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
Carsten Vahle 1, Agnes Kontny 1, Frank Dietze 1, H. Audunsson 2
1
2
Geologisch-Paläontologisches Institut, University Heidelberg, INF 234, 69120 Heidelberg, Germany
Technical University of Iceland, Hofdabakka 9, 110 Reykjavik, Iceland
[email protected]
Introduction
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 – hyaloclastite 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.
lavas 8000-3000 a
Stampahraun 4, 13th cent.
remnant of tuff cone
Stampahraun 3, 2000-3000 a
crater
lava channel
fissure
lava margin
lavas 11500-8000 a
Stampshraun
Lava north of gunna
Stampahraun 1,2
shield lava
Syrfell fissure
Melshraun
Skalafell
ridges 20000-14500 a
Sandfellshod
ca. 12500 a
Haleyjabunga picrite
14500-12500 a
lava cover
hyaloclastite breccia
pillow lava, hyalo. tuff
profile 2
altered ground
fault
steam vent
sample
63°51’ N
Krafla
profile 1
N
profile 3
profile 2
B
profile 1
Stardalur
A
63°50’ N
drill site
Reykjanes
profile 3
C
http://members.tripod.com/~AntonBerger/is_maps.htm
63°49’ N
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.
B’
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).
A’
1 km
C’
63°48’ N
22°44’ W
22°42’ W
22°40’ W
Results
Geomagnetic field intensity relative to the regional field at Reykjanes
Rock magnetic properties of Reykjanes surface samples and
from the RN-19 drilling :
40
RN7
Reykjanes - profile 2
20
15
2000
fault
fault
0
3500
3000
2500
2000
1500
1000
500
0
Háleyjabunga
crater
-2000
pillow /
hyaloclasite
ridge
R N 13
-4000
5
profile meter [m]
-6000
east of the fissure at ca. 3200 m
0
0
10
20
30
susceptibility [10E-03 SI]
100
40
6000
Stampahraun 4
Lava north of gunna
Syrfell fissure
Skalafell
Haleyjabunga picrite
pillow lava, hyalocl.
RN-19 drilling
90
80
70
Reykjanes - profile 1
SE
Skalafell
crater
4000
pillowmount
R N 13
60
NW
RN7
Q
50
2000
older Flow
0
-200
300
-2000
800
1300
1800
2300
valley
local depression
40
2800
fault
discharge of steam
-4000
30
20
profile meter [m]
-6000
fault in the eastern part at ca. 2700 m
10
6000
NW
0
10
20
30
susceptibility [10E-03 SI]
4000
Stampahraun 4
Lava north of gunna
Syrfell fissure
Skalafell
Haleyjabunga picrite
pillow lava, hyalocl.
RN-19 drilling
RN7
35
extremely vesiclerich, 1 - 3 mm diam.,
almost glassy matrix
30
25
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.
Reykjanes - profile 3
SE
40
field intensity [nT]
0
40
NRM [A/m]
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.
SE
fissure
25
10
NW
4000
field intensity [nT]
30
6000
field intensity [nT]
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 Fecontents (Jakobsson et al. 1978).
35
NRM [A/m]
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.
(measured in field with proton precession magnetometer, correcte d for daily secular variation and smoothed, IGRF10)
Stampahraun 4
Lava north of gunna
Syrfell fissure
Skalafell
Haleyjabunga picrite
pillow lava, hyalocl.
RN-19 drilling
Eldborg grynnnri
crater
fault
2000
0
0
500
1000
1500
2000
2500
3000
3500
-2000
20
valley
-4000
15
R N 13
5
profile meter [m]
-6000
highly vesicular,
lower abundance but larger
(2 - 8 mm diam.)
10
tumulus in the eastern part at ca. 3200 m
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)
0
10
RN-19 2246.16
dolerite dike
1.4
15
20
25
30
35
weight [g]
normalized susceptibility
1.2
1.0
0.8
0.6
Tc = 547°C/590°C
0.4
0.2
0.0
-200
-100
0
100
200
300
400
500
Tc = 50°C/154°C
1.2
1.0
Tc = 432°C/515°C
J/Jmax
normalized susceptibility
700
RN13-1
Syrfell fissure
1.4
0.8
0.6
0.4
0.0
-200
0.0
-100
0
100
200
300
400
500
1.8
600
0
700
20
40
60
80
100
120
1.0
RN7-1
Skalafell
1.6
140
160
RN7-1
Skalafell
NRM = 33.6 A/m
0.8
1.4
Tc = 318°C/493°C
1.2
0.6
J/Jmax
1.0
0.8
MDF = 17 mT
0.4
0.6
0.4
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 oxidized titanomagnetite, respectively.
Tc = 560°C/576°C
0.0
-200
0.0
-100
0
100
200
300
temperature [°C]
400
500
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)
600
700
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...
References:
0.2
0.2
Conclusions
0.2
Tc = 539°C/
586°C
0.2
normalized susceptibility
600
First temperature dependent magnetic susceptibility data indicate
homogeneous Ti-rich (Tc = 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
1.0
RN13-1
NRM = 12.0 A/m
Syrfell fissure
typically forming during high0.8
temperature oxidation, e.g. at
0.6
slowly cooling parts of a lava
MDF = 24 mT
flow or in an intrusion (see
0.4
sample from RN-19 drilling).
0
20
40
60
80
100
magnetic field [mT]
120
140
160
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).
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.