Sveinn P. Jakobsson

Nd and Sr isotope ratios and rare earth element abundances in Reykjanes Peninsula basalts evidence for mantle heterogeneity beneath Iceland

Post-glacial tholeiitic basalts from the western Reykjanes Peninsula range from picrite basalts (oldest) to olivine tholeiites to tholeiites (youngest). In this sequence there are large systematic variations in rare earth element (REE) abundances (La/Sm normalized to chondrites ranges from 0.33 in the picrite basalts to 1.25 in the fissure tholeiites) and corresponding variations in 143Nd/144Nd (0.51317 in the picrite basalts to 0.51299 in the fissure tholeiites). The large viaration in 143Nd/144Nd, more than one-third the total range observed in most ocean islands and mid-ocean ridge basalts (MORB), is accompanied by only a small variation in 87Sr/86Sr (0.7031–0.7032). These 87Sr/86Sr ratios are within the range of other Icelandic tholeiites, and distinct from those of MORB. We conclude that the mantle beneath the Reykjanes Peninsula is heterogeneous with respect to relative REE abundances and 143Nd/144Nd ratios. On a time-averaged basis all parts of this mantle show evidence of relative depletion in light REE. Though parts of this mantle have REE abundances and Nd isotope ratios similar to the mantle source of “normal” MORB, 87Sr/86Sr is distinctly higher. Unlike previous studies we find no evidence for chondritic relative REE abundances in the mantle beneath the Reykjanes Peninsula; in fact, the data require significant chemical heterogeneity in the hypothesized mantle plume beneath Iceland, as well as lateral mantle heterogeneity from the Reykjanes Ridge to the Reykjanes Peninsula. The compositional range of the Reykjanes Peninsula basalts is consistent with mixing of magmas produced by different degrees of melting in different parts of the heterogeneous mantle source beneath the Reykjanes Peninsula.

Chemistry and distribution pattern of recent basaltic rocks in Iceland

Abstract A survey of Recent basaltic rocks in Iceland is presented. The basalts are classified into three groups: tholeiites, transitional alkali basalts and alkali olivine basalts. The basalts can be divided into petrological regions where the composition of lavas seem to have been fairly constant throughout postglacial and possibly late-Pleistocene time. The tholeiites delineate the crest region of the Mid-Atlantic Ridge as it transects Iceland, and the mildly alkali olivine basalts and the transitional alkali basalts characterize the flank volcanic zones. Tholeiitic and alkalic diffrentiated rocks appear to have a distribution in accordance with the basalt distribution pattern. There is some correlation between the chemistry of the zones and the crustal structure of Iceland. Areal discharge of volcanic rocks varies consistently between the petrological regions being highest in the tholeiite regions. The total output of volcanic rocks along the Mid-Atlantic Ridge in the Iceland area reaches maximum in middle Iceland.

Late Pleistocene geomagnetic excursion in Icelandic lavas : Confirmation of the Laschamp excursion

In 1980 Kristjansson and Gudmundsson [1] reported a late glacial geomagnetic excursion in three hills in the Reykjanes peninsula, Iceland, with shallow negative inclinations and westerly declinations. They named it the Skalamaelifell excursion. More extensive field work has identified the same excursional paleomagnetic direction (declination= 258°,inclination= −15°) at four additional outcrops in a 10 × 10 km area in the Reykjanes peninsula. The excursion lavas are olivine tholeiites with similar petrography and chemical compositions. Paleointensity determinations by the Thellier method average 4.2 ± 0.2 μT for 8 samples, more than an order of magnitude weaker than the present geomagnetic field in Iceland. Together, these results suggest extrusion of the excursion lavas in a very brief span of time, probably less than a few hundred years. KAr dating of the excursion lavas gives a mean age for 19 determinations of 42.9 ± 7.8ka(2σ). Compilation of thirty KAr ages of the Laschamp and Olby flows by three laboratories yield a new age for the Laschamp excursion in France of 46.6 ± 2.4ka(2σ). The age of the excursion in southwestern Iceland is statistically indistinguishable from the Laschamp excursion at the 95% confidence level, and both have very low paleointensities. Therefore, we suggest that the Laschamp and Olby flows in France and the Skalamaelifell units of Iceland recorded essentially the same geomagnetic excursion. Differences in the virtual paleomagnetic poles (VGPs) of these excursions may be due to (1) the probable non-dipole character of the geomagnetic field during the excursion, (2) rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in France and Iceland, and/or (3) crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations.

Hydrothermal minerals and alteration rates at Surtsey volcano, Iceland

The volcanic island Surtsey, off the south coast of Iceland, was created by volcanic activity in 1963–1967. Core from a 181-m-deep hole extending 123 m below sea level shows the results of 12 yr of hydrothermal alteration of basaltic tephra. The primary cause of heating of the tephra and of development of the hydrothermal system was the intrusion of dikes below sea level. At present, the hottest part of the hole, at a maximum temperature of 150 °C, is cooling at ∼0.9 °C per year. Palagonitization of sideromelane glass, a dominant constituent of the tephra, is an important alteration process that is strongly temperature dependent, the rate doubling for every 12 °C increase. At 60 °C, 90% is palagonitized. Above 120 °C, olivine crystals are replaced on their edges by nontronite; the thickness of clay doubles for each 8 °C increase. Ten hydrothermal minerals have crystallized in the tephra at 25 to 150 °C; the dominant species are smectite (nontronite), analcite, phillipsite, and tobermorite. The primary clay species of palagonite is probably nontronite. Other minerals are halite, opal, calcite, chabazite, xonotlite, anhydrite, and gypsum. No major differences in mineral occurrence are noted above and below sea level, but phillipsite and tobermorite tend to grow larger below sea level, even at the same temperature. Analcite appears at lower temperature (55 °C) above sea level than below sea level (75 °C). Anhydrite is most abundant deep in the hole, where inflowing, cool sea water precipitated sulfate due to reduced sulfate solubility at higher temperatures.

Petrology of mugearite-hawaiite: Early extrusives in the 1973 Heimaey eruption, Iceland

Abstract Rocks which erupted during the first few weeks are described and chemical analyses of rocks and microprobe analyses of minerals are presented. An outline of the eruption history and of the geology of the area is given. A gradational change in chemistry from mugearitic to hawaiitic composition is recorded. Xenoliths of hypersthene gabbro not cognate with magma were discovered in the mugearite. One xenolith has reacted with the mugearite magma at depth to form pargasitic hornblende and kaersutite. It is argued that the magma reacted with the xenoliths at a total pressure well below 8–9 kbar and at a gas pressure higher 1 kbar.

Volcanism and Ice Interactions on Earth and Mars

When volcanoes and ice interact, many unique types of eruptions and geomorphic features result. Volcanism appears to occur on all planetary bodies, but of the inner solar system planets, ice is limited to Earth and Mars. Earth, the water planet, is covered by ice wherever the temperature is cold enough to freeze water for extended periods of time. Ice is found in sheets covering the Antarctic continent and Greenland, as glacial caps in high mountainous regions, as glaciers in polar and temperate regions, and as sea ice in the northern and southern oceans. With changes in climate, landmass, solar radiation, and Earth orbit, ice masses can contract or expand over great distances, as occurred during the Pleistocene Ice Age. The Earth is still in an ice age—at the beginning of the Cenozoic, 65 million years ago, our planet was ice-free. In fact, there is now so much ice that about 70% of the world’s total fresh water is contained within the Antarctic ice sheet.

Petrochemistry of the Volcanic Rocks of the North Atlantic Ridge System

The North Atlantic has been a key area since Vine [1] demonstrated the regularity of the magnetic anomalies along the Reykjanes Ridge while Iceland and the transverse ridge extending from the Faeroes to East Greenland represent the trace of a hot spot generated throughout the development of this part of the North Atlantic. It is our intention here to review the chemistry of oceanic tholeiites such as are recovered from abyssal parts of the mid-ocean rift system, contrast them with those of hot spot areas such as Iceland, and try to trace the chemical development of these lavas throughout the 60 m.y. spreading history. In addition the alkaline lavas of the area will be briefly described.

The PT phase relations of a primary oceanite from the Reykjanes peninsula, Iceland

Abstract The field relationships and petrographic features of the oceanites of the Reykjanes peninsula suggest that they might have originated as primary magmas. The principal phase relationships of primary liquids formed by the partial melting of lherzolites are defined. The phase relations obtained for oceanite RE78 between 0 and 30 kbar at dry conditions suggest that the oceanite originated as a primary magma at 25 kbar and 1580°C, and erupted at a temperature near 1300°C.

Dredged basaltic rocks from the seaward extensions of the Reykjanes and Snaefellsnes volcanic zones, Iceland

Abstract Data for basaltic rocks dredged on the Reykjanes Ridge and Jokulbanki off the southwest coast of Iceland are presented. The rocks from the Reykjanes Ridge, which are identified as recent lavas, are olivine tholeiites and quartz tholeiites very similar to those formerly described from this region. The heterogeneous Jokulbanki dredges were identified as erratics. In the light of the present and other recent data on tholeiitic rocks from Iceland and the Reykjanes Ridge, a distinct chemical gradient, together with increased scatter across Iceland is demonstrated for certain incompatible elements. This is in accordance with the mantle plume theory.

Subsidence of Surtsey volcano, 1967–1991

The Surtsey marine volcano was built on the southern insular shelf of Iceland, along the seaward extension of the east volcanic zone, during episodic explosive and effusive activity from 1963 to 1967. A 1600-m-long, east-west line of 42 bench marks was established across the island shortly after volcanic activity stopped. From 1967 to 1991 a series of leveling surveys measured the relative elevation of the original bench marks, as well as additional bench marks installed in 1979, 1982 and 1985. Concurrent measurements were made of water levels in a pit dug on the north coast, in a drill hole, and along the coastline exposed to the open ocean. These surveys indicate that the dominant vertical movement of Surtsey is a general subsidence of about 1.1±0.3 m during the 24-year period of observations. The rate of subsidence decreased from 15–20 cm/year for 1967–1968 to 1–2 cm/year in 1991. Greatest subsidence is centered about the eastern vent area. Through 1970, subsidence was locally greatest where the lava plain is thinnest, adjacent to the flanks of the eastern tephra cone. From 1982 onward, the region closest to the hydrothermal zone, which is best developed in the vicinity of the eastern vent, began showing less subsidence relative to the rest of the surveyed bench marks. The general subsidence of the island probably results from compaction of the volcanic material comprising Surtsey, compaction of the sea-floor sediments underlying the island, and possibly downwarping of the lithosphere due to the laod of Surtsey. The more localized early downwarping near the eastern tephra cone is apparently due to greater compaction of tephra relative to lava. The later diminished local subsidence near the hydrothermal zone is probably due to a minor volume increase caused by hydrous alteration of glassy tephra. However, this volume increase is concentrated at depth beneath the bottom of the 176-m-deep cased drillhole.