Submarine Basaltic Glass Research Articles

Absolute Thellier paleointensities from Ponta Grossa dikes (southern Brazil) and the early Cretaceous geomagnetic field strength


 
 
 We report a detailed rock magnetic and Thellier paleointensity study from ~ 130.5 Ma Ponta Grossa Dike Swarms in Southern Brazil. Twenty-nine samples from seven cooling units were pre-selected for paleointensity experiments based on their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. 19 samples characterized by negative pTRM tests, Arai concave-up curves or positive pTRM tests with NRM loss uncorrelated with TRM acquisition were rejected. High quality reliable paleointen- sity determinations are determined from detailed evaluation criteria, with 10 samples belonging to three dikes passing the tests. The site-mean paleointensity values obtained in this study range from 25.6 ± 4.3 to 11.3 ± 2.1 ?T and the corresponding VDM’s range from 5.7 ± 0.9 to 2.5 ± 0.5 (1022 Am2). These data yield a VDM mean value of 4.1 ± 1.6 x 1022 Am2. Significant variability of Earth’s magnetic field strength is observed for Ponta Grossa Dikes with the mean value being significantly lower as compared to the mean VDM obtained from the nearby Parana? Magmatic Province. The paleointensities for the Ponta Grossa Dikes are in agreement with abso- lute paleointensities retrieved from the submarine basaltic glasses from 130 to 120 Ma. It seems that a relativelylow field prevailed just before the Cretaceous Normal Superchron.
 
 

The Origin of Nanoscopic Grooving on Vesicle Walls in Submarine Basaltic Glass: Implications for Nanotechnology

Dendritic networks of nanoscopic grooves measuring 50–75 nm wide by <50 nm deep occur on the walls of vesicles in the glassy margins of mid‐ocean ridge pillow basalts worldwide. Until now, their exact origin and significance have remained unclear. Here we document examples of such grooved patterns on vesicle walls in rocks from beneath the North Atlantic Ocean, and give a fluid mechanical explanation for how they formed. According to this model, individual nanogrooves represent frozen viscous fingers of magmatic fluid that were injected into a thin spheroidal shell of hot glass surrounding each vesicle. The driving mechanism for this process is provided by previous numerical predictions of tangential tensile stress around some vesicles in glassy rocks upon cooling through the glass transition. The self‐assembling nature of the dendritic nanogrooves, their small size, and overall complexity in form, are interesting from the standpoint of exploring new applications in the field of nanotechnology. Replicating such structures in the laboratory would compete with state‐of‐the‐art nanolithography techniques, both in terms of pattern complexity and size, which would be useful in the fabrication of a variety of grooved nanodevices. Dendritic nanogrooving in SiO2 glass might be employed in the manufacturing of integrated circuits.

Testing the robustness and limitations of 0–1 Ma absolute paleointensity data

Absolute paleomagnetic field intensity data derived from thermally magnetized lavas and archeological objects provide information about past geomagnetic field behavior, but the average field strength, its variability, and the expected statistical distribution of these observations remain uncertain despite growing data sets. We investigate these issues for the 0–1 Ma field using data compiled in Perrin and Schnepp [Perrin, M., Schnepp, E., 2004. IAGA paleointensity database: distribution and quality of the data set. Phys. Earth Planet. Int. 147, 255–267], 1124 samples of heterogeneous quality and with restricted temporal and spatial coverage. We accommodate variable spatial sampling by using virtual axial dipole moments (VADM) in our analyses. Uneven temporal sampling results in biased estimates for the mean field and its statistical distribution. We correct for these effects using a bootstrap technique, and find an average VADM of 7.26 ± 0.14 × 1 0 22 A m 2. The associated statistical distribution appears bimodal with a subsidiary peak at approximately 5 × 1 0 22 A m 2. We evaluate a range of potential sources for this behavior. We find no visible evidence for contamination by poor quality data when considering author-supplied uncertainties in the 0–1 Ma data set. The influence of material type is assessed using independent data compilations to compare Holocene data from lava flows, submarine basaltic glass (SBG), and archeological objects. The comparison to SBG is inconclusive because of dating issues, but paleointensity estimates from lavas are on average about 10% higher than for archeological materials and show greater dispersion. Only limited tests of geographic sampling bias are possible. We compare the large number of 0–0.55 Ma Hawaiian data to the global data set with no definitive results. The possibility of over-representation of typically low intensity excursional data is discounted because exclusion of transitional data still leaves a bimodal distribution. No direct test has allowed us to rule out the idea that the observed pdf results from a mixture of two distinct distributions corresponding to two identifiable intensity states for the magnetic field. We investigate an alternative possibility that we were simply unable to recover a hypothetically smoother underlying distribution with a time span of only 1 Myr and the resolution of the current data set. Simulations from a stochastic model based on the geomagnetic field spectrum demonstrate that long period intensity variations can have a strong impact on the observed distributions and could plausibly explain the apparent bimodality. Our 0–1 Ma distribution of VADMs is consistent with that obtained for average relative paleointensity records derived from sediments.

Alteration of submarine basaltic glass from the Ontong Java Plateau: A STXM and TEM study

Frequent observations of tubular to vermicular microchannels in altered basalt glass have led to increasing appreciation of a possible significant role of microbes in the low-temperature alteration of seafloor basalt. We have examined such microchannel alteration features at the nanoscale in basalt glass shards from the Ontong Java Plateau using a combination of focused ion beam milling, transmission electron microscopy and scanning transmission X-ray microscopy. Three types of materials were found in ultrathin cross-sections cut through the microchannels by FIB milling: fresh basalt glass, amorphous Si-rich rims surrounding the microchannels, and palagonite within the microchannels. X-ray absorption spectroscopy at the C K-edge and Fe L 2,3-edges showed the presence of organic carbon in association with carbonates within the microchannels and partial oxidation of iron in palagonite compared with basalt glass. Although these observations alone cannot discriminate between a biotic or abiotic origin for the microchannels, they provide new information on their mineralogical and chemical composition and thus better constrain the physical and chemical conditions prevailing during the alteration process.

Paleointensity applications to timing and extent of eruptive activity, 9°–10°N East Pacific Rise

Placing accurate age constraints on near‐axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges. Geomagnetic paleointensity of submarine basaltic glass (SBG) holds promise for placing quantitative age constraints on near‐axis flows. In one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful SBG paleointensity estimates from 189 near‐axis (<4 km) sites at the East Pacific Rise, 9°–10°N. Paleointensities range from 6 to 53 μT and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field. Samples within and adjacent to the axial summit trough (AST) have values approximately equal to or slightly higher than the present‐day. Samples out to 1–3 km from the AST have values higher than the present‐day, and samples farther off axis have values lower than the present‐day. The on‐axis samples (<500 m from the AST) provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique. Results from samples collected near a well‐documented eruption in 1991–1992 suggest there may be a small negative bias in the paleointensity estimates, limiting resolution of the dating technique. Possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain; anomalously high magnetic unblocking temperatures, leading to a small cooling rate bias; and/or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories. Paleointensity remains useful in approximating age differences in young flows, and a clear along‐axis paleointensity contrast near 9°50′N is suggestive of a ∼150–200 year age difference. Paleointensity values of off‐axis samples are generally consistent with rough age interpretations based on side scan data. Furthermore, spatial patterns in the paleointensity suggest extensive off‐axis flow emplacement may occur infrequently, with recurrence intervals of 10–20 kyr. Results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows, with flow size linked to time between eruptions.

Long-term trends in paleointensity: The contribution of DSDP/ODP submarine basaltic glass collections

The Deep Sea Drilling Project and the Ocean Drilling Program have been collecting fresh appearing submarine basaltic glass from the world’s oceans for over three decades. This glass has proved nearly ideal for estimating paleointensity variations of the Earth’s magnetic field. We compile here data for 726 paleointensity experiments from six publications on paleointensity using DSDP/ODP glass. We also include new data for an additional 225 specimens. These were obtained through the so-called “IZZI” paleointensity experiment of [Tauxe, L., Staudigel, H., 2004. Strength of the geomagnetic field in the cretaceous normal superchron: new data from submarine basaltic glass of the troodos ophiolite. Geochem. Geophys. Geosyst. 5 (2), Q02H06, doi:10.1029/2003GC000635] whereby infield-zerofield steps are alternated with the zerofield-infield steps to enhance quality assessment of the resulting data. The entire collection of data from 951 experiments was prepared for uploading to the MagIC data base ( http://earthref.org), including original measurements, interpretations, and useful metadata. Excellent results were obtained throughout the depth ( > 1400 mbsf) and age (0–160 Ma) range sampled. DSDP/ODP glass data are compared with published paleointensity data meeting minimal acceptance criteria from the time interval 1–160 Ma. Paleolatitudes were estimated for all cooling units in a self-consistent manner for use in calculating virtual axial dipole moments. We conclude: (1) There is about a 20% difference in mean values between the SBG and the lava flow data ( 48 ± 36 and 57 ± 29 ZAm 2 respectively). The difference is caused by the fact that there are more higher values in the lava flow data than in the SBG data set rather than a difference in the minimum values. Lava flows cooling over a periods of days to months can account for the discrepancy. (2) The positive relationship between polarity interval length and average paleofield intensity first hypothesized by [Cox, A.V., 1968. Lengths of geomagnetic polarity intervals. J. Geophys. Res. 73, 3247–3260] is supported by data compiled here. The Brunhes data (for which we have only a minimum estimate for polarity interval length) are consistent with a long polarity interval, suggesting that instead of racing toward reversal [Hulot, G., Eymin, C., Langlais, B., Mandea, M., Olsen, N., 2002. Small-scale structure of the geodynamo inferred from oersted and magsat satellite data. Nature 416, 620–623], we could instead be in the midst of a long stable polarity interval. (3) Because the average value appears to be a function of polarity interval length, it is probably not useful to calculate a mean value. Nonetheless, it appears that most of the time (apart from the Brunhes and the longest polarity intervals), the average dipole moment is substantially less than the present day value as suggested by [Juarez, T., Tauxe, L., Gee, J.S., Pick, T., 1998. The intensity of the earth’s magnetic field over the past 160 million years. Nature 394, 878–881].

Sources, degassing, and contamination of CO 2, H 2O, He, Ne, and Ar in basaltic glasses from Kolbeinsey Ridge, North Atlantic

New volatile data (CO 2, H 2O, He, Ne, and Ar) are presented for 24 submarine basaltic glasses from the Kolbeinsey Ridge, Tjörnes Fracture Zone and Mohns Ridge, North Atlantic. Low CO 2 and He contents indicate that magmas were strongly outgassed with the extent of degassing increasing toward the south, as expected from shallower ridge depths. Ne and Ar are significantly more abundant in the southernmost glasses than predicted for degassed melt. The strong atmospheric isotopic signal associated with this excess Ne and Ar suggests syn- or posteruptive contamination by air. Degassing, by itself, cannot generate the large variations in δ 13C values of dissolved CO 2 or coupled CO 2-Ar variations. This suggests that δ 13C values were also affected by some other processes, most probably melt-crust interaction. Modelling indicates that degassing had a negligible influence on water owing to its higher solubility in basaltic melt than the other volatiles. Low H 2O contents in the glasses reflect melting of a mantle source that is not water-rich relative to the source of N-MORB. Before eruption, Kolbeinsey Ridge melts contained ∼400 ppm CO 2 with δ 13C of −6‰, 0.1 to 0.35 wt.% H 2O, 3He/ 4He ∼11 R A, and CO 2/ 3He of ∼2 × 10 9. We model restored volatile characteristics and find homogeneous compositions in the source of Kolbeinsey Ridge magmas. Relative to the MORB-source, He and Ne are mildly fractionated while the 40Ar/ 36Ar may be low. The 3He/ 4He ratios in Tjörnes Fracture Zone glasses are slightly higher (13.6 R A) than on Kolbeinsey Ridge, suggesting a greater contribution of Icelandic mantle from the south, but the lack of 3He/ 4He variation along the Kolbeinsey Ridge is inconsistent with active dispersal of Icelandic mantle beyond the Tjörnes Fracture Zone.

Cooling rate effects on paleointensity estimates in submarine basaltic glass and implications for dating young flows

Cooling rate effects on the intensity of thermoremanent magnetization (TRM) have been well documented in ceramics. In that case, laboratory cooling is generally more rapid than the initial cooling, leading to an overestimate of the paleofield by 5–10% in Thellier‐type paleointensity experiments. The reverse scenario, however, has never been tested. We examine the effects of cooling rate on paleointensity estimates from rapidly quenched submarine basaltic glass (SBG) samples from 13 sites at 17°30′–18°30′S on the East Pacific Rise. Absolute cooling rates determined by relaxation geospeedometry at five of these sites range from ∼10 to ∼330°C min−1 at the glass transition (∼650°C). Over the dominant range of remanence blocking temperatures (∼200–400°C), the natural cooling rates are approximately equal to or slightly slower than the laboratory cooling rates during the Thellier experiment. These results suggest that while the cooling rate effect might introduce some within‐site scatter, it should not result in a systematic bias in paleointensity from SBG. Paleointensity estimates from the 15 sites range from ∼29 to 59 μT, with an average standard error of ∼1 μT. Comparison with models of geomagnetic field intensity variations at the site indicate the youngest group of samples is very recent (indistinguishable from present‐day) and the oldest is at least 500, and probably several thousand, years old. These age estimates are consistent with available radiometric ages and geologic observations.

Anomalous uplift and subsidence of the Ontong Java Plateau inferred from CO2 contents of submarine basaltic glasses

The Ontong Java Plateau in the western Pacific is anomalous compared to other oceanic large igneous provinces in that it appears to have never formed a large subaerial plateau. Paleoeruption depths (at 122 Ma) estimated from dissolved H2O and CO2 in submarine basaltic glass pillow rims vary from 1100 m below sea level (mbsl) on the central part of the plateau to 2200–3000 mbsl on the northeastern edge. Our results suggest maximum initial uplift for the plateau of 2500–3600 m above the surrounding seafloor and 1500 ± 400 m of postemplacement subsidence since 122 Ma. Our estimates of uplift and subsidence for the plateau are significantly less than predictions from thermal models of oceanic lithosphere, and thus our results are inconsistent with formation of the plateau by a high-temperature mantle plume. Two controversial possibilities to explain the anomalous uplift and subsidence are that the plateau (1) formed as a result of a giant bolide impact, or (2) formed from a mantle plume but has a lower crust of dense garnet granulite and/or eclogite; neither of these possibilities is fully consistent with all available geological, geophysical, and geochemical data. The origin of the largest magmatic event on Earth in the past 200 m.y. thus remains an enigma.

Strength of the geomagnetic field in the Cretaceous Normal Superchron: New data from submarine basaltic glass of the Troodos Ophiolite

We present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the Troodos Ophiolite (∼92 Ma), formed during the Cretaceous Normal Superchron (CNS), a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all. Monte Carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength. Our data suggest a dipole strength equivalent to the present field or nearly twice the post‐CNS average. The mean and standard deviation of the dipole moment (81 ± 43 ZAm2; Z = 1021) from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from DSDP Site 522. The new data set for the CNS suggests a picture of a strong and stable field during the period of time when it stopped reversing. Moreover, the similarity of the CNS data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability.

Volatiles in submarine basaltic glasses from the Ontong Java Plateau (ODP Leg 192): implications for magmatic processes and source region compositions

Abstract Submarine basaltic glasses from five widely separated sites on the Ontong Java Plateau (OJP) were analysed for major and volatile elements (H 2 O, CO 2 , S, Cl). At four of the sites (1183, 1185, 1186, 1187) the glass is from pillow basalt rims, whereas at Site 1184 the glass occurs as non-vesicular glass shards in volcaniclastic rocks. Glassy pillow rims from Site 1187 and the upper group of flows at Site 1185 have 8.3–9.3 wt% MgO compared with values of 7.2–8.0 wt% MgO for glasses from Sites 1183, 1184 1186, and the lower group of flows at Site 1185. Low-MgO glasses have slightly higher H 2 O contents (average 0.22 wt% H 2 O) than high-MgO glasses (average 0.19 wt%), with the exception of Site 1184, where low-MgO glasses have lower H 2 O (average 0.16 wt%). Average S concentrations are 910 ± 60 ppm for the high-MgO glasses v. 1030 ± 60 ppm for the low-MgO glasses. When compared with mid-ocean ridge basalt (MORB), the OJP glasses have lower S at comparable FeO T . This suggests that OJP basaltic magmas were not saturated with immiscible sulphide liquid during crystallization, but small decreases in S/K 2 O and S/TiO 2 with decreasing MgO require some sulphide fractionation. Measurements of the wavelength of the S K α peak in the glasses indicate low oxygen fugacities comparable to MORB values. Chlorine contents of the glasses are very high compared with MORB, and Cl/K ratios for all glasses are relatively high (>0.7). This ratio is sensitive to assimilation of hydrothermally altered material, so the high values indicate assimilation during shallow-level crystallization of OJP magmas. Ratios of H 2 O to Ce, which have similar incompatibility to each other, are higher than most depleted and enriched MORB. However, these high H 2 O/Ce values are probably also caused by the same assimilation process that results in high Cl. The water content of the high MgO-magmas before contamination is estimated to be approximately 0.07 wt% H 2 O, corresponding to H 2 O/Ce of 135 for OJP basalts, a value at the low end of the range for Pacific MORB. There is no evidence for high H 2 O contents that would have significantly increased extents of mantle melting beneath the OJP, and the estimated H 2 O content of the OJP mantle source region (170 ± 30 ppm H 2 O) is similar to that of the depleted MORB source (140 ± 40 ppm H 2 O). Instead, large extents of melting beneath the OJP must have been caused by a relatively high mantle potential temperature, consistent with upwelling of a hot mantle plume.

Cretaceous geomagnetic paleointensities: Thellier experiments on Pillow lavas and Submarine basaltic glass from the Ontong Java Plateau

We present new Thellier paleointensity results on pillow lavas and pillow‐rim submarine basaltic glass (SBG) from 120 Ma basalts erupted on the Ontong Java Plateau. All pillow lavas have suffered low‐temperature alteration and are unsuitable for Thellier paleointensity experiments. In contrast, pillow‐rim SBG is virtually impermeable to seawater and therefore has suffered little or no low‐temperature alteration. Out of 21 selected SBG samples with magnetic moment larger than 10−10 Am2, 18 samples (from 12 chilled SBG margins) yielded paleointensity estimates of good technical quality. Rock magnetic measurements show the primary carrier of remanence is low‐Ti titanomagnetite occurring in a continuous range of compositions starting from nearly pure magnetite. As suggested by a recent study, magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400–440°C, which is in accordance with the partial thermoremanent magnetization checks failing significantly above the same temperature. We suggest that paleointensity estimates obtained below this critical temperature represent true determination of the paleomagnetic field strength. For several pillow margins two to three samples were measured, and in such cases the data agree well. Altogether we obtain reliable estimates from 12 SBG pillow‐rims, yielding a mean paleointensity of 27.1 ± 13.4 μT and a corresponding virtual axial dipole moment of 5.8 ± 2.8 · 1022 Am2. This value is slightly higher than whole rock data from similar aged subaerial lava flows. The paleosecular variation of the intensity estimates is slightly larger than previously reported for paleointensity determinations for the Cretaceous Normal Superchron.

Correction to “Paleointensity in Hawaiian Scientific Drilling Project Hole (HSDP2): Results from submarine basaltic glass”

Correction to “Paleointensity in Hawaiian Scientific Drilling Project Hole (HSDP2): Results from submarine basaltic glass”

Paleointensity in Hawaiian Scientific Drilling Project Hole (HSDP2): Results from submarine basaltic glass

Paleointensity estimates based on the high quality Thellier‐Thellier data from the early Brunhes (420–780 ka) are rare (only 30 in the published literature). The Second Hawaiian Scientific Drilling Project (HSDP2) drill hole recovered submarine volcanics spanning the approximate time period of 420–550 ka. These are of particular interest for absolute paleointensity studies owing to the abundance of fresh submarine basaltic glass, which can preserve an excellent record of ancient geomagnetic field intensity. We present here new results of Thellier‐Thellier paleointensity experiments that nearly double the number of reliable paleointensity data available for the early Brunhes. We also show that the magnetizations of the associated submarine basalts are dominated by viscous magnetizations and therefore do not reflect the true ancient geomagnetic field intensity at the time of extrusion. The viscous contamination is particularly severe because of a combination of low blocking temperatures in the basalts and relatively high temperatures in the deeper parts of the drill core. Our new data, when placed on the approximate timescale available for HSDP and HSDP2, are at odds with other contemporaneous paleointensity data. The discrepancy can be reconciled by adjusting the HSDP timescales to be younger by about 35 kyr.

Magnetic hysteresis monitoring of Cretaceous submarine basaltic glass during Thellier paleointensity experiments: evidence for alteration and attendant low field bias

On the basis of studies of Holocene samples, submarine basaltic glass (SBG) is thought to be an ideal paleointensity recorder because it contains unaltered single domain magnetic inclusions that yield Thellier paleointensity data of exceptional quality. To be useful as a recorder of the long-term geomagnetic field, older SBG must retain these optimal properties. Here, we examine this issue through rock magnetic and transmission electron microscope (TEM) analyses of Cretaceous SBG recovered at Ocean Drilling Program Site 1203 (northwestern Pacific Ocean). These SBG samples have very low natural remanent magnetization intensities (NRM<50 nAm 2/g) and TEM analyses indicate a correspondingly low concentration of crystalline inclusions. Thellier experiments on samples with the strongest NRM intensity (>5×10 −11 Am 2) show a rapid acquisition of thermoremanent magnetization (TRM) with respect to NRM demagnetization. Taken at face value, this behavior implies magnetization in a very weak (<17 μT) ambient field. But monitoring of magnetic hysteresis properties during the Thellier experiments (on subsamples of the SBG samples used for paleointensity determinations) indicates systematic variations in values over the same temperature range where the rapid TRM acquisition is observed. A similar change in properties during heating is observed on monitor SBG specimens using low-temperature data: with progressive heatings the Verwey transition becomes more distinct. We suggest that these experimental data record the partial melting and neocrystallization of magnetic grains in SBG during the thermal treatments required by the Thellier method, resulting in paleointensity values biased to low values. We further propose that this process is pronounced in Cretaceous and Jurassic SBG (relative to Holocene SBG) because devitrification on geologic time scales (i.e., tens of millions of years) lowers the transition temperature at which the neocrystallization can commence. Magnetic hysteresis monitoring may provide a straightforward means of detecting the formation of new magnetic inclusions in SBG during Thellier experiments.

Rhenium systematics in submarine MORB and back-arc basin glasses: laser ablation ICP-MS results

Rhenium and other trace elements, including the moderately chalcophile elements Mo and Cu, were determined for 37 submarine basaltic glasses from the Lau and Coriolis Troughs (CT) back-arc basins and Woodlark marginal basin, as well as 30 mid-ocean ridge basalt (MORB) glasses from the Pacific and Atlantic Oceans, using laser ablation ICP-MS. Rhenium is strongly positively correlated with Yb for all these submarine basaltic glasses. Enriched (E-) and normal (N-) MORB as well as King's Triple Junction samples show similar correlations with constant Yb/Re ratios, indicating that Re and Yb exhibit similar compatibility during melt evolution [Chem. Geol. 139 (1997) 185]. In contrast, samples from the East and Central Lau Spreading Centers have much higher ratios compared with MORB samples and form steeper arrays on Re–Yb variation diagrams, similar to komatiites. More incompatible element-depleted samples including those from the Lau and Woodlark Basin spreading centers and the more depleted (D-) MORB samples are also distinguished from E- and N-MORB and samples from King's Triple Junction and Coriolis Troughs Basin on the basis of their higher Cu/Re ratios. These observed elemental systematics are interpreted to reflect progressive melting of depleted mantle, where previous melting events result in the elimination of sulfides in the source regions of the depleted samples. Using the determined Yb/Re and Ce/Mo ratios and assuming that the abundances of Yb and Ce are 10% and 40% reduced in the DMM compared to the primitive mantle (PM), average concentrations of 0.12 ppb for Re and 34 ppb for Mo are estimated for the DMM. The partition coefficients of the analysed moderately incompatible elements are in the order of Mo<Ce<Re<Yb for the depleted samples (no residual sulfide), while for those that were derived from sources with residual sulfide, the order is Mo=Ce<Yb=Re.

The variation of intensity of earth’s magnetic field with time

A new and larger database is established to assess the variation of paleointensity with time. It is shown that submarine basaltic glass probably contains a grain-growth chemical remanent magnetization in magnetite. Intensity estimates from these samples are used to establish a lower bound estimate on intensities. Statistical tests on various subsets of the database are used to establish a more reliable database for finding long-term trends of the paleointensity. We can obtain a reasonable estimate for intensity versus time for the Cenozoic but not for the Mesozoic or late Paleozoic. The distributions of data as a function of time also provide valuable information. In particular, the two well-documented superchrons exhibit different intensity properties, which suggests that there is not a simple correlation between reversal rate and intensity.

Volatiles in Submarine Basaltic Glasses from the Northern Kerguelen Plateau (ODP Site 1140): Implications for Source Region Compositions, Magmatic Processes, and Plateau Subsidence

Submarine pillow basalts (34 Ma) recovered from the Northern INTRODUCTION Kerguelen Plateau at ODP Site 1140 contain abundant unaltered The Kerguelen Plateau and Broken Ridge formed mostly glass, providing the first opportunity to measure the volatile contents during the Cretaceous as a single, giant oceanic plateau of tholeiitic basaltic magmas related to the Kerguelen mantle plume. that is interpreted to represent voluminous magmatism The glasses have La/Sm and Nb/Zr ratios that vary from values associated with emplacement of a mantle plume head similar to Southeast Indian Ridge (SEIR) MORB (Unit 1), to below young Indian Ocean lithosphere (Duncan & slightly more enriched (Unit 6), to values transitional between Storey, 1992; Weis et al., 1992). During Ocean Drilling SEIR MORB and basaltic magmas formed by melting of the Program (ODP) Leg 183, igneous basement rock and Kerguelen plume (Units 2 and 3). Volatile contents for glasses in sediment cores were obtained from five sites on the Units 1 and 6 are similar to depleted mid-ocean ridge basalt Kerguelen Plateau and two on Broken Ridge (Fig. 1). The (MORB) values (0·25–0·27 wt % H2O, 1240–1450 ppm S, results, combined with subsequent radiometric dating, 42–54 ppm Cl). In contrast, H2O contents are higher for the demonstrate that from south to north the age of the enriched glasses (Unit 2, 0·44 wt % H2O; Unit 3, 0·69 wt %), uppermost crust decreases from 120–110 Ma in the as are S (1500 ppm) and Cl (146–206 ppm). Cl/K ratios Southern Kerguelen Plateau, to >110 to 95 Ma in the for all glasses are relatively low (0·03–0·04), indicating that Central Kerguelen Plateau, Broken Ridge and Elan Bank, assimilation of hydrothermally altered material did not occur during to Ζ35 Ma in the Northern Kerguelen Plateau (Coffin shallow-level crystallization. H2O/Ce for the enriched glasses et al., 2000; Pringle & Duncan, 2000; Duncan, 2002). At (Units 2 and 3) is significantly lower than Pacific and South six of seven basement sites, as well as at four sites drilled Atlantic MORB values, suggesting that low H2O/Ce may be an on earlier ODP cruises, subaerially erupted basalt flows inherent characteristic of the Kerguelen plume source. Vapor saturation were recovered, indicating that the growth rate of the pressures calculated using the H2O and CO2 contents of the glasses Kerguelen large igneous province (LIP) was sufficient to indicate that >1700 m of subsidence has occurred on this part of form a subaerial landmass. However, a sequence of the plateau since eruption of the basalts at 34 Ma. Subsidence submarine-erupted pillow basalt flows of 90 m thickness estimates for Site 1140 and other ODP drill sites indicate that the was drilled at Site 1140 on the Northern Kerguelen various parts of the Kerguelen Plateau subsided at a rate comparable Plateau (Coffin et al., 2000). with that for normal Indian Ocean lithosphere. The Site 1140 submarine glasses provide the first

Thellier palaeointensity experiments on Faroes flood basalts: technical aspects and geomagnetic implications

Based on palaeomagnetic and rock magnetic results, 108 samples from 27 flows from Palaeogene flood basalts of Faroe Islands were chosen for whole-rock Thellier palaeointensity experiments. Altogether 90 samples were rejected due to either chemical alterations or typical multidomain (MD) behaviour evidenced by pTRM-tails. AF pre-treatment was used to reduce the effect of MD grains on Thellier experiments. Only five flows (18 samples) yielded acceptable palaeointensity estimates, with flow mean VDMs ranging from 3.5 to 7.4×10 22 A m 2. Modest selection criteria imposed on all published 5–160 Ma palaeointensity data left only 15 palaeomagnetic dipole moments: eight from whole-rock samples, six from submarine basaltic glass and one from single plagioclase crystals. More data are needed before the intriguing differences between results from different materials can be put into a geomagnetic context.

Theory and application of the Day plot (Mrs/MsversusHcr/Hc) 2. Application to data for rocks, sediments, and soils

New theoretical curves relating the hysteresis parametersMrs/MsandHcr/Hcfor single‐domain (SD), superparamagnetic (SP), pseudo‐single‐domain (PSD), and multidomain (MD) grains and their mixtures are applied to published data for natural materials. The Day plot ofMrs/MsversusHcr/Hchas been used to crudely classify samples into box‐like SD, PSD, and MD (or sometimes incorrectly, MD + SP) regions with arbitrary boundaries. New type curves for MD, PSD/SD + MD, and SD + SP grains and mixtures permit more subtle and precise modeling. The predicted MD trend and its junction with the PSD trend are observed in two data sets: for magnetite spherules from carbonate rocks and for temperature‐varying hysteresis results spanning the Verwey transition. The latter data are the basis of a suggested new method for pinpointing the PSD‐MD threshold size. Selected data for pottery clays, soils, and paleosols generally follow SD + MD type curves and indicate intermediate‐size PSD magnetite with narrow to broad size distributions. A lake sediment section with known grain‐size progression tracks in the predicted sense along the SD + MD trend. Selected data for glaciomarine and pelagic sediments are also generally compatible with SD + MD trends. Examples of remagnetized carbonate rocks, submarine basaltic glasses, and glassy rims of pillow basalts all follow predicted SP + SD or SP + PSD mixing curves, with a large range in volume fraction of SP grains (0–75%) but a narrow range of SP particle sizes: 10 ± 2 nm. Larger SP grains spanning the range to SD size (25–30 nm) are absent for unknown reasons. Oceanic dolerites, gabbros, and serpentinized peridotites in some cases fall in a novel region of the Day plot, parallel to but below magnetite SD + MD mixing curves.