based on the FeOtrMgO ratio. The fractionation trends modeled by a linear trend in the geochemical parameter space normalized by the MgO denominator converge to a common point with ; 8% MgO. The separation vector d between the fractionation trends and this common point defines a new parameter whose sodium component shows a negative correlation significant at the 99% level with the axial depth for both the Mid-Atlantic Ridge ŽMAR. and the East Pacific Ridge ŽEPR. and with the geoid only for the MAR. The latter correlation suggests that the major-element composition of the MORB is clearly related to deep mantle convection by material and thermal coupling between the lower and upper mantle. The lack of correlation at long or medium wavelengths between the mantle enrichment inferred from the K 2 OrTiO 2 ratio and the geoid indicates that either source composition does not influence MORB chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle. q 1997 Elsevier Science B.V. Keywords: geoid; major elements; chemical composition; mid-ocean ridge basalts
1. Introduction The major-element composition of Mid-Ocean Ridge basalts ŽMORB. is the result of complex melting and mineral fractionation processes con) ´ Corresponding author. Ecole Normale Superieure de Lyon, ´ Laboratoire des Sciences de la Terre, 46, allee ´ d’Italie, F-69364 Lyon Cedex 07, France. Fax: q33-472-72-80-80. E-mail: [email protected] 1 ´ Now at: Ecole Normale Superieure de Lyon, 46, allee ´ ´ d’Italie, F-69364 Lyon Cedex 07, France.
trolled by the temperature, pressure and composition of the mantle beneath the ridges w1x. These parameters largely depend on the mantle convection regime which also influences other geophysical fields such as the local geoid height and the axial depth. A recent concern shared by geochemists and geophysicists is to identify possible relationships between the geophysical fields and geochemical variables that can be used as constraints on quantitative models of mantle convection. Klein and Langmuir w2x showed a global correlation of the sodium content in MORB with the axial depth at which the basalts were erupted.
0012-821Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 1 2 - 8 2 1 X Ž 9 7 . 0 0 1 4 0 - 4
Likewise, Humler et al. w3x and Zhang et al. w4x found that the sodium content in MORB is also correlated with the local seismic velocity anomalies at shallow depth. These observations were interpreted as suggesting deeper onset of melting in the upwelling mantle being associated with more extensive melting, thicker oceanic crust, and shallower bathymetry. Concurrently, regional correlations between geochemical parameters and the geoid Žthe equipotential surface of the Earth gravity field. have been sought at medium wavelength: Investigating the CrrŽCr q Al. ratio in spinel from abyssal peridotites as a proxy for the extent of partial melting, Dick et al. w5x found this ratio to be correlated with the Azores geoid height. Along the Mid-Atlantic Ridge, Bowin et al. w6x found some correlation between the geoid anomalies and the LarSm ratios which they believe reflect crustal thickness or density variations induced by mantle plumes. The long-wavelength signal of the geoid is dominant over that of medium and short wavelengths and is usually ascribed to deep mantle processes w7x. Along ridge axes, the long-wavelength geoid anomalies due to long-wavelength shallow density contrasts have an amplitude lower than 10 m w8x and are assumed to be insignificant. In the present study, we analyse the correlations between the major-element chemistry of MORB and the geoid anomalies at different wavelengths in order to assess the extent to which the major-element composition of the MORB may reflect some features associated with deep mantle circulation. The focus of this work is on the Mid-Atlantic Ridge ŽMAR. and the East Pacific Rise ŽEPR. because a high-quality chemical data-set exists for these ridges and because they have rather distinctive and reasonably well-understood geodynamic characteristics. After correcting for the chemical effects of low-pressure fractionation using a new method, we identify new geochemical variables and analyse their correlations with the geoid and the axial depth.
cal fields such as the geoid height. A relatively dense coverage exists, however, along the MAR and the EPR that makes it easier to investigate the detailed variations of the major-element compositions. More than 3400 glasses and whole-rock analyses of zeroage MORB have been compiled from the literature w9–42x, the Smithsonian Institution w43x, and the Joint Oceanographic Institution w44x data bases ŽFig. 1.. No data are available for the MAR segment between 108 and 208S, whereas the FAMOUS zone Ž; 368N. is overrepresented with more than 250 samples. Oxide abundances are recalculated to volatile-free 100 weight percent. Geoid anomalies represent the relative elevation of the gravity field equipotential surface, which coincides with the mean sea level, with respect to a reference ellipsoid. That reference is either a best-fit ellipsoid with the same flattening as the real Earth Žgeometric ellipsoid., or an ellipsoid having the flattening of a rotating fluid planet with the same radial density distribution as the Earth Žhydrostatic ellipsoid.. We will hereafter refer to the geoid anomalies as the geometric and hydrostatic geoid, respectively. The geoid anomaly with respect to the hydrostatic ellipsoid is dominated by a flattening term not accounted for by the Earth’s rotation which is believed to represent the overall dynamic signal of the mantle circulation w45x. The geoid data were compiled from Geosat, Topex–Poseidon, and ERS-1 altimetric data w46x. They are dominated by long-wavelength anomalies ŽFig. 1.. The geometric geoid presents maxima over the Iceland and the Azores hotspots which isotopic and trace-element studies have shown to influence MORB chemistry Že.g., w47x.. For the axial ridge depth, we used the bathymetric ETOPO5 data-set provided by the National Geophysical Data Center in Boulder, Colorado. The geoid and the axial depth were interpolated at the location of geochemical analyses from the two data-sets defined above and averaged over ridge segments of one-degree latitude.
2. The data-set
3. Correcting fractional crystallization and standardizing the data
Because each sample must be individually fetched and analyzed, the spatial coverage of geochemical data remains sparse and uneven in comparison with the wealth of data accumulated on global geophysi-
One of the critical steps that made Klein and Langmuir’s w2x approach very successful was the correction of mineral fractionation and accumulation
Fig. 1. Ža. The hydrostatic geoid and Žb. the geometric geoid map for the Atlantic and the East Pacific regions. Units are meters. Black lines represent the ridge axes, white circles the location of the sites for which major-element composition has been used in this study. The coverage along the two spreading centres is correct except along the southern Mid-Atlantic Ridge and the southern East Pacific Rise.
effects using the major-element patterns. The assumptions that Ž1. all MORB have experienced the removal of a cumulate with approximately the same mineralogy and Ž2. standardizing the data to a common MgO content of 8% would leave a residual signal that could essentially be assigned to the melting processes turned out to be very powerful. It was shown w48x, however, that Na 8.0 is still significantly correlated to the FeOrMgO ratio, taken as a differentiation index, and that fractionation and accumulation are not perfectly taken into account by a single cumulate mineralogy w48,49x. Shen and Forsyth w49x found that the Na content calculated for the parent magmas at adequate reference values of a differentiation index ŽMgO s 8 wt% or FeOrMgOs 1.2. is nearly independent of the slope assumed for the differentiation trend. They found, however, that this is no longer true for their Fe content. A filter was designed to detect the outliers, which we interpret as related to either analytical error or entrainment of a low-pressure mineral assemblage of olivine, clinopyroxene and plagioclase. Both ridge axes were subdivided into regions separated by either major transform faults with offset ) 100 km or major geochemical boundaries Že.g., Charlie–Gibbs Fracture Zone w43x.. For each sample, we first computed the chi-squared variable a defined by
a s Ž x y x m . Vy1 Ž x y x m .
T
Ž 1.
where x is the vector of a major-element composition, x m the mean vector and V the covariance matrix. Selecting a fraction f of the samples having the largest a values should identify the probable outliers in a normal population Žpractically, f is taken as being equal to 5% or 10%.. The filter defined by Eq. Ž1. was applied to each region individually. Fig. 2 shows the FeOtrMgO vs. Na 2 OrMgO for two segments of the MAR with filtering at the 10% level. The filter, however, can eliminate samples with extreme values that are not necessarily fraught with analytical problems. To take this problem into account, filters at 0%, 5% and 10% level were considered concurrently. Corrections for low-pressure crystallization were based on the FeOtrMgO differentiation index. This ratio is a reasonably well-understood fractionation parameter of basaltic magmas w48x. Crystallization of
Fig. 2. FeOt rMgO vs. Na 2 OrMgO for MORB from two regions in the Mid-Atlantic Ridge: solid circles are MORB analyses from 638N, and solid squares from 338S. Open symbols represent samples excluded by the f s10% filtering. Arrows are the fractionation of various minerals from an arbitrary initial composition Ž solid triangle ..
all major minerals but plagioclase is expected to fractionate the FeOtrMgO ratio relative to the parental melt. Only moderately fractionated lavas with a FeOtrMgO ratio in the range 0.5–3.0 were considered for further processing. In order to keep mixing, subtraction and accumulation relationships linear while removing the strong correlations induced by the closure conditions, MgO was used as a common denominator. We have considered six major-element ratios X irMgO Žwhere X i stands for SiO 2 , Al 2 O 3 , FeOt , CaO, Na 2 O, and TiO 2 . which define the major-element multispace subsequently referred to as R6 . In order to average the geochemical data over a manageable scale, each spreading centre was subdivided into segments covering one degree of latitude. The major-element variability observed for these segments has at least three components w2x ŽFig. 2.: A large part of the variability results from the lowpressure fractionation of olivine, plagioclase and clinopyroxene. As illustrated in the Fig. 2, this process can be accounted for by a linear trend in the six-dimensional space R6 with the most conspicuously variable ratio FeOtrMgO as the master variable. In addition, global systematics are observed for a given value of the differentiation index FeOtrMgO.
For example, for the same FeOtrMgO ratio, the Na 2 OrMgO ratio is larger for the 338S segment than for the 638N segment. This variability has been explained by variable temperature, pressure and composition of the mantle source w2x. Finally, individual data points scatter around the fractionation lines, which may reflect either small-scale heterogeneities in the source or small differences in the melting regime or an artefact of the ridge segmentation by one degree divisions. The interlaboratory
bias, which also contributes to the scatter of the geochemical analyses w1x, is handled as noise. In order to assess which part of the variability reflects mantle source properties, geochemical analyses have been corrected for low-pressure fractional crystallization and small-scale scattering eliminated. A fractionation line in the multispace R6 defined above is assigned to each one-degree ridge segment. Correlation coefficients were computed between the different variables. Only the segments for which a
Fig. 3. Variations of the FeOtrMgO ratio vs. Na 2 OrMgO ratio for Ža. and Žb. the Mid-Atlantic Ridge, and Žc. and Žd. the East Pacific Rise. Ža. and Žc. the data-set. The geochemical analyses diverge from a common composition. Žb. and Žd. Fractionation lines computed from each data-set with f s 5% and c s 90%.
fractionation line could be fitted through the data at a requested confidence level c w50x were kept. We found that 28 MAR segments comply with that criterion at the 90% confidence level, and 41 at 80%. We assume that, for the segments with no significant correlation, the fractionation process is more complex. For the segments which straddle major faults, correlation tend to stand out as relatively poor. In the multispace R6 , the fractionation lines appear to converge towards a common composition, a feature which is particularly clear for the MAR ŽFig. 3.. This common point composition is distinct enough from the average MORB composition that a statistical artifact can be ruled out. The estimate y of the major-element composition vector y of this common point M is determined as the point which lies closest to each fractionation straight line. For the jth ridge segment, the fractionation line d j is characterised by a mean point A j and a vector l j which was chosen as the direction of maximum spread, i.e., the eigenvector associated with the largest eigenvalue of the sample covariance matrix in R6 . The orthogonal projection matrix Pj of any point onto the fractionation line d j w51x is therefore defined as Pj s
l j l jT
Ž 2.
l jT l j
If M j represents the projection of M onto the straight line d j then, by virtue of the projection property yj y a j s Pj Ž y y a j .
Fig. 4. Sketch of the different computed parameters. The bold lines represent the fractionation trends for different ridge segments. The lower figure zooms the surrounding box of the upper figure. ssslope of the fractionation trend; M s the nearest point of the different fractionation trends; dsdistance between the fractionation trend of the Ž jq2.th ridge segment and the point M.
where n is the number of fractionation lines. Using Eq. Ž4. and the property that a projector projects onto itself Žthe projector Pj is said idempotent., we get n
Ss
Ž 6.
js1
Ž 3.
where yj and a j represent the composition vector of M j and A j , respectively. The residue vector d j connects the estimate M to its projection M j onto d j in the multispace R6 ŽFig. 4., therefore
T
Ý Ž a j y y . Ž I y Pj . Ž a j y y .
Making the differential of S equal to zero minimises the sum of squared deviations as n
dSs
Ý 2d yT Ž I y Pj . Ž a j y y . s 0
Ž 7.
js1
d j s yj y y s Ž I y Pj . Ž a j y y .
Ž 4. or
where I represents the identity matrix. The coordinate vector y of the common point M is found by minimizing the sum S of its squared-distances to each fractionation line, namely n
Ss
n
T
Ý d j2 s Ý Ž yj y y . Ž yj y y . js1
js1
Ž 5.
n
ys
y1
n
Ý Ž I y Pj .
Ý Ž I y Pj . a j
js1
js1
Ž 8.
Errors are propagated by repeating a large number of calculations in which each measurement is treated as a random deviate ŽMonte Carlo method. and a 2 s
Fig. 5. Computed common point M Ž circle . and fractionation trends Ž solid lines . for Ža. the MAR and Žb. the EPR in X irMgO vs. FeOtrMgO diagrams. f s 5% and c s 10%. Errors were propagated by Monte Carlo method Ž100 iterations.. The ellipses correspond to the 90% confidence level.
Fig. 5. Žcontinued.
Table 1 Composition Žwt%. of point M computed from different values of f and c f Ž%.
c Ž%.
Selected data Ž%.
Selected segments Ž%.
SiO 2
Al 2 O 3
66 76 64 76 67 76
29 40 28 41 28 40
50.64 50.68 50.58 50.66 50.56 50.63
15.41 15.50 15.45 15.54 15.60 15.64
85 87 90 93 91 93
42 51 47 56 48 57
50.87 50.88 50.85 50.84 50.93 50.91
14.76 14.84 14.98 14.99 14.88 14.89
FeOt
MgO
CaO
Na 2 O
TiO 2
9.79 9.82 9.75 9.71 9.54 9.56
8.54 8.41 8.58 8.47 8.64 8.52
11.94 11.85 11.99 11.93 12.05 11.97
2.38 2.44 2.37 2.42 2.36 2.43
1.28 1.31 1.28 1.28 1.24 1.26
10.85 10.68 10.32 10.35 10.46 10.49
7.36 7.45 7.64 7.62 7.52 7.51
11.60 11.65 11.83 11.82 11.78 11.77
2.81 2.80 2.75 2.76 2.77 2.77
1.76 1.71 1.62 1.63 1.66 1.67
Mid-Atlantic Ridge 0 0 5 5 10 10
90 80 90 80 90 80
East Pacific Rise 0 0 5 5 10 10
90 80 90 80 90 80
ellipse was drawn w52x for each rectangular diagram ŽFig. 5.. Finally, using the closure equation 6
100 s MgO
Ý is1
Xi MgO
Ž 9.
the X irMgO components of y and the X jirMgO components of yj are converted into major-element compositions X i and X ji , respectively. 4. Results The composition of the point M computed as the nearest point to all fractional lines is too poor in MgO to represent a mantle melt. The convergence of many fractionation lines towards M indicates the existence of a rather ubiquitous set of processes which take place prior to low-pressure fractionation. We therefore interpret M as representing the average composition of the liquids emerging from the upper mantle into shallow magma chambers. Although the number of segments for which a fractionation line can be found at a given significance level depends somewhat on the values of the filter f and confidence level c, the composition of the common point M remains remarkably stable along the same ridge
axis ŽTable 1.. Nearly parallel fractionation lines along the EPR define a rather elongated error ellipse ŽFig. 5b. and the FeOtrMgO ratio of the associated point M is consequently poorly constrained. Only the cases with f s 5% and c s 10% will therefore be discussed. We found that MgO s 8.6 wt% ŽFeOr MgO s 1.14. and MgO s 7.6 wt% ŽFeOr MgO s 1.35. for the composition of the common point M on the MAR and the EPR, respectively. These values provide an independent support for the choice of the standardization values adopted by Klein and Langmuir w2x ŽMgO s 8 and 7.3 wt% for the MAR and the EPR, respectively. and by Shen and Forsyth w49x ŽFeOrMgOs 1.2.. The shift of each fractionation line with respect to the common point M is evaluated by the residual d ji between the ith component of the jth projection M j and the point M. They define objective geochemical parameters that depend on the regional pressure, temperature, and composition variations of the mantle source. Subsequently, dNa 2 O will be used for d Na 2 O. The large proportion of ridge segments excluded ŽTable 1. mostly reflects hiatuses in data coverage. No systematic association is observed between the segments excluded and major transform faults.
5. Discussion 5.1. Correlations of MORB compositions with geoid and axial depth Fig. 6 shows that a negative correlation significant at the 99% level exists for both the MAR and the EPR between dNa 2 O and the axial depth which is essentially equivalent to the global correlation of Klein and Langmuir w2x. A negative correlation significant at the 99% level is also observed between
Fig. 6. Observed correlations between the sodium component ŽdNa 2 O. of the distance between the point M and the points M j and Ža. the axial depth or Žb. the geometric geoid. Closed symbols refer to MAR, open squares to EPR. The correlation coefficients are computed Ža. for the MAR and Žb. for both MAR and the EPR. Other symbols are used for near-hotspot segments.
dNa 2 O and the geometric geoid on the MAR. A shallow ridge segment is therefore associated with both a geoid high and a low dNa2O. This correlation does not hold with the hydrostatic geoid. Along the MAR ŽFig. 7., a significant correlation exists between dFeOt and the axial depth, while both axial depth and the geoid are weakly correlated with dCaO. No correlation is visible with the other elements and the dCaOrdAl 2 O 3 ratio. Along the EPR, chemistry, axial depth, and the geoid are essentially constant. The slopes of the fractionation lines correlate with neither the axial depth nor the geoid. No significant correlations are observed between the other components of the distance d, the dCaOrdAl 2 O 3 , the geoid and the axial depth. Geoid anomalies result from uneven density distributions within the Earth. The wavelength of the geoid anomalies strongly depends on both the vertical and lateral extent of local density heterogeneities. Geoid anomalies induced by short-scale density contrasts are filtered out by the mantle unless they are located at shallow depth. Long-wavelength geoid variations must be due to deep-mantle heterogeneities of mass distributions. The observed correlation between dNa 2 O and the geoid height at long wavelength Ž) 7000 km, Fig. 8. suggests a coupling between deep mantle circulation and the extent of melting, which controls the sodium component of the melt. Coupling may be due to massive mass transfer from the deep mantle to the upper mantle along the ridge axis such as in whole mantle convection regime. Alternatively, coupling in layered-mantle convection is essentially thermal which does not require that material from the deep mantle massively crosses the transition zone. Unfortunately, mantle tomography models do not in general have enough vertical resolution to help resolve the dilemma. Humler et al. w3x and Zhang et al. w4x compared the MORB element chemistry and the seismic velocity anomalies attributed to thermal or chemical heterogeneity. The RG5.5 tomographic model w53x used by these authors is however limited to depth - 500 km and does not sample deep-mantle anomalies. The medium-wavelength signal Ž; 2000–7000 km. of both the geoid anomalies and axial depth over the ridges and hotspot swells can be related to upper-mantle density contrasts determined by either local and regional thermal anomalies or crustal thick-
Fig. 7. Correlations of dFeOt , dCaO and dTiO 2 with the geoid and the axial depth along the MAR. Same symbols as Fig. 6.
ness variations w54x. The geometric geoid, the axial depth, and the residual d i data on the MAR have been filtered using a Gaussian filter, the errorweighted least-squares method described in Ref. w55x
and a bandpass of 2000–7000 km. The final results are nearly insensitive to the actual choice of a priori errors. Once the wavelengths in excess of 7000 km are filtered out, the hydrostatic and geometric geoids
Fig. 8. Observed Ž solid lines . and long-wavelength Ž) 7000 km. signal Žbold lines. along the MAR for the geometric geoid Župper panel., the sodium component of the points M j Žmiddle panel. and the axial depth Žlower panel..
Fig. 9. Residual medium-wavelength Ž2000–7000 km. geoid Ž solid lines ., axial depth Ž dashed lines . and d i Žsolid circles. on the MAR as a function of the latitude. Each panel shows the different component of the distance d.
are essentially identical so that they will both thereafter simply be referred to as the ‘‘geoid anomalies’’. The residual axial depth mimics the residual geoid north of 208N, whereas the signal is virtually flat along the Southern MAR ŽFig. 9.. The residual geoid highs appear to be associated with hotspots such as Iceland, Azores and Tristan da Cunha and a significant negative correlation shows up between dNa 2 O and dTiO 2 on the one hand and the geoid on the other hand. The dCaO residuals correlate with the geoid north of the equator ŽFig. 9.. Not enough ridge segments with a consistent fractionation pattern may explain the overall lack of correlation in the southern hemisphere. The lack of geochemical data between 108 and 208S does not make it possible to understand the transition between the North and South Atlantic segments. Near the Iceland hotspot, the signal associated with the dFeOt residuals is shifted southward by ; 108 with respect to that of dNa 2 O, dCaO, dTiO 2 , and to the geoid and the axial depth. A southward dAl 2 O 3 shift is also observed around the Azores hotspot. The origin of these shifts is not known. The residuals dSiO 2 with modulus - 0.2 wt% are barely significant. A negative correlation is visible between dNa 2 O and the medium-wavelength band Ž7000–2000 km. of the geoid, which is created by density anomalies in the upper mantle and the crust. Positive and negative correlations between the geoid, the axial depth, dCaO, and dTiO 2 also show up in that band while they are not normally present at ) 7000 km. The correlation of the geoid with dCaO and dTiO 2 therefore indicates that large-scale chemical variations in MORB chemistry and density anomalies may have a similar origin. 5.2. Mantle heterogeneities Langmuir et al. w1,2x argued that, for normal ridges, global variations of Na 8.0 and Fe 8.0 are largely controlled by the temperature field over a relatively homogeneous mantle. However, indications that part
of the major-element variations is inherited from mantle heterogeneities are rather strong. Natland w56x ascribes sodium variations in MORB to mantle heterogeneities. Albarede ` pointed out that Ži. once standardized to a same FeOrMgO, ocean island basalts are overall depleted in sodium with respect to MORB Žsee fig. 6 in Ref. w48x., and Žii. both Na 8.0 and isotopic anomalies correlate with the axial depth of mid-ocean ridges. Shen and Forsyth w49x find a significant correlation between the K 2 OrTiO 2 ratio and Fe 8.0 , which indicates that mantle enrichment influences the iron component of the MORB composition corrected for low-pressure fractionation. The K 2 OrTiO 2 ratios computed from the present data-set were averaged by one degree of latitude intervals ŽFig. 10a.. Besides the identified hotspots that stand out with relatively high ratios, a few ridge segments with a prominent linear crystallization trend have a K 2 OrTiO 2 ratio higher than 0.2w49x. These segments presumably contain a significant hotspot component. No significant correlation exists between the components of the residual vector d j and the K 2 OrTiO 2 ratio, whether the segments affected by enrichment ŽK 2 OrTiO 2 ) 0.2. and potentially contaminated by an OIB component are excluded ŽFig. 10b and c. or not. Then, the K 2 OrTiO 2 ratio has been filtered as discussed above ŽFig. 11.. South of the equator, the dominant signal is at short wavelength. At northern latitudes, the medium-wavelength K 2 OrTiO 2 signal shows no correlation with the geochemical residuals d i. Such a pattern may result from three non-exclusive causes: Ž1. source composition does not influence the major-element chemistry of mantle melts produced in the same conditions of T, P, and extent of melting, which is supported by the broad resemblance of magmas produced by ridge-axis hotspots and mid-ocean ridges w48x, or Ž2. hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle. The K 2 OrTiO 2 ratio appears less sensible to mantle enrichment than the isotopic composition.
Fig. 10. Ža. K 2 OrTiO 2 variations along the Mid-Atlantic Ridge segments of 18 of latitude Ž solid line .. Horizontal line represents the limit K 2 OrTiO 2 s 0.2 above which the ridge segments are assumed to be affected by mantle heterogeneities w49x. Circles: K 2 OrTiO 2 values of the ridge segments for which MORB composition has been corrected for low-pressure fractionation by the method based on the FeOtrMgO ratio. Žb. dFeOt and Žc. dNa 2 O expressed as a function of K 2 OrTiO 2 for the segments where K 2 OrTiO 2 is lower than 0.2.
Fig. 11. Medium-wavelength variations Ž2000–7000 km. of K 2 OrTiO 2 ratios Ž solid lines . and residual components of d i Ž solid circles . expressed as a function of the latitude along the MAR.
Anomalously high dTiO 2 and high K 2 OrTiO 2 ratios suggest the presence of a mantle heterogeneity ŽFigs. 9 and 10. near 148N, where previous trace-element and isotopic investigations clearly identified a prominent geochemical anomaly w12,19x. The local geoid and bathymetry show however low associated disturbance which suggests that the 148N ‘‘hotspot’’ is a narrow and shallow feature. In contrast with other hotspots ŽBouvet, St. Paul, Iceland., the Azores do not seem to be located in the neighborhood of any dFeOt high at medium-wavelength ŽFig. 9., which possibly reflects the presence of a fluid in their mantle source. Comparison of experimental data on anhydrous and hydrous melts indicates that, for a similar extent of melting, hydrous melts have less FeOt w57x. A wet mantle instead of a hotspot below the Azores was previously suggested from different lines of evidence, such as the volatiles content in basalts w58x and the lack of high values of the mantle temperature inferred from geothermometers w59x.
6. Conclusions The major-element composition of Mid-Ocean Ridge basalts is corrected for low-pressure fractionation using local cumulate control lines. In the space of major elements, these control lines are observed to converge to a common point with ; 8% MgO, which supports the overall premises of Klein and Langmuir’s w59x correction scheme. The distance of each fractionation line to the common point defines the geochemical residuals such as dNa 2 0, dFeO, etc., which are suggested to reflect melting conditions and source effects. We reproduce the correlation between dNa 2 0 and axial depth of Klein and Langmuir w59x. At long wavelength Ž) 7000 km., we also find that dNa 2 0 correlates with the geoid which supports some material or thermal coupling between the lower and the upper mantle. In contrast, the K 2 OrTiO 2 ratio which traces the contribution of plume melts does not correlate with the geoid, indicating that either source composition does not influence MORB chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle.
Acknowledgements We thank E.M. Klein, R. Batiza and an anonymous reviewer for thoughtful comments on the manuscript. This work was supported by the Institut National des Sciences de l’Univers through the program ‘‘Terre profonde’’ and by the Centre National ´ d’Etude Spatiale. The authors are particularly grateful to A. Briais for helpful discussions. [FA]
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