Jeffrey G. Ryan

The systematics of lithium abundances in young volcanic rocks

Abstract Lithium is a moderately incompatible trace element in magmatic systems. High precision analyses for lithium conducted on well characterized suites of MORB and ocean island basalts suggest a bulk distribution coefficient of 0.25−0.35 and behavior which is similar to Yb during low pressure fractionation and V during melting, as long as garnet is not an important residual phase. Data for peridotites and basalts suggest a mantle lithium content of about 1.9 ppm and show that significant concentrations of lithium reside in olivine and orthopyroxene, resulting in unusual inter-mineral partitioning of Li and complex relationships between lithium and other incompatible trace elements. The lithium abundances of arc basalts are similar to those of MORB, but their Li/Yb ratios are considerably higher. The high Li/Yb suggests the addition of a Li-rich component to arc sources; relatively low Yb abundances are consistent with the derivation of some arc magmas by larger extents of melting or from a more depleted source than MORB. Although Li is enriched at arcs, K is enriched more, leading to elevated K/Li ratios in arc volcanics. The high K/Li and relatively low La/Yb of primitive arc basalts requires either incorporation of altered ocean crust into arc magma sources, or selective removal of K and Li from subducted sediments. Bulk incorporation of sediments alone does not explain the Li systematics. Data from primitive MORB indicate a relatively low (3–4 ppm) Li content for new oceanic crust. Thus, the Li flux from the ocean crust is probably 11 g/yr, and the oceanic crust may not be an important net source in the oceanic budget of lithium.

Cross-Arc Geochemical Variations in the Kurile Arc as a Function of Slab Depth

Lavas from transects across the Kurile Islands arc showed geochemical variations related to changes in the compositions of fluids derived from the subducting slab. Enrichment factors for boron, cesium, arsenic, and antimony, all elements with strong affinities for water, decreased across the arc. This decrease is presumably related to losses of water-rich fluids during the dehydration of the subducting plate. Enrichments of potassium, barium, beryllium-10, and the light rare earth elements remained constant; these species may move in silica-rich fluids liberated from the slab at greater depths.

The systematics of boron abundances in young volcanic rocks

Abstract Boron behaves as a highly incompatible trace element in oceanic settings, while in arcs it shows unique systematics indicative of fluid-rock interactions. Boron analyses conducted on well-characterized mid-ocean ridge basalt (MORB) suites show that B approximates K most closely in its solid/ melt distribution behavior, with inferred bulk distribution coefficients of 0.004-0.009 during melting in the mantle and up to 0.07 during low-pressure crystallization. During differentiation processes in volcanic arc lavas B and K also vary similarly, but the B enrichments in basalts from different arc volcanoes are highly heterogeneous relative to those of K, Be, or other incompatibles. Boron shows strong affinities for fluids such as are liberated during the devolatilization of subducting slabs. Boron enrichments correlate directly with extents of melting in arc basalts, and inversely with the enrichments of most other lithophile trace elements. Boron enrichments at arcs are lower in those volcanoes that sample deeper portions of the slab, becoming indistinguishable from MORBs in the rearmost volcanic centers. That such B depletions are evident in lavas entails that magmatic processes and other transport mechanisms efficiently flush B through the mantle wedge and return it to surface reservoirs. The great mobility of boron apparent from the arc data precludes any long-term B enrichment in the sub-arc mantle and requires the existence of strong return fluxes for B in addition to arc volcanism.

Fractionation of Trace Elements by Subduction-Zone Metamorphism — Effect of Convergent-Margin Thermal Evolution

Differential chemical/isotopic alteration during forearc devolatilization can strongly influence the cycling of volatile components, including some trace elements, in subduction zones. The nature and magnitude of this devolatilization effect are likely to be strongly dependent on the thermal structure of individual convergent margins. A recent model for metamorphism of the Catalina Schist, involving progressive underplating (at ≤45 km depths) of rock packets metamorphosed along successively lower-T prograde P-T paths in a rapidly cooling, newly initiated subduction zone, affords a unique evaluation of the effects of varying prograde P-T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting rocks. In the Catalina Schist, the most extensive devolatilization occurred in metasedimentary rocks which experienced prograde P-T paths encountering the epidote-blueschist facies (>350°C at 9 to 12 kbar) or higher-T conditions; such rocks are depleted in ‘fluid-mobile’ elements such as N, B, Cs, As, and Sb relative to protoliths. Removal of these elements resulted in changes in B/(Be, Li, La, Zr), Cs/Th, Rb/Cs, As/Ce, Sb/Ce, and Creduced/N, and increases in δ15N and δ13C. The relative susceptibilities of the “fluid-mobile” elements to loss along increasingly higher-T P-T paths can be categorized. Boron and Cs show the greatest susceptibility to low-T removal by fluids, showing >50% depletion in even lawsonite-blueschist-facies metasedimentary rocks which experienced relatively low-T prograde metamorphic paths. In rocks which experienced higher-T paths, As and Sb (likely in sulfides) show the greatest depletions (>90%); N, Cs, and B (largely in micas) occur at ∼25% of protolith contents in even partially melted amphibolite-facies rocks. Variations in B/Be, Cs/Th, As/Ce, and Sb/Ce among arcs from differing convergent-margin thermal regimes, and conceivably some cross-arc declines in these ratios, are compatible with evidence from the Catalina Schist for varying degrees of element removal as a function of prograde thermal history. In relatively cool subduction zones (e.g., Kuriles, Marianas, Aleutians, southern Alaska) with thermal regimes similar to that which formed the low-grade units of the Catalina Schist (and blueschist-facies rocks in the Franciscan Complex), forearc devolatilization is less profound, B, Cs, As, Sb, and N are more likely to be deeply subducted, and enriched in arc lavas, and significant devolatilization occurs at the blueschist-to-eclogite transition. High-grade units could reflect thermal evolution analogous to that of relatively warm subduction zones (e.g., Cascadia) and back-arcs in which arc lavas are depleted in B, Cs, As, and Sb due to prior removal by forearc devolatilization. The results of this study also imply less efficient recycling of these elements during the warmer Archean subduction which resulted in greater slab melting and production of abundant trondhjemite-tonalite magmatic suites.

The role of hydrothermal fluids in the production of subduction zone magmas: Evidence from siderophile and chalcophile trace elements and boron

Abstract In order to evaluate the processes responsible for the enrichments of certain siderophile/ chalcophile trace elements during the production of subduction-related magmas, representative lavas from seven subduction zones have been analyzed for Pb, As, Sb, Sn, W, Mo, Tl, Cu, and Zn by inductively coupled plasma-mass spectrometry (ICP-MS), radiochemical epithermal neutron activation analysis (RENA), and atomic absorption (AA). The siderophile/chalcophile elements are compared to the highly fluid-mobile element B, the light rare earth elements (LREEs), U, and Th in order to place constraints on their behavior in subduction zones. Boron, As, Sb, and Pb are all enriched in arc lavas and continental crustal rocks more so than expected assuming normal magmatic processes (melting and crystallization). Tin, W, and Mo show little evidence of enrichment. Correlations of Pb/Ce, As/Ce, and Sb/Ce with B/La are statistically significant and have high correlation coefficients (and, more importantly, slopes approaching one) suggesting that Pb, As, and Sb behave similarly to B (i.e., that they are fluid-mobile). In addition, across-arc traverses show that B/La, As/Ce, Pb/Ce, and Sb/Ce ratios decrease dramatically with distance towards the back-arc basin. W/Th, Tl/La, Sn/Sm, and Mo/Ce ratios and Cu and Zn concentrations have much less systematic across-arc variations and correlations with B/La are not as strong (and in some cases, not statistically significant) and the regression lines have much lower slopes. Mixing models between upper mantle, slab-derived fluid, and sediment are consistent with a fluid-derived component in the arcs displaying extra enrichments of B, Pb, As, and Sb. These observations imply efficient mobilization of B, Pb, As, Sb, and possibly Tl into arc magma source regions by hydrothermal fluids derived from metamorphic dehydration reactions within the slab. Tin, W, and Mo show little, if any, evidence of hydrothermal mobilization. Copper appears to be slightly enriched in arc lavas relative to mid-ocean ridge basalts (MORBs) whereas Zn contents of arc lavas, MORB, ocean island basalts (OIBs), and continental crustal samples are similar suggesting that the bulk partition coefficient for Zn is approximately equal to one. However, Zn contents of the upper mantle are lower than these reservoirs implying an enrichment of the source region in Zn prior to melting. These nonigneous enrichments have implications not only for arc magma genesis but also for continental crust formation and crust-mantle evolution. The mobility of Pb, As, Sb, and B in hot, reducing, acidic hydrothermal fluids may be greatly enhanced relative to the large-ion lithophile elements (LILEs; including U) as a result of HS−, H2S, OH−, or other types of complexing. In the case of Pb, continued transport of Pb from subducted slabs into arc magma source regions throughout Earth history coupled with a U fluxing of the mantle a the end of the Archean may account for the depletion of Pb in the upper mantle, the low U/Pb of most arc volcanics and continental crustal rocks, and provide an explanation for the Pb- Paradox (Hofmann et al., 1986; McCulloch, 1993; Miller et al., 1994) . Recycled slabs will then retain high U/Pb ratios upon entering the deep mantle and may eventually become incorporated into the source regions of many OIBs; some with HIMU (high 238U/204Pb) signatures.

Geochemical Evidence for Magmatic Water Within Mars from Pyroxenes in the Shergotty Meteorite

Observations of martian surface morphology have been used to argue that an ancient ocean once existed on Mars. It has been thought that significant quantities of such water could have been supplied to the martian surface through volcanic outgassing, but this suggestion is contradicted by the low magmatic water content that is generally inferred from chemical analyses of igneous martian meteorites. Here, however, we report the distributions of trace elements within pyroxenes of the Shergotty meteorite—a basalt body ejected 175 million years ago from Mars—as well as hydrous and anhydrous crystallization experiments that, together, imply that water contents of pre-eruptive magma on Mars could have been up to 1.8%. We found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which formed at depth in the martian crust) are enriched in soluble trace elements when compared to the outer rims (which crystallized on or near to the martian surface). This implies that water was present in pyroxenes at depth but was largely lost as pyroxenes were carried to the surface during magma ascent. We conclude that ascending magmas possibly delivered significant quantities of water to the martian surface in recent times, reconciling geologic and petrologic constraints on the outgassing history of Mars.

Boron isotope systematics of slab fluids as inferred from a serpentine seamount, Mariana forearc

Abstract Serpentinite clasts and muds erupted from Conical Seamount, Mariana forearc, show substantial enrichment in boron (B) and 11B (δ11B up to +15‰) relative to mantle values. These elevated B isotope signatures result from chemical exchange with B-rich pore fluids that are upwelling through the seamount. If the trends of decreasing δ11B with slab depth shown by cross-arc magmatic suites in the Izu and Kurile arcs of the western Pacific are extended to shallow depths (∼25 km), they intersect the inferred δ11B of the slab-derived fluids (+13‰) at Conical Seamount. Simple mixtures of a B-rich fluid with a high δ11B and B-poor mantle with a low δ11B are insufficient to explain the combined forearc and arc data sets. The B isotope systematics of subduction-related rocks thus indicate that the fluids evolved from downgoing slabs are more enriched in 11B than the slab materials from which they originate. Progressively lower δ11B in arc lavas erupted above deep slabs reflects both the progressive depletion of 11B from the slab and progressively greater inputs of mantle-derived B. This suggests that the slab releases 11B-enriched fluids from the shallowest levels to depths greater than 200 km.

The control of lithium budgets in island arcs

Measurements of the Li isotopic compositions of lavas from magmatic arcs worldwide suggest common processes at work that lead to the retention of isotopically heavy Li in the mantle. Samples from this study derive from the Kurile arc, eastern Russia, the Sunda arc, Indonesia, and a segment of the Aleutian arc, western Alaska. The overall range in N 7 Li is very restricted (+2.1 to+5.1 ˛ 1.1, 2c) for 34 of 36 samples. These values overlap the values of unaltered normal MORB glasses. The two samples with isotopic compositions that fall outside this range in N 7 Li have B/Be 6 13, and hence do not bear classical ‘slab’ trace element signatures. Considering the high N 7 Li in altered ocean crust, marine and terrigenous sediments, and forearc fluids, aqueous components lost by subducting slabs are expected to have similarly heavy enriched Li isotope signatures. If Li behaves similarly to a fluid-mobile element such as B, N 7 Li should correlate strongly with, for example, B/Be. As such, samples with high B/Be should show elevated N 7 Li. The sample set we have examined does not show such correlations and is interpreted to reflect a globally significant process. Although Li is a fluid-mobile element, its partitioning into Mg-silicates may cause it to be effectively removed during equilibration with subarc mantle peridotite. Elements with stronger fluid/mantle partitioning behavior, such as B, are not so affected. The convergence of Li isotope ratios on MORB-like values is interpreted to result from the sequestration of slab-derived Li in the subarc mantle before it reaches the zone of melting. The results indicate conditions appropriate for mantle ‘buffering’ of slab-derived Li are widespread in magmatic arcs. Alternately, some proportion of Li could be retained on the slab in high Li/B minerals. Either way, this indicates that regions of the upper mantle with N 7 Lis MORB may be common, as a direct consequence of the subduction process. fl 2002 Elsevier Science B.V. All rights reserved.

Lithium isotope evidence for light element decoupling in the Panama subarc mantle

The systematics of fluid-mobile trace elements in arc lavas from Panama, relative to their Li isotopic compositions, provide unique evidence for the fertilization and subsequent differential extraction of mobile species from the subarc mantle. Calc-alkaline lavas that crystallized between 20 and 5 Ma (Old Group) that possess δ 7 Li as high as +11.2 have low B/Be. Otherwise identical (and similarly old) calcalkaline lavas with high B/Be (to 23), have mid-ocean ridge basalt (MORB) like δ 7 Li (+4.7 to +5.6). Adakite lavas (<3 Ma; Young Group) possess δ 7 Li from +1.4 to +4.2 and have consistently lower B/Be than Old Group lavas, consistent with derivation from melting of a devolatilized MORB slab. If Li and B had comparable fluid mobility in the subarc mantle, then slab fluids would carry both high B concentrations and elevated δ 7 Li signatures into arc sources, and samples with the highest δ 7 Li would also have the highest B/Be. Our data suggest that although both Li and B are initially derived from the slab, older δ 7 Li signatures may be preserved in the mantle beneath arcs. As a result, regions of the lithospheric mantle will develop Li isotope signatures that are heavier than typical MORB mantle.

The Boron Systematics of Intraplate Lavas: Implications for Crust and Mantle Evolution

Abstract Ocean island basalts (OIBs) possess uniformly low B contents, and lower B/Nb and B/K 2 O ratios than mid-ocean ridge basalts (MORBs). As with Pb, B enrichments in both MORBs and OIBs are substantially lower than those of arc volcanics or continental rocks. The devolatilization of subducting plates and associated arc magmatism efficiently segregate B into crustal reservoirs and return large volumes of B-depleted material to the deep mantle. Subduction processes (and presumably arc volcanism) have thus played a major role in continental crust formation. While B is depleted in OIBs relative to either MORBs or arc lavas, OIB samples representing EM and HIMU isotopic reservoirs, often ascribed to the effects of ancient subducted materials, cannot be distinguished from other OIBs in terms of B abundances or B ratios. Our results suggest either (1) the differential depletion in B of two distinct mantle reservoirs, one of which now produces MORBs, and the other OIBs or (2) the episodic or continuous mixing of OIB mantle sources with B-depleted subducted materials. The geochemical processes responsible for the isotopic heterogeneity of intraplate lavas may all serve to segregate B from the mantle into crustal rocks and other surface reservoirs.