Julien Figeac

Watching Mobile TV in Constrained Telecommunication Networks

Keywords: media consumers, reception, mobile TV, multimedia cell phone; mobility

This article shows how users adopt Mobile TV in situations of mobility by choosing TV programmes that are adapted to the size of their mobile phone screens and other constraints of daily commuting, including telephone network reception problems, short time slots, etc. It illustrates how users limit "places" where it is relevant to use Mobile TV in urban environments, in accordance with mobile telephony availability, and consequently, how they re-define the places where older forms of media are used. Consumers use media, including multimedia cell phones, and contents suitable to the resources and constraints encountered during their travel times. This article thus identifies the renewal of media preferences and diverse behavioural factors in connection to telecommunication networks. It describes how this type of engagement in media activities relates to a form of strategic media attachment consisting in exploiting utilisation opportunities in augmented urban environments.

In many countries, users have adopted Mobile TV devices because they appreciate easy and continuous access to TV contents [START_REF] Kaasinen | User acceptance of mobile TV services[END_REF]. In France, few users frequently watch the fifty television channels broadcast by multimedia cell phones. Political control of the telecommunications sector has had a significant impact on the development of this device and this business. It is especially linked to the problems of communications network availability in urban environments and public transport. This explains why this service is consulted as much at home as on the move. For example, teenagers have become accustomed to watching television programmes on their mobile phone in their room, using a wireless connection, when the TV set of the house is not available. Certain users also associate this service with their daily media practices because they use public transport systems to go to their workplace. In addition to listening to music or reading, they wish to be entertained differently when travelling, for example, by watching television.

The goal of this paper will be to analyse these consumers and describe how users develop strategies to overcome important use constraints in order to watch Mobile TV when commuting. Several studies have dealt with these questions. They showed that numerous constraints hinder users from transferring television preferences to mobile phones in situations of mobility. For example, the small size of mobile phone screens [START_REF] Chipchase | Personal TV: A Qualitative Study of Mobile TV users[END_REF], the short time slots in which they are used and the noise of urban environments [START_REF] Södergard | Mobile Television -Technology and User Experiences, Report on the Mobile-TV project[END_REF] finally prompt users to select programmes that are not at the centre of their television preferences. It has been observed that these use constraints linked to Mobile TV contribute to polarizing this televisual consumption towards the news because these programmes can be watched over a short period of time [START_REF] Knoche | Good News for Mobile TV[END_REF][START_REF] Oksman | News in Mobiles. Comparing text, audio and video[END_REF]. More generally, this factor explains why televisual consumption aligns with users' preferences in terms of usability of Mobile TV devices [START_REF] Fleury | Evaluating user preferences for video transfer methods from a mobile device to a TV screen[END_REF].

In the continuity of this report, this article will describe how users develop strategies to adopt Mobile TV viewing and adapt their choice of television consumption to mobility constraints. It is not possible to be satisfied in saying that they use this service to avoid being bored, from a desire to relax or let go, or out of simple curiosity or habit without any particular preference [START_REF] O'hara | Consuming Video on Mobile Devices[END_REF]. Users employ a strategy in selecting a TV programme that can be watched on the small screens of their mobile phone over short periods while commuting. They denote a "focus, a suspension, a pause in what is happening" [START_REF] Hennion | Pragmatics of taste[END_REF] to select a specific television genre [START_REF] Corner | Meaning, genre and context: the problematic of 'public knowledge' in the audience studies[END_REF] and to evaluate the selected TV programme. Through these strategies, they evaluate which programme is relevant to watch under these practical circumstances. They evaluate which television preferences are adapted to this new screen, which ones correspond to mobility constraints. Thus they review and renew their television preferences.

Contrary to a structuralist vision of media practices, the paper will show how users assume control of the renewal of their media practices and how the practical circumstances and the ecological dimension of daily activities intervene in the dynamic of media consumption. The article will also show how technical mediations play an active role in the daily renewal of media practices. Indeed, the principal use constraint encountered when travelling is the problem of phone network availability. First of all, the paper will show how users strategically overcome this constraint by transforming it into a resource enabling them to explore new contents and new forms of entertainment.

The structuring effects of telecommunication network availability

This article will study a specific usage constraint/resource: the availability of a phone network. It is a fact that the connectivity of this network is constraining as it makes the use of Mobile TV possible or not. And sometimes, the unavailability of the phone network forces users to switch off a programme, in particular in underground public transport. In "Ubiquitous computing" [START_REF] Weiser | The Computer for the Twenty-First Century[END_REF], from a theoretical point of view, which happens to have preceded the current developments of this service, technologies must be invisible: they must be embedded in usage environments and they must work in the background of the user's awareness. This invisibility of technical mediations can be observed in the uses of Mobile TV because the phone network waves are not perceptible to users. It is the mobile phone that manages the connection to this invisible technical network. In addition to this invisibilization of technologies, "ambient intelligence" [START_REF] Dey | A conceptual framework and a toolkit for supporting the rapid prototyping of context-aware applications[END_REF] proposes to develop technologies able to determine, by inference, what a user does in a context, or tries to do, in order to offer him use of the best-adapted service. Mobile services are not (yet) able to produce such usage opportunities in proposing the relevant service to use depending on the activity of the user, the type of phone network available, the force of the signal, the place, the time of day, etc. These adjustments must be completely made by the users. They must evaluate, among the media supports and the services available during their journey, which use is relevant in regards to the practical circumstances.

This path of research can be explored in the prolongation of "Ubiquitous computing" by observing how users resort to various opportunities of usage offered to them while commuting. The objective is to pinpoint how the invisible work of technologies develops urban environments (HARRISON & DOURISH, 1996;[START_REF] Dourish | Re-Space-ing Place: 'Place' and 'Space' Ten Years On[END_REF] by changing the way people generally move in cities and use public transport. The city can be divided into multiple territories according to their technological equipment and the diversity of usage opportunities it produces. For example, railway stations form territories rich in opportunities because they are equipped with multiple communication networks: radio, telephone, GPS and, more recently, Wi-Fi hotspots. It thus becomes interesting to describe how users demarcate "places" (HARRISON & DOURISH, 1996;[START_REF] Dourish | Re-Space-ing Place: 'Place' and 'Space' Ten Years On[END_REF] in urban environments by territorializing their various media activities around the usage opportunities produced by technology. This article will show how users delimit places to use Mobile TV. The users who replace reading the press with watching Mobile TV will move differently in urban environments. They must evaluate the phone network reception in order to watch a televised programme. To do so, they will decode the graphic indicators of his mobile phone interface. Even if these indicators show the availability of Mobile TV at this precise moment, they do not provide any information on its evolution during the itinerary. As this technological resource does not enable them to coordinate its uses with the itinerary effectively, the users must extract the indications that deliver information on the (future) availability of Mobile TV from the urban environment. The indications they will exploit are those they use to follow their routes (the name of the stations, the indicators of places, etc.). It is through them that they will identify the relevant "places" to watch Mobile TV. This paper focuses on the availability of the phone network to describe how users proceed to evaluate the connectivity of this technical mediation and, consequently, to evaluate the relevance of Mobile TV use in these practical circumstances. The paper will show how users demarcate "places" dedicated to the use of Mobile TV while describing how they index their television reception on the availability of this technical mediation. Through these two parameters of Mobile TV and its territories of use it will be shown how this service supplements, or to replaces, the media activities previously developed by the participants in their trajectory of utilization.

Methodology

The results are taken from a study made with a sample of 15 Mobile TV subscribers. This sample is primarily constituted of technophiles, i.e. users from 25 to 45 years old, college graduates, with a professional occupation in a large, French conglomeration in Paris.

To observe their uses in situations of mobility, we carried out ethnographic observations using camera glasses. These enable the analyst to record the evolution of the subjects' visual field in a specific situation and capture their engagement from their subjective perspective [START_REF] Lahlou | How can we capture the subject's perspective? An evidence-based approach for the social scientist[END_REF]. Several groups have also used them to study mobile phone uses in natural settings [START_REF] Mark | A methodology using a micro-camera for studying mobile IT usage and person mobility[END_REF][START_REF] Relieu | The 'glasscam' as an observational tool for studying screen-based mobile phone uses and management of parallel activities[END_REF], either alone or coupled, to log data [START_REF] Zouinar | Observation and capture of multimodal interaction in mobile situations[END_REF]. In this research, the users were to film their journeys in public transport for one week. The video database is based on round trips made between their residence and their workplace. It represents approximately 80 hours of recording.

These video recordings enabled to describe how travel directs Mobile TV uses and, reciprocally, how the methods of using this service orient how one travels in urban spaces. To show engagement in this form of multi-activity, first the movements of users in public transport are described: their bust and head movements, their visual commitments and their quick glances [START_REF] Sudnow | Temporal parameters of interpersonal Observation[END_REF]. While describing how a user stops looking in the direction of his telephone screen to look at the platform or the name of a station, shows how his attention is diverted from the activity of following his route to focus on the television reception activity. Then, when describing how he once again looks at the screen, which was kept within glancing range, the sequential organisation of this form of multi-activity is recomposed to show how the distribution of visual engagements, between the uses and the itinerary, takes the form of a succession of "logical" operations, preceded by body positioning.

To describe engagement in this form of multi-activity, the participants were shown the video recordings of their activities. The objective was to have them explain their intentions and their tactics, using the method of "self-confrontation interviews" [START_REF] Theureau | Le cours d'action. Méthode développée[END_REF]. This method was developed by ergonomists. It leads the user to discover the parameters contained in the situational ecology orienting his practices. Using the recordings, he can clarify his activities as he discovers them. This is why self-confrontation interviews constitute a resource in understanding how media activities and television tastes are positioned. Interviews, questionnaires and logbooks are not very useful in understanding the ecological setting of media activities. The ethnographic observations are also too far from the participants' activities to make it possible to observe these settings. For example, with them it is impossible to understand how users articulate reading a newspaper and Mobile TV reception around information from their mobile phone network indicators. Only the video recordings, coupled with self-confrontation interviews, show the action of these technological and ecological variables in the uses and in the reconfiguration of media activities.

The organisation of media practices using communication network availability

As the members of the sample chose to watch Mobile TV while commuting, they partially reconfigured their old media practices. This phenomenon is explored in examining the case of Linda, a 42-year old receptionist, because she develops the most complex media strategies, perfectly characteristic of the practices observed in the sample. The method based on video recordings invites the authors to finely tune the analysis of this one case in order to describe in detail the spatiotemporal organisation of uses during daily commuting. Instead of multiplying the examples as part of a quantitative approach, the goal here is effectively to report on contextual strategies and practical details of the organisation of media practices with one example. These detailed descriptions allow the contextual settings to be described that structure the practices of other users in the sample and, more generally, the appropriation of Mobile TV devices.

In describing the video recordings of Linda's uses, it will be shown how she re-organised her media activities by alternating her reception of various media (press, radio and Mobile TV) according to phone and radio network receptivity. These descriptions then illustrate how she redirects her media activities four times, during the 80 minutes between her residence and her workplace, according to the reception or the unavailability of telecommunication networks (see diagram 1 in the Appendix). The objective is now to describe the sequential organisation of her media practices.

Ever since she has been able to watch television on her mobile phone, Linda has not performed the same ritual before crossing the "threshold" (THIBAUT, 1994) of her residence because she no longer spends her travel time reading. She relies on her new telephone in order to replace reading magazines with watching Mobile TV. Even if she no longer takes magazines with her, she still reads while commuting. She now picks up the free daily newspapers that other passengers have left on the seats of the tube. Consequently, in the morning she enters the underground train searching for a seat where there is a free daily newspaper (photo 1).

Once she has found a newspaper, she does not read it immediately. She keeps it on her lap in order to read it later on (photo 2). In other words, she carries out a "preparatory gesture" (DATCHARY & LICOPPE, 2007): she keeps this media resource and thus shows her intention to use it at another time (during this trip or, later, at her workplace or at home). It is important to realize when and in which practical circumstances this anticipation of reading the daily newspaper will take place.

Once seated, she immediately takes her telephone to watch "Télématin", a channel France 2 newscast (photo 2). During the selfconfrontation interview, she says that she continues watching television, an activity that she had begun during breakfast with her husband and children. Furthermore, she likes to immediately watch Mobile TV because her favourite television programme (the TV news "Télématin") begins at the exact same time she takes the Metro. In regularly taking this route, she has learned that she will not be able to watch Mobile TV at the end of fifteen minutes, when the train runs beneath Paris. Consequently, she watches Mobile TV as soon as she sits down in the train because she likes this TV news programme and knows that it will be impossible for her to use this mobile service afterwards. Indeed diagram 1 (in Appendix) shows how this receptionist watched Mobile TV for 13 minutes between the moment she took the train (8.00 A.M) and the moment she did not have access to the phone network, just after the "Télématin" TV news. It is relevant to understand how this user anticipates the unavailability of the phone network to switch off Mobile TV before using another media. How is this anticipation deployed in regards to the practical circumstances of which it takes advantage? Photo 1: Linda picks up a newspaper Photo 2: Linda watches a newscast before sitting down This question will be answered using the video recordings because they allow the description of the sequential organisation of this transition between these two media activities, i.e. the end of Mobile TV reception and the beginning of her reading the daily newspaper. The goal here is to show which elements of this usage situation the receptionist perceives as relevant in operating this transition and how this transition is indexed on the availability of the phone network.

Pragmatic strategies for dealing with the availability of telecommunication networks

The transcription of the video recording of her trip to work (diagram 1) allows to answer these questions. Once seated in the underground, she watches the televised news while keeping a free daily newspaper on her lap.

The connection to the programme lasts approximately 2 minutes 40 seconds between the moment she launches it and the moment the TV news appears on the screen. She watches this programme for one minute before losing the connection. She needs one additional minute to restore it. She watches it again for four minutes and 44 seconds before losing the Mobile TV connection once again.

This second disconnection occurs 30 seconds after the underground leaves the station "Bibliothèque François Mitterrand" ("BNF"). During the self-confrontation interview, she specifies: "Here, it cannot be picked up any more (Mobile TV) so I listen to the radio". That proves that she had anticipated the unavailability of Mobile TV because she identified this area as a borderline: "after the BNF nothing can be picked up any more". When she crosses this borderline in going to the office, she says she stops using Mobile TV before launching the "radio" function of her mobile phone that she will listen to while reading a newspaper.

However, the transcription of this journey shows that she tries to connect once again to Mobile TV even if the underground has left station BNF for 45 seconds, in other words once she crossed this borderline. The launching of the connection lasts approximately one minute until she can watch the televised news again. But, as she could have predicted, she succeeds in watching it for only seven seconds before the programme is cut off. It is only after this ultimate attempt that she decides to listen to the radio with the FM Tuner of her mobile phone. As usual, she activates the radio to listen to it while she reads her free newspaper. She opens the daily newspaper 20 seconds later when the underground arrives in the Austerlitz station and when the Mobile TV reception becomes definitively impossible. It is consequently in these practical circumstances that she reactivates the reading activity that she had left on standby 13 minutes before in a preparatory gesture.

The transcription of this sequence reveals better why she was hesitant during the self-confrontation interview: "it picks up often in fact, it picks up until the BNF, from there, after passing the BNF, it does not pick up, near Austerlitz nothing can be picked up any more". She knows that connection with Mobile TV will be difficult at the BNF station, but she is not completely sure. But she is certain she cannot launch Mobile TV when she is in the following station, Austerlitz. Why is it so difficult for her to identify the exact moment when the phone network is no longer available? And, consequently, why does she try day after day to restore connection to this service in an area where phone reception is difficult?

It appears in fact that she does not try to optimize these transitions between Mobile TV, the radio and the newspaper, even if she has identified the borderline where she will be forced to change media activities. The imprecision of this border is not a problem for her. On the contrary, this imprecision enables her to test, day after day, the technological mediations which connect her to media activities: she continues to handle her telephone to test it; to understand whether the connection problems are related to her telephone or the phone networks; to spot whether the telephone operator has deployed the Mobile TV network in this area; to check if this service is usable in the underground systems in Paris; to re-evaluate the borders between Paris (where Mobile TV is unavailable in underground systems) and the suburb where she lives (where this service can be consulted in overland transport). This contribution will now describe how she tests these technological mediations, which frame her media activities, before showing, in the last part, how this test leads her to re-qualify the places of her media practices [START_REF] De | The Practice of Everyday Life[END_REF]HARRISON & DOURISH, 1996;[START_REF] Dourish | Re-Space-ing Place: 'Place' and 'Space' Ten Years On[END_REF].

Strategies related to the network indicators

Users can evaluate the intensity of the phone network with two indicators posted on mobile phone screens. The first indicates the type of phone networks to which the telephone is connected (EDGE, 3G, 3G+). The second indicates the intensity of the signal of this phone network.

Using the video recordings allows to observe that Linda is connected to Mobile TV when these indicators show her that this service is available. When she is in an area where she knows from experience that Mobile TV is unavailable, she will try to launch this service if these indicators show her that the phone network is (temporarily) available. For example, during the self-confrontation interview, she said she had identified the fact that Mobile TV was unavailable between the BNF and the Austerlitz station. The phone network indicators, located at the top left of the screen (photo 3), clearly indicated the loss of the signal to her when the TV news stopped (even if network indicators are not really visible in these screenshots, they are clearly distinguishable from video recordings). Just after-wards (photo 4), these indicators showed her that the network was available again. Consequently, she tried to connect to Mobile TV even if she knew that this service is usually unavailable at this place. This graphic information made connection to the service relevant under these practical circumstances.

Photo 3: Loss of reception Photo 4: Reception picked up again Photo 5: Loss of reception

In the same way, when the service stopped after this last connection, these indicators showed once again that phone networks were unavailable (photo 5). Thus they contributed to dissuading her from using Mobile TV. They encouraged her to read a newspaper and use the radio function on her mobile phone.

These indicators form relevant support to evaluate under which circumstances she can (or cannot) watch Mobile TV. However, they indicate to users that reception is not possible once the service is blocked, the programme stopped, the image immobilized. This information appears too late to allow users to anticipate this unavailability of Mobile TV. The limit of these indicators is to reveal instantaneous information, relating to telephone signal reception at a precise moment, without specifying the degree of receptivity of the signal in the minutes to come. This information would be useful for a user who would like to watch Mobile TV in the Metro. An indicator could provide this information to the user if his trip was traced, the orientation of his journey predictable, his stops in the stations anticipated and the speed of his movement calculated to indicate to him how long he would be in this area where Mobile TV is available. In other words, no technological mediation can fulfil this function (legally).

Consequently, users must seek other resources to anticipate the evolution of telephone signal reception to evaluate if it is relevant or not to start watching Mobile TV under these circumstances. To anticipate the connectivity of this invisible technical mediation, they identify the environmental indications that give information on telephone signal fluctuations. That will be describe in the following section.

Strategies related to the ambient environment

In addition to the indications given by network indicators, users seek indications in the surrounding environment that help them anticipate the availability of the phone network at moment T+1. Then they attribute the information given by the telephone interface to the place where they are.

A large variety of information can be used to identify a place. It is difficult to categorize all the architectural elements, or other indications, making it possible for a user to know where he is in his daily trip. For example, it is not possible for us to identify the various indications from which Linda identifies the BNF station. However, it is through them that she can identify the places where she will be able to watch Mobile TV and to define the borders where she must use another media. If it is impossible to study all the parameters that enable her to identify the places of use for Mobile TV, it is possible to isolate one. The person who often takes the same route can move while watching Mobile TV. She manages her activities using the perceptible light around the screen of her telephone. Like all users, Linda thinks that connection to Mobile TV is possible when public transport runs above ground. On the other hand, she thinks that connection is interrupted when transport moves underground. Consequently, she relies on this indication formed by light variations in order to direct her media activities: J.F.: "And there you are watching TV… L.: Exactly, there the RER is outside so I start watching TV."

If we take another look at her last connection to Mobile TV in light of this indicator, it appears that she begins this use when the underground has been above ground for more than 10 seconds. However, when connection with TV news is established, the underground has just entered a tunnel. Consequently, when the reception of this programme stopped seven seconds afterwards, this indicator dissuaded her from attempting connection once again because the train had moved underground. This example illustrates how external light is used as an indicator, which is both relevant and problematical, regarding phone network availability.

First of all, this example shows how variations in luminosity, between the light of day and the semi-obscurity of tunnels, can influence the deployment of various uses. When the user is focused on reception, he can very easily locate these variations. In this example, Linda does not move her eyes from her telephone screen. She does not need to look outside to find her location because she can identify, through the rays that light the underground train, her departure from the BNF station. Day after day, she sees in these light variations an indicator showing phone network availability. This supplements information given by network indicators while enabling her to anticipate Mobile TV connectivity at the precise moment T+1, i.e. when the RER train heads above ground. This example also shows how this information that users extract from external light is problematic. The fact that the RER moved above ground supplemented the information delivered by network indicators, leading her into error, because she could not watch the programme for more than seven seconds. For this reason, the participants test the phone network availability day after day because no indicator enables them to precisely anticipate these interruptions in connection. Uncertainty involved in the appropriation of Mobile TV shows us that the "logic" behind utilization proceeds from indications extracted from the situational ecology. However, these indications direct the course of action towards a model that differs from Gibson's theory of perception [START_REF] Gibson | The ecological approach to visual perception[END_REF]. Gibson defined the concept of "affordance" as a quality of an object, or an environment, which allows an individual to perform an action while considering the information given by an affordance directs the single activity of a person who is moving. Consequently, the light rays of an affordance overlap a coupling of perception and action without the mediation of reflexivity being necessary; this is the case when, for example, a door handle is grasped.

In this case, variations in luminosity in the underground train direct two activities at the same time. The participants make use of these variations to exploit various uses and choose their itinerary. Luminosity transmits information on the phone network availability for the person who, in an urban environment, intends to manage these two activities simultaneously. This indicator is not useful for a user who is in a stopped train because information from telephone network indicators is enough for him. The relevance of this indicator comes from this mode of involvement in this form of multi-activity. It gives information to the participants who manage their journey on a peripheral level of attention, around the Mobile TV screen on which they are focused. The light gives information to the user who awaits its appearance because he makes use of it like a reference mark that determines the orientation of its uses. Consequently, mobile TV reception is embedded in a form of multi-activity because the indicators used to manage it are also used to manage the journey.

The appropriation of mobile TV creates new forms of mobility

This contribution is based on the description of the commuting of a participant between her residence and her workplace. Since she has started watching Mobile TV, she does not wait for the underground train in the same station as the one she chooses to return home in the evening. Before using this service, she waited for the underground on the Boulainvilliers station platform. During this wait, she read a newspaper or a magazine. She let the trains that went in other directions of the underground line go by and took the one that stopped at her destination.

When she started using Mobile TV, she reconsidered the itinerary because the telephone network and the radio network were unavailable in this station. Now she enters the first train that stops at this station, no matter what underground line it serves. Then, she makes an intermediate stop in another station to take another train that stops at her destination. She selected this intermediate stop according to Mobile TV availability. As the phone network is available in the aboveground stations, she chose to change trains at the "Champs de Mars" station near the Eiffel Tower. As this station offers a beautiful view of Paris, she can take advantage of the panorama while watching her favourite television programme (photo 7).

She thought of taking this initiative once she had the opportunity of watching Mobile TV. To use this service, she reconsidered her itinerary by modifying her habits. She reconfigured her trip around her attachment to television programmes and phone network availability. In this manner, this itinerary leads to increased mobility [START_REF] Licoppe | Emergent Uses of a Location-aware Multiplayer Game: The Interactional Consequences of Mediated Encounters[END_REF] through the use of new technologies. The journey is no longer calculated according to a purely strategic logic of connecting two destinations as quickly as possible. This search for performance is secondary compared to the pleasure felt watching television because she is entertained during the time spent in transport. This is why she now reorganizes commuting time around the requirements that must be respected in order to watch Mobile TV. It is her engagement in this form of multi-activity that made these two communication networks, the phone network and the transport system, converge around this particular place. The place where she stops to watch Mobile TV (photos 6 and 7) is closely linked to this form of multi-activity. She stands near the arrival and departure board to be able to keep an eye on the arrival of the train (photo 6). Through her preoccupation with the activity which she will soon focus on (entering the train), she defines the temporal horizon of the reception activity in this intermediate place. This example shows once again how the activities of reception and commuting overlap to form a multi-activity, because the control of the first activity interferes with controlling the second, and vice versa.

The uses of Mobile TV modify the choice of intermediate stations and the way in which these places are used. It is interesting to see how these places will affect, retroactively, the uses of this service: L.: "There I stop at the Champ-de-Mars station. In fact, I could have continued to the BNF station, but I preferred to stop, the weather is nice and all, and what's more, I could watch TV. […] But it is not only because of that, it is also because the station is outside and the weather is nice. So I can watch TV while soaking up the sun. J.F.: And that is why you turned and faced the landscape? L.: Yes, it is. Here, facing the sun, facing the landscape, the Seine River… In spite of the noise of cars and the train…" She appropriated this "place" (HARRISON & DOURISH, 1996;[START_REF] Dourish | Re-Space-ing Place: 'Place' and 'Space' Ten Years On[END_REF] because it allowed her "to watch TV facing the sun […], facing the landscape, the Seine River… In spite of the noise of cars and the train…" Consequently, she did not choose this place only because the communication networks were accessible but also for its "atmosphere" (THIBAUD, 2002;[START_REF] Thomas | Architectural and urban atmospheres: shaping the way we walk in town[END_REF]. She said during the interview that the perception of the sunshine in this place appears to her, retrospectively, as the main reason for which she left the underground to continue watching TV on the platform: "the weather is nice and all, and what's more, I could watch TV".

This example shows that the atmosphere of the places where Mobile TV is used is important in understanding its appropriation. Light is not only an indicator of phone network reception. The bright sunlight over-exposes this place. It makes it prominent, on a practical level, and makes it conspicuous, on a subjective level, as a place of exposure allowing her "to get some sun".

It is necessary to show how technical mediations converge with the atmosphere of the places so that they can be suitable as "places" to use Mobile TV. If it is relevant to continue the paradigm of increased mobility, while describing how the uses of Information and Communication Technologies (ICT) transform urban mobility, it is also necessary to describe how the "emotional tonalities" (THIBAUD, 2002;[START_REF] Thomas | Architectural and urban atmospheres: shaping the way we walk in town[END_REF] of urban environments influence the uses of these technologies. These emotional tonalities do not have direct impact on the televisual content choices. However, they enter into forming media tastes because they induce pleasures which bind a user to a media practice and affect his value judgments: to enjoy one of her favourite television programs "facing the sun, facing the landscape, the Seine… In spite of the noise of cars and the train… ".

Conclusion

Analysing video recordings of Mobile TV uses in daily commuting shows how the users strategically organize their practices with the resources of telecommunication networks and the constraints of mobility settings. Users redefine their itineraries in public transport to be able to watch their favourite TV programmes. They exploit, as strategic users, the availability of the phone network by stopping at the stations where Mobile TV reception is available. They adapt to this constraint in preserving their old media practices to be used during the temporal gaps during which Mobile TV reception is impossible. Consequently, this technical mediation partially structures contemporary forms of increased mobility. It contributes to generating new itineraries in urban environments by directing Mobile TV users towards certain stations, by defining the phases and "places" (HARRISON & DOURISH, 1996;[START_REF] Dourish | Re-Space-ing Place: 'Place' and 'Space' Ten Years On[END_REF] where the uses of various media become relevant. These mediations fulfil this function if users give them this capacity, if they exploit them as strategic users to renew their media activities during their journeys. This analysis based on video recordings shows how the appropriation of a mobile device depends on the availability of cellphone networks and, especially for applications requiring the highest bandwidth capacities, how this connectivity must be understood in relation to the means of transport taken by users. This shows how the usability of such devices must be studied in relation to natural settings of daily commuting in order to account for the structuring effects of usage constraints. As it appears here with the French example, this study reminds one of how business models of mobile phone applications should also include constraints related to telecommunication infrastructure variables because these problems tend to persist despite the optimization of networks, especially when users take the underground transport system. From this point of view, an intercultural comparison is particularly relevant because it shows how the usability of a device can actually vary from one country to another depending on the availability of cell-phone networks in a specific urban environment. Business models must also include dominant patterns of mobility in each country as they are structured around the individual and collective means of transport.

  Photo 6: Linda stands near the departure board

Appendix

Diagram 1 -Linda's trip between her residence and her workplace


Nicaise Blaise

Tchuimegnie Ngongang

Pierre Kamgang

Gilles Chazot
email: gilles.chazot@univ-brest.fr

Arnaud Agranier

Hervé Bellon

Philippe Nonnotte

Kamgang Pierre

Chazot Gilles

Agranier Arnaud

Bellon Hervé

Nonnotte Philippe

Age, geochemical characteristics and petrogenesis of Cenozoic intraplate alkaline volcanic rocks in the Bafang region, West Cameroon

Keywords: Cameroon Volcanic Line, Geochemistry, Magmatism, Mantle source

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Introduction

The role of the lithospheric mantle in the petrogenesis of continental basalts is difficult to evaluate, as its nature and composition are known to be highly variable, and sometime not easily distinguishable from deeper mantle sources. The genesis of intraplate alkaline basalts remains therefore a matter of considerable debate. This volcanism is the subject of many geochemical investigations on large continental flood basalt (CFB) provinces as well as smaller volcanic fields [START_REF] Jung | Major and trace element systematics and isotope geochemistry of Cenozoic mafic volcanic from the Vogelsberg (Central Germany): constraints on the origin of continental alkaline and tholeiitic basalts and their mantle sources[END_REF][START_REF] Zou | Major, trace element, and Nd, Sr, and Pb isotope studies of Cenozoic basalts in SE China: mantle sources, regional variations, and tectonic significance[END_REF][START_REF] Barry | Petrogenesis of Cenozoic basalts from Mongolia: evidence for the role of asthenospheric versus metasomatized lithospheric mantle sources[END_REF].

The contamination of the asthenospheric mantle by lithospheric material, recognized in the geochemistry of some CFB, is classically accounted for by lithospheric delamination (e.g. Mc-Kenzie and O'Nions, 1995) or ancient subduction processes (e.g. Hofmann, 1988). However, there is increasing evidence that melts originated in the asthenosphere can interact with the lithospheric mantle en route to the surface [START_REF] Chazot | Determination of partition coefficients between apatite, clinopyroxene, amphibole, and melt in natural spinel lherzolites from Yemen: implications for wet melting of the lithospheric mantle[END_REF][START_REF] Wulff-Pedersen | The upper mantle under La Palma, Canary Islands: formation of Si-K-Na rich melt and its importance as a metasomatic agent[END_REF][START_REF] Class | Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy[END_REF] and lead to these intermediate geochemical signatures. The Cameroon Volcanic Line (CVL) appears as a SW-NE straight line (Fig. 1) displaying a ''swell and basin'' geometry. It is a chain of 12 Cenozoic and Quaternary volcanic massifs covering approximately 1600 km from Annobon Island in the Gulf of Guinea to Lake Chad, and active from Eocene to the Present (Déruelle et al., 2007 and references therein). The chain can be splitted into a continental and an oceanic part, which is a unique feature for Africa and even for the world [START_REF] Déruelle | The Cameroon Line: a review[END_REF][START_REF] Déruelle | The "Cameroon Hot Line" (CHL): a unique example of active alkaline intraplate structure in both oceanic and continental lithospheres[END_REF]. In more details three distinct zones can be identified : 1-the oceanic sector (Annobon, Sao Tome and Principe), 2-the continent/ocean boundary (c.o.b.: Bioko, Etinde and Mt. Cameroon) and 3the purely continental sector. This last part of the chain is constituted of tens to a hundred of kilometers-scale volcanic massifs including Manengouba, Bambouto, Bamenda and Oku mountains as shown on Figure 1. Early on, [START_REF] Fitton | The Cameroon Line, West-Africa, and its bearing on the origin of Oceanic and Continental alkali basalt[END_REF] recognized geochemical similarities between trace elements and Sr isotopes of basalts from both oceanic and continental sectors. Based on these data, they suggested that these melts were extracted from sub-lithospheric depths without interaction with the overlying lithosphere. Later on, [START_REF] Halliday | Mantle dynamics: a Nd, Sr, Pb and O isotopics study of the Cameroon line volcanic chain[END_REF][START_REF] Halliday | Traceelement fractionation in plumes and the origin of HIMU mantle beneath the Cameroon line[END_REF] reported anomalously high 206 Pb/ 204 Pb ratios (up to 20.5) for lavas from the c.o.b., with lower 206 Pb/ 204 Pb ratios (19)(20) for volcanic rocks from the oceanic and continental sector. These results, combined to Sr, Nd and O isotopes, suggested that the high 206 Pb/ 204 Pb signatures are due to the recent remelting of the St. Helena fossil plume head within the lithospheric mantle, after that U/Pb fractionation occurred during melt migrations as the plume head was cooling down about 125 Myr ago. As a consequence, in this model, the Pb isotope anomaly observed in volcanic rocks from the CVL, cannot be derived from metasomatized lithosphere. Moreover, this fossil plume model is consistent with a recent helium isotopes study of [START_REF] Aka | Symmetrical helium isotope distribution on the Cameroon Volcanic Line,West Africa[END_REF], who reported MORB-like 3 He/ 4 He ratios (from 7.85 to 8.31Ra, where Ra is the atmospheric ratio) for melts from both oceanic and continental sectors associated with HIMU-OIB-like He ratios for the c.o.b (from 4 to 6Ra). In this paper, we present new K-Ar ages, major and trace element concentrations and Sr-Nd-Pb-Hf isotopes of mafic lavas from Bafang in western Cameroon in order to further investigate magma generation in this area.

2-Geological setting

The Bafang area is located within the Western Cameroon highlands (Bambouto, Bamenda and Oku Mountains) in the central part of the Cameroon Volcanic Line (Fig. 2). It lies between the Manengouba mountain in the South-West and the Noun Plain in the North. The studied area is geographically bounded by the latitudes 05°4'21'' and 05°13'38'' North and the longitudes 10°8'17'' and 10°19'38'' East and represents a surface of about 500 km 2 . In this region, the Pan-African basement consists mainly of syntectonic granitoids [START_REF] Dumort | Cartes géologiques de reconnaissance à l'échelle de 1/50000[END_REF][START_REF] Nguiessi | Rb-Sr, Pb-Pb, U-Pb dating in the Bandja plutonic series of western Cameroon[END_REF][START_REF] Nguiessi | A calc-alkaline magmatic complex from Bandja in the northequatorial fold belt: a synkinematic emplacement of plutonic rocks in a sinistral strike-slip shear zone from Pan-African age[END_REF][START_REF] Nzolang | Geochemistry and preliminary data on the Neoproterozoic granitoids from the Bantoun area, West Cameroon: evidence for a derivation from a Paleoproterozoic to Archean crust[END_REF]. The lavas sampled in the Bafang area have mafic to intermediate composition, and no felsic rocks have been observed on the field. In this region, the vegetal cover is dense and the samples were therefore mostly recovered from artisanal quarries and small disseminated outcrops.

Accordingly, no spatial continuity or stratigraphic relationship can be established between the samples.

3-Analytical methods

3.1-K-Ar geochronology

Potassium-argon ages were measured at the Université de Bretagne Occidentale, in Brest (France) on chips of whole rock lavas, 0.3 to 0.15 mm in size, that were prepared after crushing and subsequent sieving of the solid samples.

One aliquot of grains was powdered in an agate grinder for chemical attack of around 0.1 g of powder by 4 cc of hydrofluoric acid, before its analysis of K content by AAS (Atomic Absorption Spectrometry).

A second aliquot of grains was reserved for argon analysis. About 0.3 g to 0.8 g of grains were heated and fused under vacuum in a molybdenum crucible, using a high frequency generator. Released gases during this step were cleaned successively on three quartz traps containing titanium sponge when their temperature was decreasing from 800°C to the ambient one during 10 minutes; at the final step the remaining gas fraction was ultra-purified with an Al-Zr SAES getter.

Isotopic composition of argon and concentration of 40 Ar R were measured in a stainless steel mass spectrometer with a 180° geometry. Isotopic dilution was realized with the fusion step, using for this process precise concentrations of 38 Ar buried as ions in aluminium targets [START_REF] Bellon | Implantation ionique d'argon dans une cible support: application au traçage isotopique de l'argon contenu dans les minéraux et les roches[END_REF].

3.2-Major and trace element analyses

Whole-rock major elements were measured on the Horiba -Jobin-Yvon ® Ultima 2 ICP-AES at the IUEM (European Institute for Marine Studies, Pôle de Spectrométrie Océan, Brest, France). The detailed description of the analytical procedure is given in [START_REF] Cotten | Origin of anomalous rare-earth element and yttrium enrichments in subaerially exposed basalts: evidence from French Polynesia[END_REF]. Major elements were determined from an H 3 BO 3 solution, boron being used as internal standard for ICP-AES analysis. For major elements, relative standard deviation is 1%

for SiO 2 and 2% for the other major elements, except for low values (<0.50 wt. %), for which the absolute standard deviation is ±0.01 wt. %.

Trace element concentrations were measured with a Thermo Element2 HR-ICP-MS in Brest (France), after a repeated HF-HClO 4 digestion, and HNO 3 dilutions (see Li and Lee, 2006 for details). The repeated analysis of the international standard BCR2 demonstrated an external reproducibility better than 5-10% depending on the element and concentration.

3.3-Sr-Nd-Pb-Hf isotopic analyses

Pb and Hf isotope compositions were obtained from single HF-HNO 3 dissolutions of 500mg-1g of sample following the protocol described by [START_REF] Blichert-Toft | Geochemical segmentation of the Mid-Atlantic Ridge north of Iceland and ridge-hot spot interaction in the North Atlantic[END_REF]. Pb and Hf isotopes were analysed using the Thermo Neptune MC-ICP-MS of Ifremer-Brest. 2σ internal errors never exceeded the last significant digit (1E -5 ) in each analysis.

Sr

4-Nomenclature and petrography

4.1-Nomenclature

All the major oxides have been normalized to 100% on an anhydrous basis. Data are plotted on the total alkali vs. silica diagram (Le [START_REF] Bas | A chemical classification of volcanic rocks based on the total alkali-silica diagram[END_REF] with subalkaline-alkaline dividing line of [START_REF] Irvine | A guide to the chemical classification of the common volcanic rocks[END_REF] for the purpose of classification (Fig. 3). According to this diagram, all the samples have alkaline affinity. Based on whole rock major element chemical compositions the Bafang samples range in composition from basanite (more than 10% normative olivine), basalt and hawaiite to mugearite. In the Na 2 O vs. K 2 O diagram of [START_REF] Middlemost | The basalt clan[END_REF] shown in Figure 3, the Bafang mafic lavas plot mainly in the Na-series field (Na 2 O/K 2 O = 1.50-2.79), and are characterized by high alkalinity index ((Na 2 O+ K 2 O) -(0.37 * SiO 2 -14.43) from +1.49 to +5.15).

4.2-Petrography

Basanites and basalts have a microlitic porphyritic texture. The groundmass is made of plagioclase, olivine, clinopyroxene and Fe-Ti oxides. They contain large olivine and clinopyroxene phenocrysts (up to 2 mm) and some rare plagioclases. Some olivine crystals present evidences for iddingsitisation. Quartz xenocrysts with wavy extinction jacketed by clinopyroxenes corona, and alkali feldspar with disequilibrium texture occasionally occur in the basanites. Some samples contain green-core clinopyroxenes.

Hawaiites are subaphyric to porphyritic with a groundmass dominated by plagioclase microlites. Phenocrysts are mainly clinopyroxene and Fe-Ti oxides except in some samples where large plagioclases with concentric zoning (from 5 to 9 mm in length) are present.

Mugearites are microlitic with a groundmass made of plagioclase, olivine, clinopyroxene, and euhedral microcrystals of apatite. One sample contains micro-phenocrysts of clinopyroxene and plagioclase.

Results

Whole-rock major and trace element compositions are given in table 1.

Major element geochemistry

As no felsic rocks have been discovered on the field, our samples cover a moderate range in SiO 2 composition (Fig. 4), ranging from 43 to 53 wt. %, but a wider range of Mg-number 4, these major element trends are compatible with an evolution of the magmas through the fractionation of olivine, clinopyroxene and Fe-Ti oxides.

Trace elements geochemistry

The samples have highly variable compatible element contents. For example, Cr St Helena, but with higher REE concentrations ([La] up to 180 times the concentration in chondrites), more akin to those of the Mount Cameroon volcanic rocks [START_REF] Yokoyama | Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa: constraints from 238 U-230 Th-226 Ra and Sr-Nd-Pb isotopes systematics[END_REF].

In Figure 5, all samples have positive europium anomalies (Eu/Eu* = 1.00 -1.29, where Eu is normalized Eu and Eu* is Eu interpolated between normalized Sm and Gd: Eu*= (Sm N xNd N ) 1/2 ). Nevertheless, phenocrysts of plagioclase are almost absent from these rocks and thus, their accumulation cannot be responsible for this excess of Eu and this feature should probably be accounted for by a source effect. On a wider primitive-mantle normalized spiderdiagram of Figure 5b, Bafang rocks still resemble OIB from St. Helena, but with higher trace element concentrations, similarly to Mt. Cameroon [START_REF] Yokoyama | Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa: constraints from 238 U-230 Th-226 Ra and Sr-Nd-Pb isotopes systematics[END_REF]. However, samples from Bafang have some geochemical peculiarities: low Th and U concentrations and high Ba values. In some samples, mostly basalts and basanites, positive Sr anomalies can also be recognized.

Geochronology

Six samples have been dated by the K-Ar method. The Ar isotopic data and calculated ages for the samples are listed in table 2. Resulting ages are distributed between 10 and 6 Ma and show three chronological stages of volcanic activity during the Upper Miocene: between 10.46 and 9.57 ± 0.25 Ma with the emplacement of basanites, at 8.47 ± 0.26 Ma and between 6.89 and 6.27 ± 0.17Ma, with the outpour of basalts and mugearites.

These data show that mafic volcanism exists in the Bafang area over a long period of time without any spatial evolution; they can be compared and agree with a few ages of volcanism in the neighboring sectors: 10.46 ± 0.28 Ma in the Noun Plain for Tchanda Boro volcano [START_REF] Wotchoko | Données pétrologiques et géochronologiques nouvelles sur le volcanisme alcalin néogène à recent de la rive oust du Noun (plaine du Noun, Ligne du Cameroun[END_REF]; 9.37 ± 0.05 Ma, a plateau age for a flow on the south-eastern border of Manengouba [START_REF] Pouclet | The Mount Manengouba, a complex volcano of the Cameroon line: volcanic history, petrological and geochemical features[END_REF] 6.5 ± 0.3 Ma [START_REF] Nana | Le complexe volcano-plutonique de Bana (Ouest Cameroun : géologie et pétrologie[END_REF] for Mont Bana, and 6.05 ± 0.05 Ma for Bambouto Mountains [START_REF] Youmen | Evolution volcanologique, pétrologique et temporelle de la caldera des monts Bambouto (Cameroun)[END_REF].

Sr, Nd, Pb and Hf isotope geochemistry

Sr, Nd, Pb, Hf isotope analyses are provided in Table 3 Compared with both oceanic and the rest of the continental part of the CVL, Bafang volcanic rocks display a larger range of Sr isotopic compositions, with values higher than 0.7038 for two basalts, two hawaiites and one mugearite (Fig. 6a). As a whole, the trend defined by Bafang lavas is parallel to the trend defined by the Bamenda lavas (just North to this study area) [START_REF] Kamgang | Mantle sources and magma evolution beneath the Cameroon Volcanic Line: Geochemistry of mafic rocks from the Bamenda Mountains (NW Cameroon)[END_REF], but with higher 143 Nd/ 144 Nd. Bafang volcanic rocks have similar Hf and Nd isotopic compositions than the oceanic part of the CVL (not illustrated here), excepted for the most differentiated samples (hawaiites and mugearites) with least radiogenic Nd and Hf signatures. In Pb isotope diagrams (Fig. 6b) the Bafang lavas also lie close to the oceanic part of the CVL, with one hawaiite and three basanites having slightly less radiogenic Pb composition, similar to some contaminated samples from the Bamenda volcanic field (Kangang et al., 2013).

Discussion

Fractional crystallization

As shown before, primary melts in equilibrium with mantle peridotites are assumed to have Mg# values of 68 -72 and high Ni (300 -500 ppm) and Cr (~1,000 ppm) contents [START_REF] Frey | Integrated models of basalts petrogenesis: a study of quartz tholeiites to olivine melilites from South Eastern Australia utilizing geochemical and experimental petrological data[END_REF][START_REF] Tatsumi | Generation of basalt magmas and the thermal structure of the mantle wedge in subduction zones[END_REF]. Even if basanite sample BAF 45 is not far from these values (Mg#=60.43, Ni=261ppm and Cr=516ppm), all the samples, even the most mafic ones, encountered crystal fractionation at some point.

As suggested on figure 4, major and compatible trace element variations are compatible with a fractionation of olivine, clinopyroxene and Fe-Ti oxide in the course of the magma differentiation. The fractionation of olivine is indicated by decreasing MgO and Ni, while fractionation of clinopyroxene is indicated by decreasing of CaO and Cr with decreasing Mg# (Fig. 4e-f). The decrease of TiO 2 , visible only for the most differentiated basalts, indicates a step of Fe-Ti oxides crystallization during the magma evolution (Fig. 4b).

The constant increase of Al 2 O 3 during nearly all the differentiation indicates that plagioclase was not an important crystallizing phase during the fractional crystallization processes. This is also consistent with the absence of Eu or Sr depletion even in the most differentiated rocks (Fig. 5a).

The relatively wide age range covered by Bafang volcanic rocks (from 10.5 to 6.3 Ma)

implies that the geochemical diversity reported here cannot be explained by a single differentiation trend but by several distinct magmatic series evolving independently, thus precluding any modelling of the fractional crystallization processes.

Crustal contamination

Some of the geochemical characteristics of Bafang volcanic rocks can be accounted for by interactions between melts and crustal rocks during the magma ascent toward the surface and its storage in magma chambers, since crustal contamination is an efficient way of modifying trace-element and isotopic composition [START_REF] Mcdermott | The petrogenesis of magmas from Mt. Bulusan and Mayon in the Bicol arc, the Philippines[END_REF][START_REF] Zellmer | Magma evolution and ascent at volcanic arcs: constraining petrogenetic processes through rates and chronologies[END_REF]. The crust along the studied part of the CVL is made of Panafrican metasedimentary units and granitoids, but Paleoproterozoic to Archean lower crustal units have also been recognized [START_REF] Marzoli | The Cameroon Volcanic Line revisited: petrogenesis of continental basaltic magmas from lithospheric and asthenospheric mantle sources[END_REF]Kwekam et al., 2010). Former studies have already suggested that contamination of the primary magmas by mature and thickened crust is important along the Cameroon Volcanic Line [START_REF] Halliday | Mantle dynamics: a Nd, Sr, Pb and O isotopics study of the Cameroon line volcanic chain[END_REF][START_REF] Marzoli | Silicic magmas from the continental Cameroon Volcanic Line (Oku, Bambouto and Ngaoundere) : 40 Ar-39 Ar dates, petrology, Sr-Nd-O isotopes and their petrogenetic significance[END_REF][START_REF] Rankenburg | The role of the continental crust and lithospheric mantle in the genesis of Cameroon volcanic line lavas: constraints from isotopic variations in lavas and megacrysts from the Biu and Jos Plateaux[END_REF][START_REF] Kamgang | Géochimie et géochronologie des laves felsiques des Monts Bamenda NW -Cameroun (Ligne du Cameroun)[END_REF][START_REF] Kamgang | Geochemistry and geochronology of mafi c rocks from Bamenda Mountains (Cameroon): Source composition and crustal contamination along the Cameroon volcanic line[END_REF][START_REF] Kamgang | Mantle sources and magma evolution beneath the Cameroon Volcanic Line: Geochemistry of mafic rocks from the Bamenda Mountains (NW Cameroon)[END_REF]. For example, this has already been clearly demonstrated for the felsic and mafic rocks from the Bamenda volcanic province nearby based on trace elements and isotopes [START_REF] Kamgang | Géochimie et géochronologie des laves felsiques des Monts Bamenda NW -Cameroun (Ligne du Cameroun)[END_REF][START_REF] Kamgang | Geochemistry and geochronology of mafi c rocks from Bamenda Mountains (Cameroon): Source composition and crustal contamination along the Cameroon volcanic line[END_REF][START_REF] Kamgang | Mantle sources and magma evolution beneath the Cameroon Volcanic Line: Geochemistry of mafic rocks from the Bamenda Mountains (NW Cameroon)[END_REF]. The effect of the CC can be recognized on figure 7, by the correlation between the MgO contents and 87 Sr/ 86 Sr and between the silica and 143 Nd/ 144 Nd ratio. In basanites, the extent of this CC appears to be limited, since their isotopic compositions are barely modified. Some silica-saturated samples

Mantle sources

The lithospheric mantle is relatively cold (<1300°C) and conductive and is often isotopically enriched (high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd values) because of its long isolation from the convective mantle and with the percolation of magmatic melts. In contrast, the asthenospheric mantle is hot and convective and is geochemically depleted, due to the extraction of crusts at subduction zones and the mid ocean ridges. One hint toward understanding whether melts were extracted from lithosphere or asthenosphere is to define if partial melting occurred in garnet or spinel stability field. If on one hand, spinel was present as a residual phase in the source, the lithospheric origin is attested. On the other hand, the presence of garnet would attest that partial melting occurred at a pressure greater than 3 GPa [START_REF] Robinson | The depth of the spinel to garnet transition at the peridotites solidus[END_REF], which corresponds to about a 70-80 km depth, while the present day (about 10 Ma after Bafang volcanic activity) depth of the lithosphere/asthenosphere boundary lies below 120 km in this area [START_REF] Fishwick | Surface wave tomography: Imaging of the lithosphere-asthenopshere boundary beneath central and southern Africa?[END_REF]De Plaen et al., 2014). Residual garnet results in HREE depletion relative to LREE due to the strong retention of HREE in this mineral (e.g. [START_REF] Green | Proton microprobe determined partitioning of Nb, Ta, Zr, Sr and Y between garnet, clinopyroxene and basaltic magma at high pressure and temperature[END_REF][START_REF] Jenner | Determination of partition coefficients for trace elements in high pressure-temperature experimental run products by laser ablation microprobe-inductively coupled plasma mass spectrometry (LAM-ICP-MS)[END_REF], and indeed, it is the case of some mafic volcanic rocks from this study, which are characterized by enrichments in the lightest, most incompatible REE, compared to the heaviest REE, with (La/Yb) N , ranging from 11.5 to 19.8. These high values, together with (Tb/Yb) N of 1.9 to 2.9 (figure 9), falling in the range of garnet-bearing mantle source of [START_REF] Wang | A mantle melting profile across the basin and range, SW USA[END_REF], demonstrate that garnet was involved during the extraction of the melts. Moreover, in order to obtain the REE patterns of Bafang mafic rocks, the CVL mantle source also needed to be geochemically richer (trace concentrations and isotopes) than the MORB asthenospheric mantle.

Even if basanites and alkali basalts share some geochemical similarities, and could therefore be believed as resulting from different degrees of partial melting of a single source, some isotopic differences among the least contaminated samples allow to distinguish two groups of samples and to testify of the geochemical non-homogeneity of the mantle source.

On figure 10, Hf isotopes show clear correlations with Th/Y and Zr/Hf : samples with the highest ε Hf have the lowest Zr/Hf, La/Yb and Th/Y ratios. The appearance of both basanites and basalts in the two isotopic trends confirms that their differences in major element compositions are more likely due to partial melting effects, than to source composition.

Because the Sr isotopes are similar for the ocean basalts of the CVL and our even most contaminated continental samples (figure 6), it is difficult to decipher between mantle source heterogeneities and crustal contamination processes, and it is suggested that the contamination processes have been often underestimated in previous studies concerning the continental volcanic rocks from Cameroon. Actually, together on figure 6, Bafang volcanic rocks and the ocean basalts of the CVL show up as a trend that could be accounted for by the mixture of three components : 1) the depleted MORB mantle (DMM) (which would not be dominant in the mixture); 2) an unidentified enriched component with radiogenic Sr and non-radiogenic Nd; and 3) what appears to be the source of Mount Cameroun volcanic rocks (at the extremity of the CVL trend on figure 6) and is particularly rich in radiogenic Pb.

On figure 11a, Pb isotopic compositions of Bafang and some oceanic volcanic samples lie on a trend pointing towards the composition of Mount Cameroon but not St Helena.

However, this contribution of the Mt Cameroon mantle source cannot be detected using Nd isotopes (Fig. 11b).

Geodynamical consequences:

As emphasized by [START_REF] Milelli | A lithospheric instability origin for the Cameroon Volcanic Line[END_REF], the fact that the magmatic activity has been sustained for about 70 Ma all along the continental part of the CVL [START_REF] Déruelle | The Cameroon Line: a review[END_REF] indicates that, unlike in the traditional hotspot -lithospheric drift model, where plate cinematic constantly changes the position of the volcanic activity at the surface, the mantle source of the magmatism along the CVL is structurally associated to the continental crust and therefore, seems to lie within the subcontinental lithospheric mantle. As seen on the figure 11, the Bafang mafic rocks have isotopic compositions similar to both those of volcanic rocks from the continental and the oceanic (Bioko, Principe, Sao Tomé and Pagalu Islands) part of the CVL [START_REF] Halliday | Mantle dynamics: a Nd, Sr, Pb and O isotopics study of the Cameroon line volcanic chain[END_REF][START_REF] Lee | Isotopic variations with distance and time in the volcanic islands of the Cameroon Line: evidence of the mantle plume origin[END_REF][START_REF] Ballentine | Hafnium isotopic studies of the Cameroon line and new HIMU paradoxes[END_REF]. Then, how could a single source has been melting in both of these environments for about 70 Ma? [START_REF] Halliday | Mantle dynamics: a Nd, Sr, Pb and O isotopics study of the Cameroon line volcanic chain[END_REF] proposed that both the subcontinental and the ocean lithospheric mantle were impregnated during the Mesozoic by veins of asthenospheric melts issued from the St. Helena mantle plume, sited just below the CVL at that time. And the later melting of this re-fertilized "plume contaminated" material would account for the HIMU-like signature of some of the CVL basalts. Even if this model has been regularly adopted in the literature [START_REF] Barfod | Noble gases in the Cameroon line and the He, Ne, and Ar isotopic compositions of high µ (HIMU) mantle[END_REF][START_REF] Rankenburg | The role of the continental crust and lithospheric mantle in the genesis of Cameroon volcanic line lavas: constraints from isotopic variations in lavas and megacrysts from the Biu and Jos Plateaux[END_REF][START_REF] Yokoyama | Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa: constraints from 238 U-230 Th-226 Ra and Sr-Nd-Pb isotopes systematics[END_REF], [START_REF] Lee | Isotopic variations with distance and time in the volcanic islands of the Cameroon Line: evidence of the mantle plume origin[END_REF] alternatively suggested, based on geochemical evidence, a different scenario involving a sublithospheric hot zone periodically re-fed by HIMU-like deep mantle plumes. Moreover, seismic velocity data in the upper mantle, led [START_REF] Reusch | Upper mantle structure beneath Cameroon from body wave tomography and the origin of the origin of the Cameroon Volcanic Line[END_REF][START_REF] Reusch | Mantle transition zone thickness beneath Cameroon: evidence for an upper mantle origin for the Cameroon Volcanic Line[END_REF] to the conclusion that there is no thermal anomaly in the upper mantle beneath the CVL deeper than the transition zone Altogether, our new data strongly advocate against an involvement of the St Helena hot spot track in the genesis of the oceanic and continental magmas along the CVL.

Nevertheless there is magmatic activity all along the CVL and it has been continue for more than 70 My along a narrow area parallel to the Benue trough, located ~200 km to the North, and perpendicularly to the continent border. Both this peculiar "Y" morphology, the timescale of its formation and evolution of CVL, and the absence of St-Helena plume geochemical imprint, suit adequately the edge flow convection [START_REF] Reusch | Upper mantle structure beneath Cameroon from body wave tomography and the origin of the origin of the Cameroon Volcanic Line[END_REF]2011) or the upwelling-downwelling [START_REF] Milelli | A lithospheric instability origin for the Cameroon Volcanic Line[END_REF]Milleli et al., 2012) models proposed by the geophysicists.

Hints about the precise petrological composition of the melting source can be looked for within the trace element compositions illustrated in figure 5. As formerly noticed on these spidergrams, many mafic samples, which are not particularly rich in Ca feature positive Eu anomalies, sometimes associated with positive Sr anomaly without phenocrysts of plagioclase being involved. Such peculiarity has already been observed in lavas from the Bambouto volcanic province [START_REF] Marzoli | The Cameroon Volcanic Line revisited: petrogenesis of continental basaltic magmas from lithospheric and asthenospheric mantle sources[END_REF]. Similar anomalies are also present in mafic rocks from the Bamenda Mountains, and [START_REF] Kamgang | Géochimie et géochronologie des laves felsiques des Monts Bamenda NW -Cameroun (Ligne du Cameroun)[END_REF][START_REF] Kamgang | Mantle sources and magma evolution beneath the Cameroon Volcanic Line: Geochemistry of mafic rocks from the Bamenda Mountains (NW Cameroon)[END_REF] argued that they are due to the melting of mafic mantle pyroxenites and the subsequent mixing of these magmas with magmas derived from the melting of lherzolites. Pyroxenites within the lithospheric mantle, could be the remnants of the crystallization of mafic magmas during either the circulation of the St Helena hot spot magmas or older magmatic events which have structured the SCLM in this region. The role of pyroxenite melting in the mantle remains to be clearly identified, and the nature and location of the source of the magmas all along the CVL is still not fully identified, however, the role of the St Helena hot spot is probably less important than previously proposed. More chemical and isotopic data on the oceanic and continental parts of the CVL as well as the modeling of the partial melting of pyroxenites, will help to further understand the origin of this peculiar and long-lasting magmatic province in West Africa.

Conclusions

Major and trace elements as well as Sr-Nd-Pb-Hf isotopes determined on mafic and intermediate lavas from the Bafang area allow us to emphasize several points about the origin of the magmatism along the Cameroon Volcanic Line:

-Lavas of alkaline affinity were emplaced between 10 and 6 Ma in the area of Bafang, in the continental part of the CVL.

-The differentiation from basalts and basanites to mugearites is accounted for by the removal of olivine, clinopyroxene and oxides. Crustal contamination is present in some samples and modifies their isotopic composition.

-The isotopic composition of the uncontaminated samples is consistent with the participation of three distinct mantle components, the depleted MORB mantle (not dominant),

an enriched component and a Pb radiogenic component similar to the source of the Mont Cameroon.

-A mantle source with the composition of the ancient St Helena plume material is not required to explain the volcanism of the CVL, but mantle pyroxenites are probably involved during partial melting processes in the garnet facies.

Figure captions

Figure 1: Location of the Bafang area (white square) along the Cameroon Volcanic Line (adapted after [START_REF] Halliday | Mantle dynamics: a Nd, Sr, Pb and O isotopics study of the Cameroon line volcanic chain[END_REF]. Location of seamounts after [START_REF] Burke | Origin of the Cameroon Line of volcano-capped swells[END_REF]. Inset, bottom right is after [START_REF] Kampunzu | Distribution of the main Phanerozoic African rifts and associated magmatism, introductory notes[END_REF]. The alkaline-subalkaline dividing line is from [START_REF] Irvine | A guide to the chemical classification of the common volcanic rocks[END_REF]. (2003). The data for N-MORB and E-MORB are from [START_REF] Sun | Chemical and isotopic systematic of oceanic basalts: implications for mantle composition and processes[END_REF]. The data for Mount Cameroon [START_REF] Yokoyama | Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa: constraints from 238 U-230 Th-226 Ra and Sr-Nd-Pb isotopes systematics[END_REF] and St Helena [START_REF] Kawabata | The Petrology and Geochemistry of St. Helena Alkali Basalts: Evaluation of the Oceanic Crustrecycling Model for HIMU OIB[END_REF] are also shown for comparison. Symbols as in Figure 3. [START_REF] Zindler | Chemical geodynamics[END_REF].

Data for the CVL and Mt Cameroon are from [START_REF] Halliday | Mantle dynamics: a Nd, Sr, Pb and O isotopics study of the Cameroon line volcanic chain[END_REF], [START_REF] Lee | Isotopic variations with distance and time in the volcanic islands of the Cameroon Line: evidence of the mantle plume origin[END_REF], [START_REF] Ballentine | Hafnium isotopic studies of the Cameroon line and new HIMU paradoxes[END_REF], [START_REF] Marzoli | The Cameroon Volcanic Line revisited: petrogenesis of continental basaltic magmas from lithospheric and asthenospheric mantle sources[END_REF], [START_REF] Rankenburg | The role of the continental crust and lithospheric mantle in the genesis of Cameroon volcanic line lavas: constraints from isotopic variations in lavas and megacrysts from the Biu and Jos Plateaux[END_REF], [START_REF] Yokoyama | Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa: constraints from 238 U-230 Th-226 Ra and Sr-Nd-Pb isotopes systematics[END_REF], [START_REF] Tsafack | The Mount Cameroon stratovolcano (Cameroon Volcanic Line, Central Africa): Petrology, geochemistry, isotope and age data[END_REF], [START_REF] Kamgang | Mantle sources and magma evolution beneath the Cameroon Volcanic Line: Geochemistry of mafic rocks from the Bamenda Mountains (NW Cameroon)[END_REF]. Data for the Atlantic MORB are from Meyzen et al. (2007). Data for St Helena are from [START_REF] Chaffey | Characterization of the St. Helena Magma Source Magmatism in the Ocean Basins[END_REF], [START_REF] Salters | Hf isotope constraints on mantle evolution[END_REF] and [START_REF] Thirlwall | Inter-laboratory and other errors in Pb isotope analyses investigated using a 207Pb-204Pb double spike[END_REF]. Symbols as in Figure 3. [START_REF] Wang | A mantle melting profile across the basin and range, SW USA[END_REF]. Symbols as in Figure 3. and [START_REF] Thirlwall | Inter-laboratory and other errors in Pb isotope analyses investigated using a 207Pb-204Pb double spike[END_REF]. Symbols as in Figure 3.

  ranges from 1.6 to 515 ppm and Ni from 1.1 to 261 ppm (Fig.4), and their content decreases with increasing Mg#, indicating that fractionation of olivine and clinopyroxene are the main factors controlling the compatible element abundances. Ni, Cr and Co contents in the most mafic lavas are generally smaller than the values assumed for primary magmas (Ni: 300 -400 ppm; Cr: 300 -500 ppm; Co: 50 -70 ppm; e.g.,[START_REF] Frey | Integrated models of basalts petrogenesis: a study of quartz tholeiites to olivine melilites from South Eastern Australia utilizing geochemical and experimental petrological data[END_REF][START_REF] Jung | Major and trace element systematics and isotope geochemistry of Cenozoic mafic volcanic from the Vogelsberg (Central Germany): constraints on the origin of continental alkaline and tholeiitic basalts and their mantle sources[END_REF], but are still high enough to indicate that for these samples the fractionation was limited.The chondrite-normalized rare earth element (REE) patterns of Figure5areveal low degrees of melting, as all the samples are highly enriched in light REE relative to heavy REE ([La/Yb] N = 11.92 -20.14, where the subscript N means chondrite-normalized). REE patterns of N-MORB, E-MORB of[START_REF] Sun | Chemical and isotopic systematic of oceanic basalts: implications for mantle composition and processes[END_REF] are also shown in Figure5for comparison. As revealed by Figure5a, REE patterns of all samples are mostly above those of N-MORB and E-MORB and strongly resemble that of alkali Ocean Island Basalts (OIB) from

(

  ~250km), and they attributed the origin of the magmatism to edge flow convection due to lateral variations of lithospheric thickness on the north-western side of the Congo craton. More recently, Milleli et al. (2012) and Fourel et al. (2013) proposed a model with similar endings, but a different driving mechanism. Based on laboratory experiments, they demonstrated that the lithosphere beneath continents can become instable, due to cooling from above. Through time, the instabilities will turn into linear upwellings and downwellings with a radial pattern from the centre of the continent. Partial melting in the SCLM could therefore appear in the upwelling regions while downwelling areas would induce compression in the crust above.

Figure 2 :

 2 Figure 2: Simplified geological map of the studied area (after Dumort, 1968, modified) and

Figure 3 :

 3 Figure 3: Total alkali vs. silica (T.A.S.) classification diagram for the Bafang lavas (Le Bas et

Figure 4 :

 4 Figure 4: SiO 2 (a), TiO 2 (b), Al 2 O 3 (c), Na 2 O (d), CaO (e) and Ni (f) vs. Mg# for the Bafang

Figure 5 :

 5 Figure 5: (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized multi-

Figure 6 :

 6 Figure 6: 143 Nd/ 144 Nd vs. 87 Sr/ 86 Sr (a) and 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb (b) diagrams for the

Figure 7 :

 7 Figure 7: 87 Sr/ 86 Sr vs. MgO (a) and 143 Nd/ 144 Nd vs. SiO 2 (b) for the Bafang volcanic rocks

Figure 8 :

 8 Figure 8: 176 Hf/ 177 Hf vs. 206 Pb/ 204 Pb diagram showing two different trends of crustal

Figure 9 :

 9 Figure 9: Primitive mantle normalized (Tb/Yb) N vs. Th for the Bafang volcanic rocks (after

Figure 10 :

 10 Figure 10: 176 Hf/ 177 Hf vs. Th/Y (a) and 176 Hf/ 177 Hf vs. Zr/Hf (b) diagrams for the Bafang

Figure 11 :

 11 Figure 11: 208 Pb/ 204 Pb vs. 206 Pb/ 204 Pb (a) and 143 Nd/ 144 Nd vs 206 Pb/ 204 Pb (b) diagrams with

  (Mg# = 100 * (MgO / 40.31) / ((MgO / 40.31 + Fe 2 O 3 * 0.8998 / (71.85 * (1 -0.15))), assuming FeOt = Fe 2 O 3 x 0.8998 Fe 2 O 3 ) ranging from 24 to 60.4. Al 2 O 3 increases from 12.3 to 17.1 wt. % corollary with the decrease of the Mg#, while CaO decreases from 11.2 to 4.3 wt.% and Na 2 O and K 2 O increases from 2.2 to 5.3 wt.%, and from 1.2 to 2.5 wt.%, respectively. As illustrated in figure

  . Because of the relatively young ages of the samples, no age correction is necessary. 87 Sr/ 86 Sr ratios range from 0.70312 to 0.70417, while 143 Nd/ 144 Nd ratios vary from 0.51281 to 0.51294, 206 Pb/ 204 Pb from 19.037 to 20.072, 207 Pb/ 204 Pb from 15.540 to 15.666, 208 Pb/ 204 Pb from 38.652 to 39.871 and 176 Hf/ 177 Hf from 0.28288 to 0.28294.

  (BAF36, BAF42) are not contaminated and have isotopic properties similar to those of basanites. Two different trends of contamination can be identified on the Hf vs Pb isotopes diagram of figure 8. Samples BAF 10-15-18-43 define a trend towards low 178 Hf/ 177 Hf and 206 Pb/ 204 Pb, this group corresponds to the samples with the highest 87 Sr/ 86 Sr (>0.704).Samples BAF 2-3-37 also have low 178 Hf/ 177 Hf but they have more radiogenic Pb signature,

similar to those of the uncontaminated rocks. These samples have probably been less affected by contamination processes, and/or have interacted with a different crustal component during differentiation. The BAF3 mugearite would appear to be uncontaminated, if it was not for its 176 Hf/ 177 Hf, which is one among the lowest in Bafang data. Overall, if present, the effect of CC cannot be detected with trace elements.

Table 1 .

 1 Major (wt.%) and trace (ppm) element data for the Bafang lavas.

	Latitude:	N05°11'47.1"	N05°10'22.5"	N05°13'19.1" N05°04'27.2" N05°10'59.8" N05°11'46.8"
	Longitude:	E10°13'08.2"	E10°12'31.9"	E10°17'41.4" E10°10'14.0" E10°17'38.8"	E10°19'22.1"
	Rocks type	Basanites
	Samples	BAF 5a	BAF 11	BAF 22	BAF 34	BAF 44	BAF 45
	SiO2	44.32	44.36	42.52	43.44	43.41	42.80
	TiO2	2.81	2.92	3.34	2.87	3.25	3.22
	Al2O3	16.25	16.28	13.04	13.44	13.16	12.22
	Fe2O3t	13.08	13.23	13.73	13.05	13.57	13.02
	MnO	0.22	0.22	0.24	0.21	0.19	0.20
	MgO	4.97	5.22	10.49	9.52	10.08	11.81
	CaO	8.74	8.91	11.04	10.19	10.53	11.09
	Na2O	3.78	4.24	2.21	2.45	2.70	3.29
	K2O	1.64	1.69	1.47	1.32	1.27	1.35
	P2O5	1.42	1.37	0.75	0.78	0.70	0.71
	LOI	1.71	0.63	1.68	1.85	1.15	0.33
	TOTAL	98.93	99.06	100.50	99.11	100.01	100.06
	Mg#	39.04	39.92	56.27	55.13	55.56	60.43
	Sc	14.28	16.84	29.50	25.36	28.54	29.73
	V	176.6	197.3	271.5	242.8	263.6	276.9
	Cr	12.51	18.89	285.0	282.5	255.7	515.6
	Mn	1689	1726	1889	1703	1563	1638
	Co	34.02	37.20	59.49	52.38	56.32	59.96
	Ni	18.91	24.72	190.3	185.7	180.2	261.4
	Cu	25.68	29.11	53.48	50.65	52.01	59.70
	Zn	125.7	134.2	108.9	118.4	113.1	107.8
	Ga	20.87	21.12	17.83	19.43	18.58	18.02
	Rb	45.34	40.41	34.56	35.23	27.87	41.22
	Li	10.57	11.57	5.92	8.14	6.57	8.42
	Be	3.09	3.14	1.48	1.88	1.55	1.77
	Sr	1366	118	925.3	994.1	973.6	920.4
	Y	32.54	34.02	31.12	33.45	32.38	32.33
	Zr	282.0	283.8	247.1	324.3	256.0	287.3
	Nb	84.92	84.34	74.99	94.93	75.09	95.83
	Cs	0.90	0.47	0.36	0.66	0.34	0.55
	Ba	659.9	648.6	480.4	559.8	497.0	557.1
	La	53.26	52.42	47.62	69.15	48.62	57.16
	Ce	109.9	107.4	98.0	135.6	100.2	112.3
	Pr	13.84	13.42	12.04	16.16	12.33	13.62
	Nd	54.83	54.36	48.72	60.34	49.35	52.71
	Sm	10.38	10.26	9.36	10.54	9.58	9.89
	Eu	3.27	3.23	3.00	3.24	3.06	3.08
	Gd	8.87	8.85	8.32	8.73	8.50	8.60
	Tb	1.18	1.19	1.14	1.20	1.16	1.17
	Dy	6.26	6.24	6.01	6.30	6.20	6.21
	Ho	1.14	1.16	1.10	1.16	1.12	1.13
	Er	2.85	2.92	2.73	2.91	2.84	2.85
	Tm	2.56	2.57	2.79	2.24	2.52	2.43
	Yb	2.29	2.32	2.16	2.38	2.18	2.25
	Lu	0.31	0.32	0.29	0.33	0.30	0.31
	Hf	5.76	5.76	5.23	6.39	5.31	5.74
	Ta	4.72	4.60	4.01	3.71	3.72	5.32
	W	0.70	0.75	0.34	0.65	0.66	1.28
	Pb	4.12	4.07	2.10	3.50	2.22	3.01
	Th	4.78	4.70	4.47	6.92	4.31	6.19
	U	1.33	1.26	1.12	1.76	1.12	1.64

Table Latitude

 Latitude

	Latitude:	N05°12'08.5''	N05°09'51.5"	N05°09'40.8" N05°04'49.4" N05°07'51.1" N05°09'40.5" N05°09'42.2"
	Longitude:	E10°11'17.5''	E10°15'04.4"	E10°14'29.2" E10°09'51.9" E10°09'14.0"	E10°10'08.4"	E10°11'51.8"
	: Rocks type	N05°09'10.9" Hawaiites	N05°09'42.9"	N05°06'14.7" N05°09'24.7" N05°09'44.3"
	Longitude: Samples	E10°17'53.4" BAF 2	E10°18'21.5" BAF 10	E10°10'10.0" E10°18'00.5" E10°17'43.7" BAF 13 BAF 33 BAF 38	BAF 40	BAF 46
	Rocks type SiO2 Samples TiO2 SiO2 Al2O3 TiO2 Fe2O3t Al2O3 MnO Fe2O3t MgO MnO CaO MgO Na2O CaO K2O Na2O P2O5 LOI K2O P2O5 TOTAL	48.60 Basalts BAF 15 2.46 46.33 16.51 3.17 13.01 15.63 0.22 12.95 3.60 0.17 5.88 6.51 4.72 8.62 2.04 3.40 1.00 1.22 1.19 0.53 99.23	47.77 BAF 18 2.55 47.12 16.75 3.27 13.30 15.91 0.23 13.12 3.86 0.20 6.70 6.34 3.94 8.79 1.99 3.76 0.76 1.24 1.22 0.51 99.05	47.44 BAF 36 2.65 45.45 16.76 3.00 13.48 15.50 0.22 13.82 3.99 0.22 6.92 5.77 3.97 7.97 1.89 3.38 0.75 1.29 1.54 1.28 99.60	47.52 BAF 42 2.66 44.93 16.68 2.96 13.80 14.19 0.23 11.44 3.70 0.18 6.22 9.12 4.27 9.42 1.93 2.77 0.98 1.90 1.29 0.68 99.28	47.38 2.47 BAF 43 15.26 46.76 12.66 3.18 0.25 15.67 5.47 12.99 6.92 0.17 3.58 6.69 2.12 8.41 1.17 3.39 1.22 2.27 0.53 99.55	46.93 2.83 16.57 14.03 0.23 4.33 7.03 3.66 1.79 0.75 2.01 100.16	47.30 2.70 16.71 13.65 0.22 3.77 7.15 4.35 1.97 0.75 0.74 99.31
	LOI	0.67	-0.41	1.79	2.26	0.59
	TOTAL	99.21	99.86	99.47	99.84	99.60
	Mg#	45.85	44.88	41.30	57.31	46.46
	Sc	20.35	21.49	17.52	19.59	21.47
	V	217.6	230.7	158.7	216.2	228.7
	Cr	125.6	115.7	67.26	273.6	124.9
	Mn	1311	1576	1654	1389	1383
	Co	46.78	47.90	37.00	47.87	47.91
	Ni	82.05	81.70	50.66	155.9	85.43
	Cu	41.21	35.83	35.64	38.41	45.59
	Zn	109.3	109.9	110.8	110.0	110.8
	Ga	19.00	19.73	18.97	20.15	20.32
	Rb	20.27	24.19	17.59	42.12	22.28
	Li	7.06	7.32	6.46	6.35	7.23
	Be	1.28	1.32	1.34	2.07	1.40
	Sr	1000	782.9	1021	1011	817.4
	Y	25.85	27.72	33.94	25.82	27.41
	Zr	202.1	206.5	218.7	263.7	212.1
	Nb	55.91	50.67	60.97	82.76	58.71
	Cs	0.64	0.43	0.23	0.46	0.34
	Ba	439.8	427.3	582.9	615.3	440.3
	La	34.71	33.83	44.69	44.80	35.97
	Ce	70.38	69.31	96.84	88.50	72.35
	Pr	8.61	8.43	12.70	10.93	8.99
	Nd	34.53	34.19	54.56	43.62	35.86
	Sm	6.99	6.98	10.82	8.57	7.16
	Eu	2.41	2.44	4.05	2.75	2.49
	Gd	6.53	6.61	9.83	7.49	6.67
	Tb	0.90	0.93	1.30	1.00	0.94
	Dy	4.91	5.04	6.75	5.15	5.05
	Ho	0.91	0.96	1.24	0.91	0.93
	Er	2.28	2.45	2.96	2.20	2.37
	Tm	2.42	2.47	2.30	2.13	2.09
	Yb	1.81	1.97	2.19	1.65	1.86
	Lu	0.25	0.27	0.30	0.22	0.26
	Hf	4.39	4.32	4.72	5.31	4.43
	Ta	3.16	2.54	3.40	4.45	3.12
	W	0.26	0.54	0.17	0.98	0.63
	Pb	2.17	2.06	1.78	2.43	2.10
	Th	3.46	3.23	3.23	4.67	3.39
	U	0.89	0.83	0.90	1.39	0.86

Acknowledgements.

We thank IRD (Institut de Recherche pour le Développement) for logistic support during field work in Cameroon. J. Langlade, C. Liorzou and C. Bassoulet are thanked for their help in the geochemical data acquisition. Comments by two reviewers were greatly appreciated and helped to improve the manuscript.

Ages are calculated using the constants proposed by Steiger and Jäger (1978).

Errors are calculated at ± 1 sigma using the calculations by [START_REF] Cox | Statistical analysis of geomagnetic reversal data and the precision of potassium-argon dating[END_REF].

Sample