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Staudigel, H, Tauxe L, Gee JS, Bogaard P, Haspels J, Kale G, Leenders A, Meijer P, Swaak B, Tuin M, Van Soest MC, Verdurmen EAT, Zevenhuizen A.  1999.  Geochemistry and Intrusive Directions In Sheeted Dikes in the Troodos Ophiolite: Implications for Mid-Ocean Ridge Spreading Centers. Geochemistry Geophysics Geosystems. 1 AbstractWebsite

Sheeted dikes at mid-ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity. As such, they are crucial for the interpretation of many observations at mid-ocean ridges or other volcanoes with pronounced rift zones, including topography, hydrothermal systems, petrology, and geochemistry. We carried out a structural, magnetic, and chemical investigation of a 4 x 10 km sheeted dike section in the Troodos ophiolite, Cyprus. On the basis of major and trace element geochemistry, we distinguish dikes that may be correlated with the basal high-Ti series (HTS) lavas from those of the overlying low-Ti series (LTS) lavas. All dikes studied are nearly parallel to each other, with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center. Anisotropy of magnetic susceptibility measurements suggests that the HTS and LTS dikes intrude in fundamentally different ways. HTS dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber. LTS dikes show a bimodal, orthogonal set of intrusive directions, one shallow and one near vertical. Near-lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system, and near-vertical dike propagation directions are probably associated with feeder dikes to down-rift surface flows. Our study suggests that the types of dike intrusive behavior in the Troodos ophiolite may also be typical for "normal" mid-ocean ridges or other major shield volcanoes with well-developed rift zones.

Gee, J, Staudigel H, Natland JH.  1991.  Geology and petrology of Jasper Seamount. Journal of Geophysical Research-Solid Earth and Planets. 96:4083-4105.   10.1029/90jb02364   AbstractWebsite

Fifteen dredges on the summit and upper flanks of Jasper Seamount (122-degrees 44'W; 30-degrees 27'N) recovered a wide variety of lithologies, including pillow lavas, vesicular lapillistones from shallow submarine explosive volcanism, and a range of xenoliths. On the basis of dredge locations, geochemical characteristics, and Ar-40/Ar-39 age data, three distinct phases of volcanism can be distinguished, a shield-building tholeiitic/transitional phase (Flank Transitional Series, FTS), followed by a flank alkalic series (FAS), and a late-stage Summit Alkalic Series (SAS). All three series consist exclusively of differentiated (Mg# = 54 to 21; Mg# = Mg2+/(Mg2+ + Fe2+)) compositions. The FTS represents a low-pressure differentiation trend from tholeiitic/transitional basalts to quartz-normative residual liquids and probably accounts for more than 90% of the volume of Jasper. Ar-40/Ar-39 age data, the dominant reversed polarity of Jasper, and a plausible duration (< 1 m.y.) for shield construction suggest FTS volcanism began about 11 Ma and ended about 10 Ma. FTS lavas probably erupted from a NW trending, hotspot track-parallel rift system. The intermediate alkalinity FAS lavas, which probably comprise 3-8% of the volume of Jasper, erupted from 8.7 to 7.5 Ma, possibly after a brief volcanic hiatus or period of reduced eruptive activity. Normative projections suggest the FAS lavas are the product of fractionation or equilibration at elevated pressures. The hawaiites and mugearites of the SAS erupted between 4.8 and 4.1 Ma, after a probable 2.7 m.y. period of volcanic quiescence, and probably constitute < 1% of the seamount volume. A suite of xenoliths incorporated in SAS lavas includes (1) tholeiitic basalt fragments from either the ocean crust or seamount interior, (2) a range of differentiated gabbros largely derived from the ocean crust, (3) residual mantle spinel lherzolites, and (4) pyroxenite and peridotite cumulates. The abundance of crustal gabbro and spinel lherzolite xenoliths in evolved lavas of the SAS suggests that these lavas probably fractionated in a magma chamber at the crust-mantle boundary. The occurrence of orthopyroxene-bearing alkalic cumulate xenoliths in these lavas, however, is enigmatic and may reflect complexities such as magma mixing or the inappropriateness of pressure estimates. The SAS vents of Jasper define a NE-SW volcanic trend which is orthogonal to the FTS rift. The pattern of volcanic activity, including periods of volcanic quiescence, and the general increase in alkalinity, as well as the structural reorganization of magmatic feeder systems of Jasper Seamount, is strikingly similar to the patterns observed on Hawaiian volcanoes. Thus our data from Jasper (690 km3) extend the concepts of structural and petrological evolution of hotspot volcanoes based on Hawaii to moderate-sized seamounts.

Blackman, DK, Karson JA, Kelley DS, Cann JR, Fruh-Green GL, Gee JS, Hurst SD, John BE, Morgan J, Nooner SL, Ross DK, Schroeder TJ, Williams EA.  2002.  Geology of the Atlantis Massif (Mid-Atlantic Ridge, 30 degrees N): Implications for the evolution of an ultramafic oceanic core complex. Marine Geophysical Research. 23:443-469.   10.1023/b:mari.0000018232.14085.75   AbstractWebsite

The oceanic core complex comprising Atlantis Massif was formed within the past 1.5-2 Myr at the intersection of the Mid-Atlantic Ridge, 30degrees N, and the Atlantis Transform Fault. The corrugated, striated central dome prominently displays morphologic and geophysical characteristics representative of an ultramafic core complex exposed via long-lived detachment faulting. Sparse volcanic features on the massif's central dome indicate that minor volcanics have penetrated the inferred footwall, which geophysical data indicates is composed predominantly of variably serpentinized peridotite. In contrast, the hanging wall to the east of the central dome is comprised of volcanic rock. The southern part of the massif has experienced the greatest uplift, shoaling to less than 700 m below sea level, and the coarsely striated surface there extends eastward to the top of the median valley wall. Steep landslide embayments along the south face of the massif expose cross sections through the core complex. Almost all of the submersible and dredge samples from this area are deformed, altered peridotite and lesser gabbro. Intense serpentinization within the south wall has likely contributed to the uplift of the southern ridge and promoted the development of the Lost City Hydrothermal Field near the summit. Differences in the distribution with depth of brittle deformation observed in microstructural analyses of outcrop samples suggest that low-temperature strain, such as would be associated with a major detachment fault, is concentrated within several tens of meters of the domal surface. However, submersible and camera imagery show that deformation is widespread along the southern face of the massif, indicating that a series of faults, rather than a single detachment, accommodated the uplift and evolution of this oceanic core complex.

Gee, JS, Cande SC, Hildebrand JA, Donnelly K, Parker RL.  2000.  Geomagnetic intensity variations over the past 780 kyr obtained from near-seafloor magnetic anomalies. Nature. 408:827-832.   10.1038/35048513   AbstractWebsite

Knowledge of past variations in the intensity of the Earth's magnetic field provides an important constraint on models of the geodynamo. A record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials, and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences. But a long-term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust. Here we show that near-seafloor magnetic anomalies recorded over the southern East Pacific Rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr. Moreover, the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well-documented dipole intensity pattern for the past 50 kyr. A comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region. The record of geomagnetic intensity preserved in the oceanic crust should provide a higher-time-resolution record of crustal accretion processes at mid-ocean ridges than has previously been obtainable.

Bowles, JA, Gee JS, Jackson MJ, Avery MS.  2015.  Geomagnetic paleointensity in historical pyroclastic density currents: Testing the effects of emplacement temperature and postemplacement alteration. Geochemistry Geophysics Geosystems. 16:3607-3625.   10.1002/2015gc005910   AbstractWebsite

Thellier-type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 Novarupta (NV) and 1980 Mt. St. Helens (MSH) pyroclastic density currents (PDCs) with the intention of evaluating their suitability for geomagnetic paleointensity studies. PDCs are common worldwide, but can have complicated thermal and alteration histories. We attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of PDCs. Results demonstrate two types of nonideal behavior: unstable remanence in multidomain (MD) titanomagnetite, and nonideal behavior linked to fumarolic and vapor phase alteration. Emplacement temperature indirectly influences MSH results by controlling the fraction of homogenous MD versus oxyexsolved pseudo-single domain titanomagnetite. NV samples are more directly influenced by vapor phase alteration. The majority of NV samples show distinct two-slope behavior in the natural remanent magnetizationpartial thermal remanent magnetization plots. We interpret this to arise from a (thermo)chemical remanent magnetization associated with vapor phase alteration, and samples with high water content (>0.75% loss on ignition) generate paleointensities that deviate most strongly from the true value. We find that PDCs can be productively used for paleointensity, but thatas with all paleointensity studiescare should be taken in identifying potential postemplacement alteration below the Curie temperature, and that large, welded flows may be more alteration-prone. One advantage in using PDCs is that they typically have greater areal (spatial) exposure than a basalt flow, allowing for more extensive sampling and better assessment of errors and uncertainty.

Kent, DV, Gee J.  1994.  Grain Size-Dependent Alteration and the Magnetization of Oceanic Basalts. Science. 265:1561-1563.   10.1126/science.265.5178.1561   AbstractWebsite

Unblocking temperatures of natural remanent magnetization were found to extend well above the dominant Curie points in samples of oceanic basalts from the axis of the East Pacific Rise. This phenomenon is attributed to the natural presence in the basalts of three related magnetic phases: an abundant fine-grained and preferentially oxidized titanomagnetite that carries most of the natural remanent magnetism, a few coarser and less oxidized grains of titanomagnetite that account for most of the high-field magnetic properties, and a small contribution to both the natural remanent magnetism and high-field magnetic properties from magnetite that may be due to the disproportionation of the oxidized titanomagnetite under sea-floor conditions. This model is consistent with evidence from the Central Anomaly magnetic high that the original magnetization acquired by oceanic basalts upon cooling is rapidly altered and accounts for the lack of sensitivity of bulk rock magnetic parameters to the degree of alteration of the remanence carrier in oceanic basalts.