A primitive mantle source for the Neoarchean mafic rocks from the Tanzania Craton

Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite (more than 8 wt.% MgO). This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios (Zr/Hf = 32–35, Ti/Zr = 107–147), producing flat REE and HFSE profiles [(La/Yb)pm = 0.9–1.3], with abundances of 3–10 times primitive mantle and with minor negative anomalies of Nb [(Nb/La)pm = 0.6–0.8] and Th [(Th/La)pm = 0.6–0.9]. Initial isotope compositions (ɛNd) range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping ∼50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between ~2830 and ~2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition

Genetic Diversity Enhances Restoration Success by Augmenting Ecosystem Services

Disturbance and habitat destruction due to human activities is a pervasive problem in near-shore marine ecosystems, and restoration is often used to mitigate losses. A common metric used to evaluate the success of restoration is the return of ecosystem services. Previous research has shown that biodiversity, including genetic diversity, is positively associated with the provision of ecosystem services. We conducted a restoration experiment using sources, techniques, and sites similar to actual large-scale seagrass restoration projects and demonstrated that a small increase in genetic diversity enhanced ecosystem services (invertebrate habitat, increased primary productivity, and nutrient retention). In our experiment, plots with elevated genetic diversity had plants that survived longer, increased in density more quickly, and provided more ecosystem services (invertebrate habitat, increased primary productivity, and nutrient retention). We used the number of alleles per locus as a measure of genetic diversity, which, unlike clonal diversity used in earlier research, can be applied to any organism. Additionally, unlike previous studies where positive impacts of diversity occurred only after a large disturbance, this study assessed the importance of diversity in response to potential environmental stresses (high temperature, low light) along a water–depth gradient. We found a positive impact of diversity along the entire depth gradient. Taken together, these results suggest that ecosystem restoration will significantly benefit from obtaining sources (transplants or seeds) with high genetic diversity and from restoration techniques that can maintain that genetic diversity

Element redistribution and mobility during upper crustal metamorphism of metasedimentary rocks: an example from the eastern Mount Lofty Ranges, South Australia

We present a detailed study on element mobility during prograde metamorphism of metasedimentary rocks of the eastern Mt. Lofty Ranges, South Australia. Mineral and bulk rock compositions were monitored across a regional metamorphic gradient from ≈350–400 °C to migmatite grade (≈650–700 °C) at ≈0.3–0.5 GPa, where pervasive up-temperature fluid flow during metamorphism has been proposed previously. Major and most trace elements (including rare earth elements) are isochemical during metamorphism as they are effectively redistributed into newly formed major and/or accessory minerals. Monazite or allanite and xenotime control the whole rock concentration of rare earth elements (REEs), whereas apatite and titanite are minor REE hosts. The only non-volatile elements that are demonstrably mobilized by metamorphic fluids are Zn, Pb, Ag, Cs, Sb, Bi and As, whose concentrations decreased with increasing metamorphic grade. Depletion of Zn, Sb and Pb was progressive with increasing temperature in staurolite-absent psammopelites, with losses of ≈80 % of the original Zn and >80 % of the protolithic Sb and ≈50 % of the original Pb from the rocks from high-grade metamorphic zones. Pronounced depletion of As and Cs occurs at the greenschist/amphibolite facies boundary and the transition to migmatite grade, respectively, while Ag and Bi contents decrease between 500 and 550 °C where >50 % of the original Ag and Bi is lost. While for most elements, unmetamorphosed sedimentary sequences can be considered chemical equivalents of metasedimentary rocks occupying deeper crust levels, in some cases, such as the extensive flow of Cl-rich fluid documented here, metals such as Zn, Pb and Ag may be stripped and may serve as a metal source for orebody formation. The decrease of As, Bi and Sb contents during prograde metamorphism might be a more universal feature that is linked with sulphide phase transitions.Johannes Hammerli, Carl Spandler and Nicholas H. S. Olive

Neodymium isotope disequilibria in subducted sediments, and potential consequences for subduction-zone recycling

Neodymium isotope geochemistry is pivotal to understanding magma genesis and crustal recycling processes at convergent plate margins. Here, we present Nd isotope compositions of rare earth element (REE)–rich minerals from blueschist- to eclogite-facies rocks that represent previously subducted continental sediments. We find Nd isotope heterogeneity in all of our samples. Apatite preserves diverse detrital isotopic compositions despite metamorphism up to at least 600 °C and 3.0 GPa, whereas lawsonite appears to be isotopically homogenous. Epidote and monazite are major REE hosts under eclogite-facies conditions; in our samples these phases also retain Nd isotope heterogeneity, even up to ultrahigh-pressure conditions of 700 °C and 4.0 GPa. These findings contest the assumption of Nd isotope equilibration during subduction-zone metamorphism, so caution is needed when using Nd isotopes to calculate subducted sediment contributions to arc magmas or for determining rates of crustal recycling through subduction zones.Carl Spandler, Johannes Hammerli, and Cassian Pirar

Neodymium isotope equilibration during crustal metamorphism revealed by in situ microanalysis of REE-rich accessory minerals

Radiogenic isotopes are widely used to investigate crustal evolutionary processes, however recent claims of Nd and Sr isotope disequilibrium during anatexis question the reliability of such information. We have conducted an in situ Sm–Nd isotope study of apatite, allanite, titanite, xenotime and monazite in metasedimentary rocks of different metamorphic grade to test Nd isotope equilibrium during metamorphism. Our results show that apatite retains an original, probably detrital, highly variable Nd isotopic signature until at least 500 °C before being isotopically homogenised, irrespective of textural context within the rock. Once equilibrated, apatite retains its Nd isotope signature throughout anatexis. In contrast, allanite and titanite are equilibrated at temperatures as low as 350–400 °C. REE-rich accessory minerals in high-grade rocks (∼600°C) show very similar initial εNd values at the time of metamorphism. We conclude that under these metamorphic conditions Nd isotope disequilibrium between crustal melts and metasedimentary sources is unlikely. Intra-grain Nd isotope zoning of monazite indicates that partial melting was open system, involving the injection of externally-derived melt into migmatites. This process, likely to be common in anatectic terranes but not always obvious at hand-specimen scale or from bulk rock geochemical data, can produce isotope variation that could potentially be misinterpreted as disequilibrium between the melt and its protolith

Cl/Br of scapolite as a fluid tracer in the earth's crust: insights into fluid sources in the Mary Kathleen Fold Belt, Mt. Isa Inlier, Australia

A combination of analytical methods, including trace element analysis of Br in scapolite by LA-ICP- MS, was employed to unravel the fluid–rock interaction history of the Mary Kathleen Fold Belt of northern Australia. Halogen ratios in the metamorphic and hydrothermally derived scapolite from a range of rock-types record interaction between the host rocks and magmatic-hydrothermal fluids derived from granite plutons and regional metamorphism. The results show that halite-dissolution supplied at best only minor chlorine to fluids in the Fold Belt. Chlorine/bromine ratios in metamorphic scapolite indicate that fluids were dominantly derived from basinal brines formed from sub-aerial evaporation of seawater beyond the point of halite saturation. This bittern fluid infiltrated the under- lying sedimentary sequences prior to regional metamorphism. Zoned scapolite in a major late meta- morphic mineralized shear-zone records three discrete pulses of magmatic and metamorphic fluid, and it is suggested that fluid mixing may have assisted mineralization along and around this shear zone. As a crucial prerequisite for halogen fluid tracer studies using scapolite, we find in our samples that Cl and Br do not fractionate when incorporated in scapolite. Furthermore, unaltered rims of heavily retrogressed scapolite show indistinguishable Cl/Br signatures compared with fresh grains from the same sample indicating retrograde metamorphism did not significantly affect Cl and Br signatures in scapolite group minerals

Laser ablation inductively coupled plasma mass spectrometry study on fluid inclusions of the Baiyinnuo'er skarn Zn-Pb deposit, North-east China

The Baiylimuo' er Zn-Pb deposit is a typical skarn deposit in NE China. Skarn and orebodies mainly occur between different units of the Lower Permian Huanggangliang Formation carbonates-sandstone-mudstone or within the contact between intrusions and marble. Skarn minerals are dominated by prograde garnet and clinopyroxene, and the main ore minerals are sphalerite and galena. We conducted single fluid inclusion compositional studies using laser ablation inductively coupled plasma mass spectrometry on clinopyroxene (pre-orestage mineral) and sphalerite and calcite (syn-ore-stage minerals). The results show that the Baiylimuo' er fluids are distinctly different from basinal brine but similar to magmatic fluid, thereby indicating that the deposit has a dominantly magmatic origin. This interpretation has also been supported by the O/Br mole ratios, which remain stable (~500) in different fluid stages. The decreases for most of the elements in the fluid inclusions from pre-ore-stage clinopyroxene to syn-ore-stage sphalerite and calcite suggest that mixing with groundwater has occurred during mineralisation, with significant Zn, Pb and Ag deposition

Tracking fluid sources for skarn formation using scapolite geochemistry: an example from the Jinshandian iron skarn deposit, Eastern China

Scapolite occurs as the major halogen-bearing phase at all paragenetic stages of skarn formation and mineralization in the Jinshandian iron skarn deposit, Eastern China. Here we integrate geochemical characteristics of scapolite with in situ B and Sr isotopes of associated tourmaline and fluorapatite, respectively, to trace the sources and evolution of the fluids responsible for mineralization in this deposit. Pre-ore stage I scapolite has molar Cl/Br ratios ranging from ~ 920 to 2200, which, together with the boron isotope composition of pre-ore stage I tourmaline, are consistent with formation from magmatic fluids from the Jinshandian intrusion. In contrast, scapolite in syn- and post-ore stages (II and III) has significantly higher Cl/Br ratios (2900–6200) that are outside the range of magmatic fluids but are consistent with involvement of fluids derived from the halite-bearing evaporite horizons that lie within regionally extensive sedimentary country rocks. The influx of sedimentary-derived fluids is also consistent with 87Sr/86Sr of syn-ore stages II fluorapatite (0.7098–0.7109), which are significantly higher than those of the skarn-related quartz diorite intrusions (0.7058–0.7061) but approach the isotopic compositions of the Middle Triassic evaporites and other continental sedimentary country rocks. These data indicate that evaporite-sourced fluids were involved in iron ore formation at the Jinshandian deposit and may be important for the formations of other iron ore deposits. Our findings also show that scapolite halogen geochemistry in combination with other fluid tracers, such as B and Sr isotopes, can be extremely useful for identifying the origins and evolution of fluids in magmatic-hydrothermal systems.Li-Ping Zeng, Xin-Fu Zhao, Johannes Hammerli, Tian-Wei-Teng Fan, Carl Spandle