Carsten Vahle 1 , Agnes Kontny 1 , Frank Dietze 1 , H. Audunsson 2 1 Geologisch-Paläontologisches Institut, University Heidelberg, INF 234, 69120 Heidelberg, Germany 2 Technical University of Iceland, Hofdabakka 9, 110 Reykjavik, Iceland [email protected] Acknowledgements We would like to thank different people at ISOR for their kind helpfulness. We’re grateful to the DFG, which funds this research (Ko1514/3). References: Fridleifsson, G.O. (2005): REYKJANES Well Report RN-17 & RN-17ST, Iceland Geosurvey internal report, Reykjavik, Iceland. Jakobsson, S.P., Jonsson, J., and F. Shido (1978): Petrology of the Western Reykjanes Peninsula, Iceland. Journal of Petrology, 19, 669-705. Karlsdottir, R. (2005): TEM-mælingar á Reykjanesi 2004, Iceland Geosurvey internal report ÍSOR-2005/002, Reykjavik, Iceland. Kiss, J., Szarka, L., and E. Prácser (2005): Second order magnetic phase transition in the Earth. Geophysical Research Letters, 32, L24310, doi: 24310.21029/22005GL024199. Saemundsson, K. (1995): Svartsengi, Eldvörp and Reykjanes geological map (bedrock) 1:25000. Orkustofnun, Hitaveita Sudurnesja and Landmaelingar Iceland. Sigurgeirsson, Th. (1975): Unpublished aeromagnetic maps. Introduction Results Krafla Krafla Stardalur Stardalur Reykjanes Reykjanes drill site drill site http://members.tripod.com/~AntonBerger/is_maps.htm profile 1 profile 2 profile 3 the field magnetic measurements show similarities with the aeromagnetic survey but reveal local anomalies of smaller scales especially the younger lava flows, where the marginal scoriaceous parts of the flows are not eroded yet, contribute to high magnetic field intensities due to their high remanent magnetization at deeper crustal levels or in lava flows with elevated temperatures (e.g. near geothermal fields), respectively, the induced component of total field magnetization could increase severely as temperatures near the susceptibility peak associated with Ti-rich titanomagnetite or titanomaghemite are reached (e.g. Kiss et al. 2005) the contribution of intrusions like the dolerite dike to the total magnetic field seems to affect mainly the induced component the different lava origins (picrite basalt lava shields, olivine tholeiite lava shields and tholeiite fissure lavas) show distinctly different magnetic properties, which could be related to magma composition; differences with regard to extrusion conditions and cooling history are the subject of further studies... Conclusions 0 5 10 15 20 25 30 35 40 0 10 20 30 40 susceptibility [10E-03 SI] NRM [A/m] Stampahraun 4 Lava north of gunna Syrfell fissure Skalafell Haleyjabunga picrite pillow lava, hyalocl. RN-19 drilling RN7 RN7 RN RN 13 13 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 susceptibility [10E-03 SI] Q Stampahraun 4 Lava north of gunna Syrfell fissure Skalafell Haleyjabunga picrite pillow lava, hyalocl. RN-19 drilling RN7 RN7 RN RN 13 13 0 5 10 15 20 25 30 35 40 10 15 20 25 30 35 weight [g] NRM [A/m] Stampahraun 4 Lava north of gunna Syrfell fissure Skalafell Haleyjabunga picrite pillow lava, hyalocl. RN-19 drilling RN7 RN7 RN RN 13 13 highly vesicular, lower abun- dance but larger (2 - 8 mm diam.) extremely vesicle- rich, 1 - 3 mm diam., almost glassy matrix Crustal magnetization and magnetic petrology from the IDDP-drilling RN-19 and its surrounding on the Reykjanes peninsula (Iceland) - discrimination of different volcanic rock units The geothermal field at Reykjanes peninsula is located at the boundary where the submarine Reykjanes Ridge passes over into the rift zone of southwestern Iceland. The geothermal field coincides with a magnetic low in the aeromagnetic anomaly map and is situated within a dense NE-SW fissure and fault zone. Surface geology is characterized by different historic fissure eruptions (youngest from 1226 AD), shield lava (12.5 – 14.5 ky) and intercalated pillow basalt – hyalo- clastite ridges probably formed during the last glacial episode (14.5 – 20 ky). During a field magnetic study in the vicinity of the geothermal field in summer 2005 different volcanic rock units have been sampled to correlate rock magnetic and magneto-mineralogical properties with magnetic field intensity. Additionally, measurements on a dense dolerite intrusion, recovered from the RN-19 borehole (2245 – 2248 m depth) in May 2005 within the frame of IDDP, should shed light on the influence of crustal rocks on the total magnetic field intensity. Generally, the natural remanent magnetization and magnetic susceptibility, measured on rock specimen, is high. The high NRM coincides with the magnetic high outside the geothermal field. The Koenigsberger ratios (Q) are also high for all surface samples, indicating the predominance of remanent magnetization. Most of the study area is covered by strongly magnetic Stampahraun and Skalafell pahoehoe and block lava stemming from fissure eruptions. The rock magnetic characteristics of theses flows are quite similar, whereas the older flow (Skalafell) shows stronger scattering. The pillow lava and especially the picritic Haleyjabunga shield lava show lower NRM intensity and magnetic susceptibility, whereas especially for the shield lavas this could be related to less Ti and total Fe in the magma, therefore constricting crystallization of Fe-Ti oxides. By contrast, the highly magnetic fissure lavas have slightly higher Ti- and total Fe- contents (Jakobsson et al. 1978). For the NRM intensity, a weak dependence on the weight (used as approximation for vesicularity) can be observed. The highly vesicular, more scoriaceous samples (lower weight) have higher NRM than massive lava. Therefore, the marginal parts of lava flows, where the lava cools and solidifies faster than in the interior (resulting in abundant Fe-Ti oxides with small grain sizes, few µm), excite high NRM and magnetic field intensities, respectively. The NRM of the doleritic dike sample from RN-19 drilling is rather low but susceptibility is high, indicating large grain sizes, formed during typically slow cooling of a intrusion. Combination map (modified from Fridleifsson 2005) of surface topography, surface structural interpretations and geothermal manifestations (from K. Sæmundsson, pers. com., revised maps), and an airborne magnetic survey from Th. Sigurgeirsson (1975) with an acquisition flight hight of 150 m above sea-level, and an interpretation of the extents of the geothermal system at Reykjanes based on a recent (2004) TEM survey (Karlsdóttir 2005). Additionally the magnetometer profiles and sample locations are shown. Geologic map of Svartsengi and surroundings; shown are the three profiles measured with a GSM-19TG proton precession magnetometer with sample locations (modified Saemundsson, 1975). First temperature dependent magnetic susceptibility data indicate homogeneous Ti-rich (T c = 60 - 240 °C) and Ti-poor titanomagnetite (T c = 350 - 520 °C), and magnetite (T c = 580 °C). Partly, titanomagnetite has been oxidized to titanomaghemite with T c ranging between 320 and 510 °C. The occurrence of magnetite and the low-temperature behavior of k(T) curves below –150 °C indicate exsolution textures typically forming during high- temperature oxidation, e.g. at slowly cooling parts of a lava flow or in an intrusion (see sample from RN-19 drilling). The MDF (median destructive field), derived from alternating field demagnetization of NRM, varies between 16 and 31 mT. This indicates small grain sizes (< 10 µm) and/or oxi- dized titanomagnetite, respec- tively. Crustal magnetization and magnetic petrology from the IDDP-drilling RN-19 and its surrounding on the Reykjanes peninsula (Iceland) - discrimination of different volcanic rock units Rock magnetic properties of Reykjanes surface samples and Rock magnetic properties of Reykjanes surface samples and from the RN-19 drilling from the RN-19 drilling: 1 km N 63°49’ N 63°50’ N 63°51’ N 63°48’ N 22°44’ W 22°42’ W 22°40’ W Stampahraun 4, 13th cent. remnant of tuff cone Stampahraun 3, 2000-3000 a Stampshraun Lava north of gunna Stampahraun 1,2 lavas 8000-3000 a Syrfell fissure Melshraun Skalafell lavas 11500-8000 a Sandfellshod ca. 12500 a Haleyjabunga picrite 14500-12500 a shield lava ridges 20000-14500 a lava cover hyaloclastite breccia pillow lava, hyalo. tuff fissure crater lava channel lava margin fault steam vent altered ground sample A A’ B’ C’ C B profile 1 profile 2 profile 3 Tc = 318°C/493°C Tc = 560°C/576°C 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 -200 -100 0 100 200 300 400 500 600 700 normalized susceptibility RN7-1 Skalafell temperature [°C] Tc = 539°C/ 586°C Tc = 50°C/154°C Tc = 432°C/515°C 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -200 -100 0 100 200 300 400 500 600 700 normalized susceptibility RN13-1 Syrfell fissure Tc = 547°C/590°C 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -200 -100 0 100 200 300 400 500 600 700 normalized susceptibility RN-19 2246.16 dolerite dike 0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 120 140 160 J/Jmax MDF = 24 mT NRM = 12.0 A/m RN13-1 Syrfell fissure 0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 120 140 160 magnetic field [mT] J/Jmax NRM = 33.6 A/m MDF = 17 mT RN7-1 Skalafell strongly influenced by topographic features, causing big scattering, but also some local anomalies can be observed, which are not displayed by the aeromagnetic survey (see map above) Geomagnetic field intensity Geomagnetic field intensity relative to the regional field at Reykjanes relative to the regional field at Reykjanes (measured in field with proton precession magnetometer, correcte (measured in field with proton precession magnetometer, correcte d for daily secular variation and smoothed, IGRF10) d for daily secular variation and smoothed, IGRF10) Reykjanes - profile 2 -6000 -4000 -2000 0 2000 4000 6000 0 500 1000 1500 2000 2500 3000 3500 profile meter [m] field intensity [nT] fissure pillow / hyaloclasite ridge Háleyjabunga crater fault fault NW SE Reykjanes - profile 3 -6000 -4000 -2000 0 2000 4000 6000 0 500 1000 1500 2000 2500 3000 3500 profile meter [m] field intensity [nT] Eldborg grynnnri crater valley fault NW SE Reykjanes - profile 1 -6000 -4000 -2000 0 2000 4000 6000 -200 300 800 1300 1800 2300 2800 profile meter [m] field intensity [nT] fault Skalafell crater discharge of steam valley pillowmount older Flow NW SE local depression east of the fissure at ca. 3200 m fault in the eastern part at ca. 2700 m tumulus in the eastern part at ca. 3200 m