Depletion Of Major Elements Research Articles

Systematic LREE enrichment of mantle harzburgites: The petrogenesis of San Carlos xenoliths revisited

The dichotomy between partial melting and metasomatism is a paradigm of mantle geochemistry since the pioneering work of Frey and Prinz (1978) on the occurrence of LREE-enriched harzburgites. However, the thermo-chemical implications of such two-stage scenarios are often poorly considered, and the latter fail to explain why trace-element enrichment and major-element depletion are often proportional. We here re-envisage the petrogenesis of the famous San Carlos peridotites based on new petrographic observations and detailed modal, major- and trace-element compositions. The lherzolites (and pyroxenites) are characterized by homogeneously fertile mineral chemistry and LREE-depleted patterns consistent with low degrees of partial melting of the lherzolitic protolith. Bulk compositions and mineral zoning suggest that opx-rich pyroxenites formed by pressure-solution creep during melt-present deformation, locally accompanied by magmatic segregations of cpx. The harzburgites are characterized by stronger mineral zoning with low-Mg# and Na-, Al- and Cr-rich cpx rims, and can be discriminated in a low-Jd and high-Jd cpx groups. The high-Jd group is interpreted as the result of local elemental redistribution in the presence of a low-degree hydrous melt, in good agreement with their wide range of LREE enrichment. In contrast, the MREE-to-HREE fractionation and increasing Cr# in spinel of the low-Jd group indicate that they experienced higher degrees of melting. Open-system melting simulations of trace-element fractionation during hydrous flux melting suggests that the high-Jd harzburgites are the result of low fluid influx producing poorly extracted melt, while higher influx led to higher melting degrees and efficient melt extraction in the low-Jd harzburgites; the lherzolites mostly remained below or near solidus during that process. The lithological and chemical heterogeneity of San Carlos mantle is thus compatible with a single-stage evolution, which is also supported by the striking consistency between Fe-Mg exchange and REE thermometric estimates (1057 and 1074 °C on average, respectively), indicating that harzburgites and lherzolites probably followed a similar P-T path and relatively little sub-solidus re-equilibration. These interpretations suggest that the development of melt extraction pathways promoted by reactive channeling instability is able to produce complex lithological heterogeneities during hydrous flux melting. This process provides a self-consistent explanation for the systematic enrichment of harzburgites observed in many mantle terranes and xenoliths worldwide. We argue that San Carlos is one of such examples where a ca 1.5-Ga continental lithosphere experienced localized flux melting and deformation during the tectonic reactivation of a Proterozoic subduction zone, providing new constraints on the mantle sources of volcanic activity in the Jemez Lineament.

Soil Physicochemical Properties and Geochemical Indices: Diagnostic Tools for Evaluating Pedogenesis in Tephra-Derived Soils along the Cameroon Volcanic Line

Physical and geochemical properties of six soils developed on tephra from Mts. Kupe and Manengouba were studied in order to assess the degree of weathering of the tephric parent materials. Physico-chemical properties were determined following standard procedures. Total elemental contents and selective extracted contents of Al and Fe by acid ammonium oxalate, dithionite-citrate-bicarbonate (DCB) and pyrophosphate were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES) after being brought into solution. Parent rocks were chemically classified as hawaiite, tephricfoidite, and basanites, while soils were classified as Andosols and Cambisols. Both the soils and parent rocks have high concentrations in SiO2, Al2O3, and Fe2O3. Texture and the evaluated related weathering indices correlate with horizon development and are important in capturing the differences between the two Reference Soil Groups. Mobility of major elements showed erratic trends in conformity with the stratigraphic breaks indicating variability of parent materials with depth. Enrichment and depletion of major elements to a certain degree explains the ongoing andosolization process in the Andosols. Physical and geochemical data can both be used concomitantly for assessment of weathering trends in these soils. Total reserve in bases (TRB) is very high in the Andosols indicating the presence of large quantities of weatherable minerals and thus the high potential of these soils for agricultural production.

Recent, deep-sourced methane/mud discharge at the most active mud volcano in the western Mediterranean

Active mud volcanism in the West Alboran Basin (WAB) is closely associated with tectonically mobilized, overpressurized shales and shale-diapirism. This appears to control mud expulsion at Carmen mud volcano, a cone-shaped structure 65 m high and 1 km in basal diameter.The presence of gas-rich mud breccia, living chemosynthetic fauna, the absence of hemipelagic draping and the abrupt transition that occurs between high dissolved sulfate in the uppermost interval and low sulfate together with high methane concentrations in the lowermost sediment interval all point to a recent expulsion of mud breccia at the summit of Carmen MV.For the lowermost interval, the depletion of major elements (i.e., Ca2+ and Mg2+) and the enrichment of trace species (i.e., Li and B) in the pore water all indicate a deep fluid source. The δ18Opw (5.7‰ VSMOW) and δDpw (−10‰ VSMOW) of pore water in the lowermost interval correspond with smectite dehydration as the main pore-water freshening mechanism. Water-formation temperatures calculated with empirical geo-thermometers (K-Na, K-Mg; δ18Opw, δDpw, and dissolved B) reveal that fluids were generated at temperatures of ~140 ± 20 °C. Taking a regional geothermal gradient for the WAB of 25–27 °C/km, this points to a fluid source from ~5 ± 1 km sediment depth. This is not only consistent with the depth of overpressurized shales and megabreccia of Lower to Middle Miocene age, but it also fits nicely with the Upper/Middle-Miocene seawater value for the porewater 87Sr/86Sr derived from dissolving carbonates. The stable carbon and hydrogen isotopic composition of methane (δ13Cmethane ~ −59.4‰ VPDB and δDmethane −184‰ VSMOW) for the deepest samples of summit-core GP05PC is consistent with the mentioned deep origin.Mud breccia expulsion of overpressurized deep sedimentary units would be accompanied by rigorous degassing, leading to rapid, ‘instantaneous’ replacement of pore fluid by bottom water in the upper sediments. The absence of oxidized sediment draping, the seawater-like pore-water composition in the uppermost part of the mud breccia interval, and the abrupt methane to sulfate transition all provide evidence for a very recent mud expulsion.The distinctively kink-shaped pore-water Cl− profile in core GP05PC has been used in a numerical transport-reaction model to derive the timing for this event. This eruptive event appears to have taken place very recently, namely 12 ± 5 yrs prior to the 2012 coring, thus in the year 2000 CE.

Giant bauxite deposits of South China: Multistage formation linked to Late Paleozoic Ice Age (LPIA) eustatic fluctuations

The Lower Carboniferous Jiujialu Formation bauxite deposits at Zunyi, northern Guizhou are distinguished by their great thickness (to 110 m) and cyclic alternation of bauxite ore and bauxitic claystone layers. These features are typical of the Kazakhstan subtype of karst bauxite deposits, although the genesis of this type of deposit is poorly understood. Here, we undertook a petrological, mineralogical, and geochemical study of the Zunyi deposit in drillcore ZK5600 to gain insights into its formation history. The Zunyi profile contains seven bauxite cycles, as recorded by petrological, mineralogical and geochemical features, indicating a multistage formation history. Each cycle is composed of a lower bauxitic claystone layer and an upper bauxite ore layer. The bauxite ore is characterized by high Al2O3, high Al-mineral (diaspore), and low SiO2 content. Positive Ce anomalies, high CIA values (>90), and strong depletion of major elements indicate that the bauxite ore formed through intense chemical weathering and leaching in a well-drained soil environment. The bauxitic claystone is characterized by relatively lower Al2O3 and higher SiO2 and clay mineral (mostly illite) content. This lithology exhibits both negative and positive Ce anomalies, moderate CIA values (70–80), and enrichment of some elements (e.g., Fe, K), indicating formation through moderate chemical weathering in a poorly drained soil environment. Secular variations in chemical weathering intensity and soil drainage conditions were linked to regional hydrologic changes. During the early Carboniferous, the Zunyi area was located in a karstified coastal upland setting, and its hydrologic system was affected by glacio-eustatic fluctuations. High groundwater table elevations during eustatic highstands led to formation of bauxitic claystone layers, whereas low groundwater table elevations during eustatic lowstands led to formation of bauxite ore layers.

Geochemical interpretation of the Precambrian basement and overlying Cambrian sandstone on Bornholm, Denmark: Implications for the weathering history

A geochemical study of the Precambrian basement granites from the Borggård borehole on Bornholm, Denmark, suggests that the granites were moderately weathered (Chemical Index of Alteration-CIA=66–71) during subaerial exposure in a humid climate. The microcline is well preserved, whereas plagioclase was thoroughly altered to clay minerals (Plagioclase Index of Alteration-PIA=93–99) which is likely due to its original Ca-rich composition. The primary Fe–Ti accessory minerals were oxidized to hematite and anatase. Evidence from REE distribution patterns and immobile element ratios, e.g. Zr/Hf and Nb/Ta, between the weathered basement granite from the Borggård borehole and regional granitoids on Bornholm, constrains the Svaneke Granite as the original basement lithology. A tau (τ) mass transport model (assuming immobile Ti) was applied to quantify the mass transfer during weathering of the basement granite. The results show a depletion of major elements in the following order: Na>Ca>Mg>Si; Al and Ti are immobile and stay constant; K shows sample dependent enrichment or depletion; Fe is slightly enriched. The Cambrian sandstone overlying the basement in the Borggård borehole, assigned to the Gadeby Member of the Nexø Formation, is feldspathic litharenite–litharenite in composition. Provenance indicators including (Gd/Yb)N, Zr/Hf and Nb/Ta ratios and petrological features indicate that source material was derived from both weathered and fresh basement granite of intermediate composition. The Gadeby Member equivalents in Germany, the basal lower Cambrian Adlergrund Konglomerat Member (AKM) in the offshore G-14 well north of Rügen, and the approximately coeval Lubmin Sandstein Formation (LSF) from the Loissin-1 borehole, mainland Germany, must have been sourced from a basement with compositions comparable to the intermediate group of the regional granitoids on Bornholm. The source materials for the AKM (CIA=71–72, PIA=94–96), the Gadeby Member in the Borggård borehole (CIA=52–69, PIA=56–99) and an outcrop in Nexø, eastern Bornholm (CIA=52–66, PIA=61–96), have endured similar degrees of weak to moderate weathering but lost most of the plagioclase. The LSF has a comparable weathering history (CIA=63–73), but the plagioclase is better preserved (PIA=65–78). The significant variation of weathering rates of plagioclase and K-feldspar in the basement granite and the provenance of sandstone from the Borggård borehole are likely due to the different permeability developed within the internal crystal structures, a Ca- rich plagioclase original composition of the plagioclase, and the occurrence of weathering in a very humid climate. ​K metasomatism occurred in the basement granite and sandstone in both the Borggård and the G14–1 boreholes, mainly through the conversion of aluminous clay minerals (e.g. kaolinite) to illite, with transformation of plagioclase to K-feldspar occurring locally. This my have taken place during deep burial in the Caledonian foreland basin.

Mineralogy and geochemistry of three weathered Lower Cambrian black shale profiles in Northeast Chongqing, China

This paper reports a geochemical study on the mineralogy and major elements of mid-ridge (A), near mountaintop (B), and valley (C) profiles developed in the Lower Cambrian black shale in Northeast Chongqing, China. The primary objective was to understand the elemental mobility, mineralogical transformation, and weathering progression during black shale chemical weathering in a subtropical climate. Profiles A, B, and C are characterized as weak, weak to moderate, and moderate to intense in terms of weathering intensity, respectively, by the Chemical Index of Alteration (CIA). Results indicate that most elements were mobilized by acidic solutions produced during the oxidation of pyrite and organic matter (OM). Among the major elements, Si was slightly enriched in profile A, but depleted through desilication in profile B and C. Al was enriched in the regolith zone in profile A and C, and Fe was enriched at the oxic front because of secondary clay and sesquioxide formation. The addition and depletion of major elements and the depth of the oxic front increased with the degree of weathering. Gypsum and Fe- (hydro-) oxides were observed to form and re-precipitate in the saprock and fractured shale zones. Clay minerals formed from dissolution of plagioclase and the transformation of other labile clay minerals during weathering. The progressive changes in mineralogical composition of weathered material from profile A to C showed the sequence of mineral decomposition with degree of weathering: first, oxidation of pyrite and OM; then Ca and Mg were removed during dissolution of carbonatite; followed by removal of Na from dissolution of plagioclase; lastly, transformation of clay minerals during weathering; meanwhile, desilication occurs at moderate to intense weathering stages.

Investigation of a diabase-derived regolith profile from Pennsylvania: Mineralogy, chemistry and Fe isotope fractionation

At Earth's surface, bedrock transforms to regolith in a process that has many implications for water storage, soil formation, and nutrient availability to ecosystems. To understand deep regolith formation, we investigate three zones demarcating changes in mineralogy, chemistry and Fe isotopic composition in a 4-m thick regolith profile developed at a ridge top in Pennsylvania (U.S.A.) that is underlain entirely by diabase. Relative to the unweathered diabase, Zone 1 at the bottom is characterized by major element depletion, abrupt oxidation of Fe(II), a rapid decrease in sulfur concentrations and the presence of short-range ordered (SRO) Fe with depleted δ56Fe values relative to unweathered diabase. We attribute many of the observations at this depth (which we refer to as the lithogenic zone) to the effects of spheroidal weathering, which may be releasing a flux of Fe(II) with low δ56Fe values as the mineral pyroxene is weathered. This Fe(II) is oxidized due to the presence of oxygen at the bottom of the profile and is likely precipitated as isotopically light SRO Fe phases, which then recrystallize with time. Zone 2 is interpreted as a region of crystallization in which minimal bulk chemical changes occur. Finally, SRO Fe isotopic compositions become lighter again near the land surface in Zone 3, the zone of most intense biogenic weathering. Biological Fe weathering likely involves enhanced microbial activity, exudation of protons and organic acids by plant roots and fungi, and plant uptake. This pattern of SRO Fe isotopic variability (with isotopically depleted values both at depth and near the surface) has been observed in another regolith profile on spheroidally weathering bedrock in Puerto Rico. We further used pore-water concentrations and soil chemistry to calculate rates of diabase weathering. While pore-water concentrations yield modern rates, soil chemistry concentrations indicate longer-term rates. The similarity in the diabase dissolution rates using modern pore fluids and soil chemistry is consistent with steady-state weathering. Similar approaches can be applied to other Appalachian soil profiles in the future.

Trace elements distributions at Datoko-Shega artisanal mining site, northern Ghana.

Environmental geochemistry classifies elements into essential, non-essential and toxic elements in relationship to human health. To assess the environmental impact of mining at Datoko-Shega area, the distributions and concentrations of trace elements in stream sediments and soil samples were carried out. X-ray fluorescence analytical technique was used to measure the major and trace element concentrations in sediments and modified fire assay absorption spectrometry in soils. The results showed general depletion of major elements except titanium oxide (TiO2) compared to the average crustal concentrations. The retention of TiO2 at the near surface environment probably was due to the intense tropical weathering accompanied by the removal of fine sediments and soil fractions during the harmattan season by the dry north-east trade winds and sheet wash deposits formed after flash floods. The results also showed extreme contamination of selenium (Se), cadmium (Cd) and mercury (Hg), plus strong contaminations of arsenic (As) and chromium (Cr) in addition to moderate contamination of lead (Pb) in the trace element samples relative to crustal averages in the upper continental crust. However Hg, Pb and Cd concentrations tend to be high around the artisanal workings. It was recognised from the analysis of the results that the artisanal mining activity harnessed and introduces some potentially toxic elements such as Hg, Cd and Pb mostly in the artisan mine sites. But the interpretation of the trace element data thus invalidates the elevation of As concentrations to be from the mine operations. It consequently noticed As values in the mine-impacted areas to be similar or sometimes lower than As values in areas outside the mine sites from the stream sediment results.

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES

Enrichment of upper mantle peridotite: petrological, trace element and isotopic evidence in xenoliths from SE China

Trace element patterns of spinel- and garnet peridotite xenoliths from the Nushan and Mingxi basalts (SE China) reflect the interplay between partial melting and subsequent enrichment by metasomatism. Trace element patterns indicate the common addition of Th, Sr and LREE, but not of significant Ti and Zr, which suggests a metasomatising agent (or agents) with characteristics of H 2O-rich and CO 2-rich fluids. Laser Ablation Microprobe linked to an Inductively Coupled Plasma Mass Spectrometer (LAM-ICPMS) analyses of volatile-bearing minerals (including amphibole, phlogopite and apatite) and glasses show that they are important hosts for K, Rb, Sr, Ba, U and rare earth elements (REE) in the mantle, and play an important role in the fractionation of these elements in mantle processes. However, mass-balance calculations require that significant proportions of LREE, Th and Sr are situated in intergranular phases. 87Sr/ 86Sr ratios of whole-rock samples from each locality are higher, and 143Nd/ 144Nd lower, than those of the corresponding acid-leached clinopyroxenes (cpx), suggesting that highly radiogenic Sr and relatively unradiogenic Nd reside in these intergranular phases. The Sr/Nd isotopic compositions of clinopyroxenes separated from the peridotites are correlated with metasomatic style, as reflected in the trace element compositions of the clinopyroxenes. Clinopyroxenes in fertile peridotites from Nushan and Mingxi have high 143Nd/ 144Nd and low 87Sr/ 86Sr, reflecting long-term depletion of the mantle. Clinopyroxenes from metasomatised peridotites define trends toward lower 143Nd/ 144Nd and higher 87Sr/ 86Sr, consistent with a metasomatic fluid having a Sr/Nd isotopic composition near Bulk Earth, and 147Sm/ 144Nd≤0.09. Metasomatism by a similar type of fluid can explain the isotopic systematics of clinopyroxenes from most eastern China localities reported in the literature, but a few localities interpreted as on-craton would require fluids with still lower 143Nd/ 144Nd and higher 87Sr/ 86Sr. The minimum age of the major element depletion may be given by a Sm/Nd regression line of 733±340 Ma for cpx from unmetasomatised peridotites at Nushan. The age of the metasomatic event(s) cannot be constrained, but a few metasomatised samples with high 143Nd/ 144Nd indicate that a metasomatic event must have occurred shortly before eruption.

Hydrological alterations with river damming in northern Sweden: Implications for weathering and river biogeochemistry

This case study tests the hypothesis that damming leads to a depletion of major elements in river systems. It determines the effect of river dams on the weathering regime, and thus on dissolved silicate (DSi) fluxes from land to the Sea by comparing two headwater areas in northern Sweden. In the pristine River Kalixälven, major dissolved elements are enriched within a few kilometers downstream from a high mountainous provenance, coming from an area low in vegetation and a thin active soil layer to a forested landscape. Also, alkalinity increased from 30 μeq L−1 to 110 μeq L−1, compared to 240 μeq L−1 measured at the river mouth. In the headwater of the River Luleälven, regulations led to inundation of the river valley and associated loss in vegetated soils. In reaches between the reservoirs, underground channeling of water and a reduction of water level fluctuations result in further decrease in soil‐water contact, and consequently diminishing weathering rates. The ratio of forest area to lentic area in the headwater was reduced dramatically with damming, from 2.65 to 0.84. As a consequence, geochemical variables in the river water show uniformity in space and in time. Alkalinity values at the River Luleälven mouth (155 μeq L−1) remained unchanged from the two main mountainous storage reservoirs (161 μeq L−1 and 166 μeq L−1). These results indicate that loss of vegetated soils through damming in river headwater critically reduces weathering fluxes and also suggest that changes in vegetation coverage in the Quaternary have altered DSi inputs significantly to the global Ocean.

Chemical and temporal variations in the Earth's lithosphere

The lithosphere is the rigid outer layer of the Earth, but its composition, and by implication its origins, are different in terrains of different ages. The continental lithosphere is typically weaker, although often much thicker, than the oceanic lithosphere. Oceanic mantle lithosphere accretes new material at its base as it cools (thermal accretion), and so it is compositionally similar to the underlying asthenosphere. Much of the continental mantle lithosphere, and particularly that beneath Archaean cratons, is relatively depleted in major elements. Thus, continental mantle lithosphere is less dense than the oceanic lithosphere, and its thickness depends on the compositional difference between it and the underlying asthenosphere, as well as on the geothermal gradient. The errors and accuracy of the mineral thermobarometry of peridotite samples are briefly reviewed, and both mineral thermobarometry and heat–flow studies consistently suggest minimum thicknesses of 150–200 km for the lithosphere beneath Archaean cratons. Os isotopes reflect the age of major–element depletion, and, hence, the stabilization of the mantle lithosphere. They confirm that in many cases the continental crust and uppermost mantle stabilized at about the same time, and have remained as a coherent unit ever since. In general, areas of Archaean lithosphere appear not to have thickened significantly since they were formed, whereas the lithosphere beneath Proterozoic and younger areas has been more prone to subsequent magmatic and tectonic events.

The petrogenesis of the eastern Pyrenean peridotites: an integrated study of their whole-rock geochemistry and Re-Os isotope composition

The Re-Os isotopic compositions of sixteen peridotites from the eastern Pyrenean ultramafic massifs have been determined in conjunction with their whole-rock major, trace, and chalcophile element contents. The data suggest that the peridotites represent residues after the extraction of up to 25% partial melt from a fertile mantle source at relatively high pressures (>20 kbar). Linear correlations between the incompatible-element contents of the peridotites and indices of major element depletion require that the initial melt extraction must have been followed by metasomatism by an incompatible-element enriched component. The S contents of the majority of the peridotites show a linear correlation with major element depletion, consistent with the extraction of a S-saturated basaltic melt from a fertile peridotite source. Low Cu/S ratios in a number of samples, however, suggest that parts of some of the massifs may have been infiltrated by an exotic sulphide component. The range of Os abundances in the peridotites is relatively limited (3.13–4.74 ppb) and unrelated to either the major or trace element contents of the peridotites. In contrast, the Re abundances vary widely (65–497 ppt) and correlate with their degree of major element depletion. Rhenium, sulfur, and Al 2O 3 are all positively correlated, suggesting all are controled by melt extraction. The 187Os/ 188Os ratios of the peridotites range from 0.1157 to 0.1294 and show good correlations with whole-rock Re, S, and Al 2O 3 abundances. From the y-intercept of the 187Os/ 188Os-S trend an initial 187Os/ 188Os ratio of ∼0.1159 is calculated for the peridotites, yielding a model age of 1.9 ± 0.3 Ga for the isotopic homogenisation of the mantle lithosphere that underlies the massifs. This model age is similar to the age of major crustal growth in Europe and implies a broadly synchronous stabilisation of both crust and lithospheric mantle at this time. The linear decrease of Re with degree of melt extraction is consistent with nonmodal melting of a Re-bearing base-metal sulphide phase. A sulphide-silicate melt partition coefficient (D sul/sil Re) of ∼325 is calculated for Re, indicating that the chalcophile behaviour of Re during melting may be similar to that of the moderately chalcophile elements, such as Cu rather than the highly chalcophile Pt-group elements.

Shifts in the source of the Paraná basalts through time

Magmatism occurred intermittently in the Paraná region of South America between 240-50 Ma, with the peak of magmatic activity between 138-126 Ma, comprising the Paraná continental flood basalts (CFB). Detailed chemical subdivision of the Paraná basalts into six units allowed stratigraphic correlations to be made, implying a northward migration of magmatism following the rifting of the South Atlantic. However, 40Ar- 39Ar ages do not uphold the chronostratigraphic nature of such magmatic units, instead they indicate a southeastwards migration. Major elements (extrapolated to 8 wt.% MgO) are used to investigate changes in magma type with time, and are interpreted in terms of parameters such as pressure, depth of melting and the nature of the source region (fertile/refractory and water content). The oldest Paraná unit originated by melting at the greatest depths (90–120 km), shifting to shallow levels (30–60 km) for the younger units. Accompanying this decrease in pressure, the source material undergoing partial melting became progressively more refractory in major elements and also more hydrous. This hydrous metasomatism occurred after major element depletion of the lithosphere at shallow levels. The data is consistent with conductive heating of a stratified source region, inferred to lie within the lithospheric mantle, which is relatively fertile but anhydrous at depth, and also appears to be younger in age than the relatively major element refractory, hydrous lithosphere at shallower depths. Thus the interaction of asthenosphere and lithosphere can be determined within an integrated age, tectonic and thermal framework.

Geochemistry of polymetamorphic ultramafics (Major, Trace, Noble and Rare Earth Elements): An example from the Helvetic basement, Central Alps, Switzerland

Polymetamorphic ultramafic rocks in orogenic terranes rarely preserve relic structures or minerals from their former mantle stages. The determination of their protoliths and their tectonic evolution by chemical discrimination methods is often difficult due to possible metasomatic processes. Ultramafics of the pre-Variscan Helvetic basement (Central Alps, Switzerland) have been investigated geochemically to address these problems. These ultramafics are partially to completely serpentinised. According to field observations several ultramafic lenses were part of an ophiolite suite, but distinct cumulate ultramafic lenses were also recognized. CIPW norms indicate that large parts of the ultramafics are harzburgites, but metasomatic CaO depletion may have produced an overestimation of the importance of the harzburgite protoliths. Major element distributions suggest a depleted mantle protolith. Close to chondritic or slightly depleted REE patterns are characteristics of the studied samples. The REE normalized patterns confirm the presence of harzburgites, lherzolites und cumulates. In some samples light REE enrichment processes have occurred. The noble metal concentrations are both affected (Pt-Pd-Au) und unaffected (Ir-Os-Ru) by melt infiltration processes. They suggest the presence of undepleted or slightly enriched harzburgites und more differentiated, probably cumulate ultramafics. Information obtained by different chemical elements leads to contrasting results. REE and noble metals show enrichment inconsistent with the major element depletion. Refertilization of depleted ultramafics is proposed.