NASA Technical Reports Server (NTRS) 19930005171: Venus steep-sided domes: Relationships between geological associations and possible petrogenetic models

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LPI Contribution No. 789 87
We have explored behavior of this model for a range of the esis and several of these models are found tobe plausible: remelting
parameters. The time-averaged thickness of the depleted layer is of basaltic or evolved crust, differentiation of basaltic melts, and
controlled in part by Tflow: decreasing Tflowto 1000°C reduces this volatile enhancement and eruption of basaltic foams.
thickness by about 50%. Decreasing Tgeto700°C reduces the time- Development of Models: Hess and Head have'shown that the
average thickness of the crust by about 20 kin. However, the cyclical full range of magma compositions existing on the Earth isplausible
variation in depleted layer thickness, along with the aecamlpanying under various environmental conditions on Venus [11]. Most of the
fluctuation in crustal thickness, is a robust feature of the models. Venera and Vega lander compositional data arc consistent with
Varying the initial temperature by :_r200°C and radioactive heating. tholeiitic basalt [3--6]; however, evidence for evolved magmas was
expressed as a fraction of that required to explain all the Earth's provided by Venera 8 data consistent with a quartz monzonite
present-day heatflow by radioactivity, by a factor of 2 influences composition [7]. Pieters et al.have examined the color of the Venus
the onset time of this behavior, but the period of the variation surface from Venera lander images and interpret the surface there to
remains on the order of 300-500 m.y. for the complete range of be oxidized [8].
conditions considered. Preliminary modeling of dome growth has provided some inter-
The parameterized convection model assumes that instabilities pretations of lava rhcoiogy. Viscosity values obtained from these
are globally synchronous. Ifthe instability described by this model models range from 10t4-10t7 Pa-s [9], and the yield strength has
is global in scale, it may take the form of episodic plate spreading been calculated to be between 104 and I06 Pa [1], consistent with
and subduction. But studies of impact crater densities on Venus [6] terrestrial silicic rocks. The apparent high viscosity of the dome
and the distribution of volcanic features [4] suggest that resurfacing lavas suggests that the domes have a silicic composition or must
may occur in patches rather than globally. Localized volcanic augment their viscosity with increased visicularity or crystal con-
resttrfacing on Venus may be aconsequence of local instability of tent.
the lithosphere or alternatively may mean that large, exceptionally Petrogenetic Models: Sixty-two percent of the Venus domes
hot plumes penetrate even thick, buoyant lithosphere. The Archean are associated with coronae, circular features that have been pro-
greeustone belts onEarth, which areflooded byhighMgO volcanics, posed as sites of mantle upwelling, and 20% of the domes arelocated
require similar mechanisms of formation. Komatiites, for example. near tessera, relatively high areas of complex deformed terrain. We
require potential temperatures of at least 1800°(2 [9,10] and mean have investigated several models that are consistent with these
depths of melt segregation of 160-330 km [11],yet average mantle geologic associations. The fast case involves the differentiation of
temperatures in the Archean are thought to be only 100°--150°C basalt in amagma reservoir inthe crust, perhaps produced by partial
higher than present [12]. These contradictions are best explained by melting within amantle plume. The second case is melting at the
amodel in which komatiites form only in plumes, whereas more base of thickened basaltic crust, and the t-real case is volatile
typical terrestrial hasalts form at spreading centers. The chemical exsolution and enhancement within abasaltic magma reservoir. The
differentiation of Venus described inthis study almost demands that association of domes with tessera might be explained by crustal
komatiite-to-picrite volcanics form the dominant portion of the remelting, while the association with coronae may be consistent
venusian crust. with chemical diffetch tiation of amagma reservoir or the exsolution
References: [1] Dupeyrat L. et al. (1992) LPSC XXIII, 319. and concentration of volatiles in the reservoir before eruption.
[2] Parmentier E. M. and Hess P. C. (1992) LPSC XXlil, 1037. Chemical Differentiation: High-silica magmas can be pro-
[31Smrekar S. E.and Phillips R.J. (1991) EPSL, 107, 582. [4] Head duced under reducing or oxidizing conditions, and regardless of
J. W. et al.(1992) LPSCXXIII, 517. [5] Solomon S. C. etal. (1992) whether the crust is wet or dry. If water is present, crystal fraction-
LPSCXXIlI, 1333. [6] Phillips R. J. et al. (1992) LPSCXXIII, 1065. ation of a basaltic magma will produce intermediate to silicie
[7] Turcotte D. L.et al.(1979) Proc. LPSC lOth, 2375.18] Olson P. magmas. Differentiation of dry oxidized basalt in amagma reser-
and Kincaid C. (1991)JGR, 96, 4347. [9] Hess P.C. (1990) LPSC voir can also produce silica-rich magma, as well as a suite of
XXI, 501. [10l Miller G. H. et al. (1991) JGR, 96, 11849. intermediate composition magmas [10]. The production of immis-
[1I] Herzberg C. (1992_JGR, 97, 4521. [12] Turcotte D.L. (1980) cible silica-rich melts and ferrobasalts occurs under reducing con-
EPSL, 48, 53. ditions, but no intermediate magma is produced [10].
N93-14359 Crustal Remeltlng: The melting of dry tholeiite basalt at
pressures of 15-25 kbaror above will result in SiO2-rich magmas for
VENUS STEEP-SIDED DOMES: RELATIONSHIPS BE- <20% partial melting [11].Depths of 53-88 km are necessary sothat
TWEEN GEOLOGICAL ASSOCIATIONS AND POSSIBLE melting occurs in the eclogite facies where garnet ispresent asalow-
PETROGENETIC MODELS. B. Pavri and J. W. Head HI, silica phase in the residue. For higher degrees of melting, andesites
Department of Geological Sciences, Brown University, Providence or basaltic andesites will form. The presence of water would allow
R102912, USA. the formation of high silica melts atshallower depths since amphi-
bole could replace garnet as alow silica residue. An excess of water
Introduction: Venus domes are characterized by steep sides, would also reduce the viscosity of ahigh silica melt, making iteasier
acircular shape, and arelatively flat summit area. In addition, they to transport. The volume of crustal melting required toproduce one
are orders of magnitude larger in volume and have alower height/ dome would be reduced considerably if tessera represents evolved
diameter ratio than terrestrial silicic lava domes [1]. The morphol- crust, as proposed by Nikolayeva [7].
ogy of the domes is consistent with formation by lava with ahigh Volat lieExsolu tion/Basalt Foam: Volatile enhancement rep-
apparent viscosity [2]. Twenty percent of the domes are located in resents an alternative mechanism for increasing magma viscosity.
or near tessera (highly deformed highlands), while most others In this model, magma viscosity is increased by two mechanisms.
(62%) are located in and near coronae (circular deformational First, as more vesicles form in the magma, the bubbles have
features thought to represent local mantle upwelling). These geo- difficulty moving past one another and second, the liquid has
logical associations provide evidence for mechanisms of petrogen- difficulty moving along the thin interbubble wails as the vesicles
88
International Colloquium on Venus
become close-packed. The maximum vesicle content that alava can differentiation and crustal remehing are common mechanisms for
sustain without disruption is 75% vesicles; this represents the generating silicic, high-viscosity magmas on the Earth, and are
maximum viscosity increase achievable with this mechanism. consistent with dome associations with coronae and tessera respec-
Model Comparisons: One difficulty with the chemical differ- tively. In both cases, further research will be necessary to under-
entiation model involves trying to concenlxate large volumes of stand how the magma is able to escape the crystal mush and migrate
silicic melt sothat the eruption can occur as asingle, steady effusion to the surface. The crustal remelting model has the additional
of lava before the magma freezes or is trapped in the crystal mush. difficulty of the lack of domes in tessera, above the supposed
It is uncertain whether the low malt fractions will be able to move melting region. The volatile exsolution model will require future
through the crust to collect in a reservoir. Work by Wickham research in order to determine if alayer enhaneext in volatiles can
indicates athreshold of >30% melt for the efficient escape of silica- form at the top of amagma reservoir, and if the shallow reservoirs
rich magmas from acrystal mush [12]. If this mechanism is active necessary for volatile exsolution can form at the low altitudes at
in forming dome lavas, then this is probably an indication that the which the domes are found. Further research will focus on refining
dome lavas arc of an intermediate composition. the models,-examining their implications for crustal evolution, and
The crustal rcmelting model hasitsdifficulties, as well. First, the developing tests todetermine which are active in different environ-
strong correlation of gravity with topography atthe scale investi- ments on Venus.
gated by Pioneer Venus [13] argues against deep isostatic compen- References: [1]Pavri B. etat. (1992) JGR, special Magellan
sation for many features on the planet. If this is true for tessera issue, in press. [2] Head J. et at. (1991) Science, 252, 276-288.
blocks, then eelogite would not be expected at thedepths necessary [3]Surkov Yu. A. et al. (1977) Space Research, 17, 659. [4] Surkov
for the formation of high silica melts. Itis possible that subduction Yu. A. et al. (1983) Proc. LPSC 13th, in JGR, 88, A481-A494.
could transport basaltic or eclogite crust to the depths necessary for [5] Surkov Yu. A. et al. (1984) Proc. LPSC 14th, in JGR, 89,
garnet tobcpresent inthe residue [14,15}, butitisdift_cuJt toinvoke B393--B402. 161Surkov Yu. A. eta]. (1987) Proc. LPSC 17th, in
this mechanism toexplain the global dome distribution. However, JGR, 92, E537-E540. I7] Nikolayeva O. V. (1990) Earth Moon
ifamphibolite is present as the low-silica melt residue, deep crustal Planets, 50151,329-341. [8] Pieters C.M. etal. (1986) Science, 234,
melting is not necessary to generate high-silica melts. An additional 1379-1383. [9] McKenzie D. et ai. (1992) JGR, special Magellan
problem with this model is its inability to explain the presence of issue. [10] Hess P. C. (1989) The Origins ofIgneous Rocks, Harvard
domes on the periphery of the tessera, but not in the tessera itself. Univ., Cambridge. [11] Hess P. C. and Head J. W. (1990) Earth
It seems most likely that the domes would be emplaced directly Moon Planets, 50151, 57--80. [12] Wickharn S. M. (1987) J. Geol.
above the melting region, not hundreds of kilometers laterally Soc., London, 144, 281-297. [13] Anad aM. P. etat. (1980) JGR, 85,
displaced from it. Itis necessary to develop amechanism that will 8308-8318. [14] Head J. W. (1990) Earth Moon Planets, 50151,
transport high-viscosity, silicic magma to the plains surrounding 25-56. [15] Burr J. D. and Head J. W. (1992) GRL, submitted.
tessera, while simu]taneously discouraging the eruption of this 1161 Head J. W. and Wilson L. (1992) JGR, 97, 3877-3903.
same magma inthe tessera. An alternative explanation might be that [1_71Parmentier E.M. and Hess P.C.(1992)LPSCXXIIi, 1037-1038.
domes are formed in the tessera, but that subsequent tectonic strain N93-14360 I
has destroyed them, and the domes on the plains survive because
they are emplaced in a less tectonically active environment. DIELECTRIC SURFACE PROPERTIES'OF VENUS. G.H.
The volatile enhancement mechanism will need to be examined Pettengill, R. J. Wilt, and P. G. Ford, Center for Space Research,
more closely to resolve some of the difficulties inherent in the Massachusetts Institute of Technology, Cambridge MA 02139,
model. First, the exsolution of volatiles should increase pressure in USA.
the chamber and prevent further exsolution unless the excess
pressure is released. At present, it is difficult to envision amecha- Ithas been known for over adecade [1]that certain high-altitude
nism that allows the concentration of the volati]es into a "foam regions on Venus exhibit bizarre radar-scattering and radiothermal-
layer" at the top of the chamber without allowing the volatiles to emission behavior. For example, observed values for normal-
escape before eruption. Perhaps an uneven chamber roof could trap incidence power reflection coefficients in these areas can exceed
pockets of volatile-rich foam that are not drawn off by earlier 0.5; enhanced backscatter in some mountainous dreas inthe Magellan
eruptions that release pressure from the chamber. An additional SAR images creates abright surface with the appearance of snow;
problem is the altitude distribution of the domes. Modeling by Head and reduced thermal emission in the anomalous areas makes the
and Wilson indicates that the necessary shallow magma chambers surface there appear hundreds of degrees cooler than the corre-
in which this volatile exsolution could occur are not likely to form sponding physical surface temperatures. The inferred radio
at altitudes at or below the mean planetary radius [16]. emmissivity in several of these regions falls to 0.3 for horizontal
We have alsoexamined the case of partial melts from the mantle. linear polarization at viewing angles in the range 200--40 °.
If the mantle of Venus is similar to Earth's (of a peridotitic Several explanation shave been offered for these linked phenom-
composition), it is impossible togenerate asilica-rich melt from the ena:
direct partial melting of the mantle without some secondary differ- 1.Single-surface reflection from asharp discontinuity separat-
entiation process occurring. If a buoyant, depleted mantle layer ing two media that have extremely disparate values of electromag-
forms under the crust, it will be even more refractory than pristine netic propagation. The mismatch may occur in either or both the real
mantle and will tend to trap rising plumes. This will encourage (associated with propagation velocity) or imaginary (associated
melting of plumes atthe base of the depleted layer, resulting in the with absorption) components of the relevant indices of refraction,
production of MgO-rlch low-viscosity melts [17]. and the discontinuity must take place over adistance appreciably
Conclusions: We have shown that there are at least three shorter than awavelength. An example of such an interaction on
plausible models for the petrogenesis of high effective viscosity Earth would occur at the surface of a body of water. At radio
magmas on Venus, and we have suggested geologic environments wavelengths, water has an index of refraction of 9 (dielectric
in which these different mechanisms might be active. Chemical perrnittivity of about 80), and an associated loss factor that varies