Browsing by Subject "Peridotite"
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Item The effect of water on partial melting in the upper mantle.(2010-06) Tenner, Travis JayThis thesis presents experimental constraints on incipient melting of mantle peridotite under hydrated conditions. High P-T experiments were performed at pressures of 3 to 13 GPa, and at temperatures of 1200-1450°C. These experiments measure mineral/melt H 2 O partitioning and storage capacity of peridotite components, as well as determine melting phase relations and the compositions of partial melts and residues of hydrated peridotite. Incipient melt H 2 O concentrations are estimated by peridotite/melt H 2 O partitioning ([Special characters omitted.] ). To parameterize [Special characters omitted.] , mineral/melt H 2 O partition coefficients were determined for all crystalline phases of the peridotite solidus assemblage (Chapter 2). Combining these [Special characters omitted.] values with corresponding modal abundances along the solidus yields a [Special characters omitted.] of 0.005-0.010 from 1 to 5 GPa, which is dependent on pressure due to varying garnet and pyroxene modal abundances, and to variable pyroxene Al content. This [Special characters omitted.] range predicts that incipient melts of MORB source (50-200 ppm H 2 O bulk ) and OIB source (300-1000 ppm H 2 O bulk ) upper mantle contain 0.5-3.8 wt.% and 3-20 wt.% dissolved H 2 O, respectively. The amount of dissolved H 2 O in incipient melt dictates hydrous solidus depression, Δ T , which ultimately controls the stability of hydrous melts at P and T . This Δ T -H 2 O melt relationship was investigated at 3.5 GPa by partially melting hydrated peridotite from 1200-1450°C (Chapter 3). Mass balance of phases allows for determination of melt fractions ( F ) from experiments, as well as estimation of H 2 O melt . Δ T values are quantified as the difference in melting temperature between dry and wet peridotite at a particular F . Parameterization of Δ T as a function of H 2 O melt predicts that solidus melts with 1.5, 5, 10, and 15 wt.% dissolved H 2 O generate Δ T values of 50, 150, 250, and 300°C, respectively. Combination of this paramterization with [Special characters omitted.] (Chapter 2) insinuates that 500 ppm H 2 O bulk is necessary to stabilize melt across the observed seismic low velocity zone (LVZ) beneath oceanic lithosphere, which is significantly greater than the MORB source upper mantle H 2 O bulk of 50-200 ppm. This observation argues against suggestions that hydrous melting is solely responsible for the LVZ. At higher pressures the aforementioned parameterizations are difficult to constrain experimentally, but the onset of hydrous melting can be determined by the peridotite H 2 O storage capacity, defined as the maximum H 2 O concentration that peridotite can store without stabilizing a hydrous fluid or melt. A new method of determining a minerals H 2 O storage capacity is employed, in which a hydrated monomineralic layer is equilibrated with a layer of hydrated peridotite and a small amount of melt (Chapter 4). Experiments were carried out at conditions near the 410 km transition zone (TZ) depth to investigate hydrous melting due to the H 2 O storage capacity contrast between the TZ and upper mantle. Measured olivine and orthopyroxene H 2 O storage capacities, combined with estimates of garnet H 2 O storage capacity, and P -dependent lherzolite modes, yields a peridotite H 2 O storage capacity of 700-1100 ppm directly above 410 km. This is not consistent with pervasive melting above 410 km, as this range is several times greater than MORB source upper mantle H 2 O bulk . However, regional melting in areas such as H 2 O-rich OIB source, or areas of recent subduction may likely occur, leaving residues with ∼1000 ppm H 2 O bulk .Item Emplacement and Crystallization Histories of Cu-Ni-PGE Sulfide-mineralized Peridotites in the Eagle and Eagle East Intrusions(2018-06) Mulcahy, ConnorThe Eagle and Eagle East intrusions, located about 40 kilometers northwest of Marquette, MI, are two small, partially exposed, sub-vertical, funnel-shaped mafic/ultramafic intrusions emplaced in Paleoproterozoic black slates. Both intrusions host economic Ni-Cu-(PGE) sulfide deposits, the Eagle intrusion in its main body and the Eagle East intrusion in its feeder at depth. The Eagle deposit has been being mined by the Lundin Mining Corporation since 2014, which is now also expanding its operation to mine the Eagle East deposit. Transmitted light petrography, scanning electron microscopy, and electron microprobe analyses were performed on samples from six drill cores in the Eagle system, three from Eagle and three from Eagle East. Lundin additionally provided whole-rock geochemistry for five of these cores at ~1.5m intervals. The concentration of Ni in olivines in the Eagle and Eagle East intrusions were measured by electron microprobe. A bimodal distribution of Ni concentration in olivine - i.e., both enriched and unenriched populations being present - may have been evidence for multiple magma pulses in the Eagle system. However, olivine in both intrusions were determined to be universally Ni-enriched, which means that this line of inquiry was not useful for determining the number of magma pulses. The cumulate nature of samples were determined by whole-rock geochemistry, wherein incompatible trace elements including Zr, and La were used as proxies for the amount of intercumulus material present in a sample, as well as by visual estimation using transmitted-light petrography. Counter to the conclusions of Ding et el. (2010), variations in incompatible trace element ratios in various rock types in the Eagle system were satisfactorily explained by the cumulate nature and high sulfide content of the samples, with no need to invoke multiple parental magmas in the explanation. The intrusive breccia (or “IBRX”) lithology present in both the Eagle and Eagle East intrusions was studied with transmitted light petrography. It was determined to occur in at least two variations. Both variations have a feldspathic lherzolite matrix with subangular clasts, but in one type heavy sulfide mineralization (up to 30% by volume) occurs in the clasts and in the other type the clasts are nearly devoid of sulfides. In both cases the clasts have high pyroxene abundances and are devoid of olivine, but clasts with high sulfide content tend to have more plagioclase and are more heavily altered. In the Eagle East intrusion, sampled clasts were only of the low-sulfide variety. The IBRX clasts may be a slower-cooling version of the PRX lithology also present in the Eagle system. Alternatively, they may be part of an older intrusion that the Eagle system parental magma cannibalized at depth during emplacement. The main body of the Eagle East intrusion was studied by petrographic examination of a core that profiled its depth. While there was no significant change in cumulate rock type, the core did show modest cryptic variation with depth. Notably, a horizon of increased olivine abundance indicated the potential recharge of the intrusion with the same, homogeneous parental magma. The lithological similarity of the Eagle and Eagle East intrusions indicates that they likely formed from the same parental magma. The main petrographic differences between the two intrusions were the poikilitic nature of clinopyroxene and the lower abundance of plagioclase in the Eagle East intrusion. These differences may be explained by the larger size and thus presumed longer cooling time of the Eagle East intrusion.Item Geology and Mineralization in the Dunka Road Copper-Nickel Mineral Deposit, St. Louis County, Minnesota(University of Minnesota Duluth, 1990-03) Monson Geerts, Stephen D; Barnes, Randal J; Hauck, Steven AThe Dunka Road Cu-Ni deposit is within the Partridge River Intrusion (T. 60 W., R. 13 W.), which is part of the Duluth Complex, and is approximately 1.1 b.y. (Keweenawan) in age. Relogging of 46 drill holes at the Dunka Road Cu-Ni deposit identified four major lithologic units and several internal ultramafic subunits that can be correlated over two miles. The ultramafic subunits (layers of picrite to peridotite) exhibit relative uniform thicknesses and are present at the same relative elevation within the major lithologic units. The major lithologic units, the same as delineated by Severson and Hauck (1990), and upward from the basal contact are: Unit I, a fine- to coursegrained a sulfide-bearing troctolite to pyroxene troctolite (450 ft. thick) with associated ultramafic layers I(a), I(b), and I(c); Unit II, a medium- to coarse-grained troctolite to pyroxene troctolite (200 ft. thick) with a basal ultramafic layer II(a); Unit III, a finegrained, mottled textured troctolitic anorthosite to anorthositic troctolite (150 ft. thick) with one minor ultramafic layer III(a); and Unit IV, a coarse-grained troctolite/pyroxene troctolite to anorthositic troctolite with associated ultramafic layers IV(a) and IV(b). Most sulfide mineralization occurs within Unit I. Within Unit I the sulfide mineralization is both widespread but variable in modal percentage (rare to 5%), continuity and thickness (few inches to tens of feet). Sulfide mineralization is somewhat related with proximity to: hornfels inclusions, the basal contact with the footwall Virginia Formation, and some of the internal ultramafic layers within Unit I. Precious metal mineralization (Pd+Pt+Au) is associated with fracturing and alteration of the host rocks. The alteration assemblage is chlorite, bleached plagioclase, serpentine and uralite. Pd+Pt values range from 100 to >2400 ppb over 10 foot intervals. These intervals can occur independently as 10 to 50 foot zones, or as part of a larger correlatable occurrence/horizon. Two mineralized subareas within the Dunka Road deposit are: 1) an area which is peripheral to a highly anomalous Pd occurrence (reported by Morton and Hauck, 1987; 1989) herein termed the "southwest area", and 2) the "northeast area" which contains several drill holes that have near surface intercepts of >1% Cu. There are four somewhat large mineralized occurrences within the study area that carry >300 ppb combined total Pt+Pd+Au. These mineralized zones appear to be stratigraphically controlled by the ultramafic subunits within Unit I. Three of the four correlatable zones are found within the southwest area, and range from 40 to 130 feet thick. High Pd values within these zones range from 10 to 20 feet thick with values of 800 to 1650 ppb Pd. In the northeast area, the fourth mineralized zone appears continuously throughout Unit I. This zone ranges from 120 to 300 feet thick. High Pd values within this zone range from 10 to 40 feet thick with values of 800 to 1500 ppb Pd. Many 5 to 30 foot intersections of >1 ppm Pd+Pt+Au occur throughout the mineral deposit. Geostatistical analysis based on 72 vertical holes and 12 angle holes suggests: 1) the base of the complex is a critical datum with the higher grade intercepts located between 100 and 400 feet above the base; 2) high inter-element correlations support local redistribution/concentration of primary mineralization by a secondary hydrothermal process and thus, polymetallic mining selectivity is possible; 3) the available drilling gives a spacial range of geologic influence at 400 foot centers, but sufficient angle drilling is not available to assess the potential of high grade, steeply dipping mineralized zones; 4) additional vertical in-fill drilling will almost certainly not discover any additional quantity of ore within the volume of rock studied; but 5) additional angle drilling to assess the potential of high grade, steeply dipping, mineralized zones would benefit a more complete geostatistical analysis.Item Geology and Mineralization of a Cyclic Layered Series, Water Hen Intrusion, St. Louis County, Minnesota(University of Minnesota Duluth, 1990-03) Strommer, James; Morton, Penelope; Hauck, Steven A; Barnes, Randal JThe Water Hen intrusion is an oxide-bearing (ilmenite + magnetite) ultramafic intrusion (OUI) that is emplaced along a pre-basement fault into the troctolitic series rocks of the Duluth Complex. The intrusion consists of medium-grained dunite and peridotite and local pegmatitic pyroxenite approximately 1,600 ft. x 500 ft. x 700 ft. in size. Oxide (>90% ilmenite) composes from 5-50% of the various lithologies. Sulfides are minor, about 2-5%, and are predominantly pyrrhotite with minor cubanite, chalcopyrite and pentlandite. Concentrations of 5-80% graphite also occur within the intrusion. Surrounding the Water Hen intrusion is a zone of mixed lithologies (Mixed Zone) consisting of the host rock troctolites, apophyses of OUI and local inclusions of footwall rocks. The Mixed Zone (M) is dominated by >60% troctolitic rocks with OUI composing the remainder. The OUI apophyses vary from 1-50 ft. thick and have sharp contacts with the country rock. The troctolitic host rocks for the Water Hen intrusion consist of medium- to coarsegrained troctolite to anorthositic troctolite (TA unit) and a troctolitic cyclically layered series (TL unit). The cyclically layered series is similar to troctolitic layered rocks at Bardon Peak. The individual cyclic layers are 6 in. to 10 ft. in thickness and the entire unit is over 300 ft. thick. The An content decreases from An80 at the bottom of the unit to An60 near the top of the unit. The individual cyclic layers are composed of ilmenite-dunite at the base and grade upward to anorthositic troctolite. The bottom contacts are sharp and each successive layer within the individual unit is identified by the occurrence of biotite or clinopyroxene. In the bottom olivine-rich layer, the oxides (<5%) are ilmenite >> magnetite. The sulfides in this same layer (3-5%) are coarse-grained with cubanite > chalcopyrite > pentlandite >> pyrrhotite. In the more feldspathic layers, the sulfides (1-3%) are fine-grained with chalcopyrite >> pentlandite = cubanite + pyrrhotite. The oxides (1- 5%) are also fine-grained with ilmenite >> magnetite. The footwall rocks in the Water Hen area consist of very fine-grained metamorphosed Virginia Formation and fine-grained hornfelsed basalt and/or troctolite. There are >100 ft. of basalt or chilled margin rocks within the footwall. This mafic hornfels commonly occurs between the Virginia Formation and the TA unit. Orthopyroxenite dikes and dikelets also occur in the mafic hornfels. These dikes contain anomalous PGEs and secondary sulfide mineralization. The copper-nickel sulfides are primary igneous sulfides associated with the troctolitic rocks. Violarite, pyrite and secondary magnetite in cross-cutting veinlets and other secondary sulfides indicate that the primary sulfides were altered and remobilized by a later event. Cu:Ni ratios have a bimodal distribution that is not followed by the PGEs. However, Cu, Ni, Ag, Au, Pt, Pd are all highly correlated with each other. This high interelement correlation suggests that the late-stage (secondary) remobilization locally redistributed and reconcentrated these elements.Item Geology of the Southern Portion of the Duluth Complex(University of Minnesota Duluth, 1995-12) Severson, Mark JThe Duluth Complex (Middle Proterozoic - 1,099 Ga) is a large intrusive body that contains numerous smaller intrusions that collectively comprise the Complex. Recent work has shown that igneous stratigraphic sections can be delineated within these intrusions through detailed relogging of drill core, e.g., for the Partridge River intrusion (Severson and Hauck, 1990; Severson, 1991) and the South Kawishiwi intrusion (Severson, 1994). This report pertains to the igneous geology of the South Complex area. More than 140 drill holes are located in the "South Complex" area. Most of these holes are relogged (112 holes, 88,000 feet of core) and are correlated into several troctolitic to gabbroic stratigraphic units for several specific areas in the South Complex that have abundant drill holes. While each individually drilled area exhibits good correlative units, these correlative units do not extend into an adjacent drilled area that is located only a few miles distant. This lack of large-scale continuity suggests: 1) the South Complex study area constitutes an area that actually includes several smaller intrusive bodies; 2) drilling is not detailed enough to delineate large-scale correlative units; 3) because most of the drill holes are located close to the basal contact, the effects of contamination to the magma, via assimilation of footwall rocks, hampers large-scale correlations; or 4) combinations of the above. Most of the holes within the South Complex were drilled during exploration for Cu-Ni sulfide mineralization. Only weak sulfide mineralization is present in these drill holes. However, many of the holes intersect small plug-like bodies of Oxide-bearing Ultramafic Intrusions (OUIs) that are intrusive into the troctolitic rocks of the Complex. The OUIs are characterized by coarsegrained to pegmatitic clinopyroxenite, picrite, peridotite, and dunite. Oxide content in the OUI varies from disseminated (15%-20%) to thick massive oxide zones. Ilmenite is the dominant oxide in some OUIs; whereas, titanomagnetite is dominant in others. In almost all instances, the OUIs are spatially arranged along linear trends, suggesting that structural control was important to their genesis. At some localities (northern end of the South Complex), an empirical link between ironformation assimilation near the basal contact and OUI formation is apparent. This relationship suggests that the OUIs were initially formed at depth followed by upward injection of OUI material along fault zones. However, other OUI (southern end of the South Complex) are situated within, or immediately below, layered oxide-rich gabbroic rocks, suggesting that the OUIs formed from a differentiated iron-rich melt that drained down into the cumulate pile along fault zones. These two different OUI groups (north and south) also show some corresponding differences in chemistry. The north OUIs are characterized by relatively higher chromium contents and the south OUIs have relatively higher vanadium contents. All of the OUIs contain titanium mineralization and some sulfide mineralization. A model of origin for the OUIs involving metasomatic replacement of preexisting igneous rock is not considered to be plausible. Also present within the South Complex area are fine-grained granular rocks that are hornfelsed inclusions of basalt and troctolitic-gabbroic-noritic rocks. One of these inclusions, referred to as the FN Unit, is only observed in drill holes in the southern half of the South Complex area. The unit exhibits vesicle-like features in drill core and has often been referred to as a hornfelsed basalt. However, several features argue against a basalt protolith for the FN Unit. These features include the presence of abundant footwall hornfels inclusions within the unit, common gradations into medium-grained intrusive rock, and a "rind-like" overall pattern of the unit at the basal contact at Water Hen. These characteristics suggest that the FN Unit represents an earlier pulse of magma (chilled?) into the footwall rocks that was later hornfelsed by subsequent intrusions of the Complex. The Bear Lake Inclusion, present in numerous outcrops and one drill hole, probably represents a large inclusion of magnetic basalt. The inclusion is a massive rock with no distinct volcanic features, but is similar to magnetic basalt inclusions described elsewhere in the Complex (Colvin Creek Inclusion, and "INCL" unit within the South Kawishiwi intrusion; Severson and Hauck, 1990; Severson, 1994; Patelke, 1996). The Bear Lake Inclusion is over 500 feet thick and dips gently to the southeast. It is located well into the interior of the Complex and is not related to the basal contact (as is the FN Unit). Geochemical plots are constructed for many of the igneous units of the South Complex area. These plots are not particularly instructive in discriminating between the units because many of the spider profiles are fairly similar, and in the X-Y plots only a few units cluster within distinct fields. However, some conclusions can still be drawn from these data. First, similarities in geochemistry indicate that some units of the nearby Partridge River intrusion are present as far south as Water Hen. Second, the FN Unit is chemically similar to both troctolitic to gabbroic rocks, even in the same drill hole. This relationship supports an earlier intrusive protolith rather than a basalt protolith. Third, the north and south OUI can be separated into two groups based on similarities in spider diagram profiles. However, the profiles for the north OUI show similar profiles that alternate with geographic location. The reason for this "leap frog" alternation in profiles is unknown at this time, but may be related to more than one OUI-forming event along a fault zone. Last, rocks of the Bear Lake Inclusion are chemically similar to rocks of the Colvin Creek inclusion (Severson & Hauck, 1990; Patelke, 1996) and the "INCL" unit of the South Kawishiwi intrusion (Severson, 1994); all of which have been inferred to be magnetic basalts. A sample of a semi-massive oxide horizon (0.8 ft. thick), associated with subhorizontal, ultramafic layers (picrite, peridotite, etc.) near the Water Hen area (drill hole SL-19A) has been found to contain anomalous PGE and chromium values (Pt = 737-786 ppb, Pd = 63-106 ppb, Cr = 46,000 ppm). This semi-massive oxide horizon is similar in many respects to PGE- and Cr-enriched semi-massive to massive oxide horizons located elsewhere within the Duluth Complex (Birch Lake and Fish Lake areas). The data suggest that the PGE in SL-19A are magmatic and have not been redistributed by hydrothermal fluids, as has been suggested for other areas within the Complex. Additional targets of vein-like PGE-enriched Cu-Ni ore are also present in the Skibo and Water Hen areas. These targets could potentially have formed via fractional crystallization of a sulfide melt in a vein-like setting.Item Geology, Geochemistry, and Stratigraphy of a Portion of the Partridge River Intrusion(University of Minnesota Duluth, 1990-03) Severson, Mark J; Hauck, Steven ADetailed 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 (<1% sulfides), five anomalous samples (>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.Item RI-70, Characterization of the Franklin Peridotite and Other Similar Intrusions in East-Central and Southwestern Minnesota(2014) Boerboom, Terrence J.Outcrops of peridotite adjacent to the Minnesota River near the town of Franklin in Renville County were sampled and petrographically characterized as part of a small study funded by the Minnesota Department of Natural Resources in 1997. That study obtained mineral separates with the intent of examining them for kimberlite indicator minerals. The results were not formally published, but rather summarized in an unpublished final report to the Minnesota Department of Natural Resources titled "Mineral Investigations of Franklin Kimberlites." In addition to petrographic and geochemical characterizations, ground magnetic traverses were made across the outcrop area in order to quantify the size and shape of the peridotite body. Based on simple ground magnetic surveys, the peridotite body is approximately 1 square kilometer (0.4 square mile) in area. The peridotite in the outcrops is extensively silicified, most likely by low-temperature alteration associated with lateritic weathering beneath Cretaceous sedimentary strata. Peridotite that is not silicified is composed of olivine (serpentinized), orthopyroxene, hornblende, magnetite, and minor spinel, phlogopite, ilmenite, and sulfide minerals; all the silicate phases are Mg-rich. The silicified peridotite contains abundant secondary quartz and chalcedony, but the silicification did not affect the Fe/Mg ratio, as both silicified and unsilicified peridotite have Mg numbers of 87 to 88. The Franklin peridotite is similar to ultramafic peridotite and pyroxenite bodies in east-central Minnesota, as well as the Cottonwood peridotite body intersected by drilling in northern Lyon County, 55 kilometers (34 miles) west–northwest of the Franklin peridotite and south of the Minnesota River valley. The peridotites in east-central Minnesota are between 1,770 and 1,791 Ma in age, whereas the age of the Franklin and Cottonwood peridotites is unknown.Item The role of partial melts of peridotite in the formation of oceanic island basalts.(2012-08) Davis, Frederick ArthurDespite the classic hypothesis that primary basalts are generated by partial melting of peridotite, several compositional characteristics of oceanic island basalts (OIB) are not easily reconciled with such an origin. Many OIB have higher FeO and lower Al2O3 concentrations than do experimentally-derived partial melts of peridotite; however experiments have not been performed at both the high pressures (3 GPa) and low melt fractions relevant to OIB formation. Trace element concentrations and ratios of many first-row transition elements (FRTE) in OIB and associated phenocrysts may also suggest that non-peridotitic melting lithologies play a role in OIB petrogenesis. This thesis presents the results of high-pressure experiments designed to test the hypothesis that oceanic island basalts (OIB) can be derived solely by partial melting of a peridotite source. Experiments were performed in a piston cylinder apparatus at 3 GPa and temperatures near the solidus of peridotite (1430-1470 °C). Experiments to determine the major-element composition of the incipient melt of garnet lherzolite employed a recently developed technique, modified iterative sandwich experiments (MISE), which simulates near-solidus partial melting while allowing for the preservation of large melt pools to facilitate chemical analysis. The resulting melts are similar to alkalic OIB in many respects, but are still higher in Al2O3 (12.7±0.2 wt.%) and lower in FeO* (9.7±0.1 wt.%) than most OIB. Trace element partitioning experiments were performed to determine mineral/melt partition coefficients for several FRTE (Sc, Ti, V, Cr, Mn, Fe, Co, Zn) and Ga and Ge during partial melting of peridotite. Experiments were cooled slowly from ≥50 °C above the intended dwell temperature to facilitate growth of large crystals for trace element analysis by LA-ICP-MS. Measured partition coefficients form the basis of a partial melting model that predicts FRTE concentrations and ratios in partial melts of peridotite at 3 GPa. Partial melts of peridotite have Fe/Mn < 62, Zn/Fe < 13*10-4, and Co/Fe > 7*10-4. Many OIB have higher Fe/Mn and lower Co/Fe than can be obtained by partial melting of peridotite, at least under relatively reducing conditions. Experiments were performed using the starting compositions from the MISE experiments but with added K2O in the melt to determine the effects of K2O on the composition of near-solidus melts of peridotite at 3 GPa. In resulting melts SiO2 increases and CaO decreases by ~0.5 wt.% for each 1 wt.% increase in the K2O content of the melt. Al2O3 increases and Cr2O3 and Na2O decrease with increasing K2O content of the melt. Increased K2O content moves the compositions of near-solidus melts of peridotite toward the low-CaO, high-SiO2 EM-type OIB, but the effect is not strong enough to match these compositions at comparable K2O concentrations. Spinel lherzolite KLB-1, which is used as a starting material in many high pressure experiments, was analyzed for major and minor elements. To perform the bulk analysis, powdered KLB-1 was glassed by laser melting of aerodynamically levitated spheroids. The glass beads and minerals separates from the powdered sample were analyzed by electron microprobe analysis. This measurement resolves conflicting FeO, CaO, and TiO2 values from two older measurements, and allows for an improved estimate of the modal proportion of mineral phases determined by mass balance.