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Clinopyroxene Growth and Dissolution Rates: High-Pressure Investigation on a Primitive Alkaline Basalt from the Campi Flegrei Volcanic District (South Italy)
No full textWith the aim to investigate the influence of time, temperature, water content and pressure on clinopyroxene growth and dissolution rate, we performed crystallization and dissolution experiments on a K-basaltic rock from Procida island (Campi Flegrei Volcanic District, south Italy). Crystallization experiments were performed at anhydrous and hydrous (1 ≤ H2O ≤ 4 wt.%) conditions, pressure of 0.8 GPa, temperature between 1030 °C and 1250 °C and dwell time between 0.25 and 9 hours. Crystallization experiments show that time is the factor that most affects the growth rate compared to temperature and water content. Clinopyroxene growth rate, indeed, varies from 10-7 to 10-8 cm/s and it reaches the maximum value in the shortest experiments (0.25 h) while it decreases increasing time (9 h). Comparing our high-pressure growth rates with the low-pressure ones available in literature related to clinopyroxene, it was possible to note that pressure does no affect the growth rate. Indeed, all the considered growth rates show similar values that vary from 10-5 to 10-9 cm/s regardless of pressure but as a function of time. Moreover, partition coefficients based on the crystal-liquid exchange demonstrate that the chemistry of minerals progressively approaches to equilibrium from the shortest to long-lasting experiments, putting forward the latter as representative of the ideal condition of crystallization in a deep magmatic reservoir. Short experiments, instead, could be representative of ascent mechanisms in disequilibrium conditions and quick times. Dissolution experiments, instead, were carried out at high pressure (0.8-2 GPa), superliquidus temperatures and different dwell times by using the seeding technique. Preliminary results show that clinopyroxene dissolution rate varies from 10-2 to 10-7 cm/s, highlighting an influence of temperature and time with respect to pressure
High-Mg basalts from Capo Marargiu (Sardinia, Italy): experimental constraints on amphibole stability in a primitive calc-alkaline magma
No full textThe composition of a high-Mg basalt (HMB) from the Capo Marargiu Volcanic District (CMVD, Sardinia, Italy)
was used for a phase relation study at high pressure and hydrous conditions.
The CMVD is a calc-alkaline complex of basaltic to intermediate hypabyssal (dikes and sills) and volcanic rocks
(lava dome and flows, pyroclastic deposits) emplaced during the Oligo-Miocene orogenic magmatism of Sardinia. The
HMB (10 wt.% MgO) occur as magmatic enclaves hosted in a basaltic-andesitic dome (BA). Minerals within the
enclaves are clinopyroxene (Cpx), plagioclase (Plg), amphibole (Amph), olivine (Ol) and oxides (Ox), and those of the
BA host are Plg + Cpx + Ox + low-Ca pyroxene.
Thermobarometric estimates indicate that Cpx and Amph from the enclaves crystallized at P = 500-800 MPa and T
= 1010-1100°C. In order to investigate the stability of Amph in the HMB magma we conducted preliminary
experiments at 800 MPa and in a range of temperatures of 1050-1110°C. Au75Pd25 capsules were loaded with ~100 mg
of a powdered HMB (sample CM42) and 2 wt.% H2O (low water experiments, LWE) or 5 wt.% H2O (high water
experiments, HWE). The experiments were run in the piston-cylinder apparatus installed at the HP-HT Laboratory of
Sapienza University of Rome and held at the target P-T conditions for 3 h.
The stable phases in the LWE are Ol + orthopyroxene (Opx) + Cpx + Plg + Ox and a hydrous basaltic andesite (~ 4
wt.%) as residual glass (Gl). The crystallization degree is higher than 50 vol.%.
Run products of HWE is Gl + Ol + Amph + Cpx + Plg + Ox. The crystallinity is lower than that of the LWE H2Osaturated
basalt (~ 7 wt.% H2O), as testified by the presence of vesicles.
This preliminary study shows that Opx is stable in a HMB at P = 800 MPa and T = 1050-1110°C and H2O ≤ 2 wt.%
(i.e., LWE), whereas its crystallization is hindered when magma water content is increased to ~5 wt.%, providing
conditions suitable for Amph saturation (i.e., HWE). This modal discrepancy is ultimately reflected in the composition
of residual melt, being more evolved in the low-water experiment and more primitive in the hydrous run.
We conclude that (i) the P–T–H2O conditions of the HWE more realistically represent those of natural enclave
crystallization, consistently with thermobarometric estimates, and (ii) at a global scale, the occurrence of amphibolebearing
cumulates in exhumed lower crustal section testify to magmatic differentiation in a water-rich environment
nearby the Moho. Importantly, the flux of H2O from this “wet and hot deep zone” (Perinelli et al., 2017) would play a
critical role in the eruptive style of the overlying volcanoes
Kinetic crystallization of clinopyroxenes from alkaline basalts: growth rate experiments at high pressure
No full textThe kinetics of crystal nucleation and growth are fundamental for the interpretation of rock textures in
terms of thermal history of a magma during its ascent to the surface, and to constrain timescales of magmatic
processes. In basaltic systems Cpx is a common phenocryst, and due to its wide crystallization range, it
contains the most complete record of evolutionary history of a magma. Nevertheless, to date, experimental
studies addressed to obtain quantitative measurements of Cpx crystallization kinetics (e.g. crystal growth rate)
are few and limited to low pressure conditions (≤ 500 MPa). In this experimental work, we investigated the
effects of temperature, water content and dwell time on clinopyroxene (Cpx) crystal growth from an alkaline
basalt of the Campi Flegrei Volcanic District (CFVD; Italy) at high pressure. With the aim to provide data on
Cpx crystallization kinetics at high pressure, we experimentally investigate a primitive alkaline basalt (APR16
sample, mg# = 0.66 and Na2O+K2O = 4.4 wt.%) from Procida island (Italy), representative of the leastevolved
rocks of the whole CFVD volcanic products at both anhydrous and hydrous conditions. Experiments
were performed at isobaric pressure (800 MPa) as function of temperature (T), water content and dwell time
by using the 1⁄2 inch piston cylinder apparatus at the HP-HT Laboratory of the Earth Sciences Department,
Sapienza, University of Rome. We performed a total of 24 experiments divided into three series. Experiments
of series 1 were carried out at anhydrous conditions at temperatures of 1250 °C and 1200 °C and dwell time
of 0.25, 3, 6 and 9 hours. Experiments of series 2 and 3, instead, were carried out at hydrous conditions (2 and
4 wt.% H2O added to the starting material, respectively), temperatures of 1220 °C and 1170 °C (series 2) and
1100 °C and 1050 °C (series 3), and the same dwell time of series 1. Preliminary results show a strong growth
rate dependence of temperature and dwell time in both the anhydrous and hydrous experiments. As expected,
the charges run at low temperature of each series are more crystallized than those run at high T; moreover,
the low-T clinopyroxene grains are generally smaller in size than those formed at high-T. This behavior is
noticeable already in the shortest dwell time (0.25 hours) experiments. As regards time, instead, we noted an
increase of crystal size with increasing the dwell time (from 0.25 to 9 hours) and a decrease of the number
of crystals: this is particularly evident in the hydrous runs (series 2 and 3). Finally, it was possible to note a
variation in crystals shape that changes from anhedral/subhedral in the shorter runs to euhedral in the longer
ones
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Uncommon K-foiditic magmas. The case study of Tufo del Palatino (Colli Albani Volcanic District, Italy)
Full text linkLeucititic rocks, K-foiditic in composition are volumetrically important in the Colli Albani (also known as Alban Hills)
volcanic district (Central Italy)especially during the most explosive phases of activity (>200 km
3
). The Colli Albani
tephra in distal (>500 km) deposits indicates that K-foiditic magma chambers fed large explosive eruptions
(i.e., tens of km
3
of pyroclastic rocks). Major oxides, trace elements and Raman spectra were measured on the glasses
and minerals occurring in the K-foiditic scoria clasts of the~530 kyr-old Tufo del Palatino, erupted in the Colli Albani
volcanic district. The Colli Albani pre-eruptive magmatic system is characterized by theaH2O<1andhighCO2activity in the melt, as testified by the CO3in the clinopyroxene melt inclusions, by the early crystallization of CO3-bearing
apatite and by the high CO2activity in the free volatile phase that led to crystallization of calcium carbonate in the
scoria clast vesicles. The K-foiditic magmas plot on the Cpx + Lc + melt divariant surface of the Ol-Cpx-Lc-MelH2O-CO2, P≥0.2 GPa andT≤1100 °C. The assimilation of cold carbonate by hot magmas is an important opensystem process allowing the establishment ofaH20< 1 condition in the volatile-rich, Colli Albani magma chambers
where the stabilityfields of the olivine and phlogopite are reduced infavor of clinopyroxene and leucite. Trace element modelling indicates large amount of carbonate assimilation (~12.4 wt%) involved in the differentiation process
that origins the K-foiditic magmas starting from a K-rich, phonotephritic parental magma. The large amount of assimilate carbonate is consistent with the peculiar distribution of the latent heat across the crystallization interval of
the phonotephritic parental magma. The isenthalpic assimilation process is very efficient in the phonotephritic
magma because the crystallization of clinopyroxene and leucite in equilibrium with a K-foiditic melt proceeds
over a relatively large temperature interval (>200 °C) and the K-foiditic melt shows low viscosity (10
4
Pa·s at
1000 °C). Actually, the low melt viscosity, that increases the growth rate, and the large temperature interval of crystallization are intrinsic factors that increase the release ofthe latent heat of crystallization from the phonotephritic
parental magma. Extrinsic factors enhancing the assimilation process efficiency are the thickness (>4 km) and the
depth (down to 5–7 km) of the carbonate substrate in the Colli Albani volcanic distric
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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