Geochemistry and Origin of Pliocene and Pleistocene Ash Layers from the Iceland Plateau, Site 907 (original) (raw)
Related papers
1998
Miocene to Holocene sediments cored at Sites 918 and 919 in the Irminger Basin contain windblown ash layers derived from Icelandic volcanoes. They represent the most explosive volcanism of the rift system. In contrast, only those eruptions that produced lava flowing far from the rift zone are now exposed on Iceland due to subsidence close to the rift axis. Sediments older than 3.2 Ma contain little fresh glass, due to the hydrating effect of diagenetic fluids, although one basaltic layer dated at 10.5 Ma and three rhyolitic layers dated at 12.1−12.7 Ma were found to contain fresh glass. Major, trace, and rare earth element data have been collected from aphyric glass shards from these ash layers, using a combination of electron and ion microprobe technology. The ashes are dominantly basaltic and tholeiitic in character and are principally derived from the rift zone, with minor input from off-axis sources, probably the Snaefellsnes Peninsula. Their trace element characteristics are consistent with derivation from a mantle source similar to that below Iceland today, although the degree of this enrichment, as modeled by Zr/ Nb, can be seen to be variable over short periods of time and to have greater range than that found in exposed Icelandic lavas of the same age. The variations are not attributable to fractional crystallization. The ashes indicate that either different volcanoes were erupting compositionally variable lavas at any one time or that the composition varied rapidly with time, or both. These variations are homogenized by magma mixing prior to eruption in the large volume flows preserved in the Tertiary lava flows exposed in Iceland. Chemical variation seen in the ash layers may be due to variation in the degree of mantle melting, mantle heterogeneity, or derivation of melt from different depths within the melting column. Melting is thought to have commenced within a garnet-bearing mantle and continued in a spinel-bearing zone.
Origin of Icelandic basalts: A review of their petrology and geochemistry
Journal of Geodynamics, 2007
The petrology and geochemistry of Icelandic basalts have been studied for more than a century. The results reveal that the Holocene basalts belong to three magma series: two sub-alkaline series (tholeiitic and transitional alkaline) and an alkali one. The alkali and the transitional basalts, which occupy the off-rift volcanic zones, are enriched in incompatible trace elements compared to the tholeiites, and have more radiogenic Sr, Pb and He isotope compositions. Compared to the tholeiites, they are most likely formed by partial melting of a lithologically heterogeneous mantle with higher proportions of melts derived from recycled oceanic crust in the form of garnet pyroxenites compared to the tholeiites. The tholeiitic basalts characterise the mid-Atlantic rift zone that transects the island, and their most enriched compositions and highest primordial (least radiogenic) He isotope signature are observed close to the centre of the presumed mantle plume. High-MgO basalts are found scattered along the rift zone and probably represent partial melting of refractory mantle already depleted of initial water-rich melts. Higher mantle temperature in the centre of the Iceland mantle plume explains the combination of higher magma productivity and diluted signatures of garnet pyroxenites in basalts from Central Iceland. A crustal component, derived from altered basalts, is evident in evolved tholeiites and indeed in most basalts; however, distinguishing between contamination by the present hydrothermally altered crust, and melting of recycled oceanic crust, remains non-trivial. Constraints from radiogenic isotope ratios suggest the presence of three principal mantle components beneath Iceland: a depleted upper mantle source, enriched mantle plume, and recycled oceanic crust.
Petrogenesis of the Sólheimar Ignimbrite (Katla, Iceland): implications for tephrostratigraphy.
Geochimica et Cosmochimica Acta, 2012
The Sólheimar ignimbrite was one of the largest eruptions from the Katla caldera (Iceland) and is important for tephra studies in the North Atlantic because of its possible linkage with the Vedde Ash, a compositionally bimodal tephra layer used for correlation of sedimentary records in the North Atlantic and Northern Europe. The composition of the Sólheimar ignimbrite extends from rhyolite to basaltic-icelandite, a trend that defines a coherent magma mixing line. Mixing is evident both in mingled textures seen in hand specimens and thin sections and as binary mixing trends in major and trace element and 87Sr/86Sr isotopes of the volcanic glasses. The Sólheimar rhyolite is slightly more radiogenic than the basaltic-icelandite in terms of Sr isotopes, which is inconsistent with generation of the rhyolite by fractionation of the basaltic-icelandite. Alternatively, the Sólheimar rhyolite may have been produced by partial melting of Icelandic crust. Major and trace element modelling indicates that partial melting of Icelandic tholeiite does not replicate the observed rhyolite composition, in particular K2O is significantly lower in the modelled melt. However, partial melting of Katla alkali basalt does produce a comparable melt. We suggest a two-stage model in which 30–40% melting of basalt generated a dacitic magma which underwent subsequent ∼30% fractionation of the observed phenocryst phases (feldspar, clinopyroxene, spinel and FeTi oxide) form rhyolite. The eruption of the Sólheimar ignimbrite was triggered by the intrusion of basaltic-icelandite magma, which mixed with resident rhyolite magma during eruption. The Sólheimar ignimbrite has been linked to the Vedde Ash (Lacasse et al., 1995), a compositionally bimodal tephra layer used to link sedimentary records in the North Atlantic and Northern Europe. Despite the importance of the Vedde Ash in late Quaternary studies, its provenance remains equivocal. We demonstrate that Vedde rhyolite glasses share the same major and trace element chemistry as the Sólheimar rhyolite, carrying the implication that these deposits may be produced by the same eruption. However, the Sólheimar ignimbrite lacks the basaltic component that is sometimes associated with rhyolitic shards of the Vedde Ash at far distal locations, therefore this correlation cannot be confirmed."
The multi‐component Hekla Ö Tephra, Iceland: a complex widespread mid‐Holocene tephra layer
Journal of Quaternary Science, 2020
Large Plinian eruptions from Hekla volcano, Iceland, produce compositionally zoned tephra used as key markers in tephrochronology. However, spatial variations in chemical composition of a tephra layer may complicate its identification. An example is the 5950-6180 cal a BP Hekla Ö tephra layer, which shows compositional spread from rhyolite, dacite and andesite to basalt. In soil sections north of Hekla, the SiO 2 content of the tephra glass reaches 76 wt% in the lowest unit of the Hekla Ö deposit and decreases to 62-63 wt% in the uppermost unit. Intermingled within the whole deposit are basalt tephra grains having 46-47 wt% SiO 2. The composition of the basalt glass includes primitive basalt and a more evolved basalt (MgO >6 and <6 wt%, respectively). Together with literature data, the Hekla Ö tephra and the so-called T-Tephra/Hekla-T are most likely from contemporaneous eruptions of different vents on the Hekla volcanic system, forming a single important marker tephra (Hekla ÖT) deposited over 80% of Iceland. Identification is complicated by its spatial compositional heterogeneity, such as systematic decrease in SiO 2 content from the east to the west of Hekla volcano. Consequently, an individual tephra layer from a large explosive eruption can have different composition at different locations.
2010
pioneered the use of tephrochronology and its application in NW Europe. The basis of this chronological tool is the use of time-parallel marker tephra (i.e., ash <2mm) horizons to correlate proximal volcanic deposits with distal tephras found in marine-lacustrine-ice cores and terrestrial sequences. The trace element geochemistry of volcanic glasses [determined by LA-ICPMS] offers an improved diagnostic tool for matching the juvenile components of proximal and distal tephras and has considerable potential for assigning provenance to cryptotephras (<100 microns) provided they are of sufficient size and thickness.
Journal of Volcanology and Geothermal Research, 1996
Sediment cores containing up to twenty-five ash layers were taken at three sites close to Vesterisbanken Seamount in the Greenland Basin. These ash layers imply frequent eruptions of the volcano within the last 60 ka. The eruptions led to airborne transport and volcaniclastic turbidity flows which transported volcanic glassy and crystalline material from the volcano into the surrounding basin. During the eruption and the transport the glass and the crystal particles were mixed. The glasses range in composition between basanites and phonolites/benmoreites with MgO contents of 8 to 0.65%. The glass analyses follow a distinct trend of fractionation suggesting the crystallization of the phases olivine, clinopyroxene, plagioclase, kaersutite, Cr-spine], Ti-magnetite and apatite. A strong zonation of clinopyroxene and kaersutite phenocrysts implies mixing processes in the magma system although the liquid compositions do not lie on mixing trends. A geochemical study of the bulk ashes shows that some ash layers possess distinct chemical compositions. The ashes are more evolved than the lavas of the volcano, suggesting fractionation of liquid from crystallized material during the eruption or transport of the ashes. Sixteen layers are statistically combined into four groups, of which several can be correlated from core to core reflecting individual eruptive events.
19. The Petrology and ^Ar/ Ar Age of Tholeiitic Basalt Recovered from Hole 907A, Iceland PLATEAU1
1996
Crystalline rock recovered from the Iceland Plateau at 69°14.989'N, 12°41.894'W in August 1993, is tholeiitic basalt. Pillow structures are well defined by glassy rims grading into vesicular rinds and aphyric interiors. Some plagioclase and augite microphenocrysts are present, and olivine may have been. Pigeonite is found in cores of some augite. Major- and trace-element abundances vary indicating magma-mixing, but the nature of the magma-mixing is not known. Trace-element ratios also vary, sometimes greatly, indicating that the 'mixed magmas' may be magmas derived from different depths in a melting column, as has been proposed by other workers. The rocks differ from many typical MORB samples in that they are relatively evolved (mg = 46); however, they are strikingly similar to the majority of basalt glasses from the Kolbeinsey Ridge. Iceland Plateau basalts are T-MORBs, transitional to N-MORB and E-MORB with (La/Sm) n 0.68-0.72, Zr/Nb < 20, and a range of major- ...
Geochemical fingerprinting of Icelandic silicic Holocene tephra layers
2012
The overall aim of this research project has been to develop a reference dataset of 19 Holocene silicic Icelandic tephra layers sourced from the Torfajökull, Askja, Katla, Öræfajökull and Hekla volcanic systems. The dataset comprises geochemical data (including major, trace and rare earth element data for bulk and glass phases collected by XRF, electron microprobe, ion probe and laser ablation ICP-MS) and physical data (including sedimentary logs, field photographs, distribution maps and GPS localities of reference sections). Results indicate that Icelandic volcanic systems show unique geochemical signatures which result from the systems proximity to the active rifting zone and the proposed upwelling mantle plume that underlies the island. Within individual volcanic systems, eruptions produce tephra with distinct geochemical characteristics, which allow for the independent confirmation of tephra identity. The identification and discrimination of tephra layers can in some cases be ac...
Chemical Geology, 2010
Micron-scale analysis of vesicular volcanic glass can be problematic because thin vesicle walls and junctions limit the area available for analysis, subsurface vesicles limit the vertical thickness available, microcrysts at or below the surface may contaminate glass analyses and some glasses show compositional banding. In addition, distal tephra are very small (10–100 μm) and material may be sparse. We have analysed the MPI-DING reference glasses and natural tephra samples (pumice, scoria and fiamme) from the Thorsmörk ignimbrite (Southern Iceland) using laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS). Three different reduction strategies are used: averaging, uncertainty weighting and log-linear regression. We then assess the data quality achieved using the various strategies.Using our technique we show that the main limiting factor on data quality is precision, particularly for natural tephra analyses. At > 20,000 cps, relative standard deviations (%RSDs) in the Thorsmörk tephra are 5–10% — approximately twice those achieved in the MPI-DING glasses (3–5%) at the same conditions. Rhyolitic pumice and fiamme from the Thorsmörk ignimbrite are compositionally homogenous. The proximal deposit also contains subordinate basalt scoria, therefore the deposit is bimodal. The Thorsmörk rhyolite correlates with the North Atlantic Ash Zone 2 (NAAZ2) tephra described in a marine sediment core (Lacasse and Garbe-Schönberg, 2001, JVGR 170, 113–147).►Data reduction by averaging is the most appropriate for natural tephra analysis. ►Signal intensity is the limiting factor for tephra analysis, dictating the minimum spot size. ►The Thorsmörk ignimbrite comprises homogenous rhyolitic pumice and subordinate basaltic scoria. ►The rhyolitic Thorsmörk shows a good statistical correlation with North Atlantic Ash Zone 2.