Petit Spot
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| Map with marking (cross) of the location of the first petit-spot basalt on the bottom of the Pacific off the island of Honshu (Japan). The red line marks the course of the axis of the Japan Rift. The Pacific plate subducts here in a west-northwest direction under the Eurasian plate . |
Petit Spot is the name for a form of volcanism that was unknown until the beginning of the 21st century and that is relatively closely related to subduction zones . The volcanic activity, which only creates relatively small volcanic structures, does not take place within the subduction zone or on the upper plate, but outside the subduction zone on the lower plate, i.e. on the descending lithospheric plate .
discovery
Petit spot volcanoes were first 2,001 Japanese by a team geologists to Naoto Hirano based on geologically very young basalt samples described that at a certain point on the upper eastern slope of the Japan trench ( 39 ° 23 'N, 144 ° 16' E , in the Literature site called A ) had been collected at a depth of around 7350 meters. This was very surprising, as the seamounts in this area of the Pacific known up to that point are more than 100 million years old ( Lower Cretaceous ) and there was no known active hotspot in the area . As a result of further exploration and sampling of this region, Hirano and colleagues published an article in the renowned science magazine Science in 2006 , in which they coined the term petit spot for this phenomenon . In the meantime, petit-spot volcanoes have also been discovered on the Tonga and Chile trenches .
properties
Petit-spot volcanoes are a maximum of a few hundred meters high, with a base area of a few square kilometers and a rock volume of less than one cubic kilometer. They are therefore also referred to in the literature as "small hills" or " knolls ". They do occur in the vicinity of subduction zones, but, unlike the previously known volcanoes associated with subduction, not on the upper plate, but on the lower plate, i.e. on the oceanic lithospheric plate that is submerging . Petit-spot volcanoes are geologically very young at a few million years, especially in relation to the oceanic crust on which they are located. The youngest ejecta, which could hardly be more than 50,000 years old, was not found in the first location on the flank of the trench ( site A ), but in a second location, about 550 kilometers east-southeast of the first ( site B ).
The rocks are “fresh”, that is, basaltic lavas that are often rich in bubbles and not changed in the mineral stock under the influence of seawater. Above all, their fresh condition clearly distinguishes them from the Cretaceous basalts in their vicinity. Petrographically , these rocks can be referred to as potassium-rich shoshonites , trachy basalts , basanites and sodium-rich basanites. These show a depletion of heavy rare earth metals (HREE) as well as an enrichment of noble gas - isotopes that come from the decay of radioactive elements (especially argon -40 and neon -21).
causes
It is assumed that magma production at petit spots is mainly caused by the deformation of the oceanic plate. Between the Japan Rift and the actual deep-sea level of the western Pacific there is a threshold running parallel to the rift , which is characterized by a positive anomaly in gravity measurements. This suggests that there the Pacific plate bulges slightly before it kinks at the subduction zone. This threshold is called outer bulge ("outer bulge") or outer rise ("outer threshold") (in this specific case also Hokkaido outer rise ).
Since the depletion at HREE suggests an origin of the lavas in the upper asthenosphere , in the so-called Low Velocity Zone (LVZ) at a depth of around 90 kilometers, it is assumed that the tectonic stresses that result from the bulge on the underside of the lithospheric plate occur, focus the small amounts of melts present in the LVZ and press them towards the surface at crevices in the lithosphere. This explained the low volume of lava production. A high 40 Ar / 36 Ar and low 20 Ne / 21 Ne ratios also indicate that the region of origin was probably not in the fertile, primitive deep mantle, but in a depleted part of the mantle in the upper asthenosphere. * On the other hand there are relatively small ratios of 143 Nd / 144 Nd and 206 Pb / 204 Pb as well as an increased 87 Sr / 86 Sr ratio, which are more characteristic of the so-called ocean island basalts of hotspot volcanism than their source deep, primitive coat applies. However, this could be explained by the local presence of enriched mantle rock (e.g. remnants of reclaimed subducted oceanic crust) in the LVZ. This has a lower melting temperature than the depleted rock, so that, in view of the overall small amount of melts present, the Nd, Pb and Sr isotope ratios shift in favor of magmas that originate from such secondary enriched areas. In contrast, noble gases are more mobile and distribute more evenly, so that their isotope ratios show the region of origin more reliably. Another indication of the origin in the upper asthenosphere is the sometimes apparently very high content of the magma in carbon dioxide (CO 2 ), which gasses out when it reaches the surface and thus causes the abundance of bubbles in some petit-spot basalts. A high CO 2 content, like a high water content, lowers the melting point of the mantle rock, so that despite the relatively high lithosphere thickness and the correspondingly high pressure, melts can exist in the upper asthenosphere.
The low age of the extraction products from the locality located on the eastern slope of the Hokkaido outer rise ( site B ) and the higher age of the basalts from the locality on the trench flank of the Japan trench ( site A ) shows that the volcanic activity apparently on the area of the slight upward movement -Knicks the Pacific plate is limited. The activity ceases as soon as the corresponding plate section has crossed the area of this kink. In view of the many times larger hotspot volcanoes, whose activity phases are also determined by the plate movements, the reason for the small size of the petit-spot volcanoes is to be found in the fact that in the absence of a hotspot immediately below the lithosphere, only a small one Amount of molten material exists that is potentially available for lava extraction.
Remarks
- * “Fertile” and “primitive” on the one hand and “depleted” on the other hand refer to the chemical composition of the mantle rock, which depends on its degree of differentiation. Rocks of the upper mantle, from which magmas have melted on mid-ocean ridges, which have risen and formed oceanic crust, are depleted in certain chemical elements (including the so-called incompatible elements , including especially REE) and isotopes, in contrast to the deeper mantle who has not experienced such a differentiation and is therefore referred to as “enriched”, “fertile” or “primitive”. The depleted mantle is also depleted of noble gas isotopes that do not originate from the radioactive decay of other elements, because these isotopes are largely "outgassed" in the course of differentiation and, in contrast to the decay products, were not formed again afterwards.
literature
- Naoto Hirano: Petit-spot volcanism: A new type of volcanic zone discovered near a trench. Geochemical Journal. Vol. 45, 2011, pp. 157-167, doi: 10.2343 / geochemj.1.0111 .
Web links
- Searching for traces in the rock: How petit spots are created Articles on scinexx.de
Individual evidence
- ↑ N. Hirano, K. Kawamura, M. Hattori, K. Saito, YA Ogawa: A new type of intra-plate volcanism; young alkali-basalts discovered from the subducting Pacific Plate, northern Japan Trench. Geophysical Research Letters. Vol. 28, No. 14, 2001, pp. 2719-2722, doi: 10.1029 / 2000GL012426 .
- ↑ Naoto Hirano, Eiichi Takahashi, Junji Yamamoto, Natsue Abe, Stephanie P. Ingle, Ichiro Kaneoka, Takafumi Hirata, Jun-Ichi Kimura, Teruaki Ishii, Yujiro Ogawa, Shiki Machida, Kiyoshi Suyehiro: Volcanism in Response to Plate Flexure. Science. Vol. 313, No. 5792, 2006, pp. 1426-1428, doi: 10.1126 / science.1128235 (alternative full-text access : ResearchGate ).