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IntroductionItokawa is a peanut-shaped near-Earth aster-oid, c. 535 m long and c. 200 m thick (Fig.4). Itokawa orbits the Sun between Earth andMars, and was generally explained to haveformed by the accretion of two smaller aster-oids that gravitated toward each other andstuck together. Fujiwara et al. (2006) sugges-ted an early collisional breakup of a preex-isting parent asteroid followed by a re-agglom-eration into a rubble-pile object.

Itokawa was selected as the target forJapan’s unmanned sampling mission. TheHayabusa probe arrived in the vicinity of Ito-kawa in 2005 and touched down, but failedto operate the soil sample collecting device.Despite technical difficulties, minor quanti-ties of dust were recovered, and Hayabusa leftthe asteroid.

In 2010, the Hayabusa craft dramaticallyfireballed in Earth’s atmosphere; however, thesample return capsule made a safe parachutelanding in Australia. The Japan AerospaceExploration Agency (JAXA) confirmed thatmost of the 1,500 rocky particles found insi-de the capsule originate from Itokawa. Con-centrations of olivine and pyroxene were iden-tified.

The sampling sites of Itokawa material inJAXA’s possession are unknown at the time ofthis writing. Detailed analyses are pending.

JAXA obtained a number of orbital andlanding site images of Itokawa. Some of theimages, published on JAXA’s internet pages,were copied, processed and interpreted by thewriter. In the following, the geological evolu-tion of mid-ocean ridge (MOR) environmentwill be reviewed and exemplified, Itokawaimages will be presented and discussed, andconclusions will be drawn on findings.

Figs. 1–3, taken by the writer, are pre-sented as an introduction to MOR basalts.Figs. 4–8, credited to JAXA, make the case.

The mid-oceanridge conceptTerrestrial seafloor spreads at MOR. A deepunderwater rift typically runs along the spineof MOR. Plates of oceanic crust are conveyedaway, and new lithosphere is created as basal-tic magma rises from the Earth’s mantle (Con-die 1985, Salters and Stracke 2004, Wilkin-son 1985). Magma fills the spreading zonewith feeder dykes that in a cross section ap-pear as a parallel dyke swarm (Kurewitz andKarson 1998).

Dykes feed >1000 ° C basaltic magma tothe ocean floor where pulses of pillow-making

Itokawa –a mid-ocean ridge asteroidKAARLO MÄKELÄ

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lava pile on previous, solidified layers. Widthsof individual feeder dykes vary from c. 30 cmup to meters. Pillow diameters in basaltic lavarange from tens of centimeters up to a meter.The Troodos ophiolite complex on Cyprus isoften referred to as type example of MOR se-quences; there pillowed flow units average 100to 200 m in thickness (Schmincke et al. 1983).

Pillow skins cool fast in seawater: glass rimsare formed. Pillow interiors cool relativelyslowly and develop cooling cracks. As pressu-re is released, gases dissolved in magma ex-pand and form vesicles (Jones 1969).

The imagesLØKKEN, NORWAY

The Løkken basalts are of MOR type, LowerOrdovigian (c. 450 million years) in age andoriginate from a shallow depth of magma re-servoirs (Grenne 1989a). At Løkken, the c. 2km thick volcanic sequence is tectonized (over-turned) and metamorphosed; however, struc-tures characteristic of MOR basalts remaindetectable. The Løkken volcanogenic massivesulphide deposit exemplifies ore forming pro-cesses that operate in MOR environments(Grenne 1989b). Figs. 1–3 represent a feederdyke complex and related metabasalts in theLøkken Mine region, Norway.

Fig. 1. Basaltic dyke in a sheeted fee-der dyke complex, Løkken Mine region,Norway. Colours were enhanced tohighlight primary structures in chemicallyhomogeneous material. Despite meta-morphism, friction banding in viscousmelt at the relatively chill margin area,and cubical cooling fractures in thecentral region, are clearly discernible.

Fig. 3. Matabasalticflow breccia, Løkken,Norway. Fragments ofbroken pillows are mixedwith hyaloclastic rubble.

Fig. 2. Metabasaltic pillow, Løkken, Norway.Hammer points towards stratigraphic bottom,gross flow direction runs from right to left. Theglassy rim, and glass in cooling cracks, stand

out. Vesicles no longer arevoids but are filled withmetamorphic mineralassemblages.

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ASTEROID ITOKAWA

A magmatic complex of asteroid size cannotform by rubble accretion in space. Instead, aplanet-size parent body is a necessity – onewith voluminous basaltic magma reservoirsand plate tectonic phenomena in operation.To the writer’s knowledge, terrestrial seafloorspreading and related volcanic processes arethe only mechanism that produce MORsequences. Figs. 4–8 represent the sequenceof sheeted feeder dykes and overlying pillowlavas in asteroid Itokawa.

Fig. 4. Asteroid Itokawa. Green lines indicatethe general strike of volcanic layering.Shadows emphasize a feeder dyke swarm atthe lowest quarter of the image, over thelength of c. 200 m. Depositional bottom isdown, layer tops are up.

Fig. 5. Asteroid Itokawa. A mushroom-shaped pillow lava unit (P), composed of c.50 separate flow pulses, is schematicallyoutlined in green. Some of the feederdykes (F) are marked in red. The youngestof dykes penetrated flows. Such dykes mayhave reached seafloor and fed another unitof pillowed lavas from a magma reservoirlocated at shallow depth. The relativelythick (10–20 m) feeder dykes suggest afast spreading rate for Hadean oceaniccrust (Kurewitz and Karson 1998).

Fig. 6. Asteroid Itokawa. This image displaysin more detail the area lined with green inFig. 5. The flow unit stands c. 70 m high and180 m wide. Flow pulses average a fewmeters in thickness. Attention should bepaid to fluidal structures in individual lavaflows and rift-like structures at the base ofthe mushrooming flow unit.

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Conclusions

Asteroid Itokawa displays features diagnos-tic of terrestrial mid-ocean ridges: There is afeeder dyke swarm (Fig. 5), a mushroomingflow unit (Fig. 6), and pillow lava (Fig. 7).Seldom, if ever, can a complete MOR sequencebe observed in a cross section on Earth.

The specific gravity of basalt ranges from1.72 to 2.76 g/cm3, depending on vesicularityin the sample (Gates 2008). The bulk specificgravity of asteroid Itokawa (1.9 g/cm3, Abe etal. 2006) falls within the terrestrial range.

The MOR origin of asteroid Itokawa pro-ves that plate tectonic processes operated onEarth already in the Hadean era. The writersuggests that early plate movements were acontinuum to convections in proto-Earth.Deep-rooted convections may have carriedfrom great depths high-pressure minerals andrare isotopes (Anders and Zinner 1993). Lo-cally, shallow-depth basaltic magma reservoirsmay have caused sanidine facies burial-thermal metamorphism (low pressure, hightemperature) – features explained shock-thermalin a basaltic micrometeorite of presumedasteroid origin (Gounelle et al. 2009).

Primitive organisms were identified inmeteorites by Hoover (2011). Since meteoritesare considered broken-up asteroids (http://www.meteorites.fr), and as asteroids originatefrom the Earth (Mäkelä 2011, and this article),it is concluded that life sparked in Hadeanoceans as soon as hydrosphere precipitatedfrom idigenous vapors. On Itokawa, signatu-res of life may exist on top of the mushroo-ming flow unit (Figs. 5 and 6) or higher instratigraphy.

Points to ponder: Detailed analysis of Ito-kawa feeder dykes would reveal the composi-tion of the Earth’s early, undepleted mantle.

Fig. 8. Asteroid Itokawa. Flow breccia.Some of the rubble may be loose; howe-ver, there is a great resemblance to theschistosed and regionally metamorpho-sed flow breccia in Fig. 3.

Fig 7. Asteroid Itokawa. A beautiful lavapillow is marked with A (compare withFig. 2). The pillow is rimmed with blackglass, while cooling cracks and vesiclepocks characterize interior. It is unclearwhether the pillow (A) lies in situ or is tornapart; however, in the image, bottom isdown and gross flow direction runs fromleft to right. B and C are identified aspillow fragments due to chill rims, arcuateoutlines, and vesicles. Dry land probablydid not exist at this part of the MOR: therelatively small size of vesicles refers todeep water conditions (Jones 1969).

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Paleomagnetic surveys on Itokawa feeder dy-kes would provide information on the prove-nance of asteroids in general (terrestrial polarvs. equatorial regions), and on the magneticpolarity of Hadean Earth.

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KAARLO MÄKELÄkaarlo.makela@saunalahti.fi

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