John C. Lassiter

Osmium isotope systematics of drilled lavas from Mauna Loa, Hawaii

We have investigated the isotopic compositions of Os, Sr, Nd, and Pb in a suite of primitive Mauna Loa lavas from the upper 280 m of the Hawaii Scientific Drilling Project pilot core drilled near Hilo, Hawaii. These lavas were probably erupted from Mauna Loas northeast rift. Correlations between Os (hosted by olivine) and other isotopes indicate that olivine crystals in these flows are closely related in space and time to the enclosing lava, despite the presence of deformation features and Fe/Mg disequilibrium in some olivines. The temporal isotopic evolution of the lavas matches published data for basalts from Mauna Loas southwest rift, indicating that the two rifts (as well as the summit) share a common magma feed zone which is distinct from that of Kilauea. The composition of the lowermost HSDP Mauna Loa sample shows some isotopic similarities to modern Kilauea compositions and in this respect compares well with published data on submarine lavas from Mauna Loas southwest rift. The good correlations among the isotopic tracers of compatible (Os) and incompatible (Sr, Nd, Pb) elements indicate that a depleted upper mantle component is very minor or nonexistent in Mauna Loa lavas. The Os isotope results definitively rule out equilibrium porous flow as a means of melt transport through the lithosphere. The isotopic variations in shield-stage lavas are most consistent with partial melting of two distinct sources within the Hawaiian plume followed by partial mixing and rapid transport of melts through the oceanic lithosphere. The passage of the Pacific lithosphere over a heterogeneous Hawaiian plume can account for the systematic differences in the compositions of volcanoes from the “Kea” and “Loa” trends as well as the geochemical evolution of individual shields on the island of Hawaii.

Helium isotopic evolution of Mauna Kea Volcano: First results from the 1-km drill core

Helium concentrations and isotopic ratios have been measured in a suite of basaltic olivines from the Hawaii Scientific Drilling Project (HSDP) core at Hilo, Hawaii, which allows a characterization of the temporal helium isotopic evolution of Mauna Kea volcano. Typically more than 85% of the helium within the olivines is released by crushing, which demonstrates that helium is dominantly contained within the melt inclusions, and strongly suggests that helium behaves as an incompatible element during silicate melting. Lavas from the base of the Mauna Loa section of the core (240–270 m depth) have 3He/4He ratios between 13.9 and 15.8 times atmospheric (Ra). These values, when combined with Sr and Nd isotopic data, correspond to those previously found in subaerial Mauna Loa lavas that are greater than 30 ka in age, place a minimum age of 30 ka for the top of the Mauna Kea section, and are consistent with radiocarbon chronology. The upper part of the Mauna Kea lava section (290–620 m) has 3He/4He ratios of 7.0 to 6.8 Ra, similar to most mid-ocean ridge basalts and is thought to represent the normal asthenospheric mantle signature. Between 620 and 670 m depth there is a transition to higher values, with 3He/4He ratios between 10.2 and 12.5 Ra at depths greater than 670 m, suggesting a greater contribution of undegassed plume material to the older Mauna Kea tholeiites. The correlations between He, Nd, and Pb isotope ratios demonstrate that the helium isotopic variations are related to mixing of source materials rather than shallow contamination effects (i.e., via addition of radiogenic 4He). The transition from high to low 3He/4He ratios occurs within the tholeiitic shield of Mauna Kea. The highest 3He/4He ratios in the Mauna Kea lavas (up to 12.5 Ra) are significantly lower than the values found in the shield tholeiites of the adjacent volcanoes (Loihi Seamount, Mauna Loa, and Kilauea); this relates to a smaller hot spot contribution to the Mauna Kea lavas. The temporal helium isotopic evolution of Hawaiian shield volcanos (with highest 3He/4He ratios found in the oldest lavas) and the inter-volcano variations (with the Mauna Kea shield having the lowest 3He/4He ratios on the island) are attributed to movement of the Pacific plate over a radially zoned hot spot.

Geochemistry of Kauai shield-stage lavas: Implications for the chemical evolution of the Hawaiian plume

We measured He, Sr, Nd, Pb, and Os isotope ratios and major and trace element concentrations in stratigraphically and paleomagnetically controlled shield-stage lavas from Kauai, Hawaii. The range of 3He/4He ratios (17–28 RA) from Kauai is similar to that reported from Loihi and thus challenges the prevailing notion that high 3He/4He ratios are restricted to the preshield stage of Hawaiian magmatism. 3He/4He ratios vary erratically with stratigraphic position, and chronostratigraphic control from paleomagnetic data indicates very rapid changes in the 3He/4He ratios (up to 8 RA in ~102 years). These variations in helium isotopic ratios are correlated with variations in radiogenic isotope ratios, suggesting rapid changes in melt composition supplying the magma reservoir. A three-component mixing model, previously proposed for Hawaiian shield lavas, does not adequately explain the isotopic data in Kauai shield lavas. The addition of a depleted-mantle (DM) component with the isotopic characteristics similar to posterosional basalts explains the isotopic variability in Kauai shield lavas. The DM component is most apparent in lavas from the Kauai shield and is present in varying proportion in other Hawaiian shield volcanoes. Shield lavas from Kauai sample a high 3He/4He end-member (Loihi component), but while lavas from western Kauai have a larger contribution from the Kea component (high 206Pb/204Pb, anomalously low 207Pb/204Pb relative to 206Pb/204Pb), lavas from eastern Kauai have a larger proportion of an enriched (Koolau) component. The systematic isotopic differences between eastern and western Kauai reflect a gradual migration of the locus of volcanism from west to east, or alternatively east and west Kauai are two distinct shield volcanoes. In the latter case, the two shield volcanoes have maintained distinct magma supply sources and plumbing systems. Our new geochemical data from Kauai are consistent with the existence of a single high 3He/4He reservoir in the Hawaiian plume and suggest that the proportion of the different mantle components in the plume have changed significantly in the past 5 Myr. The long-term evolution of the Hawaiian plume and the temporal variability recorded in Kauai lavas require more complex geochemical heterogeneities than suggested by radially zoned plume models. These complexities may arise from heterogeneities in the thermal boundary layer and through variable entrainment of ambient mantle by the upwelling plume.

Chlorine–potassium variations in melt inclusions from Raivavae and Rapa, Austral Islands: constraints on chlorine recycling in the mantle and evidence for brine-induced melting of oceanic crust

Chlorine abundance variations in oceanic basalts can provide insights into the degassing and volatile recycling history of the mantle as well as shallow melt/hydrosphere interaction. We have examined major, trace and volatile element abundances in olivine-hosted melt inclusions from the islands of Raivavae and Rapa in the Austral Island chain. The island of Raivavae sits atop a pre-existing fracture zone and thus provides the opportunity to examine the relationship between melt/hydrosphere interaction and local lithospheric structure. The majority of inclusions from both Raivavae and Rapa have well correlated chlorine and potassium concentrations consistent with a source Cl/K2O ratio of ∼0.04, similar to that of uncontaminated mid-ocean ridge and ocean island basalts. The similarity of chlorine/potassium ratios in mid-ocean ridge basalts, Austral Islands basalts, and basalts from many other ocean islands suggests that chlorine/potassium does not significantly vary in the mantle. Because the plume sources of many ocean island chains contain varying types and quantities of recycled oceanic crust and sediments, this indicates that most of the chlorine added to oceanic crust during seafloor alteration is removed during subduction and is not recycled into the deep mantle. High chlorine contents (up to 0.14 wt%) and chlorine/potassium ratios in melt inclusions from an early-erupted Raivavae lava derive from assimilation of Cl-rich brines or brine-impregnated oceanic crust. A small subset of inclusions from the same lava show more extreme chlorine enrichment (up to 2.5 wt%), are depleted in incompatible trace elements relative to normal inclusions, and show extreme fractionation of high-field-strength elements (HFSEs) relative to large-ion-lithophile or rare-earth elements. These latter inclusions derive from partial melting of the pre-existing oceanic crust under brine-saturated conditions. HFSE depletions in these inclusions reflect the stabilization of a HFSE-bearing phase in the lower crust, probably due to high chlorine fugacity. HFSE anomalies are also associated with high chlorine content in mid-ocean ridge basalts. We suggest that these anomalies are also generated by the stabilization of HFSE-bearing phases in high-chlorine-activity melts or fluids. This process may also provide a means of stabilizing rutile in the sub-arc mantle wedge.

Rhenium volatility in subaerial lavas: constraints from subaerial and submarine portions of the HSDP-2 Mauna Kea drillcore

Abstract High Re abundances in mid-ocean ridge basalts (MORB) relative to primitive mantle (ave. MORB[Re]≈930 ppt; PM[Re]≈260 ppt) indicate that Re behaves as an incompatible element during MORB generation. However, contrary to expectations for an incompatible element, average Re abundances in subaerial ocean island and arc lavas (∼330 ppt and ∼190 ppt, respectively) are much lower than in MORB. Previous studies have argued that the low Re abundances in ocean island and arc lavas reflect greater Re compatibility during melt generation, caused by higher modal abundances of garnet and sulfide. However, available partitioning data for Re suggest that reasonable variations in the modal abundances of these phases cannot fully account for the observed differences in Re abundance in different tectonic settings. Higher modal garnet in the sources of ocean island and arc lavas cannot produce the observed low Re/Yb ratios (relative to MORB), because available data suggest that Re is less compatible in garnet than Yb. High sulfide abundances can in principle produce the observed Re depletions, but would require extremely high sulfur contents in the sources of ocean island and arc lavas (>1000 ppm). Large differences in modal sulfide abundance in different tectonic settings are also contradicted by the similarity of MgO–Os trends in ocean island, arc, and mid-ocean ridge samples. An alternative explanation for the low Re abundances in ocean island and arc settings is that much of the Re initially contained in these (largely subaerial) lavas escaped as volatile Re-oxide or Re-chloride species during magma degassing prior to or during subaerial eruption. The ∼3 km deep HSDP-2 Mauna Kea drillcore on Hawaii provides a unique opportunity to examine the effects of magma degassing on Re abundances. The upper ∼800 m of the Mauna Kea portion of the drillcore is composed of subaerial lavas, whereas the lower 2 km is submarine. Os-isotopes are nearly constant throughout the core (187Os/188Os=0.128–0.130), so large variations in source composition (e.g. Re content) over the period sampled by the core are unlikely. Rhenium abundances in the subaerial lavas are consistently low (ave. Re≈180 ppt). In contrast, Re abundances in the submarine lavas increase systematically with increasing depth, ranging from an average of ∼300 ppt in samples emplaced at 1000 mbsl. Rhenium/ytterbium ratios also increase with core depth, ranging to values significantly exceeding those observed in MORB. These trends are best explained by progressive Re loss in the subaerial and shallow submarine lavas during melt degassing. Assuming that the deepest submarine lavas are unaffected by Re loss during degassing, the subaerial Mauna Kea lavas appear to have lost on average ∼80% of their initial Re. The systematic differences in Re abundance in ocean island, arc, and mid-ocean ridge lavas may therefore reflect the fact that most analyzed ocean island and arc samples are subaerial and therefore degassed, whereas MORB retain a greater fraction of their volatile inventory.

Isotopically ultradepleted domains in the convecting upper mantle: Implications for MORB petrogenesis

Mid-oceanic-ridge basalts (MORB) form by partial melting of material in the convecting upper mantle. The range in isotopic compositions observed in MORB is inconsistent with the ultradepleted isotopic compositions observed in many abyssal peridotites. These results have called into question the prevailing hypothesis that abyssal peridotites (APs) are simple residues of recent MORB melting, which should result in the two reservoirs having the same range in isotopic compositions. We examined xenoliths that, based on their chemical features (e.g., light rare earth element depleted, fertile major element compositions, Sr-Nd-Pb isotopes similar to estimates for depleted MORB mantle), are interpreted to be derived from the convecting upper mantle, in order to evaluate the potential for isotopically ultradepleted domains to contribute significantly to MORB petrogenesis. Our data support the idea that isotopically ultradepleted peridotite is widely distributed in the upper mantle, and we demonstrate that ultradepleted domains are capable of contributing to MORB petrogenesis. An isotopically enriched component, such as recycled oceanic crust, in the MORB source mantle can account for the lack of MORB with ultradepleted isotopic compositions.

Chlorine enrichment in central Rio Grande Rift basaltic melt inclusions: Evidence for subduction modification of the lithospheric mantle

Shallow subduction of the Farallon plate during the Laramide orogeny (ca. 80–40 Ma) may have resulted in metasomatism of the western North American lithospheric mantle. Olivine- and orthopyroxene-hosted melt inclusions from the central Rio Grande Rift are variably enriched in chlorine relative to fluid-immobile elements. Subparallel trends in Cl/K versus Cl/Nb for alkali basalts and tholeiites can be explained by Cl/K fractionation during low degree partial melting, with D Cl ≈ D Nb K . The observed trace element enrichment does not correlate with host Mg# or melt SiO 2 wt% as expected for crustal contamination via an assimilation–fractional crystallization (AFC) process. In addition, examples from other volcanic systems suggest that Cl/K decreases with increasing contamination, contrary to observed positive correlations between Cl/K and Ba/Nb and Sr/Nd. The positive correlation of Cl/K and Cl/Nb with typical indices of subduction enrichment (e.g., Ba/Nb and Sr/Nd) supports a model of mantle metasomatism during subduction.

Basalt volatile fluctuations during continental rifting: An example from the Rio Grande Rift, USA

Hydration and metasomatism of the lithospheric mantle potentially influences both the magmatic and tectonic evolution of southwestern North America. Prior studies have suggested that volatile enrichments to the mantle underlying western North America resulted from shallow subduction of the Farallon Plate during the Laramide (∼74–40 Ma). This study examines temporal and spatial variations in volatile elements (H2O, Cl, F, and S) determined from olivine and orthopyroxene-hosted melt inclusions along and across the Rio Grande Rift, the easternmost extent of Laramide shallow subduction. Maximum chlorine enrichments are observed in the southern rift with a Cl/Nb of ∼210 and reduce with time to MORB-OIB levels (∼5–17). Measured water abundances are <0.8 wt % in rehomogenized inclusions; however, calculated H2O, based on Cl/Nb systematics, primarily varies from 0.5 to 2 wt % H2O. Sulfur abundances (<0.61 wt %), and calculated sulfide saturation, indicate magmas with high Cl/Nb also contain oxidized sulfur. The abundance of fluorine in melt inclusions (up to 0.2 wt %) is not correlated to other volatile elements. Temporal variations in melt inclusion volatile abundances coupled with varying isotopic (Sr-Nd-Pb) whole-rock systematics suggest a transition from lithospheric to asthenospheric melt generation in the southern RGR and potential lithosphere-asthenosphere melt mixing in the central RGR. East to west decrease in volatile enrichment likely reflects a combination of varying mantle sources and early removal of metasomatized lithospheric mantle underlying regional extension. Results indicate, from multiple causes, subduction-related volatile enrichment to the lithospheric mantle is ephemeral, and strong enrichments in volatiles are not preserved in active magmatic-tectonic provenances.

Mantle melt production during the 1.4 Ga Laurentian magmatic event: Isotopic constraints from Colorado Plateau mantle xenoliths

Plutons associated with a 1.4 Ga magmatic event intrude across southwestern Laurentia. The tectonic setting of this major magmatic province is poorly understood. Proposed melting models include anorogenic heating from the mantle, continental arc or transpressive orogeny, and anatexis from radiogenic heat buildup in thickened crust. Re-Os analyses of refractory mantle xenoliths from the Navajo volcanic field (NVF; central Colorado Plateau) yield Re depletion ages of 2.1–1.7 Ga, consistent with the age of the overlying Yavapai and Mazatzal crust. However, new Sm-Nd isotope data from clinopyroxene in peridotite xenoliths from NVF diatremes show a subset of xenoliths that plot on a ca. 1.4 Ga isochron, which likely reflects mantle melt production and isotopic resetting at 1.4 Ga. This suggests that Paleoproterozoic subcontinental lithospheric mantle was involved in the 1.4 Ga magmatic event. Our constraints support a subduction model for the generation of the 1.4 Ga granites but are inconsistent with rifting and anorogenic anatexis models, both of which would require removal of ancient lithosphere.