Juliane Arnold

Facies reconstruction of a hydrothermally altered dacite extrusive sequence: Evidence from geophysical downhole logging data (ODP Leg 193)

ODP Leg 193 drilled into the PACMANUS hydrothermal field (Papua New Guinea), which is an activehydrothermal vent field associated with felsic magmatism in a convergent geodynamic setting. ThePACMANUS hydrothermal field is part of the eastern Manus Basin and is located near the crest of PualRidge, a 500 to 700 meters high felsic neo-volcanic ridge. Two sites, Snowcap (Site 1188) and RomanRuins (Site 1189), were drilled approximately 800 m apart. Although low core recovery limited theexamination of the subsurface geology, geophysical downhole measurements provided continuous recordsof in-situ physical properties with depth and were used to reconstruct lithostratigraphic profiles. Downholelogging operations included standard wire line logging and Logging While Drilling (LWD) measurements.Electrical resistivity images of the borehole wall were examined to find characteristic textures and threedifferent volcanic facies were distinguished: coherent dacite, volcaniclastic dacite and brecciated dacite. Inaddition, intervals with sulfide mineralization were identified based on characteristic responses of thephotoelectric factor log. A comparison of the reconstructed profiles from both sites shows considerabledifferences in the proportions of facies and the thickness of individual layers. Thick units of coherent faciesare predominant at Snowcap and indicate a proximal position of the site within the volcanic system of PualRidge. At Roman Ruins, thin individual layers with rapid changes in volcanic facies and a higherproportion of volcaniclastic and brecciated dacite suggest a medial position. These differences in volcanicfacies show that the volcanic architecture at Pual Ridge is characterized by small-scale facies changes,emplacement of small-volume individual lava flows and complex geometries of individual emplacementunits. Geophysical logging data suggest that subseafloor hydrothermal activity is focused to the areaaround one hole at Roman Ruins. Downhole logs from this hole show characteristic cyclic patterns in theelectrical resistivity and photoelectric factor logs indicating that hydrothermal sulfide mineralization isconcentrated at the tops of individual lava flows representing paleo-seafloor positions.

Mobile NMR for porosity analysis of drill core sections

We apply a novel mobile nuclear magnetic resonance (NMR) scanning system, the NMR-MOUSE? (NMR-MOUSE (Nuclear Magnetic Resonance Mobile Universal Surface Explorer) is a registered trademark of RWTH Aachen University), for measuring porosity of geological drill core sections. The NMR-MOUSE? is used for transverse relaxation measurements on water-saturated core sections using a CPMG sequence with a short echo time. A regularized Laplace-transform analysis by the UPEN program yields the distribution of transverse relaxation times. The signal amplitudes and the distribution integrals correlate directly with the porosity of the cores, in spite of the influence of diffusion in the strong field gradient of the NMR-MOUSE?, which is discussed. The method is particularly attractive because it neither requires a volume calibration nor the samples to be machined to fit the coil, and because the device is mobile and particularly attractive for field use such as on logging platforms and research vessels.