JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B7, 2137, /2000JB000062, 2002

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B7, 2137, /2000JB000062, 2002 Slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the Mid-Atlantic Ridge south of the Kane fracture zone (MARK) area J. Gee Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA W. P. Meurer 1 Geology Department, Göteborgs Universitet, Göteborg, Sweden Received 16 November 2000; revised 11 December 2001; accepted 16 December 2001; published 20 July [1] The remanent magnetization of gabbroic material of the Mid-Atlantic Ridge south of the Kane fracture zone (MARK) area provides constraints on both the thermal structure and tectonic history of the lower crust in this slow spreading environment. The remanence of these gabbroic samples is often complex, with the juxtaposition of intervals of apparently normal and reversed polarity rocks over small spatial scales (tens of centimeters to a few meters). Moreover, several samples when thermally demagnetized have a reversed polarity magnetization component between higher and lower stability normal polarity components. Given the nominal age (1 Ma) of the crust, we suggest that this pattern of normal/reversed-normal polarity most plausibly reflects emplacement and/ or cooling through three successive polarity intervals, Jaramillo normal polarity interval ( Ma), a portion of the Matuyama reversed polarity interval ( Ma), and the Brunhes normal polarity interval (0.78 Ma to present). A small number of samples with three well-defined magnetization components have magnetic characteristics compatible with a remanence carried by fine-grained, possibly single domain, magnetite. Laboratory unblocking temperatures in these samples therefore allow estimation of lower crustal temperatures at the time of the Jaramillo/Matuyama (0.99 Ma) and Matuyama/ Brunhes (0.78 Ma) polarity transitions. Together with depth estimates derived from fluid inclusion studies these results suggest that middle and lower crustal temperatures remained as high as C for a minimum of 0.21 m.y. after emplacement. We suggest that continued injection of liquid, in the form of sills or small magma bodies, over a broad region (half width of 3 km) is responsible for this slow cooling. In addition, inclinations of the highest stability component from these drill sites are remarkably similar to that expected from an axial geocentric dipole, suggesting that little, if any, resolvable tilt occurred during uplift of these rocks to the seafloor. INDEX TERMS: 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 1527 Geomagnetism and Paleomagnetism: Paleomagnetism applied to geologic processes; 1550 Geomagnetism and Paleomagnetism: Spatial variations attributed to seafloor spreading (3005); 3035 Marine Geology and Geophysics: Midocean ridge processes; KEYWORDS: ocean crust, gabbro, magnetization, thermal history 1 Now at Department of Geosciences, University of Houston, Houston, Texas, USA. Copyright 2002 by the American Geophysical Union /02/2000JB000062$ Introduction [2] Extensive exposures of gabbroic rocks and associated mantle-derived ultramafic lithologies have been documented at a number of localities on the flanks of slow spreading ridges (see recent reviews by Lagabrielle et al. [1998] and Tucholke et al. [1998]). Many of these originally deep-seated rocks are exposed near segment ends at inside corner highs where crustal thickness may be smaller [e.g., Lin et al., 1990; Tolstoy et al., 1993; Cannat, 1993] and fault displacements may be larger [Escartin et al., 1997; Shaw and Lin, 1996]. By analogy with the pseudostratigraphy of ophiolites, coarse-grained gabbros and mantle peridotites were presumably formed at depths greater than a few kilometers, implying significant uplift. These qualitative estimates of uplift are corroborated by direct pressure estimates from magmatic fluid inclusions that suggest a minimum uplift of a few kilometers [Kelley and Delaney, 1987]. EPM 3-1

2 EPM 3-2 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS [3] Most current models for the uplift of lower crust and upper mantle involve substantial exhumation through detachment faulting though the precise kinematic history of this fault-related thinning remains uncertain. Slip on steeply dipping faults cannot expose deep crustal units, except where the crust is initially thin, so motion is thought to be concentrated on localized, shallow-dipping shear zones with large displacements [e.g., Karson, 1998]. Geological observations, both from drilling and submersible studies, provide abundant evidence for such low-angle shear zones [e.g., Karson and Dick, 1983; Lawrence et al., 1998] although the present dips may have been subjected to flattening during uplift [Buck, 1988; Wernicke and Axen, 1988]. Substantial faulting may occur either during episodes of amagmatic extension [Tucholke and Lin, 1994; Varga and Moores, 1985] or may be a more continuous process with accompanying synkinematic intrusions [Karson, 1998]. The recent discovery of several corrugated domal structures with exposed gabbroic and ultramafic rocks in the North Atlantic [Cann et al., 1997; Tucholke et al., 1998] suggests that extreme crustal thinning may result in oceanic core complexes analogous to those formed in highly extended continental settings. [4] The thermal structure of the lithosphere plays a key role in determining the rheology and hence the style of deformation at mid-ocean ridges [e.g., Shaw and Lin, 1996]. The temperature distribution is, in turn, determined by the balance between heating by magmatic injection and cooling by hydrothermal circulation at shallow levels or by conduction deeper in the crust. At slow spreading ridges, seismic studies indicate that axial microseismic activity extends to a depth of 6 10 km [Kong et al., 1992; Wolfe et al., 1995; Toomey et al., 1988], suggesting that much of the crust deforms in a brittle manner. The brittle/ductile transition in oceanic gabbros is thought to occur at C [e.g., Hirth et al., 1998]; thus the maximum depth of seismicity provides an upper bound on the temperature of the crust and upper mantle. The relatively low temperature of the crust is generally regarded to be the result of hydrothermal cooling, a feature simulated in thermal models through incorporation of enhanced thermal conductivity in the shallow crust [e.g., Phipps Morgan and Chen, 1993]. The resulting temperature distributions near the axis of slow spreading ridges are correspondingly low. For example, the three-dimensional model of Shaw and Lin [1996] predicts maximum temperatures of <300 C for the entire axial crust within 5 km of a transform offset. [5] Samples of tectonically exposed gabbroic rocks can be used to derive additional constraints on both the thermal structure and the tectonic history of the lower crust in slow spreading environments. In this paper, we use aspects of the magnetization of gabbros recovered south of the Mid- Atlantic Ridge south of the Kane fracture zone (MARK) area to examine the thermal history of gabbroic material now exposed on the flanks of this slow spreading ridge. In particular, we document a pattern of three successive polarity intervals recorded in gabbros drilled during Ocean Drilling Program (ODP) leg 153. The presence of three distinct magnetic components, together with the nominal age (1 Ma) of the drill sites established from lineated sea surface magnetic anomaly patterns [Schulz et al., 1988], suggests that these gabbros record cooling during the normal polarity Jaramillo chron ( Ma), the intervening Matuyama reversed polarity interval and a portion of the Brunhes normal polarity chron (0.78 Ma to present; ages from Cande and Kent [1995]). Unblocking temperatures of these three components allow for an estimate of cooling rates for this section of the lower crust. 2. Geologic Setting of the MARK Area [6] The MARK area has been the subject of a number of studies that document variations in axial processes and the geophysical character of this slow spreading ridge (see Cannat et al. [1995] for a review). The northernmost segment of the ridge is characterized by a broad asymmetric axial valley with up to 4 km of relief on the western median valley wall and 1.5 km of relief on the eastern flank [Karson and Lawrence, 1997] (Figure 1a). A well-defined linear volcanic ridge, whose crest is capped by fresh basalt with little or no sediment cover, extends for nearly 40 km south from the ridge-transform intersection [Brown and Karson, 1988]. This northern ridge segment is bounded to the north by the Kane transform and to the south by a wide transition zone that coincides with a change in rift valley morphology [Karson et al., 1987; Kong et al., 1988] as well as with an increase in crustal thickness inferred from both seismic and gravity observations [Purdy and Detrick, 1986; Morris and Detrick, 1991]. This transition zone is also accompanied by an offset of several kilometers in sea surface magnetic anomaly lineations [Schulz et al., 1988] and apparently represents an accommodation zone that can be traced off axis for 7 9 m.y. [Gente et al., 1995; Pockalny et al., 1995]. Regional magnetic anomaly data suggest that spreading has been asymmetric, with an average (since 36 Ma) half spreading rate of 14 mm/yr on the west flank and 11 mm/yr on the eastern flank of the ridge [Schouten et al., 1985]. Possible duplication of near-ridge anomalies on the western flank suggests that the region may have experienced a series of small eastward ridge jumps that could account for the observed long-term asymmetry [Schulz et al., 1988]. [7] Submersible observations document extensive outcrops of gabbro, diabase, and lesser serpentinized peridotite on the ridge/transform intersection massif in addition to a narrow band of serpentinized peridotite exposed for 30 km along the western median valley wall to the south (Figure 1a) [Karson and Dick, 1983; Mével et al., 1991; Auzende et al., 1993; Karson et al., 1987]. These outcrops are characterized by moderately dipping faults and shear zones that define a nearly continuous dip slope (20 40 to east) that is cut by higher angle faults. Drilling during ODP leg 153 sampled both the peridotite massif (site 920) and gabbroic rocks (sites ) on the eastern edge of the ridge/transform intersection high (Figure 1a) [Cannat et al., 1995]. Here we are concerned only with results from the latter sites. [8] These closely spaced drill sites are all located on crust 10 km west of the present neovolcanic ridge. Sites 921 (holes 921A 921E) and 923 are located within a radius of 250 m at a depth of 2500 m. Site 922 is located at a similar depth but is offset 2 km to the south along an isochron. Lineated sea surface magnetic anomalies suggest a nominal age of 1 Ma for these sites on the basis of the

3 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS EPM 3-3 of both normal and reversed polarity material, though samples from sites 921 and 923 have predominantly reversed polarity, while samples from the remaining two sites are generally of normal polarity [Cannat et al., 1995]. [9] A total of 117 m of gabbro and olivine gabbro, and a subordinate amount of troctolite, gabbronorite, and oxide gabbro, was recovered, representing a cumulative recovery of 26% [Cannat et al., 1995]. The drill holes are all relatively shallow, with penetration ranging from 14 to 82 meters below seafloor (mbsf ). Much of the discussion that follows will focus on samples from the two deepest holes, hole 923A, which provides the most continuous record (75% recovery), and hole 921E, which has lower recovery (21%) but for which supplementary material is available from three adjacent holes that penetrated more than 40 mbsf. Crystal plastic deformation in the cores is concentrated in centimeter to decimeter shear zones, with ductile strain preferentially localized in gabbronoritic zones, and the least evolved gabbroic lithologies rarely deformed [Cannat et al., 1997]. The common association of synkinematic hornblende and recrystallized olivine, plagioclase, and pyroxene in these shear zones indicate that deformation and recrystallization took place at temperatures in excess of 700 C [Kelley, 1997; Liou et al., 1974; Cortesogno et al., 2000]. Away from these shear zones, cataclastic zones, and the margins of late-stage felsic veins, alteration is heterogeneous but generally low. Amphibole and chlorite veins suggest fluid temperatures of C during the progressive cooling and brittle deformation accompanying the formation of these veins [Dilek et al., 1997]. Figure 1. Location of sampling sites in the MARK area. (a) Simplified bathymetric map. Zones of gabbroic (light shading) and serpentinite (wavy lines) exposures and the location of the neovolcanic ridge (dark shading) are shown. Depths are in kilometers. Modified from Cannat et al. [1995]. (b) Sea surface magnetic anomaly data (R/V Conrad cruise c2511). Magnetic anomaly identifications for central anomaly (CA), Jaramillo (J), anomalies 2, and 2 0 [Schulz et al., 1988] are shown as gray lines. Dashed lines indicate location of additional highs within the central anomaly. Note the duplication of the Jaramillo on the western flank and the proximity of this anomaly to sites 921 and 923 (solid circle). identification of a small positive anomaly corresponding to the Jaramillo subchron ( Ma) [Cande and Kent, 1995] in the immediate vicinity of the drill sites [Schulz et al., 1988] (Figure 1b). Site 924 (3200 m depth) is located 1700 m to the east of sites 921 and 923 and may therefore be as much as 150 kyr younger if a model of simple linear spreading is applicable. As documented below, these age estimates are consistent with the polarity of samples recovered from the drill sites. All four sites show some evidence 3. Methods [10] Magnetic data presented here include results from 109 shipboard samples and an additional 135 gabbroic samples that were subject to shore-based study. Nearly all shore-based samples were taken as pairs of closely spaced minicores (two 1-inch diameter cores within 5 8 cm) to facilitate comparison of alternating field (AF) and thermal demagnetization results. Both sample sets were designed to provide a representative subset of the major lithologies and alteration types recovered during leg 153. [11] Approximately two thirds of the samples (including 95% of the shipboard samples) were subjected to stepwise AF demagnetization. At AF treatment steps above 40 mt, samples were routinely doubly demagnetized (i.e., samples were measured twice, once after demagnetization along three orthogonal axes and again after treatment along the opposite axial directions). The final demagnetization prior to these two measurements was along the sample +Z axis and Z axis, respectively. Averaging these two measurements should therefore minimize spurious magnetization resulting from any residual field (several hundred nanoteslas for the shipboard system) within the demagnetizing coil. However, our double demagnetization technique may introduce a gyroremanent magnetization if the samples have a significant remanence anisotropy [Stephenson, 1993]. To evaluate whether gyroremanence could be significant, we repeated the highest demagnetization step for five representative samples, measuring the remanence after each of the six axial demagnetizations. Because this test revealed a substantial gyroremanence for all five samples, we regard the

4 EPM 3-4 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS Figure 2. Variation in natural remanent magnetization (NRM) of gabbroic rocks as a function of geochemistry. (a) NRM variation with whole rock Mg# (where Mg# = 100* Mg/[Mg * Fe (total)]). Data from the MARK area include shipboard samples used for both X-ray fluorescence (XRF) and magnetic studies [Cannat et al., 1995], as well as geochemical data acquired during postcruise studies [Casey, 1997; Barling et al., 1997; Werner, 1997; Agar et al., 1997; Meurer and Gee, 2002]. Data for hole 894G (Hess Deep) and hole 735B are from shipboard samples with XRF data [Gillis et al., 1993; Dick et al., 1999]. Geochemical data are from the same piece (or rarely from an adjacent piece if core was uniform) and are typically within 5 10 cm of magnetic sample. Additional data from Hess Deep submersible samples (J. Natland, unpublished data, 1991). (b) NRM variation of gabbroic rocks from MARK area (holes 921E and 923A) as a function of olivine forsterite content (mineral data from Meurer and Gee [2002]). Circles indicate remanence data from same minicore (or adjacent minicores) used for microprobe analyses. Remaining data (pluses) indicate samples where mineralogic data [Casey, 1997; Ross and Elthon, 1997; Cannat et al., 1997; Fujibayashi et al., 1997] was acquired from an apparently uniform piece within 10 cm of the minicore used for magnetic studies. NRM intensities have been reduced to equivalent equatorial values. highest coercivity magnetization components as suspect if these components represent a small proportion of the natural remanence. [12] As illustrated more fully in section 4, detailed thermal demagnetization proved the most suitable technique for isolating the various remanence components in most samples. Samples were heated and cooled in a threechamber oven (ambient field <5 nt), typically in 50 steps to 450 C, followed by 25 C steps to 500 C, 10 C steps to 550 C, and 5 C steps, until the sample was completely demagnetized. Consistent demagnetization trajectories during the latter closely spaced steps indicate that relative temperatures are accurate to within a few degrees, with an absolute temperature accuracy estimated at 10 C. Finally, 18 minicore samples were split in half, with the a specimen taken nearest the core center and the b specimen taken nearest the borehole wall. The a specimen was AF demagnetized to 10 mt, and then both specimens were subjected to standard thermal demagnetization. Comparison of these results allows us to evaluate the effects of drilling induced magnetization, which should be more pronounced near the borehole wall. [13] All magnetization components were determined by prinicipal component analysis [Kirschvink, 1980]. Because multicomponent remanences are common, we have calculated the median destructive field (MDF ) of the vector difference sum [e.g., Gee et al., 1993] as a measure of the stability to alternating fields. The median destructive temperature (MDT ) was calculated in an analogous manner for thermally demagnetized samples. To further assess magnetic grain size and stability, hysteresis loops and acquisition of isothermal remanence (IRM) were measured using an alternating gradient force magnetometer for rock chips and single crystals from 17 representative samples. Single crystals of plagioclase and clinopyroxene were separated mechanically, and mineral identifications were based on visual inspection together with the magnitude of the high field (paramagnetic or diamagnetic) slope determined from the hysteresis data. Finally, for a subset of five specimens (used for cooling rate estimates) we imparted an anhysteretic remanent magnetization (ARM) (175 mt AF with 0.05 mt bias field) to unheated chips cut from the ends of the minicores. This ARM was then AF demagnetized to provide an estimate of magnetic grain size in these samples. 4. Magnetic Results 4.1. Compositional Dependence of Natural Remanent Magnetization [14] Natural remanent magnetization (NRM) intensities of gabbroic samples from the MARK area vary by 3 orders of magnitude. Despite this large variability, no significant difference in mean NRM intensity was discernible for the major rock types as defined by shipboard lithological descriptions (overall arithmetic mean NRM = 1.54 A/m) [Gee et al., 1997]. However, a more complete compilation of available whole rock geochemical and magnetic data for oceanic gabbroic rocks reveals a poorly defined trend of increasing NRM intensity with decreasing Mg# (where Mg# = 100* Mg/[Mg * Fe (total)]; Figure 2a). Over the range of Mg# (86 70) encompassing

5 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS EPM 3-5 [15] Mineralogical data from a subset of samples from the MARK area provide additional support for a broad petrological dependence of magnetic properties (Figure 2b). These mineral compositions may provide a more relevant measure of the degree of differentiation, particularly for the more coarse-grained samples, than do whole rock geochemical data. Although the number of evolved samples from the MARK area is small, gabbroic rocks with olivine compositions <Fo 69 tend to have higher remanent intensities (typically, >1 A/m) than do samples with more magnesianrich olivines (Figure 2b). Anorthite content in plagioclase covaries with olivine composition, so that an analogous pattern of higher NRM values in more evolved gabbroic rocks (plagioclase compositions <An 57 ) is also observed. Together with the broad trend of increasing remanence as a function of decreasing whole rock Mg#, these mineral data support earlier suggestions [Kikawa and Ozawa, 1992] that primary composition may be an important factor in controlling the remanent intensity of oceanic gabbros. Figure 3. Hysteresis data for gabbroic rocks from MARK area. (a) Day plot illustrating broad range of hysteresis parameters from near multidomain (MD) to values approaching uniaxial single domain (SD) end-member (asterisk). M rs /M s is ratio of saturation remanence to saturation magnetization. B rc /B c is ratio of remanent coercivity to coercivity. (b) Slope-corrected hysteresis loops for pyroxene single crystal in two orientations. Note the pronounced anisotropy in hysteresis parameters as a function of crystal orientation. Backfield remanence curves also shown. Hysteresis data measured on alternating gradient force magnetometer (Micromag 2900). most of the samples from slow spreading crust in the MARK area and from the Atlantis Bank (hole 735B) [Dick et al., 1999], remanent intensity does appear to increase systematically with decreasing Mg#, albeit with substantial scatter. This trend cannot be extrapolated to the more evolved compositions recovered from these slow spreading environments, nor can it be simply related to the trend suggested by submersible samples recovered from fast spread crust exposed in Hess Deep. In contrast to submarine basalts where NRM intensity of comparable age samples is well correlated with geochemistry [Gee and Kent, 1997], the bulk rock geochemistry of gabbroic samples evidently has more limited value in predicting remanent intensities. Nonetheless, the overall change in NRM intensity is more than an order of magnitude, and a significant increase in remanent intensity is suggested as gabbros evolve from troctolitic to olivine gabbroic compositions Remanence Carriers and Magnetic Stability [16] A variety of lines of evidence suggest that near endmember magnetite is the dominant remanence carrier in gabbros from the MARK area. Maximum unblocking temperatures invariably lie between 560 and 580 C, compatible with the presence of nearly pure magnetite. The dominance of magnetite is corroborated by thermomagnetic data that typically show a single magnetic phase with a Curie temperature of 580 C [Gee et al., 1997]. Isothermal remanence acquisition curves are also consistent with the presence of magnetite, as saturation is generally achieved in fields of T. Direct determinations of iron oxide compositions are sparse in these cumulate rocks, which usually have only trace amounts of discrete magnetite. However, some compositional information is available for a small number of oxide-rich gabbros that often occur as veins associated with shear zones. Titanomagnetites in these oxide gabbros are relatively Ti poor (5 11% TiO 2 ) and contain a significant amount (up to 5%) of Al 2 O 3 [Agar and Lloyd, 1997; Ross and Elthon, 1997]. Although discrete titanomagnetite grains of similar composition may be locally present, uniformly high unblocking temperatures and Curie points suggest that nearly pure magnetite, presumably through oxyexsolution of titanomagnetite and/or exsolution from silicate minerals, is the dominant phase. [17] Magnetic data suggest that the magnetite responsible for the remanence in MARK area gabbros resides primarily within silicate minerals. Hysteresis parameters from the MARK area gabbros (Figure 3a) span the range from near single domain values to values more typical of multidomain material [Day et al., 1977]. Hysteresis results from single crystals demonstrate that significant amounts of a ferromagnetic phase, presumably magnetite, occur within both plagioclase and pyroxene. Remanent intensities in plagioclase and pyroxene, measured prior to hysteresis analysis, gave median NRM intensities of 2.6 A/m (n = 16) and 0.3 A/m (n = 21), respectively. If these values are typical, magnetite within these silicate phases is sufficient to account for the NRM intensities observed in most gabbroic samples from the MARK area [cf. Davis, 1981; Evans et al., 1968]. [18] Hysteresis data from single crystals also suggest that magnetite within these silicate grains may be preferentially

6 EPM 3-6 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS samples. Approximately half of the samples have MDF values of 10 mt or less (Figure 4a), presumably reflecting a relatively large contribution from coarser grained magnetite. The remaining samples have moderate to high MDF values, including a significant number with MDF values in excess of 80 mt. The convex upward shape of the decay curve for the vector difference sum and the high MDF values in these latter samples are compatible with finegrained magnetite, in some cases approaching single domain sizes [Dunlop and Özdemir, 1997]. MDT values range from 200 to over 550 C (Figure 4b). Samples with high MDT values are often, though not exclusively, associated with companion samples having high coercivities. The discrete high unblocking temperatures and high coercivity together suggest the presence of fine-grained, possibly single domain, magnetite in some MARK area gabbros. Figure 4. Variation in (a) median destructive field and (b) median destructive temperature for MARK area gabbros. In both cases the field or temperature is that which is required to reduce the vector difference sum to half its initial value. Samples that were subjected to combined AF and thermal demagnetization (see section 3) have not been included in the lower panel. oriented along certain crystallographic directions. For 25 single crystals, hysteresis loops were measured with the field applied along two nearly orthogonal crystallographic directions. The saturation remanence in the pyroxene grain illustrated in Figure 3b varied by a factor of 2.5, with the highest remanence value obtained when the field was oriented approximately parallel to the long axis of the grain (c axis). Coercivity and remanent coercivity values were also systematically higher in this orientation. This pronounced anisotropy presumably reflects a preferred orientation of magnetite long axes subparallel to the pyroxene c axis. Comparable anisotropy was also found in plagioclase grains, where crystallographic control of magnetite needle orientations has been well documented [Sobolev, 1990; Selkin et al., 2000]. The crystallographic control of magnetite within silicate grains together with any preferred orientation of the host grains may be responsible for the moderate anisotropy of magnetic susceptibility noted in these gabbroic rocks [Cannat et al., 1995]. [19] Alternating field and thermal demagnetization data reveal a broad range of magnetic stability, as might be expected from the variation of magnetic grain size in these 4.3. Demagnetization Behavior [20] Gabbroic samples from the MARK area exhibit a range of behavior during demagnetization, from nearly univectorial remanence to complex magnetizations with up to four distinct magnetization components (Figure 5). Although not universal, most samples have a low-stability component with steep downward inclination (average 75 ) that is presumably related to drilling. This component is often removed by alternating fields of mt (Figures 5b and 5d) or by temperatures of C (Figures 5a and 5c) but may persist to higher demagnetization levels in some cases (see section 4.4). [21] About one sixth of the MARK area samples have three magnetization components in addition to a possible low-stability component related to drilling. In these samples the highest and lowest stability components are invariably of normal polarity, with an intervening component of reversed polarity (Figure 5c). We refer to the highest stability normal component as N1, the reversed polarity component as R1, and the lower stability normal component as N2. For the three component samples the R1 component is typically isolated only in thermally demagnetized samples, where it is removed over a relatively narrow temperature range ( C). Although samples with three well-defined components represent a minority (5%) of the sample collection, they contain significant information on the cooling history of the lower crust at the MARK area and will be discussed in greater detail in section 5. [22] The highest stability component for half of the gabbro samples is of reversed polarity, and by comparison with the three component samples we designate this component as R1. These samples typically also have a lower stability normal polarity component (N2) that is generally removed by 50 mt or temperatures of 475 C (Figures 5c and 5d). The remaining samples (35%, including most samples from sites 922 and 924) have only a single high stability component of normal polarity (N2). [23] Comparison of results from AF and thermal demagnetization of adjacent minicores often reveals significant discrepancies between the magnetization components recovered by these two techniques (Figure 5). In approximately half of the samples, AF and thermal demagnetization results from paired minicores yield essentially the same magnetization components. In other cases the same components are generally recognizable in both thermal and

7 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS EPM 3-7 Figure 5. Comparison of (a, c, and e) thermal and (b, d, and f ) alternating field demagnetization results for adjacent minicore samples. (a and b) Reversed polarity samples where alternating field (AF) and thermal techniques recover essentially same magnetization components. (c and d) Complex multicomponent remanence. Two specimens in Figure 5c are inner (a specimen) and outer (b specimen) halves of single core. Note that thermal demagnetization recovers a high-stability normal polarity component that is poorly defined by AF demagnetization. (e and f ) Reversed polarity sample illustrating that AF field demagnetization apparently recovers only the lower stability normal polarity overprint direction. Open (solid) circles are projections onto the vertical (horizontal) plane. AF-demagnetized samples, but the magnitude of these components may differ considerably. In the samples shown in Figures 5c and 5d both AF and thermal demagnetization reveal a similar sequence of magnetization components, a steep drilling-related component, the N2 component, and the higher stability R1 component. The highest stability ( C) N1 component is recovered only with thermal demagnetization. Although the magnetization after the highest AF treatment (140 mt) has a positive inclination, we suggest that the final two components evident in the thermal results have coercivities that overlap sufficiently to preclude isolation of the characteristic component by AF demagnetization. [24] In several sample pairs the highest stability component recovered by AF differs in polarity from the highest blocking temperature component isolated during thermal demagnetization. In some cases this polarity discrepancy may simply reflect the high coercivity of the remanence such that AF demagnetization recovers only the lower stability overprint (Figures 5e and 5f ). For 10% of the sample pairs the AF-demagnetized sample has a final reversed component apparently at odds with the highest stability normal polarity recovered by thermal demagnetization of the adjacent sample. These samples invariably have low MDF values (average of 7 mt), and the final apparently reversed magnetization component represents only a small fraction (average of 5%) of the remanence. We suggest that the final small component in these low coercivity samples most likely reflects a spurious gyroremanence acquired during static AF demagnetization Remanent Inclinations [25] Although the remanence of gabbroic rocks from the MARK area is complex, the highest stability components recovered by both AF and thermal demagnetization are generally compatible with the expected geocentric axial dipole inclination (Figure 6a). Three component samples (with N1 as final component) have a median inclination

8 EPM 3-8 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS Figure 6. Inclination of magnetization components for MARK area gabbros. (a) Inclination of highest stability components (types I, II, and III; see section 4.4) recovered by AF (dark gray) and thermal (light gray) demagnetization. (b) Inclination of intermediate (R1 is indicated by dark gray) and lower stability (N1 is indicated by light gray) components in thermally demagnetized samples with three components. (c) Inclination of lower stability normal polarity (N2) overprint in reversely magnetized samples (AF is indicated by dark gray; thermal is indicated by light gray). (50 ) slightly steeper than the expected dipole direction. This inclination difference could conceivably reflect tectonic rotation between the acquisition of this and subsequent components. Alternatively, some contamination from a steeper drilling component may be present in the least stable of these samples. The inclinations of most (70%) R1 samples lie within 10 of the expected dipole inclination ( 41 ) at the site. Samples with a single normal polarity magnetization (N2) also have inclinations that are broadly compatible with the expected dipole direction, though a small number of samples have inclinations steeper than 65, suggesting the influence of a drilling-related remanence. [26] The inclinations of lower stability components are also shown in Figure 6. For three component samples (Figure 6b) the intermediate stability component (R1) has quite variable inclination, presumably reflecting the degree to which the unblocking temperature spectrum overlaps that of the N1 and N2 components. The N2 component in these samples is typically steeper than the expected inclination, though several samples with inclinations near the expected value are also present. The inclination of the normal polarity (N2) overprint for the two component samples is also skewed toward steeper values (Figure 6c). [27] The skewness of the inclination distributions toward more positive values, particularly for the N2 component, suggests that the drilling remanence may have unblocking temperatures and coercivities that extend to quite high values. Indeed, unblocking temperatures extending to the Curie temperature might be expected if this drilling-related remanence is carried by multidomain magnetite [Dunlop and Özdemir, 1997]. Demagnetization results from the inner and outer halves (a and b specimens, respectively) of individual cores allow a more detailed evaluation of the extent to which a steep drilling-related magnetization may contaminate the inclination of the calculated components. For half of these split samples the final magnetization component isolated in the a and b specimens is essentially identical (within 3 ; Figure 5c), suggesting that no discernible drilling remanence remains at temperatures above 540 C. Lower stability components from the b specimen sometimes have more positive inclinations (Figure 5c). In the remaining split samples the highest stability component from the b specimen has systematically more positive inclinations (by 5 35 ). Because even the highest stability component in some samples is apparently affected by drilling, it is evident that the unblocking temperatures of the drilling component can extend to temperatures above 500 C and possibly to the Curie point. [28] Although drilling-related remanence does affect many samples, the distribution of inclinations for the highest stability components (Figure 6a) suggests that these components are not significantly affected by drilling for the majority of the samples. Moreover, there is no indication in any of the split samples that the unblocking temperature range of the highest stability component has been systematically affected by the presence of the drilling-related remanence. Fortunately, the inclination of magnetization components for individual samples provides a reasonable indication of the possible influence of drilling remanence. As will be illustrated below, the samples used for determining the thermal history of the MARK area appear to be minimally affected by any drilling-related remanence. [29] In view of the complexities discussed above we have assigned a subjective quality index to each final magnetization component. This quality index is based on the magnetic stability (as indicated by MDF or MDT ) and the percentage of the vector difference sum that the component represents. The highest quality (type I) samples have high stability (average MDF = 48 mt; average MDT = 497 C), and the final component represents, on average, 50 63% of the remanence (for thermally and AF-demagnetized samples, respectively). Intermediate quality (type II) samples have lower magnetic stability (average MDF = 10 mt; average MDT = 384 C), and the component represents, on average, about one quarter of the remanence. Little confidence can be placed in the lowest quality (type III)

9 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS EPM 3-9 Figure 7. Downhole variation in final (highest stability) inclination and magnetic stability for holes (a) 923A and (b) 921E from the MARK area. Results from thermally demagnetized and AF-demagnetized samples are indicated by circles and pluses, respectively. Highest quality (type I) and intermediate quality (type II) samples are indicated by larger and smaller symbols, respectively. For normal polarity components, likely N1 components are shown in gray and N2 components shown by open circles (thermal) and bold plus (AF). Core recovery is indicated by black intervals on leftmost column. Vertical bars indicate expected inclinations from geocentric axial dipole. samples that have even lower stability and where the component represents, on average, <10% of the remanence. Type III samples represent 13% of the sample collection, including samples where the final magnetization component was deemed to be predominantly either a drilling remanence or spurious gyroremanence. [30] Downhole variations in the inclination of the highest stability component (types I and II only) for the two most completely sampled holes (923A and 921E) are shown in Figure 7. Discrepancies between thermally demagnetized and AF-demagnetized samples at a given depth are evident in both holes. The upper portion of hole 923A is characterized by type I samples with negative inclinations close to the expected dipole inclination at the site. Intervals of normal polarity, typically type II samples with more complex demagnetization behavior, are common in the lower portion of the hole. Although recovery at hole 921E was significantly lower, a similar inclination pattern may be discerned. Normal polarity samples in the lower portion of this hole, as well as in additional holes from this site, have moderate coercivities and three high-stability components that are well defined by thermal demagnetization. 5. Discussion [31] Examination of thermal demagnetization results from the complete suite of MARK area gabbroic rocks reveals a pattern of magnetization components consistent with a record of emplacement/cooling during successive geomagnetic polarity intervals since 1 Ma, the nominal age of the crust at this site. Specifically, we attribute the N1, R1, and N2 magnetization components to emplacement and/ or cooling during the Jaramillo normal polarity interval ( Ma), a portion of the Matuyama reversed polarity interval ( Ma), and the Brunhes normal polarity interval (0.78 Ma to present) [Cande and Kent, 1995], respectively. [32] The samples shown in Figure 8 illustrate the continuum of demagnetization behavior observed in the MARK area gabbros that we attribute to cooling/emplacement over an extended period of time. The first four samples (Figures 8a 8d) are representative of the oldest gabbroic rocks sampled, as these samples preserve the (presumably oldest) component N1 as well as R1 and N2. The relative magnitudes of these components differ significantly as a result of their variable blocking temperature spectra and possible differences in the age of emplacement. For example, blocking temperatures of the N1 extend as low as 530 C in some samples (Figures 8a and 8b) but are restricted to higher temperatures (>560 C) in other samples (Figures 8c and 8d). The former samples may represent marginally older cumulates that cooled to lower temperatures prior to the subsequent reversed polarity interval. About half of the samples preserve the R1 component but have no record of the earliest normal polarity interval (Figure 8e). Because the unblocking temperature spectrum of these samples is similar to that of samples that do preserve N1 (cf. Figures 8d and 8e), we infer that these samples must have been emplaced

10 EPM 3-10 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS Figure 8. Vector endpoint diagrams illustrating variable record of subsequent polarity intervals in MARK area gabbroic samples. (a d) Samples with high- and low-stability normal polarity magnetization components (N1 and N2, respectively) separated by intervening reversed polarity component (R1, gray symbols). Minimum unblocking temperatures of N1 component range from 530 C (Figures 8a and 8b) to approximately 560 C (Figures 8c and 8d) and may reflect difference in age of emplacement. (e) Reversed polarity sample inferred to have been emplaced subsequent to N1 interval. (f ) Simple normal polarity magnetization inferred to represent emplacement/cooling during interval N2. Data have been rotated such that lowest stability normal component has a declination of 0. Open (solid) circles are projections onto the vertical (horizontal) plane. after normal polarity interval N1. Finally, approximately one third of the samples have simple normal polarity magnetizations (N2), consistent with emplacement during the Brunhes (Figure 8f ). The presence of samples with one, two, and three well-defined magnetization components (Figure 6a) indicates that magmatic addition to the crust occurred over at least 0.21 m.y., the duration between the end of the Jaramillo and the onset of the Brunhes Temperature and Cooling Rate Estimates [33] Remanence in the MARK area gabbros is often complex (with possible effects of drilling remanence, differences between AF, and thermal demagnetization), so we focus here on a subset of samples that exhibit a clear pattern of multiple polarity components. The inclinations of these components indicate they are not significantly affected by a drilling-related magnetization component (Table 1). Moreover, these samples have characteristics that suggest the remanence at high temperatures is carried by sufficiently fine-grained magnetite to allow the estimation of temperatures at specific times during the cooling of the gabbroic crust. [34] Néel theory [Néel, 1949] provides a rigorous basis for evaluating the thermal history of samples with a remanence carried by single domain (SD) grains. If the magnetization of an assemblage of SD particles is removed by short heating in the laboratory at a certain temperature, then this same remanence may be demagnetized by heating at a lower temperature over a longer time interval [Pullaiah et al., 1975; Williams and Walton, 1988; Worm and Jackson,

11 GEE AND MEURER: THERMAL HISTORY OF MARK AREA GABBROS EPM 3-11 Table 1. Temperature Estimates for Selected Samples From the MARK Area Inclination Breakpoint Temperature Estimated Temperature, C Sample N1 R1 N2 N1 R1 R1 N Ma 0.99 Ma a 0.78 Ma b 921C 3R1, 102 cm E 5R3, 41 cm E 7R3, 10 cm E 7R3, 76 cm B 4R2, 36 cm A 12R1, 16 cm A 8R2, 120 cm a Upper temperature bounds corresponds to upper bound of laboratory unblocking range for N1 R1 for 0.21 m.y. exposure, and lower temperature bound corresponds to lower bound of laboratory unblocking range for N1 R1 for 0.21 m.y. exposure. b Upper temperature bound corresponds to upper bound of laboratory unblocking range for R1 N2 for m.y. exposure, and lower temperature bound corresponds to lower bound of laboratory unblocking range for R1 N2 for 0.78 m.y. exposure. 1988]. Both field studies [e.g., Kent and Miller, 1987] and experimental studies on synthetic grains [Williams and Walton, 1988] have confirmed the time-temperature relationships of Pullaiah et al. [1975] for assemblages of SD grains. Anomalously high laboratory unblocking temperatures, however, have been observed in samples with coarse multidomain (MD) grains or mixtures of SD and MD grains [Dunlop and Özdemir, 1997]. [35] We have selected a subset of seven samples that have three well-defined magnetization components and magnetic characteristics consistent with the presence of fine-grained, possibly SD, magnetite to determine temperature estimates for the MARK area gabbroic rocks. A prerequisite for estimating temperatures during cooling is the presence of fine-grained (SD) particles, preferably of a single magnetic phase. As documented above, both Curie temperatures and maximum unblocking temperatures suggest that the remanence in these gabbroic samples is carried by essentially Tifree magnetite. That these samples are fresh and apparently free of hematite suggests that their thermal history is unlikely to be complicated by chemical remanence resulting from the presence of a second magnetic phase [McClelland Brown, 1982]. [36] Relatively sharp breaks between magnetization components during thermal demagnetization (Figures 8a, 8c, 8d, and 9a) provide perhaps the best indication that the remanence is a thermoremanence carried by fine-grained (SDlike) magnetite. Thermoremanence carried by MD grains commonly exhibits a broad range of unblocking, often extending to the Curie temperature [Dunlop and Özdemir, 1997], and would be expected to produce significant overlap between magnetization components. Similarly, thermal remanence carried by both MD and SD magnetite grains or chemical remanence might be expected to generate significant overlap between components [McClelland Brown, 1982]. The sharp delineation of magnetization components is therefore compatible with a thermal remanence carried primarily by fine-grained (SD-like) magnetite. Examination of the vector component removed at each demagnetization step (Figure 9b) provides further support for this interpretation. With the exception of the two temperature intervals bounding component R1 ( C; C), the vector removed at each demagnetization step is nearly coincident with the calculated principal component [Kirschvink, 1980]. This consistency, particularly for the intervening R1 component (Figure 9b), indicates that little if any overlap exists between the three magnetization components. [37] As an additional check on the relative importance of SD-like and coarser MD grains in this subset of samples, we imparted an anhysteretic remanent magnetization (175 mt AF with 0.05 mt bias field) to unheated chips cut from the ends of the minicores. Subsequent AF demagnetization of this ARM reveals initially convex upward decay for all samples, with median destructive fields that range from 25 to 42 mt (Figure 10). Comparison to AF demagnetization of thermoremanence in sized magnetite assemblages suggests that effective grain sizes in the MARK samples may range from a few microns to submicron sizes. Recall that SD-like behavior in the MARK area gabbros is also likely accentuated by the probable acicular shape of magnetite contained within silicate grains (see hysteresis results above) [Butler and Banerjee, 1975] Cooling History From a Single Sample [38] We suggest that samples such as the one illustrated in Figure 9 have a remanence carried by sufficiently finegrained magnetite from which meaningful temperature estimates can be derived. Our method for determining temperatures at specific times during the cooling history is illustrated in Figure 9c. Assuming that the N1 magnetization component represents initial emplacement and cooling during the Jaramillo normal polarity interval ( Ma), the Curie temperature of pure magnetite (580 C) constitutes an upper limit for the temperature during (at least) its latter portion. The lowest temperature at which the N1 component is removed ( C) provides a lower limit on the instantaneous temperature within this normal interval. Taken together, these results indicate that crustal temperatures between 530 and 580 C prevailed at some point during the Jaramillo. [39] Preservation of the N1 component during the subsequent reversed polarity interval imposes more stringent limits on the temperature at the time of the Jaramillo/ Matuyama boundary (0.99 Ma). We derive an upper temperature limit at 0.99 Ma as follows. The lowest unblocking temperatures of the N1 component ( C) constitute an upper bound on the crustal temperature at this time. However, this temperature estimate is implausibly high since extended exposure to reversed polarity field at lower temperatures would also result unblocking some/all of the N1 component [e.g., Pullaiah et al., 1975]. For example, crustal temperatures of 520 C for only 100 years after the reversal boundary should be sufficient enough to remove the lowest unblocking temperature portion of the N1