Effect of hydrothermal ridge flank alteration on the in situ physical properties of uppermost oceanic crust

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004jb003228, 2005 Effect of hydrothermal ridge flank alteration on the in situ physical properties of uppermost oceanic crust Anne Bartetzko 1 Applied Geophysics, Rheinisch-Westfälische Technische Hochschule Aachen, Aachen, Germany Received 14 June 2004; revised 11 March 2005; accepted 15 March 2005; published 29 June [1] The circulation of cold water through the oceanic crust at mid-ocean ridge flanks causes changes in the composition and physical properties of the crust. This study focuses on the effect of hydrothermal ridge flank alteration on in situ physical properties. P wave velocity, electrical resistivity, and natural radioactivity are compared. They are obtained from downhole logging operations in seven boreholes drilled by the Deep Sea Drilling Project and Ocean Drilling Program. Basement age ranges from 5.9 to 167 m.y. The comparison shows that the average values of P wave velocity and electrical resistivity increase with the logarithm of basement age for lava flows and pillow basalts. Natural radioactivity and potassium content do not show a statistically significant relation to the basement age. No direct influence of spreading rate or thickness of the sediment cover on the in situ physical properties can be observed. The increase in P wave velocity with age reflects the reduction in pore space due to the precipitation of secondary minerals. Electrical resistivity is low in young crust where conductive clays are important alteration minerals. It increases in old crust because electrically insulating minerals, such as carbonates, are formed as late stage alteration minerals. Natural radioactivity is influenced by the composition of the magma but also rises in altered rocks due to the formation of K- rich clay minerals. The results of this study show that ridge flank hydrothermal alteration persists over longer-term intervals than indicated by seismic experiments. Citation: Bartetzko, A. (2005), Effect of hydrothermal ridge flank alteration on the in situ physical properties of uppermost oceanic crust, J. Geophys. Res., 110,, doi: /2004jb Introduction [2] Oceanic lithosphere is created continuously at the mid-ocean ridges. As it moves away from the ridge axis, it cools and subsides. The circulation of cold water through the newly formed lithosphere removes significant amounts of heat. The interaction between the lithosphere and the circulating fluids results in considerable change in the composition of both crust and fluids. This process is known as aging of the crust. It affects composition as well as the physical properties of the crust and has been observed in a variety of studies on different scales. [3] Seafloor seismic experiments show that seismic velocity increases with crustal age within the uppermost layer of the basaltic oceanic crust [e.g., Houtz and Ewing, 1976]. This crustal layer is interpreted to be highly porous and permeable. The increase in seismic velocity is explained by the sealing of voids and fractures with secondary minerals formed by the interaction of water and rock during the circulation of seawater through the crust [e.g., Houtz and Ewing, 1976; Jacobson, 1992]. More recent studies [e.g., 1 Now at Research Center Ocean Margins, University of Bremen, Bremen, Germany. Copyright 2005 by the American Geophysical Union /05/2004JB003228$09.00 Grevemeyer and Weigel, 1996; Carlson, 1998] indicate that the seismic velocity rises rapidly in ocean crust younger than 10 m.y. and that most of the velocity increase takes place within the first 5 m.y. after the crust was formed. Velocity may rise from 2.9 km/s in 0.5 m.y. old crust to 4.3 km/s within 8 m.y. at the East Pacific Rise [Grevemeyer et al., 1999]. Furthermore, permeabilities determined from borehole tests reveal a strong decrease of permeability from m 2 in 0.9 m.y. old crust to m 2 in 3.6 m.y. old crust [Becker and Davis, 2003]. [4] Velocities and densities measured on drill cores recovered by the Deep Sea Drilling Project (DSDP) as well as by the Ocean Drilling Program (ODP) decrease with age while intergranular porosity increases [e.g., Johnson and Semyan, 1994; Johnston and Christensen, 1997; Jarrard et al., 2003]. This seems to contradict the results from seismic investigations but can be related to a scale effect. Basalt alteration, involving an increase in intergranular porosity and replacement of the original mineralogy by secondary minerals, explains the reduction in velocity with age observed on the scale of core samples. In contrast, seismic experiments have a resolution of several ten to hundreds of meters and reflect changes in porosity and the fracture network on a much larger scale than core samples. [5] Downhole measurements from wire line logging operations represent a scale that is intermediate between seismic surface experiments and core analyses, i.e., on the order of 1of9

2 Figure 1. Map showing the locations of the DSDP and ODP drill holes compared in this study. several decimeters to a few meters. Moreover, the measurements are carried out in situ, unlike core sample studies. Only a few studies exist that relate in situ physical properties to the aging of the oceanic crust. Carlson and Herrick [1990] and Jarrard et al. [2003] observed a slight increase in density and velocity with age that is linked to a decrease in porosity. Grevemeyer and Bartetzko [2004] noticed a shift toward higher values of electrical resistivity and natural radioactivity with age. They interpreted these changes to reflect the formation of secondary mineral phases. [6] This study presents a detailed comparison and interpretation of well logging data from seven boreholes drilled by the DSDP and ODP to investigate the effect of fluid circulation through the oceanic crust on its in situ physical properties. 2. Data Compilation 2.1. Selection of the Boreholes [7] Seven DSDP/ODP drill holes were selected for this study using the following criteria: [8] 1. Only holes drilled into oceanic crust formed at a midocean ridge were considered. Drill holes in back arc basins and large igneous provinces were excluded. Also excluded were rocks that could be clearly assigned to off-axis volcanism due to a more alkaline geochemical signature. [9] 2. A minimum logging program within the basement section of the holes was required, including measurements of electrical formation resistivity, sonic velocity, and natural radioactivity. [10] Seven boreholes were found to meet these criteria. Basement ages range from 5.9 to 167 m.y. (Figure 1 and Table 1). The seven holes were drilled into sections of the upper part of the igneous oceanic crust, which consists of pillow basalts and lava flows with intercalations of breccias and dikes. Only 504B reaches into the sheeted dike complex, and that deeper interval of 504B is not considered in this study. The rocks were altered at low temperatures (<100 C) and under oxidative conditions. In most of the holes alteration is nonpervasive and focused along veins. Alteration results in the formation of halos along fractures and surfaces, which have a characteristic mineralogy that is different from the basalt more distal to veins. Sequences of secondary minerals fill veins and fractures. Common secondary minerals are smectite (saponite), celadonite, and Fe-oxyhydroxides, as well as carbonate and pyrite. This low-temperature oxidative alteration results in chemical changes such as an uptake in alkalis [Alt, 1995]. As for 504B only the upper 310 m of the basement are considered in this study, because the lower section is characterized by nonoxidative alteration with more rock dominated conditions and restricted fluid flow [Alt et al., 1986]. [11] The entire logged intervals of the holes were included in this study, except for the above mentioned ones and the following sections: non-mid-ocean ridge basalt (MORB) sections within the basement, such as alkali basalts, a hydrothermal deposit, and thin sediment layers in 801C, the interval below 327 m subbasement in 395A, because it is characterized by strong gradients in the wire line logging data, and a local extreme borehole enlargement at 86 m subbasement in 395A. [12] Information on lava morphology was taken from several studies that related lithostratigraphy to downhole logging data [Bartetzko, 1999; Bartetzko et al., 2001, 2002; Barr et al., 2002]. For s 1224F and 1256D such studies were not available, and lithological classification was achieved by directly comparing core lithostratigraphy with downhole logs and high-resolution electrical images of the borehole wall Wire Line Logging Data [13] Downhole measurements of P wave velocity, electrical resistivity, and natural radioactivity (total gamma ray) are used in this study (Table 2). Detailed descriptions of the Table 1. Overview of the Drill s Total Depth, m below seafloor Location Basement Age, m.y. Sediment Cover, m Full Spreading Rate, cm/yr References 395A Mid-Atlantic Ridge Melson et al. [1979] 418A Bermuda Rise Donnelly et al. [1980] (Atlantic) 504B Costa Rica Rift Cann et al. [1983] (Pacific) 801C West Pacific Lancelot et al. [1990], Pringle [1992], Plank et al. [2000], and Bartolini and Larson [2001] 896A Costa Rica Rift Alt et al. [1993] (Pacific) 1224F Central tropical Stephen et al. [2003] N Pacific 1256D Eastern Pacific Wilson [1996] and Wilson et al. [2003] 2of9

3 Table 2. Approximate Vertical Resolution of the Wire Line Logging Tools Used in This Study Tool a Approximate Vertical Resolution, cm 395A P Wave Velocity Array Sonic Tool (SDT) 61 (120) X X Dipole Shear Imager (DSI) 107 X X X Borehole Compensated Sonic (BHC) 61 X Long Spacing Sonic (LSS) 61 X 418A 504B 801C 896A 1224F Electrical Resistivity Dual Latero Log (DLL) 61 X X X X X Spherically Focused Resistivity Log (SFL) 76 X X Natural Radioactivity Natural Gamma Ray Tool (NGT) 46 X X X X X X Scintillation Gamma Ray Tool (SGT) 45 X a All tool names are trademark of Schlumberger. 1256D logging tools and their measurement principles are given by, for example, Rider [1996] and Borehole Research Group [2000]. The logging program of the seven holes and the data quality are described by Salisbury et al. [1986], Alt et al. [1993], Becker et al. [1998], Plank et al. [2000], Stephen et al. [2003], and Wilson et al. [2003]. Logging data are available at ODP/DATABASE/. [14] Velocity measurements were carried out with different tools, a Dipole Shear Imager TM (trademark of Schlumberger), a Sonic Array Tool TM, a Long Spacing Sonic Tool TM, and a Borehole Compensated Sonic Tool TM. These tools differ in configuration and number of transmitters and receivers and thus in resolution (Table 2). The basic principle of the measurement however is the same. [15] Electrical resistivity was measured with the Dual Latero Log TM in five of seven holes. In s 418A and 1224F, the Spherically Focused Log TM was used. Both tools are based on a galvanic principle. Good agreement between measurements of electrical resistivity recorded with these two tools was observed in 395A where both tools were deployed [Becker et al., 1998]. The use of data from different resistivity tools should therefore not affect the comparison in this study. [16] Two different tools were used to measure total gamma ray. To minimize effects caused by differences in tool setup data from the Natural Gamma Ray Tool TM and the Scintillation Gamma Ray Tool TM (SGT), which both use a sodium iodide crystal, were used in this study, even though in some holes data from a tool with a better bismuth germanate dectector were available. The SGT does not provide spectral measurements, and therefore no potassium values are available from 1256D. The total gamma ray spectrum is given in a calibrated unit (gapi, gamma American Petroleum Institute). 3. Comparison of in Situ Physical Properties and Their Relation to Basement Age [17] Frequency distributions of P wave velocity, electrical resistivity, and total gamma ray are shown in Figure 2. A comparison of logging data from a hole drilled into young crust ( 504B) and one drilled into old crust ( 801C) is shown in Figure 3. Average values and standard deviation are listed in Table 3. [18] P wave velocity does not show much difference between the holes relative to the total range of values from 2.6 km/s (minimum 504B) to 7.5 km/s (maximum 1224F). There are only slight differences in the average values. It is lowest in 504B (4.7 km/s) and highest in 1256D (5.2 km/s). Comparison of electrical resistivity data shows wider differences than P wave velocity and varies on the order of one magnitude between the holes. The average resistivity is lowest in the holes drilled into young crust, s 504B and 896A (average log 10 electrical resistivity of 1.1 and 0.9 W m, respectively), and highest in the holes drilled into the oldest crust, s 418A and 801C (average log 10 electrical resistivity 1.9 and 2.0 W m, respectively). Total gamma ray also shows significant variations between the holes. The average values are lowest in s 504B and 896A (3.7 and 3.6 gapi, respectively) and highest in 1224F (13.3 gapi). [19] The relationship between in situ physical properties and basement age is displayed in Figure 4. Correlation coefficients (r) for the relationship between in situ physical properties and basement age are given in Table 4. In most cases, the correlation between the in situ physical properties with the logarithm of age is stronger than with linear age. Therefore only relations with the logarithm of age are considered below. [20] Correlation between P wave velocity and age is not strong (r = 0.71), whereas correlations for the lava morphologies are stronger and more significant (r = 0.78 and 0.87; significant at 95 and 99% confidence level for lava flows and pillow basalts, respectively), than for the total data set. Electrical resistivity shows stronger correlation with basement age than P wave velocity (r = 0.88 for the total data set; significant at 99% confidence level). Correlation is particularly strong within the lava flows (r = 0.91; significant at 99% confidence level). Despite the considerable differences in total gamma ray between the holes, correlation with age is not very strong (r = 0.71 for the total data set). However, the trend is affected by unusually high values in 1224F. If data from 1224F is excluded, correlation is higher (r = 0.82 for the 3of9

4 Figure 2. Histograms of P wave velocity, electrical resistivity, and total gamma ray. The holes are displayed with increasing basement age downward. Depth intervals are given in meters subbasement. total data set; significant at 95% confidence level). Potassium does not show a significant relationship with age (r = 0.54 for the total data set). 4. Interpretation and Discussion 4.1. Spreading Rate and Sediment Cover [21] The boreholes compared in this study do not represent a transect over a single mid-ocean ridge flank. Thus influences of differences in spreading rate, sediment thickness, etc., are possible. Spreading rate is an important factor controlling many processes at mid-ocean ridges, e.g., eruption style and ridge morphology [e.g., Perfit and Chadwick, 1998]. The seven holes in this study were drilled into crust formed at mid-ocean ridges of very different spreading rate, ranging from 3 cm/yr (full rate; Mid-Atlantic Ridge) to 20 cm/yr (full rate; East Pacific Rise; Table 1). Correlation coefficients calculated for the relationship between 4of9

5 Figure 3. Comparison of P wave velocity, electrical resistivity, and total gamma ray measured in 504B (young crust) and 801C (old crust). spreading rate and in situ physical properties do not indicate any systematic trend or relationship (Table 4). [22] Deep sea sediments have a low permeability (e.g., in the order of m 2 )[Langseth et al., 1992] compared to extrusive oceanic basement as high as to m 2, where permeability is mostly related to zones of intense fracturing [Fisher, 1998; Becker and Davis, 2003]. Thickness and completeness of the sediment cover are therefore factors controlling circulation of seawater through the crust. Both depend on the distance from the mid-ocean ridge (i.e., the age of the crust), the distance from continental margins, and biological productivity. At the seven drilling locations, thickness of the sediment cover overlying the basement varies between 28 and 462 m (Table 1). Correlation coefficients for the relationship between the thickness of sediment cover and the in situ physical properties are listed in Table 4. No systematic relation can be inferred from this data. However, differences in sediment thickness may have an influence on physical properties, e.g., the early coverage of the basement with an impermeable sedimentary blanket may prevent the influx of fresh seawater, particularly if basement outcrops are rare [e.g., Anderson and Hobart, 1976; Jacobson, 1992; Alt and Teagle, 2003]. [23] 395A is located in an isolated sediment pond surrounded by basement outcrops. Strong lateral fluid flow through the permeable basement under the impermeable sediment cover causes temperatures to remain cool and crustal fluids to be rapidly refreshed [Langseth et al., 1984]. This strong fluid flow through the basement may have resulted in an intensified alteration, which may explain why average values of in situ physical properties are higher than in the other holes drilled into young basement and are more similar to values of old crust (Figures 2 and 4 and Table 3) Primary Differences in Potassium Content [24] The content of radioactive elements is generally very low in oceanic basalts. As low-temperature oxidative seafloor weathering adds potassium (a radioisotopic contributor) to the rock during the formation of K-bearing secondary minerals [e.g., Alt, 1995], the total gamma ray and the potassium log may be used as indicators of alteration in oceanic crust [e.g., Bartetzko et al., 2001; Barr et al., 2002]. Thorium and uranium values are <0.5 ppm in most holes; with an exception in 1224F where the uranium log has an average value of 0.75 ppm and thus also contributes significantly to the total gamma ray spectrum. [25] Potassium data from core samples of volcanic glasses, lava flows, and pillow basalts are compared in Figure 5 in order to investigate a possible influence of differences in magma composition on the total gamma ray log. Analyses of fresh volcanic glasses are considered to be free of influences of alteration and inhomogeneous distribution of phenocrysts. Potassium content is lower in the glass analyses than in the whole rock analyses, with the only exceptions being s 801C and 395A. This may be explained by an inhomogeneous distribution of lava morphology downhole; values for glass analyses are not sepa- Table 3. Average Value of in Situ Physical Properties for the Total Depth Intervals and for Lava Flows, Pillow Basalts, and Breccias a P Wave Velocity, km/s Electrical Resistivity, log 10 W m Total Gamma Ray, gapi Potassium, wt % 504B ( m) Total 4.7 ± ± ± ± Lava 5.1 ± ± ± ± Pillow 4.4 ± ± ± ± A ( m) Total 4.8 ± ± ± ± Lava 5.1 ± ± ± ± Pillow 4.7 ± ± ± ± Breccias 4.4 ± ± ± ± A ( m) Total 4.9 ± ± ± ± Lava 5.5 ± ± ± ± Pillow 4.7 ± ± ± ± D ( m) Total 5.2 ± ± ± Lava 5.2 ± ± ± Pillow 4.9 ± ± ± F ( m) Total 5.1 ± ± ± ± Lava 5.2 ± ± ± ± Pillow 4.9 ± ± ± ± A (6 446 m) Total 5.1 ± ± ± ± Lava 5.8 ± ± ± ± Pillow 5.0 ± ± ± ± Breccias 4.4 ± ± ± ± C ( m) Total 5.1 ± ± ± ± Lava 5.8 ± ± ± ± Pillow 5.1 ± ± ± ± Breccias 4.6 ± ± ± ± a Average values and standard deviations are for lava flows (lava), pillow basalts (pillow), and breccias; depth is given in meters subbasement; n is number of depth points; and gapi, gamma American Petroleum Institute. n 5of9

6 Figure 4. Logarithm of basement age versus average value (±1 standard deviation as vertical line) of P wave velocity, electrical resistivity, total gamma ray, and potassium for the total average and separately for different lava morphologies. Numbers give correlation coefficients (r). rated for lava morpholgy. Potassium content is also lower in lava flows than in pillow basalts. The only exception is 896A, where extensive alteration along rims of lava flows was observed, possibly caused by focusing of fluid flow along lava flow contacts [Alt et al., 1993; Brewer et al., 1996]. [26] Potassium content from volcanic glasses from s 504B and 896A is lower (average value 0.03 and 0.02 wt %, respectively) than in s 395A (average value 0.11 wt %), 418A (average value 0.09 wt %) and 801C (average value 0.13 wt %; Figure 5). The basalts drilled in s 504B and 896A at the Costa Rica Rift are depleted in incompatible elements and particularly low in potassium [Cann et al., 1983; Alt et al., 1993]. Thus the generally lower potassium and total gamma ray values in s 504B and 896A (Figures 2 and 4 and Table 3) may be at least partly explained by a lower primary potassium content Influence of Ridge Flank Alteration on in Situ Physical Properties of Oceanic Crust [27] Changes in porosity and composition of the crust due to hydrothermal ridge flank alteration can explain much of the difference of in situ physical properties between the boreholes observed in Figures 2 and 4. Changes in porosity are reflected by P wave velocity and electrical resistivity. Particularly the presence of water-filled fractures reduces P 6of9

7 Table 4. Pearson s Correlation Coefficients r P Wave Velocity Electrical Resistivity Total Gamma Ray Potassium All Lava Morphologies Age Log 10 age Spreading rate Sediment thickness Lava Flows Age Log 10 age Spreading rate Sediment thickness Pillow Basalts Age Log 10 age Spreading rate Sediment thickness wave velocity as velocity is sensitive to changes in the aspect ratio of pore space. Cracks in rocks reduce velocities more than vugs, vesicles, or interpillow voids of the same volume [Wilkens et al., 1991]. Electrical resistivity is an indicator of porosity if pore space is connected and is filled with conductive material such as seawater or conductive minerals (e.g., clay minerals). [28] Several mineralogical studies on mid-ocean ridge basalts show that the type of sealing mineral changes with time [e.g., Alt, 1995; Alt and Teagle, 1999; Marescotti et al., 2000]. Fe- oxyhydroxide and clay minerals are the first minerals that develop at very young ages while carbonates (e.g., aragonite and calcite), zeolites and K-feldspar form later. Clay minerals are electrically conductive due to an internal structure that allows surface conduction and can lower electrical resistivity of basalt formations [Pezard, 1990]. Carbonates and K-feldspar are electric insulators and their formation within the fracture network may increase electrical resistivity. Quantitative studies indicate that the abundance of carbonate veins versus clay filled veins becomes more important in older crust. In young crust, few carbonate veins are observed, e.g., 2.6 carbonate veins per meter in 504B, 8.2 veins/m in 896A and <5 veins/m in 1256 where carbonate veins are restricted to a few depth intervals [Alt and Teagle, 1999; Wilson et al., 2003]. In old crust, carbonate veins are more abundant. In s 418A and 801C, carbonate veins constitute up to one third of all veins [Johnson, 1980; Plank et al., 2000] and the average amount of carbonate veins in the upper 50 m of 801C is 20.2 veins/m [Alt and Teagle, 1999]. [29] The difference in total gamma ray and potassium content among the seven boreholes can be partly explained by differences in primary potassium content, but the uptake of potassium from seawater during alteration and formation of secondary minerals also contributes to the total gamma ray and potassium values. This is confirmed by the higher total gamma ray and potassium values from core and in situ measurements from pillow basalts and breccias compared to lava flows (Figures 4 and 5). Alteration minerals that may contribute to an increase in total gamma ray intensity include K-bearing smectites, zeolites and K-feldspar [e.g., Alt, 1995]. Moreover, uranium may be enriched during the formation of carbonate veins in old oceanic crust [Farr et al., 2001] and also causes total gamma ray intensity to increase. This also explains the high total gamma ray values in s 418A, 801C, and 1224F. [30] The results of the correlation analysis (Table 4) show higher and more significant correlation coefficients with the logarithm of basement age than with linear basement age. A logarithmic relationship between basement age and different physical properties of the oceanic crust was also observed by Jarrard et al. [2003]. The logarithmic relation indicates that the physical properties increase more rapidly at young age and change only little in older crust. This is consistent with the observations from seismic experiments which indicate that the porosity change in the oceanic crust takes place in young crust, i.e., within the first 5 m.y. after the formation of the crust [Carlson, 1998; Grevemeyer et al., 1999]. The seven boreholes included in this study represent crust of an age where most of the porosity change observed in seismic data has already taken place. In situ P wave velocity does not show a strong increase with basement age, probably because even in the holes drilled into the youngest crust, most of the low aspect ratio pore space is already sealed. Electrical resistivity shows a strong correlation with age and Figure 4 shows that resistivity still increases in old crust. This indicates that changes in secondary mineralogy and thus changes in physical properties caused by hydrothermal alteration are continuing in much older crust than indicated by seismic experiments. [31] A comparison between P wave velocity determined by logging, from seismic experiments and from core measurements is shown in Figure 6. At young basement age, in situ P wave velocity values are within the upper range of values from seismic experiments and within the lowest values obtained from core measurements. In older crust, the values from the three types of measurements converge. The general difference in P wave velocity reflects the difference in rock and fracture volume represented by the three different methods. The convergence of P wave veloc- Figure 5. Potassium oxide from core analyses of lava flows, pillow basalts, and glass samples from s 395A, 418A, 504B, 801C, and 896A. The average values are displayed ±1 standard deviation. No glass analyses are available from s 1224F and 1256D. Data are compiled from Melson [1979], Rhodes et al. [1979], Donnelly et al. [1980], Byerly and Sinton [1980], Autio and Rhodes [1983], Hubberten et al. [1983], Natland et al. [1983], Brewer et al. [1996], Fisk et al. [1996], Plank et al. [2000], and Fisk and Kelley [2002]. 7of9

8 Figure 6. Comparison between in situ P wave velocity and velocity from seismic experiments and from core measurements. Seismic data are taken from the compilation published by Carlson [1998]; core data are from Johnson and Semyan [1994]. ity values from the three types of measurements in older crust can be explained by the effects of hydrothermal alteration on the oceanic crust on different scales. The reduction in pore space due to the sealing of fractures by hydrothermal alteration causes seismic velocity to increase, while a more pervasive background alteration increases porosity of the basalt matrix and thus reduces velocity on core sample scale. [32] Lava flows and pillow basalts differ in their in situ physical properties (Figure 4 and Table 3). Lava flows have a lower permeability and are less fractured than pillow basalt formations and breccias. Sealing may take longer in the lava flows, and this may explain why the correlation between P wave velocity and age is stronger in the lava flows than in the pillow basalts. A higher permeability in pillow basalts results in a stronger fluid flow and thus in bigger changes in mineralogy and physical properties. Most of these changes are focused along fractures, which results in a higher heterogeneity of old pillow basalt formations and may explain the higher variability (i.e., standard deviation) of particularly total gamma ray and potassium values in pillow basalts of old crust (Figure 4 and Table 3). 5. Conclusions [33] In situ measurements of P wave velocity and electrical resistivity from wire line logging operations in boreholes drilled into oceanic crust show an increase with the logarithm of the age of the crust for lava flows and pillow basalts. Natural radioactivity (total gamma ray) and potassium content do not show significant relations with basement age. These properties are also influenced by variations in magma composition, and total gamma ray values may increase in single boreholes due to the occurrence of uranium. No systematic relationship between the in situ physical properties and the thickness of sediment cover and spreading rate can be observed in this data set. [34] Changes in physical properties and composition of the oceanic crust as an effect of mid-ocean ridge flank hydrothermal alteration may explain the observed variations in the physical properties and the following scheme can be derived. Newly formed crust is inferred to consist of two components, basalt (high P wave velocity, high electrical resistivity, low radioactivity) and a connected network of void space filled with seawater (low P wave velocity, low electrical resistivity, low radioactivity). P wave velocity, electrical resistivity and total gamma ray values are expected to be low, although this cannot be proven as no downhole in situ measurements exist from zero age crust. Clay minerals (higher P wave velocity, low electrical resistivity, higher radioactivity) are formed during early alteration stages. Preferential sealing of cracks results in an increase of P wave velocity. Total gamma ray also increases but electrical resistivity does not change much because the difference in conductivity between seawater and clay minerals is low. In later alteration stages, clay minerals are still formed but particularly carbonates (high P wave velocity, high resistivity, low radioactivity) and also K- feldspar (high P wave velocity, high resistivity, high radioactivity) become important. The formation of carbonate and other resistive minerals does not have a large effect on P wave velocity because the relevant low aspect ratio fractures are already sealed. Electrical resistivity may increase, however, if the electrically insulating late stage minerals block electrically conductive pathways in the fracture network. [35] This study is based on only seven boreholes and quantitative conclusions are difficult to draw. A greater number of drill holes into oceanic crust, particularly into very young crust, would be necessary to enhance the interpretation and to improve our understanding of midocean ridge flank hydrothermal alteration. The limited results show that hydrothermal circulation through the flanks of mid-ocean ridges lasts considerably longer than indicated by seismic experiments. Therefore surveying hydrothermal circulation in the oceanic crust using marine geophysical methods requires the application of a variety of methods in order to monitor the full spectrum of changes in physical properties. [36] Acknowledgments. This research used data provided by the ODP. The ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. Funding for parts of this research was provided by the German Science Foundation (Wo 159/9). Discussions with I. Grevemeyer and comments by D. Vanko and H. Delius are appreciated. Very constructive reviews by E. Davis, R. Carlson, R. Wilkens, and Associate Editor D. Schmitt helped to significantly improve the manuscript. References Alt, J. C. (1995), Subseafloor processes in mid-ocean ridge hydrothermal systems, in Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions, Geophys. Monogr. Ser., vol. 91, edited by S. E. Humphries et al., pp , AGU, Washington, D. C. Alt, J. C., and D. A. H. Teagle (1999), The uptake of carbon during alteration of ocean crust, Geochim. Cosmochim. Acta., 63, Alt, J. C., and D. A. H. 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