Mercury's Internal Structure

We describe the current state of knowledge about Mercury's interior structure. We review the available observational constraints, including mass, size, density, gravity field, spin state, composition, and tidal response. These data enable the construction of models that represent the distribution of mass inside Mercury. In particular, we infer radial profiles of the pressure, density, and gravity in the core, mantle, and crust. We also examine Mercury's rotational dynamics and the influence of an inner core on the spin state and the determination of the moment of inertia. Finally, we discuss the wide-ranging implications of Mercury's internal structure on its thermal evolution, surface geology, capture in a unique spin-orbit resonance, and magnetic field generation.Comment: 36 pages, 11 figures, in press, to appear in "Mercury - The View after MESSENGER", S. C. Solomon, B. J. Anderson, L. R. Nittler (editors), Cambridge University Pres

Natural Resources Research Institute Technical Report

Large resources of Cu-Ni sulfides are found in troctolitic and gabbroic rocks at the base of the Duluth Complex in St. Louis and Lake Counties of northeastern Minnesota. Analysis of unpublished mining company data shows that there is a substantial reserve of PGE, Au and Ag associated with these sulfides. Weighted averages for combined Pt and Pd values vary as follows: 105 ppb in Water Hen, 278 ppb in Dunka Pit, 378 ppb in Minnamax, 570 ppb in Maturi, 651 ppb in Spruce Road to a high of 1259 ppb in Dunka Road. Au values vary from a low of 63 ppb in the Water Hen to a high of 137 ppb in the Spruce Road. Ag values vary from 1.22 ppm in Dunka Road to 3.8 ppm in the Minnamax deposit. Because recovery of PGE in copper-nickel flotation concentrates is very poor (usually less than 50%), these values add less than $5.00 to the ore. Even though these PGE and Au values are associated with the Cu-Ni sulfides, it appears that absolute values cannot be correlated with Cu, Ni and/or s contents. If sulfide values are below 0.2 wt %, then there are no appreciable PGE values. This is true for all deposits. However, if Pt+Pd/S is plotted against Cu/S, all sanples with high PGE contents appear to be related to samples with high Cu/S contents. Ag values, on the other hand, show a good correlation with absolute Cu content: r=+O.75 for all deposits and r=+0.86 for Minnamax data. The largest data base comes from the Minnamax deposit where metal values are further separated into Basal and Cloud zones. Basal zone sulfides are those that occur in the lowest 300 feet of the Duluth Complex. Cloud zone sulfides occur several hundred feet above the base of the Complex. In general, Basal zone sulfides consist of both massive and disseminated types, whereas Cloud zone sulfides are disseminated. At Minnamax, the weighted average sulfur content is 0.38% in the Cloud zone versus 2.78% in the Basal zone. The corresponding combined Pt and Pd values are, respectively, 192 and 396 ppb. Even though the absolute content in the Cloud zone is less, there is a higher metal to sulfur ratio than in the Basal zone, indicating an enrichment in PGE. This is also true for Cu and Ni contents. Ag contents, on the other hand, do not show this relationship. They are related to the absolute Cu content of the ore at Minnamax. Detailed studies of two anomalous samples, one from Water Hen and the other from Dunka Road, have identified some interesting minerals. PGE bearing minerals were only identified at Dunka Road. At Water Hen the following minerals were identified by using a reflecting microscope as well as a scanning electron microscope equipped with an EDS system: bornite, chalcopyrite, pentlandite (Ni rich), maucherite, sphalerite (pure ZnS) as inclusions in bornite, native Ag as a cross-cutting veinlet in maucherite, niccolite, parkerite (Ni3Bi2s2) , native Bi, and tentatively tetradymite (Bi2Te2s). Previous work by U.S. Steel identified the following minerals in the anomalous zone at Dunka Road: pyrrhotite, chalcopyrite, pentlandite, violarite, froodite (PdBi2), michenerite (PdTeBi), native Gold (Au,Ag), native Bi, and an unknown mineral composed of Pd, Sb and Bi. Textures within both of the samples indicate that pentlandite is being replaced by chalcopyrite and bornite at Water Hen and by violarite, chalcopyrite and the Au and Pd minerals at Dunka Road. These minerals appear to have been concentrated by later secondary copper rich fluids and are not part of the initial formation of Cu-Ni sulfides.Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442; Geology Department, University of Minnesota Duluth, Duluth MN 5581

Natural Resources Research Institute Technical Report

This project was undertaken with the objective to finish logging all drill holes from the basal contact zone of the Duluth Complex. Logging of Duluth Complex holes by Natural Resources Research Institute (NRRI) personnel began in 1989, when Severson and Hauck (1990) defined the igneous stratigraphy for most of the Partridge River intrusion (PRI). During the ensuing years the NRRI logged a total of 955 holes and defined igneous stratigraphic sections for several more intrusions of the Duluth Complex. As of 2005, a remainder of over 220 holes had yet to be logged. At the end of this project, 295 holes, which include some recently-drilled holes, were logged with about 20 holes still to be logged from the far eastern end of the Mesaba deposit. Lithologic logs for most of the holes that have been logged since 1989 are now available on the NRRI Geology Group’s website at www.nrri.umn.edu/egg/. The vast majority of holes that were logged for this project were from the Mesaba (Babbitt) Cu-Ni±PGE deposit, and thus, this report deals mostly with that deposit. A result of logging a large number of holes at the Mesaba deposit indicates that most of the deposit does not exhibit a stratigraphic package that has been recognized within the nearby Partridge River intrusion. This suggests that most of the deposit is situated within another sub-intrusion, informally called the Bathtub intrusion (BTI). The BTI appears to have been fed by a vent in the Grano Fault area on the east side of the Mesaba deposit. Forty-two cross-sections from the Mesaba deposit, showing the geology in over 450 surface holes, are presented in this report. Another 26 cross-sections, showing the geology in 219 underground holes, are also presented for the Local Boy ore zone of the Mesaba deposit. All of these cross-sections are utilized to define the igneous stratigraphy of the BTI and adjacent PRI at the deposit. All publically-available drill holes have now been logged from the Dunka Pit Cu-Ni deposit located in the South Kawishiwi intrusion (SKI). Nineteen cross-sections through the deposit are presented in this report. These cross-sections show the geology, potential Cu-Ni ore zones in the holes, and the down dip extent of potential mineable zones of the Biwabik Iron Formation at depth. Additional areas in the SKI where holes were logged for this project include the Maturi, Spruce Road, and Nokomis deposits. Cross-sections and hung stratigraphic sections are presented, and they show the geology intersected in these newly-logged holes relative to previously-logged holes. Drill holes from two Oxide-bearing Ultramafic Intrusions (OUI) were also logged for this investigation. These logs include ten holes from the Longnose deposit and ten holes from the Water Hen deposit. Six cross-sections through the Longnose deposit are presented in this report. In summary, the holes logged in this investigation have added greatly to our understanding of the geology of basal portions of the Duluth Complex. In some cases, the previously defined igneous stratigraphic sections for the various intrusions have held up remarkably well as additional holes are drilled and logged. Of course, there are always some exceptions to the rule. In other cases, e.g., the Mesaba deposit, as more holes were logged and/or drilled, the igneous stratigraphy had to be modified in order to explain differences in a group of holes that were situated in the BTI versus the nearby PRI. This change serves as an example that definition of igneous units, and modes of mineralization, in the Duluth Complex is an iterative process and has to be continuously refined as more data, in the form of new drill holes, are generated

Natural Resources Research Institute Technical Report

Detailed relogging of drill holes (83 holes totalling 100,630 feet of core) and reconnaissance mapping have delineated three major rock groups within a portion (T.58-59 N., R.13-14 W.) of the Partridge River intrusion (PRI), Duluth Complex, Northeastern Minnesota. These have been informally designated as the Partridge River Troctolitic Series (PRTS), Partridge River Gabbro Complex (PRGC) and Oxide-bearing Ultramafic Intrusions (OUI). The PRTS consists of at least eight major igneous units which are correlatable in drill holes over an indicated eleven mile strike length extending (NE to SW) from the Dunka Road Cu-Ni deposit to the Wyman Creek Cu-Ni deposit. From the base up, these units are characterized by: Unit I - sulfide-bearing augite troctolite with minor picrite to peridotite layers; Unit II - troctolite and augite troctolite, with abundant picrite to peridotite layers (Wetlegs Cu-Ni area) and/or minor sulfide-bearing zones; Unit III - mottled textured anorthositic troctolite exhibiting a highly irregular olivine oikocryst distribution; Unit IV -augite troctolite with a picritic base and grading upwards into Unit V; Unit V - coarse-grained anorthositic troctolite; Unit VI - augite troctolite to anorthositic troctolite with a picritic base; and Unit VII - augite troctolite with a well-bedded peridotite-picrite base. Field mapping suggests that an eighth unit (Unit VIII) and possibly additional units are present above Unit VII. Unit VIII consists of troctolite to anorthositic troctolite with a well-bedded peridotite base. Most of the upper units (III-VIII) represent single cooling units in that they are floored by a bedded ultramafic member; whereas, other units (I and II) near the footwall exhibit an overall heterogeneous nature and contain abundant internal members reflecting continuous magma replenishment. Some of the units also exhibit downcutting relationships and lateral "facies" changes along strike indicating a complex intrusive history. Structural studies of the basal contact of the Partridge River intrusion have indicated more structure than previously recognized. Structure contour maps of the footwall rocks at the basal contact of the Duluth Complex and on the top of the Biwabik Iron-Formation, and isopach maps of the Virginia Formation beneath the PRI indicate that pre-existing folds in the basement rocks at both Minnamax and Dunka Road exerted a strong control over the form of the base of the intrusion. Cross-sections illustrating the internal "stratigraphy" indicate that in both the Dunka Road and Wetlegs areas, numerous NE-trending normal faults parallel to the Mid-continent Rift are present. These faults support the halfgraben model (Weiblen and Morey, 1980) which envisions a step-and-riser geometry at the base of the Duluth Complex due to extensional tectonics. However, most of the faults delineated show corresponding offsets in both the troctolitic and footwall rocks and are, thus, not true half-graben faults as envisioned in the model. The only exception is within the Wetlegs area where a NE-trending fault exhibits substantial offset in the footwall rocks, but no offset is present in the overlying troctolite rocks. An inferred window of Biwabik Iron- Formation is in direct contact with the PRI along this fault. Three late-stage Oxide-bearing Ultramafic Intrusions (OUI) are also located along this zone that suggests they may be genetically related to areas where massive iron-formation assimilation has occurred. The OUIs are later pegmatitic intrusives consisting of dunite, peridotite, clinopyroxenite, and lesser picrite and melagabbro; all are oxide-bearing (> 10%) and contain semi-massive to massive oxide horizons. These bodies are intrusive into the PRTS and include the Longnose, Longear, Section 17, Wyman Creek, and Skibo Fe-Ti prospects. The PRGC is situated at the southeastern portion of the investigated area and consists dominantly of oxide-bearing gabbroic and troctolitic rocks; both locally exhibit excellent modal bedding, which may be related to magmatic density currents. The Colvin Creek "Gabbro" (CCG) is part of the PRGC and was originally interpreted to be a hornfelsed basalt. However, reconnaissance mapping indicated that similar fine-grained CCG-type "gabbro" is present within the coarse-grained rocks of the Powerline Gabbro and vice versa. Because the Powerline Gabbro is located near the CCG, the two bodies may be intricately related. Within the Colvin Creek "Gabbro" are several unusual sedimentary-like structures that are not indicative of typical North Shore Volcanic basalts. However, textures resembling vesicles/amygdules are locally present. The unusual sedimentary-like structures suggest a magmatic density current origin but the exact origin of these textures is enigmatic. Also within the Colvin Creek "Gabbro" is a mile-long 1,000 foot-thick belt of cross-bedded rocks. Several internal features of these cross-bedded rocks, e.g., lack of rock fragments, no quartz, are not indicative of typical interflow sandstones and their relationship to the surrounding rocks suggests they may have also been deposited by magmatic density currents. The unmineralized portions of all the units were sampled (155 samples) in order to establish background geochemical levels and lithogeochemical signatures for each unit and to investigate possible origins for the different units. Background Pd, Pt, and Au values in the major rock groups average 10 ppb, 20 ppb, and 5 ppb, respectively. However, slightly elevated background values are associated with Unit II (15 ppb, 24 ppb, and 9 ppb, respectively), and the OUI rock group (15 ppb, 24 ppb, and 17 ppb respectively). In the course of sampling unmineralized rock (200 ppb combined Pd and Pt) were revealed with a maximum of 910 ppb. The OUI units are the most geochemically unique in that they have elevated background values for TiO2, V, Cr, Co, Cu, Cd, C, Be, Sc, Sb, Pb, Te, Au, and W relative to the other igneous units. Geochemical data support the various rock units identified during relogging of the PRI. Units I and II exhibit a markedly different geochemical signature when compared to the other PRTS units. One interpretation of this difference is that magma contamination due to assimilation of footwall material was important in their genesis. All rock units of the PRGC have the same geochemical signature and, in turn, this geochemical signature is similar to the geochemical signature for the lower half of Unit I. The OUI units exhibit a markedly different geochemical signature when compared to all the other PRI units.Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442; Funded by Minerals Diversification Plan of the Minerals Coordinating Committe

Internal and Tectonic Evolution of Mercury

Mercury's geological and internal evolution presents an interesting enigma: are there conditions that allow for both apparently limited radial contraction over the last 4 billion years and sufficiently rapid core cooling at present to permit a hydromagnetic dynamo? To address this question, we simulate the coupled thermal, magmatic, and tectonic evolution of Mercury for a range of parameters (e.g., mantle rheology, internal heat production, core sulfur content) in order to outline the set of assumptions most consistent with these two conditions. We find that among the models tested, the only ones strictly consistent with ∼1-2 km of radial contraction since 4 Ga and a modern magnetic field generated by a core dynamo are those with a dry-olivine mantle rheology, heat production provided primarily by Th (negligible U or K), and a bulk core sulfur content >6.5 wt%. However, because of the limited coverage and resolution of Mariner 10 imaging and derived topography, the tectonic history of an entire hemisphere is unknown. The potential for other mechanisms (e.g., long-wavelength lithospheric folds) to accommodate contraction remains untested, limiting the ability to restrict models on the basis of accumulated strain. Furthermore, Mercury's magnetic field may be a consequence of a thermoelectric dynamo or even crustal remanence; neither hypothesis places strong constraints on current heat flux from the core. Spacecraft observations of Mercury are needed to elucidate further the internal structure and evolution of the planet

Mercury's Internal Structure

We describe the current state of knowledge about Mercury's interior structure. We review the available observationalconstraints, including mass, size, density, gravity eld, spin state, composition, and tidal response. These data enablethe construction of models that represent the distribution of mass inside Mercury. In particular, we infer radial prolesof the pressure, density, and gravity in the core, mantle, and crust. We also examine Mercury's rotational dynamicsand the inuence of an inner core on the spin state and the determination of the moment of inertia. Finally, we discussthe wide-ranging implications of Mercury's internal structure on its thermal evolution, surface geology, capture in aunique spin-orbit resonance, and magnetic eld generation

Thermal Evolution of Mercury as Constrained by MESSENGER Observations

Orbital observations of Mercury by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft provide new constraints on that planet's thermal and interior evolution. Specifically, MESSENGER observations have constrained the rate of radiogenic heat production via measurement of uranium, thorium, and potassium at the surface, and identified a range of surface compositions consistent with high-temperature, high-degree partial melts of the mantle. Additionally, MESSENGER data have placed new limits on the spatial and temporal variation in volcanic and tectonic activity and enabled determination that the planet's core is larger than previously estimated. Because Mercury's mantle layer is also thinner than previously thought, this result gives greater likelihood to the possibility that mantle convection is marginally supercritical or even that the mantle is not convecting. We simulate mantle convection and magma generation within Mercury's mantle under two-dimensional axisymmetry and a broad range of conditions to understand the implications of MESSENGER observations for the thermal evolution of the planet. These models demonstrate that mantle convection can persist in such a thin mantle for a substantial portion of Mercury's history, and often to the present, as long as the mantle is thicker than ~300 km. We also find that magma generation in Mercury's convecting mantle is capable of producing widespread magmas by large-degree partial melting, consistent with MESSENGER observations of the planet's surface chemistry and geology

Effect of core--mantle and tidal torques on Mercury's spin axis orientation

The rotational evolution of Mercury's mantle and its core under conservative and dissipative torques is important for understanding the planet's spin state. Dissipation results from tides and viscous, magnetic and topographic core--mantle interactions. The dissipative core--mantle torques take the system to an equilibrium state wherein both spins are fixed in the frame precessing with the orbit, and in which the mantle and core are differentially rotating. This equilibrium exhibits a mantle spin axis that is offset from the Cassini state by larger amounts for weaker core--mantle coupling for all three dissipative core--mantle coupling mechanisms, and the spin axis of the core is separated farther from that of the mantle, leading to larger differential rotation. The relatively strong core--mantle coupling necessary to bring the mantle spin axis to its observed position close to the Cassini state is not obtained by any of the three dissipative core--mantle coupling mechanisms. For a hydrostatic ellipsoidal core--mantle boundary, pressure coupling dominates the dissipative effects on the mantle and core positions, and dissipation together with pressure coupling brings the mantle spin solidly to the Cassini state. The core spin goes to a position displaced from that of the mantle by about 3.55 arcmin nearly in the plane containing the Cassini state. With the maximum viscosity considered of $\nu\sim 15.0\,{\rm cm^2/s}$ if the coupling is by the circulation through an Ekman boundary layer or $\nu\sim 8.75\times 10^5\,{\rm cm^2/s}$ for purely viscous coupling, the core spin lags the precessing Cassini plane by 23 arcsec, whereas the mantle spin lags by only 0.055 arcsec. Larger, non hydrostatic values of the CMB ellipticity also result in the mantle spin at the Cassini state, but the core spin is moved closer to the mantle spin.Comment: 35 pages, 7 figure

Mercury's Moment of Inertia from Spin and Gravity Data

Earth-based radar observations of the spin state of Mercury at 35 epochs between 2002 and 2012 reveal that its spin axis is tilted by (2.04 plus or minus 0.08) arc min with respect to the orbit normal. The direction of the tilt suggests that Mercury is in or near a Cassini state. Observed rotation rate variations clearly exhibit an 88-day libration pattern which is due to solar gravitational torques acting on the asymmetrically shaped planet. The amplitude of the forced libration, (38.5 plus or minus 1.6) arc sec, corresponds to a longitudinal displacement of ∼450 m at the equator. Combining these measurements of the spin properties with second-degree gravitational harmonics (Smith et al., 2012) provides an estimate of the polar moment of inertia of MercuryC/MR2 = 0.346 plus or minus 0.014, where M and R are Mercury's mass and radius. The fraction of the moment that corresponds to the outer librating shell, which can be used to estimate the size of the core, is Cm/C = 0.431 plus or minus 0.025