Distal Exposures of the Miocene Peach Spring Tuff in the Marble and Ship Mountains, CA

The Peach Springs Tuff (PST) is a uniquely identifiable ignimbrite deposit formed by a large caldera-forming volcanic eruption during the early Miocene era. Previous investigations (e.g., Miller et al., 1986; Neilson et al., 1990) have suggested that distal ignimbrite deposits located within the Mojave region of southern California belong to the PST eruptive event. To address this hypothesis, this study investigates ignimbrite samples collected from outcrops within the Marble Mountains (n = 6) and Ship Mountains (n = 8) of southern California. Petrographic analysis reveals the abundance of quartz, sanidine, titanite, hornblende, and biotite in most ignimbrite samples. These mineral phases agree well with previous heavy mineral studies of the PST (Gusa, 1986). Electron Probe Micro-Analysis (EPMA) reveals average biotite, hornblende, and sanidine compositions that closely match PST values. Our data suggests that the ignimbrite deposits in the Marble and Ship mountains are distal exposures of the PST

Megacrystic Potassium Feldspar Magmatism In The Southern Mojave Desert, California

Investigating the textural, chemical, and chronological records preserved within crystal populations can provide insight into the processes which operate during magma ascent, emplacement, and crystallization. K-feldspar megacrysts offer an excellent opportunity to explore these records, particularly in chemically-evolved systems. Understanding megacryst formation bears on a fundamental issue in granite petrogenesis, namely whether the textural and chemical features preserved within granitoid intrusions reflect primary magmatic processes or late-stage crystallization and subsolidus reorganization. To expand our understanding of megacryst formation, we investigated a suite of K-feldspar megacrysts from the Sheep Hole Pluton (SHP) in Southern California. SHP megacrysts are euhedral, ranging from 1-8cm in length. Petrographic analysis and SEM/EDS mapping reveals abundant plagioclase (~40%), quartz (~35%), biotite (~10%), titanite (~10%), and hornblende (~5%) inclusions. Other accessory phases include Fe-Ti oxides, apatite, allanite, and zircon. Many of these inclusions, especially euhedral plagioclase, biotite, and titanite, are preferentially orientated along diffuse oscillatory zoning boundaries in the host megacryst. EPMA analyses collected along megacryst core-to-rim traverses reveal Or78-93 compositions with dramatic fluctuations in Ba concentrations (0.89 - 2.73 wt%). Core and rim analyses of plagioclase inclusions were also collected via EPMA. These analyses reveal that plagioclase inclusions contain oligoclase to andesine cores (An19 - An34) and albite-rich rims (An3 - An10). Although SHP megacrysts are much older than megacrysts described in previous studies, the textural and chemical observations are strikingly similar. We favor a magmatic origin for these megacrysts and interpret these similarities to suggest that a common magmatic process is responsible for K-feldspar megacryst formation

The role of the axial melt lens in crustal accretion at fast-spreading mid-ocean ridges

Fast-spreading mid-ocean ridges (MOR) are underlain by a thin, quasi-steady-state melt or crystal mush body at the base of the sheeted dykes, referred to as the axial melt lens (AML). Although the AML is thought to play a key role in the development of MOR basalts (MORB), debate persists regarding the composition of the AML and the role it plays in the accretion of the lower crust. I address this question by studying a suite of varitextured gabbronorites from the Hess Deep rift valley in the equatorial Pacific Ocean which are interpreted to have formed in the AML of the East Pacific Rise. This unique sample set provides an unparalleled opportunity to conduct the first comprehensive investigation of the AML at a fast-spreading MOR. To facilitate this study, I here develop a method for the quantitative assessment of compositional distribution (QACD) in whole-thin-section element maps. QACD facilitates rapid data collection and processing to generate mineral modes, element and molar-ratio maps, and quantifying full-sample compositional distributions. My application of QACD to the Hess Deep AML suite reveals that mineral phases within the AML here are too evolved to be in equilibrium with MORB. I test the broader applicability of this conclusion by conducting detailed mapping and sampling of an analogous AML horizon in the Oman Ophiolite (Wadi Saq, Ibra Valley). This section is characterised by an evolved sheeted dyke complex rooting into a quartz diorite-hosted AML, supporting the supposition that the AML accommodates the fractionation of highly-evolved melts. I propose a model wherein the AML is predominantly fed by small volumes of evolved interstitial melts expelled from the underlying crystal mush. In the months preceding decadal eruption events, short-lived, focused injections of primitive melts into the AML mix with the extant highly-fractionated melt and trigger eruptions. This model reconciles the apparent mismatch between the volcanic and plutonic records and inferences made on geophysical and petrological grounds. I suggest that the AML is an active player in the development of MORB, permitting the fractionation and storage of evolved melts expelled from the underlying crystal mush and recording the mixing of that material with primitive melt, hence fulfilling more of a passive role with respect to lower crustal accretion than previously proposed

Paradoxical co-existing base metal sulphides in the mantle: The multi-event record preserved in Loch Roag peridotite xenoliths, North Atlantic Craton

The role of the subcontinental lithospheric mantle as a source of precious metals for mafic magmas is contentious and, given the chalcophile (and siderophile) character of metals such as the platinum-group elements (PGE), Se, Te, Re, Cu and Au, the mobility of these metals is intimately linked with that of sulphur. Hence the nature of the host phase(s), and their age and stability in the subcontinental lithospheric mantle may be of critical importance. We investigate the sulphide mineralogy and sulphide in situ trace element compositions in base metal sulphides (BMS) in a suite of spinel lherzolite mantle xenoliths from northwest Scotland (Loch Roag, Isle of Lewis). This area is situated on the margin of the North Atlantic Craton which has been overprinted by a Palaeoproterozoic orogenic belt, and occurs in a region which has undergone magmatic events from the Palaeoproterozoic to the Eocene. We identify two populations of co-existing BMS within a single spinel lherzolite xenolith (LR80) and which can also be recognised in the peridotite xenolith suite as a whole. Both populations consist of a mixture of Fe-Ni-Cu sulphide minerals, and we distinguished between these according to BMS texture, petrographic setting (i.e., location within the xenolith in terms of ‘interstitial’ or within feldspar-spinel symplectites, as demonstrated by X-ray Computed Microtomography) and in situ trace element composition. Group A BMS are coarse, metasomatic, have low concentrations of total PGE (< 40 ppm) and high (Re/Os)N (ranging 1 to 400). Group B BMS strictly occur within symplectites of spinel and feldspar, are finer-grained rounded droplets, with micron-scale PtS (cooperite), high overall total PGE concentrations (15–800 ppm) and low (Re/Os)N ranging 0.04 to 2. Group B BMS sometimes coexist with apatite, and both the Group B BMS and apatite can preserve rounded micron-scale Ca-carbonate inclusions indicative of sulphide-carbonate-phosphate immiscibility. This carbonate-phosphate metasomatic association appears to be important in forming PGE-rich sulphide liquids, although the precise mechanism for this remains obscure. As a consequence of their position within the symplectites, Group B BMS are particularly vulnerable to being incorporated in ascending mantle-derived magmas (either by melting or physical entrainment). Based on the cross-cutting relationships of the symplectites, it is possible to infer the relative ages of each metasomatic BMS population. We tally these with major tectono-magmatic events for the North Atlantic region by making comparisons to carbonatite events recorded in crustal and mantle rocks, and we suggest that the Pt-enrichment was associated with a pre-Carboniferous carbonatite episode. This method of mantle xenolith base metal sulphide documentation may ultimately permit the temporal and spatial mapping of the chalcophile metallogenic budget of the lithospheric mantle, providing a blueprint for assessing regional metallogenic potential. Abbreviations: NAC, North Atlantic Craton; GGF, Great Glen Fault; NAIP, North Atlantic Igneous Province; BPIP, British Palaeogene Igneous Province; SCLM, subcontinental lithospheric mantle; PGE, platinum-group elements; HSE, highly siderophile elements; BMS, base metal sulphid

Cobalt and precious metals in sulphides of peridotite xenoliths and inferences concerning their distribution according to geodynamic environment: A case study from the Scottish lithospheric mantle

Abundances of precious metals and cobalt in the lithospheric mantle are typically obtained by bulk geochemical analyses of mantle xenoliths. These elements are strongly chalcophile and the mineralogy, texture and trace element composition of sulphide phases in such samples must be considered. In this study we assess the mineralogy, textures and trace element compositions of sulphides in spinel lherzolites from four Scottish lithospheric terranes, which provide an ideal testing ground to examine the variability of sulphides and their precious metal endowments according to terrane age and geodynamic environment. Specifically we test differences in sulphide composition from Archaean-Palaeoproterozoic cratonic sub-continental lithospheric mantle (SCLM) in northern terranes vs. Palaeozoic lithospheric mantle in southern terranes, as divided by the Great Glen Fault (GGF). Cobalt is consistently elevated in sulphides from Palaeozoic terranes (south of the GGF) with Co concentrations > 2.9 wt.% and Co/Ni ratios > 0.048 (chondrite). In contrast, sulphides from Archaean cratonic terranes (north of the GGF) have low abundances of Co (< 3600 ppm) and low Co/Ni ratios (< 0.030). The causes for Co enrichment remain unclear, but we highlight that globally significant Co mineralisation is associated with ophiolites (e.g., Bou Azzer, Morocco and Outokumpu, Finland) or in oceanic peridotite-floored settings at slow-spreading ridges. Thus we suggest an oceanic affinity for the Co enrichment in the southern terranes of Scotland, likely directly related to the subduction of Co-enriched oceanic crust during the Caledonian Orogeny. Further, we identify a distinction between Pt/Pd ratio across the GGF, such that sulphides in the cratonic SCLM have Pt/Pd ≥ chondrite whilst Palaeozoic sulphides have Pt/Pd < chondrite. We observe that Pt-rich sulphides with discrete Pt-minerals (e.g., PtS) are associated with carbonate and phosphates in two xenolith suites north of the GGF. This three-way immiscibility (carbonate-sulphide-phosphate) indicates carbonatitic metasomatism is responsible for Pt-enrichment in this (marginal) cratonic setting. These Co and Pt-enrichments may fundamentally reflect the geodynamic setting of cratonic vs. non-cratonic lithospheric terranes and offer potential tools to facilitate geochemical mapping of the lithospheric mantle

Arc foundations and the Initiation of subduction in the Izu-Bonin forearc

Our understanding of the lower crust in the ocean basins has been inferred from geophysical studies of mid-ocean ridges (MOR), volcanic arcs, and ophiolites. Many studies of ophiolites suggest that although they exhibit characteristics of MOR’s, they may originate in supra-subduction zone (SSZ) settings. It is also suggested that the foundations and lower crust of SSZ arcs are created by a MOR, possibly in a trench-ridge-trench triple junction setting. Although many samples of the lower crust have been collected along the world’s MOR’s, few samples have been retrieved from SSZ arcs. Fifty ultramafic and gabbroic samples recovered by dredges 31 and 42 of the KH07-02 dredging cruise along the inner trench wall of the Izu-Bonin Arc have been characterized as lower crustal rocks related to MOR-like basalts (fore-arc basalts or FAB). Major element analyses of Cr-spinels indicate two distinct compositional trends. Group M consists of wehrlites and gabbros with medium Cr# (45-60) and high Al2O3 and TiO2 (12-30 & 0.1-2.25 wt. %) which reacted with MORB-like melts. Group B consists of spinels solely from dunites and peridotites with high Cr# (65-94) and low Al2O3 and TiO2 (3-21 & 0-0.12 wt. %) which reacted with boninitic melts. Boninites from the Bonin Ridge are known to be younger (44-48 Ma) than FABs (50-52 Ma). This suggests that the majority of the lower crust was related to subduction initiation. Major and trace element analyses of clinopyroxenes associated with Group M spinels indicates highly depleted compositions characteristic of high degree partial melts or boninites. A lack of orthopyroxene and association with Group M spinel suggests that D31 clinopyroxenes evidence singly depleted, un-aggregated melt fractions. This suggests that FABs are the result of mixing between a highly depleted mantle source and a MORB-like liquid. Modeling results in 5 to 7% fractional melts of DMM refertilized by the addition of ~12% N-MORB reproducing such a mixing array. This model is consistent with models for subduction initiation which suggest that asthenosphere will upwell into the void created between the down-going and overriding plates during initiation and begin to undergo decompression melting.Earth and Atmospheric Sciences, Department o

Melt stagnation in peridotites from the Godzilla Megamullion Oceanic Core Complex, Parece Vela Basin, Philippine Sea

The Godzilla Megamullion, located in the Parece Vela Backarc Basin of the Izu-Bonin-Mariana (IBM) system, is the largest known example of an Oceanic Core Complex (OCC). Peridotites recovered from the megamullion are divided petrographically into fertile (e.g. lherzolites), depleted (e.g. harzburgites), and plagioclase-bearing groups (Ohara et al., 2003a). A total of 151 thin sections were studied from the Kairei KR03-01, Hakuho Maru KH07-02, and Yokosuka YK09-05 cruises. Melt stagnation is studied via the incidence of plagioclase-bearing peridotites and the major element chemistry of Cr-spinels in the plag-bearing samples. A distinct trend in melt stagnation is evident along the length of the megamullion representing a secular evolution in the entrapment of melts rising through the lithosphere. The distal (furthest from the termination of spreading), depleted portion of the mullion represents a robust mantle section that was still producing abundant melt and can be compared to typical oceanic spreading with its relatively normal percentage of plagioclase peridotites and average spinel Cr# of 0.35. The medial, fertile portion of the mullion represents a steep falloff in melt productivity represented by fertile spinel compositions (i.e. Cr# \u3c. 0.25) and the presence of plagioclase-free lherzolites. The proximal (closest to termination of spreading), heavily plagioclase impregnated portion (with spinel Cr#s covering nearly the entire range of abyssal peridotite spinel compositions) of the mullion then represents a period of increasing stagnation of melt into a lithosphere that was undergoing progressive thickening. We infer that the processes of mantle evolution through melt stagnation and impregnation, as evidenced by the systematic variations in plag-peridotites along Godzilla Megamullion, represent a possibly common way in which the mantle reacts to OCC formation. In this case, Godzilla Megamullion may represent an extreme endmember in OCC formation. © 2013

An overview of the mantle xenoliths from Loch Roag, Northwest Scotland (the margin of the North Atlantic Craton)

At Loch Roag, on the western edge of the Outer Hebridean island of Lewis, a vertical ENE-WSW trending monchiquite dyke (50-150cm wide) cross-cuts Archaean-Palaeoproterozoic Lewisian gneisses. This fine-grained and aphanitic dyke has been dated to 45.2 ± 0.2 Ma (Faithfull et al., 2012) and is thus the youngest known magmatic intrusion in the British Isles, intruded some ~10 Ma later than the initial opening of the North Atlantic. It was discovered in the late 1970s during quarrying to widen a farm track. Aside from its recent age, the dyke has other features that mark it as unique in the British Isles, if not in western Europe. Axial portions of the dyke carry an assemblage of mantle and lower crustal xenoliths (including shonkinites (syenites), anorthoclasites, anorthosites, glimmerites, mafic granulites, pyroxenites, wehrlites and spinel lherzolites) which are distinct from those known from any other UK xenolith site, and megacrysts including gem quality sapphires (Upton et al. 1983; Menzies et al. 1987). More than 300 specimens were collected in the 1980s (exhausting the currently exposed portions of the dyke) and are now curated. For the purposes of this presentation, we will primarily focus on the spinellherzolite mantle xenoliths. The spinel lherzolite xenoliths display a range of textures, from sub-equigranular and granoblastic, to porphyroblastic. Whole-rock analyses of xenolith samples were originally presented by Hunter and Upton (1987), and further bulk geochemical analysis has beenconducted by Hughes et al. (2015), including platinum-group elements (PGE) and Au. Bulk PGE abundances are equivalent to estimates of ‘primitive upper mantle’ and are notably Ptrich. Cu abundances are elevated above cratonic or Archaean-Proterozoic mantle peridotite compositions worldwide and currently available in the published literature. The xenoliths are also light REE enriched and depleted in Nb, Ta and Ti. Thus the Loch Roag xenoliths are enriched relative to cratonic lithospheric mantle, probably because of their position on the margin of the North Atlantic Craton. Crystal sizes are typically 1-2mm but orthopyroxene porphyroblasts may be up to 10mm and olivine compositions range Fo92 to Fo88 (Upton et al. 2011). ‘Primary’ clinopyroxene compositions indicate a cryptic metasomatism event (Hughes et al. 2015). Sulphides are abundant and may be categorized into distinct ‘groups’ according to their petrographic setting within the xenoliths and trace element composition. For example, lherzolite xenoliths demonstrate modal metasomatism in the form of corroded grains of spinel mantled by anorthoclase-pyroxene symplectites. In these, the pyroxenes adjacent to the spinel-anorthoclase pairs are enriched in Na. Feldspars range from potassic-oligoclase to anorthoclase, and spinel has a wide range of Cr2O3 contents, from 6.2–31.4 wt.%. Crucially, rounded ‘droplets’ of base metal sulphides, in PGE and bearing PtS (cooperite) are found strictly within these symplectites, indicating the same metasomatic event was responsible for PGE enrichment (particularly Pt) of this portion of the SCLM. Similar PGE-rich sulphide droplets have rounded inclusions of CaCO3, possibly indicating a carbonatitic metasomatic agent, as supported by the xenolith’s light REE enrichment andother isotopic evidence. A model-age of ~2.5 Ga was obtained from the metasomatisedrocks by Long et al. (1991), suggesting that the Loch Roag-type lherzolite protolith is Archaean-Palaeoproterozoic, unlike other mantle xenoliths from western Europe south of Scandinavia where the SCLM has experienced different Phanerozoic orogenic events

An overview of the mantle xenoliths from Loch Roag, Northwest Scotland (the margin of the North Atlantic Craton)

At Loch Roag, on the western edge of the Outer Hebridean island of Lewis, a vertical ENE-WSW trending monchiquite dyke (50-150cm wide) cross-cuts Archaean-Palaeoproterozoic Lewisian gneisses. This fine-grained and aphanitic dyke has been dated to 45.2 ± 0.2 Ma (Faithfull et al., 2012) and is thus the youngest known magmatic intrusion in the British Isles, intruded some ~10 Ma later than the initial opening of the North Atlantic. It was discovered in the late 1970s during quarrying to widen a farm track. Aside from its recent age, the dyke has other features that mark it as unique in the British Isles, if not in western Europe. Axial portions of the dyke carry an assemblage of mantle and lower crustal xenoliths (including shonkinites (syenites), anorthoclasites, anorthosites, glimmerites, mafic granulites, pyroxenites, wehrlites and spinel lherzolites) which are distinct from those known from any other UK xenolith site, and megacrysts including gem quality sapphires (Upton et al. 1983; Menzies et al. 1987). More than 300 specimens were collected in the 1980s (exhausting the currently exposed portions of the dyke) and are now curated. For the purposes of this presentation, we will primarily focus on the spinellherzolite mantle xenoliths. The spinel lherzolite xenoliths display a range of textures, from sub-equigranular and granoblastic, to porphyroblastic. Whole-rock analyses of xenolith samples were originally presented by Hunter and Upton (1987), and further bulk geochemical analysis has beenconducted by Hughes et al. (2015), including platinum-group elements (PGE) and Au. Bulk PGE abundances are equivalent to estimates of ‘primitive upper mantle’ and are notably Ptrich. Cu abundances are elevated above cratonic or Archaean-Proterozoic mantle peridotite compositions worldwide and currently available in the published literature. The xenoliths are also light REE enriched and depleted in Nb, Ta and Ti. Thus the Loch Roag xenoliths are enriched relative to cratonic lithospheric mantle, probably because of their position on the margin of the North Atlantic Craton. Crystal sizes are typically 1-2mm but orthopyroxene porphyroblasts may be up to 10mm and olivine compositions range Fo92 to Fo88 (Upton et al. 2011). ‘Primary’ clinopyroxene compositions indicate a cryptic metasomatism event (Hughes et al. 2015). Sulphides are abundant and may be categorized into distinct ‘groups’ according to their petrographic setting within the xenoliths and trace element composition. For example, lherzolite xenoliths demonstrate modal metasomatism in the form of corroded grains of spinel mantled by anorthoclase-pyroxene symplectites. In these, the pyroxenes adjacent to the spinel-anorthoclase pairs are enriched in Na. Feldspars range from potassic-oligoclase to anorthoclase, and spinel has a wide range of Cr2O3 contents, from 6.2–31.4 wt.%. Crucially, rounded ‘droplets’ of base metal sulphides, in PGE and bearing PtS (cooperite) are found strictly within these symplectites, indicating the same metasomatic event was responsible for PGE enrichment (particularly Pt) of this portion of the SCLM. Similar PGE-rich sulphide droplets have rounded inclusions of CaCO3, possibly indicating a carbonatitic metasomatic agent, as supported by the xenolith’s light REE enrichment andother isotopic evidence. A model-age of ~2.5 Ga was obtained from the metasomatisedrocks by Long et al. (1991), suggesting that the Loch Roag-type lherzolite protolith is Archaean-Palaeoproterozoic, unlike other mantle xenoliths from western Europe south of Scandinavia where the SCLM has experienced different Phanerozoic orogenic events