THE ORIGIN OF S-TYPE GRANITE PLUTONS IN KUIPER BELT OBJECTS (KBOs)

Abstract:

Kuiper belt objects (KBOs) are suggested to have formed by gravitational instability against Neptune’s strongest outer resonances, with many or most forming in binary pairs due to excess angular momentum. When external perturbation induces KBO binary orbital pairs to spiral in and merge, they undergo ‘aqueous differentiation’, melting saltwater oceans which precipitate authigenic sedimentary cores. As the sedimentary cores undergo lithification, the destruction of voids expels interstitial water through hydrothermal vents into the overlying ocean. If a hydrothermal pathway becomes blocked, hydraulic pressure may cause delamination in KBO authigenic sedimentary rock, creating water blisters in the form of aqueous domes and sills, as part of a pathway to the overlying ocean through porous rock, vents or faults. The pressure and temperature drop from pressurized conduits into lower-pressure domes and sills may induce crystallization to form pegmatites and precipitation of authigenic mineral grains to form S-type granitic sediments, which lithify into granitic rock. This alternative hydrothermal model is suggested to function similar to magma in terrestrial setting, but with aqueous fluids having vastly-greater mobility than magma, particularly high-viscosity felsic magma.
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Introduction:

“Hornblende is common in the more mafic I-types and is generally present in the felsic varieties, whereas hornblende is absent, but muscovite is common, in the more felsic S-types;”

“Apatite inclusions are common in biotite and hornblende of I-types, but occur in larger individual crystals in S-types. Thus, I-types characteristically contain biotite+hornblende plus/minus sphene plus/minus monazite. S-types contain biotite plus/minus muscovite plus/minus cordierite plus/minus garnet plus/minus ilmenite plus/minus monazite.”

“One important compositional difference between the two types, not noted in 1974,
is that as a group, the S-type granites are more reduced with respect to oxygen fugacity”: lower Fe3/Fe2 in S-type granites.

Compositionally distinct with respect to Na2O vs. K2O, CaO vs. Total FeO, and Aluminium Saturation Index (for the most mafic 10% of I-type and S-type).

I-type granites lack enclaves of supracrustal origin, whereas more mafic rocks of S-type granites invariably contain a rich assemblage of supracrustal enclaves (White et al. 1999).

“The K-feldspar in S-type granites is always white in colour, never pink, provided the rock is not weathered or hydrothermally altered. However, in I-type granites the K-feldspar crystals are frequently pale pink in colour, but sometimes white.”

“However, the amount of zircon showing such inheritance is vastly different between
the I- and S-types. Williams et al. (1992 p. 503) noted that ‘Zircons with inherited cores are rare in I-type granites, but virtually every zircon in the S-types contains an older core’. Chappell et al. (1999 p. 829) pointed out that this implies that ‘the sediment component in the I-type granites, at least as indicated by the amount of inherited zircon, is trivial, a conclusion sustained by the observation that zircon was saturated in all of the low-temperature I-type magmas’.”

“The statement by Chappell and White (1974) that S-type granites are generally older than I-type granites occurring in the same batholith, is substantiated by later investigations. It is also the case that the earlier S-type granites may have a strong secondary foliation, truncated by I-type
granites that are either unfoliated or have a primary foliation.”

Above quotes from:
Chappell, B. W. and White, A. J. R., (2001), Two contrasting granite types: 25 years later, Australian Journal of Earth Sciences, Volume 48, Issue 4, pages 489–499, August 2001.
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Solar system dynamics:

The Jeans instability that formed our solar system apparently had a high degree of angular momentum, forming a quadruple star system, composed of two close binary pairs (‘binary-Sun’ and ‘binary-Companion’) in a wide-binary Sun-Companion spacing. (See section, STARS, PLANETS, MOONS, MINOR PLANETS AND COMETS)

Secular perturbation of the quadruple system caused the binary pairs to spiral in, causing a binary-Sun merger at 4,567 Ma and a binary-Companion merger 4 billion years later at 542 Ma, with the asymmetrical binary-Companion merger giving the newly merged Companion escape velocity from the Sun.

The ashes from the binary-Sun merger at 4,567 Ma condensed planetesimals by gravitational instability (GI) in at least 3 locations in the solar system; 1) rocky-iron asteroids against the Sun’s greatly expanded magnetic corotation radius near the orbit of Mercury, 2) carbonaceous chondrites against Jupiter’s strongest inner resonances and Kuiper belt objects (KBOs) against Neptune’s strongest outer resonances.

In the 4 billion year interval between the two binary spiral-in mergers, between 4,567 Ma and 542 Ma, the solar system had a ‘solar system barycenter’ (SSB) which created unusual conditions in the outer solar system by way of tidal effects.

Earth has two lunar tides, one on the near side to the Moon, caused by tidal attraction to the Moon, and one on the far side of Earth, which can be explained as the centrifugal force of Earth around the Earth-Moon barycenter. Similarly, in a Sun-Companion system with a SSB, there will be a transition point between tidal attraction and centrifugal repulsion, which is suggested to cause ‘aphelia precession’ of Kuiper belt objects (KBOs) which cross the tidal threshold, the way oceans on Earth flip flop between near-side high tide to low tide to far-side high tide to low tide. In the case of KBOs, ‘flip-flop perturbation’ aphelia precession is suggested to have caused KBO aphelia (for those KBOs which crossed the tidal thershold) to have precessed from aphelia pointing toward the Companion to pointing away from the Companion and back again, for those KBOs that repeatedly crossed the tidal threshold in their orbits around the Sun.

Additionally, as the brown-dwarf components of binary Companion spiraled in, the wide binary separation spiraled out, conserving energy by increasing the wide-binary Sun-Companion eccentricity around the SSB over time. And by Galilean relativity, it could just as well be stated that the SSB spiraled out from the Sun at an exponential rate over time, perturbing ever more distant KBOs by way of the tidal transition point reaching the semimajor axes of KBOs, with perturbation caused by flip-flop perturbation (apsidal precession). (Note, the SSB is associated with the tidal transition point but is not coincident with it. Tidal transition is defined as the semimajor axis of KBOs where flip-flop perturbation furst occurs.) Tidal transition flip-flop perturbation reached the cubewanos between the 2:3 and 1:2 resonance with Neptune between 4.1 and 3.8 Ga, causing the late heavy bombardment (LHB) of the inner solar system by KBOs.

Most KBOs are suggested to have formed as binary pairs, which were induced to spiral in and merge by the flip-flop perturbation when the exponentially-increasing reach of the tidal trasition point caught up to the semimajor axes of KBOs. Binary siral-in merger of binary KBOs initiated ‘aqueous differentiation’, melting saltwater oceans in their cores which chemically precipitated sedimentary cores. Lithification of a sedimentary KBO core is a process of destruction of voids, which expels hydrothermal fluids. As hydrothermal conduits are blocked by crystallization or by subsidence (KBO quakes), the hydrothermal fluids must force new pathways to the surface, often by delaminating layers of the sedimentary core until finding porous rock to continue the its rise to the KBO saltwater ocean above.

The periodic nature of granitic ‘line rock’, as in the Blackhills line-rock granite of the Yavapai Mazatzal craton, is suggested to be the result of tidal torquing caused by flip-flop perturbation (aphelia precession), as orbital KBO aphelia were tidally attracted toward and then centrifugally slung away from the Companion in their heliocentric orbits, causing waxing and waning of hydrothermal fluids from the lithifying sedimentary core.

The loss of the Companion at 542 Ma apparently reduced the stability of the outer solar system, causing Neptune to become the nemesis of the Kuiper belt in the Phanerozoic Eon. Phanerozoic perturbation of KBOs by Neptune may have induced the formation of authigenic Phanerozoic gneiss domes, complete with (extrusive) gneiss dome matling rock (quartzite, carbonate rock and schist), and perhaps intrusive S-type granite.
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Extraterrestrial S-type granite vs. terrestrial I-type granite:

If KBO cores are composed of authigenic sediments, as suggested here, then the hydrothermal fluids expelled during lithification and diagenesis are suggested to play a similar role in extraterrestrial KBO cores as intrusive magma and extrusive volcanic lava do on Earth.

Within mixed S-type and I-type batholiths, S-types [with whitish microcline] tend to be older, more chemically reduced, formed at lower temperature, surrounded by metasomatic skarns and pegmatites, with muscovite rather than hornblende mafic minerals, and often containing inherited zircons and supracrustal enclaves. I-types [with pinkish orthoclase], by comparison, tend to be younger, higher temperature, surrounded by contact-metamorphic hornfels and aureoles, and sometimes associated economic mineralization, with hornblende common. (Chappell and White 2001)

While metamorphic hornfels and aureoles, commonly associated with I-type granites, are clear signs of high temperature metamorphism caused by intrusive magma, S-type metasomatic skarns and pegmatites in extraterrestrial KBO cores are alternatively suggested to be caused by aqueous crystallization and metasomatism caused by lower-temperature hydrothermal fluids, which readily penetrates the surrounding porous country rock. Additionally, ‘supracrustal enclaves’ of country rock, often found in S-type granites, are much denser than the hydrothermal fluids causing hydraulic hydrothermal delamination in KBO cores promote brittle ceiling cave ins which fall through the hydrothermal fluids into the granitic sediments below to become supracrustal enclaves. By comparison, hydraulic delamination by granitic magma on Earth rarely results in ceiling collapse, due to higher temperatures which soften the country rock, reducing the probability of brittle ceiling cave ins. Additionally, the much higher viscosity of felsic magma along with the much lower density differential (of felsic magma vs. country rock compared to hydrothermal fluids vs. country rock) reduce the likelihood of country rock xenoliths in I-type granite.

So mixed S-type granites with younger I-type granites may be a combination of older extraterrestrial S-type granites followed by Earth impact in an extinction-level event, followed by terrestrial I-type granites, perhaps with terrestrial magma following and exploiting hydrothermal induced weaknesses and hydrothermal conduits.

The term ‘hydrothermal’ is a bit of a misnomer when used in an (extraterrestrial) intrusive sense, since on Earth it refers to (extrusive) hot aqueous fluids gushing from ocean plates. While extrusive hydrothermal fluids also gush into KBO saltwater oceans (beneath icy mantles) precipitating authigenic (extrusive) gneiss, schist, quartzite, carbonate rock and other types of extraterrestrial sedimentary ‘country rock’, the intrusive form is suggested to precipitate granitic sediments, which lithify into granitic (line) rock.

Low-viscosity extraterrestrial hydrothermal fluids might be expected to cause more hydraulic delamination and crosscutting dikes than much-higher-viscosity terrestrial felsic magma, while high-viscosity terrestrial magma might be expected to form more well-rounded plutons. So S-type granites might be expected to exhibit more narrow sills, dikes and veins in addition to plutons, whereas I-type granite plutons might tend to form more rounded with fewer peripheral sills, dikes and veins, although I-type batholiths are often associated with secondary, economic metasomatic mineralization, distinct from the granitic rock itself.

Aqueous solubility of mineral species is subject to ambient conditions, notably temperature, pressure, and pH. Decreasing temperature and pressure typically lower the solubility of most mineral species, promoting precipitation and (pegmatite) crystallization in intrusive hydrothermal plutons, dikes and sills, as the pressurized aqueous fluids flow down a pressure gradient to the cooler overlying KBO saltwater ocean (underlying an icy mantle).

Chemically-precipitated authigenic sediments on Earth are clay sized, sometimes forming authigenic mudrock, while in the microgravity of KBOs, mineral grains are suggested to typically fall out of aqueous suspension at sand grain size or larger, determined by the microgravitational acceleration and the local saltwater circulation rate. Thus the very gneiss which makes up the basement rocks of the continental tectonic plates on Earth is suggested to be authigenic sedimentary rock of Kuiper belt origin. S-type granite zircons typically contain older inherited ‘detrital’ cores from hydrothermal fluids emanating from older layers, deeper in the sedimentary core, whereas terrestrial I-type granites do not typically possess detrital cores.
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Why is intrusive hydrothermal S-type granite felsic in composition?:

This comparative conceptual approach does not attempt to explain the felsic nature of suggested hydrothermal intrusive granite, but merely to suggest one or two mechanisms that might come in to play.

While the terrestrial mantle has a mafic composition which may undergo igneous differentiation to ultimately form granite, or otherwise melt felsic country rock, KBO hydrothermal fluids are not necessarily chondritic in composition. Thus the mineral species most likely leached by high-temperature high-pressure hydrothermal fluids would be the very same minerals precipitated and crystallized from solution as the temperature and pressure decreases on its journey through the core to the overlying KBO ocean, and silica solubility is particularly temperature sensitive. So intrusive hydrothermal granite needn’t explain away a mafic component as terrestrial magma intrusions necessarily need to.

If silica solubility is particularly sensitive to temperature, carbon dioxide solubility in the form of carbonic acid is particularly sensitive to pressure, which can be demonstrated by removing the bottle cap from a carbonated beverage. The solubility of dissolved aluminous species is particularly pH sensitive, with a solubility trough around 6-1/2 pH, so a pressure induced drop in pH toward neutral due to conversion of carbonic acid to gaseous CO2 bubbles would tend to precipitate and crystallize aluminous mineral species in the form of felsic feldspars. (See section, AQUEOUS DIFFERENTIATION OF KUIPER BELT OBJECTS (KBOs))

In a peraluminous setting, where the proportion of aluminum oxide is higher than the combination of sodium oxide, potassium oxide and calcium oxide combined, more complex aluminous silicates would form, such as muscovite, which is common in S-type granite, and particularly with its associated pegmatites.
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Reference:

Chappell, B. W. and White, A. J. R., (2001), Two contrasting granite types: 25 years later, Australian Journal of Earth Sciences, Volume 48, Issue 4, pages 489–499, August 2001.
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