Morphology, magnetic anomalies and basalt magnetization at the ends of the Galapagos high-amplitude zone

Earth and Planetary Science Letters, 33 (1976) 145-163 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

145

MORPHOLOGY, MAGNETIC ANOMALIES AND BASALT MAGNETIZATION AT THE ENDS OF THE GALAPAGOS HIGH-AMPLITUDE ZONE P.R. VOGT Naval Research Laboratory, Washington, D.C. 20375 fUSA) and J. DE BOER Department o f Earth and Environmental Sciences, Wesleyan University, Middletown, Conn. 0645 7 (USA} Received February 13, 1976 Revised version received July 21, 1976

Detailed bathymetric and magnetic data, complemented by nine dredge stations, define the eastern and western limits of a belt of high-amplitude magnetic anomalies associated with the Galapagos hot spot. The hypothesis of "magnetic telechemistry" was tested and locally confirmed. High amplitudes correspond to high remanence, susceptibility, FeOT, TiO2, and presumably titanomagnetite concentration. The average remanence of surface samples in the high-amplitude zone is 0.027 emu/cm3 (range, 0.009-0.085 emu/cm3), about 4 times that of the normal-amplitude zone. Magnetic amplitudes are only 2-2.5 times higher, however. If the greater TiO2/FeOT ratio of high-amplitude zone basalts also characterizes the titanomagnetites, remanence in the high-amplitude zone may fall off more rapidly with depth in the crust a~ a result of reheating. Alternatively, small pillows of high remanence are more common than larger pillows at the top of the high-amplitude zone crust; FeTi basalt may also be concentrated in the upper part of the crust. Anomaly amplitudes are highest at the ends of the zone, particularly in the east. As asthenosphere crystal slushes presumably flow away from the Galapagos plume, progressive crystal fractionation may enrich residual magmas in FeOT and TiO2. The Galapagos FeTi zone terminates abruptly against transform fractures at both ends, perhaps because subaxial flow is dammed at the transforms. The FeTi-producing crystal slushes have advanced east and west at speeds up to 10 cm/yr since they first appeared at the spreading axis at least 6.6 m.y.B.P. Their progressive advance was connected with the progressive southward jumps of the spreading axis east of the Galapagos hot spot, and northward jumps to the west.

1. Introduction The amplitudes of magnetic anomalies generated by sea-floor spreading and geomagnetic reversals tend to vary regionally in response to such "geometric" variables as orientation and location of the ridge crest with respect to the paleo and present geomagnetic fields. "Geologic" variables might include such factors as spreading rate or water depth. In this paper we discuss a type of "geologic" amplitude variation that has been found on some spreading axes located close to postulated "hot spots". Magnetic anomalies of roughly twice the "normal" amplitudes occur along a 1100-km-

long section of the Galapagos spreading axis and on the Juan de Fuca Ridge between the Blanco and Sovanco fracture zones [ 1]. We shall henceforth refer to such high-amplitude zones as H-zones and the normal-amplitude regions as L-zones. Samples dredged from the H-regions yielded so-called " F e T i " basalt, i.e., basalt of unusually high FeO T and TiO2 concentration (FeO T is total iron expressed as FeO). This observation and other data led Vogt and Johnson [ 1] to postulate that the high-amplitude magnetic anomalies reflect crust relatively enriched in FeO T and TiO2 and, therefore, titanomagnetite. If correct, this hypothesis means that some magnetic anomalies contain useful information

146 spreading centers close to these hot-spot-generated platforms (the Reykjanes, Juan de Fuca, and Galapagos spreading axes). The concept of magnetic telechemistry [ 1] was intended to apply only to the latter class of features [4]. The absence of any discontinuity in spreading rate, water depth, and probably crustal thickness between the Galapagos H and L zones suggests that some initial difference in magma property for example, bulk chemistry - determines the amplitude differences. Since magmas in both provinces are exposed essentially to identical physical environments (water temperature, hydrostatic pressure, heat flow) these variables are probably unimportant. As part of a further geophysical/geological investigation of the Galapagos spreading axis, the eastern and western extremities of the H-zone were charted in detail (Fig. 1). Magnetic, bathymetric, seismic reflection and gravity data were collected, supplemented by seven rock dredges both inside and outside the eastern and western ends of the H-zone. In this paper we report on two magnetic properties (susceptibility and remanent magnetization) of dredge samples. The chemistry of dredged igneous rocks [5,6] and manganese crust [7] and the more regional geophysical data [8] are contained in separate publications. Anderson et al. [2] independently investigated the magnetic and petrologic variations along the Galapagos

not only about the paleomagnetic field, but also about the chemical/mineralogical make-up of the oceanic crust [1,2]. Watkins [3] took issue with the hypothesis of "magnetic telechemistry" [1 ] on several grounds, one of which being "that a constant Fe/Ti ratio could not be maintained reasonably during an unequal increase in the total concentrations of FeO T and TiO2 in basalt. Variation of the intensity of magnetization, J, within the FeO-Fe203-TiO2 system, shows that factors such as oxygen fugacity, degree of quenching, and exsolution, are more important controls of J than simple FeO T and TiO2 concentrations, unless order of magnitude differences are involved. Of major importance is the fact that the Curie point decreases to below room temperature with increasing TiO2 content and decreasing oxidation state of Fe, thus rendering as minimal the prospects of high remanent J in such rocks" [3]. Watkins also commented that an increase in Fe alone would not necessarily increase bulk magnetization, because the extra Fe might increase the proportion of multi-domain Fe, which is quite unstable [3]. To overcome some of Watkins' objections we must distinguish between "oceanic platforms" (Iceland, the Azores, The Galapagos Islands, and the Cocos-Carnegie Ridges) and morphologically relatively normal

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147 spreading axis. By modeling the short-wavelength magnetic anomalies observed by deep-tow methods near the ocean floor, these authors were able to confirm that the high amplitudes of sea-surface anomalies in the H-zone are due to high magnetization intensity rather than a greater thickness of magnetized material. A dredge collected in the H-zone (D-17) recovered FeTi basalt of high magnetization [2].

2. Magnetic anomalies and bathymetry at the eastern and western ends of the Galapagos high-amplitude zones Magnetic prof'fles [ 1,2,8] across the Galapagos spreading axis indicated that the H-zone is roughly football-shaped, being wider near the longitude of the islands and tapering to relatively narrow "snouts" at the two distal ends of the zone (Fig. l). Longitudinal profiles of prominent magnetic anomalies (Fig. 2) are one way to display the observation that the zone has expanded with time and that, contrary to Anderson et al. [2], the zone has existed possibly since 6.6 m.y. B.P. (Since anomaly amplitudes north of the axis were analyzed, it is possible that the older anomalies used in constructing Fig. 2 were misidentified, since extinct axes and repeated anomalies may lie north of the present axis [8,9]. In that case the H-zone may be younger than 6.6. m.y.B.P.). The two snouts of the H-zone were surveyed in sufficient detail that reasonably accurate bathymetric and magnetic anomaly contour charts could be prepared. Figs. 3 and 4 show bathymetric and magnetic contours at the eastern snout; the same parameters in the area of the western snout are contoured in Figs. 5 and 6. Values of magnetization intensity measured on dredge specimens, entered in Figs. 4 and 6, will be discussed in later parts of this paper (Table 1). The most significant discovery of the two "snout" surveys was that the H-zone terminates against fracture zones at both ends (Figs. 2 - 6 ) . In the east, what we call the Colombia spreading axis is presently offset 115 km northwards, relative to the Galapagos spreading axis, along the Inca fracture zone (Fig. 3). Although anomalies at least as old as 2 (1.8 m.y.B.P.) can be identified on both sides of these spreading centers, there is evidence from our profiles near the Galapagos Islands, as well as other recently collected data [9], that the spreading axis of 90.8°W jumped southward

a few million years ago (Fig. 1). The Inca fracture zone may, therefore, have been created or at least increased in offset by this -axisjump. The relatively minor de Steiguer fracture zone (25 km offset) forms the western terminus of the H-zone (Figs. 5 and 6). This fracture is even younger than the Inca fracture zone; our magnetic data (Fig. 6) indicate that the de Steiguer fracture zone was formed by a northward jump of the spreading axis east of it. This event occurred less than 0.9 m.y.B.P, because anomaly 1A continues uninterrupted across the southern projection of this fracture. A somewhat earlier fracture to the east at one time offset anomalies 1A and 2 (0.9 and 1.8 m.y.B.P.) but was replaced by a straight axis sometime later, as indicated by the straight course of the central anomaly. Termination of the H-zone against fracture zones is probably no coincidence. Vogt and Johnson [ 10] suggested that transform faults constitute "dams" or barriers to longitudinal flow of partial melts in the pipe-like region below the spreading axis. These postulated pipes would channel "excess" asthenosphere materials, disgorged by mantle plumes under the Galapagos Islands, Iceland, etc., to other parts of the Mid-Oceanic Ridge where the materials are used up in the plate accretion process. Thus FeTi-producing mantle could be impounded on the hot spot side of a major transform fault. Alternatively the advancing plume-derived mantle creates its own transform boundary by causing successive sections of ridge crest to jump as the flow advances [8,10]. The unusual mantle magmas responsible for the FeTi-enriched basalts of the Galapagos H-zone are presumably flowing east and west, below the spreading axis, away from the center of upwelling located in the area of the Galapagos archipelago. The westward-flowing crystal slush has to "overcome" an eastward component of plate motion (about 6 cm/yr [11] ; Fig. 1). The consequently greater viscous drag may explain why the high-amplitude (FeTi) zone does not extend as far west of the Galapagos Islands (550 km) as east (680 kin) (Figs. 1,2). Differences in magnetization and composition between the eastern and western H-zone might also relate to this flow-asymmetry. The boundaries of the Galapagos H-zone are associated with several other morphologic effects, besides fracture zones. The eastern boundary of the zone generally marks a change from a relatively smooth, elevated sea-

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Fig. 2. Longitudinal bathymetric (top) and magnetic anomaly (peak to trough) amplitude profiles along the Galapagos spreading axis. Each data point represents a ship-track crossing. Bathymetric profde follows immediate spreading axis whereas central anomaly amplitude applies to the Brunhes/Matuyama boundary. Anomaly amplitudes were measured on the north flank of the ridge, from the troughs of prominent (negative) anomaly numbers to adjacent (older) positives: 1 (0.7 m.y.B.P.), 2 (1.7 m.y.B.P.), 2' (2.4 m.y.B.P.), 3 (4.8 m.y.B.P.) and 4 (6.6 m.y.B.P.). The ages in parentheses are the average crustal age for the amplitude difference measured. Arrows and speeds show average rate of advance of H-L transition zone from the time of one anomaly to the next, relative to the crust. Values in parentheses are advance rates relative to a fixed mantle hot spot, assuming a 5.5-cm/yr eastward component of plate motion.

floor and basement inside the H-zone to rougher, deeper sea-floor outside it (Fig. 3). In the H-zone, basement depth increases more slowly with crustal age than outside the zone [12]. It would be premature to attempt more than a qualitative hypothesis to account for these morphologic details. The relatively higher, smoother basement within the H-zone may reflect the higher temperature (hence greater thermal expansion) and higher melt percentage [2] within the more fractionated asthenosphere derived from the Galapagos plume. Depth dif-

ferences along particular isochrons - for example, the Galapagos and Colombia spreading axes that represent the zero age isochron - are of the order 1 0 0 - 2 0 0 fathoms fit) ( 1 8 5 - 3 7 0 m). The generally greater roughness o f the L-basement may reflect relatively deeper isotherms. A thicker lithosphere - at the axis and elsewhere - allows the formation at the spreading axis of higher-amplitude volcanic and fault-block topography. The greatest relief on the eastern survey area is associated with the Inca fracture zone (Fig. 3). A

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Fig. 3. Bathymetry of eastern end of high-amplitude zone (H-zone), in units of 100 fm or 183 m, with high and low areas shaded. Thin dashed lines show "de Steiguer" tracks. Thick dashed line shows possible axis of Inca fracture zone, as defined by earthquake epicenters (crosses). Topography suggests axis lies slightly to the west (dash-dot line). Solid lines show spreading axis and crustal isochrons, interpolated from dated magnetic anomalies. Triangles denote dredge stations. Eastward narrowing boundary of H-zone was based on magnetic data. Hachured lines show approximate distal boundaries of crust generated by present spreading center, prior to which axis lay to the north [9].

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topographically complex belt lies west of the trace The three extreme elevations occur alon~ the northern of the fracture zone (identified on the basis of earthborder of the H-zone. No counterpart to Inca Ridge and quake epicenters and the line connecting the offset its associated rough topography is found either east of ends of the Colombia and Galapagos spreading axes). the fault trace or on either side of the fracture's inDepths range from less than 900 fm (1650 m) on the active southward extension (Fig. 3). southern crest of the Inca Ridge, to about 1950 fm At the western end of the H-zone (Fig. 5) the rela(3600 m) and 2050 fm (3750 m), 17 and 7 km northtively minor de Steiguer fracture zone does not have west and southeast Of the high point, respectively (Fig. 3). a paralleling ridge comparable to Inca Ridge. However,

152

Fig. 6. Contoured magnetic anomalies and magnetization measurements at western terminus of H-zone. Conventions as in Fig. 4.

the de Steiguer Ridge may be an incipient fracture ridge, although it more nearly parallels crustal isochrons. In fact, this ridge, the associated de Steiguer Deep, and several other bathymetric features trend obliquely at a low angle across the isochrons. Except in a few cases the track control is inadequate to make this certain, however. The oblique (time-transgressive

or diachronous) trends may have been formed as a result of westward-moving magma chambers below the axial lithosphere. They would then be analogous to the V-shaped ridges discovered on the Reykjanes Ridge [13]. The topographic relief (more than 600 fm or 1100 m) in the de Steiguer Ridge-de Steiguer Deep area is

153 exceptional for the Galapagos spreading axis. The occurrence of this rough topography in the L-H transition zone may be no coincidence, especially since the highest relief in the eastern survey area (Fig. 3) also ,occurs along the magnetic transition zone. Perhaps the high relief is related to the jumps of the ridge axis fossil spreading axes and step-like boundaries between crusts formed by different axes. In addition, it may relate in some way to the complex asthenosphere flow regime and resulting lithospheric stresses created as the anomalous Galapagos-derived mantle slushes displace the pre-existing asthenosphere.

3. Magnetic properties of dredged basalts - correlations between remanence, susceptibility, and composition Nine dredges were collected in the detailed survey areas (Figs. 3 and 5) to test for remanence differences between L- and H-zone basalts. This is one necessary step in proving that the hypothesis of "magnetic telechemistry" [1] is valid in this area. Another step, the comparison of Fe and Ti content inside and outside the H-zone, has been reported separately [5,6]. Dredging close to the boundary was to help determine whether the transition zone is broad or narrow. The evidence from sea-surface magnetic anomalies suggests a relatively narrow transition zone Width, perhaps less than 5 or 10 km. Basalts from the transition (H-L) zone might be expected to be transitional in magnetization and composition. Alternatively, the transition zone could be composed of mixtures of flows, each flow being predominately of either average (L) or high (H) magnetization. In the east, basalts were dredged at the eastern end of the H-zone (DE-I), at the western end of the normal-amplitude Colombia spreading axis (DE-2) and along the northern boundary of the H-zone (DE-3 and -4). West of the Galapagos hot spot, basalts were dredged at four sites, two inside the high-amplitude zone (DE-6 and -8) and two outside it (DE-5 and -7). A fifth site (DE-9) yielded only manganese crust [7]. A total of nine drill-cores of 10-20 cm length (1-inch or 2.54-cm diameter) were removed from larger pillow fragments of dredges 1, 2, and 3 (Fig. 4); dredge DE-4 yielded only one small basalt fragment and was not analyzed. Seventeen cores of the order 10-20 cm long were taken from pillow basalts in the western

area (Fig. 6). Fragments of glassy rhyodacite (dredge DE-6) yielded extremely low remanence; we believe such rocks constitute an extremely rare oceanic rock type not representative of the ocean crust and, therefore, inconsequential for "magnetic telechemistry". Both susceptibility (X) and remanence (Jp) were measured on each core, and the K6nigsberger ratio (Q) was computed from these (Table 1). Jp represents the peak magnetic intensity, usually a few centimeters from the pillow crust, in the zone where the grain size of completely crystallized titanomagnetites, and their degree of high- and low-temperature oxidation are minimal. Cores were taken normal to the pillow or pillow fragment margins; where the original orientation of the pillow could be estimated, an attempt was made to core along the original vertical. Further details of measurement methods are found in reference [14], Seven previously reported measurements [2] inside (four values from dredge D-17) and outside (three values from dredge D-7) the H-zone are also listed in Table 1. Since these measurements were made 5 cm inside pillow margins, they are approximately maximum values (Jp) also. (However, pillow lavas are also known for which J decreases steadily from the surface downwards [15] .) Dredge D-7 was collected on the Costa Rica rift zone (L) and D-17, near the eastern end of the H-zone, came from slightly west of our detailed eastern survey are a (Figs. 1,3,4). The results (Table 1)basically confirm one element of the "telechemistry" hypothesis [11]. Not only susceptibility, but also maximum remanence (and hence probably average remanence) is systematically higher inside the H-zone than outside it. Although remanence is a complex physical property and a function of many variables [ 2 - 4 , 1 6 - 1 8 ] , its range is great enough in the Galapagos area to reveal a correlation with X (Fig. 7). Without the samples from the H-zone the scatter in Jp and X would have made the relationship undetectable. Since chemical analyses are available for the dredged hauls analyzed [2,5] we are also able to plot both X and Jp againstTiO2 and FeO T (Fig. 8). A reasonably good correlation between X and TiOz or FeO T was to be expected [16], but it is clear that Jp also correlates with composition. The comparatively greater scatter in JpValues for given compositions (Fig. 8)may reflect short-period (~104 yr)intensity fluctuations. The two solid parallel lines drawn through the Jp vs. FeO T and Jp vs. TiO2 and FeO T plot (Fig.

154 TABLE 1 Remanent magnetization and susceptibility of basalts dredged from Galapagos spreading axis Sample No.

Remanent magnetization Jp (emu/cm 3) (DE measurements are peak values)

Susceptibility X

K6nigsberger ratio (H = 1)

(H =

5(4) x 10-4 6(4) x 10-4 6(3) x 10-4

28(28) 17(15) 18(43)

92(92) 56(50) 59(142)

1.76 3.57 2.41 2.94

10-4 10-4 10-4 10-4

44.1 21.4 25.3 21.2

146 71 83 70

7.18 x 10-4 3,15 x 10-4

11.3 20.6

" 37 68

1.09 1.34 1.1 1.2 1.0 1.4

x x x x x x

10-a 10-3 10-3 10-3 10 -3 10-3

42.4 18.5 55 71 50 29

140 61 182 234 165 96

6.61 8.61 6.09 6.30 5.98 7.30 5.98 7.24 5.88

x x × x x × x x ×

10-4 10-4 10-4 10-4 10-4 10-4 10-4 10--4 10-~

22.7 18.7 17.3 20.0 21.5 16.3 15,1 13,1 17,6

75 62 57 66 71 54 50 44 58

2.10 4.30 4.41 3.57 2.10 3.04 4.20 4.41

x x x x x x × x

10-4 10-4 10-4 10-4 10-4 10-4 10-4 10-4

15.1 12.7 11.1 4.5 31,0 30.9 6,1 3.3

50 42 37 15 102 102 20 11

0.3)

Eastern normal-amplitude zone - Costa Rica R i f t

D-7-2 D-7-3-4 I)-7-4-4

1.4(1.1) X 10-2 1.0(0.6) X 104 1.1(1.3) X 104

Eastern normal-amplitude zone - Colombia spreading axis

DE 2-4 DE 2-8 DE 2-9 DE 2-10

7.78 7.65 6.12 6.25

X X X X

10-a 10-3 10-3 10-a

x x x x

Eastern high-amplitude zone/normal-amplitude zone b o u n d a r y

DE 3-3 DE 3-4

8.15 X 10-3 6.49 X I0 -a

Eastern high-amplitude zone - Galapagos spreading center

DE 1-6 DE 1-7 D-17-1-5 D-17-2-7 13-17-3 D-17-4-4

4.63 2.49 5.5 8.5 5.0 4.0

X X X X X x

10-2 10-2 10-2 10-2 10-2 10-2

Western high-amplitude zone - Galapagos spreading center

DE 6-5 DE 6-7

1.50 X 10-2 1.61 x 10-2

DE 6-8 DE 6-9 DE 6-10

1.05 x 10-2 1.26 x 10.-2 1.28 x 10-2

DE DE DE DE

1.19 9.04 9.75 1.03

8-3 8-5 8-6 8-7

x x x x

104 10 -3 10-3 10-2

Western low-amplitude zone - Galapagos spreading center

DE 7-4 DE 7-7 DE 7-10 DE 5-5 DE 5-10 DE5-11 DE 5-12 DE 5-13

3.18 5.49 4.89 1.62 6.51 9.41 2.58 1.48

X X X X X x x x

10-3 10-a 10-3 10-3 10-3 10-3 10-3 10-a

8) b r a c k e t all d a t a p o i n t s i f g e o m a g n e t i c i n t e n s i t y varied b y a f a c t o r o f t h r e e and if t h e r e w e r e n o o t h e r sources o f scatter. Pillow size was f o u n d t o b e an ira-

p o r t a n t variable in d e t e r m i n i n g r e m a n e n c e and susc e p t i b i l i t y o n t h e c r e s t o f R e y k j a n e s Ridge [ 1 4 ] . We p r o p o s e t h a t for a given c o m p o s i t i o n t h e s c a t t e r in

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Fig. 7. Intensity of remanence magnetization, as a function of susceptibility, for basalts of the Galapagos spreading axis reported in a previous paper [2], and several "H"-type basalts from the Juan de Fuca and Carlsberg Ridges [17] (triangles). Remanent magnetization for the latter sites has been "normalized" to the geomagnetic dipole intensity for the Galapagos area. Squares connected by arrows are measurements [2] of both fresh basalt (tail of arrows) and weathered rims of same specimens (head of arrows), 2 - 4 cm distant from fresh portion of fragment.

magnetic properties is primarily due to pillow size variation (Jp and X) and paleo-intensity fluctuations (Jp) only. Although some data points from other areas [17] (Juan de Fuca Ridge, Caflsberg Ridge)have been plotted with the Galapagos data (Fig. 7), Jp values were adjusted to the average dipole intensity of the Galapagos area. Thus, there is no scatter due to latitudinal variation of geomagnetic intensity. Intensities reported for the depth range 4 - 6 cm inside pillow are taken as essentially the maximum value, Jp. In a logarithmic plot Of Jp vs. X (Fig. 7) in a given geographic area, the data would lie along any of a family of parallel lines provided Jp is proportional to X- If the constant of proportionality (Q -ratio) is the same for

all basalts the data must lie along exactly one of these lines. A Q of about 20 (at H = 1), or about 60 for the geomagnetic intensity of the Galapagos area, provides a reasonable fit to the data distribution. There is some suggestion that basalts in the H-zone 0figh Jp) have relatively higher Q values than normal, more weakly magnetized basalts - possibly because H-zone pillows are smaller. As all the tabulated measurements, except DE-3 and DE-7 were made on young (less than about 5 × 104 years) fresh-looking basalts, effects due to weathering [18] are probably not seriously large. To illustrate the possible magnitude of such effects, we have plotted in Fig. 7 measurements [2] on both weathered

156 I00

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3

157 pillow rims and fresh interiors of three pillow fragments dredged on the Costa Rica Rift, an L-zone (Fig. 1). With the exception of a slight remanence increase in one specimen, the effect of weathering reduces Jp and X. The J differences are of the same order as between-flow differences attributable to paleointensity fluctuations. The "weathering" effect on magnetization observed in the three fragments is of the same order as that noted by Marshall and Cox [18] (about 40% reduction in J). Those authors analyzed what we now know are FeTi basalts dredged in the H-zone of the Juan de Fuca Ridge and attributed the decrease of J to titanomaghematization. Although an effort was made to sample the immediate spreading axis (dredges DE-l, -2,-5, -6, -8, D-17 and D-7) and, thereby, eliminate age-dependent effects such as weathering, possible viscous demagnetization, etc., in the absence of absolute dates we cannot be sure whether a fresh-looking basalt is say, 102 or l0 s years old (topographic and magnetic data exclude ages as great as 106 years). Evidence from the much more densely sampled Mid-Atlantic Ridge suggests that the remanence of young basalts may drop by an order of magnitude within the first million years after extrusion [ 15,19,20]. If this is also true in the Galapagos area, our Jp values will exhibit greater scatter and increase more slowly with X, TiO2 and FeO T (Figs. 7 and 8) than would a hypothetical set of "zero-age" basalts. We need to review what is known about this phenomenon. Several processes have been suggested as explanations for the decrease Of Jp away from the axis of the Mid-Atlantic Ridge. Some of these processes are unlikely to be significant and others may be less important for the relatively fast-spreading Galapagos axis (3 cm/yr) than for the Mid-Atlantic Ridge (1 cm/yr). To some extent, the high Jp in axial Mid-Atlantic Ridge rocks could be due to unusually high late Heistocene/Recent paleointensity [ 15,19-21 ]. Such an effect would, of course, also be recorded by

Galapagos Ridge basalts. However, independent paleointensity data rule out increases by more than a factor of two, if indeed there was any systematic increase at all [21]. If high paleointensity were an important mechanism, magnetic anomaly amplitudes on the Galapagos spreading center would decrease as dramatically away from the axial anomaly as they do on the Mid-Atlantic Ridge [ 15,19,20]. This is not what is observed (Figs 2 and 4; References [1] and [2] and unpublished data). The decrease is only about 25% in the first few million years, and this includes the effects of increasing source depth and various possible posteruptive remanence losses. The decrease of anomaly amplitude away from the Mid-Atlantic Ridge axis [19,20], an effect not pronounced in the Galapagos area, may partly reflect "contamination" of each pre-Brunhes polarity prism by dikes and flows of opposite polarity [21,22]. The predicted effect would be most pronounced at low spreading rates, as observed. Although remanence intensity is unaffected by this process, any extrapolation from sample measurements to anomaly amplitudes must take it into account. Watkins [3] also suggested that the along-strike variations in amplitude observed in the Galapagos area (Fig. 1) might reflect varying degrees of "dike contamination". The argument might be that in the area of the Galapagos hot spot, higher mantle temperatures cause a thinner axial lithosphere, narrower injection zone, and less contamination of each block by intrusives of opposite polarities. Spreading rate itself cannot be invoked because the highamplitude zone has been formed at half-rates of 3.1 cm/yr [2], intermediate between values in the L-zones to the east (3.2 cm/yr) and west (3.0 cm/yr). Since systematic along-strike variations in remanence intensity are more than sufficient to explain concomitant amplitude variations, it appears unnecessary to postulate that a variable-width intrusion zone is a significant factor. The remaining processes that could explain the

Fig. 8. Susceptibility, x (left) and remanent magnetization, Jp, (right) as a function of weight percent TiO2 substantially and total iron as FeO (upper). Circles (this study) and squares represent Galapagos spreading axis, while triangles are relatively highly magnetized (H-type) pillows from Juan de Fuca [ 17,29] and Carlsberg [ 17] Ridges, reduced to the dipole intensity in the Galapagos area. Although only our values give peak remanence (J), Anderson et al's measurements were made 5 cm inside pillow margins, approximately where Jp occurs [ 14]. Where several values of composition or magnetic properties were obtained from a given dredge site, the numerical mean is plotted as one data point, with bars showing the total range of measurements. Small isolated symbols are cases where composition [2] and magnetic property were measured on same dredge fragment. Greater scatter in Jp, compared to x, can be explained by three-fold intensity fluctuations of the geomagnetic field (solid parallel lines).

158 magnetization peak at.the Mid-Atlantic Ridge axis include: (1) an unstable viscous magnetization component readily lost by thermal agitations [20], and (2) irreversible oxidation [20] occurring in surface flows or in covered flows near the axial vent. Irving [20] and Marshall and Cox [17] have elaborated arguments against the viscous demagnetization mechanism and we will not repeat them here. The oxidation processes - whether low-temperature (300°C) maghematization or a higher-temperature (>/400 °) twophase unmixing into magnetite and ilmenite - would proceed most readily near the ridge axis, especially for buried and reheated flows. On slow spreading ridges, such as the Mid-Atlantic Ridge, a given basalt flow will spend more time in the high heat-flow zone than at the higher half-rates (3 cm/yr) of the Galapagos spreading axis. Therefore, buried flows in the latter area should experience relatively less loss of remanence; the comparatively slight drop in magnetic amplitudes on the latter ridge (Fig. 2), compared to the Mid-Atlantic Ridge [20], makes this an attractive explanation. However, unburied lava flows are presumably the ones that are dredged, and such material would be relatively unaffected by geothermal reheating near the axis. If such flows are the only ones sampled by dredging, and if low-temperature oxidation is the predominate process that reduces initially high remanence, there would be no reason why rocks dredged along the Galapagos spreading axis should not also exhibit the order-of-magnitude decline observed on the MidAtlantic Ridge. There is, however, a morphological difference between the Mid-Atlantic and Galapagos spreading centers that may bear on the magnetization vs. age problem. The former ridge exhibits high, blockfaulted relief and a prominent medial rift valley. The range of titanomagnetite oxidation states and grain sizes at a particular site is large, suggesting that some of the specimens are not surface-quenched materials [15]. Thus, the increase in Curie and blocking temperatures, as well as decreasing remanence intensity, away from the Mid-Atlantic Ridge [15,19,20] axis may be a sampling artifact. Previously covered basalt flows already exposed to mild geothermal heating and remanence loss would be uncovered by the process of block-faulting. As dredging probably selectively oversamples fault-zone rubble and fault scarps, rather than flows, the remanence drop in Mid-Atlantic Ridge ba-

salts as revealed by dredge analysis would be quite pronounced. Flows never previously buried are less likely to be sampled except in the median valley. By contrast, the Galapagos crestal topography is relatively smooth - except near the border of the H-zone (Figs. 3 and 5), and the existing topography is primarily volcanic in origin. The higher Galapagos spreading rates guarantee that covered basalts do not spend much time in the high heat-flow zone; thus, even flows exposed away from the axis by relatively rare faulting would have lost less remanence than their Mid-Atlantic Ridge counterparts. Yet another explanation for the ten-fold reduction in remanence away from the Mid-Atlantic Ridge axis [19,20] is that low-viscosity lavas cooling to form glassy slabs, sheets and small pillows [ 14] of high remanence form at the ridge axis but are subsequently buried by later larger pillows of much lower remanence [17]. To date there is no evidence that pillow size increases away from a spreading axis. The above discussion suggests that remanence of both dredged (surface) and sub-bottom basalts may not decrease as dramatically away from the Galapagos spreading axis as away from the Mid-Atlantic Ridge. Consequently, the effect of missing the immediate spreading axis by, say, 0.2 m.y. (6 km at a spreading rate of 3 cm/yr) will not be pronounced. However, there would be great value in detailed sampling traverses at right angles to the Galapagos spreading axis, as well as deep crustal drill holes, in both H and L zones.

4. Implications for the Galapagos hot spot problem Including data from dredges D-7 and D-17 previously reported [2], we have a total of 31 measurements on eight dredges collected within the 0.5-m.y. crustal isochron (Table 1). The data are reasonably evenly distributed inside and outside the high-amplitude zone and between the eastern and western boundary areas. Unless accidental dredging bias in the two anomaly provinces is invoked (for example, systematically higher paleointensity or smaller pillow size [14] in the H-zone), our data establish beyond reasonable doubt that remanence is higher in the H-zone. From 16 measurements on four dredges an average remanence of 0.027 emu/cm 3 was obtained (range, 0.009-

159 0.085 emu/cm 3. This average is 4.1 times the average of 0.0065 emu/cm 3 (range, 0.0015-0.014 emu/cm 3) obtained for 15 samples of four dredges in the normalamplitude L-zone. Comparing results from the eastern and western transition zones (Fig. 1) suggests that at least inside the H-zone the crust at the eastern end is more intensely magnetized. Nine samples from two dredges in the western L-zone yielded an average remanence of 0.0044 emu/cm 3 (range, 0.0015-0.0094 emu/cm3), whereas 8 samples from two dredges just inside the H-zone exhibit anaverage of 0.012 emu/cm 3 (0.0090.016 emu/cm3). The seven eastern H-zone basalts had remanences of 0.023-0.085 emu/cm 3 (average 0.046 emu/cm3), almost 4 times the average for the western snout (dredges DE-6 and DE-8). The average remanence from the eastern (0.009 emu/cm 3) and western (0.0044 einu/cm 3) L-zones probably do not differ significantly, considering the range of values (Table 1). Even the difference between the eastern and western H-zones might be explained away as a sampling accident, were it not for the two other east-west differences: (1) the maximum magnetic anomaly relief is 30% higher in the east (Fig. 2) (1800 vs. 1400 nT), and (2) the FeO T and TiO2 contents of the two eastern dredges [5] range from 13.4 to 18.4% (FeO T) and from 2.14 to 3.71% (TiO2), substantially higher values than obtained in the west (dredges DE-6 and DE-8): FeO T, 11.64-15.66% and TiO2, 1.59-2.48%, excluding the rare rhyodacite fragments also recovered in dredge 6. Although the four-fold remanence difference between the eastern and western H-zone "snouts" may partially reflect fortuitous sampling, the amplitude and composition differences strongly indicate that bulk crustal magnetization in the high-amplitude H-zone is highest at its eastern end [2] - not at the longitude of the Galapagos hot spot as might have been expected. The increase in amplitude away from the hot spot, first noted by Anderson et al. [2], can be explained in terms of progressive crystal fractionation of partial melts flowing in a pipe-like partial melt region that lies along and beneath the spreading axis [5]. As the mantle magmas flow away from the hot spot they begin to crystallize; residual liquids collecting near the top of the pipe thereby become enriched in FeO T and TiO2, and upon extrusion at the spreading axis yield FeTi pillow basalts of high remanence [1,2].

The asymmetry in magnetization, composition, and anomaly amplitudes is more difficult to understand. Spreading rate is only slightly higher (3.2 cm/yr) at the eastern tip of the high-amplitude zone than in the west (3.0 cm/yr). It seems unreasonable to expect such a small difference in spreading rate (hence pipe cross-section) [10] to influence melt composition so markedly. Another more likely explanation for the asymmetry involves the absolute motions of the lithospheric plates. Both the Cocos and Nazca plates have a strong eastward component in their motion over the mantle about 5 or 6 cm/yr [11]. There should be less viscous resistance to eastward flow of crystal slushes. We suggested earlier that this postulated flow asymmetry might explain why the high-amplitude zone extends further east (680 km) than west (550 km) of the Galapagos hot spot (Fig. 1). Perhaps the eastward-flowing tongue of anomalous partial melts is thicker vertically and less mixed with normal asthenosphere. This might result in more extreme FeTi compositions and, therefore, greater remanence and anomaly amplitudes. A more complete theory for concomitant flow and crystal fractionation is required to explore this possibility.

5. Distribution of magnetization in the oceanic crust some remaining discrepancies

Although the effect of basalt composition on magnetic anomalies seems fairly clear, there are some remaining discrepancies which may bear on the problem of how bulk magnetization varies as a function of age and depth in the crust. Although remanence intensity is greater in the Hzone than outside it (Table 1), the contrast in amplitudes (2 in the west and 2.5 in the east) is much less than the contrast in remanence (4.1). The two eastern H-dredges (DE-1 and D-17) are seven times as magnetized as the average L-basalt. There are two classes of explanations for this discrepancy: (1) The dredges inside the H-zone fortuitously sampled basalts that are (a) younger, fresher, and hence less oxidized, (b) magnetized during relatively higher paleointensity, or (c) comprised of generally smaller pillows [14]. Since even rather thorough weathering (titanomaghematization) only reduces

160 remanence by 40% [18], this factor is probably insufficient. However, the average and peak remanence of fresh pillows dredged on the axis of the Reykjanes Ridge ranged over a factor of about 5, depending inversely on pillow radius [14]. If more small pillows were fortuitously dredged in the H-zone, the remanence differences would be easy enough to explain the amplitude/remanence discrepancy. It is possible that the process of dredging selectively samples flows with small pillows which are more easily broken off and collected by the dredge. (2) Lavas producing small pillows are more common on the H-zone ridge crust, especially at the eastern end of the zone. In this case there must be a compensating effect that reduces the product of bulk remanence times layer thickness, otherwise the magnetic amplitudes would still be proportional to values measured on dredge samples - higher than required to explain the amplitude difference. Such a compensating effect could be: (a) the pillow basalt layer is several times thinner in the H-zone than in the L-zone; (b) the H-zone "FeTi" flows are diluted at depth by less magnetic, more silica-rich extrusives, such as the andesite and rhyodacite recovered at site 6; (c) the H-zone pillows are more readily oxidized or demagnetized at depth by mild heating in the axial zone. Consider first the reheating mechanism. Oxidation of H-pillow could occur more readily if they are smaller because they would then be more easily fractured and in any case have more surface area per unit volume; thus sea water might have easier access to the H-zone crust. A second possibility arises from the generally higher TiO2/FeO T ratio in H-zone pillows (0.16) compared to the L-zone (0.13). A similar change in TiO2/FeO T occurs along the Reykjanes Ridge, where L-type basalts grade progressively into the FeTi basalts of Iceland [14,24]. If the H-zone titanomagnerites are similarly enriched in TiO2, their saturation magnetization and stability would affect the loss of magnetization as a result of burial and heating of pillows in the axial zone. For example, a change from FeO • Fe(Tio,TsFeo.2s)Oa reduces saturation magnetization from about 45 to 15 emu/g and the Curie temperature from 280 ° to 90°C [15]. There is a suggestion that Curie temperatures for pillows dredged along the Reykjanes Ridge, in fact, decrease approximately from

200 ° to 150°C as Iceland is approached [14]. However, only five determinations were made and the result is still inconclusive. The Ti variation in the Reykjanes Ridge titanomagnetites amounts to less than 10% (based on Curie temperatures). This result, together with microscopic analyses of augite phenocrysts, suggests that most of the "extra" Ti in the H-basalts ends up in pyroxene [14]. Similarly a few measurements [2] suggest that saturation magnetization of basalts from the Galapagos spreading axis increases in proportion to FeOT; this would not be expected if titanomagnetites in the H-basalts were significantly more titanium-rich. Magneticanomaly amplitudes do not, however, increase in proportion to FeO T or TiO2 (Fig. 9). This result is also tentative, but the increasing TiO2/FeO T ratio of basalts could explain it if the titanomagnetites also become more titanium-rich, thus reducing both Curie temperature and saturation magnetization and, therefore, remanence. There is no direct evidence for or against the hypothesis that the Galapagos H-zone pillow layer (2A) is thinner. The general association of FeTi basalts with hot spots [ 1] would suggest that, if anything, the FeTi (H-zone) crust ought to be thicker. Anderson et al. [2] speculate that the Fe-enrichment in the H-zone implies greater magma production per unit length of ridge crest, which also would produce a thicker crust. However, it may be that the proportion of pillow lavas to other crustal material (dikes, sills, pyroclastics) is less in the FeTi zones near hot spots, thus reducing bulk magnetization deeper in the crust. Reducing remanence at depth has already been discussed with respect to Curie temperatures. There are several other possibilities. Weakly magnetized breccia or weakly magnetized low-Q intrusives might be more common in the H-zone pillow layer [2]. It has also been suggested that "normal oceanic tholeiites" comprise one-half to two-thirds of Layer 2A in the Galapagos H-zone [2]. In the comparable Juan de Fuca H-zone a bimodal mixture of normal and FeTi basalts was not found [25]. Furthermore, the bulk magnetization of sea-floor topography, computed from deep-tow data [2], is 3 6 times greater for the Galapagos H-zone than for the Costa Rica rift (L). Since the peak remanence of dredged basalts from the two areas differs by the same fact o r , the recovered samples are evidently representative of the upper trends to a few hundred meters of Layer

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2A. This observation suggests less-magnetic H-zone materials, if any, must exist at depths greater than the base of small-scale topographic features. The deeper parts of Layer 2A would be exactly those most vulnerable to reheating if the remanence of H-basalts is less stable or more prone to oxidation. Our preferred explanation for the discrepancy between the H/L zone anomaly contrast and the measured remanence contrast is that in the H-zone remanence decreases more steeply with increasing depth. This in turn may reflect a combination of (1)

reduced pillow-size - hence easier alteration; (2) reduced stability - Caused by higher titanium content; (3) a greater proportion of low-Q intrusives.

6. Conclusion A rather detailed geological/geophysical survey has defined the eastern and western limits of the Galapagos high-magnetic amplitude (H) zone. Dredge sam. pies confirm the basic tenet of "magnetic telechemistry".

162 The basalts in the H-zone are richer in iron and titanium [5,6] (Fig. 9) and higher in susceptibility, remanence and, based on a few measurements, saturation magnetization. However, the iron enrichment appears to be the result of crystal fractionation [2,5,6]. Peak remanence is 4 times higher in the H-zone whereas anomaly amplitudes differ by only 2-2.5X. Possibly the H-titanomagnetites are more titanium-rich and lose their magnetization more readily at depth in Layer 2A; this can only be resolved by analyzing the titanomagnetite composition for the two zones - a difficult task that has not been accomplished [2,14,17]. Perhaps H-zone pillows are systematically smaller, hence more intensely magnetized but at the same time more easily oxidized. Deep crustal drilling in the H and L zones is needed to test such speculation. The H-zone did not appear abruptly all along the 1000-km section of the Galapagos spreading axis at anomaly 2' (3.3 m.y.B.P.) as claimed by Anderson et al. [2]. Instead, the zone was already being formed prior to 4 m.y.B.P., albeit more modest in extent (Figs. 1 and 2). Subsequently the H-conditions spread east and west, both in mantle and plate coordinate systems. In the last 1-2 m.y. the length of the H-zone seems to have stabilized. Our present interpretation of these phenomena involves relatively hot partially molten mantle materials rising in a plume below the western Galapagos Islands. These partial melts are channeled in a pipe-like region below the spreading axis. As the partially molten material cools and crystallizes, progressively more iron-rich liquids form at greater distances from the plume. (A maximum would, of course, be reached, beyond which further fractionation reduces iron in the residual liquids.) At the same time the partial melts are used up in the accretion process. In the Galapagos area the fractionation effect appears to dominate - so that the greatest magnetic amplitudes, remanence, and FeTi content appear not near the hot spot but at the ends of the H-zone. A similar situation exists on the Juan de Fuca Ridge [7]. However, southwest of Iceland FeTi concentrations [24] and remanence [14] decrease gradually, suggesting that the anomalous partial melts are used up faster than they can fractionate. It is not clear how much of the Iceland FeTi anomaly reflects primary chemistry rather than fractionation. To date there is in the Galapagos area very little information, other than magnetic anomaly amplitudes,

for crust older than the immediate spreading axis. One, therefore, cannot prove that the properties measured along the axis approximate a steady-state condition. Indeed, amplitude profiles along pre-Brunhes anomalies (Fig. 2) do not reveal distal peaks, either because present conditions along the axis are atypical or because, as discussed earlier, the most FeTi-enriched basalt comprising the distal portions of the H-zone is most vulnerable to burial and aging effects. Indirect evidence also suggests rapid advance of chemically anomalous partial melts under the Reykjanes Ridge in the last 6 m.y. [27]. The spread of the Galapagos H-zone (Fig. 2) may also reflect the flow of underlying partial melts, but further expansion of the H-zone may have been arrested in the last 1 - 2 m.y. because the flow became impeded by transform faults. There is also evidence that the expansion of FeTi-producing mantle caused the observed progressive jumping of the ridge axis [8,10]. The spread of the Galapagos H-zone has been approximately coeval with the formation of the Galapagos Islands and points to increased discharge from the Galapagos mantle plume in the last 6 m.y. Roughly simultaneous increases seem to have affected the Iceland [27,28] and Hawaii [28] hot spots, among others, and supports the "global synchronism" model for hot spot volcanism [28].

Acknowledgements

We thank N.O. Watkins and W.G. Melson for review, and S. Madosik, L. Hemler, and P. Lanasa for technical help. P. Michalco, R. Boerckel, D. O'Neill. and W. Worsley assisted in preparing this manuscript. Research was partially supported by the Office of Naval Research.

References

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(1975) 683. 3 N.D. Watkins, Magnetic Telechemistryis elegant but nature

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