Trace-element compositions of olivine from 75 mantle rocks of diverse origin, including xenoliths from kimberlites, basaltic lavas and orogenic peridotites, were determined by laser ablation ICP-MS ...to study systematic variations between mantle lithologies, partitioning mechanisms in olivine and their potential for geothermobarometry and unravelling mantle processes. Samples were selected to cover a wide range of forsterite contents (89.1–93.4), equilibration temperatures and pressures (750–1450
°C; 15–80
kbar).
Trace elements in olivine can be divided into three groups. Group I elements (Ni, Mn, Co, Cu, Zn and Li) show small concentration ranges and olivine is the major host mineral. These are mostly divalent elements and have ionic radii close to that of Mg. Group II elements (Cr, Al, V, Sc, Ca and Na) show large concentration ranges, which are mainly controlled by the equilibration temperature of the host rock. The elements are strongly concentrated in co-existing mantle minerals (garnet, clinopyroxene and spinel) and show a narrow range of bulk rock concentrations. They fit less comfortably in the olivine lattice than Group I elements because of their charge or size. Differences between garnet and spinel-facies rocks are apparent for Al, Ca and Sc. Group III elements (Ti, Zr, Nb and Y) show large ranges of concentration in olivine as well as in co-existing minerals, and are strongly dependent on bulk rock contents. Concentration differences between olivine from garnet and spinel-facies rocks are apparent for all these elements. They are strongly incompatible in olivine and other rock-forming mantle minerals because of their charge or size.
Various mantle lithologies can be discriminated using olivine composition. Spinel, garnet and garnet–spinel peridotites can be distinguished in olivine Sc–Zr and MnO–Al
2O
3 diagrams, whereas volcanic olivine is distinguished by high Ca and Al contents (picritic olivine) or high Nb contents (kimberlitic olivine). Since concentrations of Group III elements in olivine are diagnostic of whole-rock contents they can be used to trace the petrogenetic history of the rock. For instance, Ti contents and Cr# (Cr/(Cr
+
Al)) of olivine correlate with the amount of melt extracted from a mantle residue, although refertilisation may subsequently have increased Ti contents in high-
T peridotites from the base of the lithosphere.
The olivine dataset can be used to examine substitution reactions. Uptake of Al and Cr appears to be largely charge-balanced by Na in garnet-facies olivine, and olivine Cr# strongly correlates with that of co-existing minerals, in particular clinopyroxene and spinel. In spinel-facies olivine a large excess of trivalent cations is present in olivine, which can be fully attributed to excess Al. This suggests a Tschermak-style substitution, in which replacement of Mg by Al in the octahedral site is charge-balanced by replacement of Si by Al in the tetrahedral site.
Partition coefficients of Group II elements are highly temperature sensitive with most of the element variability being shown by olivine. This allows the definition of simple geothermometers based solely on the concentrations of these elements in olivine. The most widely applicable of these is Al-in-olivine for garnet peridotites, following the expression
T
Al-ol
(
°
C
)
=
9423
+
51.4
P
+
1860
Cr
#
ol
(
13.409
−
ln
Al
ol
)
−
273
with
P in kbar, Al
ol the Al concentration of olivine in ppm, and Cr#
ol is Cr/(Cr
+
Al) in olivine. This thermometer predicts the temperature with a residual of 15
°C based on calibration with two-pyroxene and Al-in-opx geothermobarometers (Brey and Köhler, 1990). Although calibrated using lherzolites only, the thermometer performs well for clinopyroxene-free harzburgites and also spinel peridotites. An alternative thermometer is presented for the case where the presence of Cr
2+ is expected, e.g., for olivine inclusions in diamonds.
The geochemical and thermobarometric information recorded by olivine can be a useful tool in studies of the petrogenesis of lithospheric mantle, olivine xenocrysts in mantle-derived magmas, the formation of diamonds and diamond exploration using detrital olivine.
This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These ...rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid–rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 μg/g B) occur in peak metamorphic phengite from a garnet–phengite quartzite. The B isotopic composition is variable (δ
11
B = − 10.3 to − 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid–rock interaction during peak metamorphic conditions. This interpretation is supported by fluid–rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in δ
11
B and B. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive δ
11
B values (δ
11
B = 20 ± 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 μg/g and δ
11
B values that cluster around 0% (δ
11
B = − 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid–rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones.
Biohybrid Polymer Capsules van Dongen, Stijn F. M; de Hoog, Hans-Peter M; Peters, Ruud J. R. W ...
Chemical reviews,
11/2009, Letnik:
109, Številka:
11
Journal Article
Recenzirano
Biohybrid polymer capsules synthesized from dendrimers are discussed. There is still tremendous potential in further research on these compounds.
Titanium- and water-rich metamorphic olivine (Fo 86–88) is reported from partially dehydrated serpentinites from the Voltri complex, Ligurian Alps. The rocks are composed of mostly antigorite and ...olivine in addition to magnetite, chlorite, clinopyroxene and Ti-clinohumite. In situ secondary ion mass spectrometry (SIMS) data show that metamorphic olivine has very high and strongly correlated H
2
O (up to 0.7 wt%) and TiO
2
contents (up to 0.85 wt%). Ti-rich olivine shows colourless to yellow pleochroism. Olivine associated with Ti-clinohumite contains low Ti, suggesting that Ti-rich olivine is not the breakdown product of Ti-clinohumite. Fourier transform infrared spectroscopy (FTIR) absorption spectra show peaks of serpentine, Ti-clinohumite and OH-related Si vacancies. Combining FTIR and SIMS data, we suggest the presence of clustered planar defects or nanoscale exsolutions of Ti-clinohumite in olivine. These defects or exsolutions contain more H
2
O (
x
~ 0.1 in the formula 4Mg
2
SiO
4
·(1−
x
)Mg(OH,F)
2
·
x
TiO
2
) than Ti-clinohumite in the sample matrix (
x
= 0.34–0.46). In addition to TiO
2
and H
2
O, secondary olivine contains significant Li (2–60 ppm), B (10–20 ppm), F (10–130 ppm) and Zr (0.9–2.1 ppm). It is enriched in
11
B (δ
11
B = +17 to +23 ‰). Our data indicate that secondary olivine may play a significant role in transporting water, high-field strength and fluid-mobile elements into the deeper mantle as well as introduce significant B isotope anomalies. Release of hydrogen from H
2
O-rich olivine subducted into the deep mantle may result in strongly reduced mantle domains.
We examined the F, Cl, Br and I abundance of minimally retrogressed lawsonite blueschists from the Tavsanli Zone in northwest Turkey to evaluate the behaviour of halogens in subduction zones, and to ...determine the role coexisting high pressure minerals may play in transporting the halogens to the Earth's mantle. The blueschists contain sodic amphibole and lawsonite, with variable amounts of phengite and chlorite, and minor apatite. A positive correlation between Cl, Br and I contents in bulk rocks suggests their overall coherent behaviour in subduction zones, although high ratios of I/Cl and Br/Cl compared to altered oceanic crust indicate that Cl is preferentially lost relative to Br and I before or during blueschist metamorphism. Iodine and F are enriched relative to altered oceanic crust, suggesting incorporation from marine sediments. In situ analyses of minerals in thin sections reveal F preferentially concentrates in apatite (avg. 3.13 wt%), over phengite (482 ppm), lawsonite (avg. 413 ppm) and Na-amphibole (257 ppm). Chlorine also preferentially resides in apatite (138 ppm), followed by equal partitioning between phengite (59 ppm) and Na-amphibole (56 ppm), and lower concentrations in lawsonite (27 ppm). Upon apatite decomposition at a depth of ∼200 km, F may redistribute into lawsonite and phengite in slabs, whilst Cl is likely expelled to the overlying mantle wedge. Given the stability of lawsonite and phengite to a depth of 280–300 km in cold subduction zones, they may transport F beyond subarc depths, contributing to the high F in magmas derived from the deep mantle.
•This is the first report of F, Cl, Br and I abundance in HP metabasites.•Chlorine is preferentially lost relative to F in the subducting slab.•F partitioning follows apatite ≫ phengite ≈ lawsonite > Na-amphibole.•Lawsonite and phengite contribute to high F in the mantle beyond subarc depths.
Whiteschists from the Dora-Maira massif (Western Alps, Italy) are Mg and K-rich metasomatised granites which experienced ultra-high-pressure metamorphism and fluid–rock interaction during Alpine ...continental subduction. The sources and timing of fluid infiltration are a source of significant debate. In this study, we present boron (B) isotopes and other fluid-mobile trace-element (FME) concentrations in various generations of phengite from whiteschists and their country rock protoliths to investigate the sources and timing of metasomatic fluid influx. Reconstructed bulk rock concentrations based on modal data and mineral compositions indicate that significant amounts B and other FME were added to the rock during prograde metamorphism, but that this fluid influx postdates the main Mg metasomatic event. High B concentrations (150–350
μ
g/g) and light
δ
11
B
values (− 16 to −4 ‰) recorded in phengite point to a B-rich sediment-derived fluid as the main source of B in the whiteschists. Further redistribution of FME during metamorphism was associated with breakdown of hydrous minerals, such as talc, phlogopite, and ellenbergerite. The source of the Mg-rich fluids cannot be constrained based on the B data in phengite, since its signature was overprinted by the later main B metasomatic event. Rare tourmaline-bearing whiteschists record additional information about B processes. Tourmaline
δ
11
B
values (− 6 to +1 ‰) are in isotopic equilibrium with similar fluids to those recorded in most phengite, but phengites in tourmaline-bearing samples record anomalous B isotope compositions that reflect later redistribution of B. This study demonstrates the utility of
in situ
analyses in unravelling complex fluid–rock interaction histories, where whole-rock analyses make it difficult to distinguish between different stages of fluid–rock interaction. Polymetasomatism may result in decoupling of different isotopic systems, thus complicating their interpretation. The Dora-Maira whiteschists interacted with multiple generations of fluids during subduction and therefore may represent a long-lived fluid pathway.
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