Earth and Planetary Science Letters 399 (2014) 103–115

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Earth and Planetary Science Letters

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Extreme mineral-scale Sr isotope heterogeneity in granitesby disequilibrium melting of the crust

Federico Farina a,∗, Andrea Dini b, Sergio Rocchi c, Gary Stevens d

a Departamento de Geologia, Universidade Federal de Ouro Preto, Ouro Preto, MG 35430000, Brazilb Istituto di Geoscienze e Georisorse, CNR, Pisa, Via Moruzzi 1, 56124, Pisa, Italyc Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, 56126, Pisa, Italyd Department of Earth Sciences, Stellenbosch University, Private Bag X1, 7602 Stellenbosch, South Africa

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 December 2013Received in revised form 18 April 2014Accepted 6 May 2014Available online xxxxEditor: T.M. Harrison

Keywords:disequilibrium meltingSr isotopesisotope heterogeneitygranitemagma batch

The broadest ranges of initial Sr isotopic ratios (87Sr/86Sri) ever reported within a single igneous rock(≈ 2 × 10−2) are preserved within the late Miocene laccolith-pluton-dyke felsic complex of Elba Island(Italy). For these units, the integration of textural and crystal-scale isotope data allows tracing theevolution of the 87Sr/86Sri of the melt from the emplacement level back to the earliest pre-emplacementcrystallization stage. The rock matrix minerals record the 87Sr/86Sri composition of the magma at theemplacement level (0.715–0.716). K-feldspar megacrysts, representing an earlier phase crystallized atdepth, record a rim-to-core increase of Sr-isotopic ratios from values similar to those of the matrixto significantly higher ones (≈ 0.719). Remarkably, biotites hosted within megacrysts, representing thefirst crystallization stage, have extreme and contrasting 87Sr/86Sri values in the different intrusive units:biotites within megacrysts from the laccolith record the lowest ratio in the intrusive complex (≈ 0.710),while those in the megacrysts from the pluton and associated felsic dyke have the highest 87Sr/86Sri(≈ 0.732). This time-transgressive record of isotopic variation in the magma reflects episodic rechargeand mixing of magma batches formed by disequilibrium melting of crustal sources that produced meltsthrough different reactions as temperature was increasing. The progression from muscovite- to biotite-dominated fluid-absent melting generates melts with increasing 87Sr/86Sr, while at higher temperatures,the progression from biotite- to hornblende-dominated melting reactions results in a decrease in the87Sr/86Sr of the melt.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Isotopic measurements at the crystal and sub-crystal scale haveled to the recognition that significant internal isotope variabilityis commonplace in igneous rocks of intermediate to acidic com-position (e.g. Davidson et al., 2007). Isotopic ranges preserved atthe micro-scale may even exceed the bulk-rock range displayedwithin an entire magmatic unit or province (Wolff et al., 2011).The value of mineral-scale isotope data as a petrogenetic tool liesin the fact that they provide a time-transgressive record of theisotopic compositional changes that the magma experienced dur-ing crystal growth, giving a unique insight into the nature andrelative timing of open-system processes (Davidson et al., 2008).Mineral-scale isotopic studies have largely focused on the determi-nation of 87Sr/86Sr values on feldspars, apatite, hornblende, biotite

* Corresponding author.E-mail address: fannak@gmail.com (F. Farina).

http://dx.doi.org/10.1016/j.epsl.2014.05.0180012-821X/© 2014 Elsevier B.V. All rights reserved.

and clinopyroxene (e.g. Charlier et al., 2007). Among these phases,plagioclase and K-feldspar are by far the most commonly inves-tigated due to their high Sr abundances, ubiquitous occurrencein igneous rocks and large range of crystallization temperature.For active volcanoes, the zero-age correction allows diversities in87Sr/86Sri as small as 10−4 to be used to unravel pre-eruptivemagma chamber processes (e.g. Martin et al., 2010). Similar studiesin felsic plutonic rocks are rare owing to the limitations imposedby the significant age corrections commonly required for grani-toids as well as by the possible role of diffusive equilibration inrocks that have cooled more slowly than their volcanic counter-parts. With respect to diffusive equilibration, the few works thathave evaluated the extent of this process in plutonic rocks haveconsistently concluded that if mineral-scale Sr isotopic diversityis acquired by minerals such as feldspars and pyroxenes duringcrystallization, it ought to be retained (Gagnevin et al., 2005a;Davidson et al., 2008). On the other hand, the likelihood of confi-dently identifying intra-rock distinctions in 87Sr/86Sri is negatively

104 F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115

Fig. 1. Compilation of data from mineral-scale 87Sr/86Sr studies and comparison with data from the Elba Island granitic complex. Isotope variations on the x-axis are expressedas �87Sr/86Sr, where � = determined initial isotopic ratio – minimum ratio in the system. Data from the literature are from rocks of intermediate to acid composition. Dataare from: (1) Tepley et al. (2000); (2) Davidson et al. (2007); (3) Martin et al. (2010); (4) Wolff et al. (1999); (5) Knesel et al. (1999); (6) Charlier et al. (2007); (7) Davidsonet al. (2008); (8) Waight et al. (2000).

correlated with the age of the rock and with the 87Rb/86Sr of theanalyzed minerals; i.e. for old ages and high 87Rb/86Sr, the errorson the calculated initial 87Sr/86Sr may obscure the original isotopevariability retained at the crystal-scale. Intrusive rocks from ElbaIsland have the advantage of coupling very young emplacementage (around 7 Ma) with extreme intra-rock 87Sr/86Sri heterogene-ity (up to 1.7 × 10−2, Fig. 1), thus offering a unique opportunityto gain insights into the processes and sources contributing to theisotopic evolution of granitic magmas.

Studies discussing the origin of isotopic heterogeneity in fel-sic rocks are based on the assumption that isotopic homogeniza-tion is achieved in the source during the pre-anatectic heatingstage and thus, that during anatexis the crust produces isotopi-cally homogeneous melts. Following this conjecture, isotopic vari-ations are generally ascribed to mixing between crustal meltsand mantle-derived magmas (e.g. Gray, 1984), with the implica-tion that crustal growth is reflected in the genesis of felsic rocksexhibiting inter- and intra-crystalline isotopic diversity. However,plutons built by tens to thousands of km3 of magma, necessar-ily tap source rock volumes that are very unlikely to have beencompletely isotopically homogenized during the prograde meta-morphic event. Therefore, the isotopic variability observed in gran-itoids may be interpreted to reflect the isotopic heterogeneity ofthe source, with source isotopic heterogeneity that has not beenobliterated by magmatic processes (e.g. Deniel et al., 1987). Inaddition, isotopic equilibrium between melt and crustal residuemight not be attained (Harris and Ayres, 1998) and a single sourcecan generate magma batches with different Sr isotopic composi-tions as the stoichiometry of the melting reaction changes withtemperature (Farina and Stevens, 2011). Although Sr-disequilibriummelting has been described (e.g. Tommasini and Davies, 1997;McLeod et al., 2012) and its consequences investigated in ex-periments (Knesel and Davidson, 1996) and models (Farina andStevens, 2011), this process is still not considered as a viable ex-planation for the occurrence of isotopic heterogeneity in granitoidrocks. In this study, we show that substantial crystal-scale vari-ations in Sr isotopic composition recorded at Elba Island can beproduced through mixing of different magma batches producedduring the progressive disequilibrium melting of metasedimentarysources.

2. Background

2.1. Geological setting

Elba Island is located at the northern end of the TyrrhenianSea (Fig. 2), a region that experienced extension after a phase

Fig. 2. Geological sketch and generalized cross-section of western and central ElbaIsland. Abbreviations: EBF – Eastern Border Fault; CEF – Central Elba Fault. Whitecircles and white squares indicate the location of samples from the Monte Capannepluton and the San Martino porphyry, respectively.

of Cretaceous to early Miocene compression related to the colli-sion between the Adria microplate and the Corsica–Sardinia block(Malinverno and Ryan, 1986). The extensional regime migratedprogressively from west to east and was intimately associatedwith the magmatic activity forming the Miocene-Quaternary Tus-can Magmatic Province (Serri et al., 1993). This magmatic provinceconsists of granitoids rocks associated with volumetrically minormantle-derived products having high-K calc-alkaline or lamproiticaffinities (Poli, 1992). At Elba Island, granitoid magmas emplacedduring the late Miocene. Magmatism started on western Elba withthe construction of a laccolith complex consisting of three mul-tilayered units: Capo Bianco aplite, Portoferraio and San Martinoporphyries (8–7.4 Ma; Rocchi et al. 2002, 2010; Roni et al., 2014).The laccolith complex was intruded by the Monte Capanne plutonand its associated late leucogranite–pegmatite dykes (Dini et al.,2002; Farina et al., 2010). Finally, the steeply-dipping Orano dykeswarm crosscuts the entire succession (Dini et al., 2008).

F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115 105

Fig. 3. Photographs of K-feldspar megacrysts. a Euhedral megacryst in the megacryst-rich facies of the Monte Capanne pluton; b Euhedral megacryst in the Cotoncello dyke;the crystal is cut very close to the core as shown by the hourglass distribution of biotite inclusions. c composite K-feldspar megacryst in the megacryst-poor facies ofthe pluton, biotites in the core are preferentially incorporated on K-feldspar {110} growth zones (i.e. hourglass zoning) while those hosted in the poikilitic overgrowth arerandomly distributed. d composite K-feldspar megacryst. The hammer in a, b, c and d is 33 cm long.

2.2. Petrogenesis of the late Miocene Elba Island intrusive complex

The petrogenesis of the Elba Island intrusive complex has beeninvestigated in many studies (Dini et al. 2002, 2004; Gagnevin etal., 2004, 2005a, 2005b, 2011; Farina et al., 2010, 2012), most ofthem focusing on the origin of the Monte Capanne pluton. Thechemical variability exhibited by this complex as well as by gran-itoids rocks of the entire Tuscan Magmatic Province has been tra-ditionally interpreted as reflecting mixing between crustal meltsand mantle-derived magmas (e.g. Poli, 1992). This interpretationis mainly based on the occurrence within the Monte Capannepluton and the San Martino porphyry of mafic microgranular en-claves (Dini et al., 2002), disequilibrium textures such as patchy-zoned and corroded An-rich plagioclase as well as multiple re-sorption surfaces in K-feldspar megacrysts (Gagnevin et al., 2004),large Sr isotopic variation preserved within K-feldspar megacrysts(Gagnevin et al., 2005a). In particular, this latter evidence ledGagnevin et al. (2005a) to propose a petrogenetic model forthe formation of the Monte Capanne pluton involving megacrystgrowth in magmas undergoing wall-rock contamination, fractionalcrystallization and mixing with both crustally- and mantle-derivedmagmas. Recently, Farina et al. (2012), based on major- and traceelement geochemical data, challenged the traditional magma mix-ing model, suggesting that mantle-derived magmas played only asubordinate role in shaping the composition of the volumetricallydominant granitic rocks (i.e. the pluton and the laccoliths). Hy-bridization with mantle-derived magma(s) is considered by theseauthors able to account for the origin of the more mafic units ofthe complex (i.e. mafic enclaves and Orano dykes), representingless than 5 vol% of the total volume of magma emplaced. Farinaet al. (2012) proposed that chemical variations in granitic rocks ofthe Elba Island complex are mostly primary; i.e. directly inheritedfrom the source.

2.3. Petrography and age of the units investigated

This study focuses on three units characterized by the wide-spread occurrence of large euhedral crystals of K-feldspar (mega-crysts hereafter) which are up to 20 cm on their longest axis. The

monzogranitic San Martino porphyry occurs as dykes and laccol-ith layers and is characterized by megacrysts of sanidine set in anaphanitic groundmass (Dini et al., 2002). The monzogranitic MonteCapanne pluton has heterogeneous megacryst distribution defin-ing three intrusive sheet-like facies with low, intermediate andhigh megacryst abundance (Farina et al., 2010). The fine-grainedsyenogranitic Cotoncello dyke has a maximum thickness of 100 mand strikes roughly parallel to the external contact of the pluton,over a distance of about 1500 m.

The three units have similar mineral assemblages consisting ofplagioclase, quartz, K-feldspar and biotite as rock-forming phasesand apatite, zircon, monazite, allanite and ilmenite as accessories.Their texture is porphyritic with K-feldspar megacrysts spanning inlength from 2 to 20 cm hosted in a distinctly finer-grained matrixthat, in the pluton, also contain anhedral interstitial K-feldspars(Fig. 3). Two main megacryst growth patterns are recognized: eu-hedral crystals mostly characterized by {010} tabular habit andelongated on the c-axis with aspect ratio 1 : 2 : 4 (Fig. 3e, f) and,only in the pluton, composite crystals having variably roundedcores surrounded by poikilitic overgrowths (Fig. 3g, h). Both euhe-dral megacrysts and rounded cores of composite crystals are richin inclusions (about 10 vol%) of plagioclase, biotite and quartz thatare characteristically smaller than crystals of the same phase in thematrix. Biotite inclusions define a well-developed hourglass distri-bution, resulting from the preferential incorporation of biotite onK-feldspar {110} growth zones (Fig. 3b, c). Plagioclase {010} facestend to attach to K-feldspar {010} growth zones.

The emplacement age for the San Martino porphyry is definedby a sanidine 40Ar/39Ar isochron date of 7.44 ± 0.08 Ma (Dini etal., 2002). For the Monte Capanne pluton two samples displayingSr isotopic equilibrium were described by Innocenti et al. (1992);these authors calculated Rb–Sr wr-Pl-Kfs-Bt cooling ages of 6.88 ±0.1 Ma and 6.75 ± 0.7 Ma. Four zircon grains from the Monte Ca-panne system (i.e. monzogranite, mafic enclaves and Orano dyke)gave a weighted mean 206Pb/238U age of 7.19±0.08 Ma (Gagnevinet al., 2011). A sanidine 40Ar/39Ar isochron age of 6.83 ± 0.06 Maobtained for a late-plutonic Orano dyke, constrain the emplace-ment age for the pluton above this value. The emplacement age ofthe Cotoncello dyke has not been determined, yet clear evidence

106 F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115

for mingling with the hosting Monte Capanne pluton supports acoeval emplacement age for these units.

3. Methods

3.1. Sampling strategy

In order to document the isotopic changes in the magmas dur-ing crystallization, we have sampled mineral phases that are likelyto have crystallized at different times along the cooling historyof the three systems: K-feldspar megacrysts and biotite includedtherein as well as biotite and K-feldspar crystals forming the ma-trix. From each megacryst, a 6–8 mm thick core-to-rim sectionwas separated perpendicular to the c-axis, i.e. perpendicular to theCarlsbad twin plane. The concentric arrangement of inclusions inthe megacrysts was used to identify the crystal cores (Fig. 3b, c).From each thick section, an internal and an external zone were cutout and crushed to be prepared for trace element and isotope anal-yses. The euhedral megacrysts were subdivided into a core and arim, while for the composite megacryst a partially resorbed innerzone was separated from the overgrowth. The resulting K-feldsparsamples were crushed, sieved, cleaned ultrasonically and passedthrough the magnetic separator. Finally, 200 mg of non-perthiticK-feldspar grains were separated from each sample by handpick-ing under binocular microscope. Matrix K-feldspars were separatedfollowing the same procedure.

Biotite crystals included within megacrysts were collected from10 samples. Due to the low Sr content of biotites, the crystals werecollected from 20–30 megacrysts from each site. Consequently,the determined Sr isotope ratio represents an average of biotitecrystals collected from many different megacrysts. The megacrystswere separated from the whole rock, crushed and sieved. Crys-tals with sizes <350 μm were selected. Grains were manuallygrounded under deionized water, passed through the magneticseparator and sieved again. This three-step process was repeateduntil pure biotite separates were obtained. The examination of thinsections revealed that biotites hosted within the megacrysts arefree of apatite inclusions.

3.2. Analytical methodologies

Sr and Nd isotope compositions of K-feldspars and whole rockswere determined using a Finnigan MAT 262V multicollector mass-spectrometer at the Istituto di Geoscienze e Georisorse (CNR, Italy)after conventional ion-exchange procedures for Sr and Nd sepa-ration from the matrix. Sr and Nd total blanks during the pe-riod of measurement were <2 ng and <1 ng, respectively. Mea-sured 87Sr/86Sr have been normalized to 86Sr/88Sr = 0.1194 and143Nd/144Nd to 146Nd/144Nd = 0.7219. During data collection, 15replicate analyses of SRM 987 (SrCO3) standard gave an average87Sr/86Sr value of 0.710200 ± 8 (2σ mean) and 14 measurementof standard JNdi-1 gave an average 143Nd/144Nd of 0.512099 ± 10(2σ mean). The Sr and Nd concentration in K-feldspars have beendetermined by ICP-MS (Fisons PQ2 Plus) at the Dipartimento diScienze della Terra, University of Pisa (Italy). Samples were dis-solved in screw-top vessels on a hotplate at 120 ◦C with HF-HNO3mixture and then analyzed following the method reported inD’Orazio (1995). External calibration was made by using the in-ternational standard BE-N (Govindaraju, 1994) as a composition-and matrix-matching calibration solution. The correction procedureincludes (i) blank subtraction, (ii) instrumental drift correction us-ing Rh–Re–Bi internal standardization and repeated analysis of adrift monitor, (iii) oxide–hydroxide interference correction. Preci-sion, evaluated by replicate dissolutions and analyses of the in-house standard HE-1 (Mt. Etna hawaiite) is between 2 and 5% RSD.

Fig. 4. Range of biotite 87Sr/86Sri values obtained by correcting the measured ratiosfor the range of emplacement ages available and taking into account 2% uncertaintyon the 87Rb/86Sr determination (gray bars). Minerals and whole-rocks form MonteCapanne pluton and Cotoncello dyke have been corrected for a 6.8–7.2 Ma interval,and those from the porphyry have for the 7.2–7.6 Ma interval. The whole range ofvariability displayed by K-feldspar megacrysts (this work and Gagnevin et al., 2005a)is plotted for comparison, together with the whole-rock Sr isotopic variability of thethree units. Data for the Monte Capanne pluton are from Farina et al. (2010).

The 87Sr/86Sr composition of biotites was determined by iso-tope dilution following the procedure of Charlier et al. (2006).Biotites were dissolved in screw-top vials using a mixture of HF(28 M) and HNO3 (14 M). The residue was taken up in 5 ml6 M HCl and split in two aliquots. One of them was spiked us-ing a mixed 87Rb–84Sr spike solution and used to determine theSr isotopic composition of the sample. From the other aliquot bothRb and Sr were separated and their concentrations determined byThermal Ionization Mass Spectrometry. Separation of Sr from thespiked aliquot was carried out using Sr Spec extraction chromato-graphic resin. Sr was eluted from the column in 2 ml 0.05 M HNO3in four 0.5 ml stages. The Sr fraction was dried and the residuewas taken up with 2 μl HNO3 and loaded on a tungsten filament.Spiked samples were corrected off-line for spike contribution us-ing standard techniques. Total Rb and Sr procedural blanks were<1 ng. An uncertainty of 1% was assigned to the 87Rb/86Sr basedon replicate analyses on natural samples.

Analyses of mineral chemistry on biotite were performed witha JEOL JXA 8600 electron microprobe at the IGG-CNR Florence, op-erating in the wavelength-dispersive mode. Analyses were carriedout at an acceleration voltage of 15 kV, using a beam current of10 nA and a 5 μm spot size. Accuracy and precision are the sameas reported by Vaggelli et al. (1999). Count time on peak was 15 sfor Si, Al, Ti. Fe, Mn, Mg, Ca and K, 10 s for Na and 40 s for Sr, Baand volatiles (F, Cl, S).

4. Mineral-scale isotope data

Unraveling intra-rock isotope heterogeneities relies on the cor-rection of measured 87Sr/86Sr values for the crystallization age,which is especially critical for biotite, owing to its high 87Rb/86Srratio (i.e. >100). To evaluate the effect of the age uncertainty onthe calculation of initial Sr isotopic ratios, we calculated and plot-ted the 87Sr/86Sri obtained using thescatter of available ages for

F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115 107

Table 1Sr isotope data for K-feldspars, biotites and whole-rocks.

Sample Unit Phase Rb Sr 87Rb/86Sr 87Sr/86Sr(m) ±2σ * 87Sr/86Sr(i)† Error§

FF04 MCp MKx-core 421 283 5.09 0.718251 13 0.71783 5FF04 MCp MKx-rim 386 351 3.23 0.715873 12 0.71556 4FF 30 MCp MKx-core 396 301 3.86 0.717537 15 0.71716 4FF 30 MCp Kfs-overgrowth 352 346 2.99 0.715180 19 0.71489 3FF30 MCp Kfs-matrix 397 229 5.09 0.715485 14 0.71499 6FF13 Cot MKx-core 466 283 4.84 0.718969 11 0.71866 6FF13 Cot MKx-rim 440 262 4.93 0.717911 12 0.71743 5PP118 SMp MKx-core 435 289 4.36 0.718760 30 0.71830 5PP118 SMp MKx-rim 499 315 4.59 0.717160 30 0.71668 5ER15a MCp Kfs-matrix 488 237 5.96 0.715400 100 0.71482 17STE 74a MCp Kfs-matrix 339 205 4.79 0.715500 100 0.71503 16FF 04 MCp Bt-included 1006 2.88 1023 0.831434 100 0.7312 35FF30 MCp Bt-included 639 6.27 296 0.761392 30 0.7324 9FF13 Cot Bt-included 1380 6.78 594 0.789860 60 0.7317 20FF143 MCp Bt-included 999 3.30 885 0.818070 40 0.7313 30FF144 MCp Bt-included 985 3.10 930 0.822300 40 0.7312 32FF145 MCp Bt-included 938 4.20 652 0.795450 80 0.7316 22FF147 MCp Bt-included 729 4.20 506 0.782010 60 0.7324 17PP330 Cot Bt-included 1299 2.90 1315 0.860080 30 0.7312 45PP126 SMp Bt-included 1058 3.90 791 0.793430 50 0.7103 29PP118b SMp Bt-included 1066 4.37 711 0.784830 40 0.7101 25PP118b SMp Bt-matrix 851 7.2 342 0.751010 70 0.7149 12PP118 SMp whole-rock 301 173 5.04 0.716680 14 0.71615 8FF13 Cot whole-rock 294 126 6.76 0.723659 11 0.72300 9FF09 Cot whole-rock 329 131 7.28 0.723744 12 0.72303 10FF04c MCp whole-rock 288 170 4.91 0.715451 10 0.71497 8FF30c MCp whole-rock 272 215 3.67 0.715436 11 0.71508 10

* The uncertainties refer to the least significant digits and are 2σ mean within-run precisions.† Initial Sr isotope ratios are calculated at 6.9 Ma for the pluton and the dyke and at 7.4 Ma for the porphyry.§ The errors refer to the least significant digits and are calculated considering the error on the87Rb/86Sr determination as well as an age uncertainties of ±0.1 Ma.a Data from Ferrara and Tonarini (1985).b Data from Westerman et al. (2004).c Data from Farina et al. (2010).

Abbreviations: Bt – biotite, Cot – Cotoncello dyke, Kfs – K-feldspar, MCp – Monte Capanne pluton, Mkfs – K-feldspar megacryst, SMp – San Martino porphyry.

the three units. The age uncertainty for the pluton (6.8–7.2 Ma)translates into an 87Sr/86Sri variability for biotites included withinmegacrysts ranging between 0.726 and 0.733 (Fig. 4). It is worthnoting that these biotites are in any case substantially more radio-genic than megacryst cores (0.719–0.715; this work and Gagnevinet al., 2005a, 2005b). In the following description of 87Sr/86Sri data,the measured isotopic ratios are corrected at 6.9 Ma for the plutonand the dyke and at 7.4 Ma for the porphyry (Dini et al., 2002).

The initial 87Sr/86Sr values for K-feldspar and biotite in the ElbaIsland complex span extremely large ranges (≈ 0.710–0.732; Ta-ble 1, Fig. 4). Biotites hosted in megacrysts from the pluton and thedyke record the highest 87Sr/86Sri, notably higher than the Sr iso-topic ratio exhibited by the rocks of the Tuscan Magmatic Province(Dini et al., 2005). In contrast, biotites included in the megacrystsfrom the porphyry show the lowest initial Sr isotope ratio of thewhole Elba Island complex. K-feldspar megacrysts in the threeunits are isotopically zoned, with cores having 87Sr/86Sri between0.7187 and 0.7172 whereas rim values vary between 0.7174 and0.7156. The 87Sr/86Sri values displayed by megacrysts’ rims forboth the porphyry and the pluton point towards the isotopic com-position of the relative whole-rocks and are thus more radiogenicin the porphyry than in the pluton. Poikilitic overgrowths of com-posite megacrysts and matrix K-feldspar crystals in the plutonshare the same 87Sr/86Sri (≈ 0.7149) approaching equilibrium withthe whole rock. In the dyke, both megacryst core and rim aresignificantly less radiogenic than the whole rock (0.7230). Theseresults are consistent with the general core-to-rim decrease in87Sr/86Sri described by Gagnevin et al. (2005a) within megacrystsfrom the Monte Capanne pluton. It is worth remarking that theSr isotope variability within megacrysts is 15–25 times greaterthan the relative calculated error (Table 1). In spite of the large Srisotope zoning within K-feldspars there is no initial 143Nd/144Nd

variation within and between K-feldspar crystals of the differentunits (Fig. 5, Table 2).

5. Discussion

5.1. Crystal-scale Sr isotopic heterogeneity

Intra-rock initial Sr isotope ranges in the Elba Island intru-sive complex are extremely wide, spanning approximately between8 × 10−3 for the porphyry and 1.7 × 10−2 for the pluton, with thelatter range representing the broadest intra-specimen heterogene-ity ever reported in intermediate to acidic igneous rocks (Fig. 1).Biotite crystals included within K-megacrysts account for most ofthe Sr isotope variability exhibited by the three units, recordingboth the highest and the lowest 87Sr/86Sri in the entire Elba Islandcomplex. Therefore, before interpreting the intra-rock Sr isotoperange described at Elba Island, it is essential to verify that thebiotites included in K-feldspar megacrysts are magmatic and thattheir age is not significantly older than the age of crystallization ofother minerals. The possibility that biotites are restitic or xenocrys-tic are discarded as there is no textural or chemical evidence sup-porting a non-magmatic origin for these biotites: the crystals areeuhedral, not altered and have major element compositions over-lapping with those of biotite in the matrix (Fig. 6). The possibilitythat the biotites included in megacrysts crystallized during a mag-matic event significantly older than the emplacement age of theunits can be discarded too. Indeed, a best-fit line of 87Rb/86Sr and87Sr/86Sr data for biotite crystals included in the megacrysts of theMonte Capanne pluton provides an age of 6.77 ± 0.06 Ma (MSWD= 0.76; 87Sr/86Sri = 0.73298; Isoplot 3.75, Ludwig, 2012) over-lapping with the emplacement age for the pluton. Although thiscalculation does not provide a reliable age, it shows that biotiteswith contrasting measured 87Rb/86Sr and 87Sr/86Sr compositions

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Fig. 5. a. Initial 87Sr/86Sr vs. 143Nd/144Nd diagram showing the composition of the Elba Island intrusive complex, K-feldspar megacrysts and other rocks of the TuscanMagmatic Province. New Sr and Nd data for the volcanic rocks of Capraia Island are also plotted (Data Repository). The average composition of Cotoncello dyke is usedas crustally-derived end-member of mixing. The mantle-derived end-members are Capraia Island shoshonites (curve 1), trachytes (curve 2) and high-K andesites (curve 3).Mixing trend 4 is from hypothetical sources having the isotopic composition of Capraia andesites and Cotoncello dyke and an r value of 0.1 (i.e. the Sr/Nd of the end-membershas been changed to obtain r = 0.1). Following the traditional mixing trend (curve 1), the Sr isotopic composition of megacryst cores is obtained adding ca. 10 wt% of anandesitic component to the composition of the Cotoncello dyke; a further addition of 10 wt% of the mafic end-member produces the 87Sr/86Sr of the rims. Source of data:Dini et al. (2002) and reference therein, Gagnevin et al. (2004), Conticelli et al. (2009) and Farina et al. (2010). The error (±2σ ) for initial 87Sr/86Sr is within the symbol,while for 143Nd/144Nd the typical error bar is shown in the key. b. Initial 87Sr/86Sr vs MgO + FeOtot and 87Sr/86Sr vs Al2O3 diagrams. Trends 1, 2 and 3 are produced bymixing Cotoncello dyke composition with shoshonites, trachytes and high-K andesites, respectively.

Table 2Nd isotope data for K-feldspars and whole rocks.

Sample Unit Phase Sm Nd 147Sm/144Nd 143Nd/144Nd(m) ±2σ * 143Nd/144Nd(i)†

FF04 MCp MKx-core 0.21 1.15 0.1113 0.512196 18 0.51219FF04 MCp MKx-rim 0.22 1.12 0.1192 0.512193 21 0.51219FF 30 MCp MKx-core 0.10 0.39 0.1556 0.512182 23 0.51218FF 30 MCp Kfs-overgrowth 0.33 1.68 0.1192 0.512181 24 0.51218FF30 MCp Kfs-matrix 0.93 5.20 0.1085 0.512176 11 0.51217FF13 Cot MKx-core 0.79 3.01 0.1592 0.512194 20 0.51219FF13 Cot MKx-rim 0.28 1.36 0.1249 0.512191 21 0.51219FF04a MCp whole-rock 5.9 29.8 0.1201 0.512198 9 0.51219FF 30a MCp whole-rock 6.70 36.00 0.1129 0.5122 10 0.51218FF13 Cot whole-rock 5.00 24.80 0.1218 0.512195 9 0.51219FF09 Cot whole-rock 4.90 24.20 0.1228 0.51214 15 0.51213

* The uncertainties refer to the least significant digits and are 2σ mean within-run precisions.† Initial Nd isotope ratios are calculated at 6.9 Ma for the pluton and the dyke and at 7.4 Ma for the porphyry.a Data from Farina et al. (2010).

Abbreviations: Cot – Cotoncello dyke, Mkfs – K-feldspar megacryst, MCp – Monte Capanne pluton.

converge to the same initial 87Sr/86Sr value when corrected fortheir emplacement age (6.9 Ma). Moreover, when biotites of theCotoncello dyke are also included in the calculation of the best-fitline, this gives an identical result (age of 6.78 ± 0.05 Ma; MSWD= 0.76; 87Sr/86Sri = 0.73289) supporting an initial 87Sr/86Sr for bi-otites hosted in megacrysts of the pluton and the dyke of 0.7330.Biotite crystals included within K-feldspar megacrysts of the SanMartino porphyry exhibit a characteristic low 87Sr/86Sri compo-sition when corrected for the emplacement age of the porphyry(7.4 Ma). The effect of correcting the measured 87Sr/86Sr of thesebiotite crystals for an age older than 7.4 Ma would decrease their87Sr/86Sri, thus increasing the Sr isotopic diversity of the system.

5.2. Crystallization sequence

In order to gain insights into the isotopic evolution of therocks of the Elba Island complex it is crucial to ascertain their se-

quence of crystallization. The reconstruction of the crystallizationsequence for the three units relies on the understanding of whenand where K-feldspar megacrysts crystallized. Based on field, tex-tural and chemical data, Farina et al. (2010) demonstrated thatthe crystallization of megacrysts in the Monte Capanne plutontook place before final emplacement from a magma that was stilllargely molten. This interpretation is consistent with the results ofcrystallization experiments showing that, although K-feldspar nu-cleates late in the sequence of mineral appearances, about 60–70%of liquid is still available in the system when this mineral beginsto crystallize (e.g. Vernon, 2010). A decisive proof advocating formegacrysts crystallization before magma emplacement comes fromthe San Martino porphyry, where megacrysts occurring in both lac-coliths and their feeder dykes are set in a fine-grained groundmasswith orientation governed by magma flow (Roni et al., 2014).

The crystallization sequence has been subdivided into fourmain steps used in Fig. 7 as a relative time-scale. Before magma

F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115 109

Fig. 6. Major-element compositions for matrix biotites (Bt-2) and biotite included in the megacrysts (Bt-1) of San Martino porphyry (SMp), Monte Capanne pluton (MCp) andCotoncello dyke (Cot dyke). Biotites of the Monte Capanne pluton are subdivided according with the facies defined by Farina et al. (2010) and abbreviated as low-, medium-and high-Mkfs to indicate the relative abundance of megacrysts in the rock. (a) Fe# vs. Al(tot) diagram. The fields outline the chemical variability of matrix biotites in theunits. Chemical formulas were calculated based on 22 cation charges. (b) Position of analyzed biotites in two core to rim/overgrowth representative thin sections for thefacies of the pluton having high and low abundance of megacrysts, respectively. An euhedral (left) and a composite megacryst (right) are shown. For those biotites wherethe number is followed by “c-r” two analyses have been performed, one in the core and one in the rim of the crystal. (c) Core to rim Fe# variation for biotites included inmegacrysts compared with Fe# of biotites in poikilitic rims and related matrix biotites.

emplacement, an early population of biotite crystals (<1 mm)formed (first step) and was subsequently included in the K-feldsparmegacrysts. We suggest that biotite crystals grew before and notsimultaneously with their hosting K-feldspar megacrysts becausethey are remarkably out of equilibrium with the megacrysts. Thisevidence suggests their crystallization from precursor magma(s).The K-feldspar megacryst cores represent the second crystallizationstep, and the rims the third one. Plagioclase and quartz crystallizedduring the second and third crystallization steps as these phasesoccur as inclusions within K-feldspar megacrysts. The fourth crys-tallization step occurred at the emplacement level where a secondgeneration of biotite, plagioclase, quartz and K-feldspar, either in-terstitial or as poikilitic overgrowths on megacrysts, nucleated andgrew.

5.3. Early wall-rock assimilation

The Monte Capanne pluton and the San Martino porphyryhost scattered metasedimentary xenoliths that consists of biotite,quartz, plagioclase, K-feldspar and a minor but variable amount ofaluminous minerals such as corundum, cordierite, hercynite, an-dalusite and sillimanite. Gagnevin et al. (2004) observed that theSr and Nd isotope variability exhibited by the Monte Capannepluton and its microgranular mafic enclaves define trends point-ing towards a low 143Nd/144Nd end-member (Fig. 5a) that couldpossibly be represented by a garnet micaschist xenolith hosted inthe San Martino porphyry. Based on this evidence, Gagnevin et al.(2004) suggested that wall-rock assimilation played a significantrole in shaping the composition of both the pluton and the en-

110 F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115

Fig. 7. Initial 87Sr/86Sr data for the porphyry, the pluton and the dyke. Mineral-scale initial isotope ratios are plotted over a relative time scale representing the crystallizationsequence. The initial Sr isotope ratios as well as error bars are from Table 1. The lines illustrate the evolution of Sr isotope ratios in the three units. The dashed ellipses showthe Sr isotope variability for megacryst cores and rims from Gagnevin et al. (2005a). Whole rock data for the Elba Island intrusive complex as well as rhyolites from theTuscan Magmatic Province are from Dini et al. (2002), Gagnevin et al. (2004) and Farina et al. (2010). The dark gray horizontal bars represent the compositions calculatedfrom the disequilibrium melting model presented in Table 3. A schematic illustration of the model proposed is presented in the lower part of the figure. With the termsMELT A, B, C, D and E we indicate melts formed by different fluid-absent reactions and thus characterized by contrasting Sr isotopic compositions as modeled in Table 3.These melts mix in different proportions giving rise to the magma batches from which the different phases crystallized. The batches are indicated with 1, 2 and 3 for boththe San Martino porphyry (SMp) and Monte Capanne pluton (MCp) systems. Abbreviations: Bt biotite, Pl plagioclase, Kfs K-feldspar. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

claves. Gagnevin et al. (2005a, 2005b) interpret the variably high87Sr/86Sr and 208Pb/204Pb composition recorded in the pluton byK-feldspar megacryst cores as the result of wall-rock contamina-tion, with this process occurring before K-feldspar crystallizationand after a first hybridization event involving a mantle-derivedcomponent. The decoupling between Sr and Nd isotopes recordedwithin K-feldspar megacrysts is at odds with this interpretation.In fact, if wall rock assimilation had been the process responsiblefor the high 87Sr/86Sr in megacryst cores, the same process wouldhave also produced a coupled decrease in 143Nd/144Nd; i.e. the iso-topic composition of megacryst cores would be pointing towardthe composition of the garnet-bearing xenolith. This is not ob-served (Fig. 5a). Moreover, early wall-rock assimilation is not ableto account for the extreme 87Sr/86Sr recorded by biotite crystalsincluded in K-feldspar megacrysts as the garnet-bearing xenolithsdescribed by Gagnevin et al. (2004) as well as micaschists from theTuscan basement (Dini et al., 2005) are typically less Sr-radiogenicthan the analyzed biotites. Our conclusion is that wall-rock assimi-lation did not control the early Sr and Nd isotopic evolution of thestudied units. This process probably played only a minor role inthe whole evolution of the Elba Island complex.

5.4. Magma mixing: a crustal perspective

The whole rock Sr and Nd isotope variability exhibited by theElba Island intrusive complex has been interpreted as the result ofmixing between crust- and mantle-derived magmas (e.g. Dini et al.,2002), with the 87Sr/86Sr core-to-rim decrease within megacrystsrecording progressive hybridization (Gagnevin et al., 2005a). Thecrustal, acidic end-member of mixing is considered a magma akinto the Cotoncello dyke and San Vincenzo rhyolites (Gagnevin et al.,2004); these rocks display the highest 87Sr/86Sr ratio of the wholeTuscan Magmatic Province (Fig. 5a). Fingerprinting the chemicaland isotopic signature of the mantle-derived, mafic component ishindered by the fact that microgranular enclaves, considered torepresent relict evidence of mixing with a mantle derived magma,are significantly hybridized (SiO2 typically greater than 62 wt%;Farina et al., 2012). Four rocks with potential to represent themantle end-member in the Tuscan Magmatic Province are tested:Pliocene lamproites from mainland Tuscany (Conticelli et al., 2009)and shoshonitic basalts as well as late Miocene latites and highK-andesites/dacites from Capraia Island, 40 km north-west of ElbaIsland (Data Repository). The lamproites are not a suitable maficend-member as their 87Sr/86Sr (ca. 0.716; Fig. 5a) is higher than

F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115 111

the Sr isotope ratio of the pluton. Moreover, mixing lamproitemelt (K2O ca. 8 wt%; Conticelli et al., 2009) with a Cotoncello-likeanatectic melt (K2O ca. 5 wt%) would have resulted in a positivecorrelation between K2O and MgO + FeO, while the opposite isobserved (Farina et al., 2012). Shoshonitic basalts (a minor productof Capraia Island volcanism) could generate the Sr and Nd iso-tope composition of the Monte Capanne pluton if added (25–35wt%) to a crust-derived end-member (Fig. 5a). However, mixingwith such a percentage of the mafic end-member would gener-ate hybrid magmas significantly higher in MgO + FeOtot and Mg#(i.e. Mg/(Mg + Fe)) than both the San Martino porphyry and theMonte Capanne pluton (Fig. 5b). Capraia Island latites have Nd iso-tope composition similar to that of the three studied units (ca.0.5122) and thus, mixing of the acidic end-member with a latite-like component would generate a sub-horizontal mixing trend inthe Sr–Nd isotope plane. This trend would account for the de-coupling between Sr and Nd isotopes recorded within K-feldsparmegacrysts, i.e large core-to-rim 87Sr/86Sr variation associated touniform 143Nd/144Nd. A latite-like end-member of mixing havehowever to be discarded based on major element evidence. Boththe Capraia Island latites and the Cotoncello dyke have lower Al2O3and higher K2O/Na2O than the pluton, hence, the composition ofhybrid granitoids produced by mixing between these two mag-mas would be remarkably lower in Al2O3 and higher in K2O/Na2Othan that of the pluton (Fig. 5b). A further indirect argument ar-guing against mixing between crust- and mantle-derived magmassimilar to Capraia latites relates to the Sr and Nd isotopic composi-tion of mafic microgranular enclaves. These enclaves exhibit largelyvariable 143Nd/144Nd associated to modest variation in 87Sr/86Sr(Gagnevin et al., 2004), defining a steeply dipping trend in Fig. 5a.This is in contrast with the sub-horizontal mixing trend producedby a latite-like mafic end-member.

The best mantle-derived candidates for a two-component mix-ing hypothesis, based on major and trace element as well asisotopic arguments, are the Capraia Island high-K andesites anddacites, as already suggested by several authors (Poli, 1992;Dini et al., 2002; Gagnevin et al., 2004). This conclusion is sup-ported by the 87Sr/86Sr and 143Nd/144Nd composition of the onlygabbroic enclave hosted in the Monte Capanne pluton (Gagnevin etal., 2004), which is close to the isotopic composition of high-K an-desites, as well as by the isotopic composition of Orano dykes,which extend towards a similar mantle-derived end-member(Fig. 5a). Although a high-K andesitic mantle-derived magmamight have been involved in the late chemical evolution of thestudied units (Farina et al., 2012), two lines of evidence sug-gest that mixing with this magma cannot account for the Srisotope variability recorded within and among mineral phases inthe Monte Capanne pluton and Cotoncello dyke. The first of theseis the decoupling between Sr and Nd isotopes recorded withinmegacrysts. The 87Sr/86Sri core-to-rim decrease in the megacrystscould be in principle produced by adding about 10 wt% of aCapraia-like andesitic component to an initially more radiogeniccrustal magma end-member having the 87Sr/86Sr recorded bymegacryst cores (Fig. 5a). However, because Capraia andesites anddacites have a substantially higher 143Nd/144Nd than crustal melts,such mixing would produce a concurrent increase in core-to-rim143Nd/144Nd. This is in contrast with the internal Nd isotopic ho-mogeneity characterizing the megacrysts. This does not definitelyrule out the involvement of a mantle-derived component of mix-ing, as it is still theoretically possible to obtain a mixing hyperbolawhose curvature describes, for the mixing percentages of interest(<20% of mafic component), large variations in 87Sr/86Sr associateto narrow changes in 143Nd/144Nd. Indeed, the curvature of themixing hyperbola is controlled by a numerical value r (Langmuir etal., 1978) that depends on the ratio between the Sr/Nd of the twoend-members (i.e. (Sr/Nd)a/(Sr/Nd)b with a and b representing the

composition of the acidic and mafic end-members, respectively).For values of r approaching 0.1 the concave-up hyperbola defines,at low degrees of mixing with a mantle-derived component, largevariation in 87Sr/86Sr coupled with homogeneous 143Nd/144Nd(Fig. 5a). Capraia andesites have Sr/Nd ranging from 8.6 to 19.6,thus an r value of 0.1 is obtained if the acidic component hasSr/Nd <2. The Roccastrada rhyolites display the highest Sr/Nd be-tween the rocks of the Tuscan Magmatic Province (Pinarelli et al.,1989), thus representing a suitable end-member. However, magmamixing between these rhyolites and the andesites is unable to re-produce the Sr and Nd isotopic signature of megacrysts nor toaccount for the whole rock 143Nd/144Nd composition of the pluton(Fig. 5a).

The importance of mantle-derived magmas during the earlystages of evolution of the units is also questioned by the similarmajor-element compositions of biotites formed at different timeswithin the pluton and the dyke. For these rocks, biotites includedin megacryst crystallized from a high 87Sr/86Sr magma (87Sr/86Sr≈0.732) while matrix biotites grew after this magma has experi-enced hybridization with 25–40 wt% of an andesitic component.Thus, the major element composition of the two biotite-type isexpected to be different (e.g. different Mg/Fe ratio) reflecting thechange in composition of the hosting magmas. In contrast withthis predicted behavior, the biotites included in the megacrystsof the pluton and the dyke, have nearly identical compositions tothose of the matrix biotites of their respective host rocks (Fig. 6).A notable exception is represented by the facies of the pluton dis-playing low megacryst abundance. In this rock, matrix biotites havelower Fe# (Fe# = Fe/(Fe + Mg)) than those crystallized earlier andincluded in the megacrysts. This evidence, together with the over-all lower whole rock 87Sr/86Sr as well as slightly “less-evolved”chemical composition (lower SiO2, higher MgO and CaO) of thisfacies (Farina et al., 2010), suggests that mantle-derived magmasmay have played a role in the petrogenesis of this rock. However,because the early isotopic evolution is identical for the three fa-cies of the pluton, we propose that mixing with mantle-derivedmagmas is a late, rather chemically inefficient process producingspectacular mingling textures: the mafic enclaves. The occurrenceof xenomorphic resorption surfaces associated to Ba zoning withinthe K-feldspar megacrysts of the pluton support dissolution and re-growth of the feldspars in higher temperature magmas (Gagnevinet al., 2005a, 2005b). However, this evidence does not imply mix-ing with mantle-derived magmas as similar disequilibrium featuresmay form by convection within individual magma bodies (Couch etal., 2001) or by interaction with magma batches of crustal origin.

The Sr isotopic evolution characterizing the San Martino por-phyry (Fig. 7) can be reconciled with a model of multiple interac-tion between crust- and mantle-derived magmas. In fact, biotitesincluded in the megacrysts of this unit display 87Sr/86Sri valuesthat are similar to the composition of Capraia high-K andesitesand can thus be regarded as crystallized from an evolved mantle-derived magma. Accordingly, the higher Sr isotopic ratio recordedby megacryst cores reflects hybridization with a more radiogeniccrustally-derived magma and the megacryst 87Sr/86Sri core-to-rimdecreases is produced by a further mixing event involving new in-puts of a mantle-derived magma. Although such a model cannotbe ruled out based on isotopic arguments, there is no evidence ofinvolvement of mantle-derived magmas reflected in the major andtrace element composition of the porphyry that has an average sil-ica content of 70 wt% and is characterized by a positive correlationbetween MgO + FeO and peraluminousity (Farina et al., 2012). Inaddition, the composition of biotites included in the megacrysts ishigh in Fe# (0.54–0.57) and identical to that of those in the matrix(Fig. 6).

In the following section, we explore the possibility that the ex-treme Sr isotope heterogeneity recorded at the crystal-scale in the

112 F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115

studied units is produced by discrete events of recharge of a tem-porary reservoir with crustally-derived magmas having different Srisotopic signatures, but similar Nd isotope ratios. It is worth re-marking that the data presented here are used to dispute the roleof mantle-derived magmas in the early stages of evolution of thethree systems and as major rock forming process, while we donot dismiss the late involvement of mantle-derived magmas in thegenesis of the Elba Island granitic complex.

5.5. Disequilibrium melting

Fluid-absent reactions involving the breakdown of micas andamphiboles, either alone or in combinations, produce graniticmelts and coexisting peritectic assemblages (Patiño Douce andHarris, 1998). When heating during the pre-anatectic metamorphicevent, as well as melting and extraction of the magma from thesource are rapid enough, complete isotopic equilibration betweenrock-forming phases in the source and between the solid residuumand the melt in the anatectic rock may not be attained (i.e. dis-equilibrium melting). The degree to which equilibrium is attainedvaries depending on many factors such as the presence or absenceof fluids during metamorphism, the nature and dynamic of fluidsas well as on the duration of the equilibration period (e.g. Farinaand Stevens, 2011). Moreover, the attainment of equilibrium alsodepends on the chemical species considered as diffusion rates ofdifferent elements in different minerals vary over many orders ofmagnitude at any given temperature. In the sense used here, dis-equilibrium melting refers to a partial melting event where themelt was not in Sr isotopic equilibrium with the residuum priorto extraction. Although this process has been documented duringcontact melting, i.e. in cases where the source–melt relationship isunambiguous (e.g. McLeod et al., 2012), its relevance during crustalanatexis is not established and this process not considered as anexplanation for the existence of isotopic heterogeneity in granitoidrocks. Two lines of evidence support the likelihood of Sr disequi-librium melting during crustal anatexis (Farina and Stevens, 2011).Firstly, Rb–Sr mineral isochrons in medium- to high-grade regionalmetamorphic rocks such as amphibolites, granulites and eclogites(e.g. Cliff and Meffan-Main, 2003), commonly yield ages that areolder than the established age of metamorphism, implying thatthese systems were not completely reset during the metamorphicevent. Thus, in high-grade rocks, that did not experience fluid-assisted re-crystallization, isotopic resetting demonstrably does notlead to full isotopic homogenization. Additionally, evidence fromgranites and migmatites indicates that melting and magma extrac-tion from crustal sources can occur rapidly enough so that traceelement and isotopic equilibration between liquid and residualphases is not achieved (e.g. Zeng et al., 2005). When Sr disequi-librium melting prevails, the Sr isotope composition of the meltdepends on (i) the time elapsed since the last isotopic equilibra-tion, (ii) the proportions of the Sr-bearing phases consumed bythe melting reaction and (iii) the extent of isotopic re-equilibrationbetween melt and residuum prior to melt segregation (Harris andAyres, 1998).

Recently, Farina et al. (2012) suggested that the San Mar-tino porphyry was formed through biotite fluid-absent melting ofmetasedimentary rocks containing a significant proportion of Al-rich clays (e.g. metapelites), while the Monte Capanne pluton wasgenerated by coupled biotite and amphibole fluid-absent meltingof layered sources. It is worth noting that both these potentialsource rocks characterize the low- to medium-grade metamor-phic Tuscan Paleozoic basement (e.g. Dini et al., 2005). Muscovite-and biotite-rich micaschists and gneisses are an important compo-nent of the poorly exposed Tuscan basement, these rocks locallycontaining layers of hornblende bearing amphibolites (Puxeddu etal., 1984). These interlayered metapelite–amphibolite rocks, con-

sidered by Farina et al. (2012) as possible sources for the MonteCapanne magmatic system, crop out in the eastern part of Elba Is-land and were sampled from boreholes in mainland Tuscany.

The possibility that mineral-scale Sr isotope variations at ElbaIsland are produced by disequilibrium melting occurring throughdifferent reactions of the inferred sources is explored here. The Srisotope behavior of muscovite, biotite, hornblende and plagioclaseis considered, as these phases are the main Sr- and Rb-bearingphases in the inferred sources and represent the key reactants inthe fluid-absent melting reactions by which granite magmas arise.These phases are assumed to have attained the same 87Sr/86Sr iso-topic composition during the latest significant metamorphic eventthat affected the area (Variscan times, ≈300 Ma) and to haveevolved in isotopic isolation until they underwent partial melt-ing (7 Ma). The 87Sr/86Sr that the mineral phases attained at thetime of melting has been calculated by means of the equation de-scribing the decay of radioactive parent 87Rb to the stable daugh-ter 87Sr. Their final 87Sr/86Sr is positively correlated with their87Rb/86Sr and is consequently high for biotite, intermediate formuscovite and low for plagioclase and hornblende (Table 3). Thecalculated Sr isotope composition of the reactant phases is usedto determine the 87Sr/86Sr of melts modeled considering changesin the stoichiometry of the melting reaction as anatexis progresses.No partial re-equilibration by diffusion between the mineral phasesof the source is considered in the calculation. The isotopic com-position of the melt (87Sr/86Srmelt) has been calculated from thestoichiometry of the melting reaction, by means of the followingequation:

87Sr/86Srmelt =(∑

x jC j87Sr/86Sr j

)/∑x jC j,

where x j is the weight proportion of mineral j (one of the reac-tant in the melting reaction), C j and 87Sr/86Srj are the concentra-tion of Sr and the isotopic composition of mineral j, respectively.The stoichiometries used are derived from the fluid-absent melt-ing experiments of Patiño Douce and Harris (1998) and Monteland Vielzeuf (1997) as described in Farina and Stevens (2011). Itis worth noting that the Sr isotopic composition of the melt ismainly dependent on the 87Rb/86Sr composition assumed for themain reactant phases as well as on the stoichiometry of the melt-ing reaction and, in particular on the ratio of biotite to plagioclaseconsumed. In addition, the model accounts for the Sr isotope vari-ability preserved in the mineral record (Fig. 7), if crystallizationbegins within discrete magma batches that accumulate to formtransient reservoirs prior to magma emplacement.

5.5.1. The San Martino porphyryIn the San Martino porphyry, biotites included in the megacrysts

crystallized from a melt (87Sr/86Sr ≈ 0.710) produced by fluid-absent partial melting of both biotite and muscovite (Melt A,Fig. 7, Table 3). The increase in 87Sr/86Sr occurring in the timegap from the crystallization of these biotites and the growthof megacryst cores reflects the progression from muscovite- tobiotite-dominated dehydration melting reactions in the source;with the muscovite-derived component in the magma decreasingfrom 85 wt% to 30 wt% (Melt B, Fig. 7, Table 3). This progressionis consistent with experimental studies showing that, at interme-diate crustal depths (i.e. ≤1 GPa), muscovite fluid-absent melt-ing begins at lower temperature than biotite fluid-absent melting(Patiño Douce and Harris, 1998). In addition, Pickering and John-ston (1998) suggested that the higher temperature portions of themuscovite melting interval overlap with the conditions required forbiotite melting (i.e. biotite and muscovite melt concurrently) forpressures close to 1 GPa. In the porphyry, the core-to-rim decreasein 87Sr/86Sr within megacrysts records changes in the stoichiom-etry of the biotite fluid-absent melting reaction and, in particular

F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115 113

Table 3Pre-emplacement Sr isotopic evolution of the porphyry (A), the pluton and the dyke (B). In A1 and B1 the time-evolved Sr isotopic composition of the minerals forming theinferred sources is calculated. In A2 and B2 the Sr isotopic composition of the melts is calculated using the relative proportion of reactant phases in the melting reactionas in Farina and Stevens (2011). In A3 and B3 the isotopic composition of the magma batches from which biotite included in the megacrysts and K-feldspar megacrystscrystallized is generated by mixing the different melts obtained in A2 and B2.

A-the San Martino Porphyry (source: muscovite- and biotite-bearing micaschists)A1 – Evolution of Sr isotope composition of reactant phases

Phase Rb Sr 87Sr/86Sr (300 Ma) 87Sr/86Sr (7 Ma)a

Biotite 600 7 0.706 1.73030Plagioclase 10 600 0.706 0.70620Muscovite 130 320 0.706 0.71085

A2 – Sr isotope composition of anatectic meltsb

Reference T(◦C)

P(GPa)

Mica/Pl 87Sr/86Sr MELT

PD&H 1998 820 10 2.71 0.70895 MELT AM&V 1997 858 10 1.54 0.72602 MELT BReactionc ≈ 880 0.692 0.71523 MELT C

A3 – Pre-emplacement isotopic evolution of San Martino porphyry

MELT A wt% MELT B wt% MELT C wt% 87Sr/86Sr Crystallizing phase

Batch SMp 1 85 15 0 0.71043 Bt (inclusions)Batch SMp 2 30 70 0 0.71844 Kfs megacryst coresBatch SMp 3 15 35 50 0.71653 Kfs megacryst rims

B-the Monte Capanne pluton and the Cotoncello dyke (source: biotite-bearing micaschists intercalated with horneblende-bearing layers)B1 – Evolution of Sr isotope composition of reactant phases

Phase Rb Sr 87Sr/86Sr (300 Ma) 87Sr/86Sr (7 Ma)

Plagioclase 5 700 0.706 0.70609Horneblende 20 70 0.706 0.70941Biotite 600 7 0.706 1.73030

B2 – Sr isotope composition of anatectic meltsb

Reference T(◦C)

P(GPa)

Hbl/Pl 87Sr/86Sr

M&V 1997 885 10 2.13 0.73343 MELT DReactiond 900 1 0.70638 MELT E

B3 – Pre-emplacement isotopic evolution of Monte Capanne pluton and Cotoncello dike

MELT D wt% MELT E wt% 87Sr/86Sr Crystallization

Batch MCp 1 100 0 0.73343 Bt (inclusions)Batch MCp 2 65 35 0.71895 Kfs megacryst coresBatch MCp 3 50.7 49.3 0.71516 Kfs megacryst rims

a The decay constant used for 87Rb is 1.42 × 10−11. The following equation has been used to calculate the atoms of 87Sr formed by the decay of 87Rb in the time intervalconsidered: 87Sr = 87Rbt0 − (87Rbt0 × e−λt ), where t0 is 300 Ma.

b Calculated from the stoichiometry of the melting reaction.c Reaction simulating a source going toward biotite consumption.d Reaction simulating horneblende fluid-absent melting; the Sr isotope composition of the melt is not strongly dependent on the stoichiometry of reaction because

plagioclase and hornblende have similar 87Rb/86Sr and thus similar Sr isotope composition at the time of melting.Abbreviations: Bt biotite, Hbl horneblende, Kfs K-feldspar, M&V 1997 Montel and Vielzeuf (1997), PD&H 1998 Patiño Douce and Harris (1998), Pl plagioclase.

the decrease in the ratio of biotite to plagioclase consumed bythe melting reaction as temperature increases (Melt C, Fig. 8).Farina and Stevens (2011) have shown, using the experimentaldata of Montel and Vielzeuf (1997) that for temperatures higherthan 875 ◦C the biotite to plagioclase ratio consumed in the melt-ing reaction declines towards the point of biotite disappearance.This behavior has been predicted by the experiments of Knesel andDavidson (1996) and observed for natural glasses formed by partialmelting of granites by intrusion of basalts (Knesel and Davidson,1999).

5.5.2. The Monte Capanne pluton and the Cotoncello dykeThe high Sr isotope ratios of biotites of the pluton and the

dyke record the composition of an initial magma produced at atemperature of approximately 850 ◦C by biotite fluid-absent melt-ing (Melt D, Fig. 7, Table 3). Subsequently, this starting magmacomposition is progressively hybridized by mixing with magmasproduced by amphibole-dominated dehydration melting reactionsas the temperature in the source increased to reach approximately

900 ◦C (batch MCp 2 and 3, Fig. 7). Hornblende is characterized bylow 87Rb/86Sr, resulting in a minor increase in 87Sr/86Sr throughtime. Therefore, the effect of hornblende fluid-absent melting in arock that is already undergoing anatexis by biotite breakdown is tolower the 87Sr/86Sr of the melt. The involvement of amphibole-derived magmas in the pluton is also indicated by the positivelinear correlation between CaO and MgO + FeO. This correlation,according to Farina et al. (2012), results from the entrainment ofperitectic clinopyroxene, mineral that is produced by incongruentmelting of hornblende in rocks of intermediate and mafic com-position. A mixing event involving mantle-derived magmas havingthe composition of high-K andesites from Capraia Island occurredlate in the evolution of the Monte Capanne pluton. We suggest thatthis process, recorded by the occurrence of microgranular enclaves,produced the minor whole-rock geochemical variability exhibitedby the three facies of the pluton defined by Farina et al. (2010). Inthis scenario, the megacryst-rich and megacryst-poor facies, whichslightly differ in silica content and Sr isotope ratios, are interpretedas variable outcomes of the hybridization process.

114 F. Farina et al. / Earth and Planetary Science Letters 399 (2014) 103–115

The more Sr-radiogenic whole rock composition exhibited bythe Cotoncello dyke with respect to the pluton suggests the in-volvement of a late input of highly radiogenic magma possibly pro-duced by a biotite-rich source. The pre-emplacement isotopic evo-lution of the two systems, recorded within K-feldspar megacrystsand their included biotites, is very similar. The only significant dif-ferent is the Sr isotopic composition of the rim of the K-feldsparmegacrysts that is higher for the megacryst in the dyke (0.7174)than for the one in the pluton (≈ 0.715). This feature, togetherwith field evidence of mixing-mingling between the pluton andthe dyke suggests that K-feldspar megacryst start crystallizing inthe Monte Capanne magmatic system and were successively in-corporated in the dyke at depth, when their cores were alreadygrown.

K-feldspar megacrysts from the pluton and the dyke displayhomogeneous initial 143Nd/144Nd composition (i.e. 0.51219) indi-cating that disequilibrium partial melting of the inferred layeredbiotite- and amphibole-bearing source generates melts having con-trasting Sr isotope signatures, but identical Nd isotope ratios. Thedecoupling between the two isotopic systematics during partialmelting matches with the model proposed. The 143Nd/144Nd com-position of anatectic melts is primarily dependent on the behaviorof accessory phases such as apatite and monazite, which containmost of the total REE fraction in the majority of crustal rocks(Ayres and Harris, 1997). The rate at which these accessories ac-quire a different Nd isotopic composition is much slower than therate of Sr isotopic divergence between biotite and plagioclase fortwo reasons. Firstly, due the overall similar chemical characteris-tic of Sm and Nd, the Nd/Sm ratios of apatite and monazite arealike, with Sm/Nd in monazite and apatite in the range 0.1–0.2 and0.2–0.5, respectively. On the other hand, biotite and plagioclasehave contrasting Rb/Sr ratios; i.e. typically greater than 100 for bi-otite, less than 1 for plagioclase. Secondly, the half time of decayfor Sm is one order of magnitude longer than that for Rb. There-fore, in a same time interval the Sr isotopic signature of biotiteand plagioclase diverge substantially, whereas the Nd compositionof apatite and monazite remains more similar.

At last, it is worth remarking that our model demonstrates thetheoretical possibility of generating large Sr isotopic variations bypartial melting of individual sources without the involvement ofjuvenile mantle magma(s). On the other hand, the possibility oftesting the model is hampered by two factors: (i) the source ofthe Elba Island magmatic complex is actually unknown and, al-though its composition can be inferred based on the chemicalcharacteristics of the magmatic complex, its isotopic compositionand degree of homogenization before partial melting cannot beassessed; (ii) partial melting experiments are not designed to de-scribe changes in the stoichiometry of the melting reaction occur-ring in response to stepwise melt extraction from a source sub-jected to progressively higher temperature, thus the stoichiometryused here may substantially depart from those relevant to describerepeated melt extraction events from individual sources.

6. Implications

The proposed model opens new perspectives in the inter-pretation of mineral-scale isotope heterogeneity within felsic ig-neous rocks. The extreme 87Sr/86Sri variability characterizing theElba Island granitic complex at the mineral scale can be ex-plained by mixing between magma batches produced by dise-quilibrium melting of individual crustal sources. These magmabatches represent discrete melting events taking place as theisotherms advance through the source: the earlier magmas rep-resent lower-temperature melts while magmas developed laterformed at higher-temperature. Crustal melts leave the source inbatches that are rapidly extracted and collected in a relatively deep

reservoir where they mix and start crystallizing before final em-placement. A prime implication of this study is that isotope varia-tions in granitoids do not necessarily call for the involvement of amantle-derived component, but may form by reworking of crustalmaterial, with no net crustal growth. Partial melting of discretecrustal volumes has been demonstrated to generate isotopicallyheterogeneous melts at sub-millimeter scale (McLeod et al., 2012).However, such initial heterogeneity could homogenize during seg-regation, storage, ascent, and emplacement of granitoid magmas.The intrusive complex at Elba Island suggests that this may notalways be the case: extreme isotopic variability, generated duringcrustal anatexis, can survive magma segregation and ascent, shap-ing the isotopic composition of minerals grown at different timesand thereby yielding an invaluable insight into the processes ofmagma generation in the crust.

Acknowledgements

We are grateful to J. Miller and an anonymous reviewer for thecritical and useful reviews provided. We thank T.M. Harrison forthe editorial guidance offered. This paper has been supported byfunding from the project PRIN2008PN8Z9K to SR and AD.

FF is grateful to S. Tonarini for skilful assistance during theanalytical work in the thermal ionization mass spectrometry labo-ratory.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.05.018.

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