uu.seUppsala University Publications
Change search
Link to record
Permanent link

Direct link
BETA
Krumbholz, Michael
Alternative names
Publications (10 of 13) Show all publications
Berg, S., Troll, V. R., Harris, C., Deegan, F., Riishuus, M. S., Burchardt, S. & Krumbholz, M. (2018). Exceptionally high whole-rock delta O-18 values in intra-caldera rhyolites from Northeast Iceland. Mineralogical magazine, 82(5), 1147-1168
Open this publication in new window or tab >>Exceptionally high whole-rock delta O-18 values in intra-caldera rhyolites from Northeast Iceland
Show others...
2018 (English)In: Mineralogical magazine, ISSN 0026-461X, E-ISSN 1471-8022, Vol. 82, no 5, p. 1147-1168Article in journal (Refereed) Published
Abstract [en]

The Icelandic crust is characterized by low delta O-18 values that originate from pervasive high-temperature hydrothermal alteration by O-18-depleted meteoric waters. Igneous rocks in Iceland with delta O-18 values significantly higher than unaltered oceanic crust (similar to 5.7 parts per thousand) are therefore rare. Here we report on rhyolitic intra-caldera samples from a cluster of Neogene central volcanoes in Borgarfjorour Eystri, Northeast Iceland, that show whole-rock delta O-18 values between +2.9 and +17.6 parts per thousand (n = 6), placing them among the highest delta O-18 values thus far recorded for Iceland. Extra-caldera rhyolite samples from the region, in turn, show delta O-18 whole-rock values between +3.7 and +7.8 parts per thousand (n = 6), consistent with the range of previously reported Icelandic rhyolites. Feldspar in the intra-caldera samples (n = 4) show delta O-18 values between +4.9 and +18.7 parts per thousand, whereas pyroxene (n = 4) shows overall low delta O-18 values of +4.0 to +4.2 parts per thousand, consistent with regional rhyolite values. In combination with the evidence from mineralogy and rock H2O contents, the high whole-rock delta O-18 values of the intra-caldera rhyolites appear to be the result of pervasive isotopic exchange during subsolidus hydrothermal alteration with O-18-enriched water. This alteration conceivably occurred in a near-surface hot spring environment at the distal end of an intra-caldera hydrothermal system. and was probably fed by waters that had already undergone significant isotope exchange with the country rock. Alternatively, O-18-enriched alteration fluids may have been produced during evaporation and boiling of standing water in former caldera lakes, which then interacted with the intra-caldera rock suites. Irrespective of the exact exchange processes involved, a previously unrecognized and highly localized delta O-18-enriched rock composition exists on Iceland and thus probably within the Icelandic crust too.

Place, publisher, year, edition, pages
MINERALOGICAL SOC, 2018
Keywords
Northeast Iceland, high(18)O values, hydrothermal alteration, intra-caldera lakes
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-372902 (URN)10.1180/mgm.2018.114 (DOI)000452016700011 ()
Funder
Swedish Research CouncilThe Royal Swedish Academy of Sciences
Available from: 2019-01-11 Created: 2019-01-11 Last updated: 2019-01-11Bibliographically approved
Berg, S. E., Troll, V. R., Deegan, F. M., Burchardt, S., Krumbholz, M., Mancini, L., . . . Brun, F. (2016). Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography. Bulletin of Volcanology, 78(12), Article ID 85.
Open this publication in new window or tab >>Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography
Show others...
2016 (English)In: Bulletin of Volcanology, ISSN 0258-8900, E-ISSN 1432-0819, Vol. 78, no 12, article id 85Article in journal (Refereed) Published
Abstract [en]

During the first week of the 2011 El Hierro submarine eruption, abundant light-coloured pumiceous, high-silica volcanic bombs coated in dark basanite were found floating on the sea. The composition of the light-coloured frothy material ('xeno-pumice') is akin to that of sedimentary rocks from the region, but the textures resemble felsic magmatic pumice, leaving their exact mode of formation unclear. To help decipher their origin, we investigated representative El Hierro xeno-pumice samples using X-ray computed microtomography for their internal vesicle shapes, volumes, and bulk porosity, as well as for the spatial arrangement and size distributions of vesicles in three dimensions (3D). We find a wide range of vesicle morphologies, which are especially variable around small fragments of rock contained in the xeno-pumice samples. Notably, these rock fragments are almost exclusively of sedimentary origin, and we therefore interpret them as relicts an the original sedimentary ocean crust protolith(s). The irregular vesiculation textures observed probably resulted from pulsatory release of volatiles from multiple sources during xeno-pumice formation, most likely by successive release of pore water and mineral water during incremental heating and decompression of the sedimentary protoliths.

Keywords
El Hierro, Xeno-pumice, X-CT imaging, Vesicle morphologies, Vesicle size distribution, Heterogeneous vesiculation, Sedimentary ocean crust
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-272316 (URN)10.1007/s00445-016-1080-x (DOI)000394130700001 ()
Funder
The Royal Swedish Academy of SciencesSwedish Research Council
Note

The manuscript version of this article was used under the name "Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by synchrotron μ-CT" in the following thesis: Silicic Magma Genesis in Basalt-dominated Oceanic Settings: Examples from Iceland and the Canary Islands http://uu.diva-portal.org/smash/record.jsf?pid=diva2:893923

Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2017-11-30Bibliographically approved
Mathieu, L., Burchardt, S., Troll, V., Krumbholz, M. & Delcamp, A. (2015). Geological constraints on the dynamic emplacement of cone-sheets - The Ardnamurchan cone-sheet swarm, NW Scotland. Journal of Structural Geology, 80, 133-141
Open this publication in new window or tab >>Geological constraints on the dynamic emplacement of cone-sheets - The Ardnamurchan cone-sheet swarm, NW Scotland
Show others...
2015 (English)In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 80, p. 133-141Article in journal (Refereed) Published
Abstract [en]

Cone-sheets are a significant constituent of many central volcanoes, where they contribute to volcano growth by intrusion and through flank eruptions, although the exact emplacement mechanisms are still controversially discussed. In particular, it is not yet fully resolved whether cone-sheets propagate as magma-driven, opening-mode fractures or as shear fractures, and to what extent pre-existing host-rock structures and different stress fields influence cone-sheet emplacement. To shed further light on the role of these parameters in cone-sheet emplacement, we use detailed field and remote sensing data of the classic Ardnamurchan cone-sheet swarm in NW-Scotland, and we show that the cone-sheets primarily propagated as opening-mode fractures in the sigma(1)-sigma(2) plane of the volcanic stress field. In addition, more than one third of the Ardnamurchan cone-sheet segments are parallel to lineaments that form a conjugate set of NNW and WNW striking fractures and probably reflect the regional NW SE orientation of sigma(1) during emplacement in the Palaeogene. Cone-sheets exploit these lineaments within the NE and SW sectors of the Ardnamurchan central complex, which indicates that the local volcanic stress field dominated during sheet propagation and only allowed exploitation of host-rock discontinuities that were approximately parallel to the sheet propagation path. In addition, outcrop-scale deflections of cone-sheets into sills and back into cone-sheets (also referred to as "staircase" geometry) are explained by the interaction of stresses at the propagating sheet tip with variations in host-rock strength, as well as the influence of sheet-induced strain. As a consequence, cone-sheets associated with sill-like segments propagate as mixed-mode I/II fractures. Hence, cone-sheet emplacement requires a dynamic model that takes into account stress fields at various scales and the way propagating magma interacts with the host rock and its inherent variations in rock strength.

Keywords
Cone-sheets, Sheet emplacement, Stress field, Host-rock structure, Ardnamurchan
National Category
Earth and Related Environmental Sciences Geosciences, Multidisciplinary Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-269255 (URN)10.1016/j.jsg.2015.08.012 (DOI)000364245100011 ()
Funder
Swedish Research Council
Available from: 2015-12-15 Created: 2015-12-15 Last updated: 2017-12-01Bibliographically approved
Berg, S., Troll, V., Burchardt, S., Deegan, F., Riishuus, M. S., Whitehouse, M. J., . . . Gústafsson, L. E. (2015). Rapid high-silica magma generation in basalt-dominated rift settings. In: : . Paper presented at EGU General Assembly 2015 (pp. 13103). , 17
Open this publication in new window or tab >>Rapid high-silica magma generation in basalt-dominated rift settings
Show others...
2015 (English)Conference paper, Oral presentation with published abstract (Other academic)
National Category
Geosciences, Multidisciplinary Geochemistry
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-270976 (URN)
Conference
EGU General Assembly 2015
Available from: 2016-01-05 Created: 2016-01-05 Last updated: 2016-01-05
Kosminska, K., Majka, J., Mazur, S., Krumbholz, M., Klonowska, I., Manecki, M., . . . Dwornik, M. (2014). Blueschist facies metamorphism in Nordenskiold Land of west-central Svalbard. Terra Nova, 26(5), 377-386
Open this publication in new window or tab >>Blueschist facies metamorphism in Nordenskiold Land of west-central Svalbard
Show others...
2014 (English)In: Terra Nova, ISSN 0954-4879, E-ISSN 1365-3121, Vol. 26, no 5, p. 377-386Article in journal (Refereed) Published
Abstract [en]

Recent fieldwork in Nordenskiold Land, Svalbard's Southwestern Basement Province, has established the presence of high-pressure (HP) lithologies. They are strongly retrogressed blueschists consisting mainly of garnet and Ca-amphibole with remnants of ferroglaucophane and phengite. The pressure-temperature (P-T) conditions were estimated using phase equilibrium modelling in the NCKFMMnASHTO system. P-T estimates based on the garnet, phengite and ferroglaucophane compositional isopleths and modelled paragenetic assemblage indicate peak metamorphism at 470-490 degrees C and 14-18 kbar. These data fall close to the 7-8 C km(-1) geo-therm, which is similar to that from Motalafjella, the only previously known occurrence of blueschists in Svalbard's Caledonides. The newly discovered blueschists could have formed during the early stage of the Caledonian Orogeny and may represent a vestige of missing marginal basins of the western Iapetus developed at the onset of subduction. The likely counterpart to Svalbard's blueschists is the ophiolitic sequence in the Pearya Terrane of northern Ellesmere Island.

National Category
Geology Geosciences, Multidisciplinary
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-232984 (URN)10.1111/ter.12110 (DOI)000341254900006 ()
Available from: 2014-10-14 Created: 2014-09-29 Last updated: 2017-12-05Bibliographically approved
Berg, S., Troll, V., Burchardt, S., Riishuus, M., Krumbholz, M. & Gústafsson, L. (2014). Iceland's best kept secret. Geology Today, 30(2), 54-60
Open this publication in new window or tab >>Iceland's best kept secret
Show others...
2014 (English)In: Geology Today, ISSN 0266-6979, E-ISSN 1365-2451, Vol. 30, no 2, p. 54-60Article in journal (Refereed) Published
Abstract [en]

The ‘forgotten fjords’ and ‘deserted inlets’ of NE-Iceland, in the region between Borgarfjörður Eystri and Loðmundarfjörður, are not only prominent because of their pristine landscape, their alleged elfin settlements, and the puffins that breed in the harbour, but also for their magnificent geology. From a geological point of view, the area may hold Iceland's best kept geological secret. The greater Borgarfjörður Eystri area hosts mountain chains that consist of voluminous and colourful silicic rocks that are concentrated within a surprisingly small area (Fig. 1), and that represent the second-most voluminous occurrence of silicic rocks in the whole of Iceland. In particular, the presence of unusually large volumes of ignimbrite sheets documents extremely violent eruptions during the Neogene, which is atypical for this geotectonic setting. As a group of geoscientists from Uppsala University (Sweden) and the Nordic Volcanological Center (NordVulk, Iceland) we set out to explore this remote place, with the aim of collecting material that may allow us to unravel the petrogenesis of these large volumes of silicic rocks. This effort could provide an answer to a long-standing petrological dilemma; the question of how silicic continental crust is initially created. Here we document on our geological journey, our field strategy, and describe our field work in the remote valleys of NE-Iceland.

National Category
Geosciences, Multidisciplinary
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-213020 (URN)DOI: 10.1111/gto.12042 (DOI)
Available from: 2013-12-17 Created: 2013-12-17 Last updated: 2017-12-06
Krumbholz, M., Vollbrecht, A. & Aschoff, M. (2014). Recent horizontal stress directions in basement rocks of southern Sweden deduced from open microcracks. Journal of Structural Geology, 65, 33-43
Open this publication in new window or tab >>Recent horizontal stress directions in basement rocks of southern Sweden deduced from open microcracks
2014 (English)In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 65, p. 33-43Article in journal (Refereed) Published
Abstract [en]

The strike direction of open intragranular microcracks in quartz and feldspar host grains was determined using optical transmission and reflection microscopy on eight oriented samples taken in two study areas in Precambrian basement rocks of southern and south-central Sweden. For an area of about 160 km(2) (SW of Vastervik) and two sample locations (W of Uppsala), the vast majority of open microcracks displays a strong preferred NW-SE strike direction. According to the common assumptions that natural cracks in crystalline rocks are predominantly extensional (mode I), and that open cracks belong to the latest microcrack generation, these strike directions should reflect the (sub-) recent main horizontal stress direction (sigma H) of the recent tectonic stress field. This conclusion is supported by corresponding directions known from in situ stress measurements and focal plane solutions in the vicinity of the study areas. It is remarkable that even in samples taken close (i.e. a few hundred metres) to recently active large scale faults the orientation of microcracks does not deviate from this common direction. This may point to slip on already softened faults, very local stress reorientations (e.g. m-scale) or that local stress relief was accomplished by other processes at microscale, e.g. mechanical twinning in favourably oriented feldspar crystals, or slip on grain boundaries.

Keywords
Microcracks, Stress directions, Precambrian, Sweden
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-228948 (URN)10.1016/j.jsg.2014.03.006 (DOI)000337778800003 ()
Available from: 2014-08-11 Created: 2014-07-24 Last updated: 2017-12-05Bibliographically approved
Krumbholz, M., Hieronymus, C., Burchardt, S., Troll, V., Tanner, D. & Friese, N. (2014). Weibull-distributed dyke thickness reflects probabilistic character of host-rock strength. Nature Communications, 5, 3272
Open this publication in new window or tab >>Weibull-distributed dyke thickness reflects probabilistic character of host-rock strength
Show others...
2014 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, p. 3272-Article in journal (Refereed) Published
Abstract [en]

Magmatic sheet intrusions (dykes) constitute the main form of magma transport in the Earth’s crust. The size distribution of dykes is a crucial parameter that controls volcanic surface deformation and eruption rates and is required to realistically model volcano deformation for eruption forecasting. Here we present statistical analyses of 3,676 dyke thickness measurements from different tectonic settings and show that dyke thickness consistently follows the Weibull distribution. Known from materials science, power law-distributed flaws in brittle materials lead to Weibull-distributed failure stress. We therefore propose a dynamic model in which dyke thickness is determined by variable magma pressure that exploits differently sized host-rock weaknesses. The observed dyke thickness distributions are thus site-specific because rock strength, rather than magma viscosity and composition, exerts the dominant control on dyke emplacement. Fundamentally, the strength of geomaterials is scale-dependent and should be approximated by a probability distribution.

Keywords
dykes, Weibull distribution, volcano tectonics
National Category
Geosciences, Multidisciplinary
Research subject
Geophysics with specialization in Solid Earth Physics; Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-218648 (URN)10.1038/ncomms4272 (DOI)000332667600019 ()
Available from: 2014-02-13 Created: 2014-02-13 Last updated: 2017-12-06Bibliographically approved
Krumbholz, M. & Burchardt, S. (2013). Qualitative and quantitative analyses of magmatic stoping in the roof of the Proterozoic Åva ring complex. In: : . Paper presented at EGU General Assembly 2013 (pp. 4795).
Open this publication in new window or tab >>Qualitative and quantitative analyses of magmatic stoping in the roof of the Proterozoic Åva ring complex
2013 (English)Conference paper, Poster (with or without abstract) (Refereed)
Abstract [en]

Daly (1903) defined magmatic stoping as magma emplacement due to the detachment of blocks of magma-chamberroof- and wall rocks and their incorporation into the magma chamber. Stoping itself involves a number of interrelated processes, e.g. hydraulic fracturing, partial melting, and explosive exfoliation, that are a product of the complex thermal, mechanical, and chemical interaction of magma and the country rocks. However, the individual processes, as well as the influence of the main controlling parameters, are poorly understood. This makes it difficult to quantify the contribution ofmagmatic stoping as a magma-emplacement process, which has resulted in vigorous debates about its efficiency and overall significance. To resolve this controversy, detailed, qualitative and quantitative studies to better understand the involved processes and the interaction of forces are essential. We studied strongly foliated amphibolite-facies volcaniclastic metasedimentary rocks that were intruded by granitic magmas of the Åva ring complex (Finland), a 1.76 Ga intrusion which formed at 5 to 6 km depth (Eklund and Shebanov, 2005). In the roof region of the main intrusion, the country rock is strongly fragmented and incorporated into the granite as xenoliths ranging in size (area) from tens of m2 to mm2. We systematically recorded subhorizontal, glacially polished coastal outcrops that contain thousands of xenoliths. The xenoliths show signs of brittle deformation resulting in intense fragmentation caused by the intrusion of granitic veins and dyklets, i.e. the fragments are angular. Bigger blocks are often split along the foliation and are surrounded by a cloud of smaller blocks. In many places, the blocks still fit to each other like a jig saw puzzle, while in other domains, they appear to have tumbled around. In contrast, some outcrops contain rounded xenolithic blocks that show signs of ductile deformation. From the outcrop maps, we carefully recorded all xenoliths to determine their size, orientation, and shape. In addition, we measured the strike of the internal foliation in relation to the undisturbed country rock for each individual xenolith. The spatial xenolith distribution pattern and the close assemblage of fragments of a wide range of sizes indicate that stoping is a rapid and efficient process. The size distribution closely resembles a power-law distribution over several orders of magnitude, even if modified by stereographic effects. The results of the shape analysis indicate that the fragmentation process is strongly controlled by the host-rock foliation, expressed in alternating aspect ratios with respect to the xenolith size. First fragmentation occurs parallel to the foliation, resulting in high aspect ratios of large xenoliths. Further fragmentation reduces block aspect ratios cracking the blocks perpendicular to their long axis, before fragmentation parallel to the foliation becomes dominant again, producing small blocks with high aspect ratios. References Daly, R. A., 1903. The mechanics of igneous intrusion. American Journal of Science 15, 269- 298. Eklund, O. and Shebanov, A. D., 2005. Prolonged postcollisional shoshonitic magmatism in the southern Svecofennian domain - a case study of the Åva granite-lamprophyre ring complex

National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-213779 (URN)
Conference
EGU General Assembly 2013
Available from: 2014-01-03 Created: 2014-01-03 Last updated: 2017-01-25
Krumbholz, M., Bock, M., Burchardt, S., Kelka, U. & Vollbrecht, A. (2012). A critical discussion of the electromagnetic radiation (EMR) method to determine stress orientations within the crust. Solid Earth, 3(2), 401-414
Open this publication in new window or tab >>A critical discussion of the electromagnetic radiation (EMR) method to determine stress orientations within the crust
Show others...
2012 (English)In: Solid Earth, ISSN 1869-9510, E-ISSN 1869-9529, Vol. 3, no 2, p. 401-414Article in journal (Refereed) Published
Abstract [en]

In recent years, the ElectroMagnetic Radiation (EMR) method has been used to detect faults and to determine main horizontal stress directions from variations in intensities and directional properties of electromagnetic emissions, which are assumed to be generated during micro-cracking. Based on a large data set taken from an area of about 250 000 km2 in Northern Germany, Denmark, and Southern Sweden with repeated measurements at one location during a time span of about 1.5 yr, the method was systematically tested. Reproducible observations of temporary changes in the signal patterns, as well as a strongly concentric spatial pattern of the main directions of the magnetic component of the EMR point to VLF transmitters as the main source and hence raise serious concerns about the applicability of the method to determine recent crustal stresses. We conclude that the EMR method, at its current stage of development, does not allow determination of the main horizontal stress directions.

National Category
Earth and Related Environmental Sciences Geology
Identifiers
urn:nbn:se:uu:diva-190682 (URN)10.5194/se-3-401-2012 (DOI)000321443000005 ()
Available from: 2013-01-08 Created: 2013-01-08 Last updated: 2017-12-06Bibliographically approved
Organisations

Search in DiVA

Show all publications