Brian L. Cousens

Multi-element and rare earth element composition of lichens, mosses, and vascular plants from the Central Barrenlands, Nunavut, Canada

Abstract Lichen (n=12) and moss (n=6) species from a remote region of northern Canada have remarkably similar multi-element patterns suggesting they are non-specific accumulators of metals under existing conditions. Within individual species the concentration of many metals analyzed range over an order of magnitude. Many elements have a positive correlation with multi-element (n=48) and REE (rare earth element) totals. Others, such as Cd, K, and Zn have relatively consistent concentrations across all lichen and moss species, and across all sampling sites, indicating different accumulation and/or retention processes. Lichens and mosses have REE concentrations 1–3 orders of magnitude less than those of the average upper continental crust (UCC) but yield identical patterns. The correlation of other poorly soluble elements and key elemental ratios in lichen and moss are also similar to UCC and modern river sediment values. Metals including Sc, V, Cr, Fe, Co, Ga, Y, Hf, W, Pb, Th, and U show strong positive correlations with REE in lichen and moss. Rare earth elements may be useful as reference elements in environmental studies because of transport in the particulate phase, lack of significant anthropogenic sources, coherent group geochemistry, generally robust concentrations, and upper crustal signatures. Further, the REE may be helpful in identifying particulate deposition related to anthropogenic activities and enrichment of other elements by biogenic processes. The multi-element compositions of vascular plants (leaves and twigs) are fundamentally different from those of lichen and moss, lack correlation with REE, and are extremely enriched for many elements (100–1000× average upper continental crust) relative to the REE; perhaps because of limited REE solubility and transport via root systems. Enrichment factors for most metals of environmental concern are low; Pb is elevated but may be an artifact of low concentrations in local bedrock. Trace metal concentrations in lichen and moss at Otter Lake are similar to those measured across the Northwest Territories over 25 a ago.

Proterozoic (1.85–1.75 Ga) igneous suites of the Western Churchill Province: granitoid and ultrapotassic magmatism in a reworked Archean hinterland

Paleoproterozoic igneous rocks in the Archean hinterland of the Paleoproterozoic Trans-Hudson orogen (THO) consist of voluminous late syn-orogenic to post-orogenic monzonite to granite (Hudson granitoids; ≈1850–1810 Ma), and contemporaneous ultrapotassic lamprophyre dykes and volcanic rocks (Dubawnt minettes) that are interbedded with alluvial fan and fluvial deposits (Baker Lake Group, lower Dubawnt Supergroup). They were followed at approximately 1750 Ma by rapakivi granite (Nueltin granite) and porphyritic rhyolite associated with aeolian sandstone (Pitz Formation, middle Dubawnt Supergroup). The tectonic cycle ended with the deposition of conglomerates and sandstones in a large sag basin (Thelon Formation, upper Dubawnt Supergroup, ≈1.72 Ga). The Hudson granitoids, which are strongly concentrated northwest of the THO, were broadly synchronous with terminal collision between the Archean Churchill and Superior cratons and the development of NE-trending ductile structures in the Western Churchill Province (WCP) that may be related to tectonic escape to the northeast. They were emplaced at mid-crustal level and no volcanic equivalents are preserved. Fault-bounded basins containing the minette volcanic rocks are located farther west in a domain dominated more by brittle faulting. The Nueltin granites, emplaced during a period of active extensional faulting, are present in a band extending southwest from the minette basins toward a preserved remnant of the sag basin (the Athabasca basin). Hudson granitoids are largely absent from this band but reappear west of it, indicating a higher crustal level of exposure in a downdropped Nueltin ‘corridor’. The Nd isotope composition of the three suites is similar (minettes: eNd,1830 Ma=−5 to −11; Hudson granitoids: eNd,1830 Ma=−7 to −13.5; Nueltin suite: eNd,1750 Ma=−7 to −10.5), and they have late Archean model ages that match those of average Archean WCP rocks. The Hudson granitoids are rich in inherited Archean zircon, and both granitoid suites are interpreted as crustal melts. Some Nueltin granites and Pitz rhyolites are mingled with basalt, and the Nueltin suite fits a commonly cited model for rapakivi granite production, which postulates injection of basalt into extending, brittly faulted crust. The Hudson granitoids are similar to late syn- to post-orogenic plutons in numerous other collisional hinterlands, which are typically associated with ultrapotassic lamprophyres. The minettes, which have high mg# and bear mantle xenocrysts, must have a mantle source component, and their source region could have been subduction-enriched lithospheric mantle. However, their source had only slightly lower time-integrated LREE enrichment than did that of the granitoids, and the incompatible element signatures of the two suites are strikingly similar. The minette source region may have been in a zone of mixed crust and upper mantle, formed during a shortening event which resulted in crustal thickening and subsequent melting at mid-crustal layers to form the Hudson granitoid plutons. The generation and emplacement of minette melts may have been promoted by extension related to a combination of slab breakoff, gravitational collapse of thickened crust, and strike-slip faulting in the deforming hinterland. Subsequent anorogenic rapakivi granite-basalt activity may have been triggered by lithospheric mantle delamination. The hinterland tectonic cycle of the WCP was repeated in other large Archean terranes that were deformed during the early Proterozoic, but the igneous and sedimentary record is unusually complete in the WCP.

Subduction-modified pelagic sediments as the enriched component in back-arc basalts from the Japan Sea: Ocean Drilling Program Sites 797 and 794

Ocean Drilling Program Legs 127 and 128 in the Yamato Basin of the Japan Sea, a Miocene-age back-arc basin in the western Pacific Ocean, recovered incompatible-element-depleted and enriched tholeiitic dolerites and basalts from the basin floor, which provide evidence of a significant sedimentary component in their mantle source. Isotopically, the volcanic rocks cover a wide range of compositions (e.g., 87Sr/86Sr=0.70369–0.70503, 204Pb/204Pb=17.65–18.36) and define a mixing trend between a depleted mantle (DM) component and an enriched component with the composition of EM II. At Site 797, the combined isotope and trace element systematics support a model of two component mixing between depleted, MORB-like mantle and Pacific pelagic sediments. A best estimate of the composition of the sedimentary component has been determined by analyzing samples of differing lithology from DSDP Sites 579 and 581 in the western Pacific, east of the Japan arc. The sediments have large depletions in the high field strength elements and are relatively enriched in the large-ion-lithophile elements, including Pb. These characteristics are mirrored, with reduced amplitudes, in Japan Sea enriched tholeiites and northeast Japan arc lavas, which strengthens the link between source enrichment and subducted sediments. However, Site 579/581 sediments have higher LILE/REE and lower HFSE/REE than the enriched component inferred from mixing trends at Site 797. Sub-arc devolatilization of the sediments is a process that will lower LILE/REE and raise HFSE/REE in the residual sediment, and thus this residual sediment may serve as the enriched component in the back-arc basalt source. Samples from other potential sources of an enriched. EM II-like component beneath Japan, such as the subcontinental lithosphere or crust, have isotopic compositions which overlap those of the Japan Sea tholeiites and are not “enriched” enough to be the EM II end-member.

Geochemical characteristics of West Molokai shield‐ and postshield‐stage lavas: Constraints on Hawaiian plume models

There are systematic geochemical differences between the < 2 Myr Hawaiian shields forming the subparallel spatial trends, known as Loa and Kea. These spatial and temporal geochemical changes provide insight into the spatial distribution of geochemical heterogeneities within the source of Hawaiian lavas, and the processes that create the Hawaiian plume. Lavas forming the similar to 1.9 Ma West Molokai volcano are important for evaluating alternative models proposed for the spatial distribution of geochemical heterogeneities because ( 1) the geochemical distinction between Loa and Kea trends may end at the Molokai Fracture Zone and ( 2) West Molokai is a Loa- trend volcano that has exposures of shield and postshield lavas. This geochemical study ( major and trace element abundances and isotopic ratios of Sr, Nd, Hf, and Pb) shows that the West Molokai shield includes lavas with Loa- and Kea- like geochemical characteristics; a mixed Loa- Kea source is required. In contrast, West Molokai postshield lavas are exclusively Kea- like. This change in source geochemistry can be explained by the observed change in strike of the Pacific plate near Molokai Island so that as West Molokai volcano moved away from a mixed Loa- Kea source it sampled only the Kea side of a bilaterally zoned plume.

Magmatic evolution of Quaternary mafic magmas at Long Valley Caldera and the Devils Postpile, California: Effects of crustal contamination on lithospheric mantle‐derived magmas

Isotopic variations in Cenozoic mafic volcanic rocks in the Basin and Range Province of the southwestern United States are considered by many to be due to differences in mantle sources, either subduction-enriched subcontinental lithosphere or the asthenosphere. This is a detailed geochemical investigation of the magmatic evolution and mantle source(s) of Quaternary mafic volcanism at Long Valley Caldera and the Devils Postpile National Monument, Western Great Basin. The Quaternary mafic lavas are dominantly post-Bishop Tuff ( 7%, assimilation of Sierra Nevada granitoids becomes increasingly important at MgO 7% MgO, 87Sr/86Sr and La/Sm decrease as 143Nd/144Nd, Nb/La, Zr/Ba and (to a lesser extent) Pb isotope ratios increase from the oldest to the youngest Quaternary lavas. The oldest lavas are chemically similar to other lavas in the western Basin and Range thought to have an enriched lithospheric mantle source. Whereas the shifts in Sr and Nd isotope ratios from oldest to youngest basalts might be consistent with a progressively increasing asthenospheric component in the lavas, the shifts in incompatible element ratios and lack of shifts in Pb isotope ratios with time are not. Instead, they indicate that with time the mafic lavas may have increasingly interacted with mafic crust, perhaps gabbroic/dioritic rocks at depth within the Sierra Nevada Batholith. Alternatively, a second, less enriched lithospheric mantle source is present beneath the Long Valley area that has only recently begun to undergo melt generation, and this source has made progressively larger contributions to the basaltic magmas erupted at the surface.

Cretaceous to Cenozoic volcanism in South Korea and in the Sea of Japan: magmatic constraints on the opening of the back-arc basin

Abstract The major element, trace element, and radiogenic isotope compositions of volcanic rocks in the back-arc area of the eastern Eurasian continental margin provide insight into the nature of the mantle wedge and constrain the magmatic evolution of the Japan Sea back-arc basin linked to its tectonic history. Different phases of post-Early Cretaceous volcanic activity are identified along the Korean margin and in the Japan Sea. Volcanic rocks from Korea include (1) Cretaceous and early Cenozoic calc-alkaline lavas of a volcanic arc at an active margin, and (2) Pliocene and Quaternary intraplate flood basalts and volcanic islands of alkaline composition. Japan Sea volcanic rocks consist of (1) early Cenozoic andesite flows of a remnant arc in the Yamato Bank, (2) early Miocene basalts of the Japan Sea basin basement, which share compositional characteristics of island arc tholeiites, continental rift tholeiites and back-arc basin basalts, (3) late Miocene seamounts of tholeiitic and mildly alkaline compositions, and (4) Pliocene and Quaternary alkaline volcanic islands. Geochemically, these rocks belong to three broad magmatic groups: (1) an arc-related, calc-alkaline group of a continental, Andean margin type, which prevailed prior to the opening of the Japan Sea between the Cretaceous and early Miocene, (2) continental rift tholeiites and back-arc basin basalts, formed during the rifting stage in the early Miocene, and (3) an intraplate alkaline group similar to OIB, erupted later during spreading, between late Miocene and Holocene times. Trace element and Sr, Nd and Pb isotopic compositions of selected samples show that the sources of magma Group 1 calc-alkaline lavas and magma Group 2 tholeiitic lavas included varying contributions of two main mantle components: an Indian Ocean MORB-like depleted mantle source (DMM) and an enriched mantle component similar to EM II. The latter component could represent DMM contaminated by subducted oceanic sediments incorporated into the lower lithosphere during the long-lived subduction of west Pacific crust. During the opening of the Japan Sea back-arc basin, the relative proportion of the DMM component dramatically increased between the rifting and spreading stages. It is also necessary to postulate a third component present in the sources of the Group 3, post-opening alkaline lavas, perhaps enriched mantle of EM I composition, which may also have resided in the subcontinental lithospheric mantle.

Carbonatite and silicate melt metasomatism of the mantle surrounding the Hawaiian plume: Evidence from volatiles, trace elements, and radiogenic isotopes in rejuvenated‐stage lavas from Niihau, Hawaii

We present new volatile, trace element, and radiogenic isotopic compositions for rejuvenated-stage lavas erupted on Niihau and its submarine northwest flank. Niihau rejuvenated-stage Kiekie Basalt lavas are mildly alkalic and are isotopically similar to, though shifted to higher 87Sr/86Sr and lower 206Pb/204Pb than, rejuvenated-stage lavas erupted on other islands and marginal seafloor settings. Kiekie lavas display trace element heterogeneity greater than that of other rejuvenated-stage lavas, with enrichments in Ba, Sr, and light-rare earth elements resulting in high and highly variable Ba/Th and Sr/Ce. The high Ba/Th lavas are among the least silica-undersaturated of the rejuvenated-stage suite, implying that the greatest enrichments are associated with the largest extents of melting. Kiekie lavas also have high and variable H2O/Ce and Cl/La, up to 620 and 39, respectively. We model the trace element concentrations of most rejuvenated-stage lavas by small degrees (∼1% to 9%) of melting of depleted peridotite recently metasomatized by a few percent of an enriched incipient melt (0.5% melting) of the Hawaiian plume. Kiekie lavas are best explained by 4% to 13% partial melting of a peridotite source metasomatized by up to 0.2% carbonatite, similar in composition to oceanic carbonatites from the Canary and Cape Verde Islands, with lower proportion of incipient melt than that for other rejuvenated-stage lavas. Primary H2O and Cl of the carbonatite component must be high, but variability in the volatile data may be caused by heterogeneity in the carbonatite composition and/or interaction with seawater. Our model is consistent with predictions based on carbonated eclogite and peridotite melting experiments in which (1) carbonated eclogite and peridotite within the Hawaiian plume are the first to melt during plume ascent; (2) carbonatite melt metasomatizes plume and surrounding depleted peridotite; (3) as the plume rises, silica-undersaturated silicate melts are also produced and contribute to the metasomatic signature. The metasomatic component is best preserved at the margins of the plume, where low extents of melting of the metasomatized depleted mantle surrounding the plume are sampled during flexural uplift. Formation of carbonatite melts may provide a mechanism to transfer plume He to the margins of the plume.

Enriched Archean lithospheric mantle beneath western Churchill Province tapped during Paleoproterozoic orogenesis

Ultrapotassic rocks of the Christopher Island Formation (Baker Lake basin) were emplaced across an enormous area (240 000 km 2 minimum) of the western Churchill Province ca. 1.83 Ga. These rocks extend across the Snowbird zone, a geophysical feature postulated by others to represent a Paleoproterozoic suture that welded the Rae and Hearne domains. Minette dikes and flows of the Rae and Hearne domains display identical ϵ Nd, 1830 values and incompatible element patterns, and thus appear to have originated from a common lithospheric-mantle source. Christopher Island Nd model ages cluster at 2.8 Ga, and ϵ Nd data from one Archean lamprophyre and three 2.45 to ca. 2.2 Ga mafic suites suggest that enriched lithospheric-mantle sources beneath both the Rae and Hearne domains existed well before ca. 1.83 Ga, inconsistent with Paleoproterozoic suturing along the Snowbird zone. In contrast to commonly invoked models that envisage melting of local enriched domains, Christopher Island ultrapotassic rocks appear to have originated from an extensive reservoir. We suggest that such a reservoir was created during an Archean metasomatic event, perhaps owing to flat subduction, and that it remained in nearly complete isolation until tapped during Paleoproterozoic extension related to squeezing of western Churchill crust between flanking Wopmay and Trans-Hudson orogens.

Mixing of magmas from enriched and depleted mantle sources in the northeast Pacific: West Valley segment, Juan de Fuca Ridge

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Smcn∼0.3, 87Sr/86Sr=0.70235–0.70242, and 206Pb/204Pb=18.22–18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Smcn∼1.8, 87Sr/86Sr=0.70245–0.70260, and 206Pb/204Pb=18.73–19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with “plums” of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the “plums”, initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the “plums” and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the “plums” could evolve to their more evolved isotopic composition after a period of 1.5–2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a less-depleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as “plums” throughout the upper mantle, transported from depth by mantle plumes.

Geology, geochronology, and geochemistry of the Miocene–Pliocene Ancestral Cascades arc, northern Sierra Nevada, California and Nevada: The roles of the upper mantle, subducting slab, and the Sierra Nevada lithosphere

The assemblage of ca. 28–3 Ma volcanic rocks exposed in the Lake Tahoe–Reno region of the northern Sierra Nevada, United States, is interpreted to be part of the Ancestral Cascades volcanic arc. The volcanic rocks are commonly highly porphyritic, including abundant plagioclase with clinopyroxene, amphibole, and rare biotite, and range from basaltic andesite to dacite in composition. Less common are poorly phyric, olivine- and clinopyroxene-bearing basalts and basaltic andesites. Porphyritic lavas dominate composite volcanic centers, whereas the poorly phyric lavas form isolated cinder cone and lava flow complexes. Tahoe-Reno arc lavas are calc-alkaline, enriched in the large ion lithophile elements but depleted in Nb and Ta relative to the light rare earth elements, and have highly variable radiogenic isotopic compositions. Compared to the modern south Cascade arc, Tahoe-Reno region basalts are enriched in the light rare earth and large ion lithophile elements and have higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd that are consistent with an old, subduction-modified lithospheric mantle source, such as that proposed for lavas of the Western Great Basin. Melting of the lithospheric mantle may be enhanced by fluid flux from the subducting slab if the Juan de Fuca slab dip is shallow. Andesites and dacites evolved from basaltic magmas by a combination of fractional crystallization and assimilation of lower crustal melts. Available geochronological data indicate that the westward sweep of Cenozoic volcanism through Nevada was associated with steepening of the slab dip, but the dip angle was lower during Miocene–Pliocene arc volcanism than it is today beneath the modern south Cascades.