Petrography and petrology of the Hawaii Scientific Drilling Project lavas: Inferences from olivine phenocryst abundances and compositions

The Mauna Loa (ML) and Mauna Kea (MK) lavas recovered by the Hawaii Scientific Drilling Project (HSDP) include aphyric to highly olivine-phyric basalts. The average olivine phenocryst abundance in the reference suite of ML flows is 14.5 vol % (vesicle-free and weighted by the flow thickness), while the average abundances of olivine in the reference suites of the MK alkalic and tholeiitic basalts are 1.1 and 14.0 vol %, respectively. Plagioclase and augite phenocrysts are rare in the ML and MK tholeiites, but the MK alkalic basalts can have up to 4 vol % plagioclase phenocrysts. Strained olivine grains, thought to represent disaggregated dunite xenoliths from the cumulate pile within the magma chamber(s), are ubiquitous in the drill core lavas. These deformed grains can comprise up to 50 % of the modal olivine in a given rock. Olivine core compositions in the lavas span forsterite contents of 80.4–90.7 (median 88.8, ML tholeiites), 75.8–86.6 (median 85.8, MK alkalic basalts), and 76.3–90.5 (median 88.0 mol %, MK tholeiites). Olivines with core compositions in the range Fo_(89–90.5) are present in tholeiitic lavas with a wide range of whole-rock MgO contents (9–30 wt %). Strained and unstrained olivines completely overlap in composition as do the compositions of spinels (100*Cr/(Cr+Al) ∼59–72; Mg# = 100*Mg/(Mg+Fe^(2+)) ∼40–66) present as inclusions in the olivine phenocrysts. The presence of Fo_(90.5) olivine in the HSDP lavas requires magmas with ∼16 wt % MgO in the ML and MK plumbing systems. Rare dunite xenoliths are also present in the drill core lavas. While compositionally homogeneous within a given xenolith, the six xenoliths contain olivines that span a wide range of forsterite contents (78.3–89.2 mol %). Spinels in these xenoliths are chrome-rich, have Mg# between 31 and 66, and define two populations on the basis of TiO_2 contents. Whole-rock compositions for the ML and MK tholeiites define olivine control lines on MgO-oxide diagrams, and the relationship between whole-rock MgO and olivine phenocryst abundance in these lavas suggests that the lavas with >12 wt % MgO have accumulated olivine. Comparing the weighted bulk composition of all of the MK tholeiites in the drill core with a calculated parental magma suggests that, on average, the MK tholeiites entrained most of the olivine phenocrysts that crystallized from their parental liquids. Although deformed olivines in Hawaiian lavas are widely thought to represent disaggregated dunite xenoliths, none of the majoror minor-element data on the strained or unstrained olivine phenocrysts suggest that the strained olivines in the HSDP lavas are exotic. We suggest that most of the olivine phenocrysts in a given flow, whether strained or unstrained, are closely related to the evolved liquid that now forms the groundmass. This is consistent with observed correlations between isotopic systems measured on olivine separates (e.g., O, He) and isotopic systems dominated by groundmass (e.g., Nd, Pb)

Widespread abiotic methane in chromitites

Recurring discoveries of abiotic methane in gas seeps and springs in ophiolites and peridotite massifs worldwide raised the question of where, in which rocks, methane was generated. Answers will impact the theories on life origin related to serpentinization of ultramafic rocks, and the origin of methane on rocky planets. Here we document, through molecular and isotopic analyses of gas liberated by rock crushing, that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites, characterized by high concentrations of chromium and ruthenium, host considerable amounts of 13C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin. Raman analyses are consistent with methane being occluded in widespread microfractures and porous serpentine- or chlorite-filled veins. Chromium and ruthenium may be key metal catalysts for methane production via Sabatier reaction. Chromitites may represent source rocks of abiotic methane on Earth and, potentially, on Mars

Ultramafic xenoliths from the Bearpaw Mountains, Montana, USA: evidence for multiple metasomatic events in the lithospheric mantle beneath the Wyoming craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzburgites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al2O3 contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al2O3 and TiO2) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0·5121 (close to the host minette values) to 0·5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd/144Nd values (0·5113) and extremely high 87Sr/86Sr ratios in their constituent phlogopite, indicating an ancient (probably mid-Proterozoic) enrichment. This enriched mantle lithosphere later contributed to the formation of the high-K Eocene host magmas. The cumulate group ranges from clinopyroxene-rich mica peridotites (including abundant mica wehrlites) to mica clinopyroxenites. Most contain >30% phlogopite. Their mineral compositions are similar to those of phenocrysts in the host minettes. Their whole-rock compositions are generally poorer in MgO but richer in incompatible trace elements than those of the tectonite peridotites. Whole-rock trace element patterns are enriched in large ion lithophile elements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb, Ta Zr and Hf) as in the host minettes, and their Sr–Nd isotopic compositions are also identical to those of the minettes. Their clinopyroxenes are LREE-enriched and formed in equilibrium with a LREE-enriched melt closely resembling the minettes. The cumulates therefore represent a much younger magmatic event, related to crystallization at mantle depths of minette magmas in Eocene times, that caused further metasomatic enrichment of the lithosphere

Ultramafic xenoliths from the Bearpaw Mountains, Montana, USA: evidence for multiple metasomatic events in the lithospheric mantle beneath the Wyoming craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzburgites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al2O3 contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al2O3 and TiO2) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0·5121 (close to the host minette values) to 0·5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd/144Nd values (0·5113) and extremely high 87Sr/86Sr ratios in their constituent phlogopite, indicating an ancient (probably mid-Proterozoic) enrichment. This enriched mantle lithosphere later contributed to the formation of the high-K Eocene host magmas. The cumulate group ranges from clinopyroxene-rich mica peridotites (including abundant mica wehrlites) to mica clinopyroxenites. Most contain >30% phlogopite. Their mineral compositions are similar to those of phenocrysts in the host minettes. Their whole-rock compositions are generally poorer in MgO but richer in incompatible trace elements than those of the tectonite peridotites. Whole-rock trace element patterns are enriched in large ion lithophile elements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb, Ta Zr and Hf) as in the host minettes, and their Sr–Nd isotopic compositions are also identical to those of the minettes. Their clinopyroxenes are LREE-enriched and formed in equilibrium with a LREE-enriched melt closely resembling the minettes. The cumulates therefore represent a much younger magmatic event, related to crystallization at mantle depths of minette magmas in Eocene times, that caused further metasomatic enrichment of the lithosphere

Platinum-group element mineralisation in the Unst ophiolite, Shetland

The ophiolitic basic and ultrabasic rocks of the island of Unst, Shetland comprise a sequence of harzburgites, dunites, clinopyroxene-rich cumulates, and gabbro, within tectonic blocks that have been thrust over a migmatite complex during the Laxer Palaeozoic. Concentrations of chromite are found in the harzburgite and dunite, and to a small extent in the pyroxene cumulate rocks. They occur as disseminations, sometimes forming millimetre scale layers, and as more massive schlieren and pods of chromitite. Five alteration or hydrothermal events have been recognised in the ultrabasic rocks. These comprise early pervasive serpentinisation, later fracture controlled serpentinisation, veining and pervasive carbonation, minor late serpentine veining and talc-carbonate alteration controlled by fault zones. Exploration for platinum group element (PGE) mineralisation uas carried out using a combination of drainage, overburden and rock sampling. Analyses of PGE were obtained by fire assay followed by either neutron activation analysis or flameless atomic absorption spectrometry, and up to 20 other elements Here determined by X-ray fluoresence analysis. Panned concentrate samples were taken from 73 drainage sites distributed throughout the complex. Ir, the only PGE determined in all samples, showed a greater concentration in samples derived from the harzburgite unit than those from other units. Lox amplitude anomalies are present in three discrete areas in the harzburgite but the maximum level of 210 ppb Ir is associated with a sample derived from a prominent N-S zone of faulting and hydrothermal activity markedly discordant to the regional trend of layering in the harzburgite and dunite. This discordant zone, which extends for at least 7 km, is also marked by samples containing enrichments in Fe, Co, Ni, Cu and As. The highest Cr levels are associated with an area in the north of the harzburgite with no previous history of chromite working but where many locally derived pieces of chromitite float have been discovered. Relatively high Cr levels are also associated nith the area of dunite containing the greatest concentration of visible chromite and old norkings. A technique of collecting panned heavy mineral concentrates from overburden samples was adopted as a reconnaissance exploration technique after orientation sampling in the harzburgite unit at Cliff, an area with high PGE levels in chromitite and associated dunite. Systematic sampling in the Cliff area outlined a zone of coincident Pd, Pt and Rh enrichment near to but separate from the chromite workings knorrn to be enriched in PGE. In contrast the distribution of Ru was entirely different with scattered lon amplitude anomalous zones and a maximum anomaly 300m from the chromite-rich zone. Reconnaissance lines were sampled at other locations within the harzburgite, dunite and cumulate units. Lore amplitude Pd and Pt anomalies were detected xithin the dunite unit, especially in 'a traverse across the trace of the prominent N-S fault zone at Helliers Uater, adjacent to the outcrop of the cumulate unit. In general the overburden data suggest some association between PGE enrichment and enhanced levels of Ni relative to typical silicate levels apparent when expressed as the ratio Ni/MgC. Rock samples Here collected from all parts of the complex, including most of the main chromitite workings. Very high levels of all PGE occur in samples of chromitite, chromite-rich dunite and dunite from the Cliff area, with a strong positive intercorrelation between all PGE. The proportions of the various PGE are very similar to those present in deposits in major layered basic/ultrabasic complexes like Bushveld and Stillwater, irith strong relative enrichment in Pd and Pt. These PGE proportions are completely different from the Ru-Ir-0s dominant assemblage typical of ophiolitic rocks. Associated with high levels of PGE are enrichments in Ni, Cu, As, Sb and Te. There is no correlation rrith Cr and some samples of chromitite from the Cliff area contain only background levels of PGE. High to moderate levels of PGE with the same proportions of elements as the Cliff samples also occur in samples of chromitite and serpentinised dunite from the dunite unit and in samples of pyroxenite from the cumulate unit, In contrast PGE-rich samples of chromitite from the harzburgite unit near Harold's Grave have entirely different proportions of PGE with Ru and Ir in greatest abundance. This PGE distribution is similar to that in some background samples of harzburgite and closely resembles the pattern found in typical ophiolites. The PGE in the Harold's Grave samples do not exhibit the Ni enhancement noted in the Cliff PGE mineralisation. In samples from the Cliff area the platinum-group minerals (PGM) sperrylite, stibiopalladinite, hollingnorthite, laurite and possibly irarsite have been identified, mostly as grains less than 10 microns in size. In chromite-rich rocks these minerals occur Rithin chlorite haloes around chromite, in the blackened altered rims of chromite grains and in interstitial Ni-rich serpentine/carbonate intergroxths in association with pentlandite, orcellite and other Ni sulphides and arsenides, sometimes spatially related to chlorite-carbonate-magnetite veins. They also occur as fine grains Rithin magnetite rims around chromite and in magnetite or carbonate veins in dunite. The Ni sulphide/arsenide assemblage associated Rith the PGH is characteristic of serpentinisation at temperatures less than 500'C, Rell belox the range of magmatic conditions. A hydrothermal origin for the PGE mineralisation is proposed, probably related to the second phase of serpentinisation. This involved the redistribution of Ni accompanied by the introduction of As, Sb and Te probably with a StrUCtUral Control. Pre-existing concentrations of chromite may have acted as a precipitation barrier causing rich PGH deposition in the alteration haloes around chromite grains. Continuous borehole or trench sections through mineralised zones are required to assess the economic significance of the PGE mineralisation. Nevertheless the high levels of PGE attained and the evidence of xidespread occurrence of the Cliff-type PGE enrichment are favourable indications. The PGE enrichments found in the cumulate complex are of potential interest as they may originally have been of magmatic origin. Larger tonnage targets may therefore be present in this unit compared Rith the likely size of structurally-controlled mineralisation elswhere in the complex

Platinum-group elements, S, Se and Cu in highly depleted abyssal peridotites from the Mid-Atlantic Ocean Ridge (ODP Hole 1274A): Influence of hydrothermal and magmatic processes

Highly depleted harzburgites and dunites were recovered from ODP Hole 1274A, near the intersection between the Mid-Atlantic Ocean Ridge and the 15°20′N Fracture Zone. In addition to high degrees of partial melting, these peridotites underwent multiple episodes of melt-rock reaction and intense serpentinization and seawater alteration close to the seafloor. Low concentrations of Se, Cu and platinum-group elements (PGE) in harzburgites drilled at around 35-85 m below seafloor are consistent with the consumption of mantle sulfides after high degrees (>15-20 %) of partial melting and redistribution of chalcophile and siderophile elements into PGE-rich residual microphases. Higher concentrations of Cu, Se, Ru, Rh and Pd in harzburgites from the uppermost and lowest cores testify to late reaction with a sulfide melt. Dunites were formed by percolation of silica- and sulfur-undersaturated melts into low-Se harzburgites. Platinum-group and chalcophile elements were not mobilized during dunite formation and mostly preserve the signature of precursor harzburgites, except for higher Ru and lower Pt contents caused by precipitation and removal of platinum-group minerals. During serpentinization at low temperature (<250 °C) and reducing conditions, mantle sulfides experienced desulfurization to S-poor sulfides (mainly heazlewoodite) and awaruite. Contrary to Se and Cu, sulfur does not record the magmatic evolution of peridotites but was mostly added in hydrothermal sulfides and sulfate from seawater. Platinum-group elements were unaffected by post-magmatic low-temperature processes, except Pt and Pd that may have been slightly remobilized during oxidative seawater alteration

Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite: Implications for melt/fluid-rock reactions at Moho level beneath oceanic spreading centers

International audience15 The Maqsad area in the Oman ophiolite exposes a >300 m thick dunitic mantle-crust 16 transition zone (DTZ) that developed above a mantle diapir. The Maqsad DTZ is primarily 17 f "p " w h scattered chromite and chromite seams) and 18 " p g " which exhibit a significant lithological variability, including various 19 kinds of clinopyroxene-, plagioclase-, orthopyroxene-, amphibole (hornblende/pargasite)-20 bearing dunites. These minerals are interstitial between olivine grains and their variable 21 abundance and distribution suggest that they crystallized from a percolating melt. Generally 22 studied through in-situ mineral characterization, the whole rock composition of dunites is 23 poorly documented. This study reports on whole rock and minerals major and trace element 24 *Manuscript contents on 79 pure to variably impregnated dunites collected systematically along cross 2

Petrologic and microstructural constraints on focused melt transport in dunites and the rheology of the shallow mantle

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2004Observations at mid-ocean ridges indicate that magmas are focused to the ridge axis by a network of porous dunites in near chemical isolation. This thesis investigates several of the outstanding questions regarding the mechanisms of melt transport and its effects on the shallow mantle. Chapter 1 details the current understanding of melt migration from observations at mid-ocean ridges and ophiolites. Chapter 2 uses the size distribution and abundance of dunites measured in the Oman ophiolite to place limits on the potential mechanisms by which dunites form and subsequently estimate the flux of chemically unequilbrated melt which a network of dunites can supply. Chapter 3 characterizes the chemical composition of dunites and harzburgites from Oman to further constrain the process by which dunites form and relates the observed trends within dunites to variations in the time-integrated meltrock ratio. Chapter 4 examines the microstructures of peridotites in Oman to constrain the deformation mechanisms which determine the viscosity of shallow mantle. Chapter 5 is a numerical investigation of advection beneath ridges incorporating the rheology inferred from the observed microstructures. Chapter 6 integrates the conclusions of the previous chapters, reevaluating the potential melt flux through dunites and constraining the permeabilty of the shallow mantle.Funding for my research was provided by The National Science Foundation through a Graduate Research Fellowship 2277600 and grants OCE-0118572, OCE- 9819666, EAR-0230267, EAR-9405845, OCE-9521113. Additional support was generously provided by the Woods Hole Oceanographic Institution through the Office of Academic Programs and the Deep Ocean Exploration Institute