V13D-1583
Geochemistry of the ~2.7 Ga Prohibition Banded Iron Formation, Meekatharra, Western Australia
At ~2.7 Ga a large quantity of continental crust was generated coinciding with a world-wide vast formation of Iron-bearing sedimentary rocks. The key depositional mechanism for iron during that time is part of an unsettled long-standing debate.Trace elements and particularly rare earth elements (REE) have been extensively applied as a proxy to approach the origin of these iron-bearing sedimentary formations. Accordingly, this study presents novel geochemical data from the Prohibition banded iron-formation (BIF) in Meekatharra, Western Australia, and its wrapping sedimentary package.The Meekatharra area belongs to the Murchison Domain that is part of the Youanmi Terrane in the Yilgarn Craton. The Youanmi Terrane is composed of north-trending greenstone belts separated by extensive granite and granitic gneiss. Lava flows at the Meekatharra-Mt Magnet Greenstone Belt has been dated at 2.7 Ga. Stratigraphically the Prohibition BIF is enclosed within a sedimentary package of volcanoclastic siltstone and fine- grained sandstone bounded by mafic chlorite-schist. Samples were collected from diamond drill core at the BIF outcrop in the Prohibition gold mine pit, comprising the iron-bearing sedimentary unit and the volcanoclastic envelope associated.Samples were normalized to bulk continental crust values. BIF samples display Fe2O3 content from 27 to 42wt%, featuring: (1) flat REE patterns systematically depleted between ~0.2-0.6; (2) slight negative Ce depletion; (3) positive Eu anomaly; and (4) large Cu, As and Sb positive anomalies up to 2.5, 62 and 7 respectively; whereas samples from the sedimentary volcanoclastic unit grouped at two populations: (1) with depleted REE profiles, La/Lu = 0.17-0.34, and V, Sc, Co, Cr and Ni enrichment; and (2) with concave REE patterns due to middle REE lower values accentuated by Eu depletion at ~0.8, coupled with La/Lu = 0.8-1. Both populations display Cu, As and Sb positive anomalies up to 5, 60 and 6 correspondingly.The geochemical fingerprint of the Prohibition BIF strikingly mimics its ~2.8 Ga BIF counterpart at the Sukumaland greenstone belt of Geita, Tanzania; interpreted as the result of mixing of hydrothermal and clastic sources. Conversely, stratigraphic observations coupled with petrological observations could envision a diagenetic component in the formation of the Prohibition iron-bearing sedimentary unit. Further geochemical study is being carried out to determine if there is any geochemical signature that could constrain the genetic processes involved.
V13D-1584
Dismembered Fragments of Meso- to Neoarchean Supra-subduction Zone Oceanic Crust: Field and Geochemical Evidence From 2800-3075 Ma Greenstone Belts in the Nuuk Region, SW Greenland
The Ivisaartoq (ca. 3075 Ma) and Storø(ca. 2800-2840 Ma) greenstone belts are the largest Meso- to Neoarchean belts in the Nuuk region, SW Greenland. Several unnamed greenstone belts (2972-3070 Ma) in the Ujarassuit Nunaat area appear to be continuation of the Ivisaartoq belt. These greenstone belts have undergone polyphase deformation and metamorphism at amphibolite facies conditions. They are dominated by metabasalts, with minor intercalations of intermediate metavolcanic rocks. They also include gabbros, picrites, cumulate rocks with relict igneous clinopyroxene, and serpentinite lenses with relict olivine. Primary magmatic features including pillow flows, inter-pillow breccias, and cumulate layering are best preserved in the Ivisaartoq belt. Metasedimentary rocks occur as thin layers (0.5-1.0 m) of aluminous paragneisses and mica schists concordant to the regional foliation. However, the Storøand Ivisaartoq belts include thick (> 200 m) units of metasediments. These supracrustal belts occur within different exotic terranes separated by high-grade mylonites. In the Ujarassuit Nunaat area, rare thrust-fold structures bring into contact the supracrustal rocks and the underlying TTG-gneisses. The metabasalts display flat to slightly fractionated REE patterns (La/Smcn = 0.9-1.8; Gd/Ybcn = 1.0-1.3) whereas the intermediate rocks possess more fractionated patterns (La/Smcn = 1.8-3.2; Gd/Ybcn = 1.9-3.1). Both types of metavolcanic rocks posses consistent negative Nb anomalies (ca. 0.2- 0.8). Major and trace elements discriminate two groups of metasediments. Mafic metasediments are geochemically similar to the associated basaltic amphibolites. Intermediate to felsic metasediments are abundant despite the dominant basaltic composition of these belts. They display enriched LREE (La/Smcn = 1.1-7.0) and near-flat HREE (Gd/Ybcn = 0.8-2.7) patterns, pronounced negative Nb anomalies, and variable concentrations of transition metals (Ni = 50-330 ppm). The geochemical characteristics of the basaltic rocks suggest partial melting of a shallow mantle source slightly modified by subduction components. The occurrence of picritic flows and rare felsic to intermediate volcanic rocks also indicate a subduction zone environment. Intermediate to felsic metasedimentary rocks are interpreted to represent volcanogenic sediments sourced from nearby arc edifices that have been eroded and are not preserved in the geological record. All these field and geochemical characteristics indicate that the studied greenstones represent relicts of Archean oceanic crust formed in a suprasubduction zone setting.
V13D-1585
Distinct Source Compositions and Genetic Environments for Archaean and Proterozoic Charnockites Across a Major Collisional Boundary in South India: Evidence From Geochemical and Sr-Nd Isotope Systematics
In the Southern Granulite Terrain (SGT), South India, a crustal scale shear zone system described as the Palghat- Cauvery Shear Zone (Pa-Ca) delineates the boundary between Late Archaean (ca. 2.52 Ga) and Neoproterozoic (ca. 0.7-0.55 Ga) high-grade terrains. In recent Gondwana Supercontinent models, the Pa-Ca is contiguous with the limit of an extensive zone of reworking of ancient crust within the Pan-African (0.75-0.5 Ga) East African Orogen which includes vast terranes covering parts of East Africa, Madagascar, southernmost India, Sri Lanka and parts of Antarctica (eg., Collins and Windley, 2002, J. Geol. 110, 325-339). A new dataset of major-trace element compositions and Sr-Nd isotopic systematics of about 180 charnockite samples has been utilized here to model source characteristics and magma genesis for the protoliths of the Archaean and Proterozoic charnockites across the Pa-Ca. The charnockites and enderbites of the Archaean domains show highly fractionated REE patterns with positive Eu-anomalies, depleted HREE, Y and near chondritic \upsilonNd0 and initial-87Sr/86Sr at ca. 2.5 Ga, consistent with hydrous partial melting of amphibolitic crust with residual garnet and hornblende for the parental melts. Their source could be either dominated by a mantle component produced by subduction process during the latest Archaean with variable extent of incorporation of mid-Archaean (upto 3.6 Ga) crustal components or may have involved an Archaean granulitic lower crust typically with low\-initial 87Sr/86Sr. On the other hand, sources of charnockites from the Proterozoic domains show greater recycling of older (upto 3.0 Ga) crustal components. Modeled at ca. 1.8 Ga and 0.8 Ga, the genesis of charnockitic rocks, which show a wider range of Ti and P, relatively lower degree of HREE depletion, commonly negative Eu-anomalies and undepleted Y, is consistent with significant intracrustal melting processes within a thickened crust at temperatures between 800 and 1000° C. Our results ascribe further fundamental importance to the Pa-Ca, and also help rationalize the evidence for some enigmatic Archaean remnants in the region south of it, which play an important role (as integral source- components) in the Proterozoic crustal accretion processes of the SGT. This interpretation is consistent with the modeled position of the SGT south of the Pa-Ca within the East African Orogen in the zone characterized by reworked pre-1.5 Ga crust.
V13D-1586
The Oil Potential Of Coal Macerals Evaluated By Spectroscopy Techniques
The constituents of coal have been categorized into three major maceral groups namely exinite (liptinite), vitrinite, and fusinite. Each maceral group, which was thought to possess different oil generation potential, has been visually confirmed by DAC-pyrolysis. This study analyzed eighteen maceral samples separated from Polish Carboniferous coals using nondestructive spectroscopy techniques include FTIR, solid-state 13C nuclear magnetic resonance (13C NMR), and XRD to obtain the information about the molecular structure in order to correlate these characteristics with the oil generative potential estimated from our visual observations. The correlation between the infrared spectroscopy with our visual observations suggests that exinite group which generated large amounts of oil has the lowest aromatics/aliphatics ratio, whereas the inert fusinite group has the highest. The proportion of aromatic carbon increases with increasing maturity within the same maceral group. The NMR spectra, similar to FTIR spectra, showed distinct representative bands for aliphatic and aromatic functional groups in the macerals. The analyses of the representative samples confirm the previous thought that exinite contains lower aromaticity value (fa) than vitrinite and fusinite. For vitrinite group which composed of different composition and structure, the aromatics/aliphatics ratios appear lower in oil-generated samples and higher in non oil-generated ones. Besides, an excellent correlation between these two analytical methods suggests that FTIR and NMR are valuable independent methods for quantitative study of organic matters. The quantitative x-ray diffraction analysis was not carried out because of the weak reflections due to small amounts of studied maceral samples. The XRD, however, showed qualitatively a considerable variation of diffraction patterns for four exinite samples but little variation for vitrinite and fusinite samples. The patterns for four exinite samples are significant different from vitrinite and fusinite. This observation is qualitatively consistent with those observed from the NMR, IR and visual studies, showing low aromatics/aliphatics ratio, thus high oil yields in exinite. In conclusion, the visual observations of oil generative potential are generally consistent with those implied from FTIR, NMR, and XRD. All of these spectroscopy analyses confirm that the oil potential of coals is related to their chemical structure, predominantly aromatic and aliphatic functional groups.
V13D-1587
Reproducibility of Raman Spectroscopy Measurements for Carbonaceous Material in Metamorphic Rocks
Raman spectroscopy shows that that carbonaceous material in metamorphic rocks shows a systematic increase in crystallinity with increasing metamorphic grade. Beyssac et al. (2002) reported a calibration curve for the "Raman spectrum of carbonaceous material" (RSCM) thermometer, from 350 to 650 C. Rahl et al. (2005) extended that calibration down to ~100 C. We report here the results of a comparative study using laser microRaman systems at Yale and Washington University to understand the reproducibility of RSCM measurements. We have started a comparative study using Raman systems at Washington and Yale Universities. Reproducible results are possible but several factors can cause problems. (1) We find that the background for the Raman spectra are commonly curved and that a second-order polynomial is needed to fully remove the background. (2) Operator differences in the peak- fitting process may produce a significant bias in results. The peak-fitting process requires the operator to separately fit for peaks in each of the ~10 spectra collected for a sample. These results are then averaged to get a composite result for the sample. We have found it is better to fit peaks to an integrated spectrum obtained from individual spectra of that particular sample. The F test is used to test for the homogeneity of a sample, and hence eliminating anomalous data. This method also allows a direct comparison of results between different labs and operators, as well as spot homogeneity estimates. (3) Laser-induced heating has been shown to distort peaks in the Raman spectra, hence, producing an upward bias in estimated metamorphic temperatures. This problem is most acute when making measurements on very low-grade metamorphic rocks. We use the Raman spectra to measure in-situ temperatures and found that heating effects can be minimized by using thin sections rather than rock chips and by greatly reducing the laser power. Our inter-lab comparison shows that RSCM measurements can be accurately reproduced, if care is given to the problems above.
V13D-1588
Re-evaluation for a geochemical significance of the fine structure in rare earth element pattern normalized by chondritic abundance: Pseudo-tetrad effect in geological materials
Rare earth element (REE) distribution pattern in geological materials normalized by chondritic abundance is one of important geochemical tools for understanding their evolution history. However, sometimes, it arouses some disputes in interpreting of formation processes for rare earth element (REE) distribution pattern such as ¢®¡ÆREE tetrad effect¢®¡¾. For example, McLennan (1994) showed that tetrad curves in normalized REE patterns are commonly very subtle and proposed that the occurrence of tetrads might be attributed to analytical problems. However, later literatures (Lee et al., 1994; Monecke et al., 2002, 2007) show that the REE tetrad effect is not analytical artifact. Recently, we observed W-type tetrad effect from mafic dike reported by Son et al. (2007). However, in re-measuring the REE concentrations for the same samples, we could not observe the REE tetrad effect. This indicates that the fine structure in rare earth element pattern normalized by chondritic abundance, sometimes, might be produced from analytical procedure of a laboratory like the preceding by McLennan (1994). In this paper, we will discuss a possibility of pseudo-tetrad effect¢®¡¾ due to REE data acquisition process during analytical procedures. Our results indicate that, despite much analytical improvements, the existence of subtle tetrads is still difficult to prove.
V13D-1589
Nd-Sr-Pb Isotopic Link Between Panarea And Sardinia Predating The Opening Of The Tyrrhenian Sea
Plio-Quaternary Tyrrhenian magmatism in the Italian peninsula and Aeolian volcanic rocks is characterized by systematic trends in Sr-Nd-Pb isotopic space, implicating different mantle end-member source contributions [1 and reference therein]. The isotopic trends also support the notion of mantle–crust interaction during magma genesis. New isotopic data on lava and tephra from Panarea island represent its entire eruptive history, and together with the GEOROC database, allow for a regional evaluation of the isotopic trends in the Tyrrhenian region. Panarea rocks vary from andesite to rhyolite, show a systematic depletion of CaO, FeOT, MnO, MgO, TiO2 wt.% and enrichment in alkalis consistent with crystal fractionation. Most Sr-Nd-Pb isotopic compositions (86Sr/87Sr:0.70464-0.70604; 143Nd/144Nd:0.512473-0.512578; 206Pb/204Pb:19.162-19.241; 207Pb/204Pb:15.628-15.709; 208Pb/204Pb:39.270-39.016) fall midway between the compositions of the calc- alkaline western sector and Stromboli and Campanian volcanic rocks (located in the Aeolian eastern sector and Southern Italian Peninsula respectively). Notably, some rocks in Panarea display comparable Pb-Sr isotope compositions but much lower Nd-isotope ratios (143Nd/144Nd:0.512235-0.512415) than previously found in the Aeolian archipelago. Nd and Sr isotopic compositions plot close to the fields of the northern and central Plio- Quaternary volcanic rocks (e.g.Mt Arci) and Oligo-Miocene rocks (143Nd/144Nd:0.51218-0.51270) from Sardinia and trend toward a mantle end-member (EM1). This newly observed isotopic similarity between Panarea and Sardinia could represent an ancient source link predating the opening of the Tyrrhenian Sea that can be explained by 1) delamination of the Sardinian lower crust during Hercynian continental collision, or 2) modification of the lithospheric mantle during Oligo-Miocene arc-type volcanism, coeval back arc extension, and counter- clockwise rotation of the Corsica-Sardinia block. [1] Peccerillo, A. Plio-Quaternary Volcanism in Italy (2005) Springer, 365p.
V13D-1590
Sr-Nd-Pb-Hf isotope geochemistry of mafic alkaline igneous rocks from the Gibeon Dicker Willem lineament, southern Namibia
The western margin of southern Africa is intersected by three NE-SW trending, age-progressive lineaments composed of alkaline igneous rocks of Late Cretaceous to Early Tertiary age. The northernmost of these is the "Gibeon-Dicker Willem" lineament of southern Namibia. The Gibeon kimberlite province (300 km inland; 72-77 Ma; Davies et al., JPet., 2001) and the Dicker Willem carbonatite complex (125 km inland; 49 Ma; Reid et al., Geol. Mag. 1990) are the two best known localities in this lineament, but it encompasses at least four other dated localities: the Blue Hills intrusive complex (75 Ma, Kurzlaukis et al., Chem. Geol. 1999) and Gross Brukkaros carbonatite complex (77 Ma, Reid et al., 1990) both located just south of and presumably related to the Gibeon kimberlite province, and the Schwarzeberg olivine nephelinites (30 Ma; Spriggs et al., PhD thesis, 1988) and Klinghardt Mountain phonolites (46 Ma; J.S. Marsh, pers. commun.), both located about 50 km inland. This lineament also includes an undated nephelinite plug at Dreyer Rucken, located within 15 km of the coast. Reid et al. (1990) proposed that the lineament may have resulted from the activity of the Vema and/or Discovery hotspots, currently located in the South Atlantic. New elemental and Sr-Nd-Pb-Hf isotope data are presented for samples from Blue Hills, Schwarzeberg and Dreyer Rucken. Like the Western Cape (WC) and Namaqualand- Bushmanland-Warmbad (NBW) lineaments further south (Janney et al., JPet. 2002 and 7IKC abstract #127 2003; Rogers et al., Lithos 1992), the isotope compositions of alkaline magmas from the Gibeon-Dicker Willem lineament extend between mild EM1 and strong HIMU compositions. However, unlike the WC and NBW lineaments, there is no clear transition from HIMU compositions near the coast to EM1 compositions in the interior. Samples from Dicker Willem and Blue Hills in the interior display extremely radiogenic 206Pb/204Pbi values of 19.9 to 21.2 (Cooper and Reid, CMP 2000, this study), whereas the Gibeon kimberlites have the least radiogenic values (18.2 to 19.0; Spriggs, PhD thesis 1988). The near-coastal Dreyer Rucken and Schwarzeberg nephelinite samples are isotopically intermediate (206Pb/204Pbi = 19.57 to 19.75). Although mixing of lithospheric and sublithospheric sources appears to have contributed to the isotopic heterogeneity displayed by the Gibeon-Dicker Willem lineament, a major role for extreme fractionation of parent-daughter elements (especially U+Th/Pb) during very low-degree partial melting is also inferred.
V13D-1591
Preliminary Multi-Isotopic Data and Potential Regional Connections for Late Cenozoic Basalts of the Western Snake River Plain, Idaho
Previous research regarding the origin and evolution of Snake River Plain (SRP) basalts west of the 116° meridian has utilized field mapping, petrographic and geochemical data, and some Sr-isotopic analyses. These studies showed that in the past 2 m.y. at least three suites of chemically and isotopically distinct basalts were produced. The oldest (1.0 Ma to 1.6 Ma) are iron-rich tholeiitic basalts (N-tholeiities); the intermediate suite (0.90 Ma) is tholeiitic with an usually high phosphorus content (P-tholeiites); and the youngest basalts (< 0.50 Ma) are mildly alkaline (A-lavas). The current study presents Sr, Nd, and Pb isotopic data for basalts collected from each of these suites. Temporal trends in isotopic systematics of western SRP basalts, from N-tholeiites (87Sr/86Sr > 0.707, epsilon Nd < -4, 206Pb/204Pb < 18.5) to younger P-tholeiites and A-lavas (87Sr/86Sr < 0.706, epsilon Nd from -2 to 0, 206Pb/204Pb > 18.5), are comparable to Late Cenozoic basalts of nearby provinces. These trends are nearly identical to those exhibited by the Boise River Group (BRG) northeast of the study area. In eastern Oregon, the Jordan Valley Volcanic Field (JVVF) also displays similar trends; however the JVVF data are slightly offset to less radiogenic Sr and more radiogenic Nd. This may be controlled by differences in the character of the underlying lithospheric mantle across the western boundary of the North American craton. Further comparisons show the N-tholeiites are isotopically similar to the Saddle Mountain basalts of the Columbia River Group (CRG), which are attributed a subcontinental lithospheric mantle source. In contrast, the P- tholeiites and A-lavas trend toward the isotopically depleted Imnaha basalts of the CRG. These, and the younger alkaline rocks of the BRG and JVVF, are interpreted to be derived from a deeper asthenospheric source. We interpret our data as recording a similar lithospheric to asthenospheric source transition for basalt magma genesis in the western SRP. The similarities in evolution of basaltic volcanism across this area imply that similar processes generate and modify magmas on a regional scale.
V13D-1592
Trace Element Geochemical Signature of Basalts and Diabases from the Bay of Islands Ophiolite in Western Newfoundland
The Bay of Islands Ophiolite in western Newfoundland lies along the Appalachian Orogenic Belt and records the generation and destruction of the Proto-Atlantic Ocean from the Late-Precambrian to the middle Ordovician Taconic Orogeny. In previous geochemical studies, Jenner et al. (1991) analyzed mainly andesites and trondhjemites dominantly from Coastal Complex, which is not considered part of the Bay of Islands Ophiolite proper. He showed that these fractionated rocks mainly contained negative Nb anomalies on multi-element variations diagrams and attributed the petrogenesis of the Coastal Complex and by extension the Bay of Islands Complex to formation within arc or supra-subduction zone environment. Likewise Elthon's (1991) analyses of diabase from the North Arm Mountain Massif of the Bay of Islands Complex showed similar, but more moderately negative Ta anomalies. In this study, 50 diabases and basalts from the \label{OLE_LINK1}North Arm Mountain and Blow Me Down Mountain massifs in the Bay of Islands Ophiolite Complex (proper)were analyzed for trace elements via ICP-MS. When our existing XRF major and trace element geochemical data were supplemented by a more complete ICP-MS trace element data set from the Bay of Islands Complex (proper), it is quite clear that the Blow Me Down samples appear more MORB-like with very small negative Nb-Ta anomalies when compared to the North Arm samples, which have somewhat more negative Nb and Ta anomalies, relative to La and Th. The mafic assemblages from the ophiolite most likely include a mixture of what are referred to as MORB-like and arc-like (or suprasubduction zone) signatures. However, the North Arm and Blow Me Down Massif basalts and diabases signatures may be similar to some MORB with arc-like signatures. We review the ambiguities created by the use of Nb-Ta anomalies in interpreting tectonic setting and classifying ophiolites. We also review the validity of using Nb-Ta anomalies in highly fractionated andesites and plagiogranites as descriminators of tectonic environment.
V13D-1593
Geochemistry of Spencer-High Point Volcanic Field Lava Flows, Idaho
Lava flows in Spencer-High Point (SHP) volcanic field, an ~1700 sq km mafic volcanic rift zone located near Yellowstone in the eastern Snake River Plain (ESRP), have been compared physically and chemically to other ESRP olivine tholeiites. Overall, SHP lavas are geochemically similar to other ESRP olivine tholeiites but their geomorphology is entirely different. The structural alignment of vents and fissures in an east-west direction in the Spencer-High Point region contrasts with most of the ESRP volcanic features aligned northwest-southeast. Numerous cinder cones at SHP, features that characterize Craters of the Moon volcanic field, are unusual on most of the eastern Snake River Plain. This study agrees with preliminary geochemical data by Leeman (1982) and Kuntz et al. (1992) suggesting that SHP lavas are typical ESRP basalts. However, a broad range of geochemical compositions exists in the SHP field that is similar to the entire range of ESRP olivine tholeiites. A few of the samples are actually closer in composition to lavas present at Craters of the Moon but only a limited number of samples from vents with physically higher relief, in the central and eastern portions of the field show these evolved chemical compositions. Typically Ta ranges 0.5-4.5ppm, La 20-90ppm, Ba 200-1100ppm and Cr 7-550ppm. Some lava flows in the central and eastern sections of SHP volcanic field also contain crustal xenoliths, implying a prolonged crustal history. These results although preliminary, suggest that the SHP system represents a possible petrologic transition between the dominant ESRP tholeiites and the evolved compositions found at Craters of the Moon.
V13D-1594
Partitioning and Phase Effects of Ni for the Martian Basalt Humphrey Composition
Ni is of particular interest because it is the most compatible cation in olivine at basaltic conditions. However, models of lunar magma ocean petrogenesis have suggested that at high temperatures and pressures Ni behaves incompatibly (Longhi and Walker 2006). Their conclusions rely on a partitioning model that was developed mainly using terrestrial studies (Jones 1984). Therefore, this study addresses whether the partitioning model (Jones 1984) can be extrapolated into planetary compositions at high temperature and pressure. It also investigates the effect of Ni on phase relations of a Martian basalt. The Martian basalt, Humphrey, was chosen because it has been studied in the absence of Ni, providing a baseline for the addition of Ni (Filiberto and Treiman 2007). Experiments were conducted using a piston cylinder apparatus on a synthetic, anhydrous, Humphrey composition + NiO at near liquidus Martian mantle pressures and temperatures. Experimental run products were analyzed by EMP and partition coefficients between olivine and melt were calculated for all samples. Although DNiol-liq determined for the Humphrey composition is always >>1 (Ni is always strongly compatible with olivine), it does match predicted values by Jones' model. Additionally, as suggested by Jones (1984), this correlation is independent of temperature and pressure. The success of the model in predicting DNiol-liq values in a Martian system is evidence that Jones' model can be applied to other planetary systems, such as the Moon; however the validity of extrapolating into ultra-mafic systems has not been fully addressed. The addition of small amounts of NiO (<< 1wt %) to the Humphrey basalt has drastic effects on the liquidus phase relations raising the olivine saturated liquidus 25 degrees. Due to NiO's greater stability in olivine's crystal structure, NiO stabilizes olivine to higher temperatures. The addition of NiO did not, however, change the location of the pigeonite-in phase boundary. This result is consistent with the near neutral partitioning of Ni and Mg into pigeonite. Therefore, the implied multiple saturation point is shifted 3 kbar higher (15.5 kbar) when compared to a nickel free system as a result of the higher temperature liquidus and the unaltered slope of the pigeonite-in line.